sustained activity of pt cathode modified by …v list of figures figure 1.2.1. schematic diagram of...
TRANSCRIPT
工學碩士學位 請求論文
DMFC 에서 염소이온으로 처리된 Pt Cathode 전극의 지속적인 활성에 관한 연구
Sustained Activity of Pt Cathode Modified by Chloride Ions in DMFC
2006 年 2 月
仁荷大學校 大學院
化學工學科 (工業化學 專攻)
徐明煥
工學碩士學位 請求論文
DMFC 에서 염소이온으로 처리된 Pt Cathode 전극의 지속적인 활성에 관한 연구
Sustained Activity of Pt Cathode Modified by Chloride Ions in DMFC
2006 年 2 月
指 導 敎 授 卓 容 奭
이 論文을 碩士學位 論文으로 提出함
이 論文을 徐明煥의 碩士學位論文으로 認定함
2006 年 2 月
主 審
副 審
委 員
Sustained Activity of Pt Cathode Modified by Chloride Ions in DMFC
by MyeongHwan Seo
A THESIS
Submitted to the faculty of
INHA UNIVERSITY
in partial fulfillment of the requirements
for the degree of
MASTER OF ENGINEERING
Department of Chemical Engineering February 2006
i
요 약
수소의 흡착탈착 공정과 백금전극의 산화환원반응의 거동 그리고
메탄올의 전기화학적 산화반응에 관한 염소이온으로 처리된 백금전극의
전기화학적 특성을 조사하였다 순수한 백금전극과 염소이온으로 처리된
백금전극을 비교해봄으로써 염소이온으로 처리된 백금전극은 양이온의
흡착능력을 유지하였고 백금전극의 산화를 억제하였다 이것은 산소와
백금전극 표면 사이에서 결합에너지의 감소를 일으키는 것으로 판단된다
순환전위법(CV)으로 캐소드 방향에서 산소환원반응의 과전압이 감소하는
것을 알 수 있었다 또한 염소이온으로 처리된 백금전극은 직접메탄올
연료전지의 성능을 향상시켰다 그리고 캐소드에 처리된 백금전극을 가진
전지의 장기 안정성에 관하여 조사하였고 그 결과 처리되지 않은
백금전극보다 5배정도의 안정성을 나타내는 것을 확인하였다
주제어 연료전지 직접메탄올연료전지 염소이온 처리 산소환원반응 메탄올 크로스오버
ii
ABSTRACT
We investigated electrochemical characteristics of chloride ion
modified Pt (PtClmminus) electrode in the electrocatalytic oxidation of methanol
oxidationreduction behaviours of Pt adsorptiondesorption phenomena of
hydrogen Comparing with pure Pt surface PtClmminus maintained the adsorption
activity of proton hindered oxidation of Pt induced less binding energy
between oxygen and Pt surface It resulted in the decrease of overpotential of
oxygen reduction reaction on the cathodic scan Thus PtClmminus could increase
power performance of direct methanol fuel cell In addition to activity of
PtClmminus the long-term durability of PtClm
minus cathode was also investigated and
it had about five times longer stability
Keywords Fuel cell DMFC methanol oxidation modified electrode
iii
TABLE OF CONTENTS
요 약helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipⅰ
Abstracthelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipⅱ
List of figureshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipⅴ
List of tablehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipVii
1 Introductionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip1
11 What is a fuel cell 1
12 Direct methanol fuel cell (DMFC) 2
13 Objectives 4
2 Basic Theoryhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip6
21 Electro-oxidation of methanol 6
22 Oxygen reduction reaction helliphelliphellip 10
23 Anionic effects 11
24 Actual performance 12
3 Experimentalhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip18
31 Polycrystalline Platinum Preparation 18
32 Electrode Preparation and the Modification 19
iv
33 Half Cell Measurements 19
34 Single cell measurements 22
4 Results and Discussionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip26
5 Conclusionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip49
6 Referenceshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip50
7 Acknowledgementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip52
v
List of Figures Figure 121 Schematic diagram of a DMFC
Figure 241 Typical Fuel Cell Polarization Curve
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
Figure 331 Schematic diagram of experimental set-up for half-cell experiments
Figure 341 Configuration of catalystdiffusion backing and membrane for type a MEA
Figure 342 Schematic diagram of experimental set-up for single cell measurements
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M H2SO4 Scan rate = 50mVs
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M CH3OH05M H2SO4 Scan rate = 50mVs
Figure 415 The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
vi
Figure 416 The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
Figure 417 Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
Figure 418 Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
Figure 419 DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Clminus (c) 0001 M Clminus Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
Figure 4111 Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
vii
List of Table
Table 21 Effect of catalyst promoters on methanol oxidation
1
11 What Is a Fuel Cell
As early as 1839 William Grove discovered the basic operating
principle of fuel cells by reversing water electrolysis to generate electricity
from hydrogen and oxygen The principle that he discovered remains
unchanged today A fuel cell is an electrochemical ldquo device rdquo that
continuously converts chemical energy into electric energy (and some heat)
for as long as fuel and oxidant are supplied Fuel cells therefore bear
similarities both to batteries with which they share the electrochemical
nature of the power generation process and to engines which unlike batteries
will work continuously consuming a fuel of some sort Here is where the
analogies stop though Unlike engines or batteries a fuel cell does not need
recharging it operates quietly and efficiently and when hydrogen is used as
fuel it generates only power and drinking water Thus it is a so-called zero
emission engine [1]
1 INTRODUCTION
2
12 Direct Methanol Fuel Cell (DMFC)
A schematic of a DMFC is shown in Figure 11 Methanol and water
electrochemically react (ie methanol is electro-oxidized) at the anode to
produce carbon dioxide protons and electrons as shown in reaction (121)
An acidic electrolyte is advantageous to aid CO2 rejection since insoluble
carbonates form in alkaline electrolytes The protons produced at the anode
migrate through the polymer electrolyte to the cathode where they react with
oxygen (usually from air) to produce water as shown in reaction (122) The
electrons produced at the anode carry the free energy change of the chemical
reaction and travel through the external circuit where they can be made to do
useful work such as powering an electric motor The overall cell reaction as
shown in equation (123) is therefore the reaction of methanol and oxygen
to produce water and carbon dioxide
Anode CH3OH + H2O rarr CO2 + 6H+ + 6e- 0AE = 0046V (121)
Cathode 32
O2 + 6H+ + 6e- rarr 3H2O 0CE = 1229V (122)
Net Reaction CH3OH + 32
O2 rarr CO2 + 2H2O (123)
0cellE = 0
CE - 0AE =1229V - 0046V = 1183V Attainable max cell potential
3
Figure 121 Schematic diagram of direct fuel cell
4
13 Objectives
Of late fuel cells have become a central focus of interest and leading
car and power companies are making large scale investments in fuel cell
systems Fuel cells are set to provide electrical vehicles small and large scale
power stations and even portable electrical applications such as laptops and
mobile phones with electrical power in a manner which is more efficient and
more environmentally friendly than ever before [2 minus6] In particular direct
methanol fuel cells (DMFCs) have been considered for use in very small to
mid-sized applications due to their high electrical efficiency long life-time
and low poisonous emissions [2 minus 6] On the other hand in the operation of
DMFCs there are several problems at a cathode caused by methanol
crossover from anode to cathode and it should be solved for the
commercialization of DMFC (i) cathode poisoning by CO adsorption and Pt
sintering (ii) significant fuel loss due to methanol crossover (iii) decrease
partial pressure of oxygen due to water flooding and (iv) temperature
dependent power performance Therefore cathode catalyst for oxygen
reduction reaction (ORR) requires (a) CO tolerance (b) proton adsorption
available (c) easy oxygen adsorption (d) ORR kinetics (e) oxygen storage
material (f) suppression of electro-oxidation of methanol To our knowledge
5
no work has been done up to now to study the influence of halide
contamination on fuel cell performance From the data presented previously
Paulus et al [2] could only point to some aspects which should be considered
during MEA production the degradation of MEA might be occurred due to
the dissolution of Nafion by H2O2 and it was mainly produced in the
reduction of oxygen on a Pt substrate containing chloride ions
In this work we tried to develop new cathode catalyst in DMFC via
adsorption of chloride ion which has been known as inhibiting species in
most electrochemical studies The electrochemical characteristics of chloride
ion modified Pt cathode in the oxidation of methanol and
adsorptiondesorption of hydrogen were studied by cyclic voltammetry
techniques The power performance of the single cell and long-term stability
of pure Pt and modified Pt cathode was also investigated
6
21 Electro-oxidation of Methanol
The electrochemical oxidation of methanol on Pt involves several
intermediate steps ie dehydrogenation CO-like species chemisorption OH
(or H2O) species adsorption chemical interaction between adsorbed CO and
OH compounds and CO2 evolution One of these steps is the rate
determining step (rds) depending on the operating temperature and
particular catalyst surface State of the art electrocatalysts for the electro-
oxidation of methanol in fuel cells are generally based on Pt alloys supported
on carbon black [8 9] The electrocatalytic activity of Pt is known to be
promoted by the presence of a second metal such as Ru or Sn acting either
as an adatom or a bimetal First a sequence of dehydrogenation steps give rise
to adsorbed methanolic residues according to the following scheme[10]
CH3OH + Pt rarr Pt-CH2OH + H+ + 1e- (211)
Pt-CH2OH + Pt rarr Pt-CHOH + H+ + 1e- (212)
Pt-CHOH + Pt rarr PtCHO + H+ + 1e- (213)
2 BASIC THEORY
7
A surface rearrangement of the methanol oxidation intermediates
gives carbon monoxide linearly or bridge-bonded to Pt sites as follows
PtCHO rarr Pt-CequivO + H+ + 1e- (214)
Or
PtCHO + Pt rarr Pt Pt C=O + H+ + 1e- (215)
In the absence of a promoting element water discharge occurs at
high anodic overpotentials on Pt with the formation of Pt-OH species at the
catalyst surface
Pt + H2O rarr PtOH + H+ + 1e- (216)
The final step is the reaction of Pt-OH groups with neighboring
methanolic residues to give carbon dioxide
PtOH + Pt CO rarr 2Pt + CO2 + H+ + 1e- (217)
The overall oxidation process of methanol to carbon dioxide
proceeds through a six electron donation process On a pure Pt surface the
dissociative chemisorption of water on Pt is the rate determining step at
potentials below 07 V vs RHE [11] It is generally accepted that an active
catalyst for methanol oxidation should give rise to water discharging at low
potentials and to labile CO chemisorption Moreover a good catalyst for
methanol oxidation should also catalyze the oxidation of carbon monoxide
8
The promotion of Pt by a range of metals (and oxides) to enhance
methanol oxidation is well known Table 21 summeries species that have a
reported positive effect on methanol oxidation Of the catalyst promoters
considered significant recent activity has focused on the use of Ru Sn and
W with Pt
On PtRu surface CH3OH adsorption would take place on the Pt site
where CH3OH chemisorption requires three neighboring sites yielding
ensembles of Pt atoms with COad and the Ru atoms would act as OHad
providing center According to this the catalytic improvement would be
bifunctional mechanism There are different reports for optimal Ru contents
According to Gasteiger el al a Ru content of 10 is optimum[12] Increasing
Ru content provides more adsorbed water species required to oxidize COad
However because Pt sites are decreased with increasing Ru contents the
oxidation rate of CH3OH is decreased Consequently the rate of reaction
decreased when the Ru contents are more than 10 But other results
referring a coverage close to 05 by Ru is optimal for enhancing CH3OH
oxidation are reported by R Dillon[13]
9
Table 21 Effect of catalyst promoters on methanol oxidation
Promotion by Catalyst promoter
comment
Alloying and
dissolution to produce
highly reticulated
surfaces
Cr
Fe
Sn
Typically less than 100 mV
lower potential than Pt
Surface adatoms Sn
Bi
Alloys
Ru
Sn Mo
Os Ir
Ti Re
Ru has the greatest effect Sn
Mo Os and Re are substantial
promoters
Metal oxides Ru Hydrous Ru oxide the most
active catalyst
Base metal oxides W
Nb Zr Ta
W oxide is a notable prometer
Small effect of other metal
oxides (typically lt 100mV)
10
22 Oxygen Reduction Reaction
Oxygen reduction reaction is considered to be one of the most
important electrocatalytic reactions because of its role in electrochemical
energy conversion several industrial processes and corrosion Consequently
it was in the focus of electrochemical interest for many years Oxygen
reduction reaction has been a challenge for electrochemists because of its
complex kinetics and the need for better electrocatalysts The most notable
need for the improvement of the catalytic activity and the development of
new better non-noble metal electrocatalysts are still remaining Despite
extensive research on oxygen reduction reaction mechanism is still not fully
understood
Oxygen reduction reaction is known to proceed either in a four-
electron step to H2O or in a two-electron step to H2O2 In general the four-
electron-step is considered to be the major reaction pathway on
polycrystalline platinum electrodes [1]
+ -2 2 0O + 4H + 4e 2H O E =1229V ( NHE)vsrarr (221)
- -2 2 0O + 2H O + 2e 4OH E =0401Vrarr (222)
The peroxide pathway is
11
+ -2 2 2 0O + 2H + 2e H O E =067Vrarr (223)
Peroxide can undergo further reduction or decomposition in acid
solutions via the following reactions
+ -2 2 2 0H O + 2H + 2e 2H O E =177Vrarr (224)
2 2 2 22H O 2H O + O rarr
However it is commonly agreed that it proceeds without formation of
surface peroxide on polycrystalline and particulate Pt in acidic media
Furthermore it is believed that the reaction proceeds
O2 harr O2ads (225)
O2ads + H+(aq) +e- rarr O2Hads (226)
O2Hads + 3H+(aq) +3e- rarr 2H2O (227)
with the first charge transfer reaction (226) as the rate determining step The
details of reaction (227) are unknown and may include several Oads or OHads
configurations [2]
23 Anionic Effects
Platinum or platinum alloy particles supported on high surface area
carbon substrates are often synthesized form halide containing reactants (eg
Cl- containing salts)[15] It is therefore possible that traces of halides are
12
present in the electrocatalyst Furthermore halides especially chloride may
also be present in the humidified anode feed-streams of PEFCs or in the
cathode air feed During the ORR remaining anions can compete with
oxygen for free adsorption sites on the catalyst Modified adsorption
conditions for oxygen molecules can have an impact on the oxygen reduction
activity of the catalyst and can alter the oxygen reduction pathway (transition
form H2O to H2O2 formation) From both Pt single crystal work (eg [14]) it
is well known that different supporting electrolytes containing anions with
different adsorption strengths influence the ORR activity tremendously It is
also well known that the presence of additional halide anions can
dramatically affect the kinetics of electrocatalytic reactions on Pt e g the
oxidation of small organic molecules or the oxygen reduction reaction (e g
[16])
24 Actual Performance
The actual cell potential is decreased from its ideal potential because
of several types of irreversible losses as shown in Figure 21 These losses
are often referred to as polarization overpotential or overvoltage though
13
only the ohmic losses actually behave as a resistance Multiple phenomena
contribute to irreversible losses in an actual fuel cell
In the V-I diagram especially for low-temperature fuel cells the
effects of the three loss categories are often easy to distinguish as illustrated
in Figure 21
Activation Losses Activation losses are caused by sluggish electrode
kinetics There is a close similarity between electrochemical and chemical
reactions in that both involve an activation energy that must be overcome by
the reacting species In reality activation losses are the result of complex
surface electrochemical reaction steps each of which have their own reaction
rate and activation energy In electrochemical reactions the activation energy
loss for overcoming this barrier is represented as Tafel equation
lnact a b iη = + (241)
where actη is activation polarization(mV) a and b are constants and i is
current density(mAcm2)
( ) ( ) ( )act cell act anode act cathodeη η η= + (242)
Ohmic Polarization Ohmic losses occur because of resistance to the
flow of ions in the electrolyte and resistance to flow of electrons through the
electrode The dominant ohmic losses through the electrolyte are reduced by
14
decreasing the electrode separation and enhancing the ionic conductivity of
the electrolyte Because both the electrolyte and fuel cell electrodes obey
Ohms law the ohmic losses can be expressed by the equation
ohm iRη = (243)
where i is the current flowing through the cell and R is the total cell
resistance which includes electronic ionic and contact resistance
R = Relectronic + Rionic + Rcontact
Mass Transport Related Losses As a reactant is consumed at the
electrode by electrochemical reaction it is often diluted by the products
when finite mass transport rates limit the supply of fresh reactant and the
evacuation of products As a consequence a concentration gradient is formed
which drives the mass transport process
Concentration polarization can be denoted by
ln(1 )consL
RT inF i
η = minus (244)
where consη is concentration polarization(mV) iL is limiting current
density(mAcm2) Same as the activation polarization the concentration
polarization occurs independently at either electrode
( ) ( ) ( )conc cell conc anode conc cathodeη η η= + (245)
15
The combined effect of the losses for a given cell and given
operating conditions can be expressed as polarizations The total polarization
at the electrodes is the sum of actη and concη or
anode acta a conc aη η η= + (246)
and
cathode acta c conc cη η η= + (247)
The effect of polarization is to shift the potential of the electrode
(Eelectrode) to a new value (Velectrode)
electrode electrode electrodeV E η= plusmn (248)
For the anode
anode anode anodeV E η= + (249)
And for the cathode
cathode cathode cathodeV E η= minus (2410)
The net result of current flow in a fuel cell is to increase the anode
potential and to decrease the cathode potential thereby reducing the cell
voltage
The cell voltage includes the contribution of the anode and cathode
potentials and ohmic polarization
16
cell cathode anodeV V V iR= minus minus (2411)
When Equations (249) and (2410) are substituted in Equation (2411)
( )cell cathode cathode anode anodeV E E iRη η= minus minus + minus (2412)
Or
cell e cathode anodeV E iRη η= ∆ minus minus minus (2413)
Where e cathode anodeE E E∆ = minus Equation (2413) shows that current
flow in a fuel cell results in a decrease in cell voltage because of losses by
electrode and ohmic polarizations The goal of fuel cell developers is to
minimize the polarization so that cellV approaches eE∆ This goal is
approached by modifications to fuel cell design (improvement in electrode
structures better electro-catalysts more conductive electrolyte thinner cell
components etc) [1]
17
Figure 241 Typical Fuel Cell Polarization Curve
18
31 Polycrystalline Platinum Preparation
A polycrystalline platinum electrode was prepared with pulse
electrodeposition on Pt- quartz crystal
1) Working electrode Pt quartz crystal counter electrode Pt wire
reference electrode saturated calomel electrode (SCE)
2) Pulse electrodeposition method ton -03V 100ms and toff 0V 300ms
3) Deposition bath 10mM H2PtCl6 + 01M HCl (65)
Figure 31 shows the cross-section diagram of the platinized
polycrystalline platinum quartz crystal
Polycrystalline Pt
Sputtered Pt Sputtered Ti
9MHz At-cut quartz crystal
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
3 EXPERIMENTAL
19
32 Electrode Preparation and the Modification
A preparation procedure of catalyst ink was well described
elsewhere [7] and the catalyst (Pt or PtRu) loading was 3 mgcm2 in the each
electrode A geometric active area was a 5cm2 and catalyst ink was directly
sparied onto carbon paper Simple dipping method was used for the chloride
ion modification Pt cathode on carbon paper was dipped in 1 times 10minus2M KCl
solution for 5min and it was rinsed in ultrapure water followed by drying in
the oven
33 Half Cell Measurements
Figure 331 shows the schematic experimental setup for measuring
of the electrocatalystic activity of Pt electrode oxidationreduction
adsorptiondesorption of hydrogen methanol oxidation The electrochemical
cell body consisted of a glass cylinder capped with a Teflon lid holding all
electrodes The Pt nanocatalyst sprayed on carbon paper (gas diffusion layer)
was used as working electrode (WE) The geometric area of the WE was
02 cm2 A concentric platinized Pt wire was used as counter electrode (CE)
The tip of a Luggin-Haber capillary hosting a SCE reference electrode (RE)
was placed in the middle of two electrodes A poteniostatgalvanostat (EG amp
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
工學碩士學位 請求論文
DMFC 에서 염소이온으로 처리된 Pt Cathode 전극의 지속적인 활성에 관한 연구
Sustained Activity of Pt Cathode Modified by Chloride Ions in DMFC
2006 年 2 月
指 導 敎 授 卓 容 奭
이 論文을 碩士學位 論文으로 提出함
이 論文을 徐明煥의 碩士學位論文으로 認定함
2006 年 2 月
主 審
副 審
委 員
Sustained Activity of Pt Cathode Modified by Chloride Ions in DMFC
by MyeongHwan Seo
A THESIS
Submitted to the faculty of
INHA UNIVERSITY
in partial fulfillment of the requirements
for the degree of
MASTER OF ENGINEERING
Department of Chemical Engineering February 2006
i
요 약
수소의 흡착탈착 공정과 백금전극의 산화환원반응의 거동 그리고
메탄올의 전기화학적 산화반응에 관한 염소이온으로 처리된 백금전극의
전기화학적 특성을 조사하였다 순수한 백금전극과 염소이온으로 처리된
백금전극을 비교해봄으로써 염소이온으로 처리된 백금전극은 양이온의
흡착능력을 유지하였고 백금전극의 산화를 억제하였다 이것은 산소와
백금전극 표면 사이에서 결합에너지의 감소를 일으키는 것으로 판단된다
순환전위법(CV)으로 캐소드 방향에서 산소환원반응의 과전압이 감소하는
것을 알 수 있었다 또한 염소이온으로 처리된 백금전극은 직접메탄올
연료전지의 성능을 향상시켰다 그리고 캐소드에 처리된 백금전극을 가진
전지의 장기 안정성에 관하여 조사하였고 그 결과 처리되지 않은
백금전극보다 5배정도의 안정성을 나타내는 것을 확인하였다
주제어 연료전지 직접메탄올연료전지 염소이온 처리 산소환원반응 메탄올 크로스오버
ii
ABSTRACT
We investigated electrochemical characteristics of chloride ion
modified Pt (PtClmminus) electrode in the electrocatalytic oxidation of methanol
oxidationreduction behaviours of Pt adsorptiondesorption phenomena of
hydrogen Comparing with pure Pt surface PtClmminus maintained the adsorption
activity of proton hindered oxidation of Pt induced less binding energy
between oxygen and Pt surface It resulted in the decrease of overpotential of
oxygen reduction reaction on the cathodic scan Thus PtClmminus could increase
power performance of direct methanol fuel cell In addition to activity of
PtClmminus the long-term durability of PtClm
minus cathode was also investigated and
it had about five times longer stability
Keywords Fuel cell DMFC methanol oxidation modified electrode
iii
TABLE OF CONTENTS
요 약helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipⅰ
Abstracthelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipⅱ
List of figureshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipⅴ
List of tablehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipVii
1 Introductionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip1
11 What is a fuel cell 1
12 Direct methanol fuel cell (DMFC) 2
13 Objectives 4
2 Basic Theoryhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip6
21 Electro-oxidation of methanol 6
22 Oxygen reduction reaction helliphelliphellip 10
23 Anionic effects 11
24 Actual performance 12
3 Experimentalhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip18
31 Polycrystalline Platinum Preparation 18
32 Electrode Preparation and the Modification 19
iv
33 Half Cell Measurements 19
34 Single cell measurements 22
4 Results and Discussionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip26
5 Conclusionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip49
6 Referenceshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip50
7 Acknowledgementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip52
v
List of Figures Figure 121 Schematic diagram of a DMFC
Figure 241 Typical Fuel Cell Polarization Curve
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
Figure 331 Schematic diagram of experimental set-up for half-cell experiments
Figure 341 Configuration of catalystdiffusion backing and membrane for type a MEA
Figure 342 Schematic diagram of experimental set-up for single cell measurements
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M H2SO4 Scan rate = 50mVs
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M CH3OH05M H2SO4 Scan rate = 50mVs
Figure 415 The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
vi
Figure 416 The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
Figure 417 Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
Figure 418 Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
Figure 419 DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Clminus (c) 0001 M Clminus Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
Figure 4111 Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
vii
List of Table
Table 21 Effect of catalyst promoters on methanol oxidation
1
11 What Is a Fuel Cell
As early as 1839 William Grove discovered the basic operating
principle of fuel cells by reversing water electrolysis to generate electricity
from hydrogen and oxygen The principle that he discovered remains
unchanged today A fuel cell is an electrochemical ldquo device rdquo that
continuously converts chemical energy into electric energy (and some heat)
for as long as fuel and oxidant are supplied Fuel cells therefore bear
similarities both to batteries with which they share the electrochemical
nature of the power generation process and to engines which unlike batteries
will work continuously consuming a fuel of some sort Here is where the
analogies stop though Unlike engines or batteries a fuel cell does not need
recharging it operates quietly and efficiently and when hydrogen is used as
fuel it generates only power and drinking water Thus it is a so-called zero
emission engine [1]
1 INTRODUCTION
2
12 Direct Methanol Fuel Cell (DMFC)
A schematic of a DMFC is shown in Figure 11 Methanol and water
electrochemically react (ie methanol is electro-oxidized) at the anode to
produce carbon dioxide protons and electrons as shown in reaction (121)
An acidic electrolyte is advantageous to aid CO2 rejection since insoluble
carbonates form in alkaline electrolytes The protons produced at the anode
migrate through the polymer electrolyte to the cathode where they react with
oxygen (usually from air) to produce water as shown in reaction (122) The
electrons produced at the anode carry the free energy change of the chemical
reaction and travel through the external circuit where they can be made to do
useful work such as powering an electric motor The overall cell reaction as
shown in equation (123) is therefore the reaction of methanol and oxygen
to produce water and carbon dioxide
Anode CH3OH + H2O rarr CO2 + 6H+ + 6e- 0AE = 0046V (121)
Cathode 32
O2 + 6H+ + 6e- rarr 3H2O 0CE = 1229V (122)
Net Reaction CH3OH + 32
O2 rarr CO2 + 2H2O (123)
0cellE = 0
CE - 0AE =1229V - 0046V = 1183V Attainable max cell potential
3
Figure 121 Schematic diagram of direct fuel cell
4
13 Objectives
Of late fuel cells have become a central focus of interest and leading
car and power companies are making large scale investments in fuel cell
systems Fuel cells are set to provide electrical vehicles small and large scale
power stations and even portable electrical applications such as laptops and
mobile phones with electrical power in a manner which is more efficient and
more environmentally friendly than ever before [2 minus6] In particular direct
methanol fuel cells (DMFCs) have been considered for use in very small to
mid-sized applications due to their high electrical efficiency long life-time
and low poisonous emissions [2 minus 6] On the other hand in the operation of
DMFCs there are several problems at a cathode caused by methanol
crossover from anode to cathode and it should be solved for the
commercialization of DMFC (i) cathode poisoning by CO adsorption and Pt
sintering (ii) significant fuel loss due to methanol crossover (iii) decrease
partial pressure of oxygen due to water flooding and (iv) temperature
dependent power performance Therefore cathode catalyst for oxygen
reduction reaction (ORR) requires (a) CO tolerance (b) proton adsorption
available (c) easy oxygen adsorption (d) ORR kinetics (e) oxygen storage
material (f) suppression of electro-oxidation of methanol To our knowledge
5
no work has been done up to now to study the influence of halide
contamination on fuel cell performance From the data presented previously
Paulus et al [2] could only point to some aspects which should be considered
during MEA production the degradation of MEA might be occurred due to
the dissolution of Nafion by H2O2 and it was mainly produced in the
reduction of oxygen on a Pt substrate containing chloride ions
In this work we tried to develop new cathode catalyst in DMFC via
adsorption of chloride ion which has been known as inhibiting species in
most electrochemical studies The electrochemical characteristics of chloride
ion modified Pt cathode in the oxidation of methanol and
adsorptiondesorption of hydrogen were studied by cyclic voltammetry
techniques The power performance of the single cell and long-term stability
of pure Pt and modified Pt cathode was also investigated
6
21 Electro-oxidation of Methanol
The electrochemical oxidation of methanol on Pt involves several
intermediate steps ie dehydrogenation CO-like species chemisorption OH
(or H2O) species adsorption chemical interaction between adsorbed CO and
OH compounds and CO2 evolution One of these steps is the rate
determining step (rds) depending on the operating temperature and
particular catalyst surface State of the art electrocatalysts for the electro-
oxidation of methanol in fuel cells are generally based on Pt alloys supported
on carbon black [8 9] The electrocatalytic activity of Pt is known to be
promoted by the presence of a second metal such as Ru or Sn acting either
as an adatom or a bimetal First a sequence of dehydrogenation steps give rise
to adsorbed methanolic residues according to the following scheme[10]
CH3OH + Pt rarr Pt-CH2OH + H+ + 1e- (211)
Pt-CH2OH + Pt rarr Pt-CHOH + H+ + 1e- (212)
Pt-CHOH + Pt rarr PtCHO + H+ + 1e- (213)
2 BASIC THEORY
7
A surface rearrangement of the methanol oxidation intermediates
gives carbon monoxide linearly or bridge-bonded to Pt sites as follows
PtCHO rarr Pt-CequivO + H+ + 1e- (214)
Or
PtCHO + Pt rarr Pt Pt C=O + H+ + 1e- (215)
In the absence of a promoting element water discharge occurs at
high anodic overpotentials on Pt with the formation of Pt-OH species at the
catalyst surface
Pt + H2O rarr PtOH + H+ + 1e- (216)
The final step is the reaction of Pt-OH groups with neighboring
methanolic residues to give carbon dioxide
PtOH + Pt CO rarr 2Pt + CO2 + H+ + 1e- (217)
The overall oxidation process of methanol to carbon dioxide
proceeds through a six electron donation process On a pure Pt surface the
dissociative chemisorption of water on Pt is the rate determining step at
potentials below 07 V vs RHE [11] It is generally accepted that an active
catalyst for methanol oxidation should give rise to water discharging at low
potentials and to labile CO chemisorption Moreover a good catalyst for
methanol oxidation should also catalyze the oxidation of carbon monoxide
8
The promotion of Pt by a range of metals (and oxides) to enhance
methanol oxidation is well known Table 21 summeries species that have a
reported positive effect on methanol oxidation Of the catalyst promoters
considered significant recent activity has focused on the use of Ru Sn and
W with Pt
On PtRu surface CH3OH adsorption would take place on the Pt site
where CH3OH chemisorption requires three neighboring sites yielding
ensembles of Pt atoms with COad and the Ru atoms would act as OHad
providing center According to this the catalytic improvement would be
bifunctional mechanism There are different reports for optimal Ru contents
According to Gasteiger el al a Ru content of 10 is optimum[12] Increasing
Ru content provides more adsorbed water species required to oxidize COad
However because Pt sites are decreased with increasing Ru contents the
oxidation rate of CH3OH is decreased Consequently the rate of reaction
decreased when the Ru contents are more than 10 But other results
referring a coverage close to 05 by Ru is optimal for enhancing CH3OH
oxidation are reported by R Dillon[13]
9
Table 21 Effect of catalyst promoters on methanol oxidation
Promotion by Catalyst promoter
comment
Alloying and
dissolution to produce
highly reticulated
surfaces
Cr
Fe
Sn
Typically less than 100 mV
lower potential than Pt
Surface adatoms Sn
Bi
Alloys
Ru
Sn Mo
Os Ir
Ti Re
Ru has the greatest effect Sn
Mo Os and Re are substantial
promoters
Metal oxides Ru Hydrous Ru oxide the most
active catalyst
Base metal oxides W
Nb Zr Ta
W oxide is a notable prometer
Small effect of other metal
oxides (typically lt 100mV)
10
22 Oxygen Reduction Reaction
Oxygen reduction reaction is considered to be one of the most
important electrocatalytic reactions because of its role in electrochemical
energy conversion several industrial processes and corrosion Consequently
it was in the focus of electrochemical interest for many years Oxygen
reduction reaction has been a challenge for electrochemists because of its
complex kinetics and the need for better electrocatalysts The most notable
need for the improvement of the catalytic activity and the development of
new better non-noble metal electrocatalysts are still remaining Despite
extensive research on oxygen reduction reaction mechanism is still not fully
understood
Oxygen reduction reaction is known to proceed either in a four-
electron step to H2O or in a two-electron step to H2O2 In general the four-
electron-step is considered to be the major reaction pathway on
polycrystalline platinum electrodes [1]
+ -2 2 0O + 4H + 4e 2H O E =1229V ( NHE)vsrarr (221)
- -2 2 0O + 2H O + 2e 4OH E =0401Vrarr (222)
The peroxide pathway is
11
+ -2 2 2 0O + 2H + 2e H O E =067Vrarr (223)
Peroxide can undergo further reduction or decomposition in acid
solutions via the following reactions
+ -2 2 2 0H O + 2H + 2e 2H O E =177Vrarr (224)
2 2 2 22H O 2H O + O rarr
However it is commonly agreed that it proceeds without formation of
surface peroxide on polycrystalline and particulate Pt in acidic media
Furthermore it is believed that the reaction proceeds
O2 harr O2ads (225)
O2ads + H+(aq) +e- rarr O2Hads (226)
O2Hads + 3H+(aq) +3e- rarr 2H2O (227)
with the first charge transfer reaction (226) as the rate determining step The
details of reaction (227) are unknown and may include several Oads or OHads
configurations [2]
23 Anionic Effects
Platinum or platinum alloy particles supported on high surface area
carbon substrates are often synthesized form halide containing reactants (eg
Cl- containing salts)[15] It is therefore possible that traces of halides are
12
present in the electrocatalyst Furthermore halides especially chloride may
also be present in the humidified anode feed-streams of PEFCs or in the
cathode air feed During the ORR remaining anions can compete with
oxygen for free adsorption sites on the catalyst Modified adsorption
conditions for oxygen molecules can have an impact on the oxygen reduction
activity of the catalyst and can alter the oxygen reduction pathway (transition
form H2O to H2O2 formation) From both Pt single crystal work (eg [14]) it
is well known that different supporting electrolytes containing anions with
different adsorption strengths influence the ORR activity tremendously It is
also well known that the presence of additional halide anions can
dramatically affect the kinetics of electrocatalytic reactions on Pt e g the
oxidation of small organic molecules or the oxygen reduction reaction (e g
[16])
24 Actual Performance
The actual cell potential is decreased from its ideal potential because
of several types of irreversible losses as shown in Figure 21 These losses
are often referred to as polarization overpotential or overvoltage though
13
only the ohmic losses actually behave as a resistance Multiple phenomena
contribute to irreversible losses in an actual fuel cell
In the V-I diagram especially for low-temperature fuel cells the
effects of the three loss categories are often easy to distinguish as illustrated
in Figure 21
Activation Losses Activation losses are caused by sluggish electrode
kinetics There is a close similarity between electrochemical and chemical
reactions in that both involve an activation energy that must be overcome by
the reacting species In reality activation losses are the result of complex
surface electrochemical reaction steps each of which have their own reaction
rate and activation energy In electrochemical reactions the activation energy
loss for overcoming this barrier is represented as Tafel equation
lnact a b iη = + (241)
where actη is activation polarization(mV) a and b are constants and i is
current density(mAcm2)
( ) ( ) ( )act cell act anode act cathodeη η η= + (242)
Ohmic Polarization Ohmic losses occur because of resistance to the
flow of ions in the electrolyte and resistance to flow of electrons through the
electrode The dominant ohmic losses through the electrolyte are reduced by
14
decreasing the electrode separation and enhancing the ionic conductivity of
the electrolyte Because both the electrolyte and fuel cell electrodes obey
Ohms law the ohmic losses can be expressed by the equation
ohm iRη = (243)
where i is the current flowing through the cell and R is the total cell
resistance which includes electronic ionic and contact resistance
R = Relectronic + Rionic + Rcontact
Mass Transport Related Losses As a reactant is consumed at the
electrode by electrochemical reaction it is often diluted by the products
when finite mass transport rates limit the supply of fresh reactant and the
evacuation of products As a consequence a concentration gradient is formed
which drives the mass transport process
Concentration polarization can be denoted by
ln(1 )consL
RT inF i
η = minus (244)
where consη is concentration polarization(mV) iL is limiting current
density(mAcm2) Same as the activation polarization the concentration
polarization occurs independently at either electrode
( ) ( ) ( )conc cell conc anode conc cathodeη η η= + (245)
15
The combined effect of the losses for a given cell and given
operating conditions can be expressed as polarizations The total polarization
at the electrodes is the sum of actη and concη or
anode acta a conc aη η η= + (246)
and
cathode acta c conc cη η η= + (247)
The effect of polarization is to shift the potential of the electrode
(Eelectrode) to a new value (Velectrode)
electrode electrode electrodeV E η= plusmn (248)
For the anode
anode anode anodeV E η= + (249)
And for the cathode
cathode cathode cathodeV E η= minus (2410)
The net result of current flow in a fuel cell is to increase the anode
potential and to decrease the cathode potential thereby reducing the cell
voltage
The cell voltage includes the contribution of the anode and cathode
potentials and ohmic polarization
16
cell cathode anodeV V V iR= minus minus (2411)
When Equations (249) and (2410) are substituted in Equation (2411)
( )cell cathode cathode anode anodeV E E iRη η= minus minus + minus (2412)
Or
cell e cathode anodeV E iRη η= ∆ minus minus minus (2413)
Where e cathode anodeE E E∆ = minus Equation (2413) shows that current
flow in a fuel cell results in a decrease in cell voltage because of losses by
electrode and ohmic polarizations The goal of fuel cell developers is to
minimize the polarization so that cellV approaches eE∆ This goal is
approached by modifications to fuel cell design (improvement in electrode
structures better electro-catalysts more conductive electrolyte thinner cell
components etc) [1]
17
Figure 241 Typical Fuel Cell Polarization Curve
18
31 Polycrystalline Platinum Preparation
A polycrystalline platinum electrode was prepared with pulse
electrodeposition on Pt- quartz crystal
1) Working electrode Pt quartz crystal counter electrode Pt wire
reference electrode saturated calomel electrode (SCE)
2) Pulse electrodeposition method ton -03V 100ms and toff 0V 300ms
3) Deposition bath 10mM H2PtCl6 + 01M HCl (65)
Figure 31 shows the cross-section diagram of the platinized
polycrystalline platinum quartz crystal
Polycrystalline Pt
Sputtered Pt Sputtered Ti
9MHz At-cut quartz crystal
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
3 EXPERIMENTAL
19
32 Electrode Preparation and the Modification
A preparation procedure of catalyst ink was well described
elsewhere [7] and the catalyst (Pt or PtRu) loading was 3 mgcm2 in the each
electrode A geometric active area was a 5cm2 and catalyst ink was directly
sparied onto carbon paper Simple dipping method was used for the chloride
ion modification Pt cathode on carbon paper was dipped in 1 times 10minus2M KCl
solution for 5min and it was rinsed in ultrapure water followed by drying in
the oven
33 Half Cell Measurements
Figure 331 shows the schematic experimental setup for measuring
of the electrocatalystic activity of Pt electrode oxidationreduction
adsorptiondesorption of hydrogen methanol oxidation The electrochemical
cell body consisted of a glass cylinder capped with a Teflon lid holding all
electrodes The Pt nanocatalyst sprayed on carbon paper (gas diffusion layer)
was used as working electrode (WE) The geometric area of the WE was
02 cm2 A concentric platinized Pt wire was used as counter electrode (CE)
The tip of a Luggin-Haber capillary hosting a SCE reference electrode (RE)
was placed in the middle of two electrodes A poteniostatgalvanostat (EG amp
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
이 論文을 徐明煥의 碩士學位論文으로 認定함
2006 年 2 月
主 審
副 審
委 員
Sustained Activity of Pt Cathode Modified by Chloride Ions in DMFC
by MyeongHwan Seo
A THESIS
Submitted to the faculty of
INHA UNIVERSITY
in partial fulfillment of the requirements
for the degree of
MASTER OF ENGINEERING
Department of Chemical Engineering February 2006
i
요 약
수소의 흡착탈착 공정과 백금전극의 산화환원반응의 거동 그리고
메탄올의 전기화학적 산화반응에 관한 염소이온으로 처리된 백금전극의
전기화학적 특성을 조사하였다 순수한 백금전극과 염소이온으로 처리된
백금전극을 비교해봄으로써 염소이온으로 처리된 백금전극은 양이온의
흡착능력을 유지하였고 백금전극의 산화를 억제하였다 이것은 산소와
백금전극 표면 사이에서 결합에너지의 감소를 일으키는 것으로 판단된다
순환전위법(CV)으로 캐소드 방향에서 산소환원반응의 과전압이 감소하는
것을 알 수 있었다 또한 염소이온으로 처리된 백금전극은 직접메탄올
연료전지의 성능을 향상시켰다 그리고 캐소드에 처리된 백금전극을 가진
전지의 장기 안정성에 관하여 조사하였고 그 결과 처리되지 않은
백금전극보다 5배정도의 안정성을 나타내는 것을 확인하였다
주제어 연료전지 직접메탄올연료전지 염소이온 처리 산소환원반응 메탄올 크로스오버
ii
ABSTRACT
We investigated electrochemical characteristics of chloride ion
modified Pt (PtClmminus) electrode in the electrocatalytic oxidation of methanol
oxidationreduction behaviours of Pt adsorptiondesorption phenomena of
hydrogen Comparing with pure Pt surface PtClmminus maintained the adsorption
activity of proton hindered oxidation of Pt induced less binding energy
between oxygen and Pt surface It resulted in the decrease of overpotential of
oxygen reduction reaction on the cathodic scan Thus PtClmminus could increase
power performance of direct methanol fuel cell In addition to activity of
PtClmminus the long-term durability of PtClm
minus cathode was also investigated and
it had about five times longer stability
Keywords Fuel cell DMFC methanol oxidation modified electrode
iii
TABLE OF CONTENTS
요 약helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipⅰ
Abstracthelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipⅱ
List of figureshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipⅴ
List of tablehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipVii
1 Introductionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip1
11 What is a fuel cell 1
12 Direct methanol fuel cell (DMFC) 2
13 Objectives 4
2 Basic Theoryhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip6
21 Electro-oxidation of methanol 6
22 Oxygen reduction reaction helliphelliphellip 10
23 Anionic effects 11
24 Actual performance 12
3 Experimentalhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip18
31 Polycrystalline Platinum Preparation 18
32 Electrode Preparation and the Modification 19
iv
33 Half Cell Measurements 19
34 Single cell measurements 22
4 Results and Discussionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip26
5 Conclusionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip49
6 Referenceshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip50
7 Acknowledgementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip52
v
List of Figures Figure 121 Schematic diagram of a DMFC
Figure 241 Typical Fuel Cell Polarization Curve
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
Figure 331 Schematic diagram of experimental set-up for half-cell experiments
Figure 341 Configuration of catalystdiffusion backing and membrane for type a MEA
Figure 342 Schematic diagram of experimental set-up for single cell measurements
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M H2SO4 Scan rate = 50mVs
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M CH3OH05M H2SO4 Scan rate = 50mVs
Figure 415 The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
vi
Figure 416 The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
Figure 417 Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
Figure 418 Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
Figure 419 DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Clminus (c) 0001 M Clminus Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
Figure 4111 Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
vii
List of Table
Table 21 Effect of catalyst promoters on methanol oxidation
1
11 What Is a Fuel Cell
As early as 1839 William Grove discovered the basic operating
principle of fuel cells by reversing water electrolysis to generate electricity
from hydrogen and oxygen The principle that he discovered remains
unchanged today A fuel cell is an electrochemical ldquo device rdquo that
continuously converts chemical energy into electric energy (and some heat)
for as long as fuel and oxidant are supplied Fuel cells therefore bear
similarities both to batteries with which they share the electrochemical
nature of the power generation process and to engines which unlike batteries
will work continuously consuming a fuel of some sort Here is where the
analogies stop though Unlike engines or batteries a fuel cell does not need
recharging it operates quietly and efficiently and when hydrogen is used as
fuel it generates only power and drinking water Thus it is a so-called zero
emission engine [1]
1 INTRODUCTION
2
12 Direct Methanol Fuel Cell (DMFC)
A schematic of a DMFC is shown in Figure 11 Methanol and water
electrochemically react (ie methanol is electro-oxidized) at the anode to
produce carbon dioxide protons and electrons as shown in reaction (121)
An acidic electrolyte is advantageous to aid CO2 rejection since insoluble
carbonates form in alkaline electrolytes The protons produced at the anode
migrate through the polymer electrolyte to the cathode where they react with
oxygen (usually from air) to produce water as shown in reaction (122) The
electrons produced at the anode carry the free energy change of the chemical
reaction and travel through the external circuit where they can be made to do
useful work such as powering an electric motor The overall cell reaction as
shown in equation (123) is therefore the reaction of methanol and oxygen
to produce water and carbon dioxide
Anode CH3OH + H2O rarr CO2 + 6H+ + 6e- 0AE = 0046V (121)
Cathode 32
O2 + 6H+ + 6e- rarr 3H2O 0CE = 1229V (122)
Net Reaction CH3OH + 32
O2 rarr CO2 + 2H2O (123)
0cellE = 0
CE - 0AE =1229V - 0046V = 1183V Attainable max cell potential
3
Figure 121 Schematic diagram of direct fuel cell
4
13 Objectives
Of late fuel cells have become a central focus of interest and leading
car and power companies are making large scale investments in fuel cell
systems Fuel cells are set to provide electrical vehicles small and large scale
power stations and even portable electrical applications such as laptops and
mobile phones with electrical power in a manner which is more efficient and
more environmentally friendly than ever before [2 minus6] In particular direct
methanol fuel cells (DMFCs) have been considered for use in very small to
mid-sized applications due to their high electrical efficiency long life-time
and low poisonous emissions [2 minus 6] On the other hand in the operation of
DMFCs there are several problems at a cathode caused by methanol
crossover from anode to cathode and it should be solved for the
commercialization of DMFC (i) cathode poisoning by CO adsorption and Pt
sintering (ii) significant fuel loss due to methanol crossover (iii) decrease
partial pressure of oxygen due to water flooding and (iv) temperature
dependent power performance Therefore cathode catalyst for oxygen
reduction reaction (ORR) requires (a) CO tolerance (b) proton adsorption
available (c) easy oxygen adsorption (d) ORR kinetics (e) oxygen storage
material (f) suppression of electro-oxidation of methanol To our knowledge
5
no work has been done up to now to study the influence of halide
contamination on fuel cell performance From the data presented previously
Paulus et al [2] could only point to some aspects which should be considered
during MEA production the degradation of MEA might be occurred due to
the dissolution of Nafion by H2O2 and it was mainly produced in the
reduction of oxygen on a Pt substrate containing chloride ions
In this work we tried to develop new cathode catalyst in DMFC via
adsorption of chloride ion which has been known as inhibiting species in
most electrochemical studies The electrochemical characteristics of chloride
ion modified Pt cathode in the oxidation of methanol and
adsorptiondesorption of hydrogen were studied by cyclic voltammetry
techniques The power performance of the single cell and long-term stability
of pure Pt and modified Pt cathode was also investigated
6
21 Electro-oxidation of Methanol
The electrochemical oxidation of methanol on Pt involves several
intermediate steps ie dehydrogenation CO-like species chemisorption OH
(or H2O) species adsorption chemical interaction between adsorbed CO and
