Transcript Fuel Cells - Sossina Haile - California Institute of Technology

Fuel Cells
for a Sustainable Energy Future
Sossina M. Haile
Materials Science / Chemical Engineering
California Institute of Technology
Graduate Students: Peter Babilo, William Chueh, Lisa Cowan, Mary
Louie, Justin Ho, Wei Lai, Mikhail Kislitsyn, Kenji Sasaki, Ayako Ikeda
Former Participants: Dane Boysen, Calum Chisholm, Tetsuya Uda,
Zongping Shao, Mary Thundathil
Funding: National Science Foundation, Department of Energy, Office
of Naval Research, (past: Kirsch Foundation, Powell Foundation)
Contents
• The Problem of Energy
– Growing consumption
– Consequences
– Sustainable energy resources
• Fuel Cell Technology Overview
– Principle of operation
– Types of fuel cells and their characteristics
• Recent (Caltech) Advances
– Too many to cover…
Towards a Sustainable Energy Future
The Problem of Energy
• The Problem
– Diminishing supply?
– Resources in unfriendly locations?
– Environmental damage?
• The Solution
– Adequate domestic supply
– Environmentally benign
– Conveniently transported
– Conveniently used
Towards a Sustainable Energy Future
Source: US Energy Information Agency
264
8.4
211
6.7
158
106
1999 totals:
2020 projections:
400 Q-Btu, 422 EJ,
630 Q-Btu, 665 EJ,
Towards a Sustainable Energy Future
Exa Joules (1018)
(annual)
5.0
3.3
52
1.7
0
0
13TW
21TW
Equivalent Power (TW, 1012)
World Energy Consumption
90% fossil
Fossil Fuel Supplies
Source: US Energy Information Agency
2.0E+05
(Exa)J
1.5E+05
Rsv = Reserves (90%)
Rsc = Resources (50%)
1.0E+05
Unconv
Conv
5.0E+04
0.0E+00
Oil
Rsv
Oil
Rsc
Gas
Rsv
Gas
Rsc
Coal
Rsv
Coal
Rsc
Source
Reserves, yrs
Resources, yrs
Total, yrs
Oil
13 - 20
10 – 35
23 - 55
Gas
11 - 25
7 – 40
18 - 65
Coal
32
270
300
56-77
287-345
Towards a Sustainable Energy Future
 400 yrs
US Energy Imports/Exports: 1949-2004
Source: US Energy Information Agency
Imports
6
25
Total
20
15
10
5
0
1950 1960 1970
Quad BTU
35
Exports
Total
5
Quad BTU
Quad BTU
35
30
Net
30
25
20
15
10
5
Petroleum
1980
1990
2000
4
Coal
3
2
Petroleum
1
0
1950
1960
1970
1980
1990
2000
• 65% of known petroleum reserves in Middle East
• 3% of reserves in USA, but 25% of world consumption
1957: Net Importer
0
1950
1960
1970
Towards a Sustainable Energy Future
1980
1990
2000
Environmental Outlook
Global CO2 levels
atmospheric CO2 [ppm]
340
330
320
310
2004: 378 ppm
Projections:
500-700 ppm by 2020
• Anthropogenic
300
Industrial
Revolution
– Fossil fuel (75%)
– Land use (25%)
290
280
270
1000
1200
1400
Source: Oak Ridge National Laboratory
Towards a Sustainable Energy Future
1600
year
1800
2000
Environmental Outlook
CO2 in 2004: 378 ppmv
300
275
250
-- CO2
-- CH4
-- T
700
+4
0
600
500
-4
200
400
-8
175
300
225
400
300
200
100
T relative to
present (°C)
CO2
CH4
(ppmv) (ppmv)
800
325
0
Thousands of years before present (Ky BP)
Intergovernmental Panel on Climate Change, 2001; http://www.ipcc.ch
N. Oreskes, Science 306, 1686, 2004; D. A. Stainforth et al, Nature 433, 403, 2005
Towards a Sustainable Energy Future
Energy Outlook
Supply
• Fossil energy sufficient
for world demand into
the forseeable future
• High geopolitical risk
• Rising costs
Environmental Impact
• Target
– Stabilize CO2 at 550 ppm
– By 2050
• Requires
– 20 TW carbon-free power
– One 1-GW power plant
daily from now until then
Urgency
• Transport of CO2 or heat into deep oceans:
– 400-1000 years; CO2 build-up is cummulative
• Must make dramatic changes within next few years
Towards a Sustainable Energy Future
The Energy Solution
1.2 x
105
Solar
The need:
~ 20 TW by 2050
TW at Earth surface
600 TW practical
Wind
Biomass
2-4 TW extractable
5-7 TW gross
all cultivatable
land not used
for food
Tide/Ocean
Currents
2 TW gross
Geothermal
12 TW gross over land
small fraction recoverable
Nuclear
Waste disposal
Hydroelectric
4.6
1.6
0.9
0.6
TW
TW
TW
TW
gross
technically feasible
economically feasible
installed capacity
Fossil with sequestration
1% / yr leakage -> lost in 100 yrs
Towards a Sustainable Energy Future
The Energy Solution
• Sufficient Domestic Supply
– Coal, Nuclear, Solar
• Environmentally Sustainable Supply
– Solar (Nuclear?)
