fuel cells for a sustainable energy future sossina m. haile materials science / chemical engineering...
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Fuel Cellsfor a Sustainable Energy Future
Sossina M. Haile
Materials Science / Chemical EngineeringCalifornia 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)
Towards a Sustainable Energy Future
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
World Energy Consumption
1999 totals: 400 Q-Btu, 422 EJ, 13TW2020 projections: 630 Q-Btu, 665 EJ, 21TW
Source: US Energy Information Agency(annual)264
211
158
106
52
0
Exa
Jou
les
(1018
)
8.4
6.7
5.0
3.3
1.7
0
Equ
ival
ent
Pow
er (
TW
, 10
12)
90% fossil
Towards a Sustainable Energy Future
Fossil Fuel Supplies
0.0E+00
5.0E+04
1.0E+05
1.5E+05
2.0E+05
(Exa
)J
OilRsv
OilRsc
GasRsv
GasRsc
CoalRsv
CoalRsc
Unconv
Conv
Rsv = Reserves (90%)Rsc = Resources (50%)
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
400 yrs
56-77 287-345
Source: US Energy Information Agency
Towards a Sustainable Energy Future
US Energy Imports/Exports: 1949-2004
1957: Net Importer
• 65% of known petroleum reserves in Middle East• 3% of reserves in USA, but 25% of world consumption
Total
Coal
Petroleum
Total
Petroleum
Net
Imports Exports
Source: US Energy Information Agency
1950 1960 1970 1980 1990 2000 1950 1960 1970 1980 1990 2000
1950 1960 1970 1980 1990 2000
Qua
d B
TU
35
30
25
20
15
10
5
0
6
5
4
3
2
1
0
Qua
d B
TU
Qua
d B
TU
35
30252015105
0
Towards a Sustainable Energy Future
Environmental Outlook
year
1000 1200 1400 1600 1800 2000
atm
osp
her
ic C
O2
[ppm
]
270
280
290
300
310
320
330
340Global CO2 levels
Source: Oak Ridge National Laboratory
2004: 378 ppm
Projections:
500-700 ppm by 2020
• Anthropogenic– Fossil fuel (75%)– Land use (25%)
Industrial Revolution
Towards a Sustainable Energy Future
Environmental Outlook
Intergovernmental Panel on Climate Change, 2001; http://www.ipcc.chN. Oreskes, Science 306, 1686, 2004; D. A. Stainforth et al, Nature 433, 403, 2005
- 8
- 4
0
+ 4
400 300 200 100
Thousands of years before present (Ky BP)
0
T r
ela
tive t
o
pre
sent
(°C
)
300
400
500
600
700
800
CH4 (ppmv
) -- CO2
-- CH4
-- T
325
300
275
250
225
200
175
CO2 (ppmv
)
CO2 in 2004: 378 ppmv
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
Solar1.2 x 105 TW at Earth surface
600 TW practical
Biomass5-7 TW gross
all cultivatable land not used
for food
HydroelectricGeothermal
Wind2-4 TW extractable
4.6 TW gross1.6 TW technically feasible0.9 TW economically feasible0.6 TW installed capacity
12 TW gross over landsmall fraction recoverable
Tide/OceanCurrents2 TW gross
The need:~ 20 TW by 2050
Fossil with sequestration1% / yr leakage -> lost in 100 yrs
NuclearWaste disposal
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
Solar or nuclear power plants
H2O
H2e-
Batteries
Capture
Storage
Delivery
Utilization
Hydrides? Liquid H2?
C-free Source
Hydrocarbon
???
