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Electrochemical Energy Systems (Fuel Cells and Batteries)

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Presentation on theme: "Electrochemical Energy Systems (Fuel Cells and Batteries)"— Presentation transcript:

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 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

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=-nFE F=Faraday constant (96 458)

7 Example Half cell potential: Anode: Zn2++2e- → Zn
Eo = V vs. SHE Half cell potential: Cathode: Cu2++2e- → Cu Eo = V vs. SHE Cell potential: EC-EA= V –( V)=1.103V DGo=-nFEo= 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 Effect of Discharge Rate on Capacity

12 Discharge Rates

13 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…

14 Energy Density (kWh/L) Specific Energy (kWh/kg)
Some energy carrier comparisons (balance of plant efficiency and weight not included) Material Energy Density (kWh/L) Specific Energy (kWh/kg) Diesel 10.9 13.7 Gasoline 9.7 12.2 LNG 7.2 12.1 Biodiesel 9.9 EtOH 6.1 7.8 MeOH 4.6 6.4 NaBH4 7.3 7.1 NH3 4.3 LH2 2.6 39 Lead Acid 0.03 0.06 Nickel Cadmium 0.05 0.1 Li Ion 0.15 0.3


16 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…)

17 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.

18 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

19 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

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

21 Li-ion Anode/Oxidation: LixGraphite (“LixC6”)
LixC6(s) → “C6”(s) +Li+(solv)+e– Cathode/Reduction: Li Spinel 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


23 Effect of Depth of Discharge on Lifetime

24 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 Self Discharge rate (%/month) Lead Acid 5 NiCd 20 Li-ion

25 Metal-Air Batteries Metal Air battery M → Mn++ne– ½O2+H2O+2e– → 2OH- Eo=0.4V Looks reasonable for specific energy Specific power is horrible (slow reaction kinetics at oxygen electrode) Atomic Mass Eo(V) Density (g/cm3) Capacity (Ah/g) Sp Energy (Wh/g) Li 6.94 3.05 0.54 3.86 13.3 Zn 65.4 0.76 7.1 0.82 1.77 Al 26.9 1.66 2.7 2.98 6.13 Mg 24.3 2.37 1.74 2.20 6.09 Na 23.0 2.71 0.97 1.16 3.61

26 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)

27 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

28 Conceptual Fuel Cell Structure

29 Closer to real fuel cell structure

30 Fuel cell types AFC PAFC PEMFC MCFC SOFC Power range (kW) 2 - 100
0.1W - 250 250 – 10k 1 – 10k Electrolyte Aq. KOH (30- 40%) Aq H3PO4 (30-40%) sulphonated organic polymer (Nafion) Molten (Li/Na/K)2CO3 YSZ Temp (°C) C Charge Carrier OH- H+ CO32- O2- Anode Ni Pt Ni/Cr2O3 nickel/YSZ Cathode Ni/Pt/Pd platinum Pt / Ru Ni/NiO SrxLa1-xMnO3 h (%) 50-60 40-45 40-50 50-75

31 Alkaline Fuel Cell Lots of experience Anode: Porous Ni
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

32 Effect of gas pressure Higher pressure leads to higher voltages: Higher Eo

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

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

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

36 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

37 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

38 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

39 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

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

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

42 An SOFC design

43 Where do fuel cells fit in?
Electrical Efficiency CHP efficiency Best for AFC 60% 80 (low qual. heat) Special applications: Needs high purity H2 PAFC 40% 85 Distributed generation: Can use low sulfur reformate PEMFC 55% (transp.) 35 %(stationary) 80% Transportation: Use DMFC instead of H2 MCFC 45% (high qual. heat) Power generation: Can use coal / NG reformate SOFC 90% Use Coal / NG reformate

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

45 Summary

46 Why AY did the course like this
Various courses in higher education: “teach you how to approach the problem like an engineer” “show you how interrelated the system is” “help you figure out your (social) responsibility” “give you fundamental information about what is done now” Assumption is you got 1-3 elsewhere, what you need to help you improve the system is more of 4 "The thinking it took to get us into this mess is not the same thinking that is going to get us out of it.“ (attributed to Albert Einstein) AY hopes some of you will get us out of the mess

47 Course Learning Objectives
1. Describe the dependence of our current industrial society on energy 2. Discuss the various approaches to conventional and alternative energy generation and describe the basic operational principles of each 3. Ability to analyze data pertaining to a certain situation and create/design an idealized energy conversion system 4. Solve quantitative, energy-related problems that use and reinforce engineering fundamentals 5. Formulate decisions on energy choices based upon consideration of the entire lifecycle of the energy source in question, socioeconomic trends, safety, and environmental impact 6. Describe and apply fundamental system calculations to predict expected system efficiency

48 We looked at Basics: What our energy system looks like right now
Why do we need energy (and how much?) Power vs. Energy Brief review of Thermo; Brayton and Rankine cycles Fundamentals of electrochemistry What our energy system looks like right now Coal, NG, Hydro, Nuclear, Wind Generators, Transformers, the Grid How we may make out power in the future Ocean, Solar, Fuel Cells

49 Thanks for your help developing this course! Course evaluation follows
And so… Thanks for your help developing this course! Course evaluation follows Note additional questions See you on Monday at 12:00 in Wilk108.

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