OH compounds and CO2 evolution One of these steps is the rate
determining step (rds) depending on the operating temperature and
particular catalyst surface State of the art electrocatalysts for the electro-
oxidation of methanol in fuel cells are generally based on Pt alloys supported
on carbon black [8 9] The electrocatalytic activity of Pt is known to be
promoted by the presence of a second metal such as Ru or Sn acting either
as an adatom or a bimetal First a sequence of dehydrogenation steps give rise
to adsorbed methanolic residues according to the following scheme[10]
CH3OH + Pt rarr Pt-CH2OH + H+ + 1e- (211)
Pt-CH2OH + Pt rarr Pt-CHOH + H+ + 1e- (212)
Pt-CHOH + Pt rarr PtCHO + H+ + 1e- (213)
2 BASIC THEORY
7
A surface rearrangement of the methanol oxidation intermediates
gives carbon monoxide linearly or bridge-bonded to Pt sites as follows
PtCHO rarr Pt-CequivO + H+ + 1e- (214)
Or
PtCHO + Pt rarr Pt Pt C=O + H+ + 1e- (215)
In the absence of a promoting element water discharge occurs at
high anodic overpotentials on Pt with the formation of Pt-OH species at the
catalyst surface
Pt + H2O rarr PtOH + H+ + 1e- (216)
The final step is the reaction of Pt-OH groups with neighboring
methanolic residues to give carbon dioxide
PtOH + Pt CO rarr 2Pt + CO2 + H+ + 1e- (217)
The overall oxidation process of methanol to carbon dioxide
proceeds through a six electron donation process On a pure Pt surface the
dissociative chemisorption of water on Pt is the rate determining step at
potentials below 07 V vs RHE [11] It is generally accepted that an active
catalyst for methanol oxidation should give rise to water discharging at low
potentials and to labile CO chemisorption Moreover a good catalyst for
methanol oxidation should also catalyze the oxidation of carbon monoxide
8
The promotion of Pt by a range of metals (and oxides) to enhance
methanol oxidation is well known Table 21 summeries species that have a
reported positive effect on methanol oxidation Of the catalyst promoters
considered significant recent activity has focused on the use of Ru Sn and
W with Pt
On PtRu surface CH3OH adsorption would take place on the Pt site
where CH3OH chemisorption requires three neighboring sites yielding
ensembles of Pt atoms with COad and the Ru atoms would act as OHad
providing center According to this the catalytic improvement would be
bifunctional mechanism There are different reports for optimal Ru contents
According to Gasteiger el al a Ru content of 10 is optimum[12] Increasing
Ru content provides more adsorbed water species required to oxidize COad
However because Pt sites are decreased with increasing Ru contents the
oxidation rate of CH3OH is decreased Consequently the rate of reaction
decreased when the Ru contents are more than 10 But other results
referring a coverage close to 05 by Ru is optimal for enhancing CH3OH
oxidation are reported by R Dillon[13]
9
Table 21 Effect of catalyst promoters on methanol oxidation
Promotion by Catalyst promoter
comment
Alloying and
dissolution to produce
highly reticulated
surfaces
Cr
Fe
Sn
Typically less than 100 mV
lower potential than Pt
Surface adatoms Sn
Bi
Alloys
Ru
Sn Mo
Os Ir
Ti Re
Ru has the greatest effect Sn
Mo Os and Re are substantial
promoters
Metal oxides Ru Hydrous Ru oxide the most
active catalyst
Base metal oxides W
Nb Zr Ta
W oxide is a notable prometer
Small effect of other metal
oxides (typically lt 100mV)
10
22 Oxygen Reduction Reaction
Oxygen reduction reaction is considered to be one of the most
important electrocatalytic reactions because of its role in electrochemical
energy conversion several industrial processes and corrosion Consequently
it was in the focus of electrochemical interest for many years Oxygen
reduction reaction has been a challenge for electrochemists because of its
complex kinetics and the need for better electrocatalysts The most notable
need for the improvement of the catalytic activity and the development of
new better non-noble metal electrocatalysts are still remaining Despite
extensive research on oxygen reduction reaction mechanism is still not fully
understood
Oxygen reduction reaction is known to proceed either in a four-
electron step to H2O or in a two-electron step to H2O2 In general the four-
electron-step is considered to be the major reaction pathway on
polycrystalline platinum electrodes [1]
+ -2 2 0O + 4H + 4e 2H O E =1229V ( NHE)vsrarr (221)
- -2 2 0O + 2H O + 2e 4OH E =0401Vrarr (222)
The peroxide pathway is
11
+ -2 2 2 0O + 2H + 2e H O E =067Vrarr (223)
Peroxide can undergo further reduction or decomposition in acid
solutions via the following reactions
+ -2 2 2 0H O + 2H + 2e 2H O E =177Vrarr (224)
2 2 2 22H O 2H O + O rarr
However it is commonly agreed that it proceeds without formation of
surface peroxide on polycrystalline and particulate Pt in acidic media
Furthermore it is believed that the reaction proceeds
O2 harr O2ads (225)
O2ads + H+(aq) +e- rarr O2Hads (226)
O2Hads + 3H+(aq) +3e- rarr 2H2O (227)
with the first charge transfer reaction (226) as the rate determining step The
details of reaction (227) are unknown and may include several Oads or OHads
configurations [2]
23 Anionic Effects
Platinum or platinum alloy particles supported on high surface area
carbon substrates are often synthesized form halide containing reactants (eg
Cl- containing salts)[15] It is therefore possible that traces of halides are
12
present in the electrocatalyst Furthermore halides especially chloride may
also be present in the humidified anode feed-streams of PEFCs or in the
cathode air feed During the ORR remaining anions can compete with
oxygen for free adsorption sites on the catalyst Modified adsorption
conditions for oxygen molecules can have an impact on the oxygen reduction
activity of the catalyst and can alter the oxygen reduction pathway (transition
form H2O to H2O2 formation) From both Pt single crystal work (eg [14]) it
is well known that different supporting electrolytes containing anions with
different adsorption strengths influence the ORR activity tremendously It is
also well known that the presence of additional halide anions can
dramatically affect the kinetics of electrocatalytic reactions on Pt e g the
oxidation of small organic molecules or the oxygen reduction reaction (e g
[16])
24 Actual Performance
The actual cell potential is decreased from its ideal potential because
of several types of irreversible losses as shown in Figure 21 These losses
are often referred to as polarization overpotential or overvoltage though
13
only the ohmic losses actually behave as a resistance Multiple phenomena
contribute to irreversible losses in an actual fuel cell
In the V-I diagram especially for low-temperature fuel cells the
effects of the three loss categories are often easy to distinguish as illustrated
in Figure 21
Activation Losses Activation losses are caused by sluggish electrode
kinetics There is a close similarity between electrochemical and chemical
reactions in that both involve an activation energy that must be overcome by
the reacting species In reality activation losses are the result of complex
surface electrochemical reaction steps each of which have their own reaction
rate and activation energy In electrochemical reactions the activation energy
loss for overcoming this barrier is represented as Tafel equation
lnact a b iη = + (241)
where actη is activation polarization(mV) a and b are constants and i is
current density(mAcm2)
( ) ( ) ( )act cell act anode act cathodeη η η= + (242)
Ohmic Polarization Ohmic losses occur because of resistance to the
flow of ions in the electrolyte and resistance to flow of electrons through the
electrode The dominant ohmic losses through the electrolyte are reduced by
14
decreasing the electrode separation and enhancing the ionic conductivity of
the electrolyte Because both the electrolyte and fuel cell electrodes obey
Ohms law the ohmic losses can be expressed by the equation
ohm iRη = (243)
where i is the current flowing through the cell and R is the total cell
resistance which includes electronic ionic and contact resistance
R = Relectronic + Rionic + Rcontact
Mass Transport Related Losses As a reactant is consumed at the
electrode by electrochemical reaction it is often diluted by the products
when finite mass transport rates limit the supply of fresh reactant and the
evacuation of products As a consequence a concentration gradient is formed
which drives the mass transport process
Concentration polarization can be denoted by
ln(1 )consL
RT inF i
η = minus (244)
where consη is concentration polarization(mV) iL is limiting current
density(mAcm2) Same as the activation polarization the concentration
polarization occurs independently at either electrode
( ) ( ) ( )conc cell conc anode conc cathodeη η η= + (245)
15
The combined effect of the losses for a given cell and given
operating conditions can be expressed as polarizations The total polarization
at the electrodes is the sum of actη and concη or
anode acta a conc aη η η= + (246)
and
cathode acta c conc cη η η= + (247)
The effect of polarization is to shift the potential of the electrode
(Eelectrode) to a new value (Velectrode)
electrode electrode electrodeV E η= plusmn (248)
For the anode
anode anode anodeV E η= + (249)
And for the cathode
cathode cathode cathodeV E η= minus (2410)
The net result of current flow in a fuel cell is to increase the anode
potential and to decrease the cathode potential thereby reducing the cell
voltage
The cell voltage includes the contribution of the anode and cathode
potentials and ohmic polarization
16
cell cathode anodeV V V iR= minus minus (2411)
When Equations (249) and (2410) are substituted in Equation (2411)
( )cell cathode cathode anode anodeV E E iRη η= minus minus + minus (2412)
Or
cell e cathode anodeV E iRη η= ∆ minus minus minus (2413)
Where e cathode anodeE E E∆ = minus Equation (2413) shows that current
flow in a fuel cell results in a decrease in cell voltage because of losses by
electrode and ohmic polarizations The goal of fuel cell developers is to
minimize the polarization so that cellV approaches eE∆ This goal is
approached by modifications to fuel cell design (improvement in electrode
structures better electro-catalysts more conductive electrolyte thinner cell
components etc) [1]
17
Figure 241 Typical Fuel Cell Polarization Curve
18
31 Polycrystalline Platinum Preparation
A polycrystalline platinum electrode was prepared with pulse
electrodeposition on Pt- quartz crystal
1) Working electrode Pt quartz crystal counter electrode Pt wire
reference electrode saturated calomel electrode (SCE)
2) Pulse electrodeposition method ton -03V 100ms and toff 0V 300ms
3) Deposition bath 10mM H2PtCl6 + 01M HCl (65)
Figure 31 shows the cross-section diagram of the platinized
polycrystalline platinum quartz crystal
Polycrystalline Pt
Sputtered Pt Sputtered Ti
9MHz At-cut quartz crystal
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
3 EXPERIMENTAL
19
32 Electrode Preparation and the Modification
A preparation procedure of catalyst ink was well described
elsewhere [7] and the catalyst (Pt or PtRu) loading was 3 mgcm2 in the each
electrode A geometric active area was a 5cm2 and catalyst ink was directly
sparied onto carbon paper Simple dipping method was used for the chloride
ion modification Pt cathode on carbon paper was dipped in 1 times 10minus2M KCl
solution for 5min and it was rinsed in ultrapure water followed by drying in
the oven
33 Half Cell Measurements
Figure 331 shows the schematic experimental setup for measuring
of the electrocatalystic activity of Pt electrode oxidationreduction
adsorptiondesorption of hydrogen methanol oxidation The electrochemical
cell body consisted of a glass cylinder capped with a Teflon lid holding all
electrodes The Pt nanocatalyst sprayed on carbon paper (gas diffusion layer)
was used as working electrode (WE) The geometric area of the WE was
02 cm2 A concentric platinized Pt wire was used as counter electrode (CE)
The tip of a Luggin-Haber capillary hosting a SCE reference electrode (RE)
was placed in the middle of two electrodes A poteniostatgalvanostat (EG amp
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
Sustained Activity of Pt Cathode Modified by Chloride Ions in DMFC
by MyeongHwan Seo
A THESIS
Submitted to the faculty of
INHA UNIVERSITY
in partial fulfillment of the requirements
for the degree of
MASTER OF ENGINEERING
Department of Chemical Engineering February 2006
i
요 약
수소의 흡착탈착 공정과 백금전극의 산화환원반응의 거동 그리고
메탄올의 전기화학적 산화반응에 관한 염소이온으로 처리된 백금전극의
전기화학적 특성을 조사하였다 순수한 백금전극과 염소이온으로 처리된
백금전극을 비교해봄으로써 염소이온으로 처리된 백금전극은 양이온의
흡착능력을 유지하였고 백금전극의 산화를 억제하였다 이것은 산소와
백금전극 표면 사이에서 결합에너지의 감소를 일으키는 것으로 판단된다
순환전위법(CV)으로 캐소드 방향에서 산소환원반응의 과전압이 감소하는
것을 알 수 있었다 또한 염소이온으로 처리된 백금전극은 직접메탄올
연료전지의 성능을 향상시켰다 그리고 캐소드에 처리된 백금전극을 가진
전지의 장기 안정성에 관하여 조사하였고 그 결과 처리되지 않은
백금전극보다 5배정도의 안정성을 나타내는 것을 확인하였다
주제어 연료전지 직접메탄올연료전지 염소이온 처리 산소환원반응 메탄올 크로스오버
ii
ABSTRACT
We investigated electrochemical characteristics of chloride ion
modified Pt (PtClmminus) electrode in the electrocatalytic oxidation of methanol
oxidationreduction behaviours of Pt adsorptiondesorption phenomena of
hydrogen Comparing with pure Pt surface PtClmminus maintained the adsorption
activity of proton hindered oxidation of Pt induced less binding energy
between oxygen and Pt surface It resulted in the decrease of overpotential of
oxygen reduction reaction on the cathodic scan Thus PtClmminus could increase
power performance of direct methanol fuel cell In addition to activity of
PtClmminus the long-term durability of PtClm
minus cathode was also investigated and
it had about five times longer stability
Keywords Fuel cell DMFC methanol oxidation modified electrode
iii
TABLE OF CONTENTS
요 약helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipⅰ
Abstracthelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipⅱ
List of figureshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipⅴ
List of tablehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipVii
1 Introductionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip1
11 What is a fuel cell 1
12 Direct methanol fuel cell (DMFC) 2
13 Objectives 4
2 Basic Theoryhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip6
21 Electro-oxidation of methanol 6
22 Oxygen reduction reaction helliphelliphellip 10
23 Anionic effects 11
24 Actual performance 12
3 Experimentalhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip18
31 Polycrystalline Platinum Preparation 18
32 Electrode Preparation and the Modification 19
iv
33 Half Cell Measurements 19
34 Single cell measurements 22
4 Results and Discussionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip26
5 Conclusionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip49
6 Referenceshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip50
7 Acknowledgementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip52
v
List of Figures Figure 121 Schematic diagram of a DMFC
Figure 241 Typical Fuel Cell Polarization Curve
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
Figure 331 Schematic diagram of experimental set-up for half-cell experiments
Figure 341 Configuration of catalystdiffusion backing and membrane for type a MEA
Figure 342 Schematic diagram of experimental set-up for single cell measurements
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M H2SO4 Scan rate = 50mVs
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M CH3OH05M H2SO4 Scan rate = 50mVs
Figure 415 The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
vi
Figure 416 The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
Figure 417 Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
Figure 418 Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
Figure 419 DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Clminus (c) 0001 M Clminus Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
Figure 4111 Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
vii
List of Table
Table 21 Effect of catalyst promoters on methanol oxidation
1
11 What Is a Fuel Cell
As early as 1839 William Grove discovered the basic operating
principle of fuel cells by reversing water electrolysis to generate electricity
from hydrogen and oxygen The principle that he discovered remains
unchanged today A fuel cell is an electrochemical ldquo device rdquo that
continuously converts chemical energy into electric energy (and some heat)
for as long as fuel and oxidant are supplied Fuel cells therefore bear
similarities both to batteries with which they share the electrochemical
nature of the power generation process and to engines which unlike batteries
will work continuously consuming a fuel of some sort Here is where the
analogies stop though Unlike engines or batteries a fuel cell does not need
recharging it operates quietly and efficiently and when hydrogen is used as
fuel it generates only power and drinking water Thus it is a so-called zero
emission engine [1]
1 INTRODUCTION
2
12 Direct Methanol Fuel Cell (DMFC)
A schematic of a DMFC is shown in Figure 11 Methanol and water
electrochemically react (ie methanol is electro-oxidized) at the anode to
produce carbon dioxide protons and electrons as shown in reaction (121)
An acidic electrolyte is advantageous to aid CO2 rejection since insoluble
carbonates form in alkaline electrolytes The protons produced at the anode
migrate through the polymer electrolyte to the cathode where they react with
oxygen (usually from air) to produce water as shown in reaction (122) The
electrons produced at the anode carry the free energy change of the chemical
reaction and travel through the external circuit where they can be made to do
useful work such as powering an electric motor The overall cell reaction as
shown in equation (123) is therefore the reaction of methanol and oxygen
to produce water and carbon dioxide
Anode CH3OH + H2O rarr CO2 + 6H+ + 6e- 0AE = 0046V (121)
Cathode 32
O2 + 6H+ + 6e- rarr 3H2O 0CE = 1229V (122)
Net Reaction CH3OH + 32
O2 rarr CO2 + 2H2O (123)
0cellE = 0
CE - 0AE =1229V - 0046V = 1183V Attainable max cell potential
3
Figure 121 Schematic diagram of direct fuel cell
4
13 Objectives
Of late fuel cells have become a central focus of interest and leading
car and power companies are making large scale investments in fuel cell
systems Fuel cells are set to provide electrical vehicles small and large scale
power stations and even portable electrical applications such as laptops and
mobile phones with electrical power in a manner which is more efficient and
more environmentally friendly than ever before [2 minus6] In particular direct
methanol fuel cells (DMFCs) have been considered for use in very small to
mid-sized applications due to their high electrical efficiency long life-time
and low poisonous emissions [2 minus 6] On the other hand in the operation of
DMFCs there are several problems at a cathode caused by methanol
crossover from anode to cathode and it should be solved for the
commercialization of DMFC (i) cathode poisoning by CO adsorption and Pt
sintering (ii) significant fuel loss due to methanol crossover (iii) decrease
partial pressure of oxygen due to water flooding and (iv) temperature
dependent power performance Therefore cathode catalyst for oxygen
reduction reaction (ORR) requires (a) CO tolerance (b) proton adsorption
available (c) easy oxygen adsorption (d) ORR kinetics (e) oxygen storage
material (f) suppression of electro-oxidation of methanol To our knowledge
5
no work has been done up to now to study the influence of halide
contamination on fuel cell performance From the data presented previously
Paulus et al [2] could only point to some aspects which should be considered
during MEA production the degradation of MEA might be occurred due to
the dissolution of Nafion by H2O2 and it was mainly produced in the
reduction of oxygen on a Pt substrate containing chloride ions
In this work we tried to develop new cathode catalyst in DMFC via
adsorption of chloride ion which has been known as inhibiting species in
most electrochemical studies The electrochemical characteristics of chloride
ion modified Pt cathode in the oxidation of methanol and
adsorptiondesorption of hydrogen were studied by cyclic voltammetry
techniques The power performance of the single cell and long-term stability
of pure Pt and modified Pt cathode was also investigated
6
21 Electro-oxidation of Methanol
The electrochemical oxidation of methanol on Pt involves several
intermediate steps ie dehydrogenation CO-like species chemisorption OH
(or H2O) species adsorption chemical interaction between adsorbed CO and
OH compounds and CO2 evolution One of these steps is the rate
determining step (rds) depending on the operating temperature and
particular catalyst surface State of the art electrocatalysts for the electro-
oxidation of methanol in fuel cells are generally based on Pt alloys supported
on carbon black [8 9] The electrocatalytic activity of Pt is known to be
promoted by the presence of a second metal such as Ru or Sn acting either
as an adatom or a bimetal First a sequence of dehydrogenation steps give rise
to adsorbed methanolic residues according to the following scheme[10]
CH3OH + Pt rarr Pt-CH2OH + H+ + 1e- (211)
Pt-CH2OH + Pt rarr Pt-CHOH + H+ + 1e- (212)
Pt-CHOH + Pt rarr PtCHO + H+ + 1e- (213)
2 BASIC THEORY
7
A surface rearrangement of the methanol oxidation intermediates
gives carbon monoxide linearly or bridge-bonded to Pt sites as follows
PtCHO rarr Pt-CequivO + H+ + 1e- (214)
Or
PtCHO + Pt rarr Pt Pt C=O + H+ + 1e- (215)
In the absence of a promoting element water discharge occurs at
high anodic overpotentials on Pt with the formation of Pt-OH species at the
catalyst surface
Pt + H2O rarr PtOH + H+ + 1e- (216)
The final step is the reaction of Pt-OH groups with neighboring
methanolic residues to give carbon dioxide
PtOH + Pt CO rarr 2Pt + CO2 + H+ + 1e- (217)
The overall oxidation process of methanol to carbon dioxide
proceeds through a six electron donation process On a pure Pt surface the
dissociative chemisorption of water on Pt is the rate determining step at
potentials below 07 V vs RHE [11] It is generally accepted that an active
catalyst for methanol oxidation should give rise to water discharging at low
potentials and to labile CO chemisorption Moreover a good catalyst for
methanol oxidation should also catalyze the oxidation of carbon monoxide
8
The promotion of Pt by a range of metals (and oxides) to enhance
methanol oxidation is well known Table 21 summeries species that have a
reported positive effect on methanol oxidation Of the catalyst promoters
considered significant recent activity has focused on the use of Ru Sn and
W with Pt
On PtRu surface CH3OH adsorption would take place on the Pt site
where CH3OH chemisorption requires three neighboring sites yielding
ensembles of Pt atoms with COad and the Ru atoms would act as OHad
providing center According to this the catalytic improvement would be
bifunctional mechanism There are different reports for optimal Ru contents
According to Gasteiger el al a Ru content of 10 is optimum[12] Increasing
Ru content provides more adsorbed water species required to oxidize COad
However because Pt sites are decreased with increasing Ru contents the
oxidation rate of CH3OH is decreased Consequently the rate of reaction
decreased when the Ru contents are more than 10 But other results
referring a coverage close to 05 by Ru is optimal for enhancing CH3OH
oxidation are reported by R Dillon[13]
9
Table 21 Effect of catalyst promoters on methanol oxidation
Promotion by Catalyst promoter
comment
Alloying and
dissolution to produce
highly reticulated
surfaces
Cr
Fe
Sn
Typically less than 100 mV
lower potential than Pt
Surface adatoms Sn
Bi
Alloys
Ru
Sn Mo
Os Ir
Ti Re
Ru has the greatest effect Sn
Mo Os and Re are substantial
promoters
Metal oxides Ru Hydrous Ru oxide the most
active catalyst
Base metal oxides W
Nb Zr Ta
W oxide is a notable prometer
Small effect of other metal
oxides (typically lt 100mV)
10
22 Oxygen Reduction Reaction
Oxygen reduction reaction is considered to be one of the most
important electrocatalytic reactions because of its role in electrochemical
energy conversion several industrial processes and corrosion Consequently
it was in the focus of electrochemical interest for many years Oxygen
reduction reaction has been a challenge for electrochemists because of its
complex kinetics and the need for better electrocatalysts The most notable
need for the improvement of the catalytic activity and the development of
new better non-noble metal electrocatalysts are still remaining Despite
extensive research on oxygen reduction reaction mechanism is still not fully
understood
Oxygen reduction reaction is known to proceed either in a four-
electron step to H2O or in a two-electron step to H2O2 In general the four-
electron-step is considered to be the major reaction pathway on
polycrystalline platinum electrodes [1]
+ -2 2 0O + 4H + 4e 2H O E =1229V ( NHE)vsrarr (221)
- -2 2 0O + 2H O + 2e 4OH E =0401Vrarr (222)
The peroxide pathway is
11
+ -2 2 2 0O + 2H + 2e H O E =067Vrarr (223)
Peroxide can undergo further reduction or decomposition in acid
solutions via the following reactions
+ -2 2 2 0H O + 2H + 2e 2H O E =177Vrarr (224)
2 2 2 22H O 2H O + O rarr
However it is commonly agreed that it proceeds without formation of
surface peroxide on polycrystalline and particulate Pt in acidic media
Furthermore it is believed that the reaction proceeds
O2 harr O2ads (225)
O2ads + H+(aq) +e- rarr O2Hads (226)
O2Hads + 3H+(aq) +3e- rarr 2H2O (227)
with the first charge transfer reaction (226) as the rate determining step The
details of reaction (227) are unknown and may include several Oads or OHads
configurations [2]
23 Anionic Effects
Platinum or platinum alloy particles supported on high surface area
carbon substrates are often synthesized form halide containing reactants (eg
Cl- containing salts)[15] It is therefore possible that traces of halides are
12
present in the electrocatalyst Furthermore halides especially chloride may
also be present in the humidified anode feed-streams of PEFCs or in the
cathode air feed During the ORR remaining anions can compete with
oxygen for free adsorption sites on the catalyst Modified adsorption
conditions for oxygen molecules can have an impact on the oxygen reduction
activity of the catalyst and can alter the oxygen reduction pathway (transition
form H2O to H2O2 formation) From both Pt single crystal work (eg [14]) it
is well known that different supporting electrolytes containing anions with
different adsorption strengths influence the ORR activity tremendously It is
also well known that the presence of additional halide anions can
dramatically affect the kinetics of electrocatalytic reactions on Pt e g the
oxidation of small organic molecules or the oxygen reduction reaction (e g
[16])
24 Actual Performance
The actual cell potential is decreased from its ideal potential because
of several types of irreversible losses as shown in Figure 21 These losses
are often referred to as polarization overpotential or overvoltage though
13
only the ohmic losses actually behave as a resistance Multiple phenomena
contribute to irreversible losses in an actual fuel cell
In the V-I diagram especially for low-temperature fuel cells the
effects of the three loss categories are often easy to distinguish as illustrated
in Figure 21
Activation Losses Activation losses are caused by sluggish electrode
kinetics There is a close similarity between electrochemical and chemical
reactions in that both involve an activation energy that must be overcome by
the reacting species In reality activation losses are the result of complex
surface electrochemical reaction steps each of which have their own reaction
rate and activation energy In electrochemical reactions the activation energy
loss for overcoming this barrier is represented as Tafel equation
lnact a b iη = + (241)
where actη is activation polarization(mV) a and b are constants and i is
current density(mAcm2)
( ) ( ) ( )act cell act anode act cathodeη η η= + (242)
Ohmic Polarization Ohmic losses occur because of resistance to the
flow of ions in the electrolyte and resistance to flow of electrons through the
electrode The dominant ohmic losses through the electrolyte are reduced by
14
decreasing the electrode separation and enhancing the ionic conductivity of
the electrolyte Because both the electrolyte and fuel cell electrodes obey
Ohms law the ohmic losses can be expressed by the equation
ohm iRη = (243)
where i is the current flowing through the cell and R is the total cell
resistance which includes electronic ionic and contact resistance
R = Relectronic + Rionic + Rcontact
Mass Transport Related Losses As a reactant is consumed at the
electrode by electrochemical reaction it is often diluted by the products
when finite mass transport rates limit the supply of fresh reactant and the
evacuation of products As a consequence a concentration gradient is formed
which drives the mass transport process
Concentration polarization can be denoted by
ln(1 )consL
RT inF i
η = minus (244)
where consη is concentration polarization(mV) iL is limiting current
density(mAcm2) Same as the activation polarization the concentration
polarization occurs independently at either electrode
( ) ( ) ( )conc cell conc anode conc cathodeη η η= + (245)
15
The combined effect of the losses for a given cell and given
operating conditions can be expressed as polarizations The total polarization
at the electrodes is the sum of actη and concη or
anode acta a conc aη η η= + (246)
and
cathode acta c conc cη η η= + (247)
The effect of polarization is to shift the potential of the electrode
(Eelectrode) to a new value (Velectrode)
electrode electrode electrodeV E η= plusmn (248)
For the anode
anode anode anodeV E η= + (249)
And for the cathode
cathode cathode cathodeV E η= minus (2410)
The net result of current flow in a fuel cell is to increase the anode
potential and to decrease the cathode potential thereby reducing the cell
voltage
The cell voltage includes the contribution of the anode and cathode
potentials and ohmic polarization
16
cell cathode anodeV V V iR= minus minus (2411)
When Equations (249) and (2410) are substituted in Equation (2411)
( )cell cathode cathode anode anodeV E E iRη η= minus minus + minus (2412)
Or
cell e cathode anodeV E iRη η= ∆ minus minus minus (2413)
Where e cathode anodeE E E∆ = minus Equation (2413) shows that current
flow in a fuel cell results in a decrease in cell voltage because of losses by
electrode and ohmic polarizations The goal of fuel cell developers is to
minimize the polarization so that cellV approaches eE∆ This goal is
approached by modifications to fuel cell design (improvement in electrode
structures better electro-catalysts more conductive electrolyte thinner cell
components etc) [1]
17
Figure 241 Typical Fuel Cell Polarization Curve
18
31 Polycrystalline Platinum Preparation
A polycrystalline platinum electrode was prepared with pulse
electrodeposition on Pt- quartz crystal
1) Working electrode Pt quartz crystal counter electrode Pt wire
reference electrode saturated calomel electrode (SCE)
2) Pulse electrodeposition method ton -03V 100ms and toff 0V 300ms
3) Deposition bath 10mM H2PtCl6 + 01M HCl (65)
Figure 31 shows the cross-section diagram of the platinized
polycrystalline platinum quartz crystal
Polycrystalline Pt
Sputtered Pt Sputtered Ti
9MHz At-cut quartz crystal
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
3 EXPERIMENTAL
19
32 Electrode Preparation and the Modification
A preparation procedure of catalyst ink was well described
elsewhere [7] and the catalyst (Pt or PtRu) loading was 3 mgcm2 in the each
electrode A geometric active area was a 5cm2 and catalyst ink was directly
sparied onto carbon paper Simple dipping method was used for the chloride
ion modification Pt cathode on carbon paper was dipped in 1 times 10minus2M KCl
solution for 5min and it was rinsed in ultrapure water followed by drying in
the oven
33 Half Cell Measurements
Figure 331 shows the schematic experimental setup for measuring
of the electrocatalystic activity of Pt electrode oxidationreduction
adsorptiondesorption of hydrogen methanol oxidation The electrochemical
cell body consisted of a glass cylinder capped with a Teflon lid holding all
electrodes The Pt nanocatalyst sprayed on carbon paper (gas diffusion layer)
was used as working electrode (WE) The geometric area of the WE was
02 cm2 A concentric platinized Pt wire was used as counter electrode (CE)
The tip of a Luggin-Haber capillary hosting a SCE reference electrode (RE)
was placed in the middle of two electrodes A poteniostatgalvanostat (EG amp
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
i
요 약
수소의 흡착탈착 공정과 백금전극의 산화환원반응의 거동 그리고
메탄올의 전기화학적 산화반응에 관한 염소이온으로 처리된 백금전극의
전기화학적 특성을 조사하였다 순수한 백금전극과 염소이온으로 처리된
백금전극을 비교해봄으로써 염소이온으로 처리된 백금전극은 양이온의
흡착능력을 유지하였고 백금전극의 산화를 억제하였다 이것은 산소와
백금전극 표면 사이에서 결합에너지의 감소를 일으키는 것으로 판단된다
순환전위법(CV)으로 캐소드 방향에서 산소환원반응의 과전압이 감소하는
것을 알 수 있었다 또한 염소이온으로 처리된 백금전극은 직접메탄올
연료전지의 성능을 향상시켰다 그리고 캐소드에 처리된 백금전극을 가진
전지의 장기 안정성에 관하여 조사하였고 그 결과 처리되지 않은
백금전극보다 5배정도의 안정성을 나타내는 것을 확인하였다
주제어 연료전지 직접메탄올연료전지 염소이온 처리 산소환원반응 메탄올 크로스오버
ii
ABSTRACT
We investigated electrochemical characteristics of chloride ion
modified Pt (PtClmminus) electrode in the electrocatalytic oxidation of methanol
oxidationreduction behaviours of Pt adsorptiondesorption phenomena of
hydrogen Comparing with pure Pt surface PtClmminus maintained the adsorption
activity of proton hindered oxidation of Pt induced less binding energy
between oxygen and Pt surface It resulted in the decrease of overpotential of
oxygen reduction reaction on the cathodic scan Thus PtClmminus could increase
power performance of direct methanol fuel cell In addition to activity of
PtClmminus the long-term durability of PtClm
minus cathode was also investigated and
it had about five times longer stability
Keywords Fuel cell DMFC methanol oxidation modified electrode
iii
TABLE OF CONTENTS
요 약helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipⅰ
Abstracthelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipⅱ
List of figureshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipⅴ
List of tablehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipVii
1 Introductionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip1
11 What is a fuel cell 1
12 Direct methanol fuel cell (DMFC) 2
13 Objectives 4
2 Basic Theoryhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip6
21 Electro-oxidation of methanol 6
22 Oxygen reduction reaction helliphelliphellip 10
23 Anionic effects 11
24 Actual performance 12
3 Experimentalhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip18
31 Polycrystalline Platinum Preparation 18
32 Electrode Preparation and the Modification 19
iv
33 Half Cell Measurements 19
34 Single cell measurements 22
4 Results and Discussionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip26
5 Conclusionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip49
6 Referenceshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip50
7 Acknowledgementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip52
v
List of Figures Figure 121 Schematic diagram of a DMFC
Figure 241 Typical Fuel Cell Polarization Curve
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
Figure 331 Schematic diagram of experimental set-up for half-cell experiments
Figure 341 Configuration of catalystdiffusion backing and membrane for type a MEA
Figure 342 Schematic diagram of experimental set-up for single cell measurements
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M H2SO4 Scan rate = 50mVs
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M CH3OH05M H2SO4 Scan rate = 50mVs
Figure 415 The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
vi
Figure 416 The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
Figure 417 Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
Figure 418 Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
Figure 419 DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Clminus (c) 0001 M Clminus Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
Figure 4111 Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
vii
List of Table
Table 21 Effect of catalyst promoters on methanol oxidation
1
11 What Is a Fuel Cell
As early as 1839 William Grove discovered the basic operating
principle of fuel cells by reversing water electrolysis to generate electricity
from hydrogen and oxygen The principle that he discovered remains
unchanged today A fuel cell is an electrochemical ldquo device rdquo that
continuously converts chemical energy into electric energy (and some heat)
for as long as fuel and oxidant are supplied Fuel cells therefore bear
similarities both to batteries with which they share the electrochemical
nature of the power generation process and to engines which unlike batteries
will work continuously consuming a fuel of some sort Here is where the
analogies stop though Unlike engines or batteries a fuel cell does not need
recharging it operates quietly and efficiently and when hydrogen is used as
fuel it generates only power and drinking water Thus it is a so-called zero
emission engine [1]
1 INTRODUCTION
2
12 Direct Methanol Fuel Cell (DMFC)
A schematic of a DMFC is shown in Figure 11 Methanol and water
electrochemically react (ie methanol is electro-oxidized) at the anode to
produce carbon dioxide protons and electrons as shown in reaction (121)
An acidic electrolyte is advantageous to aid CO2 rejection since insoluble
carbonates form in alkaline electrolytes The protons produced at the anode
migrate through the polymer electrolyte to the cathode where they react with
oxygen (usually from air) to produce water as shown in reaction (122) The
electrons produced at the anode carry the free energy change of the chemical
reaction and travel through the external circuit where they can be made to do
useful work such as powering an electric motor The overall cell reaction as
shown in equation (123) is therefore the reaction of methanol and oxygen
to produce water and carbon dioxide
Anode CH3OH + H2O rarr CO2 + 6H+ + 6e- 0AE = 0046V (121)
Cathode 32
O2 + 6H+ + 6e- rarr 3H2O 0CE = 1229V (122)
Net Reaction CH3OH + 32
O2 rarr CO2 + 2H2O (123)
0cellE = 0
CE - 0AE =1229V - 0046V = 1183V Attainable max cell potential
3
Figure 121 Schematic diagram of direct fuel cell
4
13 Objectives
Of late fuel cells have become a central focus of interest and leading
car and power companies are making large scale investments in fuel cell
systems Fuel cells are set to provide electrical vehicles small and large scale
power stations and even portable electrical applications such as laptops and
mobile phones with electrical power in a manner which is more efficient and
more environmentally friendly than ever before [2 minus6] In particular direct
methanol fuel cells (DMFCs) have been considered for use in very small to
mid-sized applications due to their high electrical efficiency long life-time
and low poisonous emissions [2 minus 6] On the other hand in the operation of
DMFCs there are several problems at a cathode caused by methanol
crossover from anode to cathode and it should be solved for the
commercialization of DMFC (i) cathode poisoning by CO adsorption and Pt
sintering (ii) significant fuel loss due to methanol crossover (iii) decrease
partial pressure of oxygen due to water flooding and (iv) temperature
dependent power performance Therefore cathode catalyst for oxygen
reduction reaction (ORR) requires (a) CO tolerance (b) proton adsorption
available (c) easy oxygen adsorption (d) ORR kinetics (e) oxygen storage
material (f) suppression of electro-oxidation of methanol To our knowledge
5
no work has been done up to now to study the influence of halide
contamination on fuel cell performance From the data presented previously
Paulus et al [2] could only point to some aspects which should be considered
during MEA production the degradation of MEA might be occurred due to
the dissolution of Nafion by H2O2 and it was mainly produced in the
reduction of oxygen on a Pt substrate containing chloride ions
In this work we tried to develop new cathode catalyst in DMFC via
adsorption of chloride ion which has been known as inhibiting species in
most electrochemical studies The electrochemical characteristics of chloride
ion modified Pt cathode in the oxidation of methanol and
adsorptiondesorption of hydrogen were studied by cyclic voltammetry
techniques The power performance of the single cell and long-term stability
of pure Pt and modified Pt cathode was also investigated
6
21 Electro-oxidation of Methanol
The electrochemical oxidation of methanol on Pt involves several
intermediate steps ie dehydrogenation CO-like species chemisorption OH
(or H2O) species adsorption chemical interaction between adsorbed CO and
OH compounds and CO2 evolution One of these steps is the rate
determining step (rds) depending on the operating temperature and
particular catalyst surface State of the art electrocatalysts for the electro-
oxidation of methanol in fuel cells are generally based on Pt alloys supported
on carbon black [8 9] The electrocatalytic activity of Pt is known to be
promoted by the presence of a second metal such as Ru or Sn acting either
as an adatom or a bimetal First a sequence of dehydrogenation steps give rise
to adsorbed methanolic residues according to the following scheme[10]
CH3OH + Pt rarr Pt-CH2OH + H+ + 1e- (211)
Pt-CH2OH + Pt rarr Pt-CHOH + H+ + 1e- (212)
Pt-CHOH + Pt rarr PtCHO + H+ + 1e- (213)
2 BASIC THEORY
7
A surface rearrangement of the methanol oxidation intermediates
gives carbon monoxide linearly or bridge-bonded to Pt sites as follows
PtCHO rarr Pt-CequivO + H+ + 1e- (214)
Or
PtCHO + Pt rarr Pt Pt C=O + H+ + 1e- (215)
In the absence of a promoting element water discharge occurs at
high anodic overpotentials on Pt with the formation of Pt-OH species at the
catalyst surface
Pt + H2O rarr PtOH + H+ + 1e- (216)
The final step is the reaction of Pt-OH groups with neighboring
methanolic residues to give carbon dioxide
PtOH + Pt CO rarr 2Pt + CO2 + H+ + 1e- (217)
The overall oxidation process of methanol to carbon dioxide
proceeds through a six electron donation process On a pure Pt surface the
dissociative chemisorption of water on Pt is the rate determining step at
potentials below 07 V vs RHE [11] It is generally accepted that an active
catalyst for methanol oxidation should give rise to water discharging at low
potentials and to labile CO chemisorption Moreover a good catalyst for
methanol oxidation should also catalyze the oxidation of carbon monoxide
8
The promotion of Pt by a range of metals (and oxides) to enhance
methanol oxidation is well known Table 21 summeries species that have a
reported positive effect on methanol oxidation Of the catalyst promoters
considered significant recent activity has focused on the use of Ru Sn and
W with Pt
On PtRu surface CH3OH adsorption would take place on the Pt site
where CH3OH chemisorption requires three neighboring sites yielding
ensembles of Pt atoms with COad and the Ru atoms would act as OHad
providing center According to this the catalytic improvement would be
bifunctional mechanism There are different reports for optimal Ru contents
According to Gasteiger el al a Ru content of 10 is optimum[12] Increasing
Ru content provides more adsorbed water species required to oxidize COad
However because Pt sites are decreased with increasing Ru contents the
oxidation rate of CH3OH is decreased Consequently the rate of reaction
decreased when the Ru contents are more than 10 But other results
referring a coverage close to 05 by Ru is optimal for enhancing CH3OH
oxidation are reported by R Dillon[13]
9
Table 21 Effect of catalyst promoters on methanol oxidation
Promotion by Catalyst promoter
comment
Alloying and
dissolution to produce
highly reticulated
surfaces
Cr
Fe
Sn
Typically less than 100 mV
lower potential than Pt
Surface adatoms Sn
Bi
Alloys
Ru
Sn Mo
Os Ir
Ti Re
Ru has the greatest effect Sn
Mo Os and Re are substantial
promoters
Metal oxides Ru Hydrous Ru oxide the most
active catalyst
Base metal oxides W
Nb Zr Ta
W oxide is a notable prometer
Small effect of other metal
oxides (typically lt 100mV)
10
22 Oxygen Reduction Reaction
Oxygen reduction reaction is considered to be one of the most
important electrocatalytic reactions because of its role in electrochemical
energy conversion several industrial processes and corrosion Consequently
it was in the focus of electrochemical interest for many years Oxygen
reduction reaction has been a challenge for electrochemists because of its
complex kinetics and the need for better electrocatalysts The most notable
need for the improvement of the catalytic activity and the development of
new better non-noble metal electrocatalysts are still remaining Despite
extensive research on oxygen reduction reaction mechanism is still not fully
understood
Oxygen reduction reaction is known to proceed either in a four-
electron step to H2O or in a two-electron step to H2O2 In general the four-
electron-step is considered to be the major reaction pathway on
polycrystalline platinum electrodes [1]
+ -2 2 0O + 4H + 4e 2H O E =1229V ( NHE)vsrarr (221)
- -2 2 0O + 2H O + 2e 4OH E =0401Vrarr (222)
The peroxide pathway is
11
+ -2 2 2 0O + 2H + 2e H O E =067Vrarr (223)
Peroxide can undergo further reduction or decomposition in acid
solutions via the following reactions
+ -2 2 2 0H O + 2H + 2e 2H O E =177Vrarr (224)
2 2 2 22H O 2H O + O rarr
However it is commonly agreed that it proceeds without formation of
surface peroxide on polycrystalline and particulate Pt in acidic media
Furthermore it is believed that the reaction proceeds
O2 harr O2ads (225)
O2ads + H+(aq) +e- rarr O2Hads (226)
O2Hads + 3H+(aq) +3e- rarr 2H2O (227)
with the first charge transfer reaction (226) as the rate determining step The
details of reaction (227) are unknown and may include several Oads or OHads
configurations [2]
23 Anionic Effects
Platinum or platinum alloy particles supported on high surface area
carbon substrates are often synthesized form halide containing reactants (eg
Cl- containing salts)[15] It is therefore possible that traces of halides are
12
present in the electrocatalyst Furthermore halides especially chloride may
also be present in the humidified anode feed-streams of PEFCs or in the
cathode air feed During the ORR remaining anions can compete with
oxygen for free adsorption sites on the catalyst Modified adsorption
conditions for oxygen molecules can have an impact on the oxygen reduction
activity of the catalyst and can alter the oxygen reduction pathway (transition
form H2O to H2O2 formation) From both Pt single crystal work (eg [14]) it
is well known that different supporting electrolytes containing anions with
different adsorption strengths influence the ORR activity tremendously It is
also well known that the presence of additional halide anions can
dramatically affect the kinetics of electrocatalytic reactions on Pt e g the
oxidation of small organic molecules or the oxygen reduction reaction (e g
[16])
24 Actual Performance
The actual cell potential is decreased from its ideal potential because
of several types of irreversible losses as shown in Figure 21 These losses
are often referred to as polarization overpotential or overvoltage though
13
only the ohmic losses actually behave as a resistance Multiple phenomena
contribute to irreversible losses in an actual fuel cell
In the V-I diagram especially for low-temperature fuel cells the
effects of the three loss categories are often easy to distinguish as illustrated
in Figure 21
Activation Losses Activation losses are caused by sluggish electrode
kinetics There is a close similarity between electrochemical and chemical
reactions in that both involve an activation energy that must be overcome by
the reacting species In reality activation losses are the result of complex
surface electrochemical reaction steps each of which have their own reaction
rate and activation energy In electrochemical reactions the activation energy
loss for overcoming this barrier is represented as Tafel equation
lnact a b iη = + (241)
where actη is activation polarization(mV) a and b are constants and i is
current density(mAcm2)
( ) ( ) ( )act cell act anode act cathodeη η η= + (242)
Ohmic Polarization Ohmic losses occur because of resistance to the
flow of ions in the electrolyte and resistance to flow of electrons through the
electrode The dominant ohmic losses through the electrolyte are reduced by
14
decreasing the electrode separation and enhancing the ionic conductivity of
the electrolyte Because both the electrolyte and fuel cell electrodes obey
Ohms law the ohmic losses can be expressed by the equation
ohm iRη = (243)
where i is the current flowing through the cell and R is the total cell
resistance which includes electronic ionic and contact resistance
R = Relectronic + Rionic + Rcontact
Mass Transport Related Losses As a reactant is consumed at the
electrode by electrochemical reaction it is often diluted by the products
when finite mass transport rates limit the supply of fresh reactant and the
evacuation of products As a consequence a concentration gradient is formed
which drives the mass transport process
Concentration polarization can be denoted by
ln(1 )consL
RT inF i
η = minus (244)
where consη is concentration polarization(mV) iL is limiting current
density(mAcm2) Same as the activation polarization the concentration
polarization occurs independently at either electrode
( ) ( ) ( )conc cell conc anode conc cathodeη η η= + (245)
15
The combined effect of the losses for a given cell and given
operating conditions can be expressed as polarizations The total polarization
at the electrodes is the sum of actη and concη or
anode acta a conc aη η η= + (246)
and
cathode acta c conc cη η η= + (247)
The effect of polarization is to shift the potential of the electrode
(Eelectrode) to a new value (Velectrode)
electrode electrode electrodeV E η= plusmn (248)
For the anode
anode anode anodeV E η= + (249)
And for the cathode
cathode cathode cathodeV E η= minus (2410)
The net result of current flow in a fuel cell is to increase the anode
potential and to decrease the cathode potential thereby reducing the cell
