Transcript Slide 1
Fuel Cells: Fundamentals, Types, and Fuel Storage Carly Reed History 1839 1889 J.H. Reid – first to use NaOH in place of acid electrolyte 1952 Term “fuel cell” coined by Ludwig Mond 1902 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 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 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 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 Fuel Cells: Types Fuel cell types can be divided in two ways: Low v. High Temperature Electrolyte Types Alkaline Polymer Electrolyte Membrane (Proton Exchange Membrane) Direct Methanol Phosphoric Acid Molten Carbonate Solid Oxide 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 Alkaline Fuel Cell Anode: C/Pt or C/Raney Ni/Pt Cathode: C/Pt r.t.-80oC H2 1 A/cm2 at 0.7 V O2 H2O OH35% KOH O2 + H2O + 2e- HO2- + OHH2 + 2OH- H2O + 2e- HO2- + H2O + 2e- 3OH- 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 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- Polymer Electrolyte Membrane Fuel Cell Used by NASA in Gemini mission Nafion – developed by Dupont (1960s) employed polystyrene sulfonate (PSS) polymer (unstable) 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 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 Polymer Electrolyte Membrane Fuel Cell Anode: C/Pt 85-105oC H2 Cathode: C/Pt O2 H+ H2O NAFION O2 + 2H+ + 2e- H2O2 H2 2H+ + 2e- H2O2 + 2H+ + 2e- H2O 1 A/cm2 at 0.7 V 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 Direct Methanol Fuel Cell Anode: Pt/Ru/C 400 mA/cm2 at 0.5V at 60oC 85-105oC Cathode: Pt/C N A F I O N O2 + 2H+ + 2e- H2O2 CH3OH + H2O CO2 + 6H+ + 6e- H2O2 + 2H+ + 2e- H2O 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 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) Direct Methanol Membrane Fuel Cell Solving the Crossover Dilemma Alter thickness of polymer membrane Cs+ doped membranes Thinner = decreases ion flow resistance Thicker = decreases MeOH crossover 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 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 Phosphoric Acid Fuel Cell Anode: Pt/C 200oC CH4 or H2 O2 H+ PTFE binding 100% H2PO4 H2 – 2e- = 2H+ Cathode: Pt/C Si matrix separator H2O O2 + 4H+ + 4e- 2H2O 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 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 Molten Carbonate Fuel Carbonate Anode: Ni/Al or Ni/Cr 580-700oC H2, CxH2x+2 O2, CO2 CO32- 150 mA/cm2 at 0.8 V at 600oC Li2CO3 and Na2CO3 CH4 + 2H2O 4H2 + CO2 + 4eH2 +CO32- H2O + CO2 + 2e- Cathode: NiO LiAlO3 used to support electrolyte O2 + 2CO2 + 4e- 2CO32- 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) 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 Solid Oxide Fuel Cell Anode = NiO-YSZ cermet 800-1000oC H2, CxH2x+2 O2 O2- 1mA at 0.7V Y doped ZrO2 H2 + O2- H2O + 2e- OR CH4 + 4O2- 2H2O + CO2 + 8e- Cathode = La1-xSrxMnO3 Interconnector material = Mg or Sr doped lanthanum chromate O2 + 2e- 2O2- 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 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. 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 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 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 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 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 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 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 References Carrette, Linds. Friedrich, K. Stimming, Ulrich. Fuel Cells: Principles, Types, Fuels, and Applications. Chemphyschem 2000, 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 Institute 2005, 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