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Fuel Cells: Fundamentals, Types, and Fuel Storage

Fuel Cells: Fundamentals, Types, and Fuel Storage. Carly Reed. History. 1839 Sir William Grove – “Gas Voltaic Battery” Two Pt strips surrounded by closed tubes containing H 2 and O 2 in dilute H 2 SO 4 Produced H 2 O and electricity, but very inconsistent 1889

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Fuel Cells: Fundamentals, Types, and Fuel Storage

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  1. Fuel Cells: Fundamentals, Types, and Fuel Storage Carly Reed

  2. History • 1839 • Sir William Grove – “Gas Voltaic Battery” • Two Pt strips surrounded by closed tubes containing H2 and O2 in dilute H2SO4 • Produced H2O and electricity, but very inconsistent • 1889 • Term “fuel cell” coined by Ludwig Mond • 1902 • J.H. Reid – first to use NaOH in place of acid electrolyte • 1952 • Alkaline fuel cell developed by Francis Bacon - later used in Apollo space missions • 1960-1965 • First successful application achieved with space technology during NASA Apollo space program

  3. Interest in Fuel Cells • Development of fuel cells has lagged behind: • Higher cost • Materials problems • Operational inadequacies • During the 20th century as need for electricity increased, primary fuel sources were still so abundant • Currently, with a desire to decrease: • Dependence on fossil fuels and foreign oil supplies • Emissions of NO2, NO3, SO2, CO2 and their effects on ozone levels, acid rain, and global warming • Fuel cells with renewable energy sources • High electrical efficiency

  4. Fuel Cells: Components and Functions • MEA = membrane electrode assembly (electrolyte and electrodes) • Anode = fuel electrode; electronic conductor and catalyst • Cathode = air electrode; electronic conductor and catalyst • Electrolyte = oxygen-ion conductor, electron inhibitor

  5. Fuel Cells: Types • Fuel cell types can be divided in two ways: • Low v. HighTemperature • Electrolyte Types • Alkaline • Polymer Electrolyte Membrane (Proton Exchange Membrane) • Direct Methanol • Phosphoric Acid • Molten Carbonate • Solid Oxide

  6. Alkaline Fuel Cell • First AFC developed by Francis Bacon (1930s) • In the Apollo missions • 85% KOH • 200-230oC • Ni anode and NiO cathode • Acidic fuel cells had been used, but alkaline had faster oxygen reduction kinetics • Fuel cells were used to provide electricity, cool the ship, and provide potable water

  7. H2 O2 H2O OH- 35% KOH Alkaline Fuel Cell Anode: C/Pt or C/Raney Ni/Pt Cathode: C/Pt r.t.-80oC 1 A/cm2 at 0.7 V O2 + H2O + 2e- HO2- + OH- HO2- + H2O + 2e- 3OH- H2 + 2OH-  H2O + 2e-

  8. Alkaline Fuel Cell • Advantages: • Low cost electrolyte solution (KOH 30-35%) • Non-noble catalyst withstand basic conditions • O2 kinetics faster in alkaline solution • OH- v. H2O

  9. Alkaline Fuel Cell • Problem Areas and Solutions: • Catalysts • Pt – expensive • Raney Ni – wettability; chemical composition - Y. Kiros, Pt/Co alloys; similar ability to reduce O2 - E.D. Geeter et. al testing Ag and Co to replace Pt • Pure gases only • CO32- builds up in electrolyte and clogs pores • CO2 + 2OH- CO32- + H2O • Fe sponges can be inserted to absorb CO2 • Circling electrolyte can slow build up of CO32-

  10. Polymer Electrolyte Membrane Fuel Cell • Used by NASA in Gemini mission • employed polystyrene sulfonate (PSS) polymer (unstable) • Nafion – developed by Dupont (1960s) • Currently used in most PEMs • Polytetrafluoroethylene (PTFE) backbone with a perfluorinated side chain that is terminated with a sulfonic acid group • More stable, higher conductivity • The Dow Chemical Company • Developed a polymer similar to Nafion • Shorter side chain and only one ether oxygen • No longer available

  11. Polymer Electrolyte Membrane Fuel Cell • Chemical structure of Nafion • Hydration of membrane dissociates proton of acid group • Solvated protons are mobile in polymer and provide conductivity

  12. Anode: C/Pt 85-105oC Cathode: C/Pt H2 O2 H+ H2O N A F I O N O2 + 2H+ + 2e-  H2O2 H2O2 + 2H+ + 2e- H2O H2 2H+ + 2e- 1 A/cm2 at 0.7 V Polymer Electrolyte Membrane Fuel Cell

