GENERAL CLASSIFICATION - Université de Mons · GENERAL CLASSIFICATION. 33 Anode: Cathode: ......

19/05/200819/05/2008

GENERAL CLASSIFICATION

M. OLIVIER

[email protected]

22

Solid oxide fuel cellSOFC

Molten carbonate fuel cellMCFC

Alkaline fuel cellAFC

Phosphoric acid fuel cellPAFC

Direct methanol fuel cellDirect ethanol fuel cell

DMFCDEFC

Proton exchange membrane fuel cellPEMFC

ElectrolyteType

GENERAL CLASSIFICATIONGENERAL CLASSIFICATION

33

Anode :

Cathode :

Global reaction :

−++→− eHH RuPt 22

/2

OHHeO Pt 222221 →−++

+−

( ) heatOHOHliquid

+→+222

21

� ELECTRODE REACTIONS

PEMFCPEMFC

44

• Electrolyte : a proton-conduction polymer electrolyte membrane, usually a perfluorinated sulfonic acid polymer The polymer membrane is thin (20-200 µm), flexible and transparent.

• Catalyst : platinum or platinum/ruthenium (deposited in the form of very fine particles)

• Electrodes : porous carbon electrode support material

MEA (membrane electrode assembly)

� MEMBRANE ELECTRODE ASSEMBLY

PEMFCPEMFC

55


PEMFCPEMFC

66

In hydrated medium, the protons settled in the sulfonicgroups become mobile and move in the membrane.

� ELECTROLYTE

PEMFCPEMFC

77

Polymers in the form of sheets, rollers or in solution poured tobe shaped.

Important parameter: ionic conductivity = f (thickness and water content λ). Too low water content = high membrane resistanceToo high water content = reduction of the catalytic activity by blocking of the poresWater content = number of water moles by sulfonic sites SO3

-

(H2O/SO3).

For Nafion, a typical value ~ 15 - 20.

� ELECTROLYTE

PEMFCPEMFC

88

� ELECTROLYTE

PEMFCPEMFC

99

Classical membranes = working at T < 100°C

A promising family : PBI [poly(benzimidazole)] doped in phosphoric acid H3PO4 or sulphuric acid H2SO4

• Stability at high temperatures (at least 200°C)• Good ionic conductivity• Weak influence of the water content on the performances

� ELECTROLYTE

PEMFCPEMFC

1010

Other characteristics required for these membranes:

- Gas tightness- Chemical and mechanical stability- Low electrical conductivity- Membrane cost

� ELECTROLYTE

PEMFCPEMFC

1111

Electrode surface = Electrochemical reactionsElectrodes allow the circulation of electrons released during the oxidation of the hydrogen

The characteristics required :- Good electrical conductivity- High contact surface with the electrolyte- Good gas diffusion- Chemical and mechanical stability

� ELECTRODES

PEMFCPEMFC

1212

Fine particles of carbon (diameter of the order of 50 nm) having

a high active surface

First way: Binder in hydrophobic PTFE used to avoid the saturation in water of the porous carbon (Nafion can be

incorporated into electrodes)Second way: a hydrophilic binder in order to improve the contact between the membrane and the catalyst

� ELECTRODES

PEMFCPEMFC

1313

The role of the catalyst = to accelerate the kinetics of electrochemical reactions especially at low temperatures Nature = only Pt for the cathode and a mixture of Pt/Ru (about 50/50) for the anode Catalyst deposited with a binder on the membrane or on the electrode First way: hydrophobic binder in order to facilitate the water evacuation Second way: hydrophilic binder in order to increase the contact with the electrolyte High cost of the catalyst: important to reduce at most the used quantities • diminishing the particles size (of the order of some nm)• Increasing the specific area of the particles (~200 - 300 m2/g of Pt)• Increasing the dispersal in the catalyst support

� CATALYST

PEMFCPEMFC

1414

Nowadays, about 1 mg of Pt per electrode cm2

The catalyst is deposited on the very thin carbon particles (electrodes).

The catalyst can be deposited either on the membrane or on the gas diffusion layer.

Numerous methods can be used : spraying, screen printing, lamination and so on.

� CATALYST

PEMFCPEMFC

1515

� CATALYST

PEMFCPEMFC

1616

The catalyst is sensitive to the chemical poisoning(molecules others than H2 or O2 settling preferentially in the surface and reducing the catalytic activity).