• Suitable Carrier
– Electricity? Hydrogen? Hydrocarbon?
• Challenges
– Convert solar (nuclear) to convenient chemical form
– Efficient consumption of chemical fuel
Towards a Sustainable Energy Future
A Sustainable Energy Cycle
H2O, CO2
C-free Source
Capture
e-
Solar or nuclear
power plants
H2
???
Storage
Batteries
Hydrocarbon
Hydrides?
Liquid H2?
Delivery
e-
Utilization
Fuel cell
Towards a Sustainable Energy Future
H2O + CO2
A Few Words on Hydrogen Fuel Cells
• Hydrogen energy density
– High energy content per unit mass of hydrogen
– But best storage technologies are at ~ 5 wt% H2
– 5x the weight of gasoline (for same energy content)
• Conventional hydrogen fuel cells require Pt
– DOE target: 1 g/kW (0.75 g/hp)
– 100 hp engine  75g Pt  $3,169
– 93% of US Pt is imported
– 80% of world reserves in one mine complex in SA
• Another 15% in one mine complex in Russia
– Converting US autos would double world consumption
– 4 yr auto lifetime, 20% recycling  out in 40 yrs
Towards a Sustainable Energy Future
Fuel Cells: Part of the Solution?
• High efficiency
– low CO2 emissions
automotive engine: 50-75 kW
80
efficiency [%]
• Size independent
60
Fuel Cells
40
Co
20
• Various applications
*
ine
g
n
E
n
o
mbusti
– stationary
s
– automotive
– portable electronics
• Controlled reactions
0
0
5
10
15
20
power plant size [MW]
25
– “Zero Emissions”
• Operable on hydrogen
– (if suitably produced)
*Can be as high as 80-90% with co-generation
Towards a Sustainable Energy Future
Fuel Cell: Principle of Operation
best of batteries,
combustion engines
conversion device,
not energy source
Anode
Cathode
e
-
H+
H2
H2  2H+ + 2e-
O2
½ O2 + 2H+ + 2e-  H2O
Electrolyte
Overall: H2 + ½ O2  H2O
Towards a Sustainable Energy Future
Fuel Cell Performance
1.2
1.17 Volts (@ no current)
cross-over
1.0
– reaction kinetics
– electrolyte resistance
– slow mass diffusion
• power = I*V
• peak efficiency at low I
• peak power at mid I
Towards a Sustainable Energy Future
Voltage [V]
• voltage losses
– fuel cross-over
0.8
theoretical voltage
slow reaction kinetics
0.8
0.6
peak power
0.6
0.4
0.4
electrolyte
resistance
0.2
0.0
0.0
0.2
slow mass
diffusion
0.4
0.8
1.2
Current [A / cm2 ]
0.0
1.6
Power [W / cm2]
H2 + ½ O2  H2O
Fuel Cell Components
• Components
electrodes
catalyst
– Electrolyte (Membrane)
• Transport ions
• Block electrons, gases
– Electrodes
• Catalyze reactions
• Transport
– Ions, electrons, gases
• May be a composite
– (electro)Catalyst +
sealant
electrolyte
– Conductors +
– Pore former
Towards a Sustainable Energy Future
Membrane-Electrode
Assembly (MEA)
Fuel Cell Types
Types differentiated by electrolyte, temperature of operation
Low T  H2 or MeOH; High T  higher hydrocarbons (HC)
Efficiency tends to  as T , due to faster electrocatalysis
Type
°C
PEM
90-110
AFC
100-250
PAFC
150-220
MCFC
SOFC
500-700
700-1000
[°F]
[200-230]
[212-500]
[300-430]
[930-1300]
[1300-1800]
H2 + H 2 O
H2
H2
HC + CO
HC + CO
Nafion
H3O+ 
KOH
OH- 
H3PO4
H+ 
Na2CO3
CO32- 
Y-ZrO2
O2- 
O2
O 2 + H2 O
O2
O2 + CO2
O2
Fuel
