Fuel Cell Technology Summer School in Energy and Environmental Catalysis

1. Fuel Cell Technology Summer School in Energy and Environmental Catalysis University of Limerick, July 2005

2. Energy – Mostly from Fossil Fuels Significant proportion of Energy for Electricity (flow of electrons) Fossil fuels  electricity via combustion, generating steam, turning of turbines, etc. Electricity from chemicals, i.e. convert energy generated during a chemical reaction (e.g. combustion) directly into electric energy. Separate chemical reaction into two reactions, one generating (pushing) electrons and one consuming (sucking) electrons, flow of electrons between two reactions  usable electricity

3. Dry Cell Batteries Anode reaction: Zn  Zn2+ + 2e- Cathode reaction: 2NH4+ + 2MnO2 + 2e- Mn2O3 + 2NH3 + H2O Overall reaction: Zn+2NH4Cl+2MnO2 ZnCl2+Mn2O3+2NH3+H2O Battery runs down once Zn is corroded away

4. Lead Storage Battery Anode reaction: Pb + HSO4- PbSO4 + H+ + 2e- Cathode reaction: PbO2 + HSO4- + 3H+ + 2e- H2O + PbSO4 Overall reaction: Pb + PbO2 +2H2SO4 2PbSO4 + 2H2O • PbSO4 adheres to both anode and cathode and is converted into Pb (on anode) and PbO2 (on cathode) by forcing current the reverse direction (via the alternator) • Batteries fail if PbSO4 is shaken from electrodes (Pb or PbO2 cannot be regenerated). • “Health” of battery measured by measuring density of electrolyte (changes as H2SO4 is consumed)

5. Rechargable Battery: Anode reaction: Cathode reaction: Cd + 2OH- Cd(OH)2 + 2e- NiO2 + 2H2O + 2e- Ni(OH)2 + 2OH- Overall reaction: Cd + NiO2 + 2H2O  Cd(OH)2 + Ni(OH)2 Products adhere to electrodes and reactants can be regenerated by forcing current in the reverse direction

6. FUEL CELLS Grove in 1839 “an electrochemical device which converts the free-energy change of an electrochemical reaction into electrical energy”. Fuel + Oxidant  Products + Energy Hydrogen (or CH4 or CH3OH) and O2 (from air) Like a battery it produces electricity using chemicals.

7. Electrolysis Fuel Cell - + e- e- e- e- O2 O2 H2 H2 H2O + electricity H2 + O2 H2 + O2 H2O + electricity

8. Conventional System THERMAL ENERGY KINETIC ENERGY BURN FUEL Electric Energy Fuel Cell System Heat Heat Heat

9. FUEL Cathode Reduction / Anode Oxidation (CROA)

10. Types Of Fuel Cell

11. Alkali H2 + 2OH-2H2O +2e  OH- O2 +2H2O + 4e  4OH- Phosphoric Acid H22H+ + 2e H+ O2 + 4H+ + 4e  2H2O Molten Carbonate H2 + CO32-CO2 +2e  CO32- O2 + 2CO2 + 4e  2CO32- Solid Polymer H22H+ + 2e H+ O2 + 4H+ + 4e  2H2O Solid Oxide H2 + O2-H2O + 2e O2- O2 + 4e  2O2- Chemical reactions in different fuel cells Anode Reactions e- Cathode Reactions electrolyte Notes: Cathode Reduction of O2 (reaction of electrons), Anode Oxidation of Fuel (generation of electrons). ELECTROLYTES – solutions, molten salts and solid polymers / oxides

12. Alkali Fuel Cell e- e- OH- H2 + 2OH-2H2O +2e O2 +2H2O + 4e  4OH- ELECTROLYTE = KOH (aq) Charge Carrier = OH- Anode / Oxidation Cathode / Reduction