• CO is the most critical (Pt and Ru in the anode in order to reduce the binding of CO on the particles of catalyst). • CO adsorbs on the surface of Pt more easily than H2 and

blocks the access.• The sulfur or the ammonia: inhibitors of the catalytic sites.

Other way = to increase the working temperature of the fuel cell. This range is limited by the temperature resistance of themembrane.

� CATALYST

PEMFCPEMFC

1717

� CATALYST

PEMFCPEMFC

1818

� Purification of the fuel and the oxygen until levels of contaminants are acceptable

� Development of a more resistant catalyst in the poisoning(Pt + Ru for the anode, for example, because the ruthenium oxidizes more CO than platinum)

� Injection of small quantities of oxygen (or air) in the anode to oxidize CO. Reduction of the efficiency of the anode

� CATALYST

PEMFCPEMFC

1919

Roles: - To allow the passage of gases towards the catalyst and the

electrolyte- To supply a mechanical support to the membrane electrode

assembly - To assure the passage of electrical current produced

towards electrodes - To evacuate the heat produced by the reactions- To allow to evacuate water produced in the cathode or

transported to the anode

� GAS DIFFUSION LAYER

PEMFCPEMFC

2020

Carbon cloth or woven fabric.Carbon fibre paper with a thickness from 0,2 to 0,4 mm and a high porosity (often >70%).

� GAS DIFFUSION LAYER

PEMFCPEMFC

2121

Incorporation in a hydrophobic material (PTFE) to facilitate the elimination of the water

Balance to be found for the hydrophobic GDL:facilitating the access of gases and the elimination of the water

� GAS LAYER DIFFUSION

PEMFCPEMFC

2222

� FUNCTIONING

PEMFCPEMFC

2323

Temperatures range: from 60 to 80°C

High current density thanks to the high ionic conductivity of the electrolyte and the high electrical conductivity of the electrodes

� FUNCTIONING

PEMFCPEMFC

2424

Fuel = practically pure H2 (max some ppm for CO and less than 1 ppm for sulphur)

Oxidant : Supply in O2 (or air) by a passive system (in the atmospheric pressure) or by an active system (compressor, ventilator or compressed gas)

� FUNCTIONING

PEMFCPEMFC

2525

� FUNCTIONING : WATER MANAGEMENT

PEMFCPEMFC

2626


PEMFCPEMFC

- Water will be produced within the cathode

- Water will be dragged from the anode to the cathode sides by protons moving though the electrolyte

- Water will be removed by evaporation into the air circulating over the cathode

2727

Non homogeneous distribution of water


PEMFCPEMFC

- Water may back-diffuse from the cathode to the anode, if the cathode side holds more water

- Water may be supplied by externally humidifying the hydrogen supply

- Water may be supplied by externally humidifying the air supply. Conversely, an insufficient evacuation of the formed water induces a reduction of the catalytic activity (blocking of the catalyst or obstruction of the pores of GDL)

2828

� Control of the operating conditions (especially T) to maintain an optimal water content in the membrane

� Low temperature (lower than 60°C) and a low gas flow, but also an efficiency strongly reduced

� At higher temperature, the water amount removed can become higher than the water produced and provoke a drying out of the membrane


PEMFCPEMFC

2929

The electrode reactions are exothermic.

The temperature increases in the reaction regions (Electrode/catalyst interface).

Maintaining a homogeneous temperature in the electrolyte is important to avoid :

- the dehydration of the membrane;- the formation of hot spots.

GDL and/or the electrode must be able to drive the heat produced during the reaction and to allow its evacuation.

� FUNCTIONING : THERMAL MANAGEMENT

PEMFCPEMFC

3030

The main causes of ageing :

� Degradation of the membrane under the effect of the temperature

� Catalytic activity loss (catalyst poisoning, aggregation of particles which become inaccessible)

� Materials heterogeneities � Water content of the membrane not perfectly controlled

� FUNCTIONING : AGEING

PEMFCPEMFC

3131

� FUNCTIONING : AGEING

PEMFCPEMFC

3232

Working temperature : 60 - 80°CWorking pressure : 1 - 3 barElectrical efficiency : 40 - 50%Real voltage : 0,6 - 0,95 VCurrent density : up to several A/cm2

