Lecture #8
Fuel Cells Revisited
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
anode cathode
H2 (fuel)
H2O
O2 (oxidant)
Se
pa
rator
(por
ou
s)
Load
Choice of Fuels
?Hydrogen
?Hydrazine
?toxic
?expensive
?Natural gas/petroleum
?catalytic stream reforming (900oC)
?remove CO by shift reaction
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)
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,< 1 ppm Nox,4 ppm CO,< 1 ppm
non-methane reactive organic gases
Fuel Cells - Why (cont’d)
?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
Efficiency
?Heat engine
?Second Law - Carnot cycle
?Top efficiency 40%
?Higher temperatures,higher efficiency
?Fuel Cell
?No such limitations
Fuel Cells - Why Not?
?High initial cost - difficult to enter market
?Technology unfamiliar to power industry
?No existing infrastructure
?Regulatory
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
?Alkali=electrolyte
?Nickel=electrodes
Hart,A.B.; Womack,G.J,Fuel Cells,Theory and
Application Chapman and Hall,London,1967.
History (Cont’d)
?1912-1942 Bauer
?Molten alkali carbonate electrolyte,solid C
anode @ 10000C
?1945 Davtyan
?Mixed carbonates and oxides with sand
separator
?work basis for post-war fuel cell work
?1950’s NASA
Classification Schemes
?Based on fuel source
?Based on electrolyte
?Dictates operating temperature
?EX,if aqueous < 2000C
?Dictates fuel
?EX,if aqueous,H2
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
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
Hydrogen-Oxygen Fuel Cell with
Alkali or Phosphoric Acid Electrolyte
H2 O2
Load
anode cathode
H2 + 2OH- =
2H2O + 2e-
H2 O2
Load
anode cathode
H2 = 2H+ + 2e-
H+OH-
OH-
OH-
OH-
H+
H+H
+
H+ + OH- = H2O
H+ + OH- = H2O
2H+ + 2e- + 1/2O2 =
H2O
H2O + 2e- + 1/2O2 =
2OH-
AFC PAFC
Note,Reactions at anode
and cathode of these cells
may be different!
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!)
PEMFC
?Thin plastic sheet permeable to H+’s,
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)
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!
AFC (cont’d)
?Long used by NASA on space missions
?High efficiency <70%
?Costly
?Significant pressure differential required across
membrane
?CO2 (from air/source of O2) poison to AFC
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 < 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 EG&G Services,
Parsons,Inc,and Science
Applications International
Corporation for the U.S,
Department of Energy,
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
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-
SOFC (cont’d)
?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)
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
Applications (cont’d)
?Military
?Navy - all electric ships?
?Army - replacement for primary Li battery
?Economics,350,000/yr @100/battery
?includes disposal $30/battery
Electrode Characteristics
?Resistant to corrosive contents
?Conduct electricity well
?Be light weight,thin
?Have high,catalytically active surface
area
?Pt,Ni
Electrolyte Characteristics
?Be ionically conducting
?Prevent the two electrodes from coming
into electrical contact
?Allow passage of ions from one electrode
to the other
Challenges
?Cost
?<$100/kWh auto,energy storage
?Device lifetime
?> 3 y auto
?> 10 y stationary energy storage
?Device performance
?Efficiency
? > 60% auto/energy storage
?Power
? > 1000 W/kg weapons
?Startup time
? < 1 min weapons
Performance
?Power density
?Power/weight ratio
?Energy density
?Energy/weight ratio
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 electrolyteCurrent Density,mA/cm2
Ce
ll V
olt
ag
e,V
Ideal1.23 V
Total loss
Mass
transport
Loss
Ohmic
Polarization
Activation
Polarization
Electrode Polarization
Curves
?Polarization Curves
?Cell Voltage
Current Density
Current Density
Pol
ari
zat
ion
,V
An
ode
Vol
tag
e,V
O2
H2
Better
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)?
