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New Developments in Electrochemical Cells

New Developments in Electrochemical Cells. Science Update Programme. Education Bureau, HKSAR & Department of Chemistry The University of Hong Kong. June 2002. References. Capacitors www.nec-tokin.net www.faradnet.com Green Energy www.greenenergy.org.uk

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New Developments in Electrochemical Cells

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  1. New Developments in Electrochemical Cells Science Update Programme Education Bureau, HKSAR & Department of Chemistry The University of Hong Kong June 2002

  2. References Capacitors www.nec-tokin.net www.faradnet.com Green Energy www.greenenergy.org.uk www.greenenergyohio.org Electric Vehicles Evworld.com Batteries www.nec-tokin.net www.duracell.com Fuel Cells www.fuelcells.com chem..hku.hk/~fuelcell Books:  A.J. Bard, L. Faulkner, “Electrochemical Methods”, 2001, Wiley. Derek Pletcher and Frank C. Walsh, “Industrial Electrochemistry”, Chapman and Hall, 1990. C.A. Vincent and B. Scrosati, “Modern Batteries : An Introduction to Electrochemical Power Sources”, Butterworth-Heinemann, 1998. James Larminie and Andrew Dicks, “Fuel Cell Systems Explained”, Wiley, 2000. Utilities www.ifc.com www.gepower.com Portable Power Sources www.nokia.com www.motorola.com Electrochemical Cells, K.Y. Chan, HKU

  3. Multidisciplinary and Integrated Science • Electrochemistry, General Chemistry • Physical Chemistry:Thermodynamics, Kinetics, Transport • Organic Chemistry • Inorganic, Solid State Chemistry • Materials Science • Basics Physics, Energy, Electricity • Environmental Science and Ecological/Biological Issues • Can be discussed with different emphasis, at different levels, and platforms. Electrochemical Cells, K.Y. Chan, HKU

  4. Fundamental Theories and Concepts • Batteries • Fuel Cells • Applications Electrochemical Cells, K.Y. Chan, HKU

  5. Fundamentals Thermodynamics • Relate Reactivity to Electrode Potential • Nernst Equation accounts for concentration(activity) effects • Calculate Electrode Potential from Free Energy Electrochemical Cells, K.Y. Chan, HKU

  6. Al/Al+3 Zn/Zn+2 H2/H+ Cu/Cu2+ H2O/O2 -1.66 -0.760.00.52 1.23 V Electrochemical Activity Series Electrochemical Cells, K.Y. Chan, HKU

  7. Fundamentals Kinetics • Current  Rate of reaction (Faraday’s law) • Rate (current) described by Tafel Equation or Butler-Volmer Equation (Bard and Faulkner, Wiley 2001) Electrochemical Cells, K.Y. Chan, HKU

  8. Fundamentals Kinetics from Absolute Rate Theory O* Free energy G nF E O + n e- n F E R Reaction co-ordinate Electrochemical Cells, K.Y. Chan, HKU

  9. Current into electrolyte Electrons out of electrode Electrochemical Cells, K.Y. Chan, HKU

  10. i Concentration or pH effect E Electrochemical Cells, K.Y. Chan, HKU

  11. E Ecell Anode Cathode Electrochemical Cells, K.Y. Chan, HKU

  12. Ref. electrode E-Eref E Ecell Anode Cathode Electrochemical Cells, K.Y. Chan, HKU

  13. Fundamentals Transport and Interfaces • Rate of supply of raw materials : diffusion of active materials • Rate of removal of: products including ions, electrons • ionic vs ohmic resistance • Change of solid interfaces: dentritic growth • Wetting/non-wetting affects gas transport into electrolyte • Selectivity of transport, e.g. cationic membrane Electrochemical Cells, K.Y. Chan, HKU

