Chemical, Biological and Environmental Engineering Electrochemical Energy Systems (Fuel Cells and Batteries)

Advanced Materials and Sustainable Energy Lab CBEE Housekeeping Final –Final on Friday AM, here at 9:30

Advanced Materials and Sustainable Energy Lab CBEE What is the major difference vs. combustion? Electrochemical systems are not heat engines! –Therefore not Carnot limited!

Advanced Materials and Sustainable Energy Lab CBEE Oxidation and Reduction Oxidation occurs at anode –Material gives up electrons –Ions dissolve into solution –E.g., Zn → Zn e - Reduction at cathode –Material takes up electrons –Ions deposited from solution –E.g., Cu e - → 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)

Advanced Materials and Sustainable Energy Lab CBEE Each material has a reduction potential  G=-nFE

Advanced Materials and Sustainable Energy Lab CBEE Cell potentials E cell =E cathode -E anode –Make sure you use the same reference. –Most tables give reference against “standard hydrogen electrode” (H + + e - -> ½ H 2 )  G=-nFE F=Faraday constant (96 458)

Advanced Materials and Sustainable Energy Lab CBEE Example Half cell potential: Anode: Zn 2+ +2e - → Zn E o = V vs. SHE Half cell potential: Cathode: Cu 2+ +2e - → Cu E o = V vs. SHE Cell potential: E C -E A = V –( V)=1.103V  G o =-nFE o = kJ∙mol -1 Since  G o <0 reaction proceeds spontaneously

Advanced Materials and Sustainable Energy Lab CBEE Internal losses depend on current density

Advanced Materials and Sustainable Energy Lab CBEE Battery terms Primary battery: Non-rechargeable (e.g., Li / SOCl 2 ) 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?)

Advanced Materials and Sustainable Energy Lab CBEE 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

Advanced Materials and Sustainable Energy Lab CBEE Effect of Discharge Rate on Capacity

Advanced Materials and Sustainable Energy Lab CBEE Discharge Rates

Advanced Materials and Sustainable Energy Lab CBEE 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/cm 2 ) in electrode assembly Power density (mW/cm 2 ) 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…

Advanced Materials and Sustainable Energy Lab CBEE Some energy carrier comparisons (balance of plant efficiency and weight not included) MaterialEnergy Density (kWh/L)Specific Energy (kWh/kg) Diesel Gasoline LNG Biodiesel EtOH MeOH NaBH NH34.3 LH Lead Acid Nickel Cadmium Li Ion

Advanced Materials and Sustainable Energy Lab CBEE

Advanced Materials and Sustainable Energy Lab CBEE 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…)

Advanced Materials and Sustainable Energy Lab CBEE The original rechargeable battery: Lead Acid Anode/Oxidation: Lead grid packed with spongy lead. Pb (s) +HSO 4 – (aq) → PbSO 4(s) +H + (aq) +2e – Cathode/Reduction: Lead grid packed with lead oxide. PbO 2(s) +3H + (aq) +HSO 4 – (aq) +2e – → PbSO 4(s) +2H 2 O (l) Electrolyte: 38% Sulfuric Acid. Cell Potential: 1.924V A typical 12 volt lead storage battery consists of six individual cells connected in series.

Advanced Materials and Sustainable Energy Lab CBEE 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) +H 2 O (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

Advanced Materials and Sustainable Energy Lab CBEE 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 LiC 8

Advanced Materials and Sustainable Energy Lab CBEE Electrochemical Potentials for Lithium Insertion (relative to Li, not SHE)

Advanced Materials and Sustainable Energy Lab CBEE Li-ion Anode/Oxidation: Li x Graphite (“Li x C 6 ”) Li x C 6(s) → “C 6 ” (s) +Li + (solv) +e – Cathode/Reduction: Li Spinel or layered oxide Li 1-x MO 2(s) +Li + (solv) +e – → LiMO 2(s) Cell Potential: 3.6V ~160Wh/kg ~300Wh/L ~300W/kg Electrolyte cannot have any water! Li salt in organic ether (LiPF 6 / LiBF 4 / LiOTf) Overcharge/Overdischarge significantly damages cells

Advanced Materials and Sustainable Energy Lab CBEE

Advanced Materials and Sustainable Energy Lab CBEE Effect of Depth of Discharge on Lifetime

Advanced Materials and Sustainable Energy Lab CBEE Parasitic Losses in Battery Most batteries have a “self discharge” rate Secondary chemical reactions –E.g., Zn (s) + H 2 O (l) → ZnO (s) + H 2(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 Self Discharge rate (%/month) Lead Acid5 NiCd20 Li-ion5

Advanced Materials and Sustainable Energy Lab CBEE Metal-Air Batteries Metal Air battery M → M n+ +ne – ½O 2 +H 2 O+2e – → 2OH - E o =0.4V Looks reasonable for specific energy Specific power is horrible (slow reaction kinetics at oxygen electrode) Atomic Mass E o (V) Density (g/cm 3 ) Capacity (Ah/g) Sp Energy (Wh/g) Li Zn Al Mg Na

