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Electrochemical Energy Systems (Fuel Cells and Batteries)

Electrochemical Energy Systems (Fuel Cells and Batteries). Housekeeping. Final Final on Friday AM, here at 9:30. What is the major difference vs. combustion?. Electrochemical systems are not heat engines! Therefore not Carnot limited!. Oxidation and Reduction.

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Electrochemical Energy Systems (Fuel Cells and Batteries)

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  1. Electrochemical Energy Systems(Fuel Cells and Batteries)

  2. Housekeeping • Final • Final on Friday AM, here at 9:30

  3. What is the major difference vs. combustion? Electrochemical systems are not heat engines! • Therefore not Carnot limited!

  4. Oxidation and Reduction • Oxidation occurs at anode • Material gives up electrons • Ions dissolve into solution • E.g., Zn → Zn+2 + 2 e- • Reduction at cathode • Material takes up electrons • Ions deposited from solution • E.g., Cu+2 + 2e-→ Cu • These are called Half-Cell reactions • Both need to happen! (electron released at anode and consumed at the cathode so net charge is conserved) (also need ion flow to maintain net charge conservation in electrolyte)

  5. Each material has a reduction potential DG=-nFE

  6. Cell potentials • Ecell=Ecathode-Eanode • Make sure you use the same reference. • Most tables give reference against “standard hydrogen electrode” (H+ + e- -> ½ H2) • DG=-nFEF=Faraday constant (96 458)

  7. Example Half cell potential: • Anode: Zn2++2e- → Zn • Eo = -0.7628 V vs. SHE Half cell potential: • Cathode: Cu2++2e- → Cu • Eo = +0.3402 V vs. SHE • Cell potential: • EC-EA=0.3402 V –(-0.7628V)=1.103V • DGo=-nFEo=-213.8 kJ∙mol-1 • Since DGo<0 reaction proceeds spontaneously

  8. Internal losses depend on current density

  9. Battery terms Primary battery: Non-rechargeable (e.g., Li / SOCl2) Secondary battery: rechargeable (e.g., Li ion, we’ll talk about this) Mechanically rechargeable: batteries are recharged by mechanical replacement of depleted electrode (e.g. metal anode in certain metal-air batteries) Voltage: Potential difference between anode and cathode. (Related to energy of reactions) Capacity: amount of charge stored in battery (usually given as Coulombs per unit mass or volume) (1A=1C.s-1↔1Ah=3600C) (Question: how does capacity relate to energy?)

  10. Rate effects Current Drain: Different batteries respond differently In general, as current is increased, the available voltage and capacity decrease Charge rate and battery capacity usually specified E.g., my laptop 5200mAh, 10.80V, 56Wh means C=5.2A

  11. Discharge Rates

  12. Important battery (and fuel cell) parameters Batteries (and Supercaps) • Specific Energy (Wh/kg) (gravimetric energy density) • Energy density (Wh/L) (volumetric energy density) • Specific power (W/kg) (gravimetric power density) • Power density (W/L) (volumetric power density) Fuel cells also discuss: • Current density (mA/cm2) in electrode assembly • Power density (mW/cm2) in electrode assembly These parameters are of primary concern for mobile systems (e.g., transportation or mobile electronics) You want all these values to be as large as possible…

  13. Some energy carrier comparisons (balance of plant efficiency and weight not included)

  14. So what makes a battery rechargeable? As long as the electrochemical reaction is reversible, the battery should be rechargeable However, other effects are important • Decay of electrode surfaces E.g., damage to electrode structural properties as ions move in and out of electrodes • Decay/contamination of electrolyte • (Cost…)

  15. The original rechargeable battery: Lead Acid Anode/Oxidation: Lead grid packed with spongy lead. Pb(s)+HSO4 –(aq)→ PbSO4(s)+H+(aq)+2e– Cathode/Reduction: Lead grid packed with lead oxide. PbO2(s)+3H+(aq)+HSO4–(aq)+2e– → PbSO4(s)+2H2O(l) Electrolyte: 38% Sulfuric Acid. Cell Potential: 1.924V A typical 12 volt lead storage battery consists of six individual cells connected in series.

  16. Nickel-Cadmium (NiCd or Nicad) Anode/Oxidation: Cadmium metal Cd(s)+2OH–(aq) → Cd(OH)2(s)+2e– Cathode/Reduction: NiO(OH) on nickel metal NiO(OH)(s)+H2O(l)+e– → Ni(OH)2(s)+OH–(aq) Cell Potential: 1.20V ~ 50Wh/kg ~100Wh/L ~150W/kg High current rates due to Grotthus transport of OH- in water

  17. Li-ion Li+ ions intercalate into the crystal structure of the electrode materials • Li metal is very reactive which causes side reactions • Recharging by growing Li metal doesn’t work well • Instead of using Li metal use LiC8

  18. Electrochemical Potentials for Lithium Insertion(relative to Li, not SHE)

  19. Li-ion Anode/Oxidation: LixGraphite (“LixC6”) LixC6(s) → “C6”(s) +Li+(solv)+e– Cathode/Reduction: LiSpinel or layered oxide Li1-xMO2(s)+Li+(solv)+e– → LiMO2(s) Cell Potential: 3.6V ~160Wh/kg ~300Wh/L ~300W/kg Electrolyte cannot have any water! Li salt in organic ether (LiPF6 / LiBF4 / LiOTf) Overcharge/Overdischarge significantly damages cells