voltage
The cell voltage includes the contribution of the anode and cathode
potentials and ohmic polarization
16
cell cathode anodeV V V iR= minus minus (2411)
When Equations (249) and (2410) are substituted in Equation (2411)
( )cell cathode cathode anode anodeV E E iRη η= minus minus + minus (2412)
Or
cell e cathode anodeV E iRη η= ∆ minus minus minus (2413)
Where e cathode anodeE E E∆ = minus Equation (2413) shows that current
flow in a fuel cell results in a decrease in cell voltage because of losses by
electrode and ohmic polarizations The goal of fuel cell developers is to
minimize the polarization so that cellV approaches eE∆ This goal is
approached by modifications to fuel cell design (improvement in electrode
structures better electro-catalysts more conductive electrolyte thinner cell
components etc) [1]
17
Figure 241 Typical Fuel Cell Polarization Curve
18
31 Polycrystalline Platinum Preparation
A polycrystalline platinum electrode was prepared with pulse
electrodeposition on Pt- quartz crystal
1) Working electrode Pt quartz crystal counter electrode Pt wire
reference electrode saturated calomel electrode (SCE)
2) Pulse electrodeposition method ton -03V 100ms and toff 0V 300ms
3) Deposition bath 10mM H2PtCl6 + 01M HCl (65)
Figure 31 shows the cross-section diagram of the platinized
polycrystalline platinum quartz crystal
Polycrystalline Pt
Sputtered Pt Sputtered Ti
9MHz At-cut quartz crystal
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
3 EXPERIMENTAL
19
32 Electrode Preparation and the Modification
A preparation procedure of catalyst ink was well described
elsewhere [7] and the catalyst (Pt or PtRu) loading was 3 mgcm2 in the each
electrode A geometric active area was a 5cm2 and catalyst ink was directly
sparied onto carbon paper Simple dipping method was used for the chloride
ion modification Pt cathode on carbon paper was dipped in 1 times 10minus2M KCl
solution for 5min and it was rinsed in ultrapure water followed by drying in
the oven
33 Half Cell Measurements
Figure 331 shows the schematic experimental setup for measuring
of the electrocatalystic activity of Pt electrode oxidationreduction
adsorptiondesorption of hydrogen methanol oxidation The electrochemical
cell body consisted of a glass cylinder capped with a Teflon lid holding all
electrodes The Pt nanocatalyst sprayed on carbon paper (gas diffusion layer)
was used as working electrode (WE) The geometric area of the WE was
02 cm2 A concentric platinized Pt wire was used as counter electrode (CE)
The tip of a Luggin-Haber capillary hosting a SCE reference electrode (RE)
was placed in the middle of two electrodes A poteniostatgalvanostat (EG amp
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
ii
ABSTRACT
We investigated electrochemical characteristics of chloride ion
modified Pt (PtClmminus) electrode in the electrocatalytic oxidation of methanol
oxidationreduction behaviours of Pt adsorptiondesorption phenomena of
hydrogen Comparing with pure Pt surface PtClmminus maintained the adsorption
activity of proton hindered oxidation of Pt induced less binding energy
between oxygen and Pt surface It resulted in the decrease of overpotential of
oxygen reduction reaction on the cathodic scan Thus PtClmminus could increase
power performance of direct methanol fuel cell In addition to activity of
PtClmminus the long-term durability of PtClm
minus cathode was also investigated and
it had about five times longer stability
Keywords Fuel cell DMFC methanol oxidation modified electrode
iii
TABLE OF CONTENTS
요 약helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipⅰ
Abstracthelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipⅱ
List of figureshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipⅴ
List of tablehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipVii
1 Introductionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip1
11 What is a fuel cell 1
12 Direct methanol fuel cell (DMFC) 2
13 Objectives 4
2 Basic Theoryhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip6
21 Electro-oxidation of methanol 6
22 Oxygen reduction reaction helliphelliphellip 10
23 Anionic effects 11
24 Actual performance 12
3 Experimentalhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip18
31 Polycrystalline Platinum Preparation 18
32 Electrode Preparation and the Modification 19
iv
33 Half Cell Measurements 19
34 Single cell measurements 22
4 Results and Discussionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip26
5 Conclusionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip49
6 Referenceshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip50
7 Acknowledgementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip52
v
List of Figures Figure 121 Schematic diagram of a DMFC
Figure 241 Typical Fuel Cell Polarization Curve
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
Figure 331 Schematic diagram of experimental set-up for half-cell experiments
Figure 341 Configuration of catalystdiffusion backing and membrane for type a MEA
Figure 342 Schematic diagram of experimental set-up for single cell measurements
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M H2SO4 Scan rate = 50mVs
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M CH3OH05M H2SO4 Scan rate = 50mVs
Figure 415 The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
vi
Figure 416 The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
Figure 417 Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
Figure 418 Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
Figure 419 DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Clminus (c) 0001 M Clminus Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
Figure 4111 Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
vii
List of Table
Table 21 Effect of catalyst promoters on methanol oxidation
1
11 What Is a Fuel Cell
As early as 1839 William Grove discovered the basic operating
principle of fuel cells by reversing water electrolysis to generate electricity
from hydrogen and oxygen The principle that he discovered remains
unchanged today A fuel cell is an electrochemical ldquo device rdquo that
continuously converts chemical energy into electric energy (and some heat)
for as long as fuel and oxidant are supplied Fuel cells therefore bear
similarities both to batteries with which they share the electrochemical
nature of the power generation process and to engines which unlike batteries
will work continuously consuming a fuel of some sort Here is where the
analogies stop though Unlike engines or batteries a fuel cell does not need
recharging it operates quietly and efficiently and when hydrogen is used as
fuel it generates only power and drinking water Thus it is a so-called zero
emission engine [1]
1 INTRODUCTION
2
12 Direct Methanol Fuel Cell (DMFC)
A schematic of a DMFC is shown in Figure 11 Methanol and water
electrochemically react (ie methanol is electro-oxidized) at the anode to
produce carbon dioxide protons and electrons as shown in reaction (121)
An acidic electrolyte is advantageous to aid CO2 rejection since insoluble
carbonates form in alkaline electrolytes The protons produced at the anode
migrate through the polymer electrolyte to the cathode where they react with
oxygen (usually from air) to produce water as shown in reaction (122) The
electrons produced at the anode carry the free energy change of the chemical
reaction and travel through the external circuit where they can be made to do
useful work such as powering an electric motor The overall cell reaction as
shown in equation (123) is therefore the reaction of methanol and oxygen
to produce water and carbon dioxide
Anode CH3OH + H2O rarr CO2 + 6H+ + 6e- 0AE = 0046V (121)
Cathode 32
O2 + 6H+ + 6e- rarr 3H2O 0CE = 1229V (122)
Net Reaction CH3OH + 32
O2 rarr CO2 + 2H2O (123)
0cellE = 0
CE - 0AE =1229V - 0046V = 1183V Attainable max cell potential
3
Figure 121 Schematic diagram of direct fuel cell
4
13 Objectives
Of late fuel cells have become a central focus of interest and leading
car and power companies are making large scale investments in fuel cell
systems Fuel cells are set to provide electrical vehicles small and large scale
power stations and even portable electrical applications such as laptops and
mobile phones with electrical power in a manner which is more efficient and
more environmentally friendly than ever before [2 minus6] In particular direct
methanol fuel cells (DMFCs) have been considered for use in very small to
mid-sized applications due to their high electrical efficiency long life-time
and low poisonous emissions [2 minus 6] On the other hand in the operation of
DMFCs there are several problems at a cathode caused by methanol
crossover from anode to cathode and it should be solved for the
commercialization of DMFC (i) cathode poisoning by CO adsorption and Pt
sintering (ii) significant fuel loss due to methanol crossover (iii) decrease
partial pressure of oxygen due to water flooding and (iv) temperature
dependent power performance Therefore cathode catalyst for oxygen
reduction reaction (ORR) requires (a) CO tolerance (b) proton adsorption
available (c) easy oxygen adsorption (d) ORR kinetics (e) oxygen storage
material (f) suppression of electro-oxidation of methanol To our knowledge
5
no work has been done up to now to study the influence of halide
contamination on fuel cell performance From the data presented previously
Paulus et al [2] could only point to some aspects which should be considered
during MEA production the degradation of MEA might be occurred due to
the dissolution of Nafion by H2O2 and it was mainly produced in the
reduction of oxygen on a Pt substrate containing chloride ions
In this work we tried to develop new cathode catalyst in DMFC via
adsorption of chloride ion which has been known as inhibiting species in
most electrochemical studies The electrochemical characteristics of chloride
ion modified Pt cathode in the oxidation of methanol and
adsorptiondesorption of hydrogen were studied by cyclic voltammetry
techniques The power performance of the single cell and long-term stability
of pure Pt and modified Pt cathode was also investigated
6
21 Electro-oxidation of Methanol
The electrochemical oxidation of methanol on Pt involves several
intermediate steps ie dehydrogenation CO-like species chemisorption OH
(or H2O) species adsorption chemical interaction between adsorbed CO and
OH compounds and CO2 evolution One of these steps is the rate
determining step (rds) depending on the operating temperature and
particular catalyst surface State of the art electrocatalysts for the electro-
oxidation of methanol in fuel cells are generally based on Pt alloys supported
on carbon black [8 9] The electrocatalytic activity of Pt is known to be
promoted by the presence of a second metal such as Ru or Sn acting either
as an adatom or a bimetal First a sequence of dehydrogenation steps give rise
to adsorbed methanolic residues according to the following scheme[10]
CH3OH + Pt rarr Pt-CH2OH + H+ + 1e- (211)
Pt-CH2OH + Pt rarr Pt-CHOH + H+ + 1e- (212)
Pt-CHOH + Pt rarr PtCHO + H+ + 1e- (213)
2 BASIC THEORY
7
A surface rearrangement of the methanol oxidation intermediates
gives carbon monoxide linearly or bridge-bonded to Pt sites as follows
PtCHO rarr Pt-CequivO + H+ + 1e- (214)
Or
PtCHO + Pt rarr Pt Pt C=O + H+ + 1e- (215)
In the absence of a promoting element water discharge occurs at
high anodic overpotentials on Pt with the formation of Pt-OH species at the
catalyst surface
Pt + H2O rarr PtOH + H+ + 1e- (216)
The final step is the reaction of Pt-OH groups with neighboring
methanolic residues to give carbon dioxide
PtOH + Pt CO rarr 2Pt + CO2 + H+ + 1e- (217)
The overall oxidation process of methanol to carbon dioxide
proceeds through a six electron donation process On a pure Pt surface the
dissociative chemisorption of water on Pt is the rate determining step at
potentials below 07 V vs RHE [11] It is generally accepted that an active
catalyst for methanol oxidation should give rise to water discharging at low
potentials and to labile CO chemisorption Moreover a good catalyst for
methanol oxidation should also catalyze the oxidation of carbon monoxide
8
The promotion of Pt by a range of metals (and oxides) to enhance
methanol oxidation is well known Table 21 summeries species that have a
reported positive effect on methanol oxidation Of the catalyst promoters
considered significant recent activity has focused on the use of Ru Sn and
W with Pt
On PtRu surface CH3OH adsorption would take place on the Pt site
where CH3OH chemisorption requires three neighboring sites yielding
ensembles of Pt atoms with COad and the Ru atoms would act as OHad
providing center According to this the catalytic improvement would be
bifunctional mechanism There are different reports for optimal Ru contents
According to Gasteiger el al a Ru content of 10 is optimum[12] Increasing
Ru content provides more adsorbed water species required to oxidize COad
However because Pt sites are decreased with increasing Ru contents the
oxidation rate of CH3OH is decreased Consequently the rate of reaction
decreased when the Ru contents are more than 10 But other results
referring a coverage close to 05 by Ru is optimal for enhancing CH3OH
oxidation are reported by R Dillon[13]
9
Table 21 Effect of catalyst promoters on methanol oxidation
Promotion by Catalyst promoter
comment
Alloying and
dissolution to produce
highly reticulated
surfaces
Cr
Fe
Sn
Typically less than 100 mV
lower potential than Pt
Surface adatoms Sn
Bi
Alloys
Ru
Sn Mo
Os Ir
Ti Re
Ru has the greatest effect Sn
Mo Os and Re are substantial
promoters
Metal oxides Ru Hydrous Ru oxide the most
active catalyst
Base metal oxides W
Nb Zr Ta
W oxide is a notable prometer
Small effect of other metal
oxides (typically lt 100mV)
10
22 Oxygen Reduction Reaction
Oxygen reduction reaction is considered to be one of the most
important electrocatalytic reactions because of its role in electrochemical
energy conversion several industrial processes and corrosion Consequently
it was in the focus of electrochemical interest for many years Oxygen
reduction reaction has been a challenge for electrochemists because of its
complex kinetics and the need for better electrocatalysts The most notable
need for the improvement of the catalytic activity and the development of
new better non-noble metal electrocatalysts are still remaining Despite
extensive research on oxygen reduction reaction mechanism is still not fully
understood
Oxygen reduction reaction is known to proceed either in a four-
electron step to H2O or in a two-electron step to H2O2 In general the four-
electron-step is considered to be the major reaction pathway on
polycrystalline platinum electrodes [1]
+ -2 2 0O + 4H + 4e 2H O E =1229V ( NHE)vsrarr (221)
- -2 2 0O + 2H O + 2e 4OH E =0401Vrarr (222)
The peroxide pathway is
11
+ -2 2 2 0O + 2H + 2e H O E =067Vrarr (223)
Peroxide can undergo further reduction or decomposition in acid
solutions via the following reactions
+ -2 2 2 0H O + 2H + 2e 2H O E =177Vrarr (224)
2 2 2 22H O 2H O + O rarr
However it is commonly agreed that it proceeds without formation of
surface peroxide on polycrystalline and particulate Pt in acidic media
Furthermore it is believed that the reaction proceeds
O2 harr O2ads (225)
O2ads + H+(aq) +e- rarr O2Hads (226)
O2Hads + 3H+(aq) +3e- rarr 2H2O (227)
with the first charge transfer reaction (226) as the rate determining step The
details of reaction (227) are unknown and may include several Oads or OHads
configurations [2]
23 Anionic Effects
Platinum or platinum alloy particles supported on high surface area
carbon substrates are often synthesized form halide containing reactants (eg
Cl- containing salts)[15] It is therefore possible that traces of halides are
12
present in the electrocatalyst Furthermore halides especially chloride may
also be present in the humidified anode feed-streams of PEFCs or in the
cathode air feed During the ORR remaining anions can compete with
oxygen for free adsorption sites on the catalyst Modified adsorption
conditions for oxygen molecules can have an impact on the oxygen reduction
activity of the catalyst and can alter the oxygen reduction pathway (transition
form H2O to H2O2 formation) From both Pt single crystal work (eg [14]) it
is well known that different supporting electrolytes containing anions with
different adsorption strengths influence the ORR activity tremendously It is
also well known that the presence of additional halide anions can
dramatically affect the kinetics of electrocatalytic reactions on Pt e g the
oxidation of small organic molecules or the oxygen reduction reaction (e g
[16])
24 Actual Performance
The actual cell potential is decreased from its ideal potential because
of several types of irreversible losses as shown in Figure 21 These losses
are often referred to as polarization overpotential or overvoltage though
13
only the ohmic losses actually behave as a resistance Multiple phenomena
contribute to irreversible losses in an actual fuel cell
In the V-I diagram especially for low-temperature fuel cells the
effects of the three loss categories are often easy to distinguish as illustrated
in Figure 21
Activation Losses Activation losses are caused by sluggish electrode
kinetics There is a close similarity between electrochemical and chemical
reactions in that both involve an activation energy that must be overcome by
the reacting species In reality activation losses are the result of complex
surface electrochemical reaction steps each of which have their own reaction
rate and activation energy In electrochemical reactions the activation energy
loss for overcoming this barrier is represented as Tafel equation
lnact a b iη = + (241)
where actη is activation polarization(mV) a and b are constants and i is
current density(mAcm2)
( ) ( ) ( )act cell act anode act cathodeη η η= + (242)
Ohmic Polarization Ohmic losses occur because of resistance to the
flow of ions in the electrolyte and resistance to flow of electrons through the
electrode The dominant ohmic losses through the electrolyte are reduced by
14
decreasing the electrode separation and enhancing the ionic conductivity of
the electrolyte Because both the electrolyte and fuel cell electrodes obey
Ohms law the ohmic losses can be expressed by the equation
ohm iRη = (243)
where i is the current flowing through the cell and R is the total cell
resistance which includes electronic ionic and contact resistance
R = Relectronic + Rionic + Rcontact
Mass Transport Related Losses As a reactant is consumed at the
electrode by electrochemical reaction it is often diluted by the products
when finite mass transport rates limit the supply of fresh reactant and the
evacuation of products As a consequence a concentration gradient is formed
which drives the mass transport process
Concentration polarization can be denoted by
ln(1 )consL
RT inF i
η = minus (244)
where consη is concentration polarization(mV) iL is limiting current
density(mAcm2) Same as the activation polarization the concentration
polarization occurs independently at either electrode
( ) ( ) ( )conc cell conc anode conc cathodeη η η= + (245)
15
The combined effect of the losses for a given cell and given
operating conditions can be expressed as polarizations The total polarization
at the electrodes is the sum of actη and concη or
anode acta a conc aη η η= + (246)
and
cathode acta c conc cη η η= + (247)
The effect of polarization is to shift the potential of the electrode
(Eelectrode) to a new value (Velectrode)
electrode electrode electrodeV E η= plusmn (248)
For the anode
anode anode anodeV E η= + (249)
And for the cathode
cathode cathode cathodeV E η= minus (2410)
The net result of current flow in a fuel cell is to increase the anode
potential and to decrease the cathode potential thereby reducing the cell
voltage
The cell voltage includes the contribution of the anode and cathode
potentials and ohmic polarization
16
cell cathode anodeV V V iR= minus minus (2411)
When Equations (249) and (2410) are substituted in Equation (2411)
( )cell cathode cathode anode anodeV E E iRη η= minus minus + minus (2412)
Or
cell e cathode anodeV E iRη η= ∆ minus minus minus (2413)
Where e cathode anodeE E E∆ = minus Equation (2413) shows that current
flow in a fuel cell results in a decrease in cell voltage because of losses by
electrode and ohmic polarizations The goal of fuel cell developers is to
minimize the polarization so that cellV approaches eE∆ This goal is
approached by modifications to fuel cell design (improvement in electrode
structures better electro-catalysts more conductive electrolyte thinner cell
components etc) [1]
17
Figure 241 Typical Fuel Cell Polarization Curve
18
31 Polycrystalline Platinum Preparation
A polycrystalline platinum electrode was prepared with pulse
electrodeposition on Pt- quartz crystal
1) Working electrode Pt quartz crystal counter electrode Pt wire
reference electrode saturated calomel electrode (SCE)
2) Pulse electrodeposition method ton -03V 100ms and toff 0V 300ms
3) Deposition bath 10mM H2PtCl6 + 01M HCl (65)
Figure 31 shows the cross-section diagram of the platinized
polycrystalline platinum quartz crystal
Polycrystalline Pt
Sputtered Pt Sputtered Ti
9MHz At-cut quartz crystal
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
3 EXPERIMENTAL
19
32 Electrode Preparation and the Modification
A preparation procedure of catalyst ink was well described
elsewhere [7] and the catalyst (Pt or PtRu) loading was 3 mgcm2 in the each
electrode A geometric active area was a 5cm2 and catalyst ink was directly
sparied onto carbon paper Simple dipping method was used for the chloride
ion modification Pt cathode on carbon paper was dipped in 1 times 10minus2M KCl
solution for 5min and it was rinsed in ultrapure water followed by drying in
the oven
33 Half Cell Measurements
Figure 331 shows the schematic experimental setup for measuring
of the electrocatalystic activity of Pt electrode oxidationreduction
adsorptiondesorption of hydrogen methanol oxidation The electrochemical
cell body consisted of a glass cylinder capped with a Teflon lid holding all
electrodes The Pt nanocatalyst sprayed on carbon paper (gas diffusion layer)
was used as working electrode (WE) The geometric area of the WE was
02 cm2 A concentric platinized Pt wire was used as counter electrode (CE)
The tip of a Luggin-Haber capillary hosting a SCE reference electrode (RE)
was placed in the middle of two electrodes A poteniostatgalvanostat (EG amp
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
iii
TABLE OF CONTENTS
요 약helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipⅰ
Abstracthelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipⅱ
List of figureshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipⅴ
List of tablehelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipVii
1 Introductionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip1
11 What is a fuel cell 1
12 Direct methanol fuel cell (DMFC) 2
13 Objectives 4
2 Basic Theoryhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip6
21 Electro-oxidation of methanol 6
22 Oxygen reduction reaction helliphelliphellip 10
23 Anionic effects 11
24 Actual performance 12
3 Experimentalhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip18
31 Polycrystalline Platinum Preparation 18
32 Electrode Preparation and the Modification 19
iv
33 Half Cell Measurements 19
34 Single cell measurements 22
4 Results and Discussionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip26
5 Conclusionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip49
6 Referenceshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip50
7 Acknowledgementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip52
v
List of Figures Figure 121 Schematic diagram of a DMFC
Figure 241 Typical Fuel Cell Polarization Curve
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
Figure 331 Schematic diagram of experimental set-up for half-cell experiments
Figure 341 Configuration of catalystdiffusion backing and membrane for type a MEA
Figure 342 Schematic diagram of experimental set-up for single cell measurements
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M H2SO4 Scan rate = 50mVs
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M CH3OH05M H2SO4 Scan rate = 50mVs
Figure 415 The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
vi
Figure 416 The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
Figure 417 Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
Figure 418 Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
Figure 419 DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Clminus (c) 0001 M Clminus Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
Figure 4111 Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
vii
List of Table
Table 21 Effect of catalyst promoters on methanol oxidation
1
11 What Is a Fuel Cell
As early as 1839 William Grove discovered the basic operating
principle of fuel cells by reversing water electrolysis to generate electricity
from hydrogen and oxygen The principle that he discovered remains
unchanged today A fuel cell is an electrochemical ldquo device rdquo that
continuously converts chemical energy into electric energy (and some heat)
for as long as fuel and oxidant are supplied Fuel cells therefore bear
similarities both to batteries with which they share the electrochemical
nature of the power generation process and to engines which unlike batteries
will work continuously consuming a fuel of some sort Here is where the
analogies stop though Unlike engines or batteries a fuel cell does not need
recharging it operates quietly and efficiently and when hydrogen is used as
fuel it generates only power and drinking water Thus it is a so-called zero
emission engine [1]
1 INTRODUCTION
2
12 Direct Methanol Fuel Cell (DMFC)
A schematic of a DMFC is shown in Figure 11 Methanol and water
electrochemically react (ie methanol is electro-oxidized) at the anode to
produce carbon dioxide protons and electrons as shown in reaction (121)
An acidic electrolyte is advantageous to aid CO2 rejection since insoluble
carbonates form in alkaline electrolytes The protons produced at the anode
migrate through the polymer electrolyte to the cathode where they react with
oxygen (usually from air) to produce water as shown in reaction (122) The
electrons produced at the anode carry the free energy change of the chemical
reaction and travel through the external circuit where they can be made to do
useful work such as powering an electric motor The overall cell reaction as
shown in equation (123) is therefore the reaction of methanol and oxygen
to produce water and carbon dioxide
Anode CH3OH + H2O rarr CO2 + 6H+ + 6e- 0AE = 0046V (121)
Cathode 32
O2 + 6H+ + 6e- rarr 3H2O 0CE = 1229V (122)
Net Reaction CH3OH + 32
O2 rarr CO2 + 2H2O (123)
0cellE = 0
CE - 0AE =1229V - 0046V = 1183V Attainable max cell potential
3
Figure 121 Schematic diagram of direct fuel cell
4
13 Objectives
Of late fuel cells have become a central focus of interest and leading
car and power companies are making large scale investments in fuel cell
systems Fuel cells are set to provide electrical vehicles small and large scale
power stations and even portable electrical applications such as laptops and
mobile phones with electrical power in a manner which is more efficient and
more environmentally friendly than ever before [2 minus6] In particular direct
methanol fuel cells (DMFCs) have been considered for use in very small to
mid-sized applications due to their high electrical efficiency long life-time
and low poisonous emissions [2 minus 6] On the other hand in the operation of
DMFCs there are several problems at a cathode caused by methanol
crossover from anode to cathode and it should be solved for the
commercialization of DMFC (i) cathode poisoning by CO adsorption and Pt
sintering (ii) significant fuel loss due to methanol crossover (iii) decrease
partial pressure of oxygen due to water flooding and (iv) temperature
dependent power performance Therefore cathode catalyst for oxygen
reduction reaction (ORR) requires (a) CO tolerance (b) proton adsorption
available (c) easy oxygen adsorption (d) ORR kinetics (e) oxygen storage
material (f) suppression of electro-oxidation of methanol To our knowledge
5
no work has been done up to now to study the influence of halide
contamination on fuel cell performance From the data presented previously
Paulus et al [2] could only point to some aspects which should be considered
during MEA production the degradation of MEA might be occurred due to
the dissolution of Nafion by H2O2 and it was mainly produced in the
reduction of oxygen on a Pt substrate containing chloride ions
In this work we tried to develop new cathode catalyst in DMFC via
adsorption of chloride ion which has been known as inhibiting species in
most electrochemical studies The electrochemical characteristics of chloride
ion modified Pt cathode in the oxidation of methanol and
adsorptiondesorption of hydrogen were studied by cyclic voltammetry
techniques The power performance of the single cell and long-term stability
of pure Pt and modified Pt cathode was also investigated
6
21 Electro-oxidation of Methanol
The electrochemical oxidation of methanol on Pt involves several
intermediate steps ie dehydrogenation CO-like species chemisorption OH
(or H2O) species adsorption chemical interaction between adsorbed CO and
OH compounds and CO2 evolution One of these steps is the rate
determining step (rds) depending on the operating temperature and
particular catalyst surface State of the art electrocatalysts for the electro-
oxidation of methanol in fuel cells are generally based on Pt alloys supported
on carbon black [8 9] The electrocatalytic activity of Pt is known to be
promoted by the presence of a second metal such as Ru or Sn acting either
as an adatom or a bimetal First a sequence of dehydrogenation steps give rise
to adsorbed methanolic residues according to the following scheme[10]
CH3OH + Pt rarr Pt-CH2OH + H+ + 1e- (211)
Pt-CH2OH + Pt rarr Pt-CHOH + H+ + 1e- (212)
Pt-CHOH + Pt rarr PtCHO + H+ + 1e- (213)
2 BASIC THEORY
7
A surface rearrangement of the methanol oxidation intermediates
gives carbon monoxide linearly or bridge-bonded to Pt sites as follows
PtCHO rarr Pt-CequivO + H+ + 1e- (214)
Or
PtCHO + Pt rarr Pt Pt C=O + H+ + 1e- (215)
In the absence of a promoting element water discharge occurs at
high anodic overpotentials on Pt with the formation of Pt-OH species at the
catalyst surface
Pt + H2O rarr PtOH + H+ + 1e- (216)
The final step is the reaction of Pt-OH groups with neighboring
methanolic residues to give carbon dioxide
PtOH + Pt CO rarr 2Pt + CO2 + H+ + 1e- (217)
The overall oxidation process of methanol to carbon dioxide
proceeds through a six electron donation process On a pure Pt surface the
dissociative chemisorption of water on Pt is the rate determining step at
potentials below 07 V vs RHE [11] It is generally accepted that an active
catalyst for methanol oxidation should give rise to water discharging at low
potentials and to labile CO chemisorption Moreover a good catalyst for
methanol oxidation should also catalyze the oxidation of carbon monoxide
8
The promotion of Pt by a range of metals (and oxides) to enhance
methanol oxidation is well known Table 21 summeries species that have a
reported positive effect on methanol oxidation Of the catalyst promoters
considered significant recent activity has focused on the use of Ru Sn and
W with Pt
On PtRu surface CH3OH adsorption would take place on the Pt site
where CH3OH chemisorption requires three neighboring sites yielding
ensembles of Pt atoms with COad and the Ru atoms would act as OHad
providing center According to this the catalytic improvement would be
bifunctional mechanism There are different reports for optimal Ru contents
According to Gasteiger el al a Ru content of 10 is optimum[12] Increasing
Ru content provides more adsorbed water species required to oxidize COad
However because Pt sites are decreased with increasing Ru contents the
oxidation rate of CH3OH is decreased Consequently the rate of reaction
decreased when the Ru contents are more than 10 But other results
referring a coverage close to 05 by Ru is optimal for enhancing CH3OH
oxidation are reported by R Dillon[13]
9
Table 21 Effect of catalyst promoters on methanol oxidation
Promotion by Catalyst promoter
comment
Alloying and
dissolution to produce
highly reticulated
surfaces
Cr
Fe
Sn
Typically less than 100 mV
lower potential than Pt
Surface adatoms Sn
Bi
Alloys
Ru
Sn Mo
Os Ir
Ti Re
Ru has the greatest effect Sn
Mo Os and Re are substantial
promoters
Metal oxides Ru Hydrous Ru oxide the most
active catalyst
Base metal oxides W
Nb Zr Ta
W oxide is a notable prometer
Small effect of other metal
oxides (typically lt 100mV)
10
22 Oxygen Reduction Reaction
Oxygen reduction reaction is considered to be one of the most
important electrocatalytic reactions because of its role in electrochemical
energy conversion several industrial processes and corrosion Consequently
it was in the focus of electrochemical interest for many years Oxygen
reduction reaction has been a challenge for electrochemists because of its
complex kinetics and the need for better electrocatalysts The most notable
need for the improvement of the catalytic activity and the development of
new better non-noble metal electrocatalysts are still remaining Despite
extensive research on oxygen reduction reaction mechanism is still not fully
understood
Oxygen reduction reaction is known to proceed either in a four-
electron step to H2O or in a two-electron step to H2O2 In general the four-
electron-step is considered to be the major reaction pathway on
polycrystalline platinum electrodes [1]
+ -2 2 0O + 4H + 4e 2H O E =1229V ( NHE)vsrarr (221)
- -2 2 0O + 2H O + 2e 4OH E =0401Vrarr (222)
The peroxide pathway is
11
+ -2 2 2 0O + 2H + 2e H O E =067Vrarr (223)
Peroxide can undergo further reduction or decomposition in acid
solutions via the following reactions
+ -2 2 2 0H O + 2H + 2e 2H O E =177Vrarr (224)
2 2 2 22H O 2H O + O rarr
However it is commonly agreed that it proceeds without formation of
surface peroxide on polycrystalline and particulate Pt in acidic media
Furthermore it is believed that the reaction proceeds
O2 harr O2ads (225)
O2ads + H+(aq) +e- rarr O2Hads (226)
O2Hads + 3H+(aq) +3e- rarr 2H2O (227)
with the first charge transfer reaction (226) as the rate determining step The
details of reaction (227) are unknown and may include several Oads or OHads
configurations [2]
23 Anionic Effects
Platinum or platinum alloy particles supported on high surface area
carbon substrates are often synthesized form halide containing reactants (eg
Cl- containing salts)[15] It is therefore possible that traces of halides are
12
present in the electrocatalyst Furthermore halides especially chloride may
also be present in the humidified anode feed-streams of PEFCs or in the
cathode air feed During the ORR remaining anions can compete with
oxygen for free adsorption sites on the catalyst Modified adsorption
conditions for oxygen molecules can have an impact on the oxygen reduction
activity of the catalyst and can alter the oxygen reduction pathway (transition
form H2O to H2O2 formation) From both Pt single crystal work (eg [14]) it
is well known that different supporting electrolytes containing anions with
different adsorption strengths influence the ORR activity tremendously It is
also well known that the presence of additional halide anions can
dramatically affect the kinetics of electrocatalytic reactions on Pt e g the
oxidation of small organic molecules or the oxygen reduction reaction (e g
[16])
24 Actual Performance
The actual cell potential is decreased from its ideal potential because
of several types of irreversible losses as shown in Figure 21 These losses
are often referred to as polarization overpotential or overvoltage though
13
only the ohmic losses actually behave as a resistance Multiple phenomena
contribute to irreversible losses in an actual fuel cell
In the V-I diagram especially for low-temperature fuel cells the
effects of the three loss categories are often easy to distinguish as illustrated
in Figure 21
Activation Losses Activation losses are caused by sluggish electrode
kinetics There is a close similarity between electrochemical and chemical
reactions in that both involve an activation energy that must be overcome by
the reacting species In reality activation losses are the result of complex
surface electrochemical reaction steps each of which have their own reaction
rate and activation energy In electrochemical reactions the activation energy
loss for overcoming this barrier is represented as Tafel equation
lnact a b iη = + (241)
where actη is activation polarization(mV) a and b are constants and i is
current density(mAcm2)
( ) ( ) ( )act cell act anode act cathodeη η η= + (242)
Ohmic Polarization Ohmic losses occur because of resistance to the
flow of ions in the electrolyte and resistance to flow of electrons through the
electrode The dominant ohmic losses through the electrolyte are reduced by
14
decreasing the electrode separation and enhancing the ionic conductivity of
the electrolyte Because both the electrolyte and fuel cell electrodes obey
Ohms law the ohmic losses can be expressed by the equation
ohm iRη = (243)
where i is the current flowing through the cell and R is the total cell
resistance which includes electronic ionic and contact resistance
R = Relectronic + Rionic + Rcontact
Mass Transport Related Losses As a reactant is consumed at the
electrode by electrochemical reaction it is often diluted by the products
when finite mass transport rates limit the supply of fresh reactant and the
evacuation of products As a consequence a concentration gradient is formed
which drives the mass transport process
Concentration polarization can be denoted by
ln(1 )consL
RT inF i
η = minus (244)
where consη is concentration polarization(mV) iL is limiting current
density(mAcm2) Same as the activation polarization the concentration
polarization occurs independently at either electrode
( ) ( ) ( )conc cell conc anode conc cathodeη η η= + (245)
15
The combined effect of the losses for a given cell and given
operating conditions can be expressed as polarizations The total polarization
at the electrodes is the sum of actη and concη or
anode acta a conc aη η η= + (246)
and
cathode acta c conc cη η η= + (247)
The effect of polarization is to shift the potential of the electrode
(Eelectrode) to a new value (Velectrode)
electrode electrode electrodeV E η= plusmn (248)
For the anode
anode anode anodeV E η= + (249)
And for the cathode
cathode cathode cathodeV E η= minus (2410)
The net result of current flow in a fuel cell is to increase the anode
potential and to decrease the cathode potential thereby reducing the cell
voltage
The cell voltage includes the contribution of the anode and cathode
potentials and ohmic polarization
16
cell cathode anodeV V V iR= minus minus (2411)
When Equations (249) and (2410) are substituted in Equation (2411)
( )cell cathode cathode anode anodeV E E iRη η= minus minus + minus (2412)
Or
cell e cathode anodeV E iRη η= ∆ minus minus minus (2413)
Where e cathode anodeE E E∆ = minus Equation (2413) shows that current
flow in a fuel cell results in a decrease in cell voltage because of losses by
electrode and ohmic polarizations The goal of fuel cell developers is to
minimize the polarization so that cellV approaches eE∆ This goal is
approached by modifications to fuel cell design (improvement in electrode
structures better electro-catalysts more conductive electrolyte thinner cell
components etc) [1]
17
Figure 241 Typical Fuel Cell Polarization Curve
18
31 Polycrystalline Platinum Preparation
A polycrystalline platinum electrode was prepared with pulse
electrodeposition on Pt- quartz crystal
1) Working electrode Pt quartz crystal counter electrode Pt wire
reference electrode saturated calomel electrode (SCE)
2) Pulse electrodeposition method ton -03V 100ms and toff 0V 300ms
3) Deposition bath 10mM H2PtCl6 + 01M HCl (65)
Figure 31 shows the cross-section diagram of the platinized
polycrystalline platinum quartz crystal
Polycrystalline Pt
Sputtered Pt Sputtered Ti
9MHz At-cut quartz crystal
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
3 EXPERIMENTAL
19
32 Electrode Preparation and the Modification
A preparation procedure of catalyst ink was well described
elsewhere [7] and the catalyst (Pt or PtRu) loading was 3 mgcm2 in the each
electrode A geometric active area was a 5cm2 and catalyst ink was directly
sparied onto carbon paper Simple dipping method was used for the chloride
ion modification Pt cathode on carbon paper was dipped in 1 times 10minus2M KCl
solution for 5min and it was rinsed in ultrapure water followed by drying in
the oven
33 Half Cell Measurements
Figure 331 shows the schematic experimental setup for measuring
of the electrocatalystic activity of Pt electrode oxidationreduction
adsorptiondesorption of hydrogen methanol oxidation The electrochemical
cell body consisted of a glass cylinder capped with a Teflon lid holding all
electrodes The Pt nanocatalyst sprayed on carbon paper (gas diffusion layer)
was used as working electrode (WE) The geometric area of the WE was
02 cm2 A concentric platinized Pt wire was used as counter electrode (CE)
The tip of a Luggin-Haber capillary hosting a SCE reference electrode (RE)
was placed in the middle of two electrodes A poteniostatgalvanostat (EG amp
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
iv
33 Half Cell Measurements 19
34 Single cell measurements 22
4 Results and Discussionhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip26
5 Conclusionshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip49
6 Referenceshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip50
7 Acknowledgementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip52
v
List of Figures Figure 121 Schematic diagram of a DMFC
Figure 241 Typical Fuel Cell Polarization Curve
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
Figure 331 Schematic diagram of experimental set-up for half-cell experiments
Figure 341 Configuration of catalystdiffusion backing and membrane for type a MEA
Figure 342 Schematic diagram of experimental set-up for single cell measurements
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M H2SO4 Scan rate = 50mVs
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M CH3OH05M H2SO4 Scan rate = 50mVs
Figure 415 The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
vi
Figure 416 The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
Figure 417 Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
Figure 418 Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
Figure 419 DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Clminus (c) 0001 M Clminus Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
Figure 4111 Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
vii
List of Table
Table 21 Effect of catalyst promoters on methanol oxidation
1
11 What Is a Fuel Cell
As early as 1839 William Grove discovered the basic operating
principle of fuel cells by reversing water electrolysis to generate electricity
from hydrogen and oxygen The principle that he discovered remains
unchanged today A fuel cell is an electrochemical ldquo device rdquo that
continuously converts chemical energy into electric energy (and some heat)
for as long as fuel and oxidant are supplied Fuel cells therefore bear
similarities both to batteries with which they share the electrochemical
nature of the power generation process and to engines which unlike batteries
will work continuously consuming a fuel of some sort Here is where the
analogies stop though Unlike engines or batteries a fuel cell does not need
recharging it operates quietly and efficiently and when hydrogen is used as
fuel it generates only power and drinking water Thus it is a so-called zero
emission engine [1]
1 INTRODUCTION
2
12 Direct Methanol Fuel Cell (DMFC)
A schematic of a DMFC is shown in Figure 11 Methanol and water
electrochemically react (ie methanol is electro-oxidized) at the anode to
produce carbon dioxide protons and electrons as shown in reaction (121)
An acidic electrolyte is advantageous to aid CO2 rejection since insoluble
carbonates form in alkaline electrolytes The protons produced at the anode
migrate through the polymer electrolyte to the cathode where they react with
oxygen (usually from air) to produce water as shown in reaction (122) The
electrons produced at the anode carry the free energy change of the chemical
reaction and travel through the external circuit where they can be made to do
useful work such as powering an electric motor The overall cell reaction as
shown in equation (123) is therefore the reaction of methanol and oxygen
to produce water and carbon dioxide
Anode CH3OH + H2O rarr CO2 + 6H+ + 6e- 0AE = 0046V (121)
Cathode 32
O2 + 6H+ + 6e- rarr 3H2O 0CE = 1229V (122)
Net Reaction CH3OH + 32
O2 rarr CO2 + 2H2O (123)
0cellE = 0
CE - 0AE =1229V - 0046V = 1183V Attainable max cell potential
3
Figure 121 Schematic diagram of direct fuel cell
4
13 Objectives
Of late fuel cells have become a central focus of interest and leading
car and power companies are making large scale investments in fuel cell
systems Fuel cells are set to provide electrical vehicles small and large scale
power stations and even portable electrical applications such as laptops and
mobile phones with electrical power in a manner which is more efficient and
more environmentally friendly than ever before [2 minus6] In particular direct
methanol fuel cells (DMFCs) have been considered for use in very small to
mid-sized applications due to their high electrical efficiency long life-time
and low poisonous emissions [2 minus 6] On the other hand in the operation of
DMFCs there are several problems at a cathode caused by methanol
crossover from anode to cathode and it should be solved for the
commercialization of DMFC (i) cathode poisoning by CO adsorption and Pt
sintering (ii) significant fuel loss due to methanol crossover (iii) decrease
partial pressure of oxygen due to water flooding and (iv) temperature
dependent power performance Therefore cathode catalyst for oxygen
reduction reaction (ORR) requires (a) CO tolerance (b) proton adsorption
available (c) easy oxygen adsorption (d) ORR kinetics (e) oxygen storage
material (f) suppression of electro-oxidation of methanol To our knowledge
5
no work has been done up to now to study the influence of halide
contamination on fuel cell performance From the data presented previously
Paulus et al [2] could only point to some aspects which should be considered
during MEA production the degradation of MEA might be occurred due to
the dissolution of Nafion by H2O2 and it was mainly produced in the
reduction of oxygen on a Pt substrate containing chloride ions
In this work we tried to develop new cathode catalyst in DMFC via
adsorption of chloride ion which has been known as inhibiting species in
most electrochemical studies The electrochemical characteristics of chloride
ion modified Pt cathode in the oxidation of methanol and
adsorptiondesorption of hydrogen were studied by cyclic voltammetry
techniques The power performance of the single cell and long-term stability
of pure Pt and modified Pt cathode was also investigated
6
21 Electro-oxidation of Methanol
The electrochemical oxidation of methanol on Pt involves several
intermediate steps ie dehydrogenation CO-like species chemisorption OH
(or H2O) species adsorption chemical interaction between adsorbed CO and
OH compounds and CO2 evolution One of these steps is the rate
determining step (rds) depending on the operating temperature and
particular catalyst surface State of the art electrocatalysts for the electro-
oxidation of methanol in fuel cells are generally based on Pt alloys supported
on carbon black [8 9] The electrocatalytic activity of Pt is known to be
promoted by the presence of a second metal such as Ru or Sn acting either
as an adatom or a bimetal First a sequence of dehydrogenation steps give rise
to adsorbed methanolic residues according to the following scheme[10]
CH3OH + Pt rarr Pt-CH2OH + H+ + 1e- (211)
Pt-CH2OH + Pt rarr Pt-CHOH + H+ + 1e- (212)
Pt-CHOH + Pt rarr PtCHO + H+ + 1e- (213)
2 BASIC THEORY
7
A surface rearrangement of the methanol oxidation intermediates
gives carbon monoxide linearly or bridge-bonded to Pt sites as follows
PtCHO rarr Pt-CequivO + H+ + 1e- (214)
Or
PtCHO + Pt rarr Pt Pt C=O + H+ + 1e- (215)
In the absence of a promoting element water discharge occurs at
high anodic overpotentials on Pt with the formation of Pt-OH species at the
catalyst surface
Pt + H2O rarr PtOH + H+ + 1e- (216)
The final step is the reaction of Pt-OH groups with neighboring
methanolic residues to give carbon dioxide
PtOH + Pt CO rarr 2Pt + CO2 + H+ + 1e- (217)
The overall oxidation process of methanol to carbon dioxide
proceeds through a six electron donation process On a pure Pt surface the
dissociative chemisorption of water on Pt is the rate determining step at
potentials below 07 V vs RHE [11] It is generally accepted that an active
catalyst for methanol oxidation should give rise to water discharging at low
potentials and to labile CO chemisorption Moreover a good catalyst for
methanol oxidation should also catalyze the oxidation of carbon monoxide
8
The promotion of Pt by a range of metals (and oxides) to enhance
methanol oxidation is well known Table 21 summeries species that have a
reported positive effect on methanol oxidation Of the catalyst promoters
considered significant recent activity has focused on the use of Ru Sn and
W with Pt
On PtRu surface CH3OH adsorption would take place on the Pt site
where CH3OH chemisorption requires three neighboring sites yielding
ensembles of Pt atoms with COad and the Ru atoms would act as OHad
providing center According to this the catalytic improvement would be
bifunctional mechanism There are different reports for optimal Ru contents
According to Gasteiger el al a Ru content of 10 is optimum[12] Increasing
Ru content provides more adsorbed water species required to oxidize COad