Set-up time : practically instantaneousResponse time : very fastLife time : 1000 to 2000 h (values in 2005)

� FUNCTIONING : CHARACTERISTICS AND PERFORMANCES

PEMFCPEMFC

3333

Advantages

- Highest power density of all the fuel cell classes- Set-up time very short- Response time very short- Compactness- Functioning at low temperature- Not sensitive to CO2

- Solid structure

Disadvantages

- Polymer membrane and ancillary components are expensive- Active water management is often required- Uses expensive platinum catalyst (Pt)- Very poor CO ( > 10-20 ppm) and sulphur resistances - Lifetime and reliability to improve

� FUNCTIONING : CHARACTERISTICS AND PERFORMANCES

PEMFCPEMFC

3434

- Applications where a very fast starting is required (power plants or the propulsion of vehicles)

- Well adapted to the weak or very weak powers by its simplicity of structure and the possibility of miniaturizing components

� FUNCTIONING : APPLICATIONS

PEMFCPEMFC

3535

Anode :

Cathode :

Global reaction :

−+++→−+ eHCOOHOHCH RuPt 66

2/23

OHHeO Pt 2236623 →−++

+−

( ) heatOHCOOOHCH liquid ++→+2223

22

3

H+ ions cross the membrane

nFEG −=∆

∆G= - 702,5 kJ.mol-1 at 25°C and n=6

Reaction kinetics rather slow, relatively high overvoltage losses and the real voltage much lower than for the PEM Fuel Cells.


DMFCDMFC

3636

• Electrolyte : thin membrane (50 - 250 µm) in perfluorinatedsulfonic acid polymer

• Catalyst : platinum or platinum/ruthenium (deposited in the form of very fine particles)

• Electrodes : generally carbon powder deposited on a support (GDL)


DMFCDMFC

3737

Methanol crossover from the anode to the cathodeMethanol is soluble in water.Diffusion phenomenon of methanol is negative for the fuel cell performances:- Energy loss because the methanol crossing the electrolyte is not oxidized- Decrease of the cathodic activity: oxidation of methanol at the cathode with CO2 production- Catalyst poisoning (Pt) in the cathode by methanol which is accompanied with a loss of catalytic activity

� CROSSOVER

DMFCDMFC

3838

Determination of the methanol amount crossing the electrolyte by measurement of the CO2volume produced in the cathode. Methanol crossover conversely proportional to the membrane thickness

A thicker membrane = higher electrical resistance

� CROSSOVER

DMFCDMFC

3939

� Pt/Ru in the anode and Pt in the cathode

� Used quantities are much greater, about several mg per cm2 (compared to less than 1 mg/cm2), the global reaction needs more energy than for pure hydrogen

� More negative effect of methanol on the ageing of catalyst than pure hydrogen in a PEM fuel cell

� CATALYST

DMFCDMFC

4040

By-products : water and carbon dioxide in gaseous state

� FUNCTIONING

DMFCDMFC

4141

Liquid fuel = simplification of the storage and supply systems.CH3OH methanol in aqueous solution (often 2 or 3M)

Methanol supply : by a passive system (circulation by gravity or capillarity) or by an active system (pump) Idea : Using the CO2 produced to pressurise the tank containing methanol

O2 or air supply : passive or active (compressor, fan or compressed air)

� FUNCTIONING: FUEL AND OXIDANT

DMFCDMFC

4242

� Methanol oxidation reaction consumes water in the anode

� In the cathode, the oxygen reduction produces water

� For one mole consumed, three moles are produced

� Global reaction = water excess must be evacuated

� FUNCTIONING: WATER MANAGEMENT

DMFCDMFC

−+++→−+ eHCOOHOHCH RuPt 66

2/23

OHHeO Pt 2236623 →−++

+−

( ) heatOHCOOOHCH liquid ++→+2223

22

3

4343

� Membrane humidification is as good in the cathode as in the anode

� Important to avoid a too high water concentration in both electrodes (supplementary dilution of methanol and blocking of active sites of the catalyst in the cathode)

� FUNCTIONING: WATER MANAGEMENT

DMFCDMFC

4444

Methanol in solution = better thermal regulation by using this fuel as fluid cooler

Elimination of CO2 produced in gaseous state in the anode (and eventually in the cathode). These bubbles reduce the methanol flow in the anode and can block the methanol circulation. The stoichiometric reaction produces 22,414 l of CO2 for 32 g of oxidized methanol (in STP).Produced bubbles management = a hydrophilic structure of GDL favours the formation of the small bubbles loosing contact more easily.