  20. Effect of Depth of Discharge on Lifetime

  21. Parasitic Losses in Battery Most batteries have a “self discharge” rate • Secondary chemical reactions • E.g., Zn(s) + H2O(l) → ZnO(s) + H2(g)↑ • Particularly important problem for some systems • Like Zn/air cells (“use it or lose it”) • Li cells have least self discharge of rechargeable batteries

  22. Metal-Air Batteries Metal Air battery M → Mn++ne– ½O2+H2O+2e– → 2OH-Eo=0.4V Looksreasonable for specificenergy Specificpower is horrible (slowreactionkineticsatoxygenelectrode)

  23. Fuel Cells Fuel Cells use externally fed fuel (H2 for now) Fuel reacts with O2 to form water. Anode/Oxidation: Carbon felt with catalyst 2H2(g) + 4OH–(aq) → 4H2O(l) + 4e– Cathode/Reduction: Carbon felt with catalyst O2(g) + 2H2O(l) + 4e– → 4OH–(aq) Overall Reaction: 2H2(g) + O2(g) → 2H2O(l) Various electrolytes (also reaction, etc)

  24. Common Fuel Cells By electrolyte: • Alkaline Fuel Cells (AFC) • Phosphoric Acid Fuel Cells (PAFC) • Polymer Electrolyte Membrane Fuel Cells (PEMFC) • Molten Carbonate Fuel Cells (MCFC) • Solid Oxide Fuel Cells (SOFC) By fuel: • Hydrogen / Air(Oxygen); Reformate Gas • Direct Methanol Fuel Cell (DMFC) • Carbon (coal?) By operating temperature: • High Temperature vs. Low Temperature Cells

  25. Conceptual Fuel Cell Structure

  26. Closer to real fuel cell structure

  27. Fuel cell types

  28. Alkaline Fuel Cell Lots of experience • UTC has been making AFCs for NASA since Apollo Anode: Porous Ni 2H2 + 4OH– → 4H2O+4e– Cathode: Porous NiO O2+2H2O+4e– → 4OH– Electrolyte is aq. KOH ~35% for low temp (120oC) ~80% for high temp (250oC) CO, CO2, H2S is harmful

  29. Effect of gas pressure Higher pressure leads to higher voltages: Higher Eo

  30. Effect of gas composition As expected 100% O2 is better than air…

  31. Effect of temperature Higher temperature leads to higher conductivity of electrolyte Lower IR losses

  32. Effect of contaminant (CO2 in AFC) CO2 + 2OH– → CO32– + H2O You trade 2 highly mobile OH– charge carriers for one low mobility CO32– ion…

  33. Phosphoric Acid Fuel Cell Several commercial designs Anode: Pt on carbon black 2H2 + 4OH– → 4H2O+4e– Cathode: Pt on carbon black O2+2H2O+4e– → 4OH– Electrolyte is aq. H3PO4 ~95% for (200oC) CO2 tolerant CO, COS, H2S poison catalyst

  34. Proton Exchange Membrane Fuel Cell Widely viewed as best bet for vehicle applications • No liquid electrolyte means safer system (?) Anode: Pt on carbon black H2 → 2H+ + 2e– Cathode: Pt on carbon black ½O2+2H++4e– → H2O Electrolyte is Sulfonated polymer “Nafion” – must keep wet! Water management is important Very sensitive to CO, COS, H2S

  35. Direct Methanol Fuel Cell Basically a PEM FC that uses MeOH instead of H2 • For transportation, methanol much easier to store than H2 • People also looking at direct hydrocarbon fuel cells Anode: Pt on carbon black MeOH +H2O → CO2+ 6H+ + 6e– Cathode: Pt on carbon black 3/2O2+6H++6e– → 3H2O Some CO produced, poisons catalyst… Fuel crossover through membrane lowers efficiencies Also note that water is consumed at anode • more water management

  36. Molten Carbonate Fuel Cell High tolerance to CO (good to use reformate gas!) • H2S is still harmful • Inefficiencies as high quality heat (cogen possible) Anode: Porous Ni/Cr H2 + CO32– → H2O+ CO2+2e– Cathode: Porous NiO ½O2+CO2+2e– → CO32– Electrolyte is (Li/Na)2CO3 melt ~50/50 and 650oC Note CO2 consumed at cathode • CO2 management needed

  37. Internal reformation possible(no need to separate CO2 and H2)

  38. Solid Oxide Fuel Cell High efficiency, especially if cogen used • Inefficiencies as high quality heat (cogen possible) Anode: Porous Ni/ZrO2cermet H2 + O2– → H2O+2e– Cathode: SrxLa1-xMnO3 ½O2+2e– → O2– Electrolyte is Yttria stabilized ZrO2 ~8% Y, T=950oC

  39. An SOFC design

  40. Where do fuel cells fit in?

  41. Combined Brayton-Fuel Cell Power System • Fuel cell efficiency about 55% (losses + wasted fuel) • Boosted to about 80% by using turbine

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