  14. H2SO4 0.6 KOH  ohm-1 cm-1 KCl CH3COOH 10 M concentration Electrochemical Cells, K.Y. Chan, HKU

  15. Electrochemical Cells, K.Y. Chan, HKU

  16. Activation Ohmic Mass-Transfer Current Density Ideal Voltage Cell Voltage Electrochemical Cells, K.Y. Chan, HKU

  17. Some Terminologies Open Circuit Voltage Equilibrium potential, Standard Potential Overpotential, underpotential Polarization (activation, ohmic, concentration) Capacity mA hr Energy Density W hr kg-1 ,W hr l-1 Power Density W kg-1 ,W l-1 ,W cm-2 Current Density mA cm-2 Electrochemical Cells, K.Y. Chan, HKU

  18. Anode: Oxidation reaction, release electrons to external circuit, negative terminal (galvanic cell) • Cathode: Reduction reaction, receive electrons from external circuit, positive terminal (galnanic cell) • Current Collector: continuous electronic conducting solid phase to collect electrons (in anode) and to distribute electrons (in cathode) • Electrolyte: ionic conducting but electronic insulating, transfer ions from/to electrodes • Separator: hydrophilic porous sheet material to hold a thin layer of electrolyte, electronic insulation Electrochemical Cells, K.Y. Chan, HKU

  19. Polymer Electrolyte: polymeric backbone with fixed charge to allow transport of either cation or anion • Porous Matrix to hold electrolyte: Ceramic, asbestos, “polymers”. • Gel/Paste electrolyte: immobilize electrolyte but allow ionic transport • Molten Salt Electrolyte:e.g. Carbonates • Solid Oxide Electrolyte: oxide ion mobiliity at elevated temperature Electrochemical Cells, K.Y. Chan, HKU

  20. Batteries A. Volta, 1880

  21. Primary Batteries: Zn/C Alkaline Zn/HgO Li metal Secondary Batteries: Lead Acid (Rechargeable) Ni-Cd Ni-MH Li ion Hybrid of Battery and Fuel Cell: Zn-Air Al-Air (Regenerative Fuel Cells) Electrochemical Cells, K.Y. Chan, HKU

  22. Batteries Zinc/Carbon (Leclanché 1880s) • Cathode: 2 MnO2 + H2O + 2e- Mn2O3 + 2OH- • Anode: Zn  Zn2+ + 2e- • Overall: 2 MnO2 + Zn + H2O  Mn2O3 + Zn2+ + 2OH- • G=-257 kJ mol-1 , Eo = 1.55 V • electrolyte: moist NH4Cl/ZnCl2/MnO2/C powder • current collectors: graphite rod and zinc • Capacity 6 A hr, energy density 80 Whr kg-1 Electrochemical Cells, K.Y. Chan, HKU

  23. Batteries Zinc/Carbon (Leclanché 1880s) Carbon rod current collector (+ve) separator MnO2 based positive paste Zinc can anode (-ve) Electrochemical Cells, K.Y. Chan, HKU

  24. Batteries Lead/Acid Discharge reactions • Cathode: PbO2 + 4H+ +SO42- + 2e- 2H2O + PbSO4 • Anode: Pb + SO42- PbSO4 + 2e- • Overall: PbO2 + Pb + 4H+ + 2SO42- 2PbSO4 + 2H2O • G= -394 kJ mol-1 , Eo = 2.05 V • electrolyte: aqueous H2SO4 • current collectors: both Pb • Capacity: 2.7 Ahr, Energy density 30 Whr kg-1 • cell voltage> 1.23 V, Electrolysis of water kinetically hindered Electrochemical Cells, K.Y. Chan, HKU

  25. Pb/PbSO4 H2/H+ H2O/O2 PbSO4/PbO2 -0.3505 0.01.23 V 1.698 Possible Electrode Pairs? Electrochemical Cells, K.Y. Chan, HKU