� FUNCTIONING: THERMAL MANAGEMENT

DMFCDMFC

� FUNCTIONING: CO2 MANAGEMENT

4545

The main causes :

� Membrane degradation under the effect of temperature

� Catalytic activity loss (catalyst poisoning, crossover, particles agglomeration and inaccessible particles)

� Heterogeneities of used materials

� FUNCTIONING: AGEING

DMFCDMFC

4646

Working temperature : ± 60°CWorking pressure : from 1 to 3 barElectrical efficiency : from 30 to 40%Real voltage : from 0,4 to 0,7 VCurrent density : from 100 to 200 mA/cm2

Set-up time : instantaneousResponse time : very short

� CHARACTERISTICS AND PERFORMANCES

DMFCDMFC

4747

Advantages

- Quite simple system- Compact design- Ease of use of methanol- Practically not supplementary humidification of the membrane

- Set-up time very short- Response time very short- Functioning at low temperature- Not sensitive to CO2


DMFCDMFC

4848

Disadvantages

- Membrane cost is expensive - Methanol crossover- Expensive catalyst (platinum)- Very low efficiency- Sensitive to CO for a concentration higher than 10-20 ppm- Production of CO2

- Transport of cartridges of methanol in planes not still

authorized (in decembre 2006)- Lifetime and reliability to improve


DMFCDMFC

4949

- Applications requiring low power under a minimal volume (mobile applications as telephones or computers)

- Methanol use, a liquid fuel quite easy to manipulate, allows to envisage also the use for mobile, portable or stationary applications of weak or average power

� APPLICATIONS

DMFCDMFC

5050

- Produced from compounds of agricultural origin so renewable

- Much less toxic

- Higher theoretical energetic density (8 kWh/kg compared to 6,1 kWh/kg for the methanol)

� ADVANTAGES OF ETHANOL

DEFCDEFC

5151

Anode :

Cathode :

Global reaction :

−+++→−+ eHCOOHOHHC RuPt 121223

2/252

OHHeO Pt 22612123 →−++

+−

( ) heatOHCOOOHHC liquid ++→+22252

323

Important decrease of efficiency due to complex secondary reactions


DEFCDEFC

5252

( )

( ) −+

−+

−+

++→−+

++→−

++→−

eHCOOHCHOHCHOCH

eHOHOH

eHCHOCHOHCHCH

Ptadsads

adsPt

adsPt

33

2

32322


DEFCDEFC

5353

Voltage

Theoretical voltage = 1,145 V

∆G = -1 325 kJ/mol at 25°C and n=12 (electrons produced by the complete oxidation of one ethanol mole)

Development

Development little advanced due to the need of having a catalyst which minimizes the secondary reactions

DEFCDEFC

5454

Anode :

Cathode :

Global reaction :

−++→− eHH alloyPtorPt 22

2

OHHeO Pt 222221 →−++

+−

( ) heatOHOH vapour +→+222 2

1


PAFCPAFC

5555

• Phosphoric acid H3PO4 (concentration up to 100%) stabilised

by a matrix in carbide of silicon (CSi) of low thickness (0,1-0,2 mm) with an organic binder like PTFE Thin porous structure stabilizing the acid by capillary action

Working temperature ~ 200°CT< 150°C: weak ionic conductivity of the electrolyteT > 210°C: decomposition of the electrolyteT < 190°C: dissolution in waterT < 42°C : the electrolyte coagulates and the volume

increases

� ELECTROLYTE

PAFCPAFC

5656

• Catalyst: Pt or Pt/metal (like Ni) used in both electrodes and deposited on thin carbon particles • T < 150°C: important poisoning by CO in the anode Decrease of the catalytic activity by sulphur (some tens of ppm)• T quite high, very weak amount of precious metals (generally < 1 mg/cm2)

• Electrodes : carbon with a binder/coating in PTFE. Porous structure to facilitate the gas circulation and water circulation produces in the cathode. Support = structure in graphite (collector of current)

� CATALYST AND ELECTRODES

PAFCPAFC

5757

Range of temperatures: between 160 and 200°C

Fuel = hydrogen The anode tolerates CO2 without any influence on its performancesUse of H2 produced by decomposition of hydrocarbons

