ti

en

les supported on carboneceramic electrode (CCE) are prepared by an

other hand, the problem of methanol crossover from the

acid (FA), a non-toxic, non-explosive and non-flammable

liquid at room temperature has considerable advantages

[6e15]. The feasibility of direct formic acid fuel cells (DFAFCs)

based on proton exchange membranes has been demon-

strated by Masel and co workers [6,16]. Hsing and co workers

is well known that platinum (Pt) is an effective electro-

in oxidation reaction [18,19]. Poisoning COads species can be

oxidatively removed from the Pt surface through a Lang-

muir-Hinshelwood-type surface reaction with neighboring

OHad species electrosorbed from water at more positive

potentials [20]. Thus, alloying of Pt with oxophilic metals

* Corresponding author. Tel./fax: 98 412 4327541.

Avai lab le a t www.sc iencedi rec t .com

w.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 9 5 8 1e9 5 9 0E-mail address: B.Habibi@azaruniv.edu (B. Habibi).anode to the cathode through membrane leads to low system

efficiency. Methanol crossover [1e5], prevents utilization of

high concentration of methanol; the limits generally less than

2 M. As an attractive candidate to replace methanol, formic

catalyst for these oxidations. However, Pt is also highly

sensitive to CO-poisoning: the catalyst surface is progres-

sively poisoned by the adsorbed CO (COads), which is formed

as a result of the stepwise dehydrogenation of these oxidantsDuring the past 20 years, direct methanol fuel cells have been

widely studied and considered as possible power sources for

the portable electronic devices and electric vehicles. These

fuel cells offer a variety of benefits such as high specific energy

and the ready availability and portability of methanol. On the

reduced by a factor of 5 and thus a higher performance can be

obtained when FA is used in place of methanol under the

same conditions.

One of the main problems in proton exchange membrane

fuel cells is the development of anodic materials with high

electroactivity towards the oxidation of FA and methanol. It6 May 2011

Accepted 8 May 2011

Available online 12 June 2011

Keywords:

Platinumenickel nanoparticles

Electrocatalyst

Electrooxidation

Formic acid

Carboneceramic

1. Introduction0360-3199/$ e see front matter Copyright doi:10.1016/j.ijhydene.2011.05.062metry. The results show that the PteNi nanoparticles (Flower like), which are uniformly

dispersed on carboneceramic, are 20e50 nm in diameters. The PteNi/CCE catalyst, which

has excellent electrocatalytic activity for formic acid (FA) electrooxidation than a compar-

ative Pt/CCE catalyst, shows great potential as less expensive electrocatalyst for FA elec-

trooxidation. On the other hand, the PteNi/CCE catalyst has satisfactory stability and

reproducibility when stored in ambient conditions or continues cycling. These results

indicate that the system studied in the present work is the most promising system for use

in direct formic acid fuel cells.

Copyright 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rightsreserved.

[17] have also reported that the rate of fuel crossover can beReceived 4 March 2011

Received in revised formelectrodeposited process. The obtained catalyst is characterized by scanning electron

microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction and cyclic voltam-Article history: The PteNi nanopartica r t i c l e i n f o a b s t r a c tCarboneceramic supported bimas an electrocatalyst for oxida

Biuck Habibi*, Nasrin Delnavaz

Electroanalytical Chemistry Laboratory, Faculty of Sciences, Departm

Tabriz, Iran

j ourna l homepage : ww2011, Hydrogen Energy Petallic PteNi nanoparticleson of formic acid

t of Chemistry, Azarbaijan University of Tarbiat Moallem,

e lsev ier . com/ loca te /heublications, LLC. Published by Elsevier Ltd. All rights reserved.

used for electrochemical experiments. A conventional three

the cyclic voltammetric responses of the PteNi/CCE, Pt/CCE,

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 9 5 8 1e9 5 9 095822. Experimental

2.1. Chemicals

Methyltrimethoxysilane (MTMOS), formic acid, methanol,

H2PtCl6.5H2O, NiCl2, HCl, H2SO4 and graphite powder of high

purity were obtained from Merck or Fluka. All solutions were

prepared with double distilled water.

2.2. Procedure of PteNi/CC preparation

Step I:

The sol-gel processing method was used for fabricating CC

substrate according to the following procedure: The amount

of 0.9 ml MTMOS was mixed with 0.6 ml methanol. After

addition of 0.6 ml HCl 0.1 M as catalyst, the mixture was

magnetically stirred (for about 15min) until it produced a clear

and homogeneous solution. Then, 0.3 g graphite powder was

added and the mixture was shaken for another 5 min.

Subsequently, the homogenized mixture was firmly packed

into a Teflon tube (with 3 mm inner diameter and 10 mm

length) and dried for at least 24 h at room temperature. A

copper wire was inserted through the other end to set up

electric contact. The electrode surface was polished with

emery paper grade 1500 and rinsed with water.

Step II:

Electrochemical deposition is an efficient method for the prep-enables electrochemical dissociation of water on oxophilic

metal sites at more negative potentials compared to pure Pt

and, therefore, allows electrocatalytic oxidation of COads at

lower anodic over potentials [21]. In order to improve the

electrocatalytic performance of the Pt catalyst for FA elec-

trooxidation, the Pt-based bimetallic catalysts, such as

PteAu [22e24], PteRu [25e27], PteBi [28,29], PteIr [30], PtePb

[31,32], PteSb [33], PteOs [34], PtePd [10,35], PteFe [36], PteSn

[36,37], and PteAg [38] have been investigated. However, to

the best of our knowledge, PteNi bimetallic catalyst for FA

electrooxidation has not been reported yet.

In this work, we have prepared CC supported PteNi

nanoparticles by a two-step procedure: (I) the CC substrate

was produced by using sol-gel technique and (II) PteNi

nanoparticles were precipitated electrochemically on the

surface of CC substrate and the resulting catalyst was

referred as PteNi/CCE. After surface, structural, and elec-

trochemical characterizations of the PteNi/CCE, the electro-

catalytic activity of PteNi/CCE towards the FA oxidation in

0.1 M H2SO4 solution was evaluated by cyclic voltammo-

metric and chronoamperometric measurements and the

obtained results were compared with those obtained on the

smooth platinum and platinum nanoparticles CC electrodes.