Fuel cell
e-
H2O, CO2
+ CO2
Towards a Sustainable Energy Future
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
• Size independent
• Various applications
– stationary
– automotive
– portable electronics
• Controlled reactions
– “Zero Emissions”
• Operable on hydrogen
– (if suitably produced)
power plant size [MW]
0 5 10 15 20 25
eff
icie
ncy
[
%]
0
20
40
60
80automotive engine: 50-75 kW
*Can be as high as 80-90% with co-generation
*
Towards a Sustainable Energy Future
Anode Cathode
Fuel Cell: Principle of Operation
H2 O2
H+
Overall: H2 + ½ O2 H2O
½ O2 + 2H+ + 2e- H2OH2 2H+ + 2e-
conversion device, not energy source
best of batteries, combustion engines
Electrolyte
e-
Towards a Sustainable Energy Future
Fuel Cell Performance
H2 + ½ O2 H2O
1.17 Volts (@ no current)
• voltage losses
– fuel cross-over
– reaction kinetics
– electrolyte resistance
– slow mass diffusion
• power = I*V
• peak efficiency at low I
• peak power at mid I Current [A / cm2 ]
0.0 0.4 0.8 1.2 1.6
Vo
ltage
[V
]
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Po
wer
[W /
cm2]
0.0
0.2
0.4
0.6
0.8theoretical voltagecross-over
slow reaction kinetics
electrolyteresistance
slow massdiffusion
peak power
Towards a Sustainable Energy Future
Fuel Cell Components
• Components– Electrolyte (Membrane)
• Transport ions
• Block electrons, gases
– Electrodes
• Catalyze reactions
• Transport
– Ions, electrons, gases
• May be a composite
– (electro)Catalyst +
– Conductors +
– Pore former Membrane-Electrode Assembly (MEA)
electrolyte
catalyst
electrodes
sealant
Towards a Sustainable Energy Future
Fuel Cell Types
Type°C[°F]
PEM90-110[200-230]
AFC100-250[212-500]
PAFC150-220[300-430]
MCFC
500-700[930-1300]
SOFC
700-1000[1300-1800]
Fuel H2 + H2O H2 H2 HC + CO HC + CO
Electrolyte
IonNafionH3O+
KOHOH-
H3PO4
H+
Na2CO3
CO32-
Y-ZrO2
O2-
Oxidant O2 O2 + H2O O2 O2 + CO2O2
By-products: H2O, CO2
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
Towards a Sustainable Energy Future
Fuel Cell Choices
• Ambient Temperature
Rapid start-up
H2 or CH3OH as fuels
Catalysts easily poisoned
• Applications
– Portable power
– Many on/off cycles
– Small size
• High Temperature
Fuel flexible
Very high efficiencies
Long start-up
• Applications
– Stationary power
– Auxiliary power in
portable systems
Temperature sets operational parameters & fuel choice
Towards a Sustainable Energy Future
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
SystemStack
Membrane electrode assembly
electrolyte
catalyst
electrodes
sealant
Electrolyte
Towards a Sustainable Energy Future
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
Towards a Sustainable Energy Future
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
New Electrolytes: Solid Acids
NSF & DOE Sponsored Program
(past ONR support)
Towards a Sustainable Energy Future
SO3- + (H2O)nH+
(CF2)n
1 nm
“PEM” Fuel Cells
Nafion (Dupont)
• saturate with H2O– inverse micelle structure
• H(H2O)n+ ion transport
Proton Exchange Membrane or Polymer Electrolyte Membrane
High conductivity Flexible, high strength
Requires humidification & water management
Operation below 90°C Permeable to methanol
Target: 120-300°C; examine inorganic H+ conductors
H(H2O)n+
H2O
Kreuer, J Membr Sci 2 (2001) 185.
Towards a Sustainable Energy Future
Solid Acids
• Chemical intermediates between normal salts and normal acids: “acid salts”½(Cs2SO4) + ½(H2SO4) CsHSO4
• Physically similar to salts
• Structural disorder at ‘warm’ temperatures
• Properties
disordereds truc ture
norm al s truc ture
s truc tura ltrans ition
1/TT
log
(co
nd
uct
ivity
)
polym er
Direct H+ transport
Humidity insensitive
Impermeable
Water soluble!! Brittle
Towards a Sustainable Energy Future
Proton Transport Mechanism
Sulfate groupreorientation
10-11 seconds
Proton transfer
10-9 seconds
H
O
S
Towards a Sustainable Energy Future
1000/T K-1
2.00 2.25 2.50 2.75 3.00 3.25
log
(con
duct
ivity
) [S
/ c
m]
-8
-7
-6
-5
-4
-3
-2
-125°C50°C100°C150°C250°C
Conductivity of Solid Acids
• CsHSO4 [Baranov, 1982]
• CsHSeO4 [Baranov, 1982]
• (NH4)3H(SO4)2 [Ramasastry, 1981]
• Rb3H(SeO4)2 [Pawlowski, 1988]
• Cs2(HSO4)(H2PO4)[Chisholm & Haile,
2000]
• -Cs3(HSO4)2(H2PO4)[Haile et al., 1997]
• K3H(SO4)2 [Chisholm & Haile,
2001]
Superprotonic transition
But sulfates and selenates are unstable under reducing conditions…
Towards a Sustainable Energy Future
Solid Acid Properties
The Good
• H+ transport only
– No electro-osmotic drag
– No electron transport
– Alcohol impermeable
• Humidity insensitive
conductivity
• Stable to ~ 250ºC
• Inexpensive
• Chemically non-aggressive
The Bad
• Known compounds are
water soluble
– Operate at T 100ºC
– Insoluble analogs?