However because Pt sites are decreased with increasing Ru contents the
oxidation rate of CH3OH is decreased Consequently the rate of reaction
decreased when the Ru contents are more than 10 But other results
referring a coverage close to 05 by Ru is optimal for enhancing CH3OH
oxidation are reported by R Dillon[13]
9
Table 21 Effect of catalyst promoters on methanol oxidation
Promotion by Catalyst promoter
comment
Alloying and
dissolution to produce
highly reticulated
surfaces
Cr
Fe
Sn
Typically less than 100 mV
lower potential than Pt
Surface adatoms Sn
Bi
Alloys
Ru
Sn Mo
Os Ir
Ti Re
Ru has the greatest effect Sn
Mo Os and Re are substantial
promoters
Metal oxides Ru Hydrous Ru oxide the most
active catalyst
Base metal oxides W
Nb Zr Ta
W oxide is a notable prometer
Small effect of other metal
oxides (typically lt 100mV)
10
22 Oxygen Reduction Reaction
Oxygen reduction reaction is considered to be one of the most
important electrocatalytic reactions because of its role in electrochemical
energy conversion several industrial processes and corrosion Consequently
it was in the focus of electrochemical interest for many years Oxygen
reduction reaction has been a challenge for electrochemists because of its
complex kinetics and the need for better electrocatalysts The most notable
need for the improvement of the catalytic activity and the development of
new better non-noble metal electrocatalysts are still remaining Despite
extensive research on oxygen reduction reaction mechanism is still not fully
understood
Oxygen reduction reaction is known to proceed either in a four-
electron step to H2O or in a two-electron step to H2O2 In general the four-
electron-step is considered to be the major reaction pathway on
polycrystalline platinum electrodes [1]
+ -2 2 0O + 4H + 4e 2H O E =1229V ( NHE)vsrarr (221)
- -2 2 0O + 2H O + 2e 4OH E =0401Vrarr (222)
The peroxide pathway is
11
+ -2 2 2 0O + 2H + 2e H O E =067Vrarr (223)
Peroxide can undergo further reduction or decomposition in acid
solutions via the following reactions
+ -2 2 2 0H O + 2H + 2e 2H O E =177Vrarr (224)
2 2 2 22H O 2H O + O rarr
However it is commonly agreed that it proceeds without formation of
surface peroxide on polycrystalline and particulate Pt in acidic media
Furthermore it is believed that the reaction proceeds
O2 harr O2ads (225)
O2ads + H+(aq) +e- rarr O2Hads (226)
O2Hads + 3H+(aq) +3e- rarr 2H2O (227)
with the first charge transfer reaction (226) as the rate determining step The
details of reaction (227) are unknown and may include several Oads or OHads
configurations [2]
23 Anionic Effects
Platinum or platinum alloy particles supported on high surface area
carbon substrates are often synthesized form halide containing reactants (eg
Cl- containing salts)[15] It is therefore possible that traces of halides are
12
present in the electrocatalyst Furthermore halides especially chloride may
also be present in the humidified anode feed-streams of PEFCs or in the
cathode air feed During the ORR remaining anions can compete with
oxygen for free adsorption sites on the catalyst Modified adsorption
conditions for oxygen molecules can have an impact on the oxygen reduction
activity of the catalyst and can alter the oxygen reduction pathway (transition
form H2O to H2O2 formation) From both Pt single crystal work (eg [14]) it
is well known that different supporting electrolytes containing anions with
different adsorption strengths influence the ORR activity tremendously It is
also well known that the presence of additional halide anions can
dramatically affect the kinetics of electrocatalytic reactions on Pt e g the
oxidation of small organic molecules or the oxygen reduction reaction (e g
[16])
24 Actual Performance
The actual cell potential is decreased from its ideal potential because
of several types of irreversible losses as shown in Figure 21 These losses
are often referred to as polarization overpotential or overvoltage though
13
only the ohmic losses actually behave as a resistance Multiple phenomena
contribute to irreversible losses in an actual fuel cell
In the V-I diagram especially for low-temperature fuel cells the
effects of the three loss categories are often easy to distinguish as illustrated
in Figure 21
Activation Losses Activation losses are caused by sluggish electrode
kinetics There is a close similarity between electrochemical and chemical
reactions in that both involve an activation energy that must be overcome by
the reacting species In reality activation losses are the result of complex
surface electrochemical reaction steps each of which have their own reaction
rate and activation energy In electrochemical reactions the activation energy
loss for overcoming this barrier is represented as Tafel equation
lnact a b iη = + (241)
where actη is activation polarization(mV) a and b are constants and i is
current density(mAcm2)
( ) ( ) ( )act cell act anode act cathodeη η η= + (242)
Ohmic Polarization Ohmic losses occur because of resistance to the
flow of ions in the electrolyte and resistance to flow of electrons through the
electrode The dominant ohmic losses through the electrolyte are reduced by
14
decreasing the electrode separation and enhancing the ionic conductivity of
the electrolyte Because both the electrolyte and fuel cell electrodes obey
Ohms law the ohmic losses can be expressed by the equation
ohm iRη = (243)
where i is the current flowing through the cell and R is the total cell
resistance which includes electronic ionic and contact resistance
R = Relectronic + Rionic + Rcontact
Mass Transport Related Losses As a reactant is consumed at the
electrode by electrochemical reaction it is often diluted by the products
when finite mass transport rates limit the supply of fresh reactant and the
evacuation of products As a consequence a concentration gradient is formed
which drives the mass transport process
Concentration polarization can be denoted by
ln(1 )consL
RT inF i
η = minus (244)
where consη is concentration polarization(mV) iL is limiting current
density(mAcm2) Same as the activation polarization the concentration
polarization occurs independently at either electrode
( ) ( ) ( )conc cell conc anode conc cathodeη η η= + (245)
15
The combined effect of the losses for a given cell and given
operating conditions can be expressed as polarizations The total polarization
at the electrodes is the sum of actη and concη or
anode acta a conc aη η η= + (246)
and
cathode acta c conc cη η η= + (247)
The effect of polarization is to shift the potential of the electrode
(Eelectrode) to a new value (Velectrode)
electrode electrode electrodeV E η= plusmn (248)
For the anode
anode anode anodeV E η= + (249)
And for the cathode
cathode cathode cathodeV E η= minus (2410)
The net result of current flow in a fuel cell is to increase the anode
potential and to decrease the cathode potential thereby reducing the cell
voltage
The cell voltage includes the contribution of the anode and cathode
potentials and ohmic polarization
16
cell cathode anodeV V V iR= minus minus (2411)
When Equations (249) and (2410) are substituted in Equation (2411)
( )cell cathode cathode anode anodeV E E iRη η= minus minus + minus (2412)
Or
cell e cathode anodeV E iRη η= ∆ minus minus minus (2413)
Where e cathode anodeE E E∆ = minus Equation (2413) shows that current
flow in a fuel cell results in a decrease in cell voltage because of losses by
electrode and ohmic polarizations The goal of fuel cell developers is to
minimize the polarization so that cellV approaches eE∆ This goal is
approached by modifications to fuel cell design (improvement in electrode
structures better electro-catalysts more conductive electrolyte thinner cell
components etc) [1]
17
Figure 241 Typical Fuel Cell Polarization Curve
18
31 Polycrystalline Platinum Preparation
A polycrystalline platinum electrode was prepared with pulse
electrodeposition on Pt- quartz crystal
1) Working electrode Pt quartz crystal counter electrode Pt wire
reference electrode saturated calomel electrode (SCE)
2) Pulse electrodeposition method ton -03V 100ms and toff 0V 300ms
3) Deposition bath 10mM H2PtCl6 + 01M HCl (65)
Figure 31 shows the cross-section diagram of the platinized
polycrystalline platinum quartz crystal
Polycrystalline Pt
Sputtered Pt Sputtered Ti
9MHz At-cut quartz crystal
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
3 EXPERIMENTAL
19
32 Electrode Preparation and the Modification
A preparation procedure of catalyst ink was well described
elsewhere [7] and the catalyst (Pt or PtRu) loading was 3 mgcm2 in the each
electrode A geometric active area was a 5cm2 and catalyst ink was directly
sparied onto carbon paper Simple dipping method was used for the chloride
ion modification Pt cathode on carbon paper was dipped in 1 times 10minus2M KCl
solution for 5min and it was rinsed in ultrapure water followed by drying in
the oven
33 Half Cell Measurements
Figure 331 shows the schematic experimental setup for measuring
of the electrocatalystic activity of Pt electrode oxidationreduction
adsorptiondesorption of hydrogen methanol oxidation The electrochemical
cell body consisted of a glass cylinder capped with a Teflon lid holding all
electrodes The Pt nanocatalyst sprayed on carbon paper (gas diffusion layer)
was used as working electrode (WE) The geometric area of the WE was
02 cm2 A concentric platinized Pt wire was used as counter electrode (CE)
The tip of a Luggin-Haber capillary hosting a SCE reference electrode (RE)
was placed in the middle of two electrodes A poteniostatgalvanostat (EG amp
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
v
List of Figures Figure 121 Schematic diagram of a DMFC
Figure 241 Typical Fuel Cell Polarization Curve
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
Figure 331 Schematic diagram of experimental set-up for half-cell experiments
Figure 341 Configuration of catalystdiffusion backing and membrane for type a MEA
Figure 342 Schematic diagram of experimental set-up for single cell measurements
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M H2SO4 Scan rate = 50mVs
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M CH3OH05M H2SO4 Scan rate = 50mVs
Figure 415 The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
vi
Figure 416 The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
Figure 417 Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
Figure 418 Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
Figure 419 DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Clminus (c) 0001 M Clminus Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
Figure 4111 Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
vii
List of Table
Table 21 Effect of catalyst promoters on methanol oxidation
1
11 What Is a Fuel Cell
As early as 1839 William Grove discovered the basic operating
principle of fuel cells by reversing water electrolysis to generate electricity
from hydrogen and oxygen The principle that he discovered remains
unchanged today A fuel cell is an electrochemical ldquo device rdquo that
continuously converts chemical energy into electric energy (and some heat)
for as long as fuel and oxidant are supplied Fuel cells therefore bear
similarities both to batteries with which they share the electrochemical
nature of the power generation process and to engines which unlike batteries
will work continuously consuming a fuel of some sort Here is where the
analogies stop though Unlike engines or batteries a fuel cell does not need
recharging it operates quietly and efficiently and when hydrogen is used as
fuel it generates only power and drinking water Thus it is a so-called zero
emission engine [1]
1 INTRODUCTION
2
12 Direct Methanol Fuel Cell (DMFC)
A schematic of a DMFC is shown in Figure 11 Methanol and water
electrochemically react (ie methanol is electro-oxidized) at the anode to
produce carbon dioxide protons and electrons as shown in reaction (121)
An acidic electrolyte is advantageous to aid CO2 rejection since insoluble
carbonates form in alkaline electrolytes The protons produced at the anode
migrate through the polymer electrolyte to the cathode where they react with
oxygen (usually from air) to produce water as shown in reaction (122) The
electrons produced at the anode carry the free energy change of the chemical
reaction and travel through the external circuit where they can be made to do
useful work such as powering an electric motor The overall cell reaction as
shown in equation (123) is therefore the reaction of methanol and oxygen
to produce water and carbon dioxide
Anode CH3OH + H2O rarr CO2 + 6H+ + 6e- 0AE = 0046V (121)
Cathode 32
O2 + 6H+ + 6e- rarr 3H2O 0CE = 1229V (122)
Net Reaction CH3OH + 32
O2 rarr CO2 + 2H2O (123)
0cellE = 0
CE - 0AE =1229V - 0046V = 1183V Attainable max cell potential
3
Figure 121 Schematic diagram of direct fuel cell
4
13 Objectives
Of late fuel cells have become a central focus of interest and leading
car and power companies are making large scale investments in fuel cell
systems Fuel cells are set to provide electrical vehicles small and large scale
power stations and even portable electrical applications such as laptops and
mobile phones with electrical power in a manner which is more efficient and
more environmentally friendly than ever before [2 minus6] In particular direct
methanol fuel cells (DMFCs) have been considered for use in very small to
mid-sized applications due to their high electrical efficiency long life-time
and low poisonous emissions [2 minus 6] On the other hand in the operation of
DMFCs there are several problems at a cathode caused by methanol
crossover from anode to cathode and it should be solved for the
commercialization of DMFC (i) cathode poisoning by CO adsorption and Pt
sintering (ii) significant fuel loss due to methanol crossover (iii) decrease
partial pressure of oxygen due to water flooding and (iv) temperature
dependent power performance Therefore cathode catalyst for oxygen
reduction reaction (ORR) requires (a) CO tolerance (b) proton adsorption
available (c) easy oxygen adsorption (d) ORR kinetics (e) oxygen storage
material (f) suppression of electro-oxidation of methanol To our knowledge
5
no work has been done up to now to study the influence of halide
contamination on fuel cell performance From the data presented previously
Paulus et al [2] could only point to some aspects which should be considered
during MEA production the degradation of MEA might be occurred due to
the dissolution of Nafion by H2O2 and it was mainly produced in the
reduction of oxygen on a Pt substrate containing chloride ions
In this work we tried to develop new cathode catalyst in DMFC via
adsorption of chloride ion which has been known as inhibiting species in
most electrochemical studies The electrochemical characteristics of chloride
ion modified Pt cathode in the oxidation of methanol and
adsorptiondesorption of hydrogen were studied by cyclic voltammetry
techniques The power performance of the single cell and long-term stability
of pure Pt and modified Pt cathode was also investigated
6
21 Electro-oxidation of Methanol
The electrochemical oxidation of methanol on Pt involves several
intermediate steps ie dehydrogenation CO-like species chemisorption OH
(or H2O) species adsorption chemical interaction between adsorbed CO and
OH compounds and CO2 evolution One of these steps is the rate
determining step (rds) depending on the operating temperature and
particular catalyst surface State of the art electrocatalysts for the electro-
oxidation of methanol in fuel cells are generally based on Pt alloys supported
on carbon black [8 9] The electrocatalytic activity of Pt is known to be
promoted by the presence of a second metal such as Ru or Sn acting either
as an adatom or a bimetal First a sequence of dehydrogenation steps give rise
to adsorbed methanolic residues according to the following scheme[10]
CH3OH + Pt rarr Pt-CH2OH + H+ + 1e- (211)
Pt-CH2OH + Pt rarr Pt-CHOH + H+ + 1e- (212)
Pt-CHOH + Pt rarr PtCHO + H+ + 1e- (213)
2 BASIC THEORY
7
A surface rearrangement of the methanol oxidation intermediates
gives carbon monoxide linearly or bridge-bonded to Pt sites as follows
PtCHO rarr Pt-CequivO + H+ + 1e- (214)
Or
PtCHO + Pt rarr Pt Pt C=O + H+ + 1e- (215)
In the absence of a promoting element water discharge occurs at
high anodic overpotentials on Pt with the formation of Pt-OH species at the
catalyst surface
Pt + H2O rarr PtOH + H+ + 1e- (216)
The final step is the reaction of Pt-OH groups with neighboring
methanolic residues to give carbon dioxide
PtOH + Pt CO rarr 2Pt + CO2 + H+ + 1e- (217)
The overall oxidation process of methanol to carbon dioxide
proceeds through a six electron donation process On a pure Pt surface the
dissociative chemisorption of water on Pt is the rate determining step at
potentials below 07 V vs RHE [11] It is generally accepted that an active
catalyst for methanol oxidation should give rise to water discharging at low
potentials and to labile CO chemisorption Moreover a good catalyst for
methanol oxidation should also catalyze the oxidation of carbon monoxide
8
The promotion of Pt by a range of metals (and oxides) to enhance
methanol oxidation is well known Table 21 summeries species that have a
reported positive effect on methanol oxidation Of the catalyst promoters
considered significant recent activity has focused on the use of Ru Sn and
W with Pt
On PtRu surface CH3OH adsorption would take place on the Pt site
where CH3OH chemisorption requires three neighboring sites yielding
ensembles of Pt atoms with COad and the Ru atoms would act as OHad
providing center According to this the catalytic improvement would be
bifunctional mechanism There are different reports for optimal Ru contents
According to Gasteiger el al a Ru content of 10 is optimum[12] Increasing
Ru content provides more adsorbed water species required to oxidize COad
However because Pt sites are decreased with increasing Ru contents the
oxidation rate of CH3OH is decreased Consequently the rate of reaction
decreased when the Ru contents are more than 10 But other results
referring a coverage close to 05 by Ru is optimal for enhancing CH3OH
oxidation are reported by R Dillon[13]
9
Table 21 Effect of catalyst promoters on methanol oxidation
Promotion by Catalyst promoter
comment
Alloying and
dissolution to produce
highly reticulated
surfaces
Cr
Fe
Sn
Typically less than 100 mV
lower potential than Pt
Surface adatoms Sn
Bi
Alloys
Ru
Sn Mo
Os Ir
Ti Re
Ru has the greatest effect Sn
Mo Os and Re are substantial
promoters
Metal oxides Ru Hydrous Ru oxide the most
active catalyst
Base metal oxides W
Nb Zr Ta
W oxide is a notable prometer
Small effect of other metal
oxides (typically lt 100mV)
10
22 Oxygen Reduction Reaction
Oxygen reduction reaction is considered to be one of the most
important electrocatalytic reactions because of its role in electrochemical
energy conversion several industrial processes and corrosion Consequently
it was in the focus of electrochemical interest for many years Oxygen
reduction reaction has been a challenge for electrochemists because of its
complex kinetics and the need for better electrocatalysts The most notable
need for the improvement of the catalytic activity and the development of
new better non-noble metal electrocatalysts are still remaining Despite
extensive research on oxygen reduction reaction mechanism is still not fully
understood
Oxygen reduction reaction is known to proceed either in a four-
electron step to H2O or in a two-electron step to H2O2 In general the four-
electron-step is considered to be the major reaction pathway on
polycrystalline platinum electrodes [1]
+ -2 2 0O + 4H + 4e 2H O E =1229V ( NHE)vsrarr (221)
- -2 2 0O + 2H O + 2e 4OH E =0401Vrarr (222)
The peroxide pathway is
11
+ -2 2 2 0O + 2H + 2e H O E =067Vrarr (223)
Peroxide can undergo further reduction or decomposition in acid
solutions via the following reactions
+ -2 2 2 0H O + 2H + 2e 2H O E =177Vrarr (224)
2 2 2 22H O 2H O + O rarr
However it is commonly agreed that it proceeds without formation of
surface peroxide on polycrystalline and particulate Pt in acidic media
Furthermore it is believed that the reaction proceeds
O2 harr O2ads (225)
O2ads + H+(aq) +e- rarr O2Hads (226)
O2Hads + 3H+(aq) +3e- rarr 2H2O (227)
with the first charge transfer reaction (226) as the rate determining step The
details of reaction (227) are unknown and may include several Oads or OHads
configurations [2]
23 Anionic Effects
Platinum or platinum alloy particles supported on high surface area
carbon substrates are often synthesized form halide containing reactants (eg
Cl- containing salts)[15] It is therefore possible that traces of halides are
12
present in the electrocatalyst Furthermore halides especially chloride may
also be present in the humidified anode feed-streams of PEFCs or in the
cathode air feed During the ORR remaining anions can compete with
oxygen for free adsorption sites on the catalyst Modified adsorption
conditions for oxygen molecules can have an impact on the oxygen reduction
activity of the catalyst and can alter the oxygen reduction pathway (transition
form H2O to H2O2 formation) From both Pt single crystal work (eg [14]) it
is well known that different supporting electrolytes containing anions with
different adsorption strengths influence the ORR activity tremendously It is
also well known that the presence of additional halide anions can
dramatically affect the kinetics of electrocatalytic reactions on Pt e g the
oxidation of small organic molecules or the oxygen reduction reaction (e g
[16])
24 Actual Performance
The actual cell potential is decreased from its ideal potential because
of several types of irreversible losses as shown in Figure 21 These losses
are often referred to as polarization overpotential or overvoltage though
13
only the ohmic losses actually behave as a resistance Multiple phenomena
contribute to irreversible losses in an actual fuel cell
In the V-I diagram especially for low-temperature fuel cells the
effects of the three loss categories are often easy to distinguish as illustrated
in Figure 21
Activation Losses Activation losses are caused by sluggish electrode
kinetics There is a close similarity between electrochemical and chemical
reactions in that both involve an activation energy that must be overcome by
the reacting species In reality activation losses are the result of complex
surface electrochemical reaction steps each of which have their own reaction
rate and activation energy In electrochemical reactions the activation energy
loss for overcoming this barrier is represented as Tafel equation
lnact a b iη = + (241)
where actη is activation polarization(mV) a and b are constants and i is
current density(mAcm2)
( ) ( ) ( )act cell act anode act cathodeη η η= + (242)
Ohmic Polarization Ohmic losses occur because of resistance to the
flow of ions in the electrolyte and resistance to flow of electrons through the
electrode The dominant ohmic losses through the electrolyte are reduced by
14
decreasing the electrode separation and enhancing the ionic conductivity of
the electrolyte Because both the electrolyte and fuel cell electrodes obey
Ohms law the ohmic losses can be expressed by the equation
ohm iRη = (243)
where i is the current flowing through the cell and R is the total cell
resistance which includes electronic ionic and contact resistance
R = Relectronic + Rionic + Rcontact
Mass Transport Related Losses As a reactant is consumed at the
electrode by electrochemical reaction it is often diluted by the products
when finite mass transport rates limit the supply of fresh reactant and the
evacuation of products As a consequence a concentration gradient is formed
which drives the mass transport process
Concentration polarization can be denoted by
ln(1 )consL
RT inF i
η = minus (244)
where consη is concentration polarization(mV) iL is limiting current
density(mAcm2) Same as the activation polarization the concentration
polarization occurs independently at either electrode
( ) ( ) ( )conc cell conc anode conc cathodeη η η= + (245)
15
The combined effect of the losses for a given cell and given
operating conditions can be expressed as polarizations The total polarization
at the electrodes is the sum of actη and concη or
anode acta a conc aη η η= + (246)
and
cathode acta c conc cη η η= + (247)
The effect of polarization is to shift the potential of the electrode
(Eelectrode) to a new value (Velectrode)
electrode electrode electrodeV E η= plusmn (248)
For the anode
anode anode anodeV E η= + (249)
And for the cathode
cathode cathode cathodeV E η= minus (2410)
The net result of current flow in a fuel cell is to increase the anode
potential and to decrease the cathode potential thereby reducing the cell
voltage
The cell voltage includes the contribution of the anode and cathode
potentials and ohmic polarization
16
cell cathode anodeV V V iR= minus minus (2411)
When Equations (249) and (2410) are substituted in Equation (2411)
( )cell cathode cathode anode anodeV E E iRη η= minus minus + minus (2412)
Or
cell e cathode anodeV E iRη η= ∆ minus minus minus (2413)
Where e cathode anodeE E E∆ = minus Equation (2413) shows that current
flow in a fuel cell results in a decrease in cell voltage because of losses by
electrode and ohmic polarizations The goal of fuel cell developers is to
minimize the polarization so that cellV approaches eE∆ This goal is
approached by modifications to fuel cell design (improvement in electrode
structures better electro-catalysts more conductive electrolyte thinner cell
components etc) [1]
17
Figure 241 Typical Fuel Cell Polarization Curve
18
31 Polycrystalline Platinum Preparation
A polycrystalline platinum electrode was prepared with pulse
electrodeposition on Pt- quartz crystal
1) Working electrode Pt quartz crystal counter electrode Pt wire
reference electrode saturated calomel electrode (SCE)
2) Pulse electrodeposition method ton -03V 100ms and toff 0V 300ms
3) Deposition bath 10mM H2PtCl6 + 01M HCl (65)
Figure 31 shows the cross-section diagram of the platinized
polycrystalline platinum quartz crystal
Polycrystalline Pt
Sputtered Pt Sputtered Ti
9MHz At-cut quartz crystal
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
3 EXPERIMENTAL
19
32 Electrode Preparation and the Modification
A preparation procedure of catalyst ink was well described
elsewhere [7] and the catalyst (Pt or PtRu) loading was 3 mgcm2 in the each
electrode A geometric active area was a 5cm2 and catalyst ink was directly
sparied onto carbon paper Simple dipping method was used for the chloride
ion modification Pt cathode on carbon paper was dipped in 1 times 10minus2M KCl
solution for 5min and it was rinsed in ultrapure water followed by drying in
the oven
33 Half Cell Measurements
Figure 331 shows the schematic experimental setup for measuring
of the electrocatalystic activity of Pt electrode oxidationreduction
adsorptiondesorption of hydrogen methanol oxidation The electrochemical
cell body consisted of a glass cylinder capped with a Teflon lid holding all
electrodes The Pt nanocatalyst sprayed on carbon paper (gas diffusion layer)
was used as working electrode (WE) The geometric area of the WE was
02 cm2 A concentric platinized Pt wire was used as counter electrode (CE)
The tip of a Luggin-Haber capillary hosting a SCE reference electrode (RE)
was placed in the middle of two electrodes A poteniostatgalvanostat (EG amp
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
vi
Figure 416 The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
Figure 417 Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
Figure 418 Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
Figure 419 DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Clminus (c) 0001 M Clminus Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
Figure 4111 Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
vii
List of Table
Table 21 Effect of catalyst promoters on methanol oxidation
1
11 What Is a Fuel Cell
As early as 1839 William Grove discovered the basic operating
principle of fuel cells by reversing water electrolysis to generate electricity
from hydrogen and oxygen The principle that he discovered remains
unchanged today A fuel cell is an electrochemical ldquo device rdquo that
continuously converts chemical energy into electric energy (and some heat)
for as long as fuel and oxidant are supplied Fuel cells therefore bear
similarities both to batteries with which they share the electrochemical
nature of the power generation process and to engines which unlike batteries
will work continuously consuming a fuel of some sort Here is where the
analogies stop though Unlike engines or batteries a fuel cell does not need
recharging it operates quietly and efficiently and when hydrogen is used as
fuel it generates only power and drinking water Thus it is a so-called zero
emission engine [1]
1 INTRODUCTION
2
12 Direct Methanol Fuel Cell (DMFC)
A schematic of a DMFC is shown in Figure 11 Methanol and water
electrochemically react (ie methanol is electro-oxidized) at the anode to
produce carbon dioxide protons and electrons as shown in reaction (121)
An acidic electrolyte is advantageous to aid CO2 rejection since insoluble
carbonates form in alkaline electrolytes The protons produced at the anode
migrate through the polymer electrolyte to the cathode where they react with
oxygen (usually from air) to produce water as shown in reaction (122) The
electrons produced at the anode carry the free energy change of the chemical
reaction and travel through the external circuit where they can be made to do
useful work such as powering an electric motor The overall cell reaction as
shown in equation (123) is therefore the reaction of methanol and oxygen
to produce water and carbon dioxide
Anode CH3OH + H2O rarr CO2 + 6H+ + 6e- 0AE = 0046V (121)
Cathode 32
O2 + 6H+ + 6e- rarr 3H2O 0CE = 1229V (122)
Net Reaction CH3OH + 32
O2 rarr CO2 + 2H2O (123)
0cellE = 0
CE - 0AE =1229V - 0046V = 1183V Attainable max cell potential
3
Figure 121 Schematic diagram of direct fuel cell
4
13 Objectives
Of late fuel cells have become a central focus of interest and leading
car and power companies are making large scale investments in fuel cell
systems Fuel cells are set to provide electrical vehicles small and large scale
power stations and even portable electrical applications such as laptops and
mobile phones with electrical power in a manner which is more efficient and
more environmentally friendly than ever before [2 minus6] In particular direct
methanol fuel cells (DMFCs) have been considered for use in very small to
mid-sized applications due to their high electrical efficiency long life-time
and low poisonous emissions [2 minus 6] On the other hand in the operation of
DMFCs there are several problems at a cathode caused by methanol
crossover from anode to cathode and it should be solved for the
commercialization of DMFC (i) cathode poisoning by CO adsorption and Pt
sintering (ii) significant fuel loss due to methanol crossover (iii) decrease
partial pressure of oxygen due to water flooding and (iv) temperature
dependent power performance Therefore cathode catalyst for oxygen
reduction reaction (ORR) requires (a) CO tolerance (b) proton adsorption
available (c) easy oxygen adsorption (d) ORR kinetics (e) oxygen storage
material (f) suppression of electro-oxidation of methanol To our knowledge
5
no work has been done up to now to study the influence of halide
contamination on fuel cell performance From the data presented previously
Paulus et al [2] could only point to some aspects which should be considered
during MEA production the degradation of MEA might be occurred due to
the dissolution of Nafion by H2O2 and it was mainly produced in the
reduction of oxygen on a Pt substrate containing chloride ions
In this work we tried to develop new cathode catalyst in DMFC via
adsorption of chloride ion which has been known as inhibiting species in
most electrochemical studies The electrochemical characteristics of chloride
ion modified Pt cathode in the oxidation of methanol and
adsorptiondesorption of hydrogen were studied by cyclic voltammetry
techniques The power performance of the single cell and long-term stability
of pure Pt and modified Pt cathode was also investigated
6
21 Electro-oxidation of Methanol
The electrochemical oxidation of methanol on Pt involves several
intermediate steps ie dehydrogenation CO-like species chemisorption OH
(or H2O) species adsorption chemical interaction between adsorbed CO and
OH compounds and CO2 evolution One of these steps is the rate
determining step (rds) depending on the operating temperature and
particular catalyst surface State of the art electrocatalysts for the electro-
oxidation of methanol in fuel cells are generally based on Pt alloys supported
on carbon black [8 9] The electrocatalytic activity of Pt is known to be
promoted by the presence of a second metal such as Ru or Sn acting either
as an adatom or a bimetal First a sequence of dehydrogenation steps give rise
to adsorbed methanolic residues according to the following scheme[10]
CH3OH + Pt rarr Pt-CH2OH + H+ + 1e- (211)
Pt-CH2OH + Pt rarr Pt-CHOH + H+ + 1e- (212)
Pt-CHOH + Pt rarr PtCHO + H+ + 1e- (213)
2 BASIC THEORY
7
A surface rearrangement of the methanol oxidation intermediates
gives carbon monoxide linearly or bridge-bonded to Pt sites as follows
PtCHO rarr Pt-CequivO + H+ + 1e- (214)
Or
PtCHO + Pt rarr Pt Pt C=O + H+ + 1e- (215)
In the absence of a promoting element water discharge occurs at
high anodic overpotentials on Pt with the formation of Pt-OH species at the
catalyst surface
Pt + H2O rarr PtOH + H+ + 1e- (216)
The final step is the reaction of Pt-OH groups with neighboring
methanolic residues to give carbon dioxide
PtOH + Pt CO rarr 2Pt + CO2 + H+ + 1e- (217)
The overall oxidation process of methanol to carbon dioxide
proceeds through a six electron donation process On a pure Pt surface the
dissociative chemisorption of water on Pt is the rate determining step at
potentials below 07 V vs RHE [11] It is generally accepted that an active
catalyst for methanol oxidation should give rise to water discharging at low
potentials and to labile CO chemisorption Moreover a good catalyst for
methanol oxidation should also catalyze the oxidation of carbon monoxide
8
The promotion of Pt by a range of metals (and oxides) to enhance
methanol oxidation is well known Table 21 summeries species that have a
reported positive effect on methanol oxidation Of the catalyst promoters
considered significant recent activity has focused on the use of Ru Sn and
W with Pt
On PtRu surface CH3OH adsorption would take place on the Pt site
where CH3OH chemisorption requires three neighboring sites yielding
ensembles of Pt atoms with COad and the Ru atoms would act as OHad
providing center According to this the catalytic improvement would be
bifunctional mechanism There are different reports for optimal Ru contents
According to Gasteiger el al a Ru content of 10 is optimum[12] Increasing
Ru content provides more adsorbed water species required to oxidize COad
However because Pt sites are decreased with increasing Ru contents the
oxidation rate of CH3OH is decreased Consequently the rate of reaction
decreased when the Ru contents are more than 10 But other results
referring a coverage close to 05 by Ru is optimal for enhancing CH3OH
oxidation are reported by R Dillon[13]
9
Table 21 Effect of catalyst promoters on methanol oxidation
Promotion by Catalyst promoter
comment
Alloying and
dissolution to produce
highly reticulated
surfaces
Cr
Fe
Sn
Typically less than 100 mV
lower potential than Pt
Surface adatoms Sn
Bi
Alloys
Ru
Sn Mo
Os Ir
Ti Re
Ru has the greatest effect Sn
Mo Os and Re are substantial
promoters
Metal oxides Ru Hydrous Ru oxide the most
active catalyst
Base metal oxides W
Nb Zr Ta
W oxide is a notable prometer
Small effect of other metal
oxides (typically lt 100mV)
10
22 Oxygen Reduction Reaction
Oxygen reduction reaction is considered to be one of the most
important electrocatalytic reactions because of its role in electrochemical
energy conversion several industrial processes and corrosion Consequently
it was in the focus of electrochemical interest for many years Oxygen
reduction reaction has been a challenge for electrochemists because of its
complex kinetics and the need for better electrocatalysts The most notable
need for the improvement of the catalytic activity and the development of
new better non-noble metal electrocatalysts are still remaining Despite
extensive research on oxygen reduction reaction mechanism is still not fully
understood
Oxygen reduction reaction is known to proceed either in a four-
electron step to H2O or in a two-electron step to H2O2 In general the four-
electron-step is considered to be the major reaction pathway on
polycrystalline platinum electrodes [1]
+ -2 2 0O + 4H + 4e 2H O E =1229V ( NHE)vsrarr (221)
- -2 2 0O + 2H O + 2e 4OH E =0401Vrarr (222)
The peroxide pathway is
11
+ -2 2 2 0O + 2H + 2e H O E =067Vrarr (223)
Peroxide can undergo further reduction or decomposition in acid
solutions via the following reactions
+ -2 2 2 0H O + 2H + 2e 2H O E =177Vrarr (224)
2 2 2 22H O 2H O + O rarr
However it is commonly agreed that it proceeds without formation of
surface peroxide on polycrystalline and particulate Pt in acidic media
Furthermore it is believed that the reaction proceeds
O2 harr O2ads (225)
O2ads + H+(aq) +e- rarr O2Hads (226)
O2Hads + 3H+(aq) +3e- rarr 2H2O (227)
with the first charge transfer reaction (226) as the rate determining step The
details of reaction (227) are unknown and may include several Oads or OHads
configurations [2]
23 Anionic Effects
Platinum or platinum alloy particles supported on high surface area
carbon substrates are often synthesized form halide containing reactants (eg
Cl- containing salts)[15] It is therefore possible that traces of halides are
12
present in the electrocatalyst Furthermore halides especially chloride may
also be present in the humidified anode feed-streams of PEFCs or in the
cathode air feed During the ORR remaining anions can compete with
oxygen for free adsorption sites on the catalyst Modified adsorption
conditions for oxygen molecules can have an impact on the oxygen reduction
activity of the catalyst and can alter the oxygen reduction pathway (transition
form H2O to H2O2 formation) From both Pt single crystal work (eg [14]) it
is well known that different supporting electrolytes containing anions with
different adsorption strengths influence the ORR activity tremendously It is
also well known that the presence of additional halide anions can
dramatically affect the kinetics of electrocatalytic reactions on Pt e g the
oxidation of small organic molecules or the oxygen reduction reaction (e g
[16])
24 Actual Performance
The actual cell potential is decreased from its ideal potential because
of several types of irreversible losses as shown in Figure 21 These losses
are often referred to as polarization overpotential or overvoltage though
13
only the ohmic losses actually behave as a resistance Multiple phenomena
contribute to irreversible losses in an actual fuel cell
In the V-I diagram especially for low-temperature fuel cells the
effects of the three loss categories are often easy to distinguish as illustrated
in Figure 21
Activation Losses Activation losses are caused by sluggish electrode
kinetics There is a close similarity between electrochemical and chemical
reactions in that both involve an activation energy that must be overcome by
the reacting species In reality activation losses are the result of complex
surface electrochemical reaction steps each of which have their own reaction
rate and activation energy In electrochemical reactions the activation energy
loss for overcoming this barrier is represented as Tafel equation
lnact a b iη = + (241)
where actη is activation polarization(mV) a and b are constants and i is
current density(mAcm2)
( ) ( ) ( )act cell act anode act cathodeη η η= + (242)
Ohmic Polarization Ohmic losses occur because of resistance to the
flow of ions in the electrolyte and resistance to flow of electrons through the
electrode The dominant ohmic losses through the electrolyte are reduced by
14
decreasing the electrode separation and enhancing the ionic conductivity of
the electrolyte Because both the electrolyte and fuel cell electrodes obey
Ohms law the ohmic losses can be expressed by the equation
ohm iRη = (243)
where i is the current flowing through the cell and R is the total cell
resistance which includes electronic ionic and contact resistance
R = Relectronic + Rionic + Rcontact
Mass Transport Related Losses As a reactant is consumed at the
electrode by electrochemical reaction it is often diluted by the products
when finite mass transport rates limit the supply of fresh reactant and the
evacuation of products As a consequence a concentration gradient is formed
which drives the mass transport process
Concentration polarization can be denoted by
ln(1 )consL
RT inF i
η = minus (244)
where consη is concentration polarization(mV) iL is limiting current
density(mAcm2) Same as the activation polarization the concentration
polarization occurs independently at either electrode
( ) ( ) ( )conc cell conc anode conc cathodeη η η= + (245)
15
The combined effect of the losses for a given cell and given
operating conditions can be expressed as polarizations The total polarization
at the electrodes is the sum of actη and concη or
anode acta a conc aη η η= + (246)
and
cathode acta c conc cη η η= + (247)
The effect of polarization is to shift the potential of the electrode
(Eelectrode) to a new value (Velectrode)
electrode electrode electrodeV E η= plusmn (248)
For the anode
anode anode anodeV E η= + (249)
And for the cathode
cathode cathode cathodeV E η= minus (2410)
The net result of current flow in a fuel cell is to increase the anode
potential and to decrease the cathode potential thereby reducing the cell
voltage
The cell voltage includes the contribution of the anode and cathode
potentials and ohmic polarization
16
cell cathode anodeV V V iR= minus minus (2411)
When Equations (249) and (2410) are substituted in Equation (2411)
( )cell cathode cathode anode anodeV E E iRη η= minus minus + minus (2412)
Or
cell e cathode anodeV E iRη η= ∆ minus minus minus (2413)
Where e cathode anodeE E E∆ = minus Equation (2413) shows that current
flow in a fuel cell results in a decrease in cell voltage because of losses by
electrode and ohmic polarizations The goal of fuel cell developers is to
minimize the polarization so that cellV approaches eE∆ This goal is
approached by modifications to fuel cell design (improvement in electrode
structures better electro-catalysts more conductive electrolyte thinner cell
components etc) [1]
17
Figure 241 Typical Fuel Cell Polarization Curve
18
31 Polycrystalline Platinum Preparation
A polycrystalline platinum electrode was prepared with pulse
electrodeposition on Pt- quartz crystal
1) Working electrode Pt quartz crystal counter electrode Pt wire
reference electrode saturated calomel electrode (SCE)
2) Pulse electrodeposition method ton -03V 100ms and toff 0V 300ms
3) Deposition bath 10mM H2PtCl6 + 01M HCl (65)
Figure 31 shows the cross-section diagram of the platinized
polycrystalline platinum quartz crystal
Polycrystalline Pt
Sputtered Pt Sputtered Ti
9MHz At-cut quartz crystal
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
3 EXPERIMENTAL
19
32 Electrode Preparation and the Modification
A preparation procedure of catalyst ink was well described
elsewhere [7] and the catalyst (Pt or PtRu) loading was 3 mgcm2 in the each
electrode A geometric active area was a 5cm2 and catalyst ink was directly
sparied onto carbon paper Simple dipping method was used for the chloride
ion modification Pt cathode on carbon paper was dipped in 1 times 10minus2M KCl
solution for 5min and it was rinsed in ultrapure water followed by drying in
the oven
33 Half Cell Measurements
Figure 331 shows the schematic experimental setup for measuring
of the electrocatalystic activity of Pt electrode oxidationreduction
adsorptiondesorption of hydrogen methanol oxidation The electrochemical
cell body consisted of a glass cylinder capped with a Teflon lid holding all
electrodes The Pt nanocatalyst sprayed on carbon paper (gas diffusion layer)
was used as working electrode (WE) The geometric area of the WE was
02 cm2 A concentric platinized Pt wire was used as counter electrode (CE)
The tip of a Luggin-Haber capillary hosting a SCE reference electrode (RE)
was placed in the middle of two electrodes A poteniostatgalvanostat (EG amp
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
vii
List of Table
Table 21 Effect of catalyst promoters on methanol oxidation
1
11 What Is a Fuel Cell
As early as 1839 William Grove discovered the basic operating
principle of fuel cells by reversing water electrolysis to generate electricity
from hydrogen and oxygen The principle that he discovered remains
unchanged today A fuel cell is an electrochemical ldquo device rdquo that
continuously converts chemical energy into electric energy (and some heat)
for as long as fuel and oxidant are supplied Fuel cells therefore bear
similarities both to batteries with which they share the electrochemical
nature of the power generation process and to engines which unlike batteries
will work continuously consuming a fuel of some sort Here is where the
analogies stop though Unlike engines or batteries a fuel cell does not need
recharging it operates quietly and efficiently and when hydrogen is used as
fuel it generates only power and drinking water Thus it is a so-called zero
emission engine [1]
1 INTRODUCTION
2
12 Direct Methanol Fuel Cell (DMFC)
A schematic of a DMFC is shown in Figure 11 Methanol and water
electrochemically react (ie methanol is electro-oxidized) at the anode to
produce carbon dioxide protons and electrons as shown in reaction (121)
An acidic electrolyte is advantageous to aid CO2 rejection since insoluble
carbonates form in alkaline electrolytes The protons produced at the anode
migrate through the polymer electrolyte to the cathode where they react with
oxygen (usually from air) to produce water as shown in reaction (122) The
electrons produced at the anode carry the free energy change of the chemical
reaction and travel through the external circuit where they can be made to do
useful work such as powering an electric motor The overall cell reaction as
shown in equation (123) is therefore the reaction of methanol and oxygen
to produce water and carbon dioxide
Anode CH3OH + H2O rarr CO2 + 6H+ + 6e- 0AE = 0046V (121)
Cathode 32
O2 + 6H+ + 6e- rarr 3H2O 0CE = 1229V (122)
Net Reaction CH3OH + 32
O2 rarr CO2 + 2H2O (123)
0cellE = 0
CE - 0AE =1229V - 0046V = 1183V Attainable max cell potential
3
Figure 121 Schematic diagram of direct fuel cell
4
13 Objectives
Of late fuel cells have become a central focus of interest and leading
car and power companies are making large scale investments in fuel cell
systems Fuel cells are set to provide electrical vehicles small and large scale
power stations and even portable electrical applications such as laptops and
mobile phones with electrical power in a manner which is more efficient and
more environmentally friendly than ever before [2 minus6] In particular direct
methanol fuel cells (DMFCs) have been considered for use in very small to
mid-sized applications due to their high electrical efficiency long life-time
and low poisonous emissions [2 minus 6] On the other hand in the operation of
DMFCs there are several problems at a cathode caused by methanol
crossover from anode to cathode and it should be solved for the
commercialization of DMFC (i) cathode poisoning by CO adsorption and Pt
sintering (ii) significant fuel loss due to methanol crossover (iii) decrease
partial pressure of oxygen due to water flooding and (iv) temperature
dependent power performance Therefore cathode catalyst for oxygen
reduction reaction (ORR) requires (a) CO tolerance (b) proton adsorption
available (c) easy oxygen adsorption (d) ORR kinetics (e) oxygen storage
material (f) suppression of electro-oxidation of methanol To our knowledge
5
no work has been done up to now to study the influence of halide
contamination on fuel cell performance From the data presented previously
Paulus et al [2] could only point to some aspects which should be considered
during MEA production the degradation of MEA might be occurred due to
the dissolution of Nafion by H2O2 and it was mainly produced in the
reduction of oxygen on a Pt substrate containing chloride ions
In this work we tried to develop new cathode catalyst in DMFC via
adsorption of chloride ion which has been known as inhibiting species in
most electrochemical studies The electrochemical characteristics of chloride
ion modified Pt cathode in the oxidation of methanol and
adsorptiondesorption of hydrogen were studied by cyclic voltammetry
techniques The power performance of the single cell and long-term stability
of pure Pt and modified Pt cathode was also investigated
6
21 Electro-oxidation of Methanol
The electrochemical oxidation of methanol on Pt involves several
intermediate steps ie dehydrogenation CO-like species chemisorption OH
(or H2O) species adsorption chemical interaction between adsorbed CO and
OH compounds and CO2 evolution One of these steps is the rate
determining step (rds) depending on the operating temperature and
particular catalyst surface State of the art electrocatalysts for the electro-
oxidation of methanol in fuel cells are generally based on Pt alloys supported
on carbon black [8 9] The electrocatalytic activity of Pt is known to be
promoted by the presence of a second metal such as Ru or Sn acting either
as an adatom or a bimetal First a sequence of dehydrogenation steps give rise
to adsorbed methanolic residues according to the following scheme[10]
CH3OH + Pt rarr Pt-CH2OH + H+ + 1e- (211)
Pt-CH2OH + Pt rarr Pt-CHOH + H+ + 1e- (212)
Pt-CHOH + Pt rarr PtCHO + H+ + 1e- (213)
2 BASIC THEORY
7
A surface rearrangement of the methanol oxidation intermediates
gives carbon monoxide linearly or bridge-bonded to Pt sites as follows
PtCHO rarr Pt-CequivO + H+ + 1e- (214)
Or
PtCHO + Pt rarr Pt Pt C=O + H+ + 1e- (215)