It was found that PteNi/CCE was catalytically more active

than smooth platinum and platinum nanoparticles sup-

ported CC electrodes and had satisfactory stability and

reproducibility when stored in ambient conditions or

continues cycling.aration of metal particles. It is widely used with different strat-

egies/methodologies. Among these, potential step depositionsmooth Pt and CC electrodes were recorded at a scan rate of

50 mV s1. Fig. 1 shows the cyclic voltammograms (CVs) of thePteNi/CCE, smooth Pt and CCE (inset A, curve 1 and 2,

respectively) and Pt/CCE (inset B) in 0.1 M H2SO4 solution.

Although the typical Pt-peaks for the hydrogen under-

potential deposition (Hupd), the oxidation of hydrogen (Hoh),

formation of platinum oxide (PtOfor), and its reduction (PtOred)

are present on the PteNi/CCE, they become ill-shaped

compared to the smooth Pt (inset A, curve 1) and even to the

unalloyed Pt modified CCE (Pt/CCE) (inset B) [40]. On the other

hand, compared with the Pt/CCE, both peaks for hydrogen

adsorption and desorption of PteNi/CCE decrease. In fact the

decrease of hydrogen peak was linear with the increase of

amount of Ni in the PteNi/CCE. So the contribution of the Ni

atoms (qNi) deposited together with Pt atoms on the CCE

surface was calculated by the following equation [41]:electrode cell was used at room temperature. The smooth (Pt or

CCE) or the modified electrode (Pt/CCE or PteNi/CCE) (3 mm

diameter) was used as a working electrode. An SCE and a plat-

inumwire were used as the reference and auxiliary electrodes,

respectively. JULABO thermostat was used to control cell

temperature at 25 C. Scanning Electron Microscopy (SEM) andEnergy-DispersiveX-rayspectroscopy (EDX)wereperformedon

a LEO 440i Oxford instrument. X-ray Diffraction (XRD) of the

PteNi nanoparticles was studied using a Brucker AXF (D8

Advance) X-ray power diffractometer with a Cu Ka radiation

source (l 0.154056 nm) generated at 40 kV and 35 mA.

3. Results and discussion

3.1. Electrochemical characteristics of PteNi/CCE

To understand the electrochemical activity of the PteNi/CCE,provides a tool to fine-tune the amount of the metal deposited,

the number of metallic sites and their size. The PteNi alloy

nanoparticles on CCE was electrodeposited (Potentiostatically)

froman aqueous solution of 0.1MNa2SO4 (pH 3.8) comprisingdifferent concentrations of H2PtCl6.5H2O and NiCl2 (total

concentration of two salts equal to 1 mM) at 25 C. Beforedeposition, the CCE was polished and subjected to electrode-

position at a fixedpotential of0.4V versus a saturated calomelelectrode (SCE) for a certain time. The charge resulting from the

complete reduction of the precursor salts at the given time is

1384mC cm2. This value corresponds to an equivalent amountof platinum of 700 mg cm2 when platinum is considered as the

only metal deposited [39]. After electrodeposition, the obtained

electrode was washed thoroughly with double distilled water

and dried before further investigation.

2.3. Instrumentation

The electrochemical experiments were carried out using an

AUTOLABPGSTAT-30 (potentiostat/galvanostat) equippedwith

aUSBelectrochemical interfaceandadrivenGEPSsoftwarewasqNi QbH QaH=QbH (1)

sL

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 9 5 8 1e9 5 9 0 9583QaH is the electric charge of PteNi/CCE evaluated from the

cyclic voltammogram in the hydrogen region charge between

Fig. 1 e CV of PteNi/CCE in 0.1 M H2SO4 at a scan rate of 50 mV

(curve 2) and (B) Pt/CCE in the same conditions.0.30 and0.10 V in Fig. 1. QbH is the electric chargemeasuredfrom Pt/CCE in the same region on the CV profile (Fig. 1, inset

B). In calculating the adsorption charge, i.e. the integrated area

under the peaks, we assume that the double layer capacitance

is constant across the entire potential range. Under the

deposition conditions [PteNi (70:30)], a stable Ni contribution

(0.31) can be easily got.

The actual active surface area of the PteNi/CCE is equiva-

lent to the number of Pt sites available for hydrogen adsorp-

tion/desorption. The PteNi/CCE produced QaH of 1.07 mC.

Accordingly, the actual active surface area (Ar) can be obtained

from charge for hydrogen adsorption as: Ar QaH/Q0 (Q0 hasbeen commonly taken as 210 mC/real cm2) 5.10 cm2.

The hydrogen adsorption charge of polycrystalline Pt was

calculated at 0.021 mC. These results show that the actual

active surface area of the PteNi/CCE is about 51 times larger

than that of polycrystalline Pt.

Curve 2 in inset A of Fig. 1 shows the CV of bare CC

substrate in 0.1 M H2SO4 solution. No adsorption/desorption

peaks of hydrogen appeared at the bare CCE.

3.2. Non-electrochemical characteristics of PteNi/CCE

In order to surface characterization of the PteNi/CCE cata-

lyst, the micrograph of CCE and PteNi/CCE has been inves-

tigated by SEM and the corresponding results were shown in

Fig. 2. Fig. 2A shows the structure of the bare CCE surface

immediately after polishing with emery paper grade 1500. As

seen in this image the surface is dense, scaly and has a high

porosity. The average pore size of CC substrate is about0.2 mm. Fig. 2B shows the SEMmicrograph of the CCE surface

after PteNi deposition. As seen, the flowers like PteNi

1. The insets are (A) CVs of smooth Pt (curve 1) and bare CCEnanoparticles were obtained. The PteNi nanoparticles have

the size of 20e50 nm. High magnification shows (Fig. 2C)

that, each flower like structures of PteNi in this image is not

the individual PteNi crystallites. They are clews consisting of

crystallite aggregates.

