Fuel Cells for a Sustainable Energy Future

1
Fuel Cellsfor 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)
2
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

3
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

4
World Energy Consumption
(annual)
Source US Energy Information Agency
1999 totals 400 Q-Btu, 422 EJ, 13TW 2020
projections 630 Q-Btu, 665 EJ, 21TW
90 fossil
5
Fossil Fuel Supplies
Source US Energy Information Agency
Rsv Reserves (90) Rsc Resources (50)
56-77
287-345
6
US Energy Imports/Exports 1949-2004
Source US Energy Information Agency
Imports
Exports
35
6
Total
30
5
25
4
Total
20
Quad BTU
Coal
3
Quad BTU
15
2
10
Petroleum
1
Petroleum
5
0
0
1950
1960
1970
1980
1990
2000
1950
1960
1970
1980
1990
2000

• 65 of known petroleum reserves in Middle East
• 3 of reserves in USA, but 25 of world
  consumption

Net
35
30
25
20
Quad BTU
15
10
5
0
1950
1960
1970
1980
1990
2000
7
Environmental Outlook
Global CO2 levels
2004 378 ppm Projections 500-700 ppm by 2020

• Anthropogenic
• Fossil fuel (75)
• Land use (25)

Industrial Revolution
Source Oak Ridge National Laboratory
8
Environmental Outlook
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
9
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

10
The Energy Solution

• Solar
• 1.2 x 105 TW at Earth surface
• 600 TW practical

The need 20 TW by 2050
Wind 2-4 TW extractable
Biomass 5-7 TW gross all cultivatable land not
used for food
Tide/Ocean Currents 2 TW gross
Hydroelectric
Geothermal
4.6 TW gross 1.6 TW technically feasible 0.9 TW
economically feasible 0.6 TW installed capacity
12 TW gross over land small fraction recoverable
Nuclear Waste disposal
Fossil with sequestration 1 / yr leakage -gt lost
in 100 yrs
11
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

12
A Sustainable Energy Cycle
H2O, CO2
C-free Source
Solar or nuclear power plants
H2
Capture
e-
???
Hydrides? Liquid H2?
Batteries
Storage
Hydrocarbon
Delivery
e-
Utilization
H2O
CO2
Fuel cell
13
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

14
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)

Can be as high as 80-90 with co-generation
15
Fuel Cell Principle of Operation
conversion device, not energy source
best of batteries, combustion engines
e-
H
H2
O2
½ O2 2H 2e- ? H2O
H2 ? 2H 2e-
Electrolyte
Overall H2 ½ O2 ? H2O
16
Fuel Cell Performance

• H2 ½ O2 ? H2O
• 1.17 Volts (_at_ no current)
• voltage losses
• fuel cross-over
• reaction kinetics
• electrolyte resistance
• slow mass diffusion
• power IV
• peak efficiency at low I
• peak power at mid I

1.2
0.8
cross-over
theoretical voltage
1.0
slow reaction kinetics
0.6
0.8
peak power
Power W / cm2
0.6
0.4
Voltage V
0.4
electrolyte
0.2
resistance
0.2
slow mass
diffusion
0.0
0.0
0.0
0.4
0.8
1.2
1.6
Current A / cm2
17
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)
18
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
By-products H2O, CO2
19
Fuel Cell Choices
Temperature sets operational parameters fuel
choice

• 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

20
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?

21
Fuel Cell System Complexity
22
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

23
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

24
New Electrolytes Solid Acids

• NSF DOE Sponsored Program(past ONR support)

25
PEM Fuel Cells
Proton Exchange Membrane or Polymer Electrolyte
Membrane

• Nafion (Dupont)
• saturate with H2O
• inverse micelle structure
• H(H2O)n ion transport ?

• High conductivity
• Flexible, high strength
• Requires humidification water management
• Operation below 90C
• Permeable to methanol

Kreuer, J Membr Sci 2 (2001) 185.
Target 120-300C examine inorganic H conductors
26
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

• Direct H transport
• Humidity insensitive
• Impermeable
• Water soluble!! Brittle

27
Proton Transport Mechanism
H
Sulfate groupreorientation 10-11 seconds
S
O
Proton transfer 10-9 seconds
28
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
• b-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
29
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

30
CsH2PO4 as a Fuel Cell Electrolyte

• Expected to have chemical stability
• 3CsH2PO4 11H2 ? Cs3PO4 3H3P 8H2O
• dG(rxn) gtgt 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

31
CsH2PO4 Dehydration
CsH2PO4 ? CsPO3 H2O
pressure detector
Tc 230
Toper 250C
CsH2PO4 H2O
P(H2O) operation
CsH2PO4
CsH2-2xPO4-x
Use water partial pressure to suppress dehydration
32
Conductivity of CsH2PO4
0.42 eV
Humidified airpH2O 0.4 atm
230 C
33
Proof of Principle
H2O, H2 Cell O2, H2O
T 235ºC
50 mW/cm2

