Fuel Cells Revisited

1
Lecture 8

• Fuel Cells Revisited

2
Fuel Cells

• Chemical Rxn ? Electricity
• Net H2 1/2O2 H2O
• A H2 - 2e- 2H OV
• C 2H 2e- 1/2O2 H2O 1.23 V
• Desired voltage achieved by stacking cells (in
  series)
• Fuel reformer
• Natural gas, alcohol, hydrocarbons ? H2 CO

H2O
anode
cathode
Separator (porous)
H2 (fuel)
O2 (oxidant)
3
Choice of Fuels

• Hydrogen
• Hydrazine
• toxic
• expensive
• Natural gas/petroleum
• catalytic stream reforming (900oC)
• remove CO by shift reaction

4
How Different from Battery?

• Battery internal supply of fuel and oxidizer
• Significance must be replenished/recharged
• EX
• Alkaline cell (primary battery)
• discharge and discard
• Car battery (primary and secondary)
• discharge (primary) and recharge (secondary)

5
Fuel Cells - Why?

• No moving parts
• Long lifetime/reliability
• High efficiency (40 - 70)
• No Carnot cycle limitations (efficiency
  independent of size)
• heat available for cogeneration
• Low emissions
• PAFC lt 1 ppm Nox, 4 ppm CO, lt 1 ppm non-methane
  reactive organic gases

6
Fuel Cells - Why (contd)

• Quiet
• No moving parts
• Long device life
• Competitive price
• 1 g Pt/1 kW cell 20-50/kW)
• Relatively low weight and small size
• 1 kg/kW

7
Efficiency

• Heat engine
• Second Law - Carnot cycle
• Top efficiency 40
• Higher temperatures, higher efficiency
• Fuel Cell
• No such limitations

8
Fuel Cells - Why Not?

• High initial cost - difficult to enter market
• Technology unfamiliar to power industry
• No existing infrastructure
• Regulatory

9
History

• 1839 Sir William Grove
• Electrolysis of water
• Father of the Fuel Cell
• 1889 Ludwig Mond and Charles Langer
• fuel cell
• First practical device based on Pt
• 1932 Francis Bacon
• Alkalielectrolyte
• Nickelelectrodes

Hart, A.B. Womack, G.J. Fuel Cells Theory and
Application Chapman and Hall London, 1967.
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History (Contd)

• 1912-1942 Bauer
• Molten alkali carbonate electrolyte, solid C
  anode _at_ 10000C
• 1945 Davtyan
• Mixed carbonates and oxides with sand separator
• work basis for post-war fuel cell work
• 1950s NASA

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Classification Schemes

• Based on fuel source
• Based on electrolyte
• Dictates operating temperature
• EX if aqueous lt 2000C
• Dictates fuel
• EX if aqueous, H2

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Classification

• Based on fuel
• Direct
• Hydrogen fed directly to anode
• Indirect
• External reformer used to supply hydrogen to
  anode
• Regenerative
• Fuel cell product reconverted into reactants and
  recycled

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Classification Based on Electrolyte

• Polymer electrolyte (PEFC) 80oC
• Proton Exchange Membrane (PEMFC)
• Alkali (AFC) 80-100oC
• Phosphoric acid (acid) (PAFC) 2000C
• Molten Carbonate (MCFC) 6500C
• Solid oxide
• Tubular solid oxide (TSOFC) 8000C
• Intermediate Temperature solid oxide (ITSOFC)
  10000C

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Hydrogen-Oxygen Fuel Cell with Alkali or
Phosphoric Acid Electrolyte
AFC
PAFC
anode
cathode
anode
cathode
H
H OH- H2O
H OH- H2O
OH-
H
H
H
OH-
OH-
OH- OH-
H2 2H 2e-
H2 2OH- 2H2O 2e-
2H 2e- 1/2O2 H2O
H2O 2e- 1/2O2 2OH-
H2
O2
H2
O2
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Note Reactions at anode and cathode of these
cells may be different!
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Direct

• Hydrogen-oxygen cell
• Extensively used in space program
• Gaseous hydrogen, oxygen fed directly
• Requires only small amount noble metal catalyst
• Generates minimal excess heat
• Pure water by-product (drinkable!)