EDX measurement was used to analysis the surface

composition of the catalyst. The EDX spectra of PteNi/CCE is

shown in Fig. 3A and the calculated chemical composition of

the some catalysts are listed in Table 1. The bare CCE data

(Fig. 3B) is also shown for comparison. It is evident that Pt and

Ni are present on the CCE support, indicating that the H2PtCl6and NiCl2 precursors can be co-reduced to their respective

metal phases by electro co-deposition method from an

aqueous solution of Na2SO4.

The XRD pattern of PteNi/CCE catalyst is shown in Fig. 4.

The peak at approx. 55 (006) corresponds to the supportingmaterial (carboneceramic). The main characteristic peaks at

41.5 (111), 46.8 (200), 68.3 (220) for Pt and at 42.6 (010), 43.7

(002), 44.8 (111), 51.0 (200), 60.1 (012), 83.8 (220) for Ni wereobserved, indicating the face-centered cubic (fcc) arrange-

ments. The X-ray diffraction patterns for Pt/CCE [42] and Ni/

CCE [43] show the diffraction peaks at 40.21, 47.12 and 68.24

correspond to the (111), (200) and (220) facets of Pt crystal and

at 42.60, 43.60, 44.76, 46.41, 52.00, 60.15 and 77.71

correspond to the (010), (002), (111), (011), (200), (012) and (220)

facets of Ni crystal, respectively. As can be seen in Fig. 4, fcc Pt

and fcc Ni have distinctive diffractions and hence can be

differentiated. It was also obvious that the XRD pattern of

PteNi/CCE catalyst combined the crystal features of Pt and Ni,

indicating the coexistence of both of them [44].

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 9 5 8 1e9 5 9 095843.3. Electrocatalytic properties of PteNi/CCE toward theoxidation of FA

Fig. 5 shows the CVs of the PteNi/CCE and smooth Pt (inset A)

in a 0.5 M FA 0.1 M H2SO4 solution. During the forward scanof the CV of smooth Pt, the first anodic peak results from FA

oxidation, while the second peak at 1.06 V can be attributed

to CO oxidation and FA oxidation on sites that were previ-

ously blocked by COads [11,30,45]. This is because COads is

generated as an intermediate during the electrochemical

oxidation of FA. The CV of PteNi/CCE shows the usual

characteristics of smooth Pt except that both for forward and

reverse scan directions the oxidation currents of FA on the

PteNi/CCE are significantly higher than on the smooth Pt. It

can be seen from CV of FA oxidation on the PteNi/CCE that

the reaction commences in the hydrogen region and

proceeds slowly in the positive direction, and then reaches

a plateau at ca. 0.28 V. At potentials with more than ca.

0.35 V, the reaction becomes accelerated and maximum rate

at ca. 0.80 V occurs. Another increase in current at potentials

more than ca. 0.95 V and a rapid increase 1.35 are assigned to

Fig. 2 e SEM images of (A) CCE surface immediately after

polishing, (B) same electrode after PteNi nanoparticles

deposition (PteNi/CCE) and (C) PteNi nanoparticles with

high magnification.Fig. 3 e EDX data for the PteNi nanoparticles modified CCEthe reaction intermediates and poisonous adsorbed species

to CO2 [46e48] and oxygen evolution reaction [49], respec-

tively. Upon reversing the potential sweep, a very steep

increase of the reaction rate at ca. 0.48 V develops and

a maximum current is observed at ca. 0.38 V. After that, the

current gradually decreases but the reaction rate is still

faster than in the forward scan. This large anodic peak in the

reverse scan is attributed to the removal of the incompletely

oxidized carbonaceous species formed in the forward

scan [32].

A closer examination of the onset potential regions in

Fig. 5 compared to the FA free electrolyte shows that the

magnitudes of the hydrogen adsorption/desorption peaks

were reduced and disappeared by the presence of FA (inset B

Fig. 5), implying that FA is adsorbed preferentially on the

electrode surface at those potentials [50e52]. It should be

noted that, for low concentration of FA (not shown), the

hydrogen adsorption/desorption peaks are still visible

(A) and bare CCE (B).

Table 1 e Surface composition of PteNi/CCE catalyst.

Element Atomiccontent, %

Masscontent, %

Measurementerror, %

C 16.18 1.94 0.55Si 27.79 6.68 0.65Pt 38.66 63.06 1.50Ni 17.37 28.32 0.35Total 100 100 e

showing that FA does not prevent completely the hydrogen

adsorption.

It has been widely accepted in the literature that FA in the

forward scan is oxidized to CO2 via a dual path mechanism

on Pt [53e55], which involves a reactive intermediate

(dehydrogenation pathway) and COads as a poisoning species

(dehydration pathway) (Scheme 1). In one pathway, direct

oxidation of FA is said to occur (dehydrogenation pathway)

whereas in the other pathway, FA is oxidized to CO

Fig. 4 e XRD patterns of PteNi/CCE.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 9 5 8 1e9 5 9 0 9585Fig. 5 e CV of 0.5 M FA on the PteNi/CCE in 0.1 M H2SO4 at

a scan rate of 50 mV sL1. Insets A and B are the CV of 0.5 M

FA on the smooth Pt in the same conditions and the

expended hydrogen adsorption/desorption region (solid

line) in the absence and (dashed line) in the presence of FA,

respectively.(dehydration pathway), which in turns must be removed by

activating water. While the net reaction is the same in both

of the pathways, the water dissociation reaction is rather

difficult. The OHads formed by the dissociation of water

molecules on Pt surface [56] aids in removing the adsorbed

surface poison COads by oxidizing it. But indeed this process

is very intricate, as a higher potential is required for water

activation (>0.5 V) on Pt surfaces. Consequently, the elec-

trode surface will be blocked by large amounts of COadsspecies thereby hindering further adsorption of other FA

molecules on the electrode surface. This drawback will make

only a few number of FA molecules to be oxidized as the

electrode poison COads remains on the electrode surface for

a long time occupying active catalyst sites thereby reducing

the overall activity of FA oxidation. Hence, the rate of FA

oxidation primarily depends on the amount of COadsremoved from the electrode surface [57].

In the backward scan, a small reduction current peak

appears due to the reduction of Pt oxide to Pt.