  13. Polymer Electrolyte Membrane Fuel Cell • Advantages: • Nonvolatile membrane • CO2 rejecting electrolyte • few material problems • Problems: • Slow O2 kinetics • Hydration of membrane is difficult (30-60%) • Formed at cathode, but difficult to keep in membrane • Too little = dehydration and loss of ion transport • Solutions - Humidify gases - Impregnate Nafion with SiO2 or TiO2

  14. Direct Methanol Fuel Cell Anode: Pt/Ru/C 85-105oC Cathode: Pt/C N A F I O N 400 mA/cm2 at 0.5V at 60oC O2 + 2H+ + 2e- H2O2 H2O2 + 2H+ + 2e- H2O CH3OH + H2O CO2 + 6H+ + 6e-

  15. Direct Methanol Fuel Cell • Pt catalyst have highest activity for MeOH oxidation thus far • Ru enhances MeOH catalytic activity OH- forms at lower voltage • CO blocks sites on Pt surface, Ru helps oxidize to CO2

  16. Direct Methanol Membrane Fuel Cell • Advantages: • Direct fuel conversion – no reformer needed, all positive aspects of PEMFC • CH3OH – natural gas or biomass • Existing infastructure for transporting petrol can be converted to MeOH • Problems: • High catalyst loading (1-3mg/cm2 v. 0.1-0.3 mg/cm2) • CH3OH hazardous • Low efficiency (MeOH crossover – lowers potential)

  17. Direct Methanol Membrane Fuel Cell • Solving the Crossover Dilemma • Alter thickness of polymer membrane • Thinner = decreases ion flow resistance • Thicker = decreases MeOH crossover • Cs+ doped membranes • Tricolli, University of Pisa, 1998 • Lower affinity for H2O • MeOH tolerant cathodes • Mo2Ru5S5 – N. Alonso-Vante, O. Solorza-Feria • Higher oxygen reduction activity in presence of MeOH • (Fe-TMPP)2O – S. Gupta, Case Western, 1997 • High oxygen reduction, insensitive to MeOH

  18. Phosphoric Acid Fuel Cell • Most commercially developed fuel cell • Mainly used in stationary power plants • More than 500 PAFC have been installed and tested around the world • Most influential developers of PAFC • UTC Fuel Cells, Toshiba, and Fuji Electric

  19. Phosphoric Acid Fuel Cell Anode: Pt/C 200oC Cathode: Pt/C CH4 or H2 O2 H+ Si matrix separator PTFE binding H2O 100%H2PO4 H2 – 2e- = 2H+ O2 + 4H+ + 4e- 2H2O

  20. Phosphoric Acid Fuel Cell • Advantages: • H2O rejecting electrolyte • high temps favor H2O2 decomposition • O2 + H2O +2e- H2O2 • Stable H2O2 lowers cell voltage and corrodes electrode • Problems: • O2 kinetic hindered • CO catalyst poison at anode • H2 only suitable fuel • low conducting electrolyte

  21. Molten Carbonate Fuel Carbonate • Developed in the mid-20th century • Developed because all carbonaceous fuel produce CO2 • Using CO32- electrolyte eliminates need to regulate CO32- build up

  22. Molten Carbonate Fuel Carbonate Anode: Ni/Al or Ni/Cr 580-700oC Cathode: NiO H2, CxH2x+2 O2, CO2 CO32- LiAlO3 used to support electrolyte 150 mA/cm2 at 0.8 V at 600oC Li2CO3 and Na2CO3 CH4 + 2H2O  4H2 + CO2 + 4e- H2 +CO32- H2O + CO2 + 2e- O2 + 2CO2 + 4e-  2CO32-

  23. Molten Carbonate Fuel Cell • Advantages: • Higher efficiency (v. PEMFC and PAFC) (50-70%) • Internal reforming (H2 or CH4) • No noble metal catalyst (High T increases O2 kinetics) • No negative effects from CO or CO2 • Problems: • Materials resistant to degradation at high T • Ni, Fe, Co steel alloys better than SS • NiO at cathode leeches into CO32- reducing efficiency or crossing over causing short circuiting • Dope electrode and electrolyte with Mg • Kucera and Myles (LiFeO2 or Li2MnO3 stabilize)

  24. Solid Oxide Fuel Cell • 1899 Nernst observed conduction in various types of stabilized zirconia at T > 600oC • 1937 Baur and Preis demonstrated a fuel cell based on zirconium oxide