Fuel Cells Revisited
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
anode cathode
H2 (fuel)
H2O
O2 (oxidant)
Se
pa
rator
(por
ou
s)
Load
Choice of Fuels
?Hydrogen
?Hydrazine
?toxic
?expensive
?Natural gas/petroleum
?catalytic stream reforming (900oC)
?remove CO by shift reaction
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)
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,< 1 ppm Nox,4 ppm CO,< 1 ppm
non-methane reactive organic gases
Fuel Cells - Why (cont’d)
?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
Efficiency
?Heat engine
?Second Law - Carnot cycle
?Top efficiency 40%
?Higher temperatures,higher efficiency
?Fuel Cell
?No such limitations
Fuel Cells - Why Not?
?High initial cost - difficult to enter market
?Technology unfamiliar to power industry
?No existing infrastructure
?Regulatory
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
?Alkali=electrolyte
?Nickel=electrodes
Hart,A.B.; Womack,G.J,Fuel Cells,Theory and
Application Chapman and Hall,London,1967.
History (Cont’d)
?1912-1942 Bauer
?Molten alkali carbonate electrolyte,solid C
anode @ 10000C
?1945 Davtyan
?Mixed carbonates and oxides with sand
separator
?work basis for post-war fuel cell work
?1950’s NASA
Classification Schemes
?Based on fuel source
?Based on electrolyte
?Dictates operating temperature
?EX,if aqueous < 2000C
?Dictates fuel
?EX,if aqueous,H2
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
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
Hydrogen-Oxygen Fuel Cell with
Alkali or Phosphoric Acid Electrolyte
H2 O2
Load
anode cathode
H2 + 2OH- =
2H2O + 2e-
H2 O2
Load
anode cathode
H2 = 2H+ + 2e-
H+OH-
OH-
OH-
OH-
H+
H+H
+
H+ + OH- = H2O
H+ + OH- = H2O
2H+ + 2e- + 1/2O2 =
H2O
H2O + 2e- + 1/2O2 =
2OH-
AFC PAFC
Note,Reactions at anode
and cathode of these cells
may be different!
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!)
PEMFC
?Thin plastic sheet permeable to H+’s,
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)
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!
AFC (cont’d)
?Long used by NASA on space missions
?High efficiency <70%
?Costly
?Significant pressure differential required across
membrane
?CO2 (from air/source of O2) poison to AFC
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 < 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 EG&G Services,
Parsons,Inc,and Science
Applications International
Corporation for the U.S,
Department of Energy,
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
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-
SOFC (cont’d)
?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)
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
Applications (cont’d)
?Military
?Navy - all electric ships?
?Army - replacement for primary Li battery
?Economics,350,000/yr @100/battery
?includes disposal $30/battery
Electrode Characteristics
?Resistant to corrosive contents
?Conduct electricity well
?Be light weight,thin
?Have high,catalytically active surface
area
?Pt,Ni
Electrolyte Characteristics
?Be ionically conducting
?Prevent the two electrodes from coming
into electrical contact
?Allow passage of ions from one electrode
to the other
Challenges
?Cost
?<$100/kWh auto,energy storage
?Device lifetime
?> 3 y auto
?> 10 y stationary energy storage
?Device performance
?Efficiency
? > 60% auto/energy storage
?Power
? > 1000 W/kg weapons
?Startup time
? < 1 min weapons
Performance
?Power density
?Power/weight ratio
?Energy density
?Energy/weight ratio
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 electrolyteCurrent Density,mA/cm2
Ce
ll V
olt
ag
e,V
Ideal1.23 V
Total loss
Mass
transport
Loss
Ohmic
Polarization
Activation
Polarization
Electrode Polarization
Curves
?Polarization Curves
?Cell Voltage
Current Density
Current Density
Pol
ari
zat
ion
,V
An
ode
Vol
tag
e,V
O2
H2
Better
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)?