O2 or air supply: passive or active (compressor, fan or compressed gas)

Water management: water in vapour state and evacuated by air or oxygen circulation

� FUNCTIONING

PAFCPAFC

5858

Thermal management

Electrolyte decomposition at ~ 210°CElectrolyte dissolution in water at T <190°C

Accurate control of the working temperature

Ageing

The main cause: high working temperature� Electrolyte degradation and evaporation� Catalytic activity loss

� FUNCTIONING

PAFCPAFC

5959

Working temperature : ± 200°CWorking pressure : from Patm to 8 barElectrical efficiency : 40 - 50%Real voltage : 0,5 - 0,8 VCurrent density : up to 800 mA/cm2

Set-up time : from 1 to 3 hResponse time : very shortLifetime : > 40 000 h (in 2005)

� CHARACTERITICS AND PERFORMANCES

PAFCPAFC

6060

Advantages

- Low working temperature- Not sensitive to CO2

- Little sensitive to CO (tolerates up to about 1%)- Possible cogeneration (recovery of heat)

Disadvantages

- High set-up time- Expensive catalyst (platinum)- Degradation (corrosive electrolyte)- Regeneration of phosphoric acid- Sensitive to sulphur- Accurate control of temperature

� CHARACTERITICS AND PERFORMANCES

PAFCPAFC

6161

- Use in stationary (electrical generator and heating) for the average powers (some tens to some hundreds of kW) or high (several megawatts)

- Use in the military domain- Only technology with proved and available commercially

industrial equipments. The UTC Power company built and already installed 300 fuel cells of an electric power of 200

kW.

� APPLICATIONS

PAFCPAFC

6262

Anode :

Cathode :

Global reaction :

−−+→−+ eOHOHH Ni 222

22

−−→−++ OHOHeO Ag 2221

22

( ) heatOHOH liquid +→+222 2

1

- OH- ions circulate in the solution- H2O produced in the anode and consumed in the cathode (ratio 2:1)

- Reaction in alkaline medium (kinetics of oxygen reduction faster than in acid medium)

- Theoretical voltage : 1,229 V


AFCAFC

6363

Concentrated KOH (30 - 85%) stabilised in a matrix or put in circulation through a pump, according to the domain of use (spatial or ground)

KOH sensitive to CO2 (reaction with formation of insoluble K2CO3 in the electrolyte blocking of pores and decrease of the fuel cell efficiency)

One part of OH- ions are not available for hydrogen oxidation

� ELECTROLYTE

AFCAFC

6464

• Electrodes :Nickel or graphite

• Catalyst: not a precious metal

Nickel in the anode or silver in the cathode can catalyse the reactions. Other possible combinations with precious metals such as Pt/Pa in the anode or Pt/Au in the cathode (Amount in precious metals lower than for the PEM fuel cells).

AFCAFC

6565

Range of temperatures : generally between 60 and 90°C

Some applications at 200 – 250°C and 5 MPa

Two modes of functioning : fixed electrolyte or circulation of electrolyte

Advantages of the electrolyte circulation with a pump :� Easier thermal management� Elimination of the impurities and the carbonates (regeneration of the electrolyte)� Water elimination� Homogenisation of the electrolyte concentration

Disadvantages:

� Corrosion by KOH (materials lifetime and working safety)� Complex system due to secondary components

� FUNCTIONING

AFCAFC

6666

Fuel cell with a fixed electrolyte :

Integration in a porous matrix which stabilises it

Simpler structure than for an electrolyte in circulation

Disadvantages :

� Evacuation of the heat more difficult to control and risks of hot spots at high temperature

� Water produced induces an electrolyte dilution and so a loss ofperformances

� Carbonates formation is possible: loss of performances

� FUNCTIONING

AFCAFC

6767

Fuel = pure hydrogenO2 or air supply = air can be used as oxidant instead of O2 but CO2 must be eliminated (air contains about 300 ppm). The CO2

elimination can be done, for example, by reaction with sodium hydroxide.