  25. Solid Oxide Fuel Cell 800-1000oC Anode = NiO-YSZ cermet Cathode = La1-xSrxMnO3 H2, CxH2x+2 O2 O2- Interconnector material = Mg or Sr doped lanthanum chromate 1mA at 0.7V Y doped ZrO2 H2 + O2-  H2O+ 2e- OR CH4 + 4O2- 2H2O + CO2 + 8e- O2 + 2e-  2O2-

  26. Solid Oxide Fuel Cell • Advantages: • Solid electrolyte eliminates leaks • H2O management, catalyst flooding, slow O2 kinetic are not problematic • CO and CO2 are not problematic • Internal reforming - almost any hydrocarbon or hydrogen fuel • Problems: • Severe material constraints due to high T • Stainless steal at lower temperatures • Alloyed metal or Lanthanum Chromite material

  27. Fuel Cell Stacks • Individual Cell 0.5-1.0V • Increase system voltage by stacking cells • Cells’ voltages are added in series; current constant over all cells • Interconnects act as flow channels for gases and connects anode of one cell to cathode of the next. Must be gas tight and made from conducting material.

  28. Applications Fuel cells are being developed for application in: • Stationary power plants • Automobiles • Portable electronics To enable mobile power source, fuel must also be portable

  29. Hydrogen Storage: Gas and Liquid • Pure H2 gas • eliminates reformer • eliminates risk of catalyst degradation from impure fuel • space limitations • explosive • Liquid H2 • highest energy density of any H2 storage method • limited by boiling point (-253oC) • 1-2% evaporation each day

  30. Hydrogen Storage: Metal Hydrides • A metal alloy exposed to H2 MH • Upon heating H2 released • 150-700 cm3/g • “Powerballs” (Powerball Technology Inc) • NaH pellets coated in waterproof skin

  31. Hydrogen Storage: Ammonia Borane • S. Shore (1955) • Ammonia Borane H3NBH3 • Advantages over MH • Air and Water Stable • Heat to release H2 • 19% wt. storage of H2 • Developed by Millennium Cell

  32. Hydrogen Storage • Carbon Nanotubes, Glass Microspheres, Zeolites • H2 can permeate at elevated P and T • At ambient T and P, H2 is trapped in structure • Heating releases H2

  33. Hydrogen Storage: Zeolites • D. Fraenkel (1977) • Tested by Fritz and Ernst (1995) • Cs3Na9(AlO2SiO2)12 • Loaded at 2.5-10.0 MPa at 573oC • 9.2cm3/g

  34. Fuel Reformation • Catalytic steam reformation • Light hydrocarbons and alcohols (highest yield reforming process) • Endothermic • Partial oxidation • Heavier hydrocarbons • Exothermic (Combustion) • Autothermal reforming • Reformed fuel must be treated to remove CO

  35. References • Carrette, Linds. Friedrich, K. Stimming, Ulrich. Fuel Cells: Principles, Types, Fuels, and Applications. Chemphyschem2000, 1, 162-193 • Winter, Martin. Brodd, Ralph. What Are Batteries, Fuel Cells, and Supercapacitors? Chem. Rev. 2004, 104, 4245-42969 • Kee, Robert J. Zhu, Huayang. Goodwin, David G. Solid-oxide fuel cells with hydrocarbon fuels. Proceedings of the Combustion Institute2005,2379-2404 • Groves, W.G. Philos Mag (14)1939 127-130 • E.D. Geeter, M.Mangan, S.Spaepen, W. Stinissen, G. Vennekens. J. Power Sources 1999, 80, 207 • Y. Kiros. J. Electrochem. Soc. 1996, 41, 2595 • Mauritz, Kenneth. Moore, Robert B. The State of Understanding Nafion Chem. Rev. 2004, 104, 4535-3585 • Tricoli, V. Journal of the Electrochemical Society 1998, 145 (11), 3798-3801 • Alonso-Vante, N. Tributsch, H. Solorza-Feria, O. Electrochim. Acta 1995, 40, 567. • Gupta, S. Tryk, D. Zecevic, S.K. Aldred, W. Guo, D. Savinelli, R.F. J.Appl. Electrochem. 1998, 28,673 • Status of Carbonate Fuel Cells J. Power Sources 56 (1995) 1-10 • Fraenkel, D. Shabtai, J. Encapsulation of hydrogen in molecular sieve zeolites JACS 1977 7074-7076 • Fritz, M. Ernst,S. Int. J. Hydrogen Energy 1995, 20 (12) 967 • Shore, Sheldon JACS 1956 78 (2) 502-503

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