Electrolyte
Ion
Oxidant
By-products: H2O, CO2
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Fuel Cell Choices
Temperature sets operational parameters & fuel choice
• Ambient Temperature
• High Temperature
 Rapid start-up
 Fuel flexible
 H2 or CH3OH as fuels
 Very high efficiencies
 Catalysts easily poisoned
 Long start-up
• Applications
• Applications
– Portable power
– Stationary power
– Many on/off cycles
– Auxiliary power in
– Small size
Towards a Sustainable Energy Future
portable systems
Technology Status
• Many, many demonstrate sites and vehicles
– Stationary PAFC (200 kW) at military sites since 1995
– Stationary SOFC (100 kW) operated for 20,000 hrs
– Toyota and Honda PEM FC vehicles released 2002
– DaimlerChrysler, Ford and GM, 2005; Hyundai planned
• Legislation is a key driver
– California zero emissions automotive standards
– China set for tough ‘CAFE’ standards
• Cost is a major barrier
– Precious metal catalysts, fabrication, complexity
• Uncertainty in future fuel infrastructure
– Gasoline for how long? Hydrogen? Methanol?
Towards a Sustainable Energy Future
Fuel Cell System Complexity
Membrane electrode assembly
sealant
electrolyte
Electrolyte
electrodes
System
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catalyst
Stack
Philosophy
Challenge
• Limitation of fuel cell materials places severe
design constraints on fuel cell systems
Approach
• Material modification for improved performance
and system simplification
• New materials discovery for next generation
fuel cell systems
• Novel system designs
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Fuel Cell Innovations
• New Electrolytes
– Intermediate temperature operation
• Lower the temperature below solid oxide fuel cells
• Raise the temperature above polymer fuel cells
• New Catalysts
– Enhance reaction kinetics (improve efficiency)
– Reduce susceptibility to poisons (reduce complexity)
• Novel integrated designs
– Dramatically improve thermal management
– Utilize micromachining technologies – micro fuel cells
Towards a Sustainable Energy Future
New Electrolytes: Solid Acids
NSF & DOE Sponsored Program
(past ONR support)
“PEM” Fuel Cells
Proton Exchange Membrane or Polymer Electrolyte Membrane
H(H2O)n+
H2O
 SO3- + (H2O)nH+
1 nm
(CF2)n 
Nafion (Dupont)
• saturate with H2O
– inverse micelle structure
• H(H2O)n+ ion transport 
 High conductivity
 Flexible, high strength
 Requires humidification &
water management
 Operation below 90°C
 Permeable to methanol
Kreuer, J Membr Sci 2 (2001) 185.
Target: 120-300°C; examine inorganic H+ conductors
Towards a Sustainable Energy Future
Solid Acids
• Chemical intermediates
between normal salts and
normal acids: “acid salts”
• Physically similar to salts
• Structural disorder at
‘warm’ temperatures
• Properties
 Direct H+ transport
 Humidity insensitive
 Impermeable
 Water soluble!! Brittle
Towards a Sustainable Energy Future
log(conductivity)
½(Cs2SO4) + ½(H2SO4)  CsHSO4
disordered
structure
polymer
normal
structure
structural
transition
T
.