In the absence of a promoting element water discharge occurs at
high anodic overpotentials on Pt with the formation of Pt-OH species at the
catalyst surface
Pt + H2O rarr PtOH + H+ + 1e- (216)
The final step is the reaction of Pt-OH groups with neighboring
methanolic residues to give carbon dioxide
PtOH + Pt CO rarr 2Pt + CO2 + H+ + 1e- (217)
The overall oxidation process of methanol to carbon dioxide
proceeds through a six electron donation process On a pure Pt surface the
dissociative chemisorption of water on Pt is the rate determining step at
potentials below 07 V vs RHE [11] It is generally accepted that an active
catalyst for methanol oxidation should give rise to water discharging at low
potentials and to labile CO chemisorption Moreover a good catalyst for
methanol oxidation should also catalyze the oxidation of carbon monoxide
8
The promotion of Pt by a range of metals (and oxides) to enhance
methanol oxidation is well known Table 21 summeries species that have a
reported positive effect on methanol oxidation Of the catalyst promoters
considered significant recent activity has focused on the use of Ru Sn and
W with Pt
On PtRu surface CH3OH adsorption would take place on the Pt site
where CH3OH chemisorption requires three neighboring sites yielding
ensembles of Pt atoms with COad and the Ru atoms would act as OHad
providing center According to this the catalytic improvement would be
bifunctional mechanism There are different reports for optimal Ru contents
According to Gasteiger el al a Ru content of 10 is optimum[12] Increasing
Ru content provides more adsorbed water species required to oxidize COad
However because Pt sites are decreased with increasing Ru contents the
oxidation rate of CH3OH is decreased Consequently the rate of reaction
decreased when the Ru contents are more than 10 But other results
referring a coverage close to 05 by Ru is optimal for enhancing CH3OH
oxidation are reported by R Dillon[13]
9
Table 21 Effect of catalyst promoters on methanol oxidation
Promotion by Catalyst promoter
comment
Alloying and
dissolution to produce
highly reticulated
surfaces
Cr
Fe
Sn
Typically less than 100 mV
lower potential than Pt
Surface adatoms Sn
Bi
Alloys
Ru
Sn Mo
Os Ir
Ti Re
Ru has the greatest effect Sn
Mo Os and Re are substantial
promoters
Metal oxides Ru Hydrous Ru oxide the most
active catalyst
Base metal oxides W
Nb Zr Ta
W oxide is a notable prometer
Small effect of other metal
oxides (typically lt 100mV)
10
22 Oxygen Reduction Reaction
Oxygen reduction reaction is considered to be one of the most
important electrocatalytic reactions because of its role in electrochemical
energy conversion several industrial processes and corrosion Consequently
it was in the focus of electrochemical interest for many years Oxygen
reduction reaction has been a challenge for electrochemists because of its
complex kinetics and the need for better electrocatalysts The most notable
need for the improvement of the catalytic activity and the development of
new better non-noble metal electrocatalysts are still remaining Despite
extensive research on oxygen reduction reaction mechanism is still not fully
understood
Oxygen reduction reaction is known to proceed either in a four-
electron step to H2O or in a two-electron step to H2O2 In general the four-
electron-step is considered to be the major reaction pathway on
polycrystalline platinum electrodes [1]
+ -2 2 0O + 4H + 4e 2H O E =1229V ( NHE)vsrarr (221)
- -2 2 0O + 2H O + 2e 4OH E =0401Vrarr (222)
The peroxide pathway is
11
+ -2 2 2 0O + 2H + 2e H O E =067Vrarr (223)
Peroxide can undergo further reduction or decomposition in acid
solutions via the following reactions
+ -2 2 2 0H O + 2H + 2e 2H O E =177Vrarr (224)
2 2 2 22H O 2H O + O rarr
However it is commonly agreed that it proceeds without formation of
surface peroxide on polycrystalline and particulate Pt in acidic media
Furthermore it is believed that the reaction proceeds
O2 harr O2ads (225)
O2ads + H+(aq) +e- rarr O2Hads (226)
O2Hads + 3H+(aq) +3e- rarr 2H2O (227)
with the first charge transfer reaction (226) as the rate determining step The
details of reaction (227) are unknown and may include several Oads or OHads
configurations [2]
23 Anionic Effects
Platinum or platinum alloy particles supported on high surface area
carbon substrates are often synthesized form halide containing reactants (eg
Cl- containing salts)[15] It is therefore possible that traces of halides are
12
present in the electrocatalyst Furthermore halides especially chloride may
also be present in the humidified anode feed-streams of PEFCs or in the
cathode air feed During the ORR remaining anions can compete with
oxygen for free adsorption sites on the catalyst Modified adsorption
conditions for oxygen molecules can have an impact on the oxygen reduction
activity of the catalyst and can alter the oxygen reduction pathway (transition
form H2O to H2O2 formation) From both Pt single crystal work (eg [14]) it
is well known that different supporting electrolytes containing anions with
different adsorption strengths influence the ORR activity tremendously It is
also well known that the presence of additional halide anions can
dramatically affect the kinetics of electrocatalytic reactions on Pt e g the
oxidation of small organic molecules or the oxygen reduction reaction (e g
[16])
24 Actual Performance
The actual cell potential is decreased from its ideal potential because
of several types of irreversible losses as shown in Figure 21 These losses
are often referred to as polarization overpotential or overvoltage though
13
only the ohmic losses actually behave as a resistance Multiple phenomena
contribute to irreversible losses in an actual fuel cell
In the V-I diagram especially for low-temperature fuel cells the
effects of the three loss categories are often easy to distinguish as illustrated
in Figure 21
Activation Losses Activation losses are caused by sluggish electrode
kinetics There is a close similarity between electrochemical and chemical
reactions in that both involve an activation energy that must be overcome by
the reacting species In reality activation losses are the result of complex
surface electrochemical reaction steps each of which have their own reaction
rate and activation energy In electrochemical reactions the activation energy
loss for overcoming this barrier is represented as Tafel equation
lnact a b iη = + (241)
where actη is activation polarization(mV) a and b are constants and i is
current density(mAcm2)
( ) ( ) ( )act cell act anode act cathodeη η η= + (242)
Ohmic Polarization Ohmic losses occur because of resistance to the
flow of ions in the electrolyte and resistance to flow of electrons through the
electrode The dominant ohmic losses through the electrolyte are reduced by
14
decreasing the electrode separation and enhancing the ionic conductivity of
the electrolyte Because both the electrolyte and fuel cell electrodes obey
Ohms law the ohmic losses can be expressed by the equation
ohm iRη = (243)
where i is the current flowing through the cell and R is the total cell
resistance which includes electronic ionic and contact resistance
R = Relectronic + Rionic + Rcontact
Mass Transport Related Losses As a reactant is consumed at the
electrode by electrochemical reaction it is often diluted by the products
when finite mass transport rates limit the supply of fresh reactant and the
evacuation of products As a consequence a concentration gradient is formed
which drives the mass transport process
Concentration polarization can be denoted by
ln(1 )consL
RT inF i
η = minus (244)
where consη is concentration polarization(mV) iL is limiting current
density(mAcm2) Same as the activation polarization the concentration
polarization occurs independently at either electrode
( ) ( ) ( )conc cell conc anode conc cathodeη η η= + (245)
15
The combined effect of the losses for a given cell and given
operating conditions can be expressed as polarizations The total polarization
at the electrodes is the sum of actη and concη or
anode acta a conc aη η η= + (246)
and
cathode acta c conc cη η η= + (247)
The effect of polarization is to shift the potential of the electrode
(Eelectrode) to a new value (Velectrode)
electrode electrode electrodeV E η= plusmn (248)
For the anode
anode anode anodeV E η= + (249)
And for the cathode
cathode cathode cathodeV E η= minus (2410)
The net result of current flow in a fuel cell is to increase the anode
potential and to decrease the cathode potential thereby reducing the cell
voltage
The cell voltage includes the contribution of the anode and cathode
potentials and ohmic polarization
16
cell cathode anodeV V V iR= minus minus (2411)
When Equations (249) and (2410) are substituted in Equation (2411)
( )cell cathode cathode anode anodeV E E iRη η= minus minus + minus (2412)
Or
cell e cathode anodeV E iRη η= ∆ minus minus minus (2413)
Where e cathode anodeE E E∆ = minus Equation (2413) shows that current
flow in a fuel cell results in a decrease in cell voltage because of losses by
electrode and ohmic polarizations The goal of fuel cell developers is to
minimize the polarization so that cellV approaches eE∆ This goal is
approached by modifications to fuel cell design (improvement in electrode
structures better electro-catalysts more conductive electrolyte thinner cell
components etc) [1]
17
Figure 241 Typical Fuel Cell Polarization Curve
18
31 Polycrystalline Platinum Preparation
A polycrystalline platinum electrode was prepared with pulse
electrodeposition on Pt- quartz crystal
1) Working electrode Pt quartz crystal counter electrode Pt wire
reference electrode saturated calomel electrode (SCE)
2) Pulse electrodeposition method ton -03V 100ms and toff 0V 300ms
3) Deposition bath 10mM H2PtCl6 + 01M HCl (65)
Figure 31 shows the cross-section diagram of the platinized
polycrystalline platinum quartz crystal
Polycrystalline Pt
Sputtered Pt Sputtered Ti
9MHz At-cut quartz crystal
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
3 EXPERIMENTAL
19
32 Electrode Preparation and the Modification
A preparation procedure of catalyst ink was well described
elsewhere [7] and the catalyst (Pt or PtRu) loading was 3 mgcm2 in the each
electrode A geometric active area was a 5cm2 and catalyst ink was directly
sparied onto carbon paper Simple dipping method was used for the chloride
ion modification Pt cathode on carbon paper was dipped in 1 times 10minus2M KCl
solution for 5min and it was rinsed in ultrapure water followed by drying in
the oven
33 Half Cell Measurements
Figure 331 shows the schematic experimental setup for measuring
of the electrocatalystic activity of Pt electrode oxidationreduction
adsorptiondesorption of hydrogen methanol oxidation The electrochemical
cell body consisted of a glass cylinder capped with a Teflon lid holding all
electrodes The Pt nanocatalyst sprayed on carbon paper (gas diffusion layer)
was used as working electrode (WE) The geometric area of the WE was
02 cm2 A concentric platinized Pt wire was used as counter electrode (CE)
The tip of a Luggin-Haber capillary hosting a SCE reference electrode (RE)
was placed in the middle of two electrodes A poteniostatgalvanostat (EG amp
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
1
11 What Is a Fuel Cell
As early as 1839 William Grove discovered the basic operating
principle of fuel cells by reversing water electrolysis to generate electricity
from hydrogen and oxygen The principle that he discovered remains
unchanged today A fuel cell is an electrochemical ldquo device rdquo that
continuously converts chemical energy into electric energy (and some heat)
for as long as fuel and oxidant are supplied Fuel cells therefore bear
similarities both to batteries with which they share the electrochemical
nature of the power generation process and to engines which unlike batteries
will work continuously consuming a fuel of some sort Here is where the
analogies stop though Unlike engines or batteries a fuel cell does not need
recharging it operates quietly and efficiently and when hydrogen is used as
fuel it generates only power and drinking water Thus it is a so-called zero
emission engine [1]
1 INTRODUCTION
2
12 Direct Methanol Fuel Cell (DMFC)
A schematic of a DMFC is shown in Figure 11 Methanol and water
electrochemically react (ie methanol is electro-oxidized) at the anode to
produce carbon dioxide protons and electrons as shown in reaction (121)
An acidic electrolyte is advantageous to aid CO2 rejection since insoluble
carbonates form in alkaline electrolytes The protons produced at the anode
migrate through the polymer electrolyte to the cathode where they react with
oxygen (usually from air) to produce water as shown in reaction (122) The
electrons produced at the anode carry the free energy change of the chemical
reaction and travel through the external circuit where they can be made to do
useful work such as powering an electric motor The overall cell reaction as
shown in equation (123) is therefore the reaction of methanol and oxygen
to produce water and carbon dioxide
Anode CH3OH + H2O rarr CO2 + 6H+ + 6e- 0AE = 0046V (121)
Cathode 32
O2 + 6H+ + 6e- rarr 3H2O 0CE = 1229V (122)
Net Reaction CH3OH + 32
O2 rarr CO2 + 2H2O (123)
0cellE = 0
CE - 0AE =1229V - 0046V = 1183V Attainable max cell potential
3
Figure 121 Schematic diagram of direct fuel cell
4
13 Objectives
Of late fuel cells have become a central focus of interest and leading
car and power companies are making large scale investments in fuel cell
systems Fuel cells are set to provide electrical vehicles small and large scale
power stations and even portable electrical applications such as laptops and
mobile phones with electrical power in a manner which is more efficient and
more environmentally friendly than ever before [2 minus6] In particular direct
methanol fuel cells (DMFCs) have been considered for use in very small to
mid-sized applications due to their high electrical efficiency long life-time
and low poisonous emissions [2 minus 6] On the other hand in the operation of
DMFCs there are several problems at a cathode caused by methanol
crossover from anode to cathode and it should be solved for the
commercialization of DMFC (i) cathode poisoning by CO adsorption and Pt
sintering (ii) significant fuel loss due to methanol crossover (iii) decrease
partial pressure of oxygen due to water flooding and (iv) temperature
dependent power performance Therefore cathode catalyst for oxygen
reduction reaction (ORR) requires (a) CO tolerance (b) proton adsorption
available (c) easy oxygen adsorption (d) ORR kinetics (e) oxygen storage
material (f) suppression of electro-oxidation of methanol To our knowledge
5
no work has been done up to now to study the influence of halide
contamination on fuel cell performance From the data presented previously
Paulus et al [2] could only point to some aspects which should be considered
during MEA production the degradation of MEA might be occurred due to
the dissolution of Nafion by H2O2 and it was mainly produced in the
reduction of oxygen on a Pt substrate containing chloride ions
In this work we tried to develop new cathode catalyst in DMFC via
adsorption of chloride ion which has been known as inhibiting species in
most electrochemical studies The electrochemical characteristics of chloride
ion modified Pt cathode in the oxidation of methanol and
adsorptiondesorption of hydrogen were studied by cyclic voltammetry
techniques The power performance of the single cell and long-term stability
of pure Pt and modified Pt cathode was also investigated
6
21 Electro-oxidation of Methanol
The electrochemical oxidation of methanol on Pt involves several
intermediate steps ie dehydrogenation CO-like species chemisorption OH
(or H2O) species adsorption chemical interaction between adsorbed CO and
OH compounds and CO2 evolution One of these steps is the rate
determining step (rds) depending on the operating temperature and
particular catalyst surface State of the art electrocatalysts for the electro-
oxidation of methanol in fuel cells are generally based on Pt alloys supported
on carbon black [8 9] The electrocatalytic activity of Pt is known to be
promoted by the presence of a second metal such as Ru or Sn acting either
as an adatom or a bimetal First a sequence of dehydrogenation steps give rise
to adsorbed methanolic residues according to the following scheme[10]
CH3OH + Pt rarr Pt-CH2OH + H+ + 1e- (211)
Pt-CH2OH + Pt rarr Pt-CHOH + H+ + 1e- (212)
Pt-CHOH + Pt rarr PtCHO + H+ + 1e- (213)
2 BASIC THEORY
7
A surface rearrangement of the methanol oxidation intermediates
gives carbon monoxide linearly or bridge-bonded to Pt sites as follows
PtCHO rarr Pt-CequivO + H+ + 1e- (214)
Or
PtCHO + Pt rarr Pt Pt C=O + H+ + 1e- (215)
In the absence of a promoting element water discharge occurs at
high anodic overpotentials on Pt with the formation of Pt-OH species at the
catalyst surface
Pt + H2O rarr PtOH + H+ + 1e- (216)
The final step is the reaction of Pt-OH groups with neighboring
methanolic residues to give carbon dioxide
PtOH + Pt CO rarr 2Pt + CO2 + H+ + 1e- (217)
The overall oxidation process of methanol to carbon dioxide
proceeds through a six electron donation process On a pure Pt surface the
dissociative chemisorption of water on Pt is the rate determining step at
potentials below 07 V vs RHE [11] It is generally accepted that an active
catalyst for methanol oxidation should give rise to water discharging at low
potentials and to labile CO chemisorption Moreover a good catalyst for
methanol oxidation should also catalyze the oxidation of carbon monoxide
8
The promotion of Pt by a range of metals (and oxides) to enhance
methanol oxidation is well known Table 21 summeries species that have a
reported positive effect on methanol oxidation Of the catalyst promoters
considered significant recent activity has focused on the use of Ru Sn and
W with Pt
On PtRu surface CH3OH adsorption would take place on the Pt site
where CH3OH chemisorption requires three neighboring sites yielding
ensembles of Pt atoms with COad and the Ru atoms would act as OHad
providing center According to this the catalytic improvement would be
bifunctional mechanism There are different reports for optimal Ru contents
According to Gasteiger el al a Ru content of 10 is optimum[12] Increasing
Ru content provides more adsorbed water species required to oxidize COad
However because Pt sites are decreased with increasing Ru contents the
oxidation rate of CH3OH is decreased Consequently the rate of reaction
decreased when the Ru contents are more than 10 But other results
referring a coverage close to 05 by Ru is optimal for enhancing CH3OH
oxidation are reported by R Dillon[13]
9
Table 21 Effect of catalyst promoters on methanol oxidation
Promotion by Catalyst promoter
comment
Alloying and
dissolution to produce
highly reticulated
surfaces
Cr
Fe
Sn
Typically less than 100 mV
lower potential than Pt
Surface adatoms Sn
Bi
Alloys
Ru
Sn Mo
Os Ir
Ti Re
Ru has the greatest effect Sn
Mo Os and Re are substantial
promoters
Metal oxides Ru Hydrous Ru oxide the most
active catalyst
Base metal oxides W
Nb Zr Ta
W oxide is a notable prometer
Small effect of other metal
oxides (typically lt 100mV)
10
22 Oxygen Reduction Reaction
Oxygen reduction reaction is considered to be one of the most
important electrocatalytic reactions because of its role in electrochemical
energy conversion several industrial processes and corrosion Consequently
it was in the focus of electrochemical interest for many years Oxygen
reduction reaction has been a challenge for electrochemists because of its
complex kinetics and the need for better electrocatalysts The most notable
need for the improvement of the catalytic activity and the development of
new better non-noble metal electrocatalysts are still remaining Despite
extensive research on oxygen reduction reaction mechanism is still not fully
understood
Oxygen reduction reaction is known to proceed either in a four-
electron step to H2O or in a two-electron step to H2O2 In general the four-
electron-step is considered to be the major reaction pathway on
polycrystalline platinum electrodes [1]
+ -2 2 0O + 4H + 4e 2H O E =1229V ( NHE)vsrarr (221)
- -2 2 0O + 2H O + 2e 4OH E =0401Vrarr (222)
The peroxide pathway is
11
+ -2 2 2 0O + 2H + 2e H O E =067Vrarr (223)
Peroxide can undergo further reduction or decomposition in acid
solutions via the following reactions
+ -2 2 2 0H O + 2H + 2e 2H O E =177Vrarr (224)
2 2 2 22H O 2H O + O rarr
However it is commonly agreed that it proceeds without formation of
surface peroxide on polycrystalline and particulate Pt in acidic media
Furthermore it is believed that the reaction proceeds
O2 harr O2ads (225)
O2ads + H+(aq) +e- rarr O2Hads (226)
O2Hads + 3H+(aq) +3e- rarr 2H2O (227)
with the first charge transfer reaction (226) as the rate determining step The
details of reaction (227) are unknown and may include several Oads or OHads
configurations [2]
23 Anionic Effects
Platinum or platinum alloy particles supported on high surface area
carbon substrates are often synthesized form halide containing reactants (eg
Cl- containing salts)[15] It is therefore possible that traces of halides are
12
present in the electrocatalyst Furthermore halides especially chloride may
also be present in the humidified anode feed-streams of PEFCs or in the
cathode air feed During the ORR remaining anions can compete with
oxygen for free adsorption sites on the catalyst Modified adsorption
conditions for oxygen molecules can have an impact on the oxygen reduction
activity of the catalyst and can alter the oxygen reduction pathway (transition
form H2O to H2O2 formation) From both Pt single crystal work (eg [14]) it
is well known that different supporting electrolytes containing anions with
different adsorption strengths influence the ORR activity tremendously It is
also well known that the presence of additional halide anions can
dramatically affect the kinetics of electrocatalytic reactions on Pt e g the
oxidation of small organic molecules or the oxygen reduction reaction (e g
[16])
24 Actual Performance
The actual cell potential is decreased from its ideal potential because
of several types of irreversible losses as shown in Figure 21 These losses
are often referred to as polarization overpotential or overvoltage though
13
only the ohmic losses actually behave as a resistance Multiple phenomena
contribute to irreversible losses in an actual fuel cell
In the V-I diagram especially for low-temperature fuel cells the
effects of the three loss categories are often easy to distinguish as illustrated
in Figure 21
Activation Losses Activation losses are caused by sluggish electrode
kinetics There is a close similarity between electrochemical and chemical
reactions in that both involve an activation energy that must be overcome by
the reacting species In reality activation losses are the result of complex
surface electrochemical reaction steps each of which have their own reaction
rate and activation energy In electrochemical reactions the activation energy
loss for overcoming this barrier is represented as Tafel equation
lnact a b iη = + (241)
where actη is activation polarization(mV) a and b are constants and i is
current density(mAcm2)
( ) ( ) ( )act cell act anode act cathodeη η η= + (242)
Ohmic Polarization Ohmic losses occur because of resistance to the
flow of ions in the electrolyte and resistance to flow of electrons through the
electrode The dominant ohmic losses through the electrolyte are reduced by
14
decreasing the electrode separation and enhancing the ionic conductivity of
the electrolyte Because both the electrolyte and fuel cell electrodes obey
Ohms law the ohmic losses can be expressed by the equation
ohm iRη = (243)
where i is the current flowing through the cell and R is the total cell
resistance which includes electronic ionic and contact resistance
R = Relectronic + Rionic + Rcontact
Mass Transport Related Losses As a reactant is consumed at the
electrode by electrochemical reaction it is often diluted by the products
when finite mass transport rates limit the supply of fresh reactant and the
evacuation of products As a consequence a concentration gradient is formed
which drives the mass transport process
Concentration polarization can be denoted by
ln(1 )consL
RT inF i
η = minus (244)
where consη is concentration polarization(mV) iL is limiting current
density(mAcm2) Same as the activation polarization the concentration
polarization occurs independently at either electrode
( ) ( ) ( )conc cell conc anode conc cathodeη η η= + (245)
15
The combined effect of the losses for a given cell and given
operating conditions can be expressed as polarizations The total polarization
at the electrodes is the sum of actη and concη or
anode acta a conc aη η η= + (246)
and
cathode acta c conc cη η η= + (247)
The effect of polarization is to shift the potential of the electrode
(Eelectrode) to a new value (Velectrode)
electrode electrode electrodeV E η= plusmn (248)
For the anode
anode anode anodeV E η= + (249)
And for the cathode
cathode cathode cathodeV E η= minus (2410)
The net result of current flow in a fuel cell is to increase the anode
potential and to decrease the cathode potential thereby reducing the cell
voltage
The cell voltage includes the contribution of the anode and cathode
potentials and ohmic polarization
16
cell cathode anodeV V V iR= minus minus (2411)
When Equations (249) and (2410) are substituted in Equation (2411)
( )cell cathode cathode anode anodeV E E iRη η= minus minus + minus (2412)
Or
cell e cathode anodeV E iRη η= ∆ minus minus minus (2413)
Where e cathode anodeE E E∆ = minus Equation (2413) shows that current
flow in a fuel cell results in a decrease in cell voltage because of losses by
electrode and ohmic polarizations The goal of fuel cell developers is to
minimize the polarization so that cellV approaches eE∆ This goal is
approached by modifications to fuel cell design (improvement in electrode
structures better electro-catalysts more conductive electrolyte thinner cell
components etc) [1]
17
Figure 241 Typical Fuel Cell Polarization Curve
18
31 Polycrystalline Platinum Preparation
A polycrystalline platinum electrode was prepared with pulse
electrodeposition on Pt- quartz crystal
1) Working electrode Pt quartz crystal counter electrode Pt wire
reference electrode saturated calomel electrode (SCE)
2) Pulse electrodeposition method ton -03V 100ms and toff 0V 300ms
3) Deposition bath 10mM H2PtCl6 + 01M HCl (65)
Figure 31 shows the cross-section diagram of the platinized
polycrystalline platinum quartz crystal
Polycrystalline Pt
Sputtered Pt Sputtered Ti
9MHz At-cut quartz crystal
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
3 EXPERIMENTAL
19
32 Electrode Preparation and the Modification
A preparation procedure of catalyst ink was well described
elsewhere [7] and the catalyst (Pt or PtRu) loading was 3 mgcm2 in the each
electrode A geometric active area was a 5cm2 and catalyst ink was directly
sparied onto carbon paper Simple dipping method was used for the chloride
ion modification Pt cathode on carbon paper was dipped in 1 times 10minus2M KCl
solution for 5min and it was rinsed in ultrapure water followed by drying in
the oven
33 Half Cell Measurements
Figure 331 shows the schematic experimental setup for measuring
of the electrocatalystic activity of Pt electrode oxidationreduction
adsorptiondesorption of hydrogen methanol oxidation The electrochemical
cell body consisted of a glass cylinder capped with a Teflon lid holding all
electrodes The Pt nanocatalyst sprayed on carbon paper (gas diffusion layer)
was used as working electrode (WE) The geometric area of the WE was
02 cm2 A concentric platinized Pt wire was used as counter electrode (CE)
The tip of a Luggin-Haber capillary hosting a SCE reference electrode (RE)
was placed in the middle of two electrodes A poteniostatgalvanostat (EG amp
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
2
12 Direct Methanol Fuel Cell (DMFC)
A schematic of a DMFC is shown in Figure 11 Methanol and water
electrochemically react (ie methanol is electro-oxidized) at the anode to
produce carbon dioxide protons and electrons as shown in reaction (121)
An acidic electrolyte is advantageous to aid CO2 rejection since insoluble
carbonates form in alkaline electrolytes The protons produced at the anode
migrate through the polymer electrolyte to the cathode where they react with
oxygen (usually from air) to produce water as shown in reaction (122) The
electrons produced at the anode carry the free energy change of the chemical
reaction and travel through the external circuit where they can be made to do
useful work such as powering an electric motor The overall cell reaction as
shown in equation (123) is therefore the reaction of methanol and oxygen
to produce water and carbon dioxide
Anode CH3OH + H2O rarr CO2 + 6H+ + 6e- 0AE = 0046V (121)
Cathode 32
O2 + 6H+ + 6e- rarr 3H2O 0CE = 1229V (122)
Net Reaction CH3OH + 32
O2 rarr CO2 + 2H2O (123)
0cellE = 0
CE - 0AE =1229V - 0046V = 1183V Attainable max cell potential
3
Figure 121 Schematic diagram of direct fuel cell
4
13 Objectives
Of late fuel cells have become a central focus of interest and leading
car and power companies are making large scale investments in fuel cell
systems Fuel cells are set to provide electrical vehicles small and large scale
power stations and even portable electrical applications such as laptops and
mobile phones with electrical power in a manner which is more efficient and
more environmentally friendly than ever before [2 minus6] In particular direct
methanol fuel cells (DMFCs) have been considered for use in very small to
mid-sized applications due to their high electrical efficiency long life-time
and low poisonous emissions [2 minus 6] On the other hand in the operation of
DMFCs there are several problems at a cathode caused by methanol
crossover from anode to cathode and it should be solved for the
commercialization of DMFC (i) cathode poisoning by CO adsorption and Pt
sintering (ii) significant fuel loss due to methanol crossover (iii) decrease
partial pressure of oxygen due to water flooding and (iv) temperature
dependent power performance Therefore cathode catalyst for oxygen
reduction reaction (ORR) requires (a) CO tolerance (b) proton adsorption
available (c) easy oxygen adsorption (d) ORR kinetics (e) oxygen storage
material (f) suppression of electro-oxidation of methanol To our knowledge
5
no work has been done up to now to study the influence of halide
contamination on fuel cell performance From the data presented previously
Paulus et al [2] could only point to some aspects which should be considered
during MEA production the degradation of MEA might be occurred due to
the dissolution of Nafion by H2O2 and it was mainly produced in the
reduction of oxygen on a Pt substrate containing chloride ions
In this work we tried to develop new cathode catalyst in DMFC via
adsorption of chloride ion which has been known as inhibiting species in
most electrochemical studies The electrochemical characteristics of chloride
ion modified Pt cathode in the oxidation of methanol and
adsorptiondesorption of hydrogen were studied by cyclic voltammetry
techniques The power performance of the single cell and long-term stability
of pure Pt and modified Pt cathode was also investigated
6
21 Electro-oxidation of Methanol
The electrochemical oxidation of methanol on Pt involves several
intermediate steps ie dehydrogenation CO-like species chemisorption OH
(or H2O) species adsorption chemical interaction between adsorbed CO and
OH compounds and CO2 evolution One of these steps is the rate
determining step (rds) depending on the operating temperature and
particular catalyst surface State of the art electrocatalysts for the electro-
oxidation of methanol in fuel cells are generally based on Pt alloys supported
on carbon black [8 9] The electrocatalytic activity of Pt is known to be
promoted by the presence of a second metal such as Ru or Sn acting either
as an adatom or a bimetal First a sequence of dehydrogenation steps give rise
to adsorbed methanolic residues according to the following scheme[10]
CH3OH + Pt rarr Pt-CH2OH + H+ + 1e- (211)
Pt-CH2OH + Pt rarr Pt-CHOH + H+ + 1e- (212)
Pt-CHOH + Pt rarr PtCHO + H+ + 1e- (213)
2 BASIC THEORY
7
A surface rearrangement of the methanol oxidation intermediates
gives carbon monoxide linearly or bridge-bonded to Pt sites as follows
PtCHO rarr Pt-CequivO + H+ + 1e- (214)
Or
PtCHO + Pt rarr Pt Pt C=O + H+ + 1e- (215)
In the absence of a promoting element water discharge occurs at
high anodic overpotentials on Pt with the formation of Pt-OH species at the
catalyst surface
Pt + H2O rarr PtOH + H+ + 1e- (216)
The final step is the reaction of Pt-OH groups with neighboring
methanolic residues to give carbon dioxide
PtOH + Pt CO rarr 2Pt + CO2 + H+ + 1e- (217)
The overall oxidation process of methanol to carbon dioxide
proceeds through a six electron donation process On a pure Pt surface the
dissociative chemisorption of water on Pt is the rate determining step at
potentials below 07 V vs RHE [11] It is generally accepted that an active
catalyst for methanol oxidation should give rise to water discharging at low
potentials and to labile CO chemisorption Moreover a good catalyst for
methanol oxidation should also catalyze the oxidation of carbon monoxide
8
The promotion of Pt by a range of metals (and oxides) to enhance
methanol oxidation is well known Table 21 summeries species that have a
reported positive effect on methanol oxidation Of the catalyst promoters
considered significant recent activity has focused on the use of Ru Sn and
W with Pt
On PtRu surface CH3OH adsorption would take place on the Pt site
where CH3OH chemisorption requires three neighboring sites yielding
ensembles of Pt atoms with COad and the Ru atoms would act as OHad
providing center According to this the catalytic improvement would be
bifunctional mechanism There are different reports for optimal Ru contents
According to Gasteiger el al a Ru content of 10 is optimum[12] Increasing
Ru content provides more adsorbed water species required to oxidize COad
However because Pt sites are decreased with increasing Ru contents the
oxidation rate of CH3OH is decreased Consequently the rate of reaction
decreased when the Ru contents are more than 10 But other results
referring a coverage close to 05 by Ru is optimal for enhancing CH3OH
oxidation are reported by R Dillon[13]
9
Table 21 Effect of catalyst promoters on methanol oxidation
Promotion by Catalyst promoter
comment
Alloying and
dissolution to produce
highly reticulated
surfaces
Cr
Fe
Sn
Typically less than 100 mV
lower potential than Pt
Surface adatoms Sn
Bi
Alloys
Ru
Sn Mo
Os Ir
Ti Re
Ru has the greatest effect Sn
Mo Os and Re are substantial
promoters
Metal oxides Ru Hydrous Ru oxide the most
active catalyst
Base metal oxides W
Nb Zr Ta
W oxide is a notable prometer
Small effect of other metal
oxides (typically lt 100mV)
10
22 Oxygen Reduction Reaction
Oxygen reduction reaction is considered to be one of the most
important electrocatalytic reactions because of its role in electrochemical
energy conversion several industrial processes and corrosion Consequently
it was in the focus of electrochemical interest for many years Oxygen
reduction reaction has been a challenge for electrochemists because of its
complex kinetics and the need for better electrocatalysts The most notable
need for the improvement of the catalytic activity and the development of
new better non-noble metal electrocatalysts are still remaining Despite
extensive research on oxygen reduction reaction mechanism is still not fully
understood
Oxygen reduction reaction is known to proceed either in a four-
electron step to H2O or in a two-electron step to H2O2 In general the four-
electron-step is considered to be the major reaction pathway on
polycrystalline platinum electrodes [1]
+ -2 2 0O + 4H + 4e 2H O E =1229V ( NHE)vsrarr (221)
- -2 2 0O + 2H O + 2e 4OH E =0401Vrarr (222)
The peroxide pathway is
11
+ -2 2 2 0O + 2H + 2e H O E =067Vrarr (223)
Peroxide can undergo further reduction or decomposition in acid
solutions via the following reactions
+ -2 2 2 0H O + 2H + 2e 2H O E =177Vrarr (224)
2 2 2 22H O 2H O + O rarr
However it is commonly agreed that it proceeds without formation of
surface peroxide on polycrystalline and particulate Pt in acidic media
Furthermore it is believed that the reaction proceeds
O2 harr O2ads (225)
O2ads + H+(aq) +e- rarr O2Hads (226)
O2Hads + 3H+(aq) +3e- rarr 2H2O (227)
with the first charge transfer reaction (226) as the rate determining step The
details of reaction (227) are unknown and may include several Oads or OHads
configurations [2]
23 Anionic Effects
Platinum or platinum alloy particles supported on high surface area
carbon substrates are often synthesized form halide containing reactants (eg
Cl- containing salts)[15] It is therefore possible that traces of halides are
12
present in the electrocatalyst Furthermore halides especially chloride may
also be present in the humidified anode feed-streams of PEFCs or in the
cathode air feed During the ORR remaining anions can compete with
oxygen for free adsorption sites on the catalyst Modified adsorption
conditions for oxygen molecules can have an impact on the oxygen reduction
activity of the catalyst and can alter the oxygen reduction pathway (transition
form H2O to H2O2 formation) From both Pt single crystal work (eg [14]) it
is well known that different supporting electrolytes containing anions with
different adsorption strengths influence the ORR activity tremendously It is
also well known that the presence of additional halide anions can
dramatically affect the kinetics of electrocatalytic reactions on Pt e g the
oxidation of small organic molecules or the oxygen reduction reaction (e g
[16])
24 Actual Performance
The actual cell potential is decreased from its ideal potential because
of several types of irreversible losses as shown in Figure 21 These losses
are often referred to as polarization overpotential or overvoltage though
13
only the ohmic losses actually behave as a resistance Multiple phenomena
contribute to irreversible losses in an actual fuel cell
In the V-I diagram especially for low-temperature fuel cells the
effects of the three loss categories are often easy to distinguish as illustrated
in Figure 21
Activation Losses Activation losses are caused by sluggish electrode
kinetics There is a close similarity between electrochemical and chemical
reactions in that both involve an activation energy that must be overcome by
the reacting species In reality activation losses are the result of complex
surface electrochemical reaction steps each of which have their own reaction
rate and activation energy In electrochemical reactions the activation energy
loss for overcoming this barrier is represented as Tafel equation
lnact a b iη = + (241)
where actη is activation polarization(mV) a and b are constants and i is
current density(mAcm2)
( ) ( ) ( )act cell act anode act cathodeη η η= + (242)
Ohmic Polarization Ohmic losses occur because of resistance to the
flow of ions in the electrolyte and resistance to flow of electrons through the
electrode The dominant ohmic losses through the electrolyte are reduced by
14
decreasing the electrode separation and enhancing the ionic conductivity of
the electrolyte Because both the electrolyte and fuel cell electrodes obey
Ohms law the ohmic losses can be expressed by the equation
ohm iRη = (243)
where i is the current flowing through the cell and R is the total cell
resistance which includes electronic ionic and contact resistance
R = Relectronic + Rionic + Rcontact
Mass Transport Related Losses As a reactant is consumed at the
electrode by electrochemical reaction it is often diluted by the products
when finite mass transport rates limit the supply of fresh reactant and the
evacuation of products As a consequence a concentration gradient is formed
which drives the mass transport process
Concentration polarization can be denoted by
ln(1 )consL
RT inF i
η = minus (244)
where consη is concentration polarization(mV) iL is limiting current
density(mAcm2) Same as the activation polarization the concentration
polarization occurs independently at either electrode
( ) ( ) ( )conc cell conc anode conc cathodeη η η= + (245)
15
The combined effect of the losses for a given cell and given
operating conditions can be expressed as polarizations The total polarization
at the electrodes is the sum of actη and concη or
anode acta a conc aη η η= + (246)
and
cathode acta c conc cη η η= + (247)
The effect of polarization is to shift the potential of the electrode
(Eelectrode) to a new value (Velectrode)
electrode electrode electrodeV E η= plusmn (248)
For the anode
anode anode anodeV E η= + (249)
And for the cathode
cathode cathode cathodeV E η= minus (2410)
The net result of current flow in a fuel cell is to increase the anode
potential and to decrease the cathode potential thereby reducing the cell
voltage
The cell voltage includes the contribution of the anode and cathode
potentials and ohmic polarization
16
cell cathode anodeV V V iR= minus minus (2411)
When Equations (249) and (2410) are substituted in Equation (2411)
( )cell cathode cathode anode anodeV E E iRη η= minus minus + minus (2412)
Or
cell e cathode anodeV E iRη η= ∆ minus minus minus (2413)
Where e cathode anodeE E E∆ = minus Equation (2413) shows that current
flow in a fuel cell results in a decrease in cell voltage because of losses by
electrode and ohmic polarizations The goal of fuel cell developers is to
minimize the polarization so that cellV approaches eE∆ This goal is
approached by modifications to fuel cell design (improvement in electrode
structures better electro-catalysts more conductive electrolyte thinner cell
components etc) [1]
17
Figure 241 Typical Fuel Cell Polarization Curve
18
31 Polycrystalline Platinum Preparation
A polycrystalline platinum electrode was prepared with pulse
electrodeposition on Pt- quartz crystal
1) Working electrode Pt quartz crystal counter electrode Pt wire
reference electrode saturated calomel electrode (SCE)
2) Pulse electrodeposition method ton -03V 100ms and toff 0V 300ms
3) Deposition bath 10mM H2PtCl6 + 01M HCl (65)
Figure 31 shows the cross-section diagram of the platinized
polycrystalline platinum quartz crystal
Polycrystalline Pt
Sputtered Pt Sputtered Ti
9MHz At-cut quartz crystal
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
3 EXPERIMENTAL
19
32 Electrode Preparation and the Modification
A preparation procedure of catalyst ink was well described
elsewhere [7] and the catalyst (Pt or PtRu) loading was 3 mgcm2 in the each
electrode A geometric active area was a 5cm2 and catalyst ink was directly
sparied onto carbon paper Simple dipping method was used for the chloride
ion modification Pt cathode on carbon paper was dipped in 1 times 10minus2M KCl
solution for 5min and it was rinsed in ultrapure water followed by drying in
the oven
33 Half Cell Measurements
Figure 331 shows the schematic experimental setup for measuring
of the electrocatalystic activity of Pt electrode oxidationreduction
adsorptiondesorption of hydrogen methanol oxidation The electrochemical
cell body consisted of a glass cylinder capped with a Teflon lid holding all
electrodes The Pt nanocatalyst sprayed on carbon paper (gas diffusion layer)
was used as working electrode (WE) The geometric area of the WE was
02 cm2 A concentric platinized Pt wire was used as counter electrode (CE)
The tip of a Luggin-Haber capillary hosting a SCE reference electrode (RE)
was placed in the middle of two electrodes A poteniostatgalvanostat (EG amp
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
3
Figure 121 Schematic diagram of direct fuel cell
4
13 Objectives
Of late fuel cells have become a central focus of interest and leading
car and power companies are making large scale investments in fuel cell
systems Fuel cells are set to provide electrical vehicles small and large scale
power stations and even portable electrical applications such as laptops and
mobile phones with electrical power in a manner which is more efficient and
more environmentally friendly than ever before [2 minus6] In particular direct
methanol fuel cells (DMFCs) have been considered for use in very small to
mid-sized applications due to their high electrical efficiency long life-time
and low poisonous emissions [2 minus 6] On the other hand in the operation of
DMFCs there are several problems at a cathode caused by methanol
crossover from anode to cathode and it should be solved for the
commercialization of DMFC (i) cathode poisoning by CO adsorption and Pt
sintering (ii) significant fuel loss due to methanol crossover (iii) decrease
partial pressure of oxygen due to water flooding and (iv) temperature
dependent power performance Therefore cathode catalyst for oxygen
reduction reaction (ORR) requires (a) CO tolerance (b) proton adsorption
available (c) easy oxygen adsorption (d) ORR kinetics (e) oxygen storage
material (f) suppression of electro-oxidation of methanol To our knowledge
5
no work has been done up to now to study the influence of halide
contamination on fuel cell performance From the data presented previously
Paulus et al [2] could only point to some aspects which should be considered
during MEA production the degradation of MEA might be occurred due to
the dissolution of Nafion by H2O2 and it was mainly produced in the
reduction of oxygen on a Pt substrate containing chloride ions
In this work we tried to develop new cathode catalyst in DMFC via
adsorption of chloride ion which has been known as inhibiting species in
most electrochemical studies The electrochemical characteristics of chloride
ion modified Pt cathode in the oxidation of methanol and
adsorptiondesorption of hydrogen were studied by cyclic voltammetry
techniques The power performance of the single cell and long-term stability
of pure Pt and modified Pt cathode was also investigated
6
21 Electro-oxidation of Methanol
The electrochemical oxidation of methanol on Pt involves several
intermediate steps ie dehydrogenation CO-like species chemisorption OH
(or H2O) species adsorption chemical interaction between adsorbed CO and
OH compounds and CO2 evolution One of these steps is the rate
determining step (rds) depending on the operating temperature and
particular catalyst surface State of the art electrocatalysts for the electro-
oxidation of methanol in fuel cells are generally based on Pt alloys supported
on carbon black [8 9] The electrocatalytic activity of Pt is known to be
promoted by the presence of a second metal such as Ru or Sn acting either
as an adatom or a bimetal First a sequence of dehydrogenation steps give rise
to adsorbed methanolic residues according to the following scheme[10]
CH3OH + Pt rarr Pt-CH2OH + H+ + 1e- (211)
Pt-CH2OH + Pt rarr Pt-CHOH + H+ + 1e- (212)
Pt-CHOH + Pt rarr PtCHO + H+ + 1e- (213)
2 BASIC THEORY
7
A surface rearrangement of the methanol oxidation intermediates
gives carbon monoxide linearly or bridge-bonded to Pt sites as follows
PtCHO rarr Pt-CequivO + H+ + 1e- (214)
Or
PtCHO + Pt rarr Pt Pt C=O + H+ + 1e- (215)
In the absence of a promoting element water discharge occurs at
high anodic overpotentials on Pt with the formation of Pt-OH species at the
catalyst surface
Pt + H2O rarr PtOH + H+ + 1e- (216)
The final step is the reaction of Pt-OH groups with neighboring
methanolic residues to give carbon dioxide
PtOH + Pt CO rarr 2Pt + CO2 + H+ + 1e- (217)
The overall oxidation process of methanol to carbon dioxide
proceeds through a six electron donation process On a pure Pt surface the
dissociative chemisorption of water on Pt is the rate determining step at
potentials below 07 V vs RHE [11] It is generally accepted that an active
catalyst for methanol oxidation should give rise to water discharging at low
potentials and to labile CO chemisorption Moreover a good catalyst for
methanol oxidation should also catalyze the oxidation of carbon monoxide
8
The promotion of Pt by a range of metals (and oxides) to enhance
methanol oxidation is well known Table 21 summeries species that have a
reported positive effect on methanol oxidation Of the catalyst promoters
considered significant recent activity has focused on the use of Ru Sn and
W with Pt
On PtRu surface CH3OH adsorption would take place on the Pt site
where CH3OH chemisorption requires three neighboring sites yielding
ensembles of Pt atoms with COad and the Ru atoms would act as OHad
providing center According to this the catalytic improvement would be
bifunctional mechanism There are different reports for optimal Ru contents
According to Gasteiger el al a Ru content of 10 is optimum[12] Increasing
Ru content provides more adsorbed water species required to oxidize COad
However because Pt sites are decreased with increasing Ru contents the
oxidation rate of CH3OH is decreased Consequently the rate of reaction
decreased when the Ru contents are more than 10 But other results
referring a coverage close to 05 by Ru is optimal for enhancing CH3OH
oxidation are reported by R Dillon[13]
9
Table 21 Effect of catalyst promoters on methanol oxidation
Promotion by Catalyst promoter
comment
Alloying and
dissolution to produce
highly reticulated
surfaces
Cr
Fe
Sn
Typically less than 100 mV
lower potential than Pt
Surface adatoms Sn
Bi
Alloys
Ru
Sn Mo
Os Ir
Ti Re
Ru has the greatest effect Sn
Mo Os and Re are substantial
promoters
Metal oxides Ru Hydrous Ru oxide the most
active catalyst
Base metal oxides W
Nb Zr Ta
W oxide is a notable prometer
Small effect of other metal
oxides (typically lt 100mV)
10
22 Oxygen Reduction Reaction
Oxygen reduction reaction is considered to be one of the most
important electrocatalytic reactions because of its role in electrochemical
energy conversion several industrial processes and corrosion Consequently
it was in the focus of electrochemical interest for many years Oxygen
reduction reaction has been a challenge for electrochemists because of its
complex kinetics and the need for better electrocatalysts The most notable
need for the improvement of the catalytic activity and the development of