PtO 2H e/ PtH2O (2)The backward oxidation peak has been attributed to the addi-

tional oxidation of the adsorbed carbonaceous species to CO :

Scheme 1 e Schematic representation of FA oxidation on

the PteNi nanoparticles modified CCE.2

PtCOads H2O/Pt CO2 2H 2e (3)In comparison with Pt nanoparticles modified CCE [40];

PteNi nanoparticles modified CCE showed an enhanced

electrocatalytic activity towards the FA oxidation. The anodic

current density of FA oxidation on the PteNi nanoparticles in

forward scan is found increased to the 1.82 mA/cm2 (actual

surface area Ar), which is 0.31 mA/cm2 (Ar) for Pt/CCE in thesame condition [40]. While in the backward scan the anodic

current density is found 1.82 mA/cm2 (Ar), which is 0.64 mA/

cm2 (Ar) for Pt/CCE.

The effects of the alloying element (Ni) in the Pt-based

binary alloy on the electrooxidation reaction of FA can be

satisfactorily explained on the basis of (a) the bifunctional

mechanism, which should consider adsorption properties of

CO and OH surface species, (b) the electronic interaction

between the alloying element and Pt and (c) the increase in

electrochemical active area due to the leaching out effect of

the alloying element [58]. According to the bifunctional theory

[59], an efficient catalyst favors CO adsorption on Pt and OHadsformation takes place on the second metal. Hence, the binary

combination yields the best overall activity for FA oxidation.

Therefore, CO adsorption mainly occurs on Pt, while OHadsspecies easily interact with Ni surface [60].

NiH2O/NiOHads H e (4)

NiOHads PtCOads/CO2 PtNiH e (5)Based on the above deduction, the ratio of the forward peak

current (Ipf) to the backward peak current (Ipb) reflects the ratio

of the amount of FA oxidized to CO2 to the amount of CO.

Hence the ratio of the Ipf to the Ipb, Ipf/Ipb, can be used to

describe the catalyst tolerance to carbonaceous species accu-

mulation. Basically, a higher Ipf/Ipb value represents a relatively

complete oxidation of FA, producing CO2, while; a low Ipf/Ipbratio indicates poor oxidation of FA to CO2 during the anodic

scan and excessive accumulation of carbonaceous residues on

the catalyst surface [61]. In other words, this ratio essentially

reflects the fraction of the catalyst surface that is not poisoned

by COads and can be used to measure the catalyst tolerance to

CO-poisoning [50]. The ratio of Ipf/Ipb for PteNi/CCE in FA

oxidation is 1.0 which is more than 2 multiple higher than

those of Pt/CCE and Pt (0.49 and 0.39, respectively), which

indicated that more intermediate carbonaceous species were

oxidized to CO2 in the forward scan on PteNi/CCE surface than

those on Pt/CCE and Pt surfaces. It is clear that, in PteNi

or dehydrogenation mechanism wherein the reaction

proceeds without any poisonous intermediate. Therefore, an

enhanced oxidation rate towards the electrooxidation of FA is

observed due to the fact that Ni can increase the rate of FA via

a direct CO2 pathway [62].

It should be noted that, in addition to the presence of

second metal (Ni), the final potential in the cyclic voltam-

metric method can also affect the ratio of Ipf/Ipb in the elec-

trooxidation of FA on the PteNi/CCE (Fig. 6). With an increase

in the final potential, the Ipf remains constant while Ipbdecrease and consequently the ratio of Ipf/Ipb increase (Fig. 6,

insets A, B and C). In fact, increasing of the final potential

accelerates the formation of Pt oxide. Acceleration of PtO

formation cases the acceleration of the dehydration pathway

and decreasing of COads and consequently decreasing of the

Ipb. This phenomenon again shows that the ratio of Ipf/Ipb is

a sign of electrocatalytic and promoter activity of catalyst

toward the poisonous intermediate COads oxidation. In other

words by increasing the final potential the conversion of

metal to metal oxides is accelerated and as a result, an

increase in reduction current peak of Pt oxide to Pt happens.

In case of potential of peaks, it can be seen (inset B) that the

potential of FA oxidation peak in the forward scan (Epf)

remains invariable, while the potential of oxidation peak in

backward scan (Epb) shifts positively and consequently

difference between Epf-Epb increases by increasing of final

potential (inset C). In fact, the presence of high clean metal

nanoparticles surface improves poisonous intermediate

the

e1.

orw

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 9 5 8 1e9 5 9 09586nanoparticles modified electrode, the presence of Ni provokes

the reaction to proceed through a direct pathway mechanism

Fig. 6 e Effect of upper limit of potential scanning region on

H2SO4. (1)L0.3e1.0 V, (2)L0.3e1.1 V, (3)L0.3e1.2 V, (4)L0.3

rate 50 mV sL1. Insets are (A) Plot of anodic peak current in fanodic peak potential in forward (A) and backward (-) and (C)

a function final potential.COads oxidation reaction thermodynamically and the lowers

potential peak of their oxidation in backward scan.

electrooxidation of 0.5 M FA on the PteNi/CCE in 0.1 M

3 V, (5)L0.3e1.4 V, (6)L0.3e1.5 V, and (7)L0.3e1.6 V. Scan

ard (Ipf) (A) and in backward scan (Ipb) (-), (B) Variation ofplot of the ratio of Ipf/Ipb and difference between EpfeEpb as