1/T
Proton Transport Mechanism
H
S
O
Sulfate group
reorientation
10-11 seconds
Proton transfer
10-9 seconds
Towards a Sustainable Energy Future
Conductivity of Solid Acids
• CsHSO4 [Baranov, 1982]
• CsHSeO4 [Baranov, 1982]
-1
• (NH4)3H(SO4)2
-2
• Rb3H(SeO4)2
[Pawlowski, 1988]
• Cs2(HSO4)(H2PO4)
[Chisholm & Haile, 2000]
•
b-Cs3(HSO4)2(H2PO4)
[Haile et al., 1997]
• K3H(SO4)2
[Chisholm & Haile, 2001]
log(conductivity) [S / cm]
[Ramasastry, 1981]
250°C
150°C 100°C
50°C 25°C
Superprotonic
transition
-3
-4
-5
-6
-7
-8
2.00
2.25
2.50
2.75
3.00
3.25
1000/T K-1
But sulfates and selenates are unstable under reducing conditions…
Towards a Sustainable Energy Future
Solid Acid Properties
The Good
The Bad
• H+ transport only
• Known compounds are
– No electro-osmotic drag
water soluble
– No electron transport
– Operate at T  100ºC
– Alcohol impermeable
– Insoluble analogs?
• Humidity insensitive
conductivity
• Stable to ~ 250ºC
• Inexpensive
• Chemically non-aggressive
• Few are chemically stable
The Ugly
• Poor processability and
mechanical properties
– Composite membranes
with inert polymers
Towards a Sustainable Energy Future
CsH2PO4 as a Fuel Cell Electrolyte
• Expected to have chemical stability
– 3CsH2PO4 + 11H2  Cs3PO4 + 3H3P + 8H2O
– dG(rxn) >> 0
• But does it have high conductivity?
• Does it have sufficient thermal stability?
• Literature dispute
– High conductivity on heating due to H2O loss
– High conductivity due to transition to a cubic phase
Towards a Sustainable Energy Future
CsH2PO4 Dehydration
CsH2PO4  CsPO3 + H2O
pressure detector
0.4
Tc = 230
Toper = 250°C
77
CsH2PO4 + H2O
P(H2O) operation
71
2
0.3
0.3
o
0.4
Equilibrium pH O(atm)
Water partial pressure (atm)
0.5
Td(C)
0.2
0.1
0.2
62
0.1
47
0.0
200
0.0
150
200
250
300
225
250
275
Temperature(C)
Temperature (C)
Use water partial pressure to suppress dehydration
Towards a Sustainable Energy Future
Water temperature ( C)
CsH2PO4 dehydration onset
300
Conductivity of CsH2PO4
o
Temperature [ C]
260
240
220
200
180
1st heating
1st cooling
2nd heating
2nd cooling
-2
-1
-1
log(conductivity) [ cm ]
0.42 eV
160
Humidified air
p[H2O] = 0.4 atm
-4
230 °C
-6
1.9
2.0
2.1
2.2
-1
1000/T [K ]
Towards a Sustainable Energy Future
2.3
2.4
Proof of Principle
H2O, H2 | Cell | O2, H2O
T = 235ºC
50 mW/cm2
Cell voltage (V)
-2
before
after
1.0
50
0.8
40
power
0.6
30
0.4
20
voltage
0.2
0.0
10
0
0
50
100
150
200
250
300
-2
Current density (mA cm )
260mm membrane; 18 mg Pt/cm2
Power density (mW cm )
60
Compared to polymers
 High open circuit voltage
– Theoretical: 1.15V
– Measured: 1.00 V
– Polymers: 0.8-0.9 V
 Power density
– Polymers: > 1 W/cm2
• Platinum content
– Polymers: ~ 0.1 mg/cm2
D. A. Boysen, T. Uda, C. R.I. Chisholm and S. M. Haile, Science 303, 68-70 (2004)
Towards a Sustainable Energy Future
Fuel Cell Longevity: Stable Performance
H2, H2O | cell | O2, H2O
Cell voltage (V)
1.0
260 mm thick CsH2PO4 electrolyte
T = 235ºC
Current = 100 mA/cm2
0.8
0.6
0.4
2
0.2
0.0
on, 100 mA/cm
0
20
off
40
60
80
100
Time (h)
• CsH2PO4 – no degradation in 110 hr measurement
• CsHSO4 – functions for only ~ 30 mins (recoverable degradation)
Towards a Sustainable Energy Future
3rd Generation Fuel Cell
Fine CsH2PO4
100 sccm
200 sccm
2 mm
Slurry deposit
Voltage ( V )
T = 248°C
1.0 8 mg Pt/cm2
0.5
0.4
0.8
0.3
0.6
0.2
0.4
0.1
0.2
36 mm electrolyte
0.0
0.0
0.5
1.0
0.0
2.0
1.5
2
Current density ( A / cm )
10-40 mm pores, ~40% porosity
Open circuit voltage: 0.9-1.0 V
Towards a Sustainable Energy Future
T. Uda & S.M. Haile, Electrochem &
Solid State Lett. 8 (2005) A245-A246
Peak power density: 285-415 mW/cm2
2
1.2
Power density ( W /cm )
H2, H2O | cell | O2, H2O
Impact
S. M. Haile, D. A. Boysen, C. R. I. Chisholm and R. B. Merle,
“Solid Acids as Fuel Cell Electrolytes,” Nature 410, 910-913 (2001).