new better non-noble metal electrocatalysts are still remaining Despite
extensive research on oxygen reduction reaction mechanism is still not fully
understood
Oxygen reduction reaction is known to proceed either in a four-
electron step to H2O or in a two-electron step to H2O2 In general the four-
electron-step is considered to be the major reaction pathway on
polycrystalline platinum electrodes [1]
+ -2 2 0O + 4H + 4e 2H O E =1229V ( NHE)vsrarr (221)
- -2 2 0O + 2H O + 2e 4OH E =0401Vrarr (222)
The peroxide pathway is
11
+ -2 2 2 0O + 2H + 2e H O E =067Vrarr (223)
Peroxide can undergo further reduction or decomposition in acid
solutions via the following reactions
+ -2 2 2 0H O + 2H + 2e 2H O E =177Vrarr (224)
2 2 2 22H O 2H O + O rarr
However it is commonly agreed that it proceeds without formation of
surface peroxide on polycrystalline and particulate Pt in acidic media
Furthermore it is believed that the reaction proceeds
O2 harr O2ads (225)
O2ads + H+(aq) +e- rarr O2Hads (226)
O2Hads + 3H+(aq) +3e- rarr 2H2O (227)
with the first charge transfer reaction (226) as the rate determining step The
details of reaction (227) are unknown and may include several Oads or OHads
configurations [2]
23 Anionic Effects
Platinum or platinum alloy particles supported on high surface area
carbon substrates are often synthesized form halide containing reactants (eg
Cl- containing salts)[15] It is therefore possible that traces of halides are
12
present in the electrocatalyst Furthermore halides especially chloride may
also be present in the humidified anode feed-streams of PEFCs or in the
cathode air feed During the ORR remaining anions can compete with
oxygen for free adsorption sites on the catalyst Modified adsorption
conditions for oxygen molecules can have an impact on the oxygen reduction
activity of the catalyst and can alter the oxygen reduction pathway (transition
form H2O to H2O2 formation) From both Pt single crystal work (eg [14]) it
is well known that different supporting electrolytes containing anions with
different adsorption strengths influence the ORR activity tremendously It is
also well known that the presence of additional halide anions can
dramatically affect the kinetics of electrocatalytic reactions on Pt e g the
oxidation of small organic molecules or the oxygen reduction reaction (e g
[16])
24 Actual Performance
The actual cell potential is decreased from its ideal potential because
of several types of irreversible losses as shown in Figure 21 These losses
are often referred to as polarization overpotential or overvoltage though
13
only the ohmic losses actually behave as a resistance Multiple phenomena
contribute to irreversible losses in an actual fuel cell
In the V-I diagram especially for low-temperature fuel cells the
effects of the three loss categories are often easy to distinguish as illustrated
in Figure 21
Activation Losses Activation losses are caused by sluggish electrode
kinetics There is a close similarity between electrochemical and chemical
reactions in that both involve an activation energy that must be overcome by
the reacting species In reality activation losses are the result of complex
surface electrochemical reaction steps each of which have their own reaction
rate and activation energy In electrochemical reactions the activation energy
loss for overcoming this barrier is represented as Tafel equation
lnact a b iη = + (241)
where actη is activation polarization(mV) a and b are constants and i is
current density(mAcm2)
( ) ( ) ( )act cell act anode act cathodeη η η= + (242)
Ohmic Polarization Ohmic losses occur because of resistance to the
flow of ions in the electrolyte and resistance to flow of electrons through the
electrode The dominant ohmic losses through the electrolyte are reduced by
14
decreasing the electrode separation and enhancing the ionic conductivity of
the electrolyte Because both the electrolyte and fuel cell electrodes obey
Ohms law the ohmic losses can be expressed by the equation
ohm iRη = (243)
where i is the current flowing through the cell and R is the total cell
resistance which includes electronic ionic and contact resistance
R = Relectronic + Rionic + Rcontact
Mass Transport Related Losses As a reactant is consumed at the
electrode by electrochemical reaction it is often diluted by the products
when finite mass transport rates limit the supply of fresh reactant and the
evacuation of products As a consequence a concentration gradient is formed
which drives the mass transport process
Concentration polarization can be denoted by
ln(1 )consL
RT inF i
η = minus (244)
where consη is concentration polarization(mV) iL is limiting current
density(mAcm2) Same as the activation polarization the concentration
polarization occurs independently at either electrode
( ) ( ) ( )conc cell conc anode conc cathodeη η η= + (245)
15
The combined effect of the losses for a given cell and given
operating conditions can be expressed as polarizations The total polarization
at the electrodes is the sum of actη and concη or
anode acta a conc aη η η= + (246)
and
cathode acta c conc cη η η= + (247)
The effect of polarization is to shift the potential of the electrode
(Eelectrode) to a new value (Velectrode)
electrode electrode electrodeV E η= plusmn (248)
For the anode
anode anode anodeV E η= + (249)
And for the cathode
cathode cathode cathodeV E η= minus (2410)
The net result of current flow in a fuel cell is to increase the anode
potential and to decrease the cathode potential thereby reducing the cell
voltage
The cell voltage includes the contribution of the anode and cathode
potentials and ohmic polarization
16
cell cathode anodeV V V iR= minus minus (2411)
When Equations (249) and (2410) are substituted in Equation (2411)
( )cell cathode cathode anode anodeV E E iRη η= minus minus + minus (2412)
Or
cell e cathode anodeV E iRη η= ∆ minus minus minus (2413)
Where e cathode anodeE E E∆ = minus Equation (2413) shows that current
flow in a fuel cell results in a decrease in cell voltage because of losses by
electrode and ohmic polarizations The goal of fuel cell developers is to
minimize the polarization so that cellV approaches eE∆ This goal is
approached by modifications to fuel cell design (improvement in electrode
structures better electro-catalysts more conductive electrolyte thinner cell
components etc) [1]
17
Figure 241 Typical Fuel Cell Polarization Curve
18
31 Polycrystalline Platinum Preparation
A polycrystalline platinum electrode was prepared with pulse
electrodeposition on Pt- quartz crystal
1) Working electrode Pt quartz crystal counter electrode Pt wire
reference electrode saturated calomel electrode (SCE)
2) Pulse electrodeposition method ton -03V 100ms and toff 0V 300ms
3) Deposition bath 10mM H2PtCl6 + 01M HCl (65)
Figure 31 shows the cross-section diagram of the platinized
polycrystalline platinum quartz crystal
Polycrystalline Pt
Sputtered Pt Sputtered Ti
9MHz At-cut quartz crystal
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
3 EXPERIMENTAL
19
32 Electrode Preparation and the Modification
A preparation procedure of catalyst ink was well described
elsewhere [7] and the catalyst (Pt or PtRu) loading was 3 mgcm2 in the each
electrode A geometric active area was a 5cm2 and catalyst ink was directly
sparied onto carbon paper Simple dipping method was used for the chloride
ion modification Pt cathode on carbon paper was dipped in 1 times 10minus2M KCl
solution for 5min and it was rinsed in ultrapure water followed by drying in
the oven
33 Half Cell Measurements
Figure 331 shows the schematic experimental setup for measuring
of the electrocatalystic activity of Pt electrode oxidationreduction
adsorptiondesorption of hydrogen methanol oxidation The electrochemical
cell body consisted of a glass cylinder capped with a Teflon lid holding all
electrodes The Pt nanocatalyst sprayed on carbon paper (gas diffusion layer)
was used as working electrode (WE) The geometric area of the WE was
02 cm2 A concentric platinized Pt wire was used as counter electrode (CE)
The tip of a Luggin-Haber capillary hosting a SCE reference electrode (RE)
was placed in the middle of two electrodes A poteniostatgalvanostat (EG amp
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
4
13 Objectives
Of late fuel cells have become a central focus of interest and leading
car and power companies are making large scale investments in fuel cell
systems Fuel cells are set to provide electrical vehicles small and large scale
power stations and even portable electrical applications such as laptops and
mobile phones with electrical power in a manner which is more efficient and
more environmentally friendly than ever before [2 minus6] In particular direct
methanol fuel cells (DMFCs) have been considered for use in very small to
mid-sized applications due to their high electrical efficiency long life-time
and low poisonous emissions [2 minus 6] On the other hand in the operation of
DMFCs there are several problems at a cathode caused by methanol
crossover from anode to cathode and it should be solved for the
commercialization of DMFC (i) cathode poisoning by CO adsorption and Pt
sintering (ii) significant fuel loss due to methanol crossover (iii) decrease
partial pressure of oxygen due to water flooding and (iv) temperature
dependent power performance Therefore cathode catalyst for oxygen
reduction reaction (ORR) requires (a) CO tolerance (b) proton adsorption
available (c) easy oxygen adsorption (d) ORR kinetics (e) oxygen storage
material (f) suppression of electro-oxidation of methanol To our knowledge
5
no work has been done up to now to study the influence of halide
contamination on fuel cell performance From the data presented previously
Paulus et al [2] could only point to some aspects which should be considered
during MEA production the degradation of MEA might be occurred due to
the dissolution of Nafion by H2O2 and it was mainly produced in the
reduction of oxygen on a Pt substrate containing chloride ions
In this work we tried to develop new cathode catalyst in DMFC via
adsorption of chloride ion which has been known as inhibiting species in
most electrochemical studies The electrochemical characteristics of chloride
ion modified Pt cathode in the oxidation of methanol and
adsorptiondesorption of hydrogen were studied by cyclic voltammetry
techniques The power performance of the single cell and long-term stability
of pure Pt and modified Pt cathode was also investigated
6
21 Electro-oxidation of Methanol
The electrochemical oxidation of methanol on Pt involves several
intermediate steps ie dehydrogenation CO-like species chemisorption OH
(or H2O) species adsorption chemical interaction between adsorbed CO and
OH compounds and CO2 evolution One of these steps is the rate
determining step (rds) depending on the operating temperature and
particular catalyst surface State of the art electrocatalysts for the electro-
oxidation of methanol in fuel cells are generally based on Pt alloys supported
on carbon black [8 9] The electrocatalytic activity of Pt is known to be
promoted by the presence of a second metal such as Ru or Sn acting either
as an adatom or a bimetal First a sequence of dehydrogenation steps give rise
to adsorbed methanolic residues according to the following scheme[10]
CH3OH + Pt rarr Pt-CH2OH + H+ + 1e- (211)
Pt-CH2OH + Pt rarr Pt-CHOH + H+ + 1e- (212)
Pt-CHOH + Pt rarr PtCHO + H+ + 1e- (213)
2 BASIC THEORY
7
A surface rearrangement of the methanol oxidation intermediates
gives carbon monoxide linearly or bridge-bonded to Pt sites as follows
PtCHO rarr Pt-CequivO + H+ + 1e- (214)
Or
PtCHO + Pt rarr Pt Pt C=O + H+ + 1e- (215)
In the absence of a promoting element water discharge occurs at
high anodic overpotentials on Pt with the formation of Pt-OH species at the
catalyst surface
Pt + H2O rarr PtOH + H+ + 1e- (216)
The final step is the reaction of Pt-OH groups with neighboring
methanolic residues to give carbon dioxide
PtOH + Pt CO rarr 2Pt + CO2 + H+ + 1e- (217)
The overall oxidation process of methanol to carbon dioxide
proceeds through a six electron donation process On a pure Pt surface the
dissociative chemisorption of water on Pt is the rate determining step at
potentials below 07 V vs RHE [11] It is generally accepted that an active
catalyst for methanol oxidation should give rise to water discharging at low
potentials and to labile CO chemisorption Moreover a good catalyst for
methanol oxidation should also catalyze the oxidation of carbon monoxide
8
The promotion of Pt by a range of metals (and oxides) to enhance
methanol oxidation is well known Table 21 summeries species that have a
reported positive effect on methanol oxidation Of the catalyst promoters
considered significant recent activity has focused on the use of Ru Sn and
W with Pt
On PtRu surface CH3OH adsorption would take place on the Pt site
where CH3OH chemisorption requires three neighboring sites yielding
ensembles of Pt atoms with COad and the Ru atoms would act as OHad
providing center According to this the catalytic improvement would be
bifunctional mechanism There are different reports for optimal Ru contents
According to Gasteiger el al a Ru content of 10 is optimum[12] Increasing
Ru content provides more adsorbed water species required to oxidize COad
However because Pt sites are decreased with increasing Ru contents the
oxidation rate of CH3OH is decreased Consequently the rate of reaction
decreased when the Ru contents are more than 10 But other results
referring a coverage close to 05 by Ru is optimal for enhancing CH3OH
oxidation are reported by R Dillon[13]
9
Table 21 Effect of catalyst promoters on methanol oxidation
Promotion by Catalyst promoter
comment
Alloying and
dissolution to produce
highly reticulated
surfaces
Cr
Fe
Sn
Typically less than 100 mV
lower potential than Pt
Surface adatoms Sn
Bi
Alloys
Ru
Sn Mo
Os Ir
Ti Re
Ru has the greatest effect Sn
Mo Os and Re are substantial
promoters
Metal oxides Ru Hydrous Ru oxide the most
active catalyst
Base metal oxides W
Nb Zr Ta
W oxide is a notable prometer
Small effect of other metal
oxides (typically lt 100mV)
10
22 Oxygen Reduction Reaction
Oxygen reduction reaction is considered to be one of the most
important electrocatalytic reactions because of its role in electrochemical
energy conversion several industrial processes and corrosion Consequently
it was in the focus of electrochemical interest for many years Oxygen
reduction reaction has been a challenge for electrochemists because of its
complex kinetics and the need for better electrocatalysts The most notable
need for the improvement of the catalytic activity and the development of
new better non-noble metal electrocatalysts are still remaining Despite
extensive research on oxygen reduction reaction mechanism is still not fully
understood
Oxygen reduction reaction is known to proceed either in a four-
electron step to H2O or in a two-electron step to H2O2 In general the four-
electron-step is considered to be the major reaction pathway on
polycrystalline platinum electrodes [1]
+ -2 2 0O + 4H + 4e 2H O E =1229V ( NHE)vsrarr (221)
- -2 2 0O + 2H O + 2e 4OH E =0401Vrarr (222)
The peroxide pathway is
11
+ -2 2 2 0O + 2H + 2e H O E =067Vrarr (223)
Peroxide can undergo further reduction or decomposition in acid
solutions via the following reactions
+ -2 2 2 0H O + 2H + 2e 2H O E =177Vrarr (224)
2 2 2 22H O 2H O + O rarr
However it is commonly agreed that it proceeds without formation of
surface peroxide on polycrystalline and particulate Pt in acidic media
Furthermore it is believed that the reaction proceeds
O2 harr O2ads (225)
O2ads + H+(aq) +e- rarr O2Hads (226)
O2Hads + 3H+(aq) +3e- rarr 2H2O (227)
with the first charge transfer reaction (226) as the rate determining step The
details of reaction (227) are unknown and may include several Oads or OHads
configurations [2]
23 Anionic Effects
Platinum or platinum alloy particles supported on high surface area
carbon substrates are often synthesized form halide containing reactants (eg
Cl- containing salts)[15] It is therefore possible that traces of halides are
12
present in the electrocatalyst Furthermore halides especially chloride may
also be present in the humidified anode feed-streams of PEFCs or in the
cathode air feed During the ORR remaining anions can compete with
oxygen for free adsorption sites on the catalyst Modified adsorption
conditions for oxygen molecules can have an impact on the oxygen reduction
activity of the catalyst and can alter the oxygen reduction pathway (transition
form H2O to H2O2 formation) From both Pt single crystal work (eg [14]) it
is well known that different supporting electrolytes containing anions with
different adsorption strengths influence the ORR activity tremendously It is
also well known that the presence of additional halide anions can
dramatically affect the kinetics of electrocatalytic reactions on Pt e g the
oxidation of small organic molecules or the oxygen reduction reaction (e g
[16])
24 Actual Performance
The actual cell potential is decreased from its ideal potential because
of several types of irreversible losses as shown in Figure 21 These losses
are often referred to as polarization overpotential or overvoltage though
13
only the ohmic losses actually behave as a resistance Multiple phenomena
contribute to irreversible losses in an actual fuel cell
In the V-I diagram especially for low-temperature fuel cells the
effects of the three loss categories are often easy to distinguish as illustrated
in Figure 21
Activation Losses Activation losses are caused by sluggish electrode
kinetics There is a close similarity between electrochemical and chemical
reactions in that both involve an activation energy that must be overcome by
the reacting species In reality activation losses are the result of complex
surface electrochemical reaction steps each of which have their own reaction
rate and activation energy In electrochemical reactions the activation energy
loss for overcoming this barrier is represented as Tafel equation
lnact a b iη = + (241)
where actη is activation polarization(mV) a and b are constants and i is
current density(mAcm2)
( ) ( ) ( )act cell act anode act cathodeη η η= + (242)
Ohmic Polarization Ohmic losses occur because of resistance to the
flow of ions in the electrolyte and resistance to flow of electrons through the
electrode The dominant ohmic losses through the electrolyte are reduced by
14
decreasing the electrode separation and enhancing the ionic conductivity of
the electrolyte Because both the electrolyte and fuel cell electrodes obey
Ohms law the ohmic losses can be expressed by the equation
ohm iRη = (243)
where i is the current flowing through the cell and R is the total cell
resistance which includes electronic ionic and contact resistance
R = Relectronic + Rionic + Rcontact
Mass Transport Related Losses As a reactant is consumed at the
electrode by electrochemical reaction it is often diluted by the products
when finite mass transport rates limit the supply of fresh reactant and the
evacuation of products As a consequence a concentration gradient is formed
which drives the mass transport process
Concentration polarization can be denoted by
ln(1 )consL
RT inF i
η = minus (244)
where consη is concentration polarization(mV) iL is limiting current
density(mAcm2) Same as the activation polarization the concentration
polarization occurs independently at either electrode
( ) ( ) ( )conc cell conc anode conc cathodeη η η= + (245)
15
The combined effect of the losses for a given cell and given
operating conditions can be expressed as polarizations The total polarization
at the electrodes is the sum of actη and concη or
anode acta a conc aη η η= + (246)
and
cathode acta c conc cη η η= + (247)
The effect of polarization is to shift the potential of the electrode
(Eelectrode) to a new value (Velectrode)
electrode electrode electrodeV E η= plusmn (248)
For the anode
anode anode anodeV E η= + (249)
And for the cathode
cathode cathode cathodeV E η= minus (2410)
The net result of current flow in a fuel cell is to increase the anode
potential and to decrease the cathode potential thereby reducing the cell
voltage
The cell voltage includes the contribution of the anode and cathode
potentials and ohmic polarization
16
cell cathode anodeV V V iR= minus minus (2411)
When Equations (249) and (2410) are substituted in Equation (2411)
( )cell cathode cathode anode anodeV E E iRη η= minus minus + minus (2412)
Or
cell e cathode anodeV E iRη η= ∆ minus minus minus (2413)
Where e cathode anodeE E E∆ = minus Equation (2413) shows that current
flow in a fuel cell results in a decrease in cell voltage because of losses by
electrode and ohmic polarizations The goal of fuel cell developers is to
minimize the polarization so that cellV approaches eE∆ This goal is
approached by modifications to fuel cell design (improvement in electrode
structures better electro-catalysts more conductive electrolyte thinner cell
components etc) [1]
17
Figure 241 Typical Fuel Cell Polarization Curve
18
31 Polycrystalline Platinum Preparation
A polycrystalline platinum electrode was prepared with pulse
electrodeposition on Pt- quartz crystal
1) Working electrode Pt quartz crystal counter electrode Pt wire
reference electrode saturated calomel electrode (SCE)
2) Pulse electrodeposition method ton -03V 100ms and toff 0V 300ms
3) Deposition bath 10mM H2PtCl6 + 01M HCl (65)
Figure 31 shows the cross-section diagram of the platinized
polycrystalline platinum quartz crystal
Polycrystalline Pt
Sputtered Pt Sputtered Ti
9MHz At-cut quartz crystal
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
3 EXPERIMENTAL
19
32 Electrode Preparation and the Modification
A preparation procedure of catalyst ink was well described
elsewhere [7] and the catalyst (Pt or PtRu) loading was 3 mgcm2 in the each
electrode A geometric active area was a 5cm2 and catalyst ink was directly
sparied onto carbon paper Simple dipping method was used for the chloride
ion modification Pt cathode on carbon paper was dipped in 1 times 10minus2M KCl
solution for 5min and it was rinsed in ultrapure water followed by drying in
the oven
33 Half Cell Measurements
Figure 331 shows the schematic experimental setup for measuring
of the electrocatalystic activity of Pt electrode oxidationreduction
adsorptiondesorption of hydrogen methanol oxidation The electrochemical
cell body consisted of a glass cylinder capped with a Teflon lid holding all
electrodes The Pt nanocatalyst sprayed on carbon paper (gas diffusion layer)
was used as working electrode (WE) The geometric area of the WE was
02 cm2 A concentric platinized Pt wire was used as counter electrode (CE)
The tip of a Luggin-Haber capillary hosting a SCE reference electrode (RE)
was placed in the middle of two electrodes A poteniostatgalvanostat (EG amp
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
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51
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(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
5
no work has been done up to now to study the influence of halide
contamination on fuel cell performance From the data presented previously
Paulus et al [2] could only point to some aspects which should be considered
during MEA production the degradation of MEA might be occurred due to
the dissolution of Nafion by H2O2 and it was mainly produced in the
reduction of oxygen on a Pt substrate containing chloride ions
In this work we tried to develop new cathode catalyst in DMFC via
adsorption of chloride ion which has been known as inhibiting species in
most electrochemical studies The electrochemical characteristics of chloride
ion modified Pt cathode in the oxidation of methanol and
adsorptiondesorption of hydrogen were studied by cyclic voltammetry
techniques The power performance of the single cell and long-term stability
of pure Pt and modified Pt cathode was also investigated
6
21 Electro-oxidation of Methanol
The electrochemical oxidation of methanol on Pt involves several
intermediate steps ie dehydrogenation CO-like species chemisorption OH
(or H2O) species adsorption chemical interaction between adsorbed CO and
OH compounds and CO2 evolution One of these steps is the rate
determining step (rds) depending on the operating temperature and
particular catalyst surface State of the art electrocatalysts for the electro-
oxidation of methanol in fuel cells are generally based on Pt alloys supported
on carbon black [8 9] The electrocatalytic activity of Pt is known to be
promoted by the presence of a second metal such as Ru or Sn acting either
as an adatom or a bimetal First a sequence of dehydrogenation steps give rise
to adsorbed methanolic residues according to the following scheme[10]
CH3OH + Pt rarr Pt-CH2OH + H+ + 1e- (211)
Pt-CH2OH + Pt rarr Pt-CHOH + H+ + 1e- (212)
Pt-CHOH + Pt rarr PtCHO + H+ + 1e- (213)
2 BASIC THEORY
7
A surface rearrangement of the methanol oxidation intermediates
gives carbon monoxide linearly or bridge-bonded to Pt sites as follows
PtCHO rarr Pt-CequivO + H+ + 1e- (214)
Or
PtCHO + Pt rarr Pt Pt C=O + H+ + 1e- (215)
In the absence of a promoting element water discharge occurs at
high anodic overpotentials on Pt with the formation of Pt-OH species at the
catalyst surface
Pt + H2O rarr PtOH + H+ + 1e- (216)
The final step is the reaction of Pt-OH groups with neighboring
methanolic residues to give carbon dioxide
PtOH + Pt CO rarr 2Pt + CO2 + H+ + 1e- (217)
The overall oxidation process of methanol to carbon dioxide
proceeds through a six electron donation process On a pure Pt surface the
dissociative chemisorption of water on Pt is the rate determining step at
potentials below 07 V vs RHE [11] It is generally accepted that an active
catalyst for methanol oxidation should give rise to water discharging at low
potentials and to labile CO chemisorption Moreover a good catalyst for
methanol oxidation should also catalyze the oxidation of carbon monoxide
8
The promotion of Pt by a range of metals (and oxides) to enhance
methanol oxidation is well known Table 21 summeries species that have a
reported positive effect on methanol oxidation Of the catalyst promoters
considered significant recent activity has focused on the use of Ru Sn and
W with Pt
On PtRu surface CH3OH adsorption would take place on the Pt site
where CH3OH chemisorption requires three neighboring sites yielding
ensembles of Pt atoms with COad and the Ru atoms would act as OHad
providing center According to this the catalytic improvement would be
bifunctional mechanism There are different reports for optimal Ru contents
According to Gasteiger el al a Ru content of 10 is optimum[12] Increasing
Ru content provides more adsorbed water species required to oxidize COad
However because Pt sites are decreased with increasing Ru contents the
oxidation rate of CH3OH is decreased Consequently the rate of reaction
decreased when the Ru contents are more than 10 But other results
referring a coverage close to 05 by Ru is optimal for enhancing CH3OH
oxidation are reported by R Dillon[13]
9
Table 21 Effect of catalyst promoters on methanol oxidation
Promotion by Catalyst promoter
comment
Alloying and
dissolution to produce
highly reticulated
surfaces
Cr
Fe
Sn
Typically less than 100 mV
lower potential than Pt
Surface adatoms Sn
Bi
Alloys
Ru
Sn Mo
Os Ir
Ti Re
Ru has the greatest effect Sn
Mo Os and Re are substantial
promoters
Metal oxides Ru Hydrous Ru oxide the most
active catalyst
Base metal oxides W
Nb Zr Ta
W oxide is a notable prometer
Small effect of other metal
oxides (typically lt 100mV)
10
22 Oxygen Reduction Reaction
Oxygen reduction reaction is considered to be one of the most
important electrocatalytic reactions because of its role in electrochemical
energy conversion several industrial processes and corrosion Consequently
it was in the focus of electrochemical interest for many years Oxygen
reduction reaction has been a challenge for electrochemists because of its
complex kinetics and the need for better electrocatalysts The most notable
need for the improvement of the catalytic activity and the development of
new better non-noble metal electrocatalysts are still remaining Despite
extensive research on oxygen reduction reaction mechanism is still not fully
understood
Oxygen reduction reaction is known to proceed either in a four-
electron step to H2O or in a two-electron step to H2O2 In general the four-
electron-step is considered to be the major reaction pathway on
polycrystalline platinum electrodes [1]
+ -2 2 0O + 4H + 4e 2H O E =1229V ( NHE)vsrarr (221)
- -2 2 0O + 2H O + 2e 4OH E =0401Vrarr (222)
The peroxide pathway is
11
+ -2 2 2 0O + 2H + 2e H O E =067Vrarr (223)
Peroxide can undergo further reduction or decomposition in acid
solutions via the following reactions
+ -2 2 2 0H O + 2H + 2e 2H O E =177Vrarr (224)
2 2 2 22H O 2H O + O rarr
However it is commonly agreed that it proceeds without formation of
surface peroxide on polycrystalline and particulate Pt in acidic media
Furthermore it is believed that the reaction proceeds
O2 harr O2ads (225)
O2ads + H+(aq) +e- rarr O2Hads (226)
O2Hads + 3H+(aq) +3e- rarr 2H2O (227)
with the first charge transfer reaction (226) as the rate determining step The
details of reaction (227) are unknown and may include several Oads or OHads
configurations [2]
23 Anionic Effects
Platinum or platinum alloy particles supported on high surface area
carbon substrates are often synthesized form halide containing reactants (eg
Cl- containing salts)[15] It is therefore possible that traces of halides are
12
present in the electrocatalyst Furthermore halides especially chloride may
also be present in the humidified anode feed-streams of PEFCs or in the
cathode air feed During the ORR remaining anions can compete with
oxygen for free adsorption sites on the catalyst Modified adsorption
conditions for oxygen molecules can have an impact on the oxygen reduction
activity of the catalyst and can alter the oxygen reduction pathway (transition
form H2O to H2O2 formation) From both Pt single crystal work (eg [14]) it
is well known that different supporting electrolytes containing anions with
different adsorption strengths influence the ORR activity tremendously It is
also well known that the presence of additional halide anions can
dramatically affect the kinetics of electrocatalytic reactions on Pt e g the
oxidation of small organic molecules or the oxygen reduction reaction (e g
[16])
24 Actual Performance
The actual cell potential is decreased from its ideal potential because
of several types of irreversible losses as shown in Figure 21 These losses
are often referred to as polarization overpotential or overvoltage though
13
only the ohmic losses actually behave as a resistance Multiple phenomena
contribute to irreversible losses in an actual fuel cell
In the V-I diagram especially for low-temperature fuel cells the
effects of the three loss categories are often easy to distinguish as illustrated
in Figure 21
Activation Losses Activation losses are caused by sluggish electrode
kinetics There is a close similarity between electrochemical and chemical
reactions in that both involve an activation energy that must be overcome by
the reacting species In reality activation losses are the result of complex
surface electrochemical reaction steps each of which have their own reaction
rate and activation energy In electrochemical reactions the activation energy
loss for overcoming this barrier is represented as Tafel equation
lnact a b iη = + (241)
where actη is activation polarization(mV) a and b are constants and i is
current density(mAcm2)
( ) ( ) ( )act cell act anode act cathodeη η η= + (242)
Ohmic Polarization Ohmic losses occur because of resistance to the
flow of ions in the electrolyte and resistance to flow of electrons through the
electrode The dominant ohmic losses through the electrolyte are reduced by
14
decreasing the electrode separation and enhancing the ionic conductivity of
the electrolyte Because both the electrolyte and fuel cell electrodes obey
Ohms law the ohmic losses can be expressed by the equation
ohm iRη = (243)
where i is the current flowing through the cell and R is the total cell
resistance which includes electronic ionic and contact resistance
R = Relectronic + Rionic + Rcontact
Mass Transport Related Losses As a reactant is consumed at the
electrode by electrochemical reaction it is often diluted by the products
when finite mass transport rates limit the supply of fresh reactant and the
evacuation of products As a consequence a concentration gradient is formed
which drives the mass transport process
Concentration polarization can be denoted by
ln(1 )consL
RT inF i
η = minus (244)
where consη is concentration polarization(mV) iL is limiting current
density(mAcm2) Same as the activation polarization the concentration
polarization occurs independently at either electrode
( ) ( ) ( )conc cell conc anode conc cathodeη η η= + (245)
15
The combined effect of the losses for a given cell and given
operating conditions can be expressed as polarizations The total polarization
at the electrodes is the sum of actη and concη or
anode acta a conc aη η η= + (246)
and
cathode acta c conc cη η η= + (247)
The effect of polarization is to shift the potential of the electrode
(Eelectrode) to a new value (Velectrode)
electrode electrode electrodeV E η= plusmn (248)
For the anode
anode anode anodeV E η= + (249)
And for the cathode
cathode cathode cathodeV E η= minus (2410)
The net result of current flow in a fuel cell is to increase the anode
potential and to decrease the cathode potential thereby reducing the cell
voltage
The cell voltage includes the contribution of the anode and cathode
potentials and ohmic polarization
16
cell cathode anodeV V V iR= minus minus (2411)
When Equations (249) and (2410) are substituted in Equation (2411)
( )cell cathode cathode anode anodeV E E iRη η= minus minus + minus (2412)
Or
cell e cathode anodeV E iRη η= ∆ minus minus minus (2413)
Where e cathode anodeE E E∆ = minus Equation (2413) shows that current
flow in a fuel cell results in a decrease in cell voltage because of losses by
electrode and ohmic polarizations The goal of fuel cell developers is to
minimize the polarization so that cellV approaches eE∆ This goal is
approached by modifications to fuel cell design (improvement in electrode
structures better electro-catalysts more conductive electrolyte thinner cell
components etc) [1]
17
Figure 241 Typical Fuel Cell Polarization Curve
18
31 Polycrystalline Platinum Preparation
A polycrystalline platinum electrode was prepared with pulse
electrodeposition on Pt- quartz crystal
1) Working electrode Pt quartz crystal counter electrode Pt wire
reference electrode saturated calomel electrode (SCE)
2) Pulse electrodeposition method ton -03V 100ms and toff 0V 300ms
3) Deposition bath 10mM H2PtCl6 + 01M HCl (65)
Figure 31 shows the cross-section diagram of the platinized
polycrystalline platinum quartz crystal
Polycrystalline Pt
Sputtered Pt Sputtered Ti
9MHz At-cut quartz crystal
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
3 EXPERIMENTAL
19
32 Electrode Preparation and the Modification
A preparation procedure of catalyst ink was well described
elsewhere [7] and the catalyst (Pt or PtRu) loading was 3 mgcm2 in the each
electrode A geometric active area was a 5cm2 and catalyst ink was directly
sparied onto carbon paper Simple dipping method was used for the chloride
ion modification Pt cathode on carbon paper was dipped in 1 times 10minus2M KCl
solution for 5min and it was rinsed in ultrapure water followed by drying in
the oven
33 Half Cell Measurements
Figure 331 shows the schematic experimental setup for measuring
of the electrocatalystic activity of Pt electrode oxidationreduction
adsorptiondesorption of hydrogen methanol oxidation The electrochemical
cell body consisted of a glass cylinder capped with a Teflon lid holding all
electrodes The Pt nanocatalyst sprayed on carbon paper (gas diffusion layer)
was used as working electrode (WE) The geometric area of the WE was
02 cm2 A concentric platinized Pt wire was used as counter electrode (CE)
The tip of a Luggin-Haber capillary hosting a SCE reference electrode (RE)
was placed in the middle of two electrodes A poteniostatgalvanostat (EG amp
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
6
21 Electro-oxidation of Methanol
The electrochemical oxidation of methanol on Pt involves several
intermediate steps ie dehydrogenation CO-like species chemisorption OH
(or H2O) species adsorption chemical interaction between adsorbed CO and
OH compounds and CO2 evolution One of these steps is the rate
determining step (rds) depending on the operating temperature and
particular catalyst surface State of the art electrocatalysts for the electro-
oxidation of methanol in fuel cells are generally based on Pt alloys supported
on carbon black [8 9] The electrocatalytic activity of Pt is known to be
promoted by the presence of a second metal such as Ru or Sn acting either
as an adatom or a bimetal First a sequence of dehydrogenation steps give rise
to adsorbed methanolic residues according to the following scheme[10]
CH3OH + Pt rarr Pt-CH2OH + H+ + 1e- (211)
Pt-CH2OH + Pt rarr Pt-CHOH + H+ + 1e- (212)
Pt-CHOH + Pt rarr PtCHO + H+ + 1e- (213)
2 BASIC THEORY
7
A surface rearrangement of the methanol oxidation intermediates
gives carbon monoxide linearly or bridge-bonded to Pt sites as follows
PtCHO rarr Pt-CequivO + H+ + 1e- (214)
Or
PtCHO + Pt rarr Pt Pt C=O + H+ + 1e- (215)
In the absence of a promoting element water discharge occurs at
high anodic overpotentials on Pt with the formation of Pt-OH species at the
catalyst surface
Pt + H2O rarr PtOH + H+ + 1e- (216)
The final step is the reaction of Pt-OH groups with neighboring
methanolic residues to give carbon dioxide
PtOH + Pt CO rarr 2Pt + CO2 + H+ + 1e- (217)
The overall oxidation process of methanol to carbon dioxide
proceeds through a six electron donation process On a pure Pt surface the
dissociative chemisorption of water on Pt is the rate determining step at
potentials below 07 V vs RHE [11] It is generally accepted that an active
catalyst for methanol oxidation should give rise to water discharging at low
potentials and to labile CO chemisorption Moreover a good catalyst for
methanol oxidation should also catalyze the oxidation of carbon monoxide
8
The promotion of Pt by a range of metals (and oxides) to enhance
methanol oxidation is well known Table 21 summeries species that have a
reported positive effect on methanol oxidation Of the catalyst promoters
considered significant recent activity has focused on the use of Ru Sn and
W with Pt
On PtRu surface CH3OH adsorption would take place on the Pt site
where CH3OH chemisorption requires three neighboring sites yielding
ensembles of Pt atoms with COad and the Ru atoms would act as OHad
providing center According to this the catalytic improvement would be
bifunctional mechanism There are different reports for optimal Ru contents
According to Gasteiger el al a Ru content of 10 is optimum[12] Increasing
Ru content provides more adsorbed water species required to oxidize COad
However because Pt sites are decreased with increasing Ru contents the
oxidation rate of CH3OH is decreased Consequently the rate of reaction
decreased when the Ru contents are more than 10 But other results
referring a coverage close to 05 by Ru is optimal for enhancing CH3OH
oxidation are reported by R Dillon[13]
9
Table 21 Effect of catalyst promoters on methanol oxidation
Promotion by Catalyst promoter
comment
Alloying and
dissolution to produce
highly reticulated
surfaces
Cr
Fe
Sn
Typically less than 100 mV
lower potential than Pt
Surface adatoms Sn
Bi
Alloys
Ru
Sn Mo
Os Ir
Ti Re
Ru has the greatest effect Sn
Mo Os and Re are substantial
promoters
Metal oxides Ru Hydrous Ru oxide the most
active catalyst
Base metal oxides W
Nb Zr Ta
W oxide is a notable prometer
Small effect of other metal
oxides (typically lt 100mV)
10
22 Oxygen Reduction Reaction
Oxygen reduction reaction is considered to be one of the most
important electrocatalytic reactions because of its role in electrochemical
energy conversion several industrial processes and corrosion Consequently
it was in the focus of electrochemical interest for many years Oxygen
reduction reaction has been a challenge for electrochemists because of its
complex kinetics and the need for better electrocatalysts The most notable
need for the improvement of the catalytic activity and the development of
new better non-noble metal electrocatalysts are still remaining Despite
extensive research on oxygen reduction reaction mechanism is still not fully
understood
Oxygen reduction reaction is known to proceed either in a four-
electron step to H2O or in a two-electron step to H2O2 In general the four-
electron-step is considered to be the major reaction pathway on
polycrystalline platinum electrodes [1]
+ -2 2 0O + 4H + 4e 2H O E =1229V ( NHE)vsrarr (221)
- -2 2 0O + 2H O + 2e 4OH E =0401Vrarr (222)
The peroxide pathway is
11
+ -2 2 2 0O + 2H + 2e H O E =067Vrarr (223)
Peroxide can undergo further reduction or decomposition in acid
solutions via the following reactions
+ -2 2 2 0H O + 2H + 2e 2H O E =177Vrarr (224)
2 2 2 22H O 2H O + O rarr
However it is commonly agreed that it proceeds without formation of
surface peroxide on polycrystalline and particulate Pt in acidic media
Furthermore it is believed that the reaction proceeds
O2 harr O2ads (225)
O2ads + H+(aq) +e- rarr O2Hads (226)
O2Hads + 3H+(aq) +3e- rarr 2H2O (227)
with the first charge transfer reaction (226) as the rate determining step The
details of reaction (227) are unknown and may include several Oads or OHads
configurations [2]
23 Anionic Effects
Platinum or platinum alloy particles supported on high surface area
carbon substrates are often synthesized form halide containing reactants (eg
Cl- containing salts)[15] It is therefore possible that traces of halides are
12
present in the electrocatalyst Furthermore halides especially chloride may
also be present in the humidified anode feed-streams of PEFCs or in the
cathode air feed During the ORR remaining anions can compete with
oxygen for free adsorption sites on the catalyst Modified adsorption
conditions for oxygen molecules can have an impact on the oxygen reduction
activity of the catalyst and can alter the oxygen reduction pathway (transition
form H2O to H2O2 formation) From both Pt single crystal work (eg [14]) it
is well known that different supporting electrolytes containing anions with
different adsorption strengths influence the ORR activity tremendously It is
also well known that the presence of additional halide anions can
dramatically affect the kinetics of electrocatalytic reactions on Pt e g the
oxidation of small organic molecules or the oxygen reduction reaction (e g
[16])
24 Actual Performance
The actual cell potential is decreased from its ideal potential because
of several types of irreversible losses as shown in Figure 21 These losses
are often referred to as polarization overpotential or overvoltage though
13
only the ohmic losses actually behave as a resistance Multiple phenomena
contribute to irreversible losses in an actual fuel cell
In the V-I diagram especially for low-temperature fuel cells the
effects of the three loss categories are often easy to distinguish as illustrated
in Figure 21
Activation Losses Activation losses are caused by sluggish electrode
kinetics There is a close similarity between electrochemical and chemical
reactions in that both involve an activation energy that must be overcome by
the reacting species In reality activation losses are the result of complex
surface electrochemical reaction steps each of which have their own reaction
rate and activation energy In electrochemical reactions the activation energy
loss for overcoming this barrier is represented as Tafel equation
lnact a b iη = + (241)
where actη is activation polarization(mV) a and b are constants and i is
current density(mAcm2)
( ) ( ) ( )act cell act anode act cathodeη η η= + (242)
Ohmic Polarization Ohmic losses occur because of resistance to the
flow of ions in the electrolyte and resistance to flow of electrons through the
electrode The dominant ohmic losses through the electrolyte are reduced by
14
decreasing the electrode separation and enhancing the ionic conductivity of
the electrolyte Because both the electrolyte and fuel cell electrodes obey
Ohms law the ohmic losses can be expressed by the equation
ohm iRη = (243)
where i is the current flowing through the cell and R is the total cell
resistance which includes electronic ionic and contact resistance
R = Relectronic + Rionic + Rcontact
Mass Transport Related Losses As a reactant is consumed at the
electrode by electrochemical reaction it is often diluted by the products
when finite mass transport rates limit the supply of fresh reactant and the
evacuation of products As a consequence a concentration gradient is formed
which drives the mass transport process
Concentration polarization can be denoted by
ln(1 )consL
RT inF i
η = minus (244)
where consη is concentration polarization(mV) iL is limiting current
density(mAcm2) Same as the activation polarization the concentration
polarization occurs independently at either electrode
( ) ( ) ( )conc cell conc anode conc cathodeη η η= + (245)
15
The combined effect of the losses for a given cell and given
operating conditions can be expressed as polarizations The total polarization
at the electrodes is the sum of actη and concη or
anode acta a conc aη η η= + (246)
and
cathode acta c conc cη η η= + (247)
The effect of polarization is to shift the potential of the electrode
(Eelectrode) to a new value (Velectrode)
electrode electrode electrodeV E η= plusmn (248)
For the anode
anode anode anodeV E η= + (249)
And for the cathode
cathode cathode cathodeV E η= minus (2410)
The net result of current flow in a fuel cell is to increase the anode
potential and to decrease the cathode potential thereby reducing the cell
voltage
The cell voltage includes the contribution of the anode and cathode
potentials and ohmic polarization
16
cell cathode anodeV V V iR= minus minus (2411)
When Equations (249) and (2410) are substituted in Equation (2411)
( )cell cathode cathode anode anodeV E E iRη η= minus minus + minus (2412)
Or
cell e cathode anodeV E iRη η= ∆ minus minus minus (2413)
Where e cathode anodeE E E∆ = minus Equation (2413) shows that current
flow in a fuel cell results in a decrease in cell voltage because of losses by
electrode and ohmic polarizations The goal of fuel cell developers is to
minimize the polarization so that cellV approaches eE∆ This goal is
approached by modifications to fuel cell design (improvement in electrode
structures better electro-catalysts more conductive electrolyte thinner cell
components etc) [1]
17
Figure 241 Typical Fuel Cell Polarization Curve
18
31 Polycrystalline Platinum Preparation
A polycrystalline platinum electrode was prepared with pulse
electrodeposition on Pt- quartz crystal
1) Working electrode Pt quartz crystal counter electrode Pt wire
reference electrode saturated calomel electrode (SCE)
2) Pulse electrodeposition method ton -03V 100ms and toff 0V 300ms
3) Deposition bath 10mM H2PtCl6 + 01M HCl (65)
Figure 31 shows the cross-section diagram of the platinized
polycrystalline platinum quartz crystal
Polycrystalline Pt
Sputtered Pt Sputtered Ti
9MHz At-cut quartz crystal
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
3 EXPERIMENTAL
19
32 Electrode Preparation and the Modification
A preparation procedure of catalyst ink was well described
elsewhere [7] and the catalyst (Pt or PtRu) loading was 3 mgcm2 in the each
electrode A geometric active area was a 5cm2 and catalyst ink was directly
sparied onto carbon paper Simple dipping method was used for the chloride
ion modification Pt cathode on carbon paper was dipped in 1 times 10minus2M KCl
solution for 5min and it was rinsed in ultrapure water followed by drying in
the oven
33 Half Cell Measurements
Figure 331 shows the schematic experimental setup for measuring
of the electrocatalystic activity of Pt electrode oxidationreduction
adsorptiondesorption of hydrogen methanol oxidation The electrochemical
cell body consisted of a glass cylinder capped with a Teflon lid holding all
electrodes The Pt nanocatalyst sprayed on carbon paper (gas diffusion layer)
was used as working electrode (WE) The geometric area of the WE was
02 cm2 A concentric platinized Pt wire was used as counter electrode (CE)
The tip of a Luggin-Haber capillary hosting a SCE reference electrode (RE)
was placed in the middle of two electrodes A poteniostatgalvanostat (EG amp
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
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51
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(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
7
A surface rearrangement of the methanol oxidation intermediates
gives carbon monoxide linearly or bridge-bonded to Pt sites as follows
PtCHO rarr Pt-CequivO + H+ + 1e- (214)
Or
PtCHO + Pt rarr Pt Pt C=O + H+ + 1e- (215)
In the absence of a promoting element water discharge occurs at
high anodic overpotentials on Pt with the formation of Pt-OH species at the
catalyst surface
Pt + H2O rarr PtOH + H+ + 1e- (216)
The final step is the reaction of Pt-OH groups with neighboring
methanolic residues to give carbon dioxide
PtOH + Pt CO rarr 2Pt + CO2 + H+ + 1e- (217)
The overall oxidation process of methanol to carbon dioxide
proceeds through a six electron donation process On a pure Pt surface the
dissociative chemisorption of water on Pt is the rate determining step at
potentials below 07 V vs RHE [11] It is generally accepted that an active
catalyst for methanol oxidation should give rise to water discharging at low
potentials and to labile CO chemisorption Moreover a good catalyst for
methanol oxidation should also catalyze the oxidation of carbon monoxide
8
The promotion of Pt by a range of metals (and oxides) to enhance
methanol oxidation is well known Table 21 summeries species that have a
reported positive effect on methanol oxidation Of the catalyst promoters
considered significant recent activity has focused on the use of Ru Sn and