• Compared to polymers
• High open circuit voltage
• Theoretical 1.15V
• Measured 1.00 V
• Polymers 0.8-0.9 V
• Power density
• Polymers gt 1 W/cm2
• Platinum content
• Polymers 0.1 mg/cm2

260mm membrane 18 mg Pt/cm2
D. A. Boysen, T. Uda, C. R.I. Chisholm and S. M.
Haile, Science 303, 68-70 (2004)
34
Fuel Cell Longevity Stable Performance
H2, H2O cell O2, H2O
260 mm thick CsH2PO4 electrolyte T
235ºC Current 100 mA/cm2

• CsH2PO4 no degradation in 110 hr measurement
• CsHSO4 functions for only 30 mins
  (recoverable degradation)

35
3rd Generation Fuel Cell
H2, H2O cell O2, H2O
T 248C 8 mg Pt/cm2
Slurry deposit
T. Uda S.M. Haile, Electrochem Solid State
Lett. 8 (2005) A245-A246
10-40 mm pores, 40 porosity
Open circuit voltage 0.9-1.0 V Peak power
density 285-415 mW/cm2
36
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
37
Fuel Cell Stack
38
Direct Alcohol Fuel Cells
Methanol in proton exchange membrane fuel
cells CH3OH H2O ? 6H CO2 6e- CH3OH H2O ?
3H2 CO2

• SAFCS ideal thermal match
• Reforming rxn 200 300C
• Electrolyte 240 280C
• Steam reforming endothermic
• Fuel cell rxns exothermic
• Integrated design
• Incorporate alcohol reforming catalyst in anode
  chamber

39
Direct Methanol Fuel Cells
Without reformer
With reformer
T 260 C 47 mm membrane
T 240 C 34 mm membrane
hydrogen
hydrogen
reformate 1CO 24CO2
methanol 42 vol
methanol
1.5

• Methanol power 85 H2 power
• For polymer fuel cells 10

• Reformate power 90 H2 power
• Methanol power 45 H2 power

40
Direct Alcohol Fuel Cells
With reformer
T 260 C 47 mm membrane
ethanol 36 vol
Vodka 80 proof

• Ethanol power 40 H2 power

41
New Cathodesfor Solid Oxide Fuel Cells

• NSF DOE Sponsored Program(past DARPA support)

42
State-of-the-Art SOFCs
Component Materials
cathode (air electrode)
(La,Sr)MnO3
Zr0.92Y0.08O2.96 yttria stabilized zirconia
(YSZ)
electrolyte
Ni YSZ composite
anode (fuel electrode)

• 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

43
Solid Oxide Fuel Cell Cathodes
(Ba0.5Sr0.5)(Co0.8Fe0.2)O2.3

• Traditional cathodes
• A3B3O3 perovskites
• Poor O2- transport
• Limited reaction sites
• Our approach
• High O2- flux materials
• Extended reaction sites
• A2B4O3 perovskites

almost 1 in 4 vacant
electrode bulk path
triple-point path
O2
O2
Oad
Oad
cathode
2e-
cathode
O2-
2e-
Oad
O2-
O2-
electrolyte
electrolyte
44
Cathode Electrocatalysis
½ O2

2e-
O2, Ar, (CO2)

• Symmetric cell resistance measurements

cathode layers
O
Electrolyte
2e-
-
Ag current collectors
½ O2

• Equivalent circuit
• Distinguish resistance contributions using
  frequency dependent measurements

Rcathode
Relectrolyte
Rcathode
-
45
Cathode Electrocatalysis
O2
Oad
slow
2e-
O2-
fast
cathode
O2-
electrolyte
P(O2) 0.21 atm
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 Wcm2 (LaSr)(CoFe)O3 48 Wcm2
0.5 0.6 Wcm2
46
Cell Fabrication
Sinter, 1350oC 5h
Anode supported
Dual dry press
NiO SDC (Ce0.85Sm0.15O2)
SDC
NiO SDC
600oC 5h, 15H2
Spray cathode
Calcine, 950oC 5h, inert gas
Porous anode
0.71 cm2
1.3 cm
47
Fuel Cell Power Output
H2, 3 H2O fuel cell Air
gt 1 W/cm2 at 600C!!!
Comparison literature cathode material ? 350
mW/cm2 at 600C
48
Impact
Z. Shao and S. M. Haile, A High Performance
Cathode for the Next Generation Solid-Oxide Fuel
Cells, Nature 431, 170-173 (2004).