13. Phosphoric Acid Fuel Cell e- e- H+ H22H+ +2e O2 +4H+ + 4e  2H2O ELECTROLYTE = H3PO4 (aq) Charge Carrier = H+ Anode / Oxidation Cathode / Reduction

14. Molten Carbonate Fuel Cell e- e- CO32- O2 + 2CO2 + 4e  2CO32- H2 + CO32-CO2 +2e ELECTROLYTE = Na2CO3(l) Charge Carrier = CO32- Anode / Oxidation Cathode / Reduction

15. Proton Exchange Membrane Fuel Cell e- e- H+ H22H+ +2e O2 +4H+ + 4e  2H2O Same reactions / charge carriers as H3PO4 – different operating conditions ELECTROLYTE = “Plastic” Membrane Charge Carrier = H+ Anode / Oxidation Cathode / Reduction

16. Solid Oxide Fuel Cell e- e- O2- H2 + O2-H2O +2e O2 + 4e  2O2- ELECTROLYTE = YSZ (same as l sensor in TWC technology) Charge Carrier = O2- Anode / Oxidation Cathode / Reduction

18. Medium term - Potential Uses for Fuel Cells • Combined Heat and Power Plants – for apartment blocks. More efficient than electricity generating stations, Quieter than gas or diesel turbines, Inherent Reliability. • Transport – such as cars / buses etc. Zero Emission Vehicles. • Mobile / Portable power sources – e.g. instead of batteries for mobile phones / PCs / radio communications / military applications. A cartridge containing methanol would be used which would be equivalent to immediate battery recharging

19. Cheap • High power density • Developed fuel infrastructure • Reliable • Expensive • Low power density • H2 as fuel • Not well developed INTERNAL COMBUSTION FUEL CELL • High Efficiency • Not load dependent • Zero emissions • No moving parts • Low Efficiency • very load dependent • NOx, CO, HC, particulates • Several moving parts

20. Processes in a PEM fuel Cell • H2 2H+ + 2e- Oxidation on anode • H+ travels through membrane to anode • e- travels through circuit to anode (doing work) • O2 reacts with H+ and e- to form H2O (and heat)

21. PEM Animations

24. Fuel Cell Stack using PEM fuel cells

25. Allows protons to move through the membrane – must be moist to operate

26. ANODE CATALYST (H2 2H+ + 2e-) Pt/Ru/WO3/SnO2 on Carbon electrode. Ru decreases CO chemisorption / SnO2 and WO3 enhance CO oxidation CATHODE CATALYST (O2 + 4H+ + 4e- 2H2O) Pt/ Carbon with Co / Cr and Ni – all of which are incorporated into the FCC lattice of the Pt. This aids in the adsorption and dissociation of O2 (mechanism unclear). Both electrodes are prepared from an aqueous slurry of the metals of interest (as salts) followed by drying, reduction and heat treatment Electrodes must be porous in order to allow gas molecules (and H2O) through to the electrolyte

27. Fuel for powering fuel cells CH3OH would be desirable Easily synthesised, lots of chemical energy released during “combustion” Liquid Can be used but is limited Direct Methanol Fuel Cell

28. e- e- e- e- e- e- H+ CH3OH + H2O   O2 H+ H+ H+ CO2 H2O Anode reaction: CH3OH + H2O  CO2 + 6H+ +6e- Cathode reaction; 1.5 O2 + 6H+ + 6e-  3H2O Overall Reaction CH3OH + 1.5 O2  CO2 + 2 H2O Problems 1 anodes not very active / stable 2 Methanol diffuses through electrolyte (short circuiting the cell)

29. Fuel for powering Fuel Cells • H2 is the fuel of choice since it is easily activated, produces only H2O as a by-product and does not harm the anode. • Hydrogen • Is abundant on earth • Can be produced from fossil fuels (CxHy) or from Water (H2O) • However it is a gas and therefore has very low energy density at STP • 1 Litre Petrol  9,100 Wh • 1 Litre H2  2.8 Wh