W with Pt
On PtRu surface CH3OH adsorption would take place on the Pt site
where CH3OH chemisorption requires three neighboring sites yielding
ensembles of Pt atoms with COad and the Ru atoms would act as OHad
providing center According to this the catalytic improvement would be
bifunctional mechanism There are different reports for optimal Ru contents
According to Gasteiger el al a Ru content of 10 is optimum[12] Increasing
Ru content provides more adsorbed water species required to oxidize COad
However because Pt sites are decreased with increasing Ru contents the
oxidation rate of CH3OH is decreased Consequently the rate of reaction
decreased when the Ru contents are more than 10 But other results
referring a coverage close to 05 by Ru is optimal for enhancing CH3OH
oxidation are reported by R Dillon[13]
9
Table 21 Effect of catalyst promoters on methanol oxidation
Promotion by Catalyst promoter
comment
Alloying and
dissolution to produce
highly reticulated
surfaces
Cr
Fe
Sn
Typically less than 100 mV
lower potential than Pt
Surface adatoms Sn
Bi
Alloys
Ru
Sn Mo
Os Ir
Ti Re
Ru has the greatest effect Sn
Mo Os and Re are substantial
promoters
Metal oxides Ru Hydrous Ru oxide the most
active catalyst
Base metal oxides W
Nb Zr Ta
W oxide is a notable prometer
Small effect of other metal
oxides (typically lt 100mV)
10
22 Oxygen Reduction Reaction
Oxygen reduction reaction is considered to be one of the most
important electrocatalytic reactions because of its role in electrochemical
energy conversion several industrial processes and corrosion Consequently
it was in the focus of electrochemical interest for many years Oxygen
reduction reaction has been a challenge for electrochemists because of its
complex kinetics and the need for better electrocatalysts The most notable
need for the improvement of the catalytic activity and the development of
new better non-noble metal electrocatalysts are still remaining Despite
extensive research on oxygen reduction reaction mechanism is still not fully
understood
Oxygen reduction reaction is known to proceed either in a four-
electron step to H2O or in a two-electron step to H2O2 In general the four-
electron-step is considered to be the major reaction pathway on
polycrystalline platinum electrodes [1]
+ -2 2 0O + 4H + 4e 2H O E =1229V ( NHE)vsrarr (221)
- -2 2 0O + 2H O + 2e 4OH E =0401Vrarr (222)
The peroxide pathway is
11
+ -2 2 2 0O + 2H + 2e H O E =067Vrarr (223)
Peroxide can undergo further reduction or decomposition in acid
solutions via the following reactions
+ -2 2 2 0H O + 2H + 2e 2H O E =177Vrarr (224)
2 2 2 22H O 2H O + O rarr
However it is commonly agreed that it proceeds without formation of
surface peroxide on polycrystalline and particulate Pt in acidic media
Furthermore it is believed that the reaction proceeds
O2 harr O2ads (225)
O2ads + H+(aq) +e- rarr O2Hads (226)
O2Hads + 3H+(aq) +3e- rarr 2H2O (227)
with the first charge transfer reaction (226) as the rate determining step The
details of reaction (227) are unknown and may include several Oads or OHads
configurations [2]
23 Anionic Effects
Platinum or platinum alloy particles supported on high surface area
carbon substrates are often synthesized form halide containing reactants (eg
Cl- containing salts)[15] It is therefore possible that traces of halides are
12
present in the electrocatalyst Furthermore halides especially chloride may
also be present in the humidified anode feed-streams of PEFCs or in the
cathode air feed During the ORR remaining anions can compete with
oxygen for free adsorption sites on the catalyst Modified adsorption
conditions for oxygen molecules can have an impact on the oxygen reduction
activity of the catalyst and can alter the oxygen reduction pathway (transition
form H2O to H2O2 formation) From both Pt single crystal work (eg [14]) it
is well known that different supporting electrolytes containing anions with
different adsorption strengths influence the ORR activity tremendously It is
also well known that the presence of additional halide anions can
dramatically affect the kinetics of electrocatalytic reactions on Pt e g the
oxidation of small organic molecules or the oxygen reduction reaction (e g
[16])
24 Actual Performance
The actual cell potential is decreased from its ideal potential because
of several types of irreversible losses as shown in Figure 21 These losses
are often referred to as polarization overpotential or overvoltage though
13
only the ohmic losses actually behave as a resistance Multiple phenomena
contribute to irreversible losses in an actual fuel cell
In the V-I diagram especially for low-temperature fuel cells the
effects of the three loss categories are often easy to distinguish as illustrated
in Figure 21
Activation Losses Activation losses are caused by sluggish electrode
kinetics There is a close similarity between electrochemical and chemical
reactions in that both involve an activation energy that must be overcome by
the reacting species In reality activation losses are the result of complex
surface electrochemical reaction steps each of which have their own reaction
rate and activation energy In electrochemical reactions the activation energy
loss for overcoming this barrier is represented as Tafel equation
lnact a b iη = + (241)
where actη is activation polarization(mV) a and b are constants and i is
current density(mAcm2)
( ) ( ) ( )act cell act anode act cathodeη η η= + (242)
Ohmic Polarization Ohmic losses occur because of resistance to the
flow of ions in the electrolyte and resistance to flow of electrons through the
electrode The dominant ohmic losses through the electrolyte are reduced by
14
decreasing the electrode separation and enhancing the ionic conductivity of
the electrolyte Because both the electrolyte and fuel cell electrodes obey
Ohms law the ohmic losses can be expressed by the equation
ohm iRη = (243)
where i is the current flowing through the cell and R is the total cell
resistance which includes electronic ionic and contact resistance
R = Relectronic + Rionic + Rcontact
Mass Transport Related Losses As a reactant is consumed at the
electrode by electrochemical reaction it is often diluted by the products
when finite mass transport rates limit the supply of fresh reactant and the
evacuation of products As a consequence a concentration gradient is formed
which drives the mass transport process
Concentration polarization can be denoted by
ln(1 )consL
RT inF i
η = minus (244)
where consη is concentration polarization(mV) iL is limiting current
density(mAcm2) Same as the activation polarization the concentration
polarization occurs independently at either electrode
( ) ( ) ( )conc cell conc anode conc cathodeη η η= + (245)
15
The combined effect of the losses for a given cell and given
operating conditions can be expressed as polarizations The total polarization
at the electrodes is the sum of actη and concη or
anode acta a conc aη η η= + (246)
and
cathode acta c conc cη η η= + (247)
The effect of polarization is to shift the potential of the electrode
(Eelectrode) to a new value (Velectrode)
electrode electrode electrodeV E η= plusmn (248)
For the anode
anode anode anodeV E η= + (249)
And for the cathode
cathode cathode cathodeV E η= minus (2410)
The net result of current flow in a fuel cell is to increase the anode
potential and to decrease the cathode potential thereby reducing the cell
voltage
The cell voltage includes the contribution of the anode and cathode
potentials and ohmic polarization
16
cell cathode anodeV V V iR= minus minus (2411)
When Equations (249) and (2410) are substituted in Equation (2411)
( )cell cathode cathode anode anodeV E E iRη η= minus minus + minus (2412)
Or
cell e cathode anodeV E iRη η= ∆ minus minus minus (2413)
Where e cathode anodeE E E∆ = minus Equation (2413) shows that current
flow in a fuel cell results in a decrease in cell voltage because of losses by
electrode and ohmic polarizations The goal of fuel cell developers is to
minimize the polarization so that cellV approaches eE∆ This goal is
approached by modifications to fuel cell design (improvement in electrode
structures better electro-catalysts more conductive electrolyte thinner cell
components etc) [1]
17
Figure 241 Typical Fuel Cell Polarization Curve
18
31 Polycrystalline Platinum Preparation
A polycrystalline platinum electrode was prepared with pulse
electrodeposition on Pt- quartz crystal
1) Working electrode Pt quartz crystal counter electrode Pt wire
reference electrode saturated calomel electrode (SCE)
2) Pulse electrodeposition method ton -03V 100ms and toff 0V 300ms
3) Deposition bath 10mM H2PtCl6 + 01M HCl (65)
Figure 31 shows the cross-section diagram of the platinized
polycrystalline platinum quartz crystal
Polycrystalline Pt
Sputtered Pt Sputtered Ti
9MHz At-cut quartz crystal
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
3 EXPERIMENTAL
19
32 Electrode Preparation and the Modification
A preparation procedure of catalyst ink was well described
elsewhere [7] and the catalyst (Pt or PtRu) loading was 3 mgcm2 in the each
electrode A geometric active area was a 5cm2 and catalyst ink was directly
sparied onto carbon paper Simple dipping method was used for the chloride
ion modification Pt cathode on carbon paper was dipped in 1 times 10minus2M KCl
solution for 5min and it was rinsed in ultrapure water followed by drying in
the oven
33 Half Cell Measurements
Figure 331 shows the schematic experimental setup for measuring
of the electrocatalystic activity of Pt electrode oxidationreduction
adsorptiondesorption of hydrogen methanol oxidation The electrochemical
cell body consisted of a glass cylinder capped with a Teflon lid holding all
electrodes The Pt nanocatalyst sprayed on carbon paper (gas diffusion layer)
was used as working electrode (WE) The geometric area of the WE was
02 cm2 A concentric platinized Pt wire was used as counter electrode (CE)
The tip of a Luggin-Haber capillary hosting a SCE reference electrode (RE)
was placed in the middle of two electrodes A poteniostatgalvanostat (EG amp
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
8
The promotion of Pt by a range of metals (and oxides) to enhance
methanol oxidation is well known Table 21 summeries species that have a
reported positive effect on methanol oxidation Of the catalyst promoters
considered significant recent activity has focused on the use of Ru Sn and
W with Pt
On PtRu surface CH3OH adsorption would take place on the Pt site
where CH3OH chemisorption requires three neighboring sites yielding
ensembles of Pt atoms with COad and the Ru atoms would act as OHad
providing center According to this the catalytic improvement would be
bifunctional mechanism There are different reports for optimal Ru contents
According to Gasteiger el al a Ru content of 10 is optimum[12] Increasing
Ru content provides more adsorbed water species required to oxidize COad
However because Pt sites are decreased with increasing Ru contents the
oxidation rate of CH3OH is decreased Consequently the rate of reaction
decreased when the Ru contents are more than 10 But other results
referring a coverage close to 05 by Ru is optimal for enhancing CH3OH
oxidation are reported by R Dillon[13]
9
Table 21 Effect of catalyst promoters on methanol oxidation
Promotion by Catalyst promoter
comment
Alloying and
dissolution to produce
highly reticulated
surfaces
Cr
Fe
Sn
Typically less than 100 mV
lower potential than Pt
Surface adatoms Sn
Bi
Alloys
Ru
Sn Mo
Os Ir
Ti Re
Ru has the greatest effect Sn
Mo Os and Re are substantial
promoters
Metal oxides Ru Hydrous Ru oxide the most
active catalyst
Base metal oxides W
Nb Zr Ta
W oxide is a notable prometer
Small effect of other metal
oxides (typically lt 100mV)
10
22 Oxygen Reduction Reaction
Oxygen reduction reaction is considered to be one of the most
important electrocatalytic reactions because of its role in electrochemical
energy conversion several industrial processes and corrosion Consequently
it was in the focus of electrochemical interest for many years Oxygen
reduction reaction has been a challenge for electrochemists because of its
complex kinetics and the need for better electrocatalysts The most notable
need for the improvement of the catalytic activity and the development of
new better non-noble metal electrocatalysts are still remaining Despite
extensive research on oxygen reduction reaction mechanism is still not fully
understood
Oxygen reduction reaction is known to proceed either in a four-
electron step to H2O or in a two-electron step to H2O2 In general the four-
electron-step is considered to be the major reaction pathway on
polycrystalline platinum electrodes [1]
+ -2 2 0O + 4H + 4e 2H O E =1229V ( NHE)vsrarr (221)
- -2 2 0O + 2H O + 2e 4OH E =0401Vrarr (222)
The peroxide pathway is
11
+ -2 2 2 0O + 2H + 2e H O E =067Vrarr (223)
Peroxide can undergo further reduction or decomposition in acid
solutions via the following reactions
+ -2 2 2 0H O + 2H + 2e 2H O E =177Vrarr (224)
2 2 2 22H O 2H O + O rarr
However it is commonly agreed that it proceeds without formation of
surface peroxide on polycrystalline and particulate Pt in acidic media
Furthermore it is believed that the reaction proceeds
O2 harr O2ads (225)
O2ads + H+(aq) +e- rarr O2Hads (226)
O2Hads + 3H+(aq) +3e- rarr 2H2O (227)
with the first charge transfer reaction (226) as the rate determining step The
details of reaction (227) are unknown and may include several Oads or OHads
configurations [2]
23 Anionic Effects
Platinum or platinum alloy particles supported on high surface area
carbon substrates are often synthesized form halide containing reactants (eg
Cl- containing salts)[15] It is therefore possible that traces of halides are
12
present in the electrocatalyst Furthermore halides especially chloride may
also be present in the humidified anode feed-streams of PEFCs or in the
cathode air feed During the ORR remaining anions can compete with
oxygen for free adsorption sites on the catalyst Modified adsorption
conditions for oxygen molecules can have an impact on the oxygen reduction
activity of the catalyst and can alter the oxygen reduction pathway (transition
form H2O to H2O2 formation) From both Pt single crystal work (eg [14]) it
is well known that different supporting electrolytes containing anions with
different adsorption strengths influence the ORR activity tremendously It is
also well known that the presence of additional halide anions can
dramatically affect the kinetics of electrocatalytic reactions on Pt e g the
oxidation of small organic molecules or the oxygen reduction reaction (e g
[16])
24 Actual Performance
The actual cell potential is decreased from its ideal potential because
of several types of irreversible losses as shown in Figure 21 These losses
are often referred to as polarization overpotential or overvoltage though
13
only the ohmic losses actually behave as a resistance Multiple phenomena
contribute to irreversible losses in an actual fuel cell
In the V-I diagram especially for low-temperature fuel cells the
effects of the three loss categories are often easy to distinguish as illustrated
in Figure 21
Activation Losses Activation losses are caused by sluggish electrode
kinetics There is a close similarity between electrochemical and chemical
reactions in that both involve an activation energy that must be overcome by
the reacting species In reality activation losses are the result of complex
surface electrochemical reaction steps each of which have their own reaction
rate and activation energy In electrochemical reactions the activation energy
loss for overcoming this barrier is represented as Tafel equation
lnact a b iη = + (241)
where actη is activation polarization(mV) a and b are constants and i is
current density(mAcm2)
( ) ( ) ( )act cell act anode act cathodeη η η= + (242)
Ohmic Polarization Ohmic losses occur because of resistance to the
flow of ions in the electrolyte and resistance to flow of electrons through the
electrode The dominant ohmic losses through the electrolyte are reduced by
14
decreasing the electrode separation and enhancing the ionic conductivity of
the electrolyte Because both the electrolyte and fuel cell electrodes obey
Ohms law the ohmic losses can be expressed by the equation
ohm iRη = (243)
where i is the current flowing through the cell and R is the total cell
resistance which includes electronic ionic and contact resistance
R = Relectronic + Rionic + Rcontact
Mass Transport Related Losses As a reactant is consumed at the
electrode by electrochemical reaction it is often diluted by the products
when finite mass transport rates limit the supply of fresh reactant and the
evacuation of products As a consequence a concentration gradient is formed
which drives the mass transport process
Concentration polarization can be denoted by
ln(1 )consL
RT inF i
η = minus (244)
where consη is concentration polarization(mV) iL is limiting current
density(mAcm2) Same as the activation polarization the concentration
polarization occurs independently at either electrode
( ) ( ) ( )conc cell conc anode conc cathodeη η η= + (245)
15
The combined effect of the losses for a given cell and given
operating conditions can be expressed as polarizations The total polarization
at the electrodes is the sum of actη and concη or
anode acta a conc aη η η= + (246)
and
cathode acta c conc cη η η= + (247)
The effect of polarization is to shift the potential of the electrode
(Eelectrode) to a new value (Velectrode)
electrode electrode electrodeV E η= plusmn (248)
For the anode
anode anode anodeV E η= + (249)
And for the cathode
cathode cathode cathodeV E η= minus (2410)
The net result of current flow in a fuel cell is to increase the anode
potential and to decrease the cathode potential thereby reducing the cell
voltage
The cell voltage includes the contribution of the anode and cathode
potentials and ohmic polarization
16
cell cathode anodeV V V iR= minus minus (2411)
When Equations (249) and (2410) are substituted in Equation (2411)
( )cell cathode cathode anode anodeV E E iRη η= minus minus + minus (2412)
Or
cell e cathode anodeV E iRη η= ∆ minus minus minus (2413)
Where e cathode anodeE E E∆ = minus Equation (2413) shows that current
flow in a fuel cell results in a decrease in cell voltage because of losses by
electrode and ohmic polarizations The goal of fuel cell developers is to
minimize the polarization so that cellV approaches eE∆ This goal is
approached by modifications to fuel cell design (improvement in electrode
structures better electro-catalysts more conductive electrolyte thinner cell
components etc) [1]
17
Figure 241 Typical Fuel Cell Polarization Curve
18
31 Polycrystalline Platinum Preparation
A polycrystalline platinum electrode was prepared with pulse
electrodeposition on Pt- quartz crystal
1) Working electrode Pt quartz crystal counter electrode Pt wire
reference electrode saturated calomel electrode (SCE)
2) Pulse electrodeposition method ton -03V 100ms and toff 0V 300ms
3) Deposition bath 10mM H2PtCl6 + 01M HCl (65)
Figure 31 shows the cross-section diagram of the platinized
polycrystalline platinum quartz crystal
Polycrystalline Pt
Sputtered Pt Sputtered Ti
9MHz At-cut quartz crystal
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
3 EXPERIMENTAL
19
32 Electrode Preparation and the Modification
A preparation procedure of catalyst ink was well described
elsewhere [7] and the catalyst (Pt or PtRu) loading was 3 mgcm2 in the each
electrode A geometric active area was a 5cm2 and catalyst ink was directly
sparied onto carbon paper Simple dipping method was used for the chloride
ion modification Pt cathode on carbon paper was dipped in 1 times 10minus2M KCl
solution for 5min and it was rinsed in ultrapure water followed by drying in
the oven
33 Half Cell Measurements
Figure 331 shows the schematic experimental setup for measuring
of the electrocatalystic activity of Pt electrode oxidationreduction
adsorptiondesorption of hydrogen methanol oxidation The electrochemical
cell body consisted of a glass cylinder capped with a Teflon lid holding all
electrodes The Pt nanocatalyst sprayed on carbon paper (gas diffusion layer)
was used as working electrode (WE) The geometric area of the WE was
02 cm2 A concentric platinized Pt wire was used as counter electrode (CE)
The tip of a Luggin-Haber capillary hosting a SCE reference electrode (RE)
was placed in the middle of two electrodes A poteniostatgalvanostat (EG amp
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
9
Table 21 Effect of catalyst promoters on methanol oxidation
Promotion by Catalyst promoter
comment
Alloying and
dissolution to produce
highly reticulated
surfaces
Cr
Fe
Sn
Typically less than 100 mV
lower potential than Pt
Surface adatoms Sn
Bi
Alloys
Ru
Sn Mo
Os Ir
Ti Re
Ru has the greatest effect Sn
Mo Os and Re are substantial
promoters
Metal oxides Ru Hydrous Ru oxide the most
active catalyst
Base metal oxides W
Nb Zr Ta
W oxide is a notable prometer
Small effect of other metal
oxides (typically lt 100mV)
10
22 Oxygen Reduction Reaction
Oxygen reduction reaction is considered to be one of the most
important electrocatalytic reactions because of its role in electrochemical
energy conversion several industrial processes and corrosion Consequently
it was in the focus of electrochemical interest for many years Oxygen
reduction reaction has been a challenge for electrochemists because of its
complex kinetics and the need for better electrocatalysts The most notable
need for the improvement of the catalytic activity and the development of
new better non-noble metal electrocatalysts are still remaining Despite
extensive research on oxygen reduction reaction mechanism is still not fully
understood
Oxygen reduction reaction is known to proceed either in a four-
electron step to H2O or in a two-electron step to H2O2 In general the four-
electron-step is considered to be the major reaction pathway on
polycrystalline platinum electrodes [1]
+ -2 2 0O + 4H + 4e 2H O E =1229V ( NHE)vsrarr (221)
- -2 2 0O + 2H O + 2e 4OH E =0401Vrarr (222)
The peroxide pathway is
11
+ -2 2 2 0O + 2H + 2e H O E =067Vrarr (223)
Peroxide can undergo further reduction or decomposition in acid
solutions via the following reactions
+ -2 2 2 0H O + 2H + 2e 2H O E =177Vrarr (224)
2 2 2 22H O 2H O + O rarr
However it is commonly agreed that it proceeds without formation of
surface peroxide on polycrystalline and particulate Pt in acidic media
Furthermore it is believed that the reaction proceeds
O2 harr O2ads (225)
O2ads + H+(aq) +e- rarr O2Hads (226)
O2Hads + 3H+(aq) +3e- rarr 2H2O (227)
with the first charge transfer reaction (226) as the rate determining step The
details of reaction (227) are unknown and may include several Oads or OHads
configurations [2]
23 Anionic Effects
Platinum or platinum alloy particles supported on high surface area
carbon substrates are often synthesized form halide containing reactants (eg
Cl- containing salts)[15] It is therefore possible that traces of halides are
12
present in the electrocatalyst Furthermore halides especially chloride may
also be present in the humidified anode feed-streams of PEFCs or in the
cathode air feed During the ORR remaining anions can compete with
oxygen for free adsorption sites on the catalyst Modified adsorption
conditions for oxygen molecules can have an impact on the oxygen reduction
activity of the catalyst and can alter the oxygen reduction pathway (transition
form H2O to H2O2 formation) From both Pt single crystal work (eg [14]) it
is well known that different supporting electrolytes containing anions with
different adsorption strengths influence the ORR activity tremendously It is
also well known that the presence of additional halide anions can
dramatically affect the kinetics of electrocatalytic reactions on Pt e g the
oxidation of small organic molecules or the oxygen reduction reaction (e g
[16])
24 Actual Performance
The actual cell potential is decreased from its ideal potential because
of several types of irreversible losses as shown in Figure 21 These losses
are often referred to as polarization overpotential or overvoltage though
13
only the ohmic losses actually behave as a resistance Multiple phenomena
contribute to irreversible losses in an actual fuel cell
In the V-I diagram especially for low-temperature fuel cells the
effects of the three loss categories are often easy to distinguish as illustrated
in Figure 21
Activation Losses Activation losses are caused by sluggish electrode
kinetics There is a close similarity between electrochemical and chemical
reactions in that both involve an activation energy that must be overcome by
the reacting species In reality activation losses are the result of complex
surface electrochemical reaction steps each of which have their own reaction
rate and activation energy In electrochemical reactions the activation energy
loss for overcoming this barrier is represented as Tafel equation
lnact a b iη = + (241)
where actη is activation polarization(mV) a and b are constants and i is
current density(mAcm2)
( ) ( ) ( )act cell act anode act cathodeη η η= + (242)
Ohmic Polarization Ohmic losses occur because of resistance to the
flow of ions in the electrolyte and resistance to flow of electrons through the
electrode The dominant ohmic losses through the electrolyte are reduced by
14
decreasing the electrode separation and enhancing the ionic conductivity of
the electrolyte Because both the electrolyte and fuel cell electrodes obey
Ohms law the ohmic losses can be expressed by the equation
ohm iRη = (243)
where i is the current flowing through the cell and R is the total cell
resistance which includes electronic ionic and contact resistance
R = Relectronic + Rionic + Rcontact
Mass Transport Related Losses As a reactant is consumed at the
electrode by electrochemical reaction it is often diluted by the products
when finite mass transport rates limit the supply of fresh reactant and the
evacuation of products As a consequence a concentration gradient is formed
which drives the mass transport process
Concentration polarization can be denoted by
ln(1 )consL
RT inF i
η = minus (244)
where consη is concentration polarization(mV) iL is limiting current
density(mAcm2) Same as the activation polarization the concentration
polarization occurs independently at either electrode
( ) ( ) ( )conc cell conc anode conc cathodeη η η= + (245)
15
The combined effect of the losses for a given cell and given
operating conditions can be expressed as polarizations The total polarization
at the electrodes is the sum of actη and concη or
anode acta a conc aη η η= + (246)
and
cathode acta c conc cη η η= + (247)
The effect of polarization is to shift the potential of the electrode
(Eelectrode) to a new value (Velectrode)
electrode electrode electrodeV E η= plusmn (248)
For the anode
anode anode anodeV E η= + (249)
And for the cathode
cathode cathode cathodeV E η= minus (2410)
The net result of current flow in a fuel cell is to increase the anode
potential and to decrease the cathode potential thereby reducing the cell
voltage
The cell voltage includes the contribution of the anode and cathode
potentials and ohmic polarization
16
cell cathode anodeV V V iR= minus minus (2411)
When Equations (249) and (2410) are substituted in Equation (2411)
( )cell cathode cathode anode anodeV E E iRη η= minus minus + minus (2412)
Or
cell e cathode anodeV E iRη η= ∆ minus minus minus (2413)
Where e cathode anodeE E E∆ = minus Equation (2413) shows that current
flow in a fuel cell results in a decrease in cell voltage because of losses by
electrode and ohmic polarizations The goal of fuel cell developers is to
minimize the polarization so that cellV approaches eE∆ This goal is
approached by modifications to fuel cell design (improvement in electrode
structures better electro-catalysts more conductive electrolyte thinner cell
components etc) [1]
17
Figure 241 Typical Fuel Cell Polarization Curve
18
31 Polycrystalline Platinum Preparation
A polycrystalline platinum electrode was prepared with pulse
electrodeposition on Pt- quartz crystal
1) Working electrode Pt quartz crystal counter electrode Pt wire
reference electrode saturated calomel electrode (SCE)
2) Pulse electrodeposition method ton -03V 100ms and toff 0V 300ms
3) Deposition bath 10mM H2PtCl6 + 01M HCl (65)
Figure 31 shows the cross-section diagram of the platinized
polycrystalline platinum quartz crystal
Polycrystalline Pt
Sputtered Pt Sputtered Ti
9MHz At-cut quartz crystal
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
3 EXPERIMENTAL
19
32 Electrode Preparation and the Modification
A preparation procedure of catalyst ink was well described
elsewhere [7] and the catalyst (Pt or PtRu) loading was 3 mgcm2 in the each
electrode A geometric active area was a 5cm2 and catalyst ink was directly
sparied onto carbon paper Simple dipping method was used for the chloride
ion modification Pt cathode on carbon paper was dipped in 1 times 10minus2M KCl
solution for 5min and it was rinsed in ultrapure water followed by drying in
the oven
33 Half Cell Measurements
Figure 331 shows the schematic experimental setup for measuring
of the electrocatalystic activity of Pt electrode oxidationreduction
adsorptiondesorption of hydrogen methanol oxidation The electrochemical
cell body consisted of a glass cylinder capped with a Teflon lid holding all
electrodes The Pt nanocatalyst sprayed on carbon paper (gas diffusion layer)
was used as working electrode (WE) The geometric area of the WE was
02 cm2 A concentric platinized Pt wire was used as counter electrode (CE)
The tip of a Luggin-Haber capillary hosting a SCE reference electrode (RE)
was placed in the middle of two electrodes A poteniostatgalvanostat (EG amp
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
10
22 Oxygen Reduction Reaction
Oxygen reduction reaction is considered to be one of the most
important electrocatalytic reactions because of its role in electrochemical
energy conversion several industrial processes and corrosion Consequently
it was in the focus of electrochemical interest for many years Oxygen
reduction reaction has been a challenge for electrochemists because of its
complex kinetics and the need for better electrocatalysts The most notable
need for the improvement of the catalytic activity and the development of
new better non-noble metal electrocatalysts are still remaining Despite
extensive research on oxygen reduction reaction mechanism is still not fully
understood
Oxygen reduction reaction is known to proceed either in a four-
electron step to H2O or in a two-electron step to H2O2 In general the four-
electron-step is considered to be the major reaction pathway on
polycrystalline platinum electrodes [1]
+ -2 2 0O + 4H + 4e 2H O E =1229V ( NHE)vsrarr (221)
- -2 2 0O + 2H O + 2e 4OH E =0401Vrarr (222)
The peroxide pathway is
11
+ -2 2 2 0O + 2H + 2e H O E =067Vrarr (223)
Peroxide can undergo further reduction or decomposition in acid
solutions via the following reactions
+ -2 2 2 0H O + 2H + 2e 2H O E =177Vrarr (224)
2 2 2 22H O 2H O + O rarr
However it is commonly agreed that it proceeds without formation of
surface peroxide on polycrystalline and particulate Pt in acidic media
Furthermore it is believed that the reaction proceeds
O2 harr O2ads (225)
O2ads + H+(aq) +e- rarr O2Hads (226)
O2Hads + 3H+(aq) +3e- rarr 2H2O (227)
with the first charge transfer reaction (226) as the rate determining step The
details of reaction (227) are unknown and may include several Oads or OHads
configurations [2]
23 Anionic Effects
Platinum or platinum alloy particles supported on high surface area
carbon substrates are often synthesized form halide containing reactants (eg
Cl- containing salts)[15] It is therefore possible that traces of halides are
12
present in the electrocatalyst Furthermore halides especially chloride may
also be present in the humidified anode feed-streams of PEFCs or in the
cathode air feed During the ORR remaining anions can compete with
oxygen for free adsorption sites on the catalyst Modified adsorption
conditions for oxygen molecules can have an impact on the oxygen reduction
activity of the catalyst and can alter the oxygen reduction pathway (transition
form H2O to H2O2 formation) From both Pt single crystal work (eg [14]) it
is well known that different supporting electrolytes containing anions with
different adsorption strengths influence the ORR activity tremendously It is
also well known that the presence of additional halide anions can
dramatically affect the kinetics of electrocatalytic reactions on Pt e g the
oxidation of small organic molecules or the oxygen reduction reaction (e g
[16])
24 Actual Performance
The actual cell potential is decreased from its ideal potential because
of several types of irreversible losses as shown in Figure 21 These losses
are often referred to as polarization overpotential or overvoltage though
13
only the ohmic losses actually behave as a resistance Multiple phenomena
contribute to irreversible losses in an actual fuel cell
In the V-I diagram especially for low-temperature fuel cells the
effects of the three loss categories are often easy to distinguish as illustrated
in Figure 21
Activation Losses Activation losses are caused by sluggish electrode
kinetics There is a close similarity between electrochemical and chemical
reactions in that both involve an activation energy that must be overcome by
the reacting species In reality activation losses are the result of complex
surface electrochemical reaction steps each of which have their own reaction
rate and activation energy In electrochemical reactions the activation energy
loss for overcoming this barrier is represented as Tafel equation
lnact a b iη = + (241)
where actη is activation polarization(mV) a and b are constants and i is
current density(mAcm2)
( ) ( ) ( )act cell act anode act cathodeη η η= + (242)
Ohmic Polarization Ohmic losses occur because of resistance to the
flow of ions in the electrolyte and resistance to flow of electrons through the
electrode The dominant ohmic losses through the electrolyte are reduced by
14
decreasing the electrode separation and enhancing the ionic conductivity of
the electrolyte Because both the electrolyte and fuel cell electrodes obey
Ohms law the ohmic losses can be expressed by the equation
ohm iRη = (243)
where i is the current flowing through the cell and R is the total cell
resistance which includes electronic ionic and contact resistance
R = Relectronic + Rionic + Rcontact
Mass Transport Related Losses As a reactant is consumed at the
electrode by electrochemical reaction it is often diluted by the products
when finite mass transport rates limit the supply of fresh reactant and the
evacuation of products As a consequence a concentration gradient is formed
which drives the mass transport process
Concentration polarization can be denoted by
ln(1 )consL
RT inF i
η = minus (244)
where consη is concentration polarization(mV) iL is limiting current
density(mAcm2) Same as the activation polarization the concentration
polarization occurs independently at either electrode
( ) ( ) ( )conc cell conc anode conc cathodeη η η= + (245)
15
The combined effect of the losses for a given cell and given
operating conditions can be expressed as polarizations The total polarization
at the electrodes is the sum of actη and concη or
anode acta a conc aη η η= + (246)
and
cathode acta c conc cη η η= + (247)
The effect of polarization is to shift the potential of the electrode
(Eelectrode) to a new value (Velectrode)
electrode electrode electrodeV E η= plusmn (248)
For the anode
anode anode anodeV E η= + (249)
And for the cathode
cathode cathode cathodeV E η= minus (2410)
The net result of current flow in a fuel cell is to increase the anode
potential and to decrease the cathode potential thereby reducing the cell
voltage
The cell voltage includes the contribution of the anode and cathode
potentials and ohmic polarization
16
cell cathode anodeV V V iR= minus minus (2411)
When Equations (249) and (2410) are substituted in Equation (2411)
( )cell cathode cathode anode anodeV E E iRη η= minus minus + minus (2412)
Or
cell e cathode anodeV E iRη η= ∆ minus minus minus (2413)
Where e cathode anodeE E E∆ = minus Equation (2413) shows that current
flow in a fuel cell results in a decrease in cell voltage because of losses by
electrode and ohmic polarizations The goal of fuel cell developers is to
minimize the polarization so that cellV approaches eE∆ This goal is
approached by modifications to fuel cell design (improvement in electrode
structures better electro-catalysts more conductive electrolyte thinner cell
components etc) [1]
17
Figure 241 Typical Fuel Cell Polarization Curve
18
31 Polycrystalline Platinum Preparation
A polycrystalline platinum electrode was prepared with pulse
electrodeposition on Pt- quartz crystal
1) Working electrode Pt quartz crystal counter electrode Pt wire
reference electrode saturated calomel electrode (SCE)
2) Pulse electrodeposition method ton -03V 100ms and toff 0V 300ms
3) Deposition bath 10mM H2PtCl6 + 01M HCl (65)
Figure 31 shows the cross-section diagram of the platinized
polycrystalline platinum quartz crystal
Polycrystalline Pt
Sputtered Pt Sputtered Ti
9MHz At-cut quartz crystal
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
3 EXPERIMENTAL
19
32 Electrode Preparation and the Modification
A preparation procedure of catalyst ink was well described
elsewhere [7] and the catalyst (Pt or PtRu) loading was 3 mgcm2 in the each
electrode A geometric active area was a 5cm2 and catalyst ink was directly
sparied onto carbon paper Simple dipping method was used for the chloride
ion modification Pt cathode on carbon paper was dipped in 1 times 10minus2M KCl
solution for 5min and it was rinsed in ultrapure water followed by drying in
the oven
33 Half Cell Measurements
Figure 331 shows the schematic experimental setup for measuring
of the electrocatalystic activity of Pt electrode oxidationreduction
adsorptiondesorption of hydrogen methanol oxidation The electrochemical
cell body consisted of a glass cylinder capped with a Teflon lid holding all
electrodes The Pt nanocatalyst sprayed on carbon paper (gas diffusion layer)
was used as working electrode (WE) The geometric area of the WE was
02 cm2 A concentric platinized Pt wire was used as counter electrode (CE)
The tip of a Luggin-Haber capillary hosting a SCE reference electrode (RE)
was placed in the middle of two electrodes A poteniostatgalvanostat (EG amp
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
11
+ -2 2 2 0O + 2H + 2e H O E =067Vrarr (223)
Peroxide can undergo further reduction or decomposition in acid
solutions via the following reactions
+ -2 2 2 0H O + 2H + 2e 2H O E =177Vrarr (224)
2 2 2 22H O 2H O + O rarr
However it is commonly agreed that it proceeds without formation of
surface peroxide on polycrystalline and particulate Pt in acidic media
Furthermore it is believed that the reaction proceeds
O2 harr O2ads (225)
O2ads + H+(aq) +e- rarr O2Hads (226)
O2Hads + 3H+(aq) +3e- rarr 2H2O (227)
with the first charge transfer reaction (226) as the rate determining step The
details of reaction (227) are unknown and may include several Oads or OHads
configurations [2]
23 Anionic Effects
Platinum or platinum alloy particles supported on high surface area
carbon substrates are often synthesized form halide containing reactants (eg
Cl- containing salts)[15] It is therefore possible that traces of halides are
12
present in the electrocatalyst Furthermore halides especially chloride may
also be present in the humidified anode feed-streams of PEFCs or in the
cathode air feed During the ORR remaining anions can compete with
oxygen for free adsorption sites on the catalyst Modified adsorption
conditions for oxygen molecules can have an impact on the oxygen reduction
activity of the catalyst and can alter the oxygen reduction pathway (transition
form H2O to H2O2 formation) From both Pt single crystal work (eg [14]) it
is well known that different supporting electrolytes containing anions with
different adsorption strengths influence the ORR activity tremendously It is
also well known that the presence of additional halide anions can
dramatically affect the kinetics of electrocatalytic reactions on Pt e g the
oxidation of small organic molecules or the oxygen reduction reaction (e g
[16])
24 Actual Performance
The actual cell potential is decreased from its ideal potential because
of several types of irreversible losses as shown in Figure 21 These losses
are often referred to as polarization overpotential or overvoltage though
13
only the ohmic losses actually behave as a resistance Multiple phenomena
contribute to irreversible losses in an actual fuel cell
In the V-I diagram especially for low-temperature fuel cells the
effects of the three loss categories are often easy to distinguish as illustrated
in Figure 21
Activation Losses Activation losses are caused by sluggish electrode
kinetics There is a close similarity between electrochemical and chemical
reactions in that both involve an activation energy that must be overcome by
the reacting species In reality activation losses are the result of complex
surface electrochemical reaction steps each of which have their own reaction
rate and activation energy In electrochemical reactions the activation energy
loss for overcoming this barrier is represented as Tafel equation
lnact a b iη = + (241)
where actη is activation polarization(mV) a and b are constants and i is
current density(mAcm2)
( ) ( ) ( )act cell act anode act cathodeη η η= + (242)
Ohmic Polarization Ohmic losses occur because of resistance to the
flow of ions in the electrolyte and resistance to flow of electrons through the
electrode The dominant ohmic losses through the electrolyte are reduced by
14
decreasing the electrode separation and enhancing the ionic conductivity of
the electrolyte Because both the electrolyte and fuel cell electrodes obey
Ohms law the ohmic losses can be expressed by the equation
ohm iRη = (243)
where i is the current flowing through the cell and R is the total cell
resistance which includes electronic ionic and contact resistance
R = Relectronic + Rionic + Rcontact
Mass Transport Related Losses As a reactant is consumed at the
electrode by electrochemical reaction it is often diluted by the products
when finite mass transport rates limit the supply of fresh reactant and the
evacuation of products As a consequence a concentration gradient is formed
which drives the mass transport process
Concentration polarization can be denoted by
ln(1 )consL
RT inF i
η = minus (244)
where consη is concentration polarization(mV) iL is limiting current
density(mAcm2) Same as the activation polarization the concentration
polarization occurs independently at either electrode
( ) ( ) ( )conc cell conc anode conc cathodeη η η= + (245)
15
The combined effect of the losses for a given cell and given
operating conditions can be expressed as polarizations The total polarization
at the electrodes is the sum of actη and concη or
anode acta a conc aη η η= + (246)
and
cathode acta c conc cη η η= + (247)
The effect of polarization is to shift the potential of the electrode
(Eelectrode) to a new value (Velectrode)
electrode electrode electrodeV E η= plusmn (248)
For the anode
anode anode anodeV E η= + (249)
And for the cathode
cathode cathode cathodeV E η= minus (2410)
The net result of current flow in a fuel cell is to increase the anode
potential and to decrease the cathode potential thereby reducing the cell
voltage
The cell voltage includes the contribution of the anode and cathode
potentials and ohmic polarization
16
cell cathode anodeV V V iR= minus minus (2411)
When Equations (249) and (2410) are substituted in Equation (2411)
( )cell cathode cathode anode anodeV E E iRη η= minus minus + minus (2412)
Or
cell e cathode anodeV E iRη η= ∆ minus minus minus (2413)
Where e cathode anodeE E E∆ = minus Equation (2413) shows that current
flow in a fuel cell results in a decrease in cell voltage because of losses by
electrode and ohmic polarizations The goal of fuel cell developers is to
minimize the polarization so that cellV approaches eE∆ This goal is
approached by modifications to fuel cell design (improvement in electrode
structures better electro-catalysts more conductive electrolyte thinner cell
components etc) [1]
17
Figure 241 Typical Fuel Cell Polarization Curve
18
31 Polycrystalline Platinum Preparation
A polycrystalline platinum electrode was prepared with pulse
electrodeposition on Pt- quartz crystal
1) Working electrode Pt quartz crystal counter electrode Pt wire
reference electrode saturated calomel electrode (SCE)
2) Pulse electrodeposition method ton -03V 100ms and toff 0V 300ms
3) Deposition bath 10mM H2PtCl6 + 01M HCl (65)
Figure 31 shows the cross-section diagram of the platinized
polycrystalline platinum quartz crystal
Polycrystalline Pt
Sputtered Pt Sputtered Ti
9MHz At-cut quartz crystal
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
3 EXPERIMENTAL
19
32 Electrode Preparation and the Modification
A preparation procedure of catalyst ink was well described
elsewhere [7] and the catalyst (Pt or PtRu) loading was 3 mgcm2 in the each
electrode A geometric active area was a 5cm2 and catalyst ink was directly
sparied onto carbon paper Simple dipping method was used for the chloride
ion modification Pt cathode on carbon paper was dipped in 1 times 10minus2M KCl
solution for 5min and it was rinsed in ultrapure water followed by drying in
the oven
33 Half Cell Measurements
Figure 331 shows the schematic experimental setup for measuring
of the electrocatalystic activity of Pt electrode oxidationreduction
adsorptiondesorption of hydrogen methanol oxidation The electrochemical
cell body consisted of a glass cylinder capped with a Teflon lid holding all
electrodes The Pt nanocatalyst sprayed on carbon paper (gas diffusion layer)
was used as working electrode (WE) The geometric area of the WE was
02 cm2 A concentric platinized Pt wire was used as counter electrode (CE)
The tip of a Luggin-Haber capillary hosting a SCE reference electrode (RE)
was placed in the middle of two electrodes A poteniostatgalvanostat (EG amp
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
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[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
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[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
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[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
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[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
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[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
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[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
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[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
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[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
12
present in the electrocatalyst Furthermore halides especially chloride may
also be present in the humidified anode feed-streams of PEFCs or in the
cathode air feed During the ORR remaining anions can compete with
oxygen for free adsorption sites on the catalyst Modified adsorption
conditions for oxygen molecules can have an impact on the oxygen reduction
activity of the catalyst and can alter the oxygen reduction pathway (transition
form H2O to H2O2 formation) From both Pt single crystal work (eg [14]) it
is well known that different supporting electrolytes containing anions with
different adsorption strengths influence the ORR activity tremendously It is
also well known that the presence of additional halide anions can
dramatically affect the kinetics of electrocatalytic reactions on Pt e g the
oxidation of small organic molecules or the oxygen reduction reaction (e g
[16])
24 Actual Performance
The actual cell potential is decreased from its ideal potential because
of several types of irreversible losses as shown in Figure 21 These losses
are often referred to as polarization overpotential or overvoltage though
13
only the ohmic losses actually behave as a resistance Multiple phenomena
contribute to irreversible losses in an actual fuel cell
In the V-I diagram especially for low-temperature fuel cells the
effects of the three loss categories are often easy to distinguish as illustrated
in Figure 21
Activation Losses Activation losses are caused by sluggish electrode
kinetics There is a close similarity between electrochemical and chemical
reactions in that both involve an activation energy that must be overcome by
the reacting species In reality activation losses are the result of complex
surface electrochemical reaction steps each of which have their own reaction
rate and activation energy In electrochemical reactions the activation energy
loss for overcoming this barrier is represented as Tafel equation
lnact a b iη = + (241)
where actη is activation polarization(mV) a and b are constants and i is
current density(mAcm2)
( ) ( ) ( )act cell act anode act cathodeη η η= + (242)
Ohmic Polarization Ohmic losses occur because of resistance to the
flow of ions in the electrolyte and resistance to flow of electrons through the
electrode The dominant ohmic losses through the electrolyte are reduced by
14
decreasing the electrode separation and enhancing the ionic conductivity of
the electrolyte Because both the electrolyte and fuel cell electrodes obey
Ohms law the ohmic losses can be expressed by the equation
ohm iRη = (243)
where i is the current flowing through the cell and R is the total cell
resistance which includes electronic ionic and contact resistance
R = Relectronic + Rionic + Rcontact
Mass Transport Related Losses As a reactant is consumed at the
electrode by electrochemical reaction it is often diluted by the products
when finite mass transport rates limit the supply of fresh reactant and the
evacuation of products As a consequence a concentration gradient is formed
which drives the mass transport process
Concentration polarization can be denoted by
ln(1 )consL
