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  Zn 2+ + 2e - Cathode reaction: 2NH 4 + + 2MnO 2 + 2e -  Mn 2 O 3 + 2NH 3 + H 2 O Overall reaction: Zn+2NH 4 Cl+2MnO 2  ZnCl 2 +Mn 2 O 3 +2NH 3 +H 2 O Battery runs down once Zn is corroded away

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

5. Rechargable Battery: Anode reaction:Cathode reaction: Cd + 2OH -  Cd(OH) 2 + 2e - NiO 2 + 2H 2 O + 2e -  Ni(OH) 2 + 2OH - Overall reaction: Cd + NiO 2 + 2H 2 O  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 CH 4 or CH 3 OH) and O 2 (from air) Like a battery it produces electricity using chemicals.

7. H2H2 O2O2 - + e-e- e-e- H2H2 O2O2 e-e- e-e- H 2 O + electricity  H 2 + O 2 H 2 + O 2  H 2 O + electricity ElectrolysisFuel Cell

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

9. FUEL Cathode Reduction / Anode Oxidation (CROA)

10. Types Of Fuel Cell

11. e -  electrolyte Alkali H 2 + 2OH -  2H 2 O +2e  OH - O 2 +2H 2 O + 4e  4OH - Phosphoric Acid H 2  2H + + 2e H+H+ O 2 + 4H + + 4e  2H 2 O Molten Carbonate H 2 + CO 3 2-  CO 2 +2e  CO 3 2- O 2 + 2CO 2 + 4e  2CO 3 2- Solid Polymer H 2  2H + + 2e H+H+ O 2 + 4H + + 4e  2H 2 O Solid Oxide H 2 + O 2-  H 2 O + 2e  O 2- O 2 + 4e  2O 2- Anode Reactions Cathode Reactions Chemical reactions in different fuel cells Notes:Cathode Reduction of O 2 (reaction of electrons), Anode Oxidation of Fuel (generation of electrons). ELECTROLYTES – solutions, molten salts and solid polymers / oxides

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

13. H+H+ Anode / Oxidation Cathode / Reduction ELECTROLYTE= H 3 PO 4 (aq) Charge Carrier = H + O 2 +4H + + 4e  2H 2 OH 2  2H + +2e Phosphoric Acid Fuel Cell e-e- e-e-

14.  CO 3 2- Anode / Oxidation Cathode / Reduction ELECTROLYTE= Na 2 CO 3 (l) Charge Carrier = CO 3 2- O 2 + 2CO 2 + 4e  2CO 3 2- H 2 + CO 3 2-  CO 2 +2e Molten Carbonate Fuel Cell e-e- e-e-

15. H+H+ Anode / Oxidation Cathode / Reduction ELECTROLYTE= “Plastic” Membrane Charge Carrier = H + O 2 +4H + + 4e  2H 2 OH 2  2H + +2e Proton Exchange Membrane Fuel Cell e-e- e-e- Same reactions / charge carriers as H 3 PO 4 – different operating conditions

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

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. High Efficiency Not load dependent Zero emissions No moving parts FUEL CELL INTERNAL COMBUSTION Low Efficiency very load dependent NOx, CO, HC, particulates Several moving parts Expensive Low power density H 2 as fuel Not well developed Cheap High power density Developed fuel infrastructure Reliable

20. Processes in a PEM fuel Cell H 2  2H + + 2e - Oxidation on anode H + travels through membrane to anode e - travels through circuit to anode (doing work) O 2 reacts with H + and e - to form H 2 O (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 (H 2  2H + + 2e - ) Pt/Ru/WO 3 /SnO 2 on Carbon electrode. Ru decreases CO chemisorption / SnO 2 and WO 3 enhance CO oxidation CATHODE CATALYST (O 2 + 4H + + 4e -  2H 2 O) 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 O 2 (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 H 2 O) through to the electrolyte

27. Fuel for powering fuel cells CH 3 OH would be desirable Easily synthesised, lots of chemical energy released during “combustion” Liquid Can be used but is limited Direct Methanol Fuel Cell