The promise of protonics
Solid Acids Show Promise...
Some Like It Medium Hot
Nature: News & Views
Physics Today Online
Science Now Magazine
Towards a Sustainable Energy Future
Fuel Cell Stack
Towards a Sustainable Energy Future
‘Direct’ Alcohol Fuel Cells
Methanol in proton exchange membrane fuel cells
CH3OH + H2O  6H+ + CO2 + 6eCH3OH + H2O  3H2 + CO2
• SAFCS ideal thermal match
–
–
–
–
Reforming rxn: 200 – 300°C
Electrolyte: 240 – 280°C
Steam reforming: endothermic
Fuel cell rxns: exothermic
• Integrated design
– Incorporate alcohol reforming
catalyst in anode chamber
Towards a Sustainable Energy Future
‘Direct’ Methanol Fuel Cells
Cell voltage (V)
T = 260 °C; 47 mm membrane
T = 240 °C; 34 mm membrane
0.4
2
1.0
hydrogen
0.8
hydrogen
0.3
reformate
0.6
1%CO
24%CO2
0.4
0.2
methanol
42 vol%
0.1
0.2
methanol
0.0
0.0
0.0
0.5
1.0
1.50.0
1.5
0.5
1.0
1.5
2
Current density (A/cm )
•
•
Methanol power ~ 85% H2 power
For polymer fuel cells ~ 10%
Towards a Sustainable Energy Future
•
•
Power density (W /cm )
Without reformer
With reformer
Reformate power ~ 90% H2 power
Methanol power ~ 45% H2 power
‘Direct’ Alcohol Fuel Cells
With reformer
2
T = 260 °C; 47 mm membrane
Power density (mW /cm )
Cell voltage (V)
1.0
125
ethanol
0.8
Vodka
36 vol%
80 proof
0.6
100
75
50
0.4
0.2
25
0.0
0.0
0
0.1
0.2
0.3
0.4
0.5
2
Current density (A/cm )
•
Towards a Sustainable Energy Future
Ethanol power ~ 40% H2 power
New Cathodes
for Solid Oxide Fuel Cells
NSF & DOE Sponsored Program
(past DARPA support)
State-of-the-Art SOFCs
Component Materials
cathode (air electrode)
electrolyte
(La,Sr)MnO3
Zr0.92Y0.08O2.96 = yttria stabilized zirconia (YSZ)
Ni + YSZ composite
anode (fuel electrode)
• Operation: ~1000 °C
• Fuel flexible, efficient
• But…
– All high temp materials
– Costly (manufacture)
– Poor thermal cyclability
Towards a Sustainable Energy Future
• Goal: 500 – 800 °C
• Challenges
–
–
–
–
Slower kinetics 
Electrolyte resistance
Poor anode activity
Poor cathode activity
Solid Oxide Fuel Cell Cathodes
• Traditional cathodes
(Ba0.5Sr0.5)(Co0.8Fe0.2)O2.3
– A3+B3+O3 perovskites
– Poor O2- transport
– Limited reaction sites
• Our approach
almost
1 in 4 vacant
– High O2- flux materials
– Extended reaction sites
– A2+B4+O3 perovskites
‘triple-point’ path
O2
O2
Oad
O2-
electrode bulk path
Oad
2e-
cathode
electrolyte
Towards a Sustainable Energy Future
Oad
O2-
2ecathode
O2-
electrolyte
Cathode Electrocatalysis
½ O2
+
2e-
• Symmetric cell
resistance
measurements
O2,
Ar,
(CO2)
O=
Ag current
collectors
• Equivalent circuit
– Distinguish resistance
contributions using
frequency dependent
measurements
cathode
layers