Water managementFor the fuel cells where the electrolyte is in circulation, water in excess (produced in the anode) dilutes the electrolyte and can be recuperated at a next stepIn the case of fuel cells with a solid electrolyte, an entrainment by hydrogen in excess allows to recuperate water (used as a drink for the astronauts in the American spatial missions)

� FUNCTIONING

AFCAFC

6868

Thermal management

For a fuel cell with an electrolyte in circulation, the produced heat can be eliminated by the use of a heat exchanger.For those in solid matrix, a management system of the produced heat must be integrated.

Ageing

In a closed loop (without circulation), the electrolyte dilutes due to the incomplete elimination of the produced water. The used electrolyte is corrosive and can attack the components with which it is in contact.

� FUNCTIONING

AFCAFC

6969

Working temperature : between 60 and 90°CWorking pressure : from 1 to 5 barElectrical efficiency : > 60%Real voltage : from 0,7 to 1 VCurrent density : from 100 to 200 mA/cm2

Set-up time : some tens of minResponse time : quite shortLifetime : about 5000 h (in 2006)


AFCAFC

7070

Advantages

- Working at low temperature- Working at atmospheric pressure- Low cost of the electrolyte- Low cost of the catalyst- Short response time - Short set-up time- High electrical efficiency - Working at low temperature (below 0°C)

Disadvantages

- Sensitive to CO2

- Corrosive electrolyte- Required to use pure gases


AFCAFC

7171

Applications

- Potentially, better ratio cost/power delivered- Nowadays, use for applications of average power (up to

some kW)


AFCAFC

7272

Anode :

Cathode :

Global reaction :

(CO3)2- ions circulate in electrolyte and a « transfer » of CO2

between the anode and the cathode is necessary. The delivered voltage depends on the partial pressures of the reactants and the products (H2, O2, H2O, CO2).

( ) −−++→+ eCOOHCOH 2

22

2

32

( ) −−→++

2

322212 COOeCO

( ) heatCOOHCOOH vapor ++→++22222 2

1

� ELECTROCHEMICAL REACTIONS

MCFCMCFC

7373

� ELECTROCHEMICAL REACTIONS

MCFCMCFC

7474

Mixture of carbonates (Li2CO3, K2CO3) in a porous matrix of aluminium and lithium oxides (LiAlO2) in form of sheets with a thickness between 0,5 – 0,1 mm.Good ionic conductivity of carbonates at a temperature range from 600 to 700°C (about 0,5 - 2 S.cm-1 at 700°C). At these temperatures, the electrolyte is liquid. The melting temperature is between 450 and 500°C.

� ELECTROLYTE.

MCFCMCFC

7575

�High temperatures allow to avoid the use of precious metals like catalyst. �Nickel catalyst = good compromise to have the electrode and the catalyst in the same material. � Ni/Cr or Ni/Al alloys in the anode and a porous nickel oxidedoped with lithium in the cathode.

� CATALYST

MCFCMCFC

7676

Temperature : generally between 600 and 700°C

Fuel : Hydrogen generally resulting from reactions of

decomposition of hydrocarbons within the same fuel cellThe sulphur must be eliminated (inhibitor of the anodic reaction at a concentration of a few ppm)Oxidant = mixture of CO2 and O2 in a ratio 2 : 1 according to the stoichiometric reaction Water management: the produced steam is got back in the anode

� FUNCTIONING.

MCFCMCFC

7777

Thermal management

The heat produced by the electrode reactions must be evacuated to maintain an uniform temperature.

Ageing

Unwanted reactions due to the high temperatures and to the

corrosive aspect of the electrolyteExample: � Dissolution of Ni2+ ions in the electrolyte in the cathode and diffusion towards the anode � Mechanical stability of the electrodes is affected� Change in the structure of the LiAlO2 electrolyte matrix (Increase of the particles size and the porosity)The functioning at atmospheric pressure minimizes the nickel dissolution

� FUNCTIONING.

MCFCMCFC

7878

Working temperature : from 650 to 700°CWorking pressure : from 1 to several barElectrical efficiency : 55%Real voltage : from 0,75 to 0,9 VCurrent density : up to 200 mA/cm2

Set-up time : up to several hoursResponse time : Size dependingLifetime : several thousands hours

� CHARATERISTICS AND PERFORMANCES

MCFCMCFC

7979

Advantages

• High efficiency• Not sensitive to CO• Catalyst in nickel• Hydrogen production within the fuel cell from hydrocarbons• Cogeneration

Disadvantages

• High set-up time• Electrolyte control (carbonates ions are consumed)• Corrosion of the anode and the cathode by the electrolyte• Sensitive to sulphur• CO2 management• Electrolyte loss• Dissolution of the cathode (in nickel)• Low current density

� CHARATERISTICS AND PERFORMANCES

MCFCMCFC

8080

- Stationary industrial use of high power (up to several MW of electricity and heat) and military applications (standby

power)

� APPLICATIONS

MCFCMCFC

8181

Anode :

Cathode :

Global reaction :

O2- ions circulate in the electrolyte.

