er100/200 & public policy 184/284 lecture 21: fuel … cells...3. fuel cell basics -...

ER100/200 & Public Policy 184/284

Lecture 21: Fuel Cell Systems

November 12, 2015

Topics

1. Brief History

2. Electrolysis

3. Fuel Cell Basics

- Electrolysis in Reverse

- Thermodynamics

- Components

- Putting It Together

4. Types of Fuel Cells

- Alkali

- Molten Carbonate

- Phosphoric Acid

- Proton Exchange Membrane

- Solid Oxide

5. Benefits

6. Current Initiatives

- Automotive Industry

- Stationary Power Supply Units

- Residential Power Units

7. Future

Fuel Cells

6

2

Fuel Cells provide flexible Voltage & Current

5

8

4

3

7

7

Applications

A Very Brief History

Considered a curiosity in the 1800’s. The first fuel cell was built in 1839 by Sir William Grove, a

lawyer and gentleman scientist.

Serious interest in the fuel cell as a practical generator did not begin until the 1960's, when the U.S.

space program chose fuel cells over riskier nuclear power and more expensive solar energy.

Fuel cells furnished power for the Gemini and Apollo spacecraft, and still provide electricity and

water for the space shuttle.(1)

Jules Verne, Mysterious Island, 1874“l believe that water will one day be employed as a fuel, that hydrogen and oxygen will furnish an inexhaustible source of heat and light.”

Energy Density kWh/kgGasoline 14

Lead Acid Batteries 0.04

Flywheel (silica) 1

Hydrogen 38

Compressed air 2/m3

Dams (hydrostorage) 0.3/m3

Electrolysis“What does this have to do with fuel cells?”

By providing energy from a battery,

water (H2O) can be dissociated into

the diatomic molecules of hydrogen

(H2)

and oxygen (O2).

Fuel Cell BasicsComponents

Anode: Where the fuel reacts or "oxidizes", and releases electrons.

Cathode: Where oxygen (usually from the air) "reduction" occurs.

Electrolyte: A chemical compound that conducts ions from one electrode to the other inside a fuel cell.

Catalyst: A substance that causes or speeds a chemical reaction without itself being affected.

Cogeneration: The use of waste heat to generate electricity. Harnessing otherwise wasted heat boosts the efficiency of

power-generating systems.

Reformer: A device that extracts pure hydrogen from hydrocarbons.

Direct Fuel Cell: A type of fuel cell in which a hydrocarbon fuel is fed directly to the fuel cell stack, without requiring

an external "reformer" to generate hydrogen.

Fuel Cell Operation

• Pressurized hydrogen gas (H2) enters cell on anode side. • Gas is forced through catalyst by pressure.

– When H2 molecule comes contacts platinum catalyst, it splits into two H+ ions and two electrons (e-).

• Electrons are conducted through the anode– Make their way through the external circuit (doing useful work

such as turning a motor) and return to the cathode side of the fuel cell.

• On the cathode side, oxygen gas (O2) is forced through the catalyst– Forms two oxygen atoms, each with a strong negative charge. – Negative charge attracts the two H+ ions through the membrane, – Combine with an oxygen atom and two electrons from the external

circuit to form a water molecule (H2O).

Fuel cell thermodynamics (extra)The first law of thermodynamics:• The energy of a system is conserved

ΔQ - ΔW = ΔE

•The energy is a relative quantity Q – W = ΔE

• For a closed system (control mass system), such as a piston

ΔE = ΔU + ΔK + ΔP

(The total energy change equals the sum of the change in internal energy, the change in kinetic energy, and the change in potential energy

Change of heat provided to the

system

Change of work provided by the system

Change of system’s total

energyChange of

system energy

Fuel cell thermodynamics (extra)

From:

ΔE = ΔU + ΔK + ΔP

ΔE = ΔU + ΔK + Δ(pV)

H = U + pV

Or ΔH = ΔE – ΔK - Δ(pV)

If there is no change in volume so the kinetic and potential energies are constant:

H = Q – W

Enthalpy is the difference between the heat and the work involved in a system

Change of system’s total

energy

Fuel Cell Basics

fuel

cell

H2O

O2

H2

heat

work

The familiar process of electrolysis requires work to proceed, if the process is put in reverse, it should be able to do

work for us spontaneously.

The most basic “black box” representation of a fuel cell in action is shown below:

Figure 2

Fuel Cell ThermodynamicsH2(g) + ½O2(g) H2O(l)

Enthalpy is defined as the energy of a system plus the work needed to make room for it in an environment with constant

pressure.