It is well known that the oxidation of FA like as other C1molecules on Pt alloys nanoparticles is strongly enhanced or

decreased by the change of heavymetals deposited (i.e., Ni). In

order to optimize the contents of electro co-deposited PteNi

nanoparticles on the CCE for FA oxidation, solutions with

various Pt/Ni (total concentration of two salts equal to 1 mM)

molar ratios were chosen. Themetals loading were controlled

at 700 mg/cm2. The CV curve of FA oxidation on the eight

PteNi/CCE samples are drawn (not shown) and compared

with together. The peak current in the forward scan, the ratio

of Ipf/Ipb and the onset potential are three key parameters used

to evaluate electrocatalytic activity of different PteNi nano-

particles in the FA electrooxidation (Table 2). Because, in

addition to the peak current in the forward scan and ratio of

Ipf/Ipb it is generally recognized that the onset potential is an

indicator in electrocatalytic activity [61]. Table 2 shows that

the current of FA oxidation in forward scan are significantly

enhanced after the introduction of second metal (i.e., Ni) and

all bimetallic PteNi catalysts exhibit high Ipf/Ipb values by at

least 12e104% higher than these in Pt/CCE catalyst. Such high

current ratio represents that most of the intermediate carbo-

naceous species can be oxidized to CO2 in the forward scan on

PteNi catalysts [47,60]. Additionally, it should be noted that

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 9 5 8 1e9 5 9 0 9587the onset potential of FA oxidation depends on the content of

alloy (Table 2). On the other hand, the results indicate that the

PteNi nanoparticles electro co-deposited with the ratio of Pt/

Ni 70:30, exhibit the highest electrocatalytic activity, i.e. thehighest peak current in forward scan (9.4 mA), highest ratio of

Ipf/Ipb (1.0) and the lowest onset potential (0.15 V). Based onthe above results, it can be concluded that the PteNi/CCE

(70:30) catalyst has the greatest catalysis ability in the elec-

trochemically oxidation of FA. It is clear that all the PteNi alloy

catalysts present an activity enhancement for the FA oxida-

tion and the maximum electrocatalytic activity is found with

the Pt:Ni atomic composition (70:30), which is similar to that

Table 2 e Comparison of electrochemical activities of thedifferent PteNi/CCE catalysts.

Catalysttype

Ipf(mA)

Onsetpotential(V vs. SCE)

Ipf/Ipb at1st cycle

(dimensionless)

% (Ipf/Ipb)

PteNi/CCE/(Ipf/Ipb)Pt/CCE

Pt (100) 1.8 0.0 0.49 e

PteNi/CCE

(90:10)a4.3 0.09 0.55 12

PteNi/CCE

(80:20)

7.2 0.12 0.82 67

PteNi/CCE

(70:30)

9.4 0.15 1.0 104

PteNi/CCE

(60:40)

7.4 0.11 0.94 91

PteNi/CCE

(50:50)

6.4 0.10 0.86 75

PteNi/CCE

(40:60)

5.0 0.09 0.80 63

PteNi/CCE

(30:70)

4.2 0.05 0.74 51

PteNi/CCE

(20:80)

3.1 0.01 0.70 43a 0.9 103 M PtCl6.5H2O 0.1 103 M NiCl2.obtained on the other Pt alloy [11,63e65]. In general, the

activity enhancement on Pt-based alloy catalysts could be

ascribed to the changes in structural parameters and the Pt-M

composition. In our work, it can be seen that the maximum

activity is foundwith the Pt/Ni (70:30) atomic composition. i.e.,

When the content of Ni in the PteNi/CCE catalyst is increased

(0-30), in the ratio of Pt:Ni (70:30) the highest peak current in

forward scan, the highest ratio of Ipf/Ipb and lowest onset

potential are observed, suggesting that the increase in the

content of Ni in the PteNi/CCE catalyst changes the amount

and strength oxidation of FAmost probably via increase in the

nanoparticles surface area. However, the electrocatalytic

activity of the PteNi/CCE catalyst would be decreased when

the content of Ni in the PteNi/CCE catalysts is high (>40). This

may be attributed, on one hand, to the agglomeration of

formed particles to form clusters with lower surface area and,

on the other hand, to the low electrocatalytic activity on Ni for

the oxidation of FA. i.e., only when the atomic ratio of Pt and

Ni is suitable, such as PteNi/CCE (70:30) catalyst, its electro-

catalytic activity can be better than that of the other PteNi

mole ratio catalysts. So the activity enhancement in the

PteNi/CCE not only depends to addition of Ni to Pt/CCE cata-

lyst but also depends on the ratio of PteNi. Therefore the

combination of optimal PteNi nanoparticles with excellent

catalytic activity and CCE support with high electrochemical

accessible surface areas and stability can become a promising

electrocatalyst for FA oxidation in DFAFCs.

A further investigation was done to find out the transport

characteristics of FA PteNi/CCE. The influence of the scan rate

(y) on the electrooxidation of FA at the PteNi/CCE was inves-

tigated (Fig. 7). As shown in Fig. 7, the dependence of the

voltammetric profile with the scan rate is different in the

forward and backward scans. Whereas the backward scan is

almost insensitive to the scan rate (only small changes in the

maximum current obtained) [66], the currents recorded in the

forward scan increase as the scan rate increases. The anodic

peak currents are linearly proportional to y1/2 (inset Fig. 7)

which suggests that the electrocatalytic oxidation of FA on

PteNi/CCE is a diffusion-controlled process [67,68].

Finally, in order to further assess the activity of PteNi/CCE,

the peak potential and peak current were determined as

a function of bulk concentration of FA. The obtained results

show that, the peak potential is shifted positively as the FA

concentration, but remained fairly constant, within a 50 mV

range over the whole change in concentration; meanwhile,

the anodic current at forward scan increased linearly as the FA

concentration is increased.

3.4. Long-term stability of the PteNi/CCE

In practical view, long-term stability of the electrode is

important. The long-term stability of PteNi/CCE was exam-

ined in 0.1 M H2SO4 solution containing 0.5 M FA (Fig. 8). It

can be observed that the anodic current remains constant

with an increase in the scan number at the initial stage and

then starts to decrease after 60 scans. The peak current of the

250th scan is about 90% than that of the first scan. In general,

the loss of the catalytic activity after successive number ofscans may result from the consumption of FA during the CV

scan. It may also be due to poisoning and the structure

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 9 5 8 1e9 5 9 09588change of the metal nanoparticles as a result of the pertur-

bation of the potentials during the scanning in aqueous

solutions, especially in presence of the organic compound.