RT inF i
η = minus (244)
where consη is concentration polarization(mV) iL is limiting current
density(mAcm2) Same as the activation polarization the concentration
polarization occurs independently at either electrode
( ) ( ) ( )conc cell conc anode conc cathodeη η η= + (245)
15
The combined effect of the losses for a given cell and given
operating conditions can be expressed as polarizations The total polarization
at the electrodes is the sum of actη and concη or
anode acta a conc aη η η= + (246)
and
cathode acta c conc cη η η= + (247)
The effect of polarization is to shift the potential of the electrode
(Eelectrode) to a new value (Velectrode)
electrode electrode electrodeV E η= plusmn (248)
For the anode
anode anode anodeV E η= + (249)
And for the cathode
cathode cathode cathodeV E η= minus (2410)
The net result of current flow in a fuel cell is to increase the anode
potential and to decrease the cathode potential thereby reducing the cell
voltage
The cell voltage includes the contribution of the anode and cathode
potentials and ohmic polarization
16
cell cathode anodeV V V iR= minus minus (2411)
When Equations (249) and (2410) are substituted in Equation (2411)
( )cell cathode cathode anode anodeV E E iRη η= minus minus + minus (2412)
Or
cell e cathode anodeV E iRη η= ∆ minus minus minus (2413)
Where e cathode anodeE E E∆ = minus Equation (2413) shows that current
flow in a fuel cell results in a decrease in cell voltage because of losses by
electrode and ohmic polarizations The goal of fuel cell developers is to
minimize the polarization so that cellV approaches eE∆ This goal is
approached by modifications to fuel cell design (improvement in electrode
structures better electro-catalysts more conductive electrolyte thinner cell
components etc) [1]
17
Figure 241 Typical Fuel Cell Polarization Curve
18
31 Polycrystalline Platinum Preparation
A polycrystalline platinum electrode was prepared with pulse
electrodeposition on Pt- quartz crystal
1) Working electrode Pt quartz crystal counter electrode Pt wire
reference electrode saturated calomel electrode (SCE)
2) Pulse electrodeposition method ton -03V 100ms and toff 0V 300ms
3) Deposition bath 10mM H2PtCl6 + 01M HCl (65)
Figure 31 shows the cross-section diagram of the platinized
polycrystalline platinum quartz crystal
Polycrystalline Pt
Sputtered Pt Sputtered Ti
9MHz At-cut quartz crystal
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
3 EXPERIMENTAL
19
32 Electrode Preparation and the Modification
A preparation procedure of catalyst ink was well described
elsewhere [7] and the catalyst (Pt or PtRu) loading was 3 mgcm2 in the each
electrode A geometric active area was a 5cm2 and catalyst ink was directly
sparied onto carbon paper Simple dipping method was used for the chloride
ion modification Pt cathode on carbon paper was dipped in 1 times 10minus2M KCl
solution for 5min and it was rinsed in ultrapure water followed by drying in
the oven
33 Half Cell Measurements
Figure 331 shows the schematic experimental setup for measuring
of the electrocatalystic activity of Pt electrode oxidationreduction
adsorptiondesorption of hydrogen methanol oxidation The electrochemical
cell body consisted of a glass cylinder capped with a Teflon lid holding all
electrodes The Pt nanocatalyst sprayed on carbon paper (gas diffusion layer)
was used as working electrode (WE) The geometric area of the WE was
02 cm2 A concentric platinized Pt wire was used as counter electrode (CE)
The tip of a Luggin-Haber capillary hosting a SCE reference electrode (RE)
was placed in the middle of two electrodes A poteniostatgalvanostat (EG amp
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
13
only the ohmic losses actually behave as a resistance Multiple phenomena
contribute to irreversible losses in an actual fuel cell
In the V-I diagram especially for low-temperature fuel cells the
effects of the three loss categories are often easy to distinguish as illustrated
in Figure 21
Activation Losses Activation losses are caused by sluggish electrode
kinetics There is a close similarity between electrochemical and chemical
reactions in that both involve an activation energy that must be overcome by
the reacting species In reality activation losses are the result of complex
surface electrochemical reaction steps each of which have their own reaction
rate and activation energy In electrochemical reactions the activation energy
loss for overcoming this barrier is represented as Tafel equation
lnact a b iη = + (241)
where actη is activation polarization(mV) a and b are constants and i is
current density(mAcm2)
( ) ( ) ( )act cell act anode act cathodeη η η= + (242)
Ohmic Polarization Ohmic losses occur because of resistance to the
flow of ions in the electrolyte and resistance to flow of electrons through the
electrode The dominant ohmic losses through the electrolyte are reduced by
14
decreasing the electrode separation and enhancing the ionic conductivity of
the electrolyte Because both the electrolyte and fuel cell electrodes obey
Ohms law the ohmic losses can be expressed by the equation
ohm iRη = (243)
where i is the current flowing through the cell and R is the total cell
resistance which includes electronic ionic and contact resistance
R = Relectronic + Rionic + Rcontact
Mass Transport Related Losses As a reactant is consumed at the
electrode by electrochemical reaction it is often diluted by the products
when finite mass transport rates limit the supply of fresh reactant and the
evacuation of products As a consequence a concentration gradient is formed
which drives the mass transport process
Concentration polarization can be denoted by
ln(1 )consL
RT inF i
η = minus (244)
where consη is concentration polarization(mV) iL is limiting current
density(mAcm2) Same as the activation polarization the concentration
polarization occurs independently at either electrode
( ) ( ) ( )conc cell conc anode conc cathodeη η η= + (245)
15
The combined effect of the losses for a given cell and given
operating conditions can be expressed as polarizations The total polarization
at the electrodes is the sum of actη and concη or
anode acta a conc aη η η= + (246)
and
cathode acta c conc cη η η= + (247)
The effect of polarization is to shift the potential of the electrode
(Eelectrode) to a new value (Velectrode)
electrode electrode electrodeV E η= plusmn (248)
For the anode
anode anode anodeV E η= + (249)
And for the cathode
cathode cathode cathodeV E η= minus (2410)
The net result of current flow in a fuel cell is to increase the anode
potential and to decrease the cathode potential thereby reducing the cell
voltage
The cell voltage includes the contribution of the anode and cathode
potentials and ohmic polarization
16
cell cathode anodeV V V iR= minus minus (2411)
When Equations (249) and (2410) are substituted in Equation (2411)
( )cell cathode cathode anode anodeV E E iRη η= minus minus + minus (2412)
Or
cell e cathode anodeV E iRη η= ∆ minus minus minus (2413)
Where e cathode anodeE E E∆ = minus Equation (2413) shows that current
flow in a fuel cell results in a decrease in cell voltage because of losses by
electrode and ohmic polarizations The goal of fuel cell developers is to
minimize the polarization so that cellV approaches eE∆ This goal is
approached by modifications to fuel cell design (improvement in electrode
structures better electro-catalysts more conductive electrolyte thinner cell
components etc) [1]
17
Figure 241 Typical Fuel Cell Polarization Curve
18
31 Polycrystalline Platinum Preparation
A polycrystalline platinum electrode was prepared with pulse
electrodeposition on Pt- quartz crystal
1) Working electrode Pt quartz crystal counter electrode Pt wire
reference electrode saturated calomel electrode (SCE)
2) Pulse electrodeposition method ton -03V 100ms and toff 0V 300ms
3) Deposition bath 10mM H2PtCl6 + 01M HCl (65)
Figure 31 shows the cross-section diagram of the platinized
polycrystalline platinum quartz crystal
Polycrystalline Pt
Sputtered Pt Sputtered Ti
9MHz At-cut quartz crystal
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
3 EXPERIMENTAL
19
32 Electrode Preparation and the Modification
A preparation procedure of catalyst ink was well described
elsewhere [7] and the catalyst (Pt or PtRu) loading was 3 mgcm2 in the each
electrode A geometric active area was a 5cm2 and catalyst ink was directly
sparied onto carbon paper Simple dipping method was used for the chloride
ion modification Pt cathode on carbon paper was dipped in 1 times 10minus2M KCl
solution for 5min and it was rinsed in ultrapure water followed by drying in
the oven
33 Half Cell Measurements
Figure 331 shows the schematic experimental setup for measuring
of the electrocatalystic activity of Pt electrode oxidationreduction
adsorptiondesorption of hydrogen methanol oxidation The electrochemical
cell body consisted of a glass cylinder capped with a Teflon lid holding all
electrodes The Pt nanocatalyst sprayed on carbon paper (gas diffusion layer)
was used as working electrode (WE) The geometric area of the WE was
02 cm2 A concentric platinized Pt wire was used as counter electrode (CE)
The tip of a Luggin-Haber capillary hosting a SCE reference electrode (RE)
was placed in the middle of two electrodes A poteniostatgalvanostat (EG amp
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
14
decreasing the electrode separation and enhancing the ionic conductivity of
the electrolyte Because both the electrolyte and fuel cell electrodes obey
Ohms law the ohmic losses can be expressed by the equation
ohm iRη = (243)
where i is the current flowing through the cell and R is the total cell
resistance which includes electronic ionic and contact resistance
R = Relectronic + Rionic + Rcontact
Mass Transport Related Losses As a reactant is consumed at the
electrode by electrochemical reaction it is often diluted by the products
when finite mass transport rates limit the supply of fresh reactant and the
evacuation of products As a consequence a concentration gradient is formed
which drives the mass transport process
Concentration polarization can be denoted by
ln(1 )consL
RT inF i
η = minus (244)
where consη is concentration polarization(mV) iL is limiting current
density(mAcm2) Same as the activation polarization the concentration
polarization occurs independently at either electrode
( ) ( ) ( )conc cell conc anode conc cathodeη η η= + (245)
15
The combined effect of the losses for a given cell and given
operating conditions can be expressed as polarizations The total polarization
at the electrodes is the sum of actη and concη or
anode acta a conc aη η η= + (246)
and
cathode acta c conc cη η η= + (247)
The effect of polarization is to shift the potential of the electrode
(Eelectrode) to a new value (Velectrode)
electrode electrode electrodeV E η= plusmn (248)
For the anode
anode anode anodeV E η= + (249)
And for the cathode
cathode cathode cathodeV E η= minus (2410)
The net result of current flow in a fuel cell is to increase the anode
potential and to decrease the cathode potential thereby reducing the cell
voltage
The cell voltage includes the contribution of the anode and cathode
potentials and ohmic polarization
16
cell cathode anodeV V V iR= minus minus (2411)
When Equations (249) and (2410) are substituted in Equation (2411)
( )cell cathode cathode anode anodeV E E iRη η= minus minus + minus (2412)
Or
cell e cathode anodeV E iRη η= ∆ minus minus minus (2413)
Where e cathode anodeE E E∆ = minus Equation (2413) shows that current
flow in a fuel cell results in a decrease in cell voltage because of losses by
electrode and ohmic polarizations The goal of fuel cell developers is to
minimize the polarization so that cellV approaches eE∆ This goal is
approached by modifications to fuel cell design (improvement in electrode
structures better electro-catalysts more conductive electrolyte thinner cell
components etc) [1]
17
Figure 241 Typical Fuel Cell Polarization Curve
18
31 Polycrystalline Platinum Preparation
A polycrystalline platinum electrode was prepared with pulse
electrodeposition on Pt- quartz crystal
1) Working electrode Pt quartz crystal counter electrode Pt wire
reference electrode saturated calomel electrode (SCE)
2) Pulse electrodeposition method ton -03V 100ms and toff 0V 300ms
3) Deposition bath 10mM H2PtCl6 + 01M HCl (65)
Figure 31 shows the cross-section diagram of the platinized
polycrystalline platinum quartz crystal
Polycrystalline Pt
Sputtered Pt Sputtered Ti
9MHz At-cut quartz crystal
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
3 EXPERIMENTAL
19
32 Electrode Preparation and the Modification
A preparation procedure of catalyst ink was well described
elsewhere [7] and the catalyst (Pt or PtRu) loading was 3 mgcm2 in the each
electrode A geometric active area was a 5cm2 and catalyst ink was directly
sparied onto carbon paper Simple dipping method was used for the chloride
ion modification Pt cathode on carbon paper was dipped in 1 times 10minus2M KCl
solution for 5min and it was rinsed in ultrapure water followed by drying in
the oven
33 Half Cell Measurements
Figure 331 shows the schematic experimental setup for measuring
of the electrocatalystic activity of Pt electrode oxidationreduction
adsorptiondesorption of hydrogen methanol oxidation The electrochemical
cell body consisted of a glass cylinder capped with a Teflon lid holding all
electrodes The Pt nanocatalyst sprayed on carbon paper (gas diffusion layer)
was used as working electrode (WE) The geometric area of the WE was
02 cm2 A concentric platinized Pt wire was used as counter electrode (CE)
The tip of a Luggin-Haber capillary hosting a SCE reference electrode (RE)
was placed in the middle of two electrodes A poteniostatgalvanostat (EG amp
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
15
The combined effect of the losses for a given cell and given
operating conditions can be expressed as polarizations The total polarization
at the electrodes is the sum of actη and concη or
anode acta a conc aη η η= + (246)
and
cathode acta c conc cη η η= + (247)
The effect of polarization is to shift the potential of the electrode
(Eelectrode) to a new value (Velectrode)
electrode electrode electrodeV E η= plusmn (248)
For the anode
anode anode anodeV E η= + (249)
And for the cathode
cathode cathode cathodeV E η= minus (2410)
The net result of current flow in a fuel cell is to increase the anode
potential and to decrease the cathode potential thereby reducing the cell
voltage
The cell voltage includes the contribution of the anode and cathode
potentials and ohmic polarization
16
cell cathode anodeV V V iR= minus minus (2411)
When Equations (249) and (2410) are substituted in Equation (2411)
( )cell cathode cathode anode anodeV E E iRη η= minus minus + minus (2412)
Or
cell e cathode anodeV E iRη η= ∆ minus minus minus (2413)
Where e cathode anodeE E E∆ = minus Equation (2413) shows that current
flow in a fuel cell results in a decrease in cell voltage because of losses by
electrode and ohmic polarizations The goal of fuel cell developers is to
minimize the polarization so that cellV approaches eE∆ This goal is
approached by modifications to fuel cell design (improvement in electrode
structures better electro-catalysts more conductive electrolyte thinner cell
components etc) [1]
17
Figure 241 Typical Fuel Cell Polarization Curve
18
31 Polycrystalline Platinum Preparation
A polycrystalline platinum electrode was prepared with pulse
electrodeposition on Pt- quartz crystal
1) Working electrode Pt quartz crystal counter electrode Pt wire
reference electrode saturated calomel electrode (SCE)
2) Pulse electrodeposition method ton -03V 100ms and toff 0V 300ms
3) Deposition bath 10mM H2PtCl6 + 01M HCl (65)
Figure 31 shows the cross-section diagram of the platinized
polycrystalline platinum quartz crystal
Polycrystalline Pt
Sputtered Pt Sputtered Ti
9MHz At-cut quartz crystal
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
3 EXPERIMENTAL
19
32 Electrode Preparation and the Modification
A preparation procedure of catalyst ink was well described
elsewhere [7] and the catalyst (Pt or PtRu) loading was 3 mgcm2 in the each
electrode A geometric active area was a 5cm2 and catalyst ink was directly
sparied onto carbon paper Simple dipping method was used for the chloride
ion modification Pt cathode on carbon paper was dipped in 1 times 10minus2M KCl
solution for 5min and it was rinsed in ultrapure water followed by drying in
the oven
33 Half Cell Measurements
Figure 331 shows the schematic experimental setup for measuring
of the electrocatalystic activity of Pt electrode oxidationreduction
adsorptiondesorption of hydrogen methanol oxidation The electrochemical
cell body consisted of a glass cylinder capped with a Teflon lid holding all
electrodes The Pt nanocatalyst sprayed on carbon paper (gas diffusion layer)
was used as working electrode (WE) The geometric area of the WE was
02 cm2 A concentric platinized Pt wire was used as counter electrode (CE)
The tip of a Luggin-Haber capillary hosting a SCE reference electrode (RE)
was placed in the middle of two electrodes A poteniostatgalvanostat (EG amp
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
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51
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[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
16
cell cathode anodeV V V iR= minus minus (2411)
When Equations (249) and (2410) are substituted in Equation (2411)
( )cell cathode cathode anode anodeV E E iRη η= minus minus + minus (2412)
Or
cell e cathode anodeV E iRη η= ∆ minus minus minus (2413)
Where e cathode anodeE E E∆ = minus Equation (2413) shows that current
flow in a fuel cell results in a decrease in cell voltage because of losses by
electrode and ohmic polarizations The goal of fuel cell developers is to
minimize the polarization so that cellV approaches eE∆ This goal is
approached by modifications to fuel cell design (improvement in electrode
structures better electro-catalysts more conductive electrolyte thinner cell
components etc) [1]
17
Figure 241 Typical Fuel Cell Polarization Curve
18
31 Polycrystalline Platinum Preparation
A polycrystalline platinum electrode was prepared with pulse
electrodeposition on Pt- quartz crystal
1) Working electrode Pt quartz crystal counter electrode Pt wire
reference electrode saturated calomel electrode (SCE)
2) Pulse electrodeposition method ton -03V 100ms and toff 0V 300ms
3) Deposition bath 10mM H2PtCl6 + 01M HCl (65)
Figure 31 shows the cross-section diagram of the platinized
polycrystalline platinum quartz crystal
Polycrystalline Pt
Sputtered Pt Sputtered Ti
9MHz At-cut quartz crystal
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
3 EXPERIMENTAL
19
32 Electrode Preparation and the Modification
A preparation procedure of catalyst ink was well described
elsewhere [7] and the catalyst (Pt or PtRu) loading was 3 mgcm2 in the each
electrode A geometric active area was a 5cm2 and catalyst ink was directly
sparied onto carbon paper Simple dipping method was used for the chloride
ion modification Pt cathode on carbon paper was dipped in 1 times 10minus2M KCl
solution for 5min and it was rinsed in ultrapure water followed by drying in
the oven
33 Half Cell Measurements
Figure 331 shows the schematic experimental setup for measuring
of the electrocatalystic activity of Pt electrode oxidationreduction
adsorptiondesorption of hydrogen methanol oxidation The electrochemical
cell body consisted of a glass cylinder capped with a Teflon lid holding all
electrodes The Pt nanocatalyst sprayed on carbon paper (gas diffusion layer)
was used as working electrode (WE) The geometric area of the WE was
02 cm2 A concentric platinized Pt wire was used as counter electrode (CE)
The tip of a Luggin-Haber capillary hosting a SCE reference electrode (RE)
was placed in the middle of two electrodes A poteniostatgalvanostat (EG amp
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
17
Figure 241 Typical Fuel Cell Polarization Curve
18
31 Polycrystalline Platinum Preparation
A polycrystalline platinum electrode was prepared with pulse
electrodeposition on Pt- quartz crystal
1) Working electrode Pt quartz crystal counter electrode Pt wire
reference electrode saturated calomel electrode (SCE)
2) Pulse electrodeposition method ton -03V 100ms and toff 0V 300ms
3) Deposition bath 10mM H2PtCl6 + 01M HCl (65)
Figure 31 shows the cross-section diagram of the platinized
polycrystalline platinum quartz crystal
Polycrystalline Pt
Sputtered Pt Sputtered Ti
9MHz At-cut quartz crystal
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
3 EXPERIMENTAL
19
32 Electrode Preparation and the Modification
A preparation procedure of catalyst ink was well described
elsewhere [7] and the catalyst (Pt or PtRu) loading was 3 mgcm2 in the each
electrode A geometric active area was a 5cm2 and catalyst ink was directly
sparied onto carbon paper Simple dipping method was used for the chloride
ion modification Pt cathode on carbon paper was dipped in 1 times 10minus2M KCl
solution for 5min and it was rinsed in ultrapure water followed by drying in
the oven
33 Half Cell Measurements
Figure 331 shows the schematic experimental setup for measuring
of the electrocatalystic activity of Pt electrode oxidationreduction
adsorptiondesorption of hydrogen methanol oxidation The electrochemical
cell body consisted of a glass cylinder capped with a Teflon lid holding all
electrodes The Pt nanocatalyst sprayed on carbon paper (gas diffusion layer)
was used as working electrode (WE) The geometric area of the WE was
02 cm2 A concentric platinized Pt wire was used as counter electrode (CE)
The tip of a Luggin-Haber capillary hosting a SCE reference electrode (RE)
was placed in the middle of two electrodes A poteniostatgalvanostat (EG amp
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
18
31 Polycrystalline Platinum Preparation
A polycrystalline platinum electrode was prepared with pulse
electrodeposition on Pt- quartz crystal
1) Working electrode Pt quartz crystal counter electrode Pt wire
reference electrode saturated calomel electrode (SCE)
2) Pulse electrodeposition method ton -03V 100ms and toff 0V 300ms
3) Deposition bath 10mM H2PtCl6 + 01M HCl (65)
Figure 31 shows the cross-section diagram of the platinized
polycrystalline platinum quartz crystal
Polycrystalline Pt
Sputtered Pt Sputtered Ti
9MHz At-cut quartz crystal
Figure 311 Cross-section diagram of the platinized polycrystalline
Platinum on Platinum quartz crystal
3 EXPERIMENTAL
19
32 Electrode Preparation and the Modification
A preparation procedure of catalyst ink was well described
elsewhere [7] and the catalyst (Pt or PtRu) loading was 3 mgcm2 in the each
electrode A geometric active area was a 5cm2 and catalyst ink was directly
sparied onto carbon paper Simple dipping method was used for the chloride
ion modification Pt cathode on carbon paper was dipped in 1 times 10minus2M KCl
solution for 5min and it was rinsed in ultrapure water followed by drying in
the oven
33 Half Cell Measurements
Figure 331 shows the schematic experimental setup for measuring
of the electrocatalystic activity of Pt electrode oxidationreduction
adsorptiondesorption of hydrogen methanol oxidation The electrochemical
cell body consisted of a glass cylinder capped with a Teflon lid holding all
electrodes The Pt nanocatalyst sprayed on carbon paper (gas diffusion layer)
was used as working electrode (WE) The geometric area of the WE was
02 cm2 A concentric platinized Pt wire was used as counter electrode (CE)
The tip of a Luggin-Haber capillary hosting a SCE reference electrode (RE)
was placed in the middle of two electrodes A poteniostatgalvanostat (EG amp
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
19
32 Electrode Preparation and the Modification
A preparation procedure of catalyst ink was well described
elsewhere [7] and the catalyst (Pt or PtRu) loading was 3 mgcm2 in the each
electrode A geometric active area was a 5cm2 and catalyst ink was directly
sparied onto carbon paper Simple dipping method was used for the chloride
ion modification Pt cathode on carbon paper was dipped in 1 times 10minus2M KCl
solution for 5min and it was rinsed in ultrapure water followed by drying in
the oven
33 Half Cell Measurements
Figure 331 shows the schematic experimental setup for measuring
of the electrocatalystic activity of Pt electrode oxidationreduction
adsorptiondesorption of hydrogen methanol oxidation The electrochemical
cell body consisted of a glass cylinder capped with a Teflon lid holding all
electrodes The Pt nanocatalyst sprayed on carbon paper (gas diffusion layer)
was used as working electrode (WE) The geometric area of the WE was
02 cm2 A concentric platinized Pt wire was used as counter electrode (CE)
The tip of a Luggin-Haber capillary hosting a SCE reference electrode (RE)
was placed in the middle of two electrodes A poteniostatgalvanostat (EG amp
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
20
G 273A) was used for all cyclic voltammerty (CV) experiments and the data
were transferred to an IBM compatible PC controlled by a GPIB interface
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
21
Figure 331 Schematic diagram of experimental set-up for half-cell
experiments
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
22
34 Single cell measurements
Prior to fabricating electrodes the Nafion 115 (DuPont) membrane
was boiled in 2wt H2O2 solution for 1 h then it was rinsed in boiling
deionized (DI) water for 2h In order to remove metallic contaminants on the
membrane surface and exchange Na+ for H+ in the membrane it was boiled
in 3M H2SO4 for 1h Finally it was rinsed again in boiling deionized (DI)
water for 2h In order to fabricate MEAs the electrocatalysts and 50 wt
nafion solution (DuPont) were thoroughly mixed in an ultrasonic bath The
anode and a cathode catalyst used was PtRu Black and Pt Black The
catalytic layer is applied by spraying the catalytic ink on the gas diffusion
layer The catalyst ink is prepared by catalyst mixing with ionomer solution
water and iso-propyl alcohol in an ultrasonic bath for 15min The resulting
slurry is sprayed onto the gas diffusion layer up to a loading of 3mgPtcm2
for each electrode and dried at 60 in a convection oven To improve the
interface contact between the catalytic coated backing and the membrane a
thin layer of ionomer solution is sprayed on the electrode surface The MEA
is assembled by hot-pressing the catalytic coated substrate on a pre-treated
polymer electrolyte membrane at a temperature of 140 and with a pressure
of 160 kgfcm2 for 240 sec The electrode area of the MEA was 5cm2
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
23
All experiments including electrochemical measurements were
conducted with cells which consisted of MEAs sandwiched between two
graphite flow field plates Current-voltage curves were measured
galvanostatically by using an electronic load (EL-200P Daegil Electronics)
Flow rate of oxygen and air was controlled by mass flow meter And
temperature controller adjusted humidifier of anode and cathode In a long-
term operation constant current density was loaded and galvanostatic
current-potential profile Figure 342 shows the schematic experimental
setup for measuring of power performance of the system
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
24
Fig 341 Configuration of catalystdiffusion backing and membrane for
type a MEA
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
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[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
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[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
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[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
25
Fig 342 Schematic diagram of experimental set-up for single cell
measurements
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
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[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
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[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
26
A cyclic voltammogram of polycrystalline Pt in 05M H2SO4
electrolyte is shown in Figure 411 The geometric of electrode is 0196cm2
In the voltammogram it is possible to identify different processes that occur
during the potential range between -025 and 125 V (vs SCE)
At potentials more negative than the under potential limit -025V
hydrogen evolution takes place Likewise at potentials more positive than the
upper limit 125 V oxygen evolution starts
Prior to the hydrogen evolution two peaks are due to hydrogen
adsorption onto the Pt surface The hydrogen ions in the electrolyte are
reduced by the reaction
H3O+ + e- rarr Pt-Hads + H2O (411)
The peak to the right is attributed to the so-called strongly bonded
[17] hydrogen and is also assigned to (100) sites The left peak is similarly
attributed to weakly bonded [17] hydrogen and is assigned to (110) sites As
this is a reversible reaction two peaks appear symmetrically in the reverse
4 RESULTS AND DISCUSSION
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
27
scan which are marked as region B These peaks are due to the hydrogen
desorption from the Pt surface
After the hydrogen region when looking in the direction of the
positive potential there is a region between approximately 015 and 05V C
where almost no current flows This region is called the double layer region
This is the region where very limited reactions take place When the potential
of the working electrode is changed during the scan a capacitive current
flows through the system The electrolytic double layer at the electrode
surface can be considered as a parallel plate capacitor The current that flows
in this potential region charges and discharges the double layer When the
potential is further increased formation of the monolayer oxide commences
just above 06 V which is marked as region D in Figure 7 This behavior is
close to that expected on the basis of thermodynamic data [18] The oxide
formation reaction whose Edeg = 07V in acid solution can be written as
Pt + H2O rarr Pt-O + 2H+ + 2e- (412)
This hydrous Pt oxide film can be further oxidized From the
thermodynamic viewpoint this will happen at 0765 V according to [18]
Pt-O + H2O rarr Pt-O2 + 2H+ + 2e- (413)
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
28
After reversing the scan at 125 V the oxide is reduced at ca 085V
which is indicated as region E in Figure 411
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
29
-04 -02 00 02 04 06 08 10 12 14
-10
-8
-6
-4
-2
0
2
4
6
Cur
rent
den
sity
(mA
cm2 )
E
D
C
Potential (V vs SCE)
A
BO2
H2
Figure 411 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 at a scan rate of 50 mVs
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
30
Figures 412 (a) and (b) are cyclic current-potential profile of a pure
Pt and chloride ion modified Pt surface (PtClmminus) in 05 M H2SO4 with scan
rate 50mVs respectively Figure 412 (a) shows typical hydrogen
adsorptiondesorption peaks and oxidationreduction peaks of Pt on the
anodic and cathodic scan On the other hand we obtain slightly different
current-potential profile of modified Pt surface The desorptionadsorption of
chloride ion changes the hydrogen adsorptiondesorption reaction and PtClmminus
suppress the adsorption of oxygen containing species consistent with
previous reports on polycrystalline Pt [8 19] In addition the smaller
reduction peak occurs at a little bit earlier cathodic overpotential of 043 V
which might be attributed to the weaker binding energy between Pt surface
and oxygen containing species and PtClmminus has more doubly layer charge
compared with pure Pt surface
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
31
-03 00 03 06 09 12
-15
-10
-5
0
5
10 (b)(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 412 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 05 M
H2SO4 Scan rate = 50mVs
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
32
On Pt surface Figure 413 shows that methanol dissociates to COad
like reaction (121) With anodic scanning COad reacts with OHad
generated from adsorbed H2O Thus high current generated through methanol
oxidation At ca 095V current peak rapidly decreases because of the Pt
oxide formation On cathodic scan high current peak re-occured since Pt
surface is cleared due to Pt oxide removal and in this case the same reaction
mechanism applied Since the CO poisoning on the Pt surface blocks the
hydrogen adsorption and desorption in methanol solution current was
decreased at hydrogen adsorption and desorption region in case of methanol
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
33
-04 -02 00 02 04 06 08 10 12 14-20
0
20
40
60
80
100
120
140
160
Cur
rent
den
sity
(mA
cm
2 )
Potential vs SCE
Figure 413 Basic voltammogram of polycrystalline platinum in 05 M
H2SO4 05M CH3OH at a scan rate of 50 mVs
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
34
Figure 414 illustrates the effect of chloride ion modification in the
methanol oxidation on Pt electrode Figure 414 (a) shows the typical CV
profile with internal resistance on an unmodified Pt electrode while no
electro-oxidation current of CH3OH on chloride ion modified electrode is
observed (see Figure 414 (b)) In order to oxidize methanol andor CO on a
Pt surface via Langmuir-Hinselwood mechanism the adsorption of oxygen
containing species is needed Therefore the experimental observations in
Figure 414 (b) are clearly understood that adsorbed chloride ions inhibits
the adsorption of oxygenated species on the anodic scan and it further
suppress the oxidation reaction of methanol
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
35
-03 00 03 06 09 12 15 18-30
0
30
60
90
120
(b)
(a)
Cur
rent
den
sity
(mA
cm
2 )
Potential (V vs SCE)
Figure 414 Cyclic voltammogram of (a) Pt and (b) PtClmminus in 10M
CH3OH05M H2SO4 Scan rate = 50mVs
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
36
The result indicates that in a single cell measurement crossover
methanol could be not oxidized by hindering effect of adsorbed chloride ion
on a Pt surface It could improve the cell performance and induces less fuel
loss As mentioned in previous study [1] no one tried to study the influence
of chloride ions contamination on fuel cell performance This experiment
investigated effects of the modified electrode operation condition in
performance Operation condition of DMFC is temperature concentration of
fuel and using modified electrode humidity of cathode and gas diffusion
layer Also life time of DMFC investigated at constant current density under
above operation condition
For the performance test of direct methanol fuel cell active area of
MEA was 5cm2 In this experiments methanol solutions (05M 1M and 2M)
were flowed through the anode at a rate of 5mLmin Air was supplied to the
cathode at a flow rate of 500mLmin without backpressure humidified to
80 The temperature of single cell was controlled 80 Before the single
cell test the cell was conditioned with methanol For the conditioning the
cell was operated in polarization condition 05M methanol was fed to the
anode at the rate of 5mLmin and O2 humidified to 80 was then fed to the
cathode at the rate of 250mLmin After the cell polarization was carried out
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
37
The methanol was flushed from the cell with deionized water before
performance test
Figure 415 shows single cell performances at 80 with different
methanol concentration which is 05M 1M 2M When oxygen feed to
cathode performances were 100 120 200mWcm2 reduced about 25
because of oxygen partial pressure Figure 416 shows also single cell
performance above same condition at air feed for cathode But max power
density was reduced half of oxygen feed because of oxygen partial pressure
And at air feed Max power density was almost same value both 1M and 2M
methanol concentration It was caused methanol crossover to cathode
inhibited oxygen reduction
We investigated effects of different concentration with modified
chloride ion at single cell test Figure 417 shows concentration decreased to
001M 0001M and 00001M at 2M methanol feed the performance reduced
gradually And we founded proper concentration of modified solution to
001M KCl because of performance using 001M KCl modified electrode
was max powerdensity 18mWcm2 than it which was using pure Pt electrode
And when 2M methanol feed to anode the performance was the better than
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
38
1M methanol Because crossoverd methanol oxidation was inhibited by
chloride modified Pt cathode
Figure 418(a) is voltage change and power density with current
density when PtRu anode and unmodified Pt cathode are used The open-
circuit-voltage and maximum power density are 066 V and ca 134 mWcm2
respectively On the other hand chloride ion modified Pt (PtClmminus) increases
the power density of the cell up to 173 mAcm2 even if OCV is lower than a
pure Pt cathode It is about 30 higher performance than that used a
conventional cathode Its results are due to suppressed electrochemical
oxidation rate of crossover methanol on a cathode surface In other words the
performance of a single cell is strongly dependent of the amount of crossover
methanol ie the activity of cathode to oxygen reduction reaction in the
presence of methanol on a Pt cathode and inactivity to electrochemical
oxidation of methanol
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
39
00 01 02 03 04 05 06 07 08 09 10 1100
01
02
03
04
05
06
07
08
09
0
20
40
60
80
100
120
140
160
180
200
220
Vol
tage
(V)
Current density(Acm2)
(a)
(b)
(c)
pow
er d
ensi
ty (m
Wc
m2 )
Figure 415 The influence of various concentration on single cell
performance from the DMFC at 80degC and oxygen feed
(a) 05M (b) 1M (c) 2M Methanol Methanol flow rate
= 5 ccmin Humidified air flow rate = 250 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
40
00 01 02 03 04 05 06 0701
02
03
04
05
06
07
Vol
tage
(V)
Current density (mAcm2)
(a)
(b)(c)
0
20
40
60
80
100
120
140
Pow
er D
ensi
ty (m
Wc
m2 )
Figure 416 The influence of various concentration on single cell
performance from the DMFC at 80degC and air feed (a)
05M (b) 1M (c) 2M Methanol Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
41
00 01 02 03 04 05 06 07
0
20
40
60
80
100
120
01
02
03
04
05
06
Pow
er d
ensi
ty (m
Wc
m2 )
Current density(Acm2)
(a)(b)(c)
(d)
Vol
tage
(V)
Figure 417 Single cell performance of DMFC at 80degC (a) without and
(b) with 001M KCl (c) with 0001M KCl (d) with
00001M KCl cathode modification 2M methanol flow
rate = 5 ccmin Humidified air flow rate = 1000 ccmin
Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt
(cathode) = 3 mgcm2
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
42
00 01 02 03 04 05 06 07 08
01
02
03
04
05
06
07 (b)
(a)
Current density (mAcm2)
Vol
tage
(V)
0
30
60
90
120
150
180
Pow
er d
ensi
ty (m
Wc
m2 )
Figure 418 Single cell performance of DMFC at 80degC (a) without and
(b) with cathode modification Methanol flow rate =
5 ccmin Humidified air flow rate = 1000 ccmin Electrode
area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
43
Impedance measurements was useful method as a diagnostic tool to
investigate DMFC performance issues This method enables researchers to
gain insight into the effects of electrode morphology and operating
parameters to obtain detailed information about degradation phenomena and
will help to tailor-make DMFC electrodes with improved performance
Figure 419 shows the impedance plot for the DMFC with chloride
ion electrode and without electrode measured at a 05Acm2 And it shows
the electrode impedance plots obtained at single cell with chloride ion
modified electrode and pure electrode when using air and oxygen
respectively The size of the plot decreased with increasing chloride
concentration up to 0001M and 001M for air respectively and the shape of
the plot remained similar in the figure At high frequency the plots are
obviously related to an ohmic process And the size of the semicircle at low
frequency for modified electrode was significantly smaller than that for the
pure electrode This may be related to the charge transfer of oxygen reduction
reation at the cathode However further study is needed to relate the shape of
the impedance plot and the reaction mechanism in detail
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
44
50 100 150 2000
50
100
(c)(b) (a)
-Im Z
m
Ω c
m2
ReZ mΩ cm2
Figure 419 DMFC impedance plots measured at different concentration
chloride ion modification at cathode (a) wo chloride ion
modification (b) 001M Clminus (c) 0001 M Clminus Electrode area
5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M
CH3OH feed = 5 mlmin air feed = 500 ccmin
temperature 60
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
45
Figure 4110 shows the cell voltage of 034 V and it continuously
decreases until 030 V within two days and thus its degradation rate is about
083 mVhr Figure 4110(b) is the voltage transients at a loading of
100 mAcm2 and 150 mAcm2 as shown in region (i) and (ii) At a 100 and
150 mAcm2 the cell voltage maintains 037 and 035 V representing the
power performance between 37 and 53 mWcm2 Comparing with a long-
term stability of the cell used pure Pt cathode chloride ions modified Pt
cathode sustains at least 5 times longer activity
The life test was carried out at a variety of operational condition as
shown in figure 4111 temperature of 60C The cell voltage decreases with
test time and the voltage loss can be partially recovered after every
intermission (for exchanging current density) Although the intermission may
be as short as several second after that the cell voltage immediately turned
back nearly to the beginning value So it is suggested that there exists a short-
term performance loss which is a rapid and reversible process The
mechanism of short-term performance loss is not very clear
On the other hand there is a slow performance loss that is
irrecoverable which might relate to the degradation of electrocatalysts and
the polymer electrolyte membrane
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
46
Fig 4111 showed cell performance from three operational condition
which is that 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2
(c) 05M CH3OH 100mAcm2 during the time range of 350h There is a
long-term stability of the cell used chloride ions modified Pt cathode
sustained at least 2 times longer activity in each condition
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
47
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
0 25 50 75 100 125 150 175 200 22501
02
03
04
05
06
(ii)(i)(b)
(a)Volta
ge (V
)
Time (hrs)
Figure 4110 Cell potential measurement at 100 and 150 mAcm2 (a)
without and (b) with cathode modification Electrode
area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3
mgcm2 Methanol flow rate = 5 ccmin Humidified air
feed rate = 500 ccmin
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
48
0 50 100 150 200 250 300 35000
01
02
03
04
05
06
07
Volta
ge (V
)
Time (hr)
(a) (b) (c)
Figure 4111 Cell potential measurement at various operational
condition (a) 1M CH3OH 150mAcm2 (b) 2M
CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2
CH3OH feed = 5mlmin air feed = 500 ccmin Temp
60 Humidity 80
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
49
Methanol crossover is major problem for the commercialization of
DMFC since it leads to a serious decrease in the performance of the cathode
ie lower overall fuel cell efficiency ORR on the cathode in DMFC
operation is greatly affected by the ability of Pt to electrochemical oxidation
of methanol reached from anode This problem could be overcome by the
development of new cathode that is inactive for methanol oxidation
Modified cathode tolerance to electro-oxidation of methanol was developed
by simple dipping method Both of single-cell activity and long-term stability
of PtClmminus are better than those from a pure Pt cathode
5 CONCLUSIONS
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
50
[1] Gregor Hoogers Fuel Cell Technology Handbook (2003)
[2] Handbook of Fuel Cells Fundamentals Technology and Applications
Volume 2 Electrocatlysis Editors Wolf Vielstich Arnold Lamm Hubert
A Gasteiger John Wiley amp Sons Ltd (2003)
[3] Carrette K A Friedrich U Stimming Fuel Cells 1 (2001) 5
[4] K Scott W M Taama P Argyropoulos K Sundmacher J Power
Sources 83 (1999) 204-216
[5] C K Dyer J Power Sources 106 (2002) 31-34
[6] A Heinzel C Hebling M Muumlller M Zedda C Muumlller J Power
Sources 105 (2002) 250-255
[7] J Pavio J Hallmark J Bostaph A Fisher B Mylan C G Xie Fuel
Cells Bulletin 2002 (2002) 8
[8] A Hamnett Catalysis Today 39 (1997) 445
[9] M P Hogarth and G A Hards Platinum Metal Rev 40(1996) 150
[10] Arico Srinivasan Antonucci FUEL CELLS 1 (2001) No 2
6 REFERENCES
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
51
[11] K Chandrasekaran J C Wass and J O M Bockris J Electrochem
Soc 137 (1990) 518
[12] H AGasteiger N Markovic P Ross and E Cairns J Phys Chem 97
(1993) 12020
[13] HN Dinh X Ren FH Garzon P Zelenay S Gottesfeld J
Electroanal Chem 491(2000) 222
[14] J Clavilier in A Wieckowski (Ed) Interfacial Electrochemistry
Marcel Dekker New York (1999) 231
[15] J Clavilier D Armand SG Sun M Petit J Electroanal Chem 205
(1986) 267
[16] F GLOAGUEN J-M LEA GER C LAMY Journal of Applied
Electrochemistry 27 (1997) 1052
[17] J Lipkowski PN Ross Adsorption of Molecules at Metal Electrodes
Chapter 7 VCH Publishers Inc (1992)
[18] LD Burke DT Buckley Russian Journal of Electrochemistry31
(1995) 957
[19] M W Breiter Electrochim Acta 8 (1963) 925-935
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
52
2003 년도 겨울 재료 및 전기화학연구실 문을 열고 들어온 기억이
선한데 벌써 2 년이라는 시간이 흘러 졸업을 앞두게 되었습니다 이기간은
제 인생에서 큰 전환점이 되었습니다 학문적 사회적으로 많은 견문을 넓힐
수 있는 계기였다고 생각합니다
항상 ldquo일은 만들어 나가는 거죠ldquo라며 말씀하셨던 지도교수님이신
탁용석교수님께 머리 숙여 감사 드리며 교수님의 말씀 언제나
기억하겠습니다 항상 따뜻한 가르침으로 제자들을 대해주신 화학공학과
교수님과 늘 대학원생들을 챙겨주신 고동윤조교님께도 감사 드립니다
대학원과정동안 실험실 살림살이를 하시며 저를 더욱 아껴 주셨던
진욱형님과 늘 후배들을 따뜻하게 맞아 주셨던 재광형께 감사 드리며
진욱형의 결혼을 진심으로 축하 드립니다 그리고 2 년동안 동고동락했던
종민형 용식형 은성 성진 은경이와 앞으로 한 식구가 될 재민 유진
준희에게 고맙다는 말을 전하며 하는 일(연구) 모두 잘 되시길
기원하겠습니다
연료전지를 공부하면서 늘 가르침을 주시며 항상 후배들을
챙겨주셨던 재영형과 실험실 모든 선배님들께 감사 드립니다
7 ACKNOELEDGEMENTS
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
53
학부 졸업하기까지 많은 도움을 주었던 대학 동기들 진한 고향의
향기를 느낄 수 있었던 울산 향우회의 선후배님들 그리고 제가 2 년 동안
미술의 세계에 심취할 수 있었던 화우회 선후배님들께 감사 드리며 특히
대학 과정 동안 늘 곁에서 저와 같이 생활했던 우일에게는 고맙다는 말을
전하고 싶습니다
학부 대학원까지 인천에서 생활하였지만 늘 마음의 고향인 웅상
지음회 친구들 동현 상문 상오 동호 정호 민환 호균 성호 일현 경백
재형 기주 형근에게도 고맙다는 말을 전하고 싶습니다
항상 저를 믿고 뒷바라지를 해주신 가족 분들께 감사 드립니다
늘 인천에 있는 조카를 신경 쓰셨던 서울 작은아버지 내외분 조카를 항상
믿고 응원해주신 울산 작은아버지 내외분 서창 작은아버지 내외분 막내
작은아버지 내외분 어릴 적부터 저를 키웠다는 큰 고모님 내외분
작은고모님 내외분께 감사 드리며 항상 가정에 평온함과 즐거운 일만 있길
큰 조카가 바라겠습니다 또한 우리 사촌동생들 모두 건강하고 바르게
자라서 자기가 하고 싶은 일 모두 이루었으면 좋겠습니다
항상 형이자 오빠인 저 때문에 고생한 우리 동생 경화 성환에게
미안하고 고맙다는 말을 전합니다
마지막으로 항상 큰 손자 큰 손자하고 자랑하고 다니시는 우리
할머니와 지금은 약간 편찮으시지만 늘 든든한 믿음으로 보살펴 주신 우리
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-
54
할아버지 할아버지 할머니께 진심으로 감사의 마음을 전합니다 또한 늘
곁에 붙어있지 못했던 큰 아들을 두신 우리 어머니 항상 아들 걱정하시는
마음 이젠 잠시 접어 두시고 건강하시기 바랍니다 앞으로 우리 가족들에게
잘 하는 든든한 큰 아들이 되겠다고 다짐하며 이 논문을 가족에게 바치면서
감사의 글을 마치겠습니다
2006 년 1 월 16 일
또 다른 비상을 위하여
서 명 환
- 목차
-
- 1 Introduction
-
- 11 What is a fuel cell
- 12 Direct methanol fuel cell (DMFC)
- 13 Objectives
-
- 2 Basic Theory
-
- 21 Electro-oxidation of methanol
- 22 Oxygen reduction reaction
- 23 Anionic effects
- 24 Actual performance
-
- 3 Experimental
-
- 31 Polycrystalline Platinum Preparation
- 32 Electrode Preparation and the Modification
- 33 Half Cell Measurements
- 34 Single cell measurements
-
- 4 Results and Discussion
- 5 Conclusions
- 6 References
- 7 Acknowledgements
-
- 표목차
-
- [Table 21] Effect of catalyst promoters on methanol oxidation
-
- 그림목차
-
- [Figure 121] Schematic diagram of a DMFC
- [Figure 241] Typical Fuel Cell Polarization Curve
- [Figure 311] Cross-section diagram of the platinized polycrystalline Platinum on Platinum quartz crystal
- [Figure 331] Schematic diagram of experimental set-up for half-cell experiments
- [Figure 341] Configuration of catalystdiffusion backing and membrane for type a MEA
- [Figure 342] Schematic diagram of experiental set-up for single cell measurements
- [Figure 411] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 at a scan rate of 50 mVs
- [Figure 412] Cyclic voltammogram of (a) Pt and (b) PtClm- in 05 M H2SO4 Scan rate = 50mVs
- [Figure 413] Basic voltammogram of polycrystalline platinum in 05 M H2SO4 05M CH3OH at a scan rate of 50 mVs
- [Figure 414] Cyclic voltammogram of (a) Pt and (b) PtClm- in 10M CH3OH05M H2SO4 Scan rate = 50mVs
- [Figure 415] The influence of various concentration on single cell performance from the DMFC at 80degC and oxygen feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5ccmin Humidified air flow rate = 250 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 416] The influence of various concentration on single cell performance from the DMFC at 80degC and air feed (a) 05M (b) 1M (c) 2M Methanol Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 417] Single cell performance of DMFC at 80degC (a) without and (b) with 001M KCl (c) with 0001M KCl (d) with 00001M KCl cathode modification 2M methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 418] Single cell performance of DMFC at 80degC (a) without and (b) with cathode modification Methanol flow rate = 5 ccmin Humidified air flow rate = 1000 ccmin Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2
- [Figure 419] DMFC impedance plots measured at different concentration chloride ion modification at cathode (a) without chloride ion modification (b) 001M Cl- (c) 0001 M Cl- Electrode area 5 cm2 RuPt (anode) = 3 mgcm2 Pt (cathode) 3 mgcm2 2M CH3OH feed = 5 mlmin air feed = 500 ccmin temperature 60
- [Figure 4110] Cell potential measurement at 100 and 150 mAcm2 (a) without and (b) with cathode modification Electrode area = 5 cm2 PtRu (anode) = 3 mgcm2 Pt (cathode) = 3 mgcm2 Methanol flow rate = 5 ccmin Humidified air feed rate = 500 ccmin
- [Figure 4111] Cell potential measurement at various operational condition (a) 1M CH3OH 150mAcm2 (b) 2M CH3OH 100mAcm2 (c) 05M CH3OH 100mAcm2 CH3OH feed = 5mlmin air feed = 500 ccmin Temp 60 Humidity 80
-