Enthalpy = H = U + PV

Entropy can be considered as the measure of disorganization of a system, or as a measure of the amount of energy that is

unavailable to do work.

ΔS = Q/T

Entropy gain > Entropy Loss

(Q/T) + Sproducts ≥ Sreactants

Q ≥ T [Sproducts - Sreactants]

Gibbs Free Energy, G = H – Qliberated, = H – TΔS

Ηmax = ΔG / ΔH

Fuel Cell BasicsThermodynamics

H2(g) + ½O2(g) H2O(l)

Other gases in the fuel and air inputs (such as N2 and CO2) may be present, but as they are not involved in the

electrochemical reaction, they do not need to be considered in the energy calculations.

69.91 J/mol·K205.14 J/mol·K130.68 J/mol·KEntropy (S)

-285.83 kJ/mol00Enthalpy (H)

H2O (l)O2H2

Table 1 Thermodynamic properties at 1Atm and 298K

Enthalpy is defined as the energy of a system plus the work needed to make room for it in an environment with constant

pressure.

Entropy can be considered as the measure of disorganization of a system, or as a measure of the amount of energy that is

unavailable to do work.

Fuel Cell ThermodynamicsH2(g) + ½O2(g) H2O(l)


Enthalpy of the chemical reaction:

ΔH = ΔHreaction = ΣHproducts – ΣHreactants

= (1mol)(-285.83 kJ/mol) – (0)

= -285.83 kJ

Entropy of chemical reaction:

ΔS = ΔSreaction = ΣSproducts – ΣSreactants

= [(1mol)(69.91 J/mol·K)] – [(1mol)(130.68 J/mol·K) + (½mol)(205.14 J/mol·K)]

= -163.34 J/K

Heat gained by the system:

ΔQ = TΔS= (298K)(-163.34 J/K)

= -48.7 kJ


The Gibbs free energy is then calculated by:

ΔG = ΔH – TΔS= (-285.83 kJ) – (-48.7 kJ)

= -237 kJ

The external work done on the reaction, assuming reversibility and constant temp.

W = ΔG

The work done on the reaction by the environment is:

The heat transferred to the reaction by the environment is:

W = ΔG = -237 kJ

ΔQ = TΔS = -48.7 kJ

More simply stated:

The chemical reaction can do 237 kJ of work and produces 48.7 kJ of heat to the environment.

Fuel Cell BasicsPutting it together.

Figure 3

• Anode:2H2 => 4H+ + 4e-

• Cathode: O2 + 4H+ + 4e- => 2H2O

• Net Reaction:2H2 + O2 => 2H2O

• Exact opposite of electrolysis

Energy Conversion: PEM Fuel Cell

Reduction-Oxidation Rxn (redox)

−+ +→ eHH 442 2

OHeHO 22 244 →++ −+

ProductOxidant →+ −e

−+→ eProductReductant

energyOHOH +→+ 222 22

Anode Half-Reaction

Cathode Half-Reaction

Hydrogen and oxygen are combined in a non-combustion process Electricity, heat and water are produced

Proton Exchange Membrane Fuel Cell(Polymer Electrolyte Membrane)

Between the reduction and oxidation stages, the

electrons are routed through a circuit

−+ +→ eHH 442 2

Hydrogen ions (protons) permeate through the

electrolyte membrane

Reduction reaction

Oxidation reaction facilitated by a catalyst -

typically Pt ($$$)

OHeHO 22 244 →++ −+

1.23 V

Power Produced – Watts/m2

• Activation Loss– potential difference above

the equilibrium value required to produce a current (depends on activation energy of the reaction)

– energy is lost as heat• Ohmic Loss

– voltage drop due to resistance of the cell components and interconnects

• Mass Transport Loss– depletion of reactants at

catalyst sites under high loads

PEM (Polymer Electrolyte Membrane)

• Polymers such as polyphenylenes, Nafion are used

• Water is a crucial participant in the process

•absorption of water increases the proton conductivity

•membrane is confined – not free to swell – pushes electrodes

Platinum needs to be placed to maximize surface area

Needs to be encased in engineered components

• Thickness of the membrane and catalyst in the PEM can vary …

• Example: catalyst layers containing about 0.15 milligrams (mg) Pt/cm2

• thickness of the catalyst layer is close to 10 micrometers

•yields a MEA with a total thickness of about 200μm (or 0.2 mm or 20 sheets of paper)