Another factor might be due to the diffusion process occur-

ring between the surface of the electrode and the bulk solu-

tion. With an increase in scan number, FA diffuses gradually

from the bulk solution to the surface of the electrode. After

the long-term CV experiments, the PteNi/CCE was stored in

water for a week; then FA oxidation was carried out again by

CV, and excellent catalytic activity towards FA oxidation was

still observed. This indicates that the PteNi/CCE composite

prepared in our experiment has good long-term stability and

storage properties. In order to further evaluate the stability of

the electrocatalytic activity of the PteNi/CCE towards FA,

chronoamperometric measurements were performed. Insets

(A and B) of Fig. 8 show the chronoamperometric curves of

0.5M FA 0.1MH2SO4 solution on the PteNi/CCE and smoothPt electrode at 0.9 V for 4000s, respectively. It was found that

the currents observed from chronoamperograms were in

good agreement with the currents observed from cyclic vol-

tammetry. On the other hand, as can be seen the current on

the PteNi/CCE and smooth Pt electrode decreased with time

and reached at 4000s to 6.3 and 0.04 mA, respectively.

Fig. 7 e Effect of scan rate on 0.5 M FA electrooxidation on

the PteNi/CCE in 0.1 M H2SO4. Scan rates are shown on

CVs. The inset shows the dependence of the forward

anodic peak currents on the square root of scan rates.Compared to the smooth Pt electrode, the relative value of

the decrease for the PteNi/CCE is much lower. These results

show that the current represent less decay at the applied

Fig. 8 e Plot of anodic peak current in forward scan (Ipf) in

the electrooxidation of 0.5 M FA as a function of scan

number in cyclic voltammetric method at a scan rate of

50 mV sL1 (Long-term stability of PteNi/CCE). Insets are

chronoamperometric curves in the same condition on the

PteNi/CCE (A) and smooth Pt electrode (B), respectively.constant potential on the PteNi/CCE for a long duration

(4000 s), indicating that PteNi/CCE exhibits a steady-state

electrolysis activity for FA oxidation. A further investigation

was done to find out the effect of applied potential in chro-

noamperometric response of oxidation of FA at the PteNi/

CCE. Chronoamperometric data of the PteNi/CCE were

recorded by potential steps from 0 mV for 60 s to the various

potentials for 500 s. The current-time transient for the FA

oxidation at different anodic potentials shows that the

steady-state current at the ending time (t 500 s) increaseswith an increase in applied potential. Phenomenologically,

the oxidation current increases slowly with time at 300 and

400 mV, while at higher potentials the current increases

sharply during short times and then decreases slowly at

longer times. This may be caused by the CO-poisoning effect

which results in deactivation of the catalyst surface and

blocks further oxidation of.

4. Conclusions

A bimetallic PteNi nanoelectrocatalyst with the PteNi atomic

ratio in surface layer 2.2:1 is synthesized by co-depositing on

highly prose carboneceramic from a solution of the corre-

sponding metals. FA electrooxidation at this catalyst was

studied. It was found that the catalytic activity of the

synthesized catalyst exceeds that of Pt nanoparticles (alone)

on CCE and smooth Pt electrode. The prepared catalyst

exhibits satisfactory stability and reproducibility when stored

[12] Liu Z, Zhang X. Carbon-supported PdSn nanoparticles ascatalysts for formic acid oxidation. Electrochem Commun

carbon paper for formic acid oxidation. Int J Hydrogen Energy

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 9 5 8 1e9 5 9 0 95892009;34:8270e5.[15] Nekooi P, Akbari M, Amini MK. CoSe nanoparticles prepared

by the microwave-assisted polyol method as an alcohol andformic acid tolerant oxygen reduction catalyst. Int J2009;11:1667e70.[13] Ren F, Zhou W, Du Y, Yang P, Wang C, Xu J. High efficient

electrocatalytic oxidation of formic acid at Pt dispersed onporous poly(o-methoxyaniline). Int J Hydrogen Energy Articlein press.

[14] Wang J, Yin G, Chen Y, Li R, Sun X. Pd nanoparticlesdeposited on vertically aligned carbon nanotubes grown onin ambient conditions or continues cycling, which makes it

attractive as anode in DFAFCs and applications.

Acknowledgment

The authors gratefully acknowledge the research council of

Azarbaijan University of Tarbiat Moallem for financial support

of this research via grant/research fund number 401/475.

r e f e r e n c e s

[1] Xu C, Faghri A, Li X, Ward T. Methanol and water crossoverin a passive liquid-feed direct methanol fuel cell. Int JHydrogen Energy 2010;35:1769e77.

[2] Jung EH, Jung UH, Yang TH, Peak DH, Jung DH, Kim SH.Methanol crossover through PtRu/Nafion compositemembrane for a direct methanol fuel cell. Int J HydrogenEnergy 2007;32:903e7.

[3] Oliveira VB, Rangel CM, Pinto AMFR. Modelling andexperimental studies on a direct methanol fuel cell workingunder low methanol crossover and high methanolconcentrations. Int J Hydrogen Energy 2009;34:6443e51.

[4] Liang ZX, Shi JY, Liao SJ, Zeng JH. Noble metal nanowiresincorporated Nafion membranes for reduction of methanolcrossover in direct methanol fuel cells. Int J Hydrogen Energy2010;35:9182e5.

[5] Brandao L, Rodrigues J, Madeira LM, Mendes A. Methanolcrossover reduction by Nafion modification with palladiumcomposite nanoparticles: application to direct methanol fuelcells. Int J Hydrogen Energy 2010;35:11561e7.

[6] Rice C, Ha S, Masel RI, Waszczuk P, Wieckowski A, Barnard T.Direct formic acid fuel cells. J Power Sources 2002;111:83e9.

[7] Ha S, Adams B, Masel RI. A miniature air breathing directformic acid fuel cell. J Power Sources 2004;128(2):119e24.

[8] Yu XW, Pickup PG. Recent advances in direct formic acid fuelcells (DFAFC). J Power Sources 2008;182:124e32.

[9] Wang SY, Kristian N, Jiang SP, Wang X. Controlled depositionof Pt on Au nanorods and their catalytic activity towardsformic acid oxidation. Electrochem Commun 2008;10:961e4.

[10] Liu B, Li HY, Die L, Zhang XH, Fan Z, Chen JH. Carbonnanotubes supported PtPd hollow nanospheres for formicacid electrooxidation. J Power Sources 2009;186:62e6.