Theoretical voltage at 900°C is about 0,95 V (fuel cell using pure H2 and O2).

−−+→+ eOHOH 2

2

2

2

−−−−++→++→+ eCOOHOCHeCOOCO 824/2

22

2

42

2

−−→+

2

2221 OeO

( ) heatOHOH vapor +→+222 2

1

� ELECTRODES REACTION

SOFCSOFC

8282

� ELECTRODES REACTION

SOFCSOFC

8383

� TUBULAR SOFC DESIGN

SOFCSOFC

Air is fed through the inside of the tubes while the fuel stream is fed along the outside of the tubes

8484

Important compactness

Ionic conductivity of the electrolyte = f(T). A decrease of the electrolyte thickness allows a decrease of temperature.

Development of a structure where the anode supports the electrolyte which can be deposited in a thin layer.

� PLANAR SOFC DESIGN

SOFCSOFC

8585

Two approaches for the elementary cells:

- Classic structure where components (electrodes and electrolyte) piled in the form of sheets, the access of the fueland the oxidant by both opposite faces.

- Concentric structure with access of fuel by the center.


SOFCSOFC

8686


SOFCSOFC

8787

Mixture of oxides called YSZ (Yttrium Stabilized Zirconia) composed of zirconium oxide (ZrO2) stabilised by yttrium (Y2O3, from 8 to 10%).

Good ionic conductivity at very high temperature (about 0,13 S.cm-1

at 1000°C)

� ELECTROLYTE

SOFCSOFC

8888

Ionic conductivity due to defects in the

crystalline structure.

� ELECTROLYTE

SOFCSOFC

8989

For cells with a tubular structure, the electrolyte thickness deposited by chemical vapour deposition process ~ 40 µm.

In the planar cells where the electrolyte is the support, the thickness is about from 100 to 200 µm and the electrodes (about 30 - 80 µm) are deposited on each faces.When the anode is the support, the electrolyte thickness is about 5 - 30 µm.

Current researches: minimizing the working temperature.

� ELECTROLYTE

SOFCSOFC

9090

� High electrical conductivity and very good mechanical and chemical stability � Porous electrodes to make the fuel and the oxidant diffusing towards the electrolyte � Ceramic composite (cermet or nickel stabilised by the YSZ mixture) for the anode and oxides mixture of lanthanum-strontium-magnesium (LSM) for the cathode.

At high working temperatures, it is not necessary to have precious metals like catalysts.

� ELECTRODES

SOFCSOFC

� CATALYST

9191

Temperature : between 850 and 1000°C.

At these high working temperatures, other fuels such as carbonmonoxide CO and methane (CH4) can be directly used.

Water management: the water is produced in vapour phase and can be used to activate a turbine or to heat buildings or houses(cogeneration).

−−

−−

++→+

+→+

eCOOHOCH

eCOOCO

224

2

22

2

4

2

2

� FUNCTIONING

SOFCSOFC

9292

Thermal management : Isolate the cell of the atmosphere in order to reduce the thermal losses.

� FUNCTIONING

SOFCSOFC

9393

Working temperature : 900 - 1000°CWorking pressure : 1 - 10 barElectrical efficiency : 60 %Real voltage : 0,7 - 1,15 VCurrent density : up to 1000 mA/cm2

Set-up time : up to several hoursResponse time : slowLifetime : > 30.000 h


SOFCSOFC

9494

Advantages

• Stability of the electrolyte• Very efficient cogeneration• High electrical efficiency• Use of other fuels that the hydrogen• Cheap catalysts in nickel or oxides mixture

Disadvantages

• Materials resistance (high temperature)• Sensitive to sulphur • Very long set-up time• Sensitive to changes of the working temperature• Evacuation of the heat


SOFCSOFC

9595

Applications

- Use for stationary applications (or mobile with long working time) from several kW to several hundreds of kW.