•generates more than half an ampere of current per cm2 at a voltage of 0.7 volts

PEM (Polymer Electrolyte Membrane)

Parts of a Fuel CellBipolar Plates

• Serpentine channels for hydrogen and oxygen to flow through device

• Acts as a current collector –electrons enter and exit cell through the plate

Anode• Conducts electrons away from

catalyst to external circuit

• Channels to supply H2 evenly to the surface of the catalyst

Cathode• Channels to supply O2 evenly

to the surface of the catalyst

• Conducts electrons back to catalyst for recombining

Parts of a PEM Fuel CellMembrane Electrode Assembly• Anode

• Cathode

• PEM (Polymer Electrolyte Membrane)• conducts only positively charged ions

• blocks electrons and other substances

• Catalyst• thin coat of platinum powder applied to

carbon paper or cloth•maximizes surface area

• Backing Layers• porous carbon cloth conducts electrons

away from catalyst to external circuit• allows right amount of water vapor to

enter/exit• too much blocks the pores• membrane needs to be humidified

Schematic of Fuel Cell Operation

energyOHOH +→+ 222 21

1.2 V = theoretical maximum voltage generated by this reaction

Typical output = 0.7V – 0.9V ….. (1 W per cm2)

• Anode

Schematic of Fuel Cell Operation

Electron is stripped from Hydrogen as it makes contact with Pt catalyst which is embedded in a carbon nanoparticle

Electron conducted away through circuit

Hydrogen nucleus (proton) passes through PEM membrane to cathode

Hydrogen gas is circulated through ‘serpentine’channelsHydrogen from

channels passes through porous medium

(gas diffusion backing)

Types of Fuel Cells

The five most common types:

• Alkali

• Molten Carbonate

• Phosphoric Acid

• Proton Exchange Membrane

• Solid Oxide

A Wide Range of Types of Fuel Cells

Types of Fuel Cells

• Polymer Electrolyte Membrane (PEM) Fuel Cells

• Direct Methanol Fuel Cells • Alkaline Fuel Cells • Phosphoric Acid Fuel Cells • Molten Carbonate Fuel Cells • Solid Oxide Fuel Cells • Regenerative Fuel Cells

Alkali Fuel Cell

compressed hydrogen and oxygen fuel

potassium hydroxide (KOH) electrolyte

~70% efficiency

150˚C - 200˚C operating temp.

300W to 5kW output

requires pure hydrogen fuel and platinum catylist → ($$)

liquid filled container → corrosive leaks

Figure 4

Molten Carbonate Fuel Cell (MCFC)

carbonate salt electrolyte

60 – 80% efficiency

~650˚C operating temp.

cheap nickel electrode catylist

up to 2 MW constructed, up to 100 MW designs exist

Figure 5

The operating temperature is too hot for many applications.

carbonate ions are consumed in the reaction → inject CO2 to compensate

Phosphoric Acid Fuel Cell (PAFC)

phosphoric acid electrolyte


150˚C - 200˚C operating temp

11 MW units have been tested

sulphur free gasoline can be used as a fuel

Figure 6

The electrolyte is very corrosive

Platinum catalyst is very expensive

Proton Exchange Membrane (PEM)

thin permeable polymer sheet electrolyte


50 – 250 kW

80˚C operating temperature

electrolyte will not leak or crack

temperature good for home or vehicle use

platinum catalyst on both sides of membrane → $$

Figure 7

Solid Oxide Fuel Cell (SOFC)

hard ceramic oxide electrolyte

~60% efficient

~1000˚C operating temperature

cells output up to 100 kW

high temp / catalyst can extract the hydrogen from the fuel at the electrode

high temp allows for power generation using the heat, but limits use

SOFC units are very large

solid electrolyte won’t leak, but can crack

Figure 8

Benefits

Efficient: in theory and in practice

Portable: modular units

Reliable: few moving parts to wear out or break

Fuel Flexible: With a fuel reformer, fuels such as natural gas, ethanol, methanol,

propane, gasoline, diesel, landfill gas,wastewater, treatment digester gas, or even ammonia can be used

Environmental: produces heat and water (less than combustion in both cases)

near zero emission of CO and NOx

reduced emission of CO2 (zero emission if pure H2 fuel)

Material‘s challenges of the PEM Fuel Cell

11/12/2015 Fuel Cell Fundamentals 40

Review of Membrane (Nafion) Properties

• Chemical Structure

• Proton Conduction Process

• Water Transport and Interface Reactions

PSSApoly(styrene-co-styrenesulfonic acid)