30. Storing H2 on Board PHYSICAL STORAGE compress 200 - 600 bar – Size issues, safety issues, weight issues  extra cost • Liquefy – 20 K, 2 Bar – again size / bulk + expensive refrigeration etc. – extra cost • Store in carbon nanotubes – tubes made from C60 units can reversibly store large amounts of H2 lighter than the metal hydrides but still too heavy • Store as a Metal Hydride – Ti2Ni-H2..5, FeTi-H2 etc. which can be dehydrogenated as needed and regenerated when used – these are very heavy – since the empty “tank” will be full of dehydrogenated metal • no H2 economy / infrastructure

31. CHEMICAL STORAGE Far higher energy density if stored as another chemical and then “reformed” to H2 on board – Hydrocarbons / methanol / ammonia. Infrastructure is already in place

32. Reforming of Hydrocarbons (or Methanol) to H2 (A) Steam reforming: CxHy + H2O  H2 + CO + CO2 An endothermic process (B) Partial Oxidation xs CxHy + O2  CO + CO2 + H2 An exothermic process (C) Auto thermal reforming – a combination of both approaches which is self-sustaining. Followed in Both cases by water Gas shift to (a) remove CO and (b) generate more H2

33. PROBLEMS  (a) as well as H2 significant amounts of CO are formed (2%) and these poison the anode – resulting in far decreased performance and eventually the fuel cell stops working (b) the fuel must be free of sulphur as this poisons BOTH catalysts (reforming catalyst AND electrocatalysts)

34. Effect of CO in the reformate on anode performance CO irreversibly adsorbs on the catalytic Pt Particles Pt + CO  Pt-COads.

35. Methods for dealing with CO in the reformate • Improve the tolerance of the anode • This is done in 2 ways • Alloy the Pt component with Ru. This reduces the CO chemisorption strength and therefore the CO Coverage • Add an Oxide Component to promote the electro-oxidation of CO (SnO2 or WO3). This needs a slight air bleed and is not ideal. • (B) Selectively removing the CO from the Reformate • This can be done in 4 ways • Selective oxidation – through adding O2 • Selective methanation using H2 present in the reformate • Permeable membrane that allows H2 through but not CO • Pressure swing adsorption Needs High Pressure

36. CO selective oxidation • Generate a catalyst that will CO+O2 CO2 but not 2H2 + O2  2H2O • Au/Fe2O3/TiO2 CO adsorbs selectively on Au Oads delivered from Fe2O3 rather than from gas mechanism prohibits H2H2O

37. H2 / H2O N2, O2, H2O e- Anode Exhaust Cathode Exhaust Cathode Anode Air / O2 H2 / H2O Need to generate H2

38. H2 / H2O N2, O2, H2O e- Anode Exhaust Cathode Exhaust Cathode Anode Air / O2 H2 / H2O Needs to be removed H2O H2, H2O, CO2, N2, CO CH3OH REFORMER Air

39. Excess H2 should be used (burn it to generate heat and add extra “power” to reformer) H2 / H2O N2, O2, H2O Anode Exhaust e- Cathode Exhaust Cathode Anode H2O H2, H2O, CO2, N2, CO Air / O2 CH3OH CO Clean Up H2 / H2O REFORMER Air Air Bleed (for electro-oxidation)

40. Start Up CH3OH combustion Anode Exhaust Burner H2 / H2O N2, O2, H2O Anode Exhaust e- Cathode Exhaust Cathode Anode H2O H2, H2O, CO2, N2, CO Air / O2 CH3OH CO Clean Up H2 / H2O REFORMER Air

41. To Battery, Car, CHP etc. Anode Exhaust Burner H2 / H2O N2, O2, H2O Anode Exhaust e- Cathode Exhaust Cathode Anode H2O H2, H2O, CO2, N2, CO Air / O2 CH3OH CO Clean Up H2 / H2O REFORMER Air CATALYSTS