- Cogeneration


SOFCSOFC

9696

O2 or air-← O2-H2O, CO2

H2, CO, CH4

1 000SOFC

O2 or airCO2←

(CO3)2-

H20H2650MCFC

O2 or air-← OH-H20H280AFC

O2 or airH2OH+ →-H2200PAFC

O2 or airH20H+ →CO2CH3OH110DMFC

O2 or airH2OH+ →-H280PEMFC

OxidantSub-product

CATHODE

IonsANODE

Sub-products

FuelsT (°C)Type

CRITERIA OF COMPARISONCRITERIA OF COMPARISON

9797

SOFC

MCFC

AFC

PAFC

DMFC

PEMFC

100 kW – 10 MW10 – 100 kW1 – 10 kW1 – 100 WPower

CentralTransportsResidentialPortableApplication


9898

PEMFC : High energy density, very short set-up and reaction times and low working temperature Expensive catalysts and membrane and water management are critical points. DMFC : comparable to the PEM fuel cell but the liquid fuel is easier to use and the water management is less complex. Same limitations but a lower efficiency and a problem due to the methanol crossover PAFC : Proved technology and cheap electrolyte. But the electrolyte is corrosive and the catalysts expensive. AFC : Cheap electrolyte and catalysts. Complex system for the electrolyte management (circulation of the electrolyte) and use of pure hydrogen and oxygen


9999

MCFC : Cheap catalysts, large choice of the fuel and possibility of the cogeneration are positive factors. The global system is complex, the electrolyte is corrosive and this fuel cell needs ahigh set-up time.

SOFC : Same advantages than the MCFC. The same limitations. The cost of components is quite high because they must resist at very high temperatures.


100100

Supplied voltage is generally lower than 1 V.To obtain high voltages, several elementary electrode

assemblies (electrolyte, electrodes, gas diffusion layer) are used in series to constitute a stack.

STACKSTACK

101101

Stack of 30 SOFC for a volume of 2,5 L and a weight of 9 kg

STACKSTACK

102102

Fuel cell interconnection = bipolar plates (the plates serve as the anode in one cell and the cathode in the next cell)

Bipolar plates have to meet the following requirements: mechanical resistance (assembly), thermal transfer (cooling or heating as a function of the technology) and efficient gases distribution.

STACK DESIGNSTACK DESIGN

103103


104104

- Low weight (materials density and thicknesses)- Durability- Corrosion resistance- Very good electrical conductivity (> 100 S cm-1)- Very good thermal conductivity- Gas impermeability (permeability cm3.cm-2.s-1)- Hydrophobic (PEMFC, DMFC, etc..)- Low cost


105105

- Composites (graphite in a binder) and metals (steel, aluminium, titanum)

- Graphite plates (thicknesses between 1 and 3 mm) and metallic plates obtained from metal sheets (only a few tenths mm)

- The graphite plates obtained by machining are very expensive (prototypes).

- The metallic plates obtained by drawing are more compact, slighter and have a low product cost (repetitive manufacturing).

� MATERIALS

BIPOLAR PLATESBIPOLAR PLATES

106106

Important weight: up to 80 - 90% of the global weight for a PEMFC

� MATERIALS


107107

LowHighCost

with surface treatmentHighChemical resistance

with surface treatmentHighCorrosion resistance

From 2,7 to 8,8 (depending on the

metal)

1,6 - 2,0Density

120 (Al) à 400 (Cu)Up to 50Thermal conductivity (W/mK)

38 106 (Al) à 60 106

(Cu)200 à 300Electrical Conductivity

(S.cm-1)

MetalGraphite composite

� MATERIALS


108108

To avoid the mixture between the different fluids and to reduce the losses: use of fluid seals

� HYDRAULIC ROLE


109109

The quantity of water increases along channels

The pressure loss induces a reduction of the activity and a heterogeneous functioning

These parameters depend on the design (section, dimensions), the surface state and the changes of direction.

� HYDRAULIC ROLE


110110

� HYDRAULIC ROLE


111111

� HYDRAULIC ROLE


112112

� HYDRAULIC ROLE


113113

� ELECTRICAL ROLE


114114

� THERMAL ROLE


115115

� THERMAL ROLE


116116

� THERMAL ROLE


117117

Bipolar plates in moulded graphite

� THERMAL ROLE


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