(PSSA)

Nafion,TM

Membrane C

Dow

PESA(Polyepoxy-

succinic Acid)

α,β,β-Trifluorostyrene grafted onto poly(tetrafluoro-ethylene) with post-

sulfonation)

Poly – AMPS

Poly(2-acrylamido-

2-methylpropane sulfonate)

Chemical structures of some membrane materials

Water Transport (& Interface Reactions)

in Nafion Membrane of the PEM Fuel Cell

Material‘s challenges of the SOFC

SOFCSolid Oxide Fuel Cell

Air side = cathode: High oxygen partial pressure

1conductanced

σ= ∝

Fuel side= anode: H2 + H2O= low oxygen partial pressure

H2 + 1/2O2 H2O

H2

O2

H2O

SOFCElectromotive Force (EMF)

Chemical Reactions in 2 separated compartements:- Cathode (Oxidation): - Anode (Reduction):

½O2 + 2e- O2-

H2 + O2- H2O + 2e-

EMF of a galvanic Cell:(1) EMF = ∆Gr /-z F

∆G = Free Enthalpie

z = number of charge carriers

F = Faraday Constant

∆G0= Free Enthalpie in standart state

R = Gas Constant

SOFC: ½O2 + H2 H2O( )2

0 0.52 2

ln( ) ( )

a H OG G RT

a H a O∆ = ∆ +(2)

difference of ∆G between anode und cathode

( )( )

2

2

ln4

p ORTEMKF p O

=K

A

Nernst Equation:

SOFCElektrochemische Potential

Oxygen ions migrate due to an electrical and chemical gradient

2 2( ) ( ) 2O O Fµ µ ϕ− −∆ = ∆ − ∆

2( )Oµ −∆

ChemicalPotential

ElectricalPotential

Electrochemichal Potential

Driving force for the O2- Diffusion through the electrolyte are the different oxygen partial pressures at the anode and the cathode side:

2( )2

iij O

Fσ µ −= − ∆

ji = ionic current

σi= ionic conductivity

SOFCengl. Open Circuit Voltage (OCV)

2( )2

iij O

Fσ µ −= − ∆ 2 2( ) ( ) 2O O Fµ µ ϕ− −∆ = ∆ − ∆

2( ) 0Oµ −∆ =

What happems in case :

0ij =No currentElectrical potential difference = chemical potetialOCV

SOFCLeistungs-Verluste

Under load decrease of cell voltageand internal losses

U(I) = OCV - I(RE+ RC+RA) - ηC - ηA

(RE+ RC+RA)OCV

ηC

ηA

cell current I [mA/cm2]cell

volta

ge U

(I) [V

] Ohmic resistances

Non ohmic resistances= over voltages

SOFCÜberspannungen

Over voltages exist at interfaces of• Elektrolyte - Cathode• Elektrolyte - Anode

Reasons:

• Kinetic hindrance of the electrochemical reactions• Bad adheasion of electrode and electrolyte• Diffusion limitations at high current densities

SOFCOhm‘s losses

800nm

Kathode Anode

Reduce electrolyte thickness

Past Future

SOFCLeistungs-Verluste

(1)Open circuit voltage (OCV), I = 0(2)SOFC under Load U-I curve(3) Short circuit, Vcell = 0

0.0 0.5 1.0 1.5 2.00.0

0.2

0.4

0.6

0.8

1.0

900°Cin Luft/Wasserstoff

Stromdichte [A/cm2]

Zells

pann

ung

[V]

0.0

0.1

0.2

0.3

0.4

0.5

Leistung [W/cm

2]

(1)

(2)

(3)

(RE+ RC+RA)OCV

ηCηA


volta

geU

(I) [V

] (RE+ RC+RA)OCV

ηCηA


volta

geU

(I) [V

]

1

23

SOFC

( )*

U LR f TI A σ

∆= = =

0 log( )aET kTσσ = −

1. aT vs ET

σ ⇒

Electrical resistance:

Electrical conductivity: U : voltage [V]I : current [A]R : resistivity [ohm]∆L : distance between both

inner wires [cm]A : sample surface [cm2]σ : conductivity [S/m]Ea : activation energy [eV]T : temperature [K]K : Boltzmann constant

How to determine the electrical conductance

Iinput

Um

easured

SOFC

Tubular designi.e. Siemens-Westinghouse design

Planar designi.e. Sulzer Hexis, BMW design

Segment-type tubular design

SOFC Design

SOFCTubular Design – Siemens-Westinghouse

air flow anode (fuel)

cathode interconnection

cathode (air)

Why was tubular design developed in 1960s by Westinghouse?• Planar cell: Thermal expansion mismatch between ceramic and support structures leads to problems with the gas sealing tubular design was invented

Advantages of tubular design:• At cell plenum: depleted air and fuel react heat is generated incoming oxidant can be pre-heated. • No leak-free gas manifolding needed in this design !