  26. Batteries Nickel/Cadmium • Cathode: 2NiO(OH) + 2H2O + 2e- 2Ni(OH)2 + 2OH- • Anode: Cd + 2OH- Cd(OH)2+ 2e- • Overall: 2NiO(OH) + Cd + 2H2O  2Ni(OH)2 + Cd(OH)2 • G= -283 kJ mol-1 , Eo = 1.48 V • electrolyte: aqueous KOH • current collectors: Ni foam and peforated nickel sheet • Capacity: 4 Ahr, energy density: 33 Whr kg-1 Electrochemical Cells, K.Y. Chan, HKU

  27. Batteries Nickel/Metal hydride • Cathode: NiO(OH) + H2O + e- Ni(OH)2 + OH- • Anode: MH + OH- M + H2O + 2e- • Overall: MH + NiO(OH)  M + Ni(OH)2 • Metal hydride: AB5 e.g. LaNi5 or AB2, e.g. TiMn2 , ZnMn2 • electrolyte: aqueous KOH • current collectors: Ni foam and peforated nickel sheet • Capacity: 4 Ahr, energy density: 80 Whr kg-1 Electrochemical Cells, K.Y. Chan, HKU

  28. Batteries Nickel/Metal Hydride • Overcharging • Cathode: 2 OH- H2O + ½O2 + 2e- • Anode: charge reserve M + H2O + 2e- MH + OH- • Oxygen dissolves to Anode: 2MH + ½ O2 2M + H2O • Prevent gassing and build up of pressure Electrochemical Cells, K.Y. Chan, HKU

  29. Batteries Lithium Ion • Cathode: xLi+ + LiM2O4 + xe- Li1+xM2O4 • M=Mn,Ti • xLi+ + LiMO2 + xe- Li1+xMO2 • M=Co, Ni • Anode: LiC6 x Li++ x e- + Li1-xC6 • Overall: C6 + LiMO2 LixC6 + Li1-xMO2 • LiMn2O4G= -287 kJ mol-1 , Eo = 2.97 V • Energy density > 100 Whr/kg Electrochemical Cells, K.Y. Chan, HKU

  30. Batteries Lithium Ion Electrolyte Anode • Aprotic Solvent • Gel • Polymer (lower weight) • Li in graphite lattice • Lower activity but safer than Li metal Cathode • Solid Structures for storing Li • Spinels, Olivines, rhombohedral NASICON Electrochemical Cells, K.Y. Chan, HKU

  31. Batteries and Fuel Cells Fuel Cells • ReFuel • Continuous • Open system • Mostly Gas/Liquid Fuel • High energy density • Micro to Mega Watts Batteries • Recharge • Intermittent • Closed system • Mostly solid • High power density Electrochemical Cells, K.Y. Chan, HKU

  32. Fuel Cells • Efficient conversion of Chemical Energy to useful energy (without losing to heat, mechanical linkages) • Environmentally friendly • Flexible: from micro to mega • Materials and Nanotechnology Electrochemical Cells, K.Y. Chan, HKU

  33. Fuel Cells Classification according to electrolyte • Alkaline Fue Cells • Proton Exchange Membrane (PEM) • Phosphoric Acid • Molten Carbonate • Solid Oxide Electrolyte Electrochemical Cells, K.Y. Chan, HKU

  34. 燃料電池發電的原理 CxHyOz ===> CO2 + H2O + e- 負極﹕燃料(氫氣﹐酒精﹐ 葡萄糖等) 負極 電解液 電能 正極 正極 ﹕氧氣 ( 氧化劑 ) O2 + e- ===> H2O Electrochemical Cells, K.Y. Chan, HKU

  35. Fuel Cells Chemical Energy Electrical Energy Electrochemical Cells, K.Y. Chan, HKU

  36. Activation Ohmic Mass-Transfer Current Density Ideal Voltage Cell Voltage Electrochemical Cells, K.Y. Chan, HKU

  37. Diversity of Technology andMaterials Problems in Fuel Cells • Fuel • Oxidant • Catalyst • Container • Control • Transport • Storage Electrochemical Cells, K.Y. Chan, HKU