28. CH 3 OH + H 2 O  CO 2 Anode reaction: CH 3 OH + H 2 O  CO 2 + 6H + +6e - H+ H+  H+ H+  H+ H+  H+ H+  e - e - e -   O2 O2 H2OH2O Cathode reaction;1.5 O 2 + 6H + + 6e -  3H 2 O Problems1 anodes not very active / stable 2 Methanol diffuses through electrolyte (short circuiting the cell) Overall Reaction CH 3 OH + 1.5 O 2  CO 2 + 2 H 2 O

29. Fuel for powering Fuel Cells H 2 is the fuel of choice since it is easily activated, produces only H 2 O as a by-product and does not harm the anode. Hydrogen Is abundant on earth Can be produced from fossil fuels (C x H y ) or from Water (H 2 O) However it is a gas and therefore has very low energy density at STP 1 Litre Petrol  9,100 Wh 1 Litre H 2  2.8 Wh

30. Storing H 2 on Board Store in carbon nanotubes – tubes made from C 60 units can reversibly store large amounts of H 2  lighter than the metal hydrides but still too heavy Store as a Metal Hydride – Ti 2 Ni-H 2..5, FeTi-H 2 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 Liquefy – 20 K, 2 Bar – again size / bulk + expensive refrigeration etc. – extra cost no H 2 economy / infrastructure PHYSICAL STORAGE compress 200 - 600 bar – Size issues, safety issues, weight issues  extra cost

31. CHEMICAL STORAGE Far higher energy density if stored as another chemical and then “reformed” to H 2 on board – Hydrocarbons / methanol / ammonia. Infrastructure is already in place

32. Reforming of Hydrocarbons (or Methanol) to H 2 (A) Steam reforming: C x H y + H 2 O  H 2 + CO + CO 2 An endothermic process (B) Partial Oxidation xs C x H y + O 2  CO + CO 2 + H 2 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 H 2

33. PROBLEMS  (a) as well as H 2 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-CO ads.

35. Methods for dealing with CO in the reformate (A)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 (SnO 2 or WO 3 ). 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 O 2 Selective methanation using H 2 present in the reformate Permeable membrane that allows H 2 through but not CO Pressure swing adsorption Needs High Pressure

36. CO selective oxidation Generate a catalyst that will CO+O 2  CO 2 but not 2H 2 + O 2  2H 2 O Au/Fe 2 O 3 /TiO 2 CO adsorbs selectively on Au O ads delivered from Fe 2 O 3 rather than from gas mechanism prohibits H 2  H 2 O

37. e-e- Air / O 2 H 2 / H 2 O N 2, O 2, H 2 OH 2 / H 2 O Cathode Exhaust Anode Exhaust Anode Cathode Need to generate H 2

38. REFORMER e-e- Air / O 2 H 2 / H 2 O N 2, O 2, H 2 OH 2 / H 2 O Cathode Exhaust Anode Exhaust Anode Cathode Air CH 3 OH H2OH2O H 2, H 2 O, CO 2, N 2, CO Needs to be removed

39. Excess H 2 should be used (burn it to generate heat and add extra “power” to reformer) REFORMER e-e- Air / O 2 H 2 / H 2 O N 2, O 2, H 2 OH 2 / H 2 O Cathode ExhaustAnode Exhaust Anode Cathode Air CH 3 OH H2OH2O CO Clean Up Air Bleed (for electro- oxidation) H 2, H 2 O, CO 2, N 2, CO

40. Anode Exhaust Burner Start Up CH 3 OH combustion REFORMER e-e- Air / O 2 H 2 / H 2 O N 2, O 2, H 2 OH 2 / H 2 O Cathode ExhaustAnode Exhaust Anode Cathode Air CH 3 OH H2OH2O CO Clean Up H 2, H 2 O, CO 2, N 2, CO

41. Anode Exhaust Burner REFORMER e-e- Air / O 2 H 2 / H 2 O N 2, O 2, H 2 OH 2 / H 2 O Cathode ExhaustAnode Exhaust Anode Cathode Air CH 3 OH H2OH2O CO Clean Up H 2, H 2 O, CO 2, N 2, CO CATALYSTS To Battery, Car, CHP etc.