[11] Jayashree RS, Spendelow JS, Yeom J, Rastogi C, Shannon MA,Kenis PJA. Characterization and application ofelectrodeposited Pt, Pt/Pd, and Pd catalyst structures fordirect formic acid micro fuel cells. Electrochim Acta 2005;50:4674e82.Hydrogen Energy 2010;35:6392e8.[16] Zhu Y, Ha S, Masel RI. High power density direct formic acidfuel cells. J Power Sources 2004;130:8e14.

[17] Wang X, Hu JM, Hsing IM. Electrochemical investigation offormic acid electro-oxidation and its crossover througha Nafion membrane. J Electroanal Chem 2004;562:73e80.

[18] Wasmus S, Kuver A. Methanol oxidation and directmethanol fuel cells: a selective review. J Electroanal Chem1999;461:14e31.

[19] Mrozek MF, Luo H, Weaver MJ. Formic acid electrooxidationon platinum-Group metals: is adsorbed carbon monoxideSolely a catalytic poison? Langmuir 2000;16:8463e9.

[20] Mikhailova AA, Pasynskii AA, Grinberg VA, Velikodnyi YA,Khazova OA. CO and methanol oxidation at platinum-tinelectrodes. Russ J Electrochem 2010;46:26e33.

[21] Lu S, Zhang C, Liu Y. Carbon nanotube supported PteNicatalysts for preferential oxidation of CO in hydrogen-richgases. Int J Hydrogen Energy 2011;36:1939e48.

[22] Obradovic MD, Tripkovic AV, Gojkovic SL. The origin of highactivity of Pt-Au surfaces in the formic acid oxidation.Electrochim Acta 2009;55:204e9.

[23] Zhang S, Shao Y, Yin G, Lin Y. Facile synthesis of PtAu alloynanoparticles with high activity for formic acid oxidation.J Power Sources 2009;195:1103e6.

[24] Xu JB, Zhao TS, Liang ZX. Carbon supported platinum-goldalloy catalyst for direct formic acid fuel cells. J Power Sources2008;185:857e61.

[25] Sieben JM, Duarte MME, Mayer CE, Bazan JC. Influence ofethylene glycol, ethanol and formic acid on platinum andruthenium electrodeposition on carbon support material.J Appl Electrochem 2009;39:1045e51.

[26] Jiang J, Kucernak A. Electrocatalytic properties ofnanoporous PtRu alloy towards the electrooxidation offormic acid. J Electroanal Chem 2009;630:10e8.

[27] Zhou W, Xu J, Du Y, Yang P. Polycarbazole as an efficientpromoter for electrocatalytic oxidation of formic acid on Ptand PteRu nanoparticles. Int J Hydrogen Energy 2011;36:1903e12.

[28] Tripkovic AV, Popovic KDj, Stevanovic RM, Socha R, Kowal A.Activity of a PtBi alloy in the electrochemical oxidation offormic acid. Electrochem Commun 2006;8:1492e8.

[29] Kang S, Lee J, Lee JK, Chung S-Y, Tak Y. Influence of Bimodification of Pt anode catalyst in direct formic acid fuelcells. J Phys Chem B 2006;110:7270e4.

[30] Yi Q, Chen A, Huang W, Zhang J, Liu X, Xu G, et al.Titanium-supported nanoporous bimetallic Pt-Irelectrocatalysts for formic acid oxidation. ElectrochemCommun 2007;9:1513e8.

[31] Matsumoto F, Roychowdhury C, DiSalvo FJ, Abruna HD.Electrocatalytic activity of ordered intermetallic PtPbnanoparticles prepared by borohydride reduction towardformic acid oxidation. J Electrochem Soc 2008;155:B148e54.

[32] Zhang LJ, Wang ZY, Xia DG. Bimetallic PtPb for formic acidelectro-oxidation. J Alloys Comp 2006;426:268e71.

[33] Matsui H, Saitou K, Kashu K. Electrocatalytic activity of Pt-Sballoys in oxidation of formic acid and methanol. ECS Trans2008;6:225e30.

[34] Liu W, Huang J. Electro-oxidation of formic acid on carbonsupported Pt-Os catalyst. J Power Sources 2009;189:1012e5.

[35] Blair S, Lycke D, Iordache C. Palladium-platinum alloy anodecatalysts for direct formic acid fuels. ECS Trans 2006;3:1325e32.

[36] Chen W, Kim J, Sun S, Chen S. Electro-oxidation of formicacid catalyzed by FePt nanoparticles. Phys Chem Chem Phys2006;8:2779e86.

[37] Yi Q, Zhang J, Chen A, Liu X, Xu G, Zhou Z. Activity of a noveltitanium-supported bimetallic PtSn/Ti electrode forelectrocatalytic oxidation of formic acid and methanol.

J Appl Electrochem 2008;38:695e701.

[38] Xu JB, Zhao TS, Liang ZX. Synthesis of active platinum-silveralloy electrocatalyst toward the formic acid oxidationreaction. J Phys Chem C 2008;112:17362e7.

[39] Golabi SM, Nozad A. Electrocatalytic oxidation of methanolat lower potentials on glassy carbon electrode modified byplatinum and platinum alloys incorporated in Poly(o-Aminophenol) Film. Electroanalysis 2003;15:278e86.

[40] Habibi B, Delnavaz N. Electrocatalytic oxidation of formicacid and formaldehyde on platinum nanoparticles decoratedcarbon-ceramic substrate. Int J Hydrogen Energy 2010;35:8831e40.

electroless cementations and its use for the electrooxidationof methanol. J Electroanal Chem 2005;580:23e34.

[53] Capon A, Parsons R. The oxidation of formic acid at noblemetal electrodes Part III. Intermediates and mechanism onplatinum electrodes. J Electroanal Chem 1973;45:205e31.

[54] Jovanovic VM, Tripkovic D, Tripkovic A, Kowal A, Stoch J.Oxidation of formic acid on platinum electrodeposited onpolished and oxidized glassy carbon. Electrochem Commun2005;7:1039e44.

[55] Seland F, Tunold R, Harrington DA. Impedance study offormic acid oxidation on platinum electrodes. Electrochim

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 9 5 8 1e9 5 9 09590[41] Lamy-Pitara E, Barvier J. Platinum modified byelectrochemical deposition of adatoms. Appl Catal A 1997;149:49e87.