• 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
200 225 250 275 3000.0
0.1
0.2
0.3
0.4
47
62
71
77
Equ
ilibr
ium
pH
2O(a
tm)
Temperature(C)
Wat
er te
mpe
ratu
re (
o C)
CsH2PO4 Dehydration
CsH2PO4 CsPO3 + H2O
Tc = 230 Toper = 250°C
P(H2O) operation
Use water partial pressure to suppress dehydration
CsH
2-2x
PO 4-
x
CsH 2
PO4
pressure detector
CsH2PO4 + H2O
Towards a Sustainable Energy Future
260 240 220 200 180 160
1.9 2.0 2.1 2.2 2.3 2.4
-6
-4
-2
log
(co
nd
uct
ivity
)
[-1cm
-1]
1000/T [K-1]
1st heating 1st cooling 2nd heating 2nd cooling
Temperature [oC]
Conductivity of CsH2PO4
230 °C
0.42 eV
Humidified airp[H2O] = 0.4 atm
Towards a Sustainable Energy Future
0 50 100 150 200 250 3000.0
0.2
0.4
0.6
0.8
1.0
0
10
20
30
40
50
60
Cel
l vol
tage
(V
)
Current density (mA cm-2)
power
voltage
before after
Pow
er d
ensi
ty (
mW
cm
-2)
Proof of Principle
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
H2O, H2 | Cell | O2, H2O
260m membrane; 18 mg Pt/cm2
T = 235ºC 50 mW/cm2
D. A. Boysen, T. Uda, C. R.I. Chisholm and S. M. Haile, Science 303, 68-70 (2004)
Towards a Sustainable Energy Future
0 20 40 60 80 1000.0
0.2
0.4
0.6
0.8
1.0
offon, 100 mA/cm2
Cel
l vol
tage
(V
)
Time (h)
Fuel Cell Longevity: Stable Performance
H2, H2O | cell | O2, H2O
260 m thick CsH2PO4 electrolyteT = 235ºCCurrent = 100 mA/cm2
• CsH2PO4 – no degradation in 110 hr measurement
• CsHSO4 – functions for only ~ 30 mins (recoverable degradation)
Towards a Sustainable Energy Future
3rd Generation Fuel Cell
T. Uda & S.M. Haile, Electrochem & Solid State Lett. 8 (2005) A245-A246
H2, H2O | cell | O2, H2O
T = 248°C8 mg Pt/cm2
10-40 m pores, ~40% porosity
Slurry deposit
Fine CsH2PO4
2 m
Open circuit voltage: 0.9-1.0 V Peak power density: 285-415 mW/cm2
Towards a Sustainable Energy Future
Impact
Some Like It Medium Hot
The promise of protonics Solid Acids Show Promise...
Nature: News & ViewsScience Now Magazine
Physics Today Online
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).
Towards a Sustainable Energy Future
Fuel Cell Stack
Towards a Sustainable Energy Future
‘Direct’ Alcohol Fuel Cells
• 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
Methanol in proton exchange membrane fuel cellsCH3OH + H2O 6H+ + CO2 + 6e-
CH3OH + H2O 3H2 + CO2
Towards a Sustainable Energy Future
‘Direct’ Methanol Fuel Cells
0.0
0.1
0.2
0.3
0.4
0.0 0.5 1.0 1.50.0
0.2
0.4
0.6
0.8
1.0
Ce
ll vo
ltag
e (
V)
0.0 0.5 1.0 1.5
Current density (A/cm2)
Po
we
r d
en
sity
(W
/cm
2)
With reformer Without reformer
hydrogen
methanol42 vol%
hydrogen
methanol
reformate1%CO
24%CO2
• Methanol power ~ 85% H2 power
• For polymer fuel cells ~ 10%
• Reformate power ~ 90% H2 power
• Methanol power ~ 45% H2 power
T = 260 °C; 47 m membrane T = 240 °C; 34 m membrane
1.5
Towards a Sustainable Energy Future
‘Direct’ Alcohol Fuel Cells
T = 260 °C; 47 m membrane
With reformer
ethanol36 vol%
Vodka80 proof
• Ethanol power ~ 40% H2 power
New Cathodesfor Solid Oxide Fuel Cells
NSF & DOE Sponsored Program
(past DARPA support)
Towards a Sustainable Energy Future
State-of-the-Art SOFCs
• Operation: ~1000 °C• Fuel flexible, efficient• But…
– All high temp materials– Costly (manufacture)– Poor thermal cyclability
• Goal: 500 – 800 °C
• Challenges– Slower kinetics – Electrolyte resistance– Poor anode activity– Poor cathode activity
cathode (air electrode)
anode (fuel electrode)
Component Materials
(La,Sr)MnO3
Zr0.92Y0.08O2.96 = yttria stabilized zirconia (YSZ)
Ni + YSZ composite
electrolyte
Towards a Sustainable Energy Future
Solid Oxide Fuel Cell Cathodes
• Traditional cathodes– A3+B3+O3 perovskites
– Poor O2- transport– Limited reaction sites
• Our approach– High O2- flux materials– Extended reaction sites
– A2+B4+O3 perovskites
electrolyteO2-
O2
Oad
2e-cathode
Oad
electrolyteO2-
O2Oad
2e- cathodeO2-
‘triple-point’ path electrode bulk path
(Ba0.5Sr0.5)(Co0.8Fe0.2)O2.3
almost 1 in 4 vacant
Towards a Sustainable Energy Future
Cathode Electrocatalysis
• Symmetric cell resistance measurements
Ag current collectors
ElectrolyteO2, Ar,
(CO2)
cathode layers
+
-
½ O2
2e-
O=
½ O2
2e-
+
-
Rcathode
Rcathode
Relectrolyte
• Equivalent circuit– Distinguish resistance
contributions using frequency dependent measurements
Towards a Sustainable Energy Future
1.0 1.1 1.2 1.3 1.4 1.5
0.01
0.1
1
10750 700 650 600 550 500 450 400
0.01
0.1
1
10
Temperature (oC)
Cat
hode
are
a sp
ecifi
c re
sist
ance
(.