SOFC

anode (fuel)

cathode (air)

electrolyte

Tubular Design – Siemens-Westinghouse

To overcome problems new Siemens-Westinghouse „HPD-SOFC“ design:

New: Flat cathode tube with ligaments

Advantages of HPD-SOFC:• Ligaments within cathode short current pathways decrease of ohmic resistance• High packaging density of cells compared to tubular designSiemens-Westinghouse shifted from

basic technology to cost reduction and scale up.

Power output: Some 100 kW can be produced.

SOFCPlanar Design – Sulzer Hexis

anode (fuel)electrolytecathode (air)

interconnect Advantages of planar design:• Planer cell design of bipolar plates easy stacking no long current pathways• Low-cost fabrication methods, i.e. Screen printing and tape casting can be used.

Drawback of tubular design:• Life time of the cells 3000-7000h needs to be improved by optimization of mechanical and electrochemical stability of used materials.

SOFCPlanar Design – BMW

electrolyteanode

porous metallic substrateFe-26Cr-(Mo, Ti, Mn, Y2O3) alloy

cathodeCathode current collector

bipolar plate

bipolar plate

Air channel

Fuel channel

20-50 µm5-20 µm

15-50 µm

Plasma sprayPlasma spray

Plasma spray

ApplicationBatterie replacement in the BMW cars of the 7-series.

Power output: 135 kW is aimed.

Current InitiativesAutomotive Industry

Most of the major auto manufacturers have fuel cell vehicle (FCV) projects currently under way, which involve all sorts of fuel cells and

hybrid combinations of conventional combustion, fuel reformers and battery power.

Considered to be the first gasoline powered fuel cell vehicle is the H20 by GM:

GMC S-10 (2001)

fuel cell battery hybrid

low sulfur gasoline fuel

25 kW PEM

40 mpg

112 km/h top speed

Figure 9

Fords Adavanced Focus FCV (2002)

fuel cell battery hybrid

85 kW PEM

~50 mpg (equivalent)

4 kg of compressed H2 @ 5000 psi

Approximately 40 fleet vehicles are planned as a market

introduction for Germany, Vancouver and California for

2004.


Figure 10

Figure 11

Daimler-Chrysler NECAR 5 (introduced in 2000)

85 kW PEM fuel cell

methanol fuel

reformer required

150 km/h top speed

version 5.2 of this model completed a California to Washington DC drive

awarded road permit for Japanese roads


Figure 12

Mitsubishi Grandis FCV minivan

fuel cell / battery hybrid

68 kW PEM

compressed hydrogen fuel

140 km/h top speed

Plans are to launch as a production vehicle for Europe in 2004.


Figure 13

Current InitiativesStationary Power Supply Units

A fuel cell installed at McDonald’s restaurant, Long Island Power Authority to install 45 more fuel cells across Long Island,

including homes.(2) Feb 26, 2013

More than 2500 stationary fuel cell systems have been installed all over the world - in hospitals, nursing homes, hotels,

office buildings, schools, utility power plants, and an airport terminal, providing primary power or backup. In large-scale

building systems, fuel cells can reduce facility energy service costs by 20% to 40% over conventional energy service.

Figure 14

Current InitiativesResidential Power Units

There are few residential fuel cell power units on the market but many designs are undergoing testing and should be

available within the next few years. The major technical difficulty in producing residential fuel cells is that they must be

safe to install in a home, and be easily maintained by the average homeowner.

Residential fuel cells are typically the size of

a large deep freezer or furnace, such as the

Plug Power 7000 unit shown here, and cost

$5000 - $10,000.

If a power company was to install a residential fuel cell power unit in a home, it would have to charge the homeowner at

least 40 ¢/kWh to be economically profitable.(3) They will have to remain a backup power supply for the near future.

Figure 15

er100/200 & public policy 184/284 lecture 21: fuel … cells...3. fuel cell basics -...

Documents