Advanced Materials and Sustainable Energy Lab CBEE Fuel Cells Fuel Cells use externally fed fuel (H 2 for now) Fuel reacts with O 2 to form water. Anode/Oxidation: Carbon felt with catalyst 2H 2(g) + 4OH – (aq) → 4H 2 O (l) + 4e – Cathode/Reduction: Carbon felt with catalyst O 2(g) + 2H 2 O (l) + 4e – → 4OH – (aq) Overall Reaction: 2H 2(g) + O 2(g) → 2H 2 O (l) Various electrolytes (also reaction, etc)

Advanced Materials and Sustainable Energy Lab CBEE 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

Advanced Materials and Sustainable Energy Lab CBEE Conceptual Fuel Cell Structure

Advanced Materials and Sustainable Energy Lab CBEE Closer to real fuel cell structure

Advanced Materials and Sustainable Energy Lab CBEE Fuel cell types AFC PAFCPEMFC MCFC SOFC Power range (kW) W – 10k1 – 10k ElectrolyteAq. KOH (30- 40%) Aq H 3 PO 4 (30-40%) sulphonated organic polymer (Nafion) Molten (Li/Na/K) 2 CO 3 YSZ Temp (°C) C Charge Carrier OH - H+H+ H+H+ CO 3 2- O 2- AnodeNiPt Ni/Cr2O3nickel/YSZ CathodeNi/Pt/PdplatinumPt / RuNi/NiOSr x La 1-x MnO 3  %)

Advanced Materials and Sustainable Energy Lab CBEE Alkaline Fuel Cell Lots of experience –UTC has been making AFCs for NASA since Apollo Anode: Porous Ni 2H 2 + 4OH – → 4H 2 O+4e – Cathode: Porous NiO O 2 +2H 2 O+4e – → 4OH – Electrolyte is aq. KOH ~35% for low temp (120 o C) ~80% for high temp (250 o C) CO, CO 2, H 2 S is harmful

Advanced Materials and Sustainable Energy Lab CBEE Effect of gas pressure Higher pressure leads to higher voltages: Higher E o

Advanced Materials and Sustainable Energy Lab CBEE Effect of gas composition As expected 100% O 2 is better than air…

Advanced Materials and Sustainable Energy Lab CBEE Effect of temperature Higher temperature leads to higher conductivity of electrolyte Lower IR losses

Advanced Materials and Sustainable Energy Lab CBEE Effect of contaminant (CO 2 in AFC) CO 2 + 2OH – → CO 3 2– + H 2 O You trade 2 highly mobile OH – charge carriers for one low mobility CO 3 2– ion…

Advanced Materials and Sustainable Energy Lab CBEE Phosphoric Acid Fuel Cell Several commercial designs Anode: Pt on carbon black 2H 2 + 4OH – → 4H 2 O+4e – Cathode: Pt on carbon black O 2 +2H 2 O+4e – → 4OH – Electrolyte is aq. H 3 PO 4 ~95% for (200 o C) CO 2 tolerant CO, COS, H 2 S poison catalyst

Advanced Materials and Sustainable Energy Lab CBEE Proton Exchange Membrane Fuel Cell Widely viewed as best bet for vehicle applications –No liquid electrolyte means safer system (?) Anode: Pt on carbon black H 2 → 2H + + 2e – Cathode: Pt on carbon black ½O 2 +2H + +4e – → H 2 O Electrolyte is Sulfonated polymer “Nafion” – must keep wet! Water management is important Very sensitive to CO, COS, H 2 S

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

Advanced Materials and Sustainable Energy Lab CBEE Molten Carbonate Fuel Cell High tolerance to CO (good to use reformate gas!) –H 2 S is still harmful –Inefficiencies as high quality heat (cogen possible) Anode: Porous Ni/Cr H 2 + CO 3 2– → H 2 O+ CO 2 +2e – Cathode: Porous NiO ½O 2 +CO 2 +2e – → CO 3 2– Electrolyte is (Li/Na) 2 CO 3 melt ~50/50 and 650 o C Note CO 2 consumed at cathode –CO 2 management needed

Advanced Materials and Sustainable Energy Lab CBEE Internal reformation possible (no need to separate CO 2 and H 2 )

Advanced Materials and Sustainable Energy Lab CBEE Solid Oxide Fuel Cell High efficiency, especially if cogen used –Inefficiencies as high quality heat (cogen possible) Anode: Porous Ni/ZrO 2 cermet H 2 + O 2– → H 2 O+2e – Cathode: Sr x La 1-x MnO 3 ½O 2 +2e – → O 2– Electrolyte is Yttria stabilized ZrO 2 ~8% Y, T=950 o C

Advanced Materials and Sustainable Energy Lab CBEE An SOFC design

Advanced Materials and Sustainable Energy Lab CBEE Where do fuel cells fit in? Electrical Efficiency CHP efficiencyBest for AFC60%80 (low qual. heat) Special applications: Needs high purity H2 PAFC40%85 (low qual. heat) Distributed generation: Can use low sulfur reformate PEMFC55% (transp.) 35 %(stationary) 80% (low qual. heat) Transportation: Use DMFC instead of H 2 MCFC45%80% (high qual. heat) Power generation: Can use coal / NG reformate SOFC40%90% (high qual. heat) Power generation: Use Coal / NG reformate

Advanced Materials and Sustainable Energy Lab CBEE Combined Brayton-Fuel Cell Power System Fuel cell efficiency about 55% (losses + wasted fuel) Boosted to about 80% by using turbine