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
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=-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.