cm2 )
1000/T (K-1
Symmetric cell (2-electrode) Half cell (3-electrode)
Ea=116 kJ.mol-1
Cathode Electrocatalysis
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
0.5 – 0.6 cm2
P(O2) = 0.21 atm electrolyteO2-
O2 Oad
cathodeO2-
slowfast
2e-
Towards a Sustainable Energy Future
Cell Fabrication
Spray cathode
NiO + SDC (Ce0.85Sm0.15O2)
NiO + SDC
SDC
Dual dry press
Sinter, 1350oC 5h
600oC 5h, 15%H2
Porous anode
Calcine, 950oC 5h, inert gascathode
electrolyte
anode
Anode supported
Anode: 700 m
Cathode: 20 m
Electrolyte
~ 20m
Electrolyte surface
2 m
1.3
cm
0.71 cm2
Towards a Sustainable Energy Future
0 1000 2000 3000 40000.0
0.2
0.4
0.6
0.8
1.0
0
200
400
600
800
1000
Pow
er d
ensi
ty (
mW
.cm
-2)
Vol
tage
(V
olts
)
Current density (mA.cm-2)
600oC
550oC
500oC
445oC
400oC
Fuel Cell Power Output
> 1 W/cm2 at 600°C!!!H2, 3% H2O | fuel cell | Air
Comparison: literature cathode material 350 mW/cm2 at 600°C
Towards a Sustainable Energy Future
Impact
Cooler Material Boosts Fuel Cells
SOFC cathode is hot stuff…Next generation of fuel cells…
Tech Research News
R & D FocusFuel Cell Works
Z. Shao and S. M. Haile, “A High Performance Cathode for the Next Generation Solid-Oxide Fuel Cells,” Nature 431, 170-173 (2004).
Towards a Sustainable Energy Future
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
• The agencies– National Science Foundation, Office of Naval Research,
DARPA, California Energy Commission, Department of Energy, Kirsch Foundation, Powell Foundation
Calum Dane
TetsuyaMary
Kenji
Zongping
Justin
Wei
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 Optimal Temperatures,” Electrochem. Solid State Lett. 9, A261-A264 (2006).
• T. Uda and S. M. Haile, “Thin-Membrane Solid-Acid Fuel Cell,” Electrochem. Solid State Lett. 8, A245-A246 (2005).
• D. A. Boysen, T. Uda, C. R.I. Chisholm and S. M. Haile, “High performance Solid Acid Fuel Cells through humidity stabilization,” Science Online Express, Nov 20, 2003; Science 303, 68-70 (2004).
• D. A. Boysen, S. M. Haile, H. Lui and R. A. Secco “High-temperature Behavior of CsH2PO4 under both Ambient and High Pressure Conditions,” Chem. Mat. 15, 727-736 (2003).
• R. B. Merle, C. R. I. Chisholm, D. A. Boysen and S. M. Haile, “Instability of Sulfate and Selenate Solid Acids in Fuel Cell Environments,” Energy and Fuels 17, 210-215 (2003).
• 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).
• Z. P. Shao, S. M. Haile, J. M. Ahn, P.D. Ronney, Z. L. Zhan and S. A. Barnett, “A thermally self-sustained micro Solid-Oxide Fuel Cell stack with high power density,” Nature 435, 795-798 (2005).
• Z. Shao and S. M. Haile, “A High Performance Cathode for the Next Generation Solid-Oxide Fuel Cells,” Nature 431, 170-173 (2004).
• S. M. Haile, “Fuel Cell Materials and Components,” (invited) Acta. Met. 51, 5981-6000 (2003).
• S. M. Haile, “Materials for Fuel Cells,” (invited) Materials Today 18, 24-29 (2003).