[42] Razmi H, Es Habibi, Heidari H. Electrocatalytic oxidation ofmethanol and ethanol at carbon ceramic electrode modifiedwith platinum nanoparticles. Electrochim Acta 2008;53:8178e85.

[43] Habibi B, Gahramanzadeh R. Fabrication andcharacterization of non-platinum electrocatalyst formethanol oxidation in alkaline medium: nickelnanoparticles modified carbon-ceramic electrode. Int JHydrogen Energy 2011;36:1913e23.

[44] Geng X, Zhang H, Ye W, Ma Y, Zhong H. Ni-Pt/C as anodeelectrocatalyst for a direct borohydride fuel cell. J PowerSources 2008;185:627e32.

[45] Brummer SB. The use of large anodic Galvanostatictransients to evaluate the maximum adsorption on platinumfrom formic acid solutions. J Phys Chem 1965;69:562e71.

[46] Zhang XG, Arikawa T, Murakami Y, Yahikozawa K, Takasu Y.Electrocatalytic oxidation of formic acid on ultrafinepalladium particles supported on a glassy carbon.Electrochim Acta 1995;40:1889e97.

[47] Antolini E, Salgado JRC, Gonzalez ER. The methanoloxidation reaction on platinum alloys with the first rowtransition metals The case of PteCo and eNi alloyelectrocatalysts for DMFCs: a short review. Appl Catal B 2006;63:137e49.

[48] Zhao Y, E Y, Fan L, Qiu Y, Yang S. A new route for theelectrodeposition of platinum-nickel alloy nanoparticles onmulti-walled carbon nanotubes. Electrochim Acta 2007;52:5873e8.

[49] Wang Z-B, Yin G-P, Shao Y-Y, Yang B-Q, Shi P-F, Feng P-X.Electrochemical impedance studies on carbon supportedPtRuNi and PtRu anode catalysts in acid medium for directmethanol fuel cell. J Power Sources 2007;165:9e15.

[50] Napporn WT, Leger J-M, Lamy C. Electrocatalytic oxidation ofcarbon monoxide at lower potentials on platinum-basedalloys incorporated in polyaniline. J Electroanal Chem 1996;408:141e7.

[51] Kita H, Lei H-W. Oxidation of formic acid in acid solution onPt single-crystal electrodes. J Electroanal Chem 1995;388:167e77.

[52] Pournaghi-Azar MH, Habibi AB. Preparation of a platinumlayer-modified aluminum electrode by electrochemical andActa 2008;53:6851e64.[56] Iwasita T, Xia SH. Adsorption of water at Pt(111) electrode in

HClO4 solutions. The potential of zero charge. J ElectroanalChem 1996;411:95e102.

[57] Selvaraj V, Nirmala Grace A, Alagar M. Electrocatalyticoxidation of formic acid and formaldehyde on nanoparticledecorated single walled carbon nanotubes. J Colloid InterfaceSci 2009;333:254e62.

[58] Carrette L, Friedrich KA, Stimming U. Fuel cells: Principles,types, fuels, and applications. Chem Phys Chem 2000;1:162e93.

[59] Watanabe M, Motoo S. Electrocatalysis by ad-atoms: Part II.Enhancement of the oxidation of methanol on platinum byruthenium ad-atoms. J Electroanal Chem 1975;60:267e73.

[60] Wang Z-B, Yin G-P, Zhang J, Sun Y-C, Shi P-F. Investigation ofethanol electrooxidation on a Pt-Ru-Ni/C catalyst for a directethanol fuel cell. J Power Sources 2006;160:37e43.

[61] Liu ZL, Ling XY, Su XD. Carbon-Supported Pt and PtRuNanoparticles As catalysts for a direct methanol fuel cell.J Phys Chem B 2004;108:8234e40.

[62] Mathiyarasu J, Remona AM, Mani A, Phani KLN,Yegnaraman V. Exploration of electrodeposited platinumalloy catalysts for methanol electro-oxidation in 0.5 M H2SO4:PteNi system. J Solid State Electrochem 2004;8:968e75.

[63] Yang H, Vogel W, Lamy C, Alonso-Vante Structure N,Activity Electrocatalytic. Of carbon-supported Pt-Ni alloynanoparticles toward the oxygen reduction reaction. J PhysChem B 2004;108:11024e34.

[64] Gokaliler F, Gocmen BA, Aksoylu AE. The effect of Ni:Pt ratioon oxidative steam reforming performance of Pt-Ni/Al2O3catalyst. Int J Hydrogen Energy 2008;33:4358e66.

[65] Li Wenzhen, Xin Qin. Yushan Yan, Nanostructured Pt-Fe/Ccathode catalysts for direct methanol fuel cell: the effect ofcatalyst composition. Int J Hydrogen Energy 2010;35:2530e8.

[66] Macia MD, Herrero E, Feliu JM. Formic acid oxidation on Bi-/Pt(111) electrode in perchloric acid media. A kinetic study.J Electroanal Chem 2003;554-5:25e34.

[67] Wang R, Liao S, Ji S. High performance Pd-based catalysts foroxidation of formic acid. J Power Sources 2008;180:205e8.

[68] Morales-Acosta D, Ledesma-Garcia J, Godinez LA,Rodrguez HG, Alvarez-Contreras L, Arriaga LG. Developmentof Pd and Pd-Co catalysts supported on multi-walled carbonnanotubes for formic acid oxidation. J Power Sources 2010;195:461e5.

Carbonceramic supported bimetallic PtNi nanoparticles as an electrocatalyst for oxidation of formic acid1 Introduction2 Experimental2.1 Chemicals2.2 Procedure of PtNi/CC preparation2.3 Instrumentation

3 Results and discussion3.1 Electrochemical characteristics of PtNi/CCE3.2 Non-electrochemical characteristics of PtNi/CCE3.3 Electrocatalytic properties of PtNi/CCE toward the oxidation of FA3.4 Long-term stability of the PtNi/CCE

4 Conclusions Acknowledgment References