Electrolyte
2e½ O2
Rcathode
-
+
Relectrolyte
Rcathode
-
Towards a Sustainable Energy Future
Cathode Electrocatalysis
O2 Oad
2eslow
O2fast
o
2
Cathode area specific resistance ( .cm )
Temperature ( C)
10
750 700 650 600 550
500
450
400
10
P(O2) = 0.21 atm
1
1
0.5 – 0.6 cm2
0.1
Ea=116 kJ.mol
0.01
-1
Symmetric cell (2-electrode)
Half cell (3-electrode)
1.0
1.1
1.2
1.3
-1
1000/T (K 
Towards a Sustainable Energy Future
0.1
1.4
1.5
0.01
O2-
cathode
electrolyte
Ea same as oxygen surface
exchange (113 kJ.mol-1)
Bulk diffusion is fast
(46 kJ.mol-1)
Other ‘advanced’ cathodes
(PrSm)CoO3: 5.5 cm2
(LaSr)(CoFe)O3: 48 cm2
Cell Fabrication
Anode supported
Dual dry
press
NiO + SDC
(Ce0.85Sm0.15O2)
SDC
Sinter,
1350oC 5h
NiO + SDC
Spray
cathode
Calcine, 950oC
5h, inert gas
cathode
600oC 5h,
15%H2
Porous
anode
electrolyte
anode
Anode: 700 mm
0.71 cm2
Electrolyte surface
1.3 cm
~ 20mm
Electrolyte
Cathode: 20 mm
Towards a Sustainable Energy Future
2 mm
Fuel Cell Power Output
H2, 3% H2O | fuel cell | Air
> 1 W/cm2 at 600°C!!!
o
0.8
0.6
0.4
400
0.2
200
0.0
0
0
1000
2000
3000
-2
600 C
1000
o
550 C
o
500 C
o
445 C 800
o
400 C
600
Power density (mW.cm )
Voltage (Volts)
1.0
4000
-2
Current density (mA.cm )
Comparison: literature cathode material  350 mW/cm2 at 600°C
Towards a Sustainable Energy Future
Impact
Cooler Material Boosts Fuel Cells
Z. Shao and S. M. Haile, “A High Performance
Cathode for the Next Generation Solid-Oxide
Fuel Cells,” Nature 431, 170-173 (2004).
SOFC cathode is hot stuff…
Next generation of fuel cells…
Tech Research News
R & D Focus
Towards a Sustainable Energy Future
Fuel Cell Works
Summary & Conclusions
• Sustainable energy is the ‘grand challenge’ of
the 21st century
– Solutions must meet the need, not the hype
– Fuel cells can play an important role
• Solid acid fuel cells
– Radical alternatives to state-of-the-art
– Viability demonstrated; spin-off company established
• Solid oxide fuel cells
– Promising alternative cathode discovered
• Still plenty of need for fundamental research
“The stone age didn’t end because we ran out of stones.”
-Anonymous
Towards a Sustainable Energy Future
Acknowledgments
• The people
Zongping
Mary
Tetsuya
Justin
Wei
Calum
Dane
Kenji
• The agencies
– National Science Foundation, Office of Naval Research,
DARPA, California Energy Commission, Department of
Energy, Kirsch Foundation, Powell Foundation
Towards a Sustainable Energy Future
Selected Relevant Publications
•
•
•
•
•
•
•
•
•
•
T. Uda, D. A. Boysen, C. R. I. Chisholm and S. M. Haile, “Alcohol Fuel Cells at
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Towards a Sustainable Energy Future