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PEMFC

• Thin plastic sheet permeable to Hs, coated on
  both sides with Pt catalyst
• Anode H2 ? 2H 2e-
• Cathode 1/2O2 2H 2e- ? H2O
• High power density (power/weight)
• Quick startup
• Primary candidates for auto industry
• Disadvantage low CO tolerance (ppm)

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AFC

• Concentrated KOH (35-85 wt) in asbestos matrix
• Anode H2 2OH- ? 2H2O e-
• Cathode 1/2O2 H2O 2e- ? 2OH-
• CO poison CO2 KOH produces K2CO3 altering
  electrolyte!

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AFC (contd)

• Long used by NASA on space missions
• High efficiency lt70
• Costly
• Significant pressure differential required across
  membrane
• CO2 (from air/source of O2) poison to AFC

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PAFC

• Up to 100 concentrated H3PO4 in SiC matrix, Pt
  electrocatalyst (expensive)
• Anode H2 ? 2H 2e-
• Cathode 1/2O2 2H 2e- ? H2O
• High temperatures required - H3PO4 poor conductor
• CO lt 3-5 vol or Pt poisoned (water gas shift
  reaction)
• In commercial use
• EX Toshiba PC-25 Fuel Cell (shown at left)
• Efficiency 37-42

Fuel Cell Handbook Fifth Edition, October 2000,
PDF version - by EGG Services, Parsons, Inc. and
Science Applications International Corporation
for the U.S. Department of Energy.
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MCFC

• Mixture of alkali carbonates in ceramic matrix of
  LiAlO2 at high T (600-8000C) where corrosive mess
  becomes highly conductive molten salt
• Anode H2 CO32- ? H2O CO2 2e-
• Anode CO CO32- ? 2CO2 2e-
• Cathode 1/2O2 CO2 2e- ? CO32-
• Ni (anode) and NiO (cathode)
• Promise high fuel-to-electricity efficiencies
• Fuels H2, CO, natural gas, propane, and diesel

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SOFC

• Hard ceramic material usually Y2O3-stabilized
  ZrO2
• Anode H2 O2- ? H2O 2e-
• Anode CO O2- ? CO2 2e-
• Anode CH4 4O2- ? 2H2O CO2 8e-
• Cathode 1/2O2 2e- ? O2-

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SOFC (contd)

• Co-ZrO2 or Ni-ZrO2(anode) and Sr-doped LaMnO3
  (cathode)
• Two geometries
• tubular - array of meter-long tubes
• compressed disc
• Large high-power applications (electricity
  generating stations)

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Applications

• Transportation
• Cars
• Spaceflight
• Q What are waste products? How might these be
  useful in spaceflight?
• Power generation
• Stationary Power Plants
• Weapons
• Telecommunications
• Cell phones

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Applications (contd)

• Military
• Navy - all electric ships?
• Army - replacement for primary Li battery
• Economics 350,000/yr _at_100/battery
• includes disposal 30/battery

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Electrode Characteristics

• Resistant to corrosive contents
• Conduct electricity well
• Be light weight, thin
• Have high, catalytically active surface area
• Pt, Ni

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Electrolyte Characteristics

• Be ionically conducting
• Prevent the two electrodes from coming into
  electrical contact
• Allow passage of ions from one electrode to the
  other

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Challenges

• Cost
• lt100/kWh auto, energy storage
• Device lifetime
• gt 3 y auto
• gt 10 y stationary energy storage
• Device performance
• Efficiency
• gt 60 auto/energy storage
• Power
• gt 1000 W/kg weapons
• Startup time
• lt 1 min weapons

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Performance

• Power density
• Power/weight ratio
• Energy density
• Energy/weight ratio

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Ideal vs. Actual Cell Voltage/Current
Characteristic

• Ohmic Polarization
• decrease electrode separation
• enhance ionic conductivity of electrolyte
• Concentration Polarization
• slow diffusion in electrode pores
• slow diffusion of reactants through electrolyte

Ideal
1.23 V
Total loss
Activation Polarization
Ohmic Polarization
Cell Voltage, V
Mass transport Loss
Current Density, mA/cm2
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Electrode Polarization Curves
Polarization, V
H2

• Polarization Curves
• Cell Voltage

O2
Current Density
Better
Anode Voltage, V
Current Density
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Homework

• The ideal standard potential for a H2/O2 fuel
  cell is 1.23 V with H2O(l) as product and 1.18 V
  for H2O(g) as product. How is the ideal
  potential of the cell expected to vary with
  temperature. Since fuel cells have different
  characteristic operating temperatures, what
  effect is this expected to have on the ideal
  operating voltage of a PEFC (353K) as versus a
  SOFC cell (1373K)?