GENERAL CLASSIFICATION

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1 GENERAL CLASSIFICATION M. OLIVIER 19/05/2008

2 GENERAL CLASSIFICATION Type Electrolyte PEMFC DMFC DEFC PAFC AFC MCFC SOFC Proton exchange membrane fuel cell Direct methanol fuel cell Direct ethanol fuel cell Phosphoric acid fuel cell Alkaline fuel cell Molten carbonate fuel cell Solid oxide fuel cell 2

3 PEMFC ELECTRODE REACTIONS Anode : H 2 + Pt / Ru 2 H + 2 e Cathode : + 1 2O2 + 2 e + 2 H Pt H 2O Global reaction : H 1 O H O + ( ) heat liquid

4 PEMFC MEMBRANE ELECTRODE ASSEMBLY Electrolyte : a proton-conduction polymer electrolyte membrane, usually a perfluorinated sulfonic acid polymer The polymer membrane is thin ( 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) 4

5 PEMFC MEMBRANE ELECTRODE ASSEMBLY 5

6 PEMFC ELECTROLYTE In hydrated medium, the protons settled in the sulfonic groups become mobile and move in the membrane. 6

7 PEMFC ELECTROLYTE Polymers in the form of sheets, rollers or in solution poured to be shaped. Important parameter: ionic conductivity = f (thickness and water content λ). Too low water content = high membrane resistance Too high water content = reduction of the catalytic activity by blocking of the pores Water content = number of water moles by sulfonic sites SO 3 - (H 2 O/SO 3 ). For Nafion, a typical value ~

8 PEMFC ELECTROLYTE 8

9 PEMFC ELECTROLYTE Classical membranes = working at T < 100 C A promising family : PBI [poly(benzimidazole)] doped in phosphoric acid H 3 PO 4 or sulphuric acid H 2 SO 4 Stability at high temperatures (at least 200 C) Good ionic conductivity Weak influence of the water content on the performances 9

10 PEMFC ELECTROLYTE Other characteristics required for these membranes: - Gas tightness - Chemical and mechanical stability - Low electrical conductivity - Membrane cost 10

11 PEMFC ELECTRODES Electrode surface = Electrochemical reactions Electrodes 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 11

12 PEMFC ELECTRODES 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 12

13 PEMFC CATALYST 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 (~ m 2 /g of Pt) Increasing the dispersal in the catalyst support 13

14 PEMFC CATALYST Nowadays, about 1 mg of Pt per electrode cm 2 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. 14

15 PEMFC CATALYST 15

16 PEMFC CATALYST The catalyst is sensitive to the chemical poisoning (molecules others than H 2 or O 2 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 H 2 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 the membrane. 16

17 PEMFC CATALYST 17

18 PEMFC CATALYST 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 18

19 PEMFC GAS DIFFUSION LAYER 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 19

20 PEMFC GAS DIFFUSION LAYER Carbon cloth or woven fabric. Carbon fibre paper with a thickness from 0,2 to 0,4 mm and a high porosity (often >70%). 20

21 PEMFC GAS LAYER DIFFUSION 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 21

22 PEMFC FUNCTIONING 22

23 FUNCTIONING PEMFC 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 23

24 PEMFC FUNCTIONING Fuel = practically pure H 2 (max some ppm for CO and less than 1 ppm for sulphur) Oxidant : Supply in O 2 (or air) by a passive system (in the atmospheric pressure) or by an active system (compressor, ventilator or compressed gas) 24

25 PEMFC FUNCTIONING : WATER MANAGEMENT 25

26 PEMFC FUNCTIONING : WATER MANAGEMENT - 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 26

27 PEMFC FUNCTIONING : WATER MANAGEMENT Non homogeneous distribution of water - 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) 27

28 PEMFC FUNCTIONING : WATER MANAGEMENT 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 28

29 PEMFC FUNCTIONING : THERMAL MANAGEMENT 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. 29

30 PEMFC FUNCTIONING : AGEING 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 30

31 PEMFC FUNCTIONING : AGEING 31

32 PEMFC FUNCTIONING : CHARACTERISTICS AND PERFORMANCES Working temperature : C Working pressure : 1-3 bar Electrical efficiency : 40-50% Real voltage : 0,6-0,95 V Current density : up to several A/cm 2 Set-up time : practically instantaneous Response time : very fast Life time : 1000 to 2000 h (values in 2005) 32

33 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 CO 2 - Solid structure Disadvantages PEMFC FUNCTIONING : CHARACTERISTICS AND PERFORMANCES - Polymer membrane and ancillary components are expensive - Active water management is often required - Uses expensive platinum catalyst (Pt) - Very poor CO ( > ppm) and sulphur resistances - Lifetime and reliability to improve 33

34 PEMFC FUNCTIONING : APPLICATIONS - 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 34

35 DMFC ELECTRODE REACTIONS Anode : Cathode : Global reaction : CH 3 OH + + H 2O Pt / Ru CO2 + 6 H + 6 e + 3 2O2 + 6 e + 6 H Pt 3 H 2O CH OH + 3 O CO + H O ( liquid ) heat 2 H + ions cross the membrane G = nfe 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. 35

36 DMFC MEMBRANE ELECTRODE ASSEMBLY Electrolyte : thin membrane ( µm) in perfluorinated sulfonic acid polymer Catalyst : platinum or platinum/ruthenium (deposited in the form of very fine particles) Electrodes : generally carbon powder deposited on a support (GDL) 36

37 DMFC CROSSOVER Methanol crossover from the anode to the cathode Methanol 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 CO 2 production - Catalyst poisoning (Pt) in the cathode by methanol which is accompanied with a loss of catalytic activity 37

38 DMFC CROSSOVER Determination of the methanol amount crossing the electrolyte by measurement of the CO 2 volume produced in the cathode. Methanol crossover conversely proportional to the membrane thickness A thicker membrane = higher electrical resistance 38

39 DMFC CATALYST Pt/Ru in the anode and Pt in the cathode Used quantities are much greater, about several mg per cm 2 (compared to less than 1 mg/cm 2 ), 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 39

40 DMFC FUNCTIONING By-products : water and carbon dioxide in gaseous state 40

41 DMFC FUNCTIONING: FUEL AND OXIDANT Liquid fuel = simplification of the storage and supply systems. CH 3 OH 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 CO 2 produced to pressurise the tank containing methanol O 2 or air supply : passive or active (compressor, fan or compressed air) 41

42 DMFC FUNCTIONING: WATER MANAGEMENT Methanol oxidation reaction consumes water in the anode CH 3 OH + + H 2O Pt / Ru CO2 + 6 H + 6e In the cathode, the oxygen reduction produces water + 3 2O2 + 6 e + 6 H Pt 3 H 2O For one mole consumed, three moles are produced CH OH O2 CO2 + 2 H 2O ( liquid ) + heat 2 Global reaction = water excess must be evacuated 42

43 DMFC FUNCTIONING: WATER MANAGEMENT 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) 43

44 DMFC FUNCTIONING: THERMAL MANAGEMENT Methanol in solution = better thermal regulation by using this fuel as fluid cooler FUNCTIONING: CO 2 MANAGEMENT Elimination of CO 2 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 CO 2 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. 44

45 DMFC FUNCTIONING: AGEING 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 45

46 DMFC CHARACTERISTICS AND PERFORMANCES Working temperature : ± 60 C Working pressure : from 1 to 3 bar Electrical efficiency : from 30 to 40% Real voltage : from 0,4 to 0,7 V Current density : from 100 to 200 ma/cm 2 Set-up time : instantaneous Response time : very short 46

47 Advantages DMFC CHARACTERISTICS AND PERFORMANCES - 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 CO 2 47

48 Disadvantages DMFC CHARACTERISTICS AND PERFORMANCES - Membrane cost is expensive - Methanol crossover - Expensive catalyst (platinum) - Very low efficiency - Sensitive to CO for a concentration higher than ppm - Production of CO 2 - Transport of cartridges of methanol in planes not still authorized (in decembre 2006) - Lifetime and reliability to improve 48

49 DMFC APPLICATIONS - 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 49

50 ADVANTAGES OF ETHANOL DEFC - 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) 50

51 DEFC ELECTRODE REACTIONS Anode : C 2 H 5 OH H 2O Pt / Ru 2CO H + 12 e Cathode : + 3O e + 12 H Pt 6 H 2O Global reaction : C H OH 3O 2CO + H O ( liquid ) + heat Important decrease of efficiency due to complex secondary reactions 51

52 DEFC ELECTRODE REACTIONS CH H 2 ( CH CHO) Pt ads O CH OH OH + H + + e ( ) + CH CHO + OH CH COOH + H + e 3 Pt ads ads ads Pt + 3 H + e 52

53 DEFC Voltage Theoretical voltage = 1,145 V G = 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 53

54 PAFC ELECTRODE REACTIONS Anode : H 2 + Pt or Pt alloy 2 H + 2 e Cathode : Global reaction : + 1 2O2 + 2 e + 2 H Pt H 2O H 1 O H O ( vapour ) + heat

55 PAFC ELECTROLYTE Phosphoric acid H 3 PO 4 (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 C T< 150 C: weak ionic conductivity of the electrolyte T > 210 C: decomposition of the electrolyte T < 190 C: dissolution in water T < 42 C : the electrolyte coagulates and the volume increases 55

56 PAFC CATALYST AND ELECTRODES 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/cm 2 ) 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) 56

57 PAFC FUNCTIONING Range of temperatures: between 160 and 200 C Fuel = hydrogen The anode tolerates CO 2 without any influence on its performances Use of H 2 produced by decomposition of hydrocarbons O 2 or air supply: passive or active (compressor, fan or compressed gas) Water management: water in vapour state and evacuated by air or oxygen circulation 57

58 PAFC FUNCTIONING Thermal management Electrolyte decomposition at ~ 210 C Electrolyte 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 58

59 PAFC CHARACTERITICS AND PERFORMANCES Working temperature : ± 200 C Working pressure : from P atm to 8 bar Electrical efficiency : 40-50% Real voltage : 0,5-0,8 V Current density : up to 800 ma/cm 2 Set-up time : from 1 to 3 h Response time : very short Lifetime : > h (in 2005) 59

60 Advantages PAFC CHARACTERITICS AND PERFORMANCES - Low working temperature - Not sensitive to CO 2 - 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 60

61 PAFC APPLICATIONS - 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. 61

62 AFC ELECTRODE REACTIONS Anode : Cathode : Global reaction : H Ni OH 2 H 2O + e 1 2O2 + 2e + H 2O Ag 2OH H 1 O H O ( liquid ) + heat OH - ions circulate in the solution - H 2 O 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 62

63 AFC ELECTROLYTE 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 CO 2 (reaction with formation of insoluble K 2 CO 3 in the electrolyte blocking of pores and decrease of the fuel cell efficiency) One part of OH - ions are not available for hydrogen oxidation 63

64 AFC 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). 64

65 AFC FUNCTIONING Range of temperatures : generally between 60 and 90 C Some applications at 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 65

66 AFC FUNCTIONING 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 of performances Carbonates formation is possible: loss of performances 66

67 AFC FUNCTIONING Fuel = pure hydrogen O 2 or air supply = air can be used as oxidant instead of O 2 but CO 2 must be eliminated (air contains about 300 ppm). The CO 2 elimination can be done, for example, by reaction with sodium hydroxide. Water management For 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 step In 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) 67

68 AFC FUNCTIONING 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. 68

69 AFC CHARACTERISTICS AND PERFORMANCES Working temperature : between 60 and 90 C Working pressure : from 1 to 5 bar Electrical efficiency : > 60% Real voltage : from 0,7 to 1 V Current density : from 100 to 200 ma/cm 2 Set-up time : some tens of min Response time : quite short Lifetime : about 5000 h (in 2006) 69

70 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 AFC CHARACTERISTICS AND PERFORMANCES - Sensitive to CO 2 - Corrosive electrolyte - Required to use pure gases 70

71 Applications AFC CHARACTERISTICS AND PERFORMANCES - Potentially, better ratio cost/power delivered - Nowadays, use for applications of average power (up to some kw) 71

72 MCFC ELECTROCHEMICAL REACTIONS Anode : 2 ( CO ) H O + CO + H + 2e Cathode : CO 2 + e ( ) 2 O CO 2 3 Global reaction : H + 1 O2 + CO2 H 2O( vapor ) + CO2 + heat 2 2 (CO 3 ) 2- ions circulate in electrolyte and a «transfer» of CO 2 between the anode and the cathode is necessary. The delivered voltage depends on the partial pressures of the reactants and the products (H 2, O 2, H 2 O, CO 2 ). 72

73 MCFC ELECTROCHEMICAL REACTIONS 73

74 MCFC ELECTROLYTE. Mixture of carbonates (Li 2 CO 3, K 2 CO 3 ) in a porous matrix of aluminium and lithium oxides (LiAlO 2 ) 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. 74

75 MCFC CATALYST 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 oxide doped with lithium in the cathode. 75

76 MCFC FUNCTIONING. Temperature : generally between 600 and 700 C Fuel : Hydrogen generally resulting from reactions of decomposition of hydrocarbons within the same fuel cell The sulphur must be eliminated (inhibitor of the anodic reaction at a concentration of a few ppm) Oxidant = mixture of CO 2 and O 2 in a ratio 2 : 1 according to the stoichiometric reaction Water management: the produced steam is got back in the anode 76

77 MCFC FUNCTIONING. 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 electrolyte Example: Dissolution of Ni 2+ 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 LiAlO 2 electrolyte matrix (Increase of the particles size and the porosity) The functioning at atmospheric pressure minimizes the nickel dissolution 77

78 MCFC CHARATERISTICS AND PERFORMANCES Working temperature : from 650 to 700 C Working pressure : from 1 to several bar Electrical efficiency : 55% Real voltage : from 0,75 to 0,9 V Current density : up to 200 ma/cm 2 Set-up time : up to several hours Response time : Size depending Lifetime : several thousands hours 78

79 CHARATERISTICS AND PERFORMANCES 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 CO 2 management Electrolyte loss Dissolution of the cathode (in nickel) Low current density 79 MCFC

80 MCFC APPLICATIONS - Stationary industrial use of high power (up to several MW of electricity and heat) and military applications (standby power) 80

81 SOFC ELECTRODES REACTION Anode : Cathode : H O H 2O + e CO O CO2 + 2e / CH 4 + 4O 2 H 2O + CO2 + e 1 2O e O 2 Global reaction : H 1 O H O ( vapor ) + heat O 2- ions circulate in the electrolyte. Theoretical voltage at 900 C is about 0,95 V (fuel cell using pure H 2 and O 2 ). 81

82 SOFC ELECTRODES REACTION 82

83 SOFC TUBULAR SOFC DESIGN Air is fed through the inside of the tubes while the fuel stream is fed along the outside of the tubes 83

84 SOFC PLANAR SOFC DESIGN 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. 84

85 SOFC PLANAR SOFC DESIGN Two approaches for the elementary cells: - Classic structure where components (electrodes and electrolyte) piled in the form of sheets, the access of the fuel and the oxidant by both opposite faces. - Concentric structure with access of fuel by the center. 85

86 SOFC PLANAR SOFC DESIGN 86

87 SOFC ELECTROLYTE Mixture of oxides called YSZ (Yttrium Stabilized Zirconia) composed of zirconium oxide (ZrO 2 ) stabilised by yttrium (Y 2 O 3, from 8 to 10%). Good ionic conductivity at very high temperature (about 0,13 S.cm -1 at 1000 C) 87

88 SOFC ELECTROLYTE Ionic conductivity due to defects in the crystalline structure. 88

89 SOFC ELECTROLYTE 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 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. 89

90 SOFC ELECTRODES 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 lanthanumstrontium-magnesium (LSM) for the cathode. CATALYST At high working temperatures, it is not necessary to have precious metals like catalysts. 90

91 SOFC FUNCTIONING Temperature : between 850 and 1000 C. At these high working temperatures, other fuels such as carbon monoxide CO and methane (CH 4 ) can be directly used. CH 4 CO + O + 4O 2 2 CO 2 H e O + CO 2 + 2e Water management: the water is produced in vapour phase and can be used to activate a turbine or to heat buildings or houses (cogeneration). 91

92 SOFC FUNCTIONING Thermal management : Isolate the cell of the atmosphere in order to reduce the thermal losses. 92

93 SOFC CHARACTERISTICS AND PERFORMANCES Working temperature : C Working pressure : 1-10 bar Electrical efficiency : 60 % Real voltage : 0,7-1,15 V Current density : up to 1000 ma/cm 2 Set-up time : up to several hours Response time : slow Lifetime : > h 93

94 SOFC CHARACTERISTICS AND PERFORMANCES 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 94

95 Applications - Use for stationary applications (or mobile with long working time) from several kw to several hundreds of kw. - Cogeneration SOFC CHARACTERISTICS AND PERFORMANCES 95

96 CRITERIA OF COMPARISON Type PEMFC T ( C) 80 Fuels H 2 Subproduct Subproducts - A N O D E Ions H + C A T H O D E H 2 O Oxidant O 2 or air DMFC 110 CH 3 OH CO 2 H + H 2 0 O 2 or air PAFC 200 H 2 - H + H 2 O O 2 or air AFC 80 H 2 H 2 0 OH - - O 2 or air MCFC 650 H 2 H 2 0 (CO 3 ) 2- CO 2 O 2 or air SOFC H 2, CO, CH 4 H 2 O, CO 2 O 2- - O 2 or air 96

97 CRITERIA OF COMPARISON Application Portable Residential Transports Central Power W 1 10 kw kw 100 kw 10 MW PEMFC DMFC PAFC AFC MCFC SOFC 97

98 CRITERIA OF COMPARISON 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 98

99 CRITERIA OF COMPARISON 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 a high 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. 99

100 STACK 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. 100

101 STACK Stack of 30 SOFC for a volume of 2,5 L and a weight of 9 kg 101

102 STACK DESIGN 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. 102

103 103 STACK DESIGN

104 STACK DESIGN - Low weight (materials density and thicknesses) - Durability - Corrosion resistance - Very good electrical conductivity (> 100 S cm-1) - Very good thermal conductivity - Gas impermeability (permeability cm 3.cm -2.s -1 ) - Hydrophobic (PEMFC, DMFC, etc..) - Low cost 104

105 BIPOLAR PLATES MATERIALS - 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). 105

106 BIPOLAR PLATES MATERIALS Important weight: up to 80-90% of the global weight for a PEMFC 106

107 BIPOLAR PLATES MATERIALS Electrical Conductivity (S.cm -1 ) Thermal conductivity (W/mK) Density Corrosion resistance Chemical resistance Cost Graphite composite 200 à 300 Up to 50 1,6-2,0 High High High Metal (Al) à (Cu) 120 (Al) à 400 (Cu) From 2,7 to 8,8 (depending on the metal) with surface treatment with surface treatment Low 107

108 BIPOLAR PLATES HYDRAULIC ROLE To avoid the mixture between the different fluids and to reduce the losses: use of fluid seals 108

109 BIPOLAR PLATES HYDRAULIC ROLE 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. 109

110 BIPOLAR PLATES HYDRAULIC ROLE 110

111 BIPOLAR PLATES HYDRAULIC ROLE 111

112 BIPOLAR PLATES HYDRAULIC ROLE 112

113 BIPOLAR PLATES ELECTRICAL ROLE 113

114 BIPOLAR PLATES THERMAL ROLE 114

115 BIPOLAR PLATES THERMAL ROLE 115

116 BIPOLAR PLATES THERMAL ROLE 116

117 BIPOLAR PLATES THERMAL ROLE Bipolar plates in moulded graphite 117