  38. Fuels: Hydrogen Metals Natural Gas Small Hydrocarbons (methanol, glucose) Oxidant: air oxygen halides oxides Catalysts: platinum metals metal oxides macrocycles Catalyst Support: Porous Carbon Ceramic Matrix Molecular Sieves Polymer Container and Movable Parts: Alloys Ceramic Polymers Transport/Electrolyte: Proton Exchange Membranes PTFE (Teflon) Solid Electrolyte Storage: Metal Hydride Electrochemical Cells, K.Y. Chan, HKU

  39. Fuels • Hydrogen H2+2OH-2H2O +2e- 2e- +½ O2+H2O  2 OH- • Methanol CH3OH + H2O  CO2 + 6H+ +6e- 6e- +1½ O2+6H+  3H2O • Aluminium Al + 4OH-Al(OH)4- +3e- 4e- +O2+2H2O  4 OH- • Borohydride NaBH4 + 8 OH-  NaBO2 + 6H2O + 8e- • Methane (natural gas) • Octane : demonstrated in SOFC half cell Electrochemical Cells, K.Y. Chan, HKU

  40. Thermochemistry Electrochemical Cells, K.Y. Chan, HKU

  41. Micro and Nanostructured Electrodes: • Catalyst Support: High Surface Carbon • Size Effects of Catalysts • Controlled Porosity • Controlled Wetting • Maxinum Gas-Liquid-Solid Interface • Minimize ohmic resistance • Minimize ionic resistance Electrochemical Cells, K.Y. Chan, HKU

  42. Scanning Tunneling Spectroscopy Electrochemical Cells, K.Y. Chan, HKU

  43. Catalysts • Platinum is the most important for both anode and cathode • Platinum can be replaced by Ag, Mn, Co, only for oxygen reduction in alkaline medium • Platinum subject to CO poisoning (impure H2) • Binary/Ternary system, macrocycle, bifunctional • Stability/Life of nanometals Electrochemical Cells, K.Y. Chan, HKU

  44. Maximum peak current density at 52.5~77.6% Co, one order of magnitude higher than that of pure Pt particles. One possible role of cobalt in promoting the catalysis of platinum, is the removal of COadCOOHad intermediates. Chi et al., Catalysis Letters, 71 (2001) 21. Electrochemical Cells, K.Y. Chan, HKU

  45. Catalysts • Oxygen Cathode is most limiting and is present in most fuel cells • Non-platinum cathode catalyst can tolerant cross over effect. • At high temperature, no precious metal or no catalysts is needed in MCFC and SOFC Electrochemical Cells, K.Y. Chan, HKU

  46. Performances of different air cathode Electrochemical Cells, K.Y. Chan, HKU

  47. H2 H+ e- Gas Diffusion Electrodes Electronic circuit: continuous solid phase Ionic circuit: Continuous electrolyte phase Materials flow circuit: feed of reactancts Chan et al. , Electrochimica Acta, 32 (1987), 1227;33 (1988) 1767. Tang and Chan, Electroanal. Chem. 334 (1992) 65. Electrochemical Cells, K.Y. Chan, HKU

  48. Single air cathode Electrochemical Cells, K.Y. Chan, HKU

  49. Electrolyte • Alkaline electrolyte (first deployed for Apollo mission) • Phosphoric Acid 180 C • Polymer Electrolyte • Cross Over • Stability (CO2 removal in alkaline) • Solid Oxide (YSZ, doped Ceria) • Shunt Current / Leak Current Electrochemical Cells, K.Y. Chan, HKU

  50. Ce Ce Ce Ce Y Y Ce Ce Ce Ce Ce Ce O2- O2- O2- Zr O2- O2- O2- O2- O2- O2- SOFC Electrolyte • Ytrium Stabilized Zirconia • Doped Ceria (Cerium Oxide) • O2- conductivity at 600~800 C Electrochemical Cells, K.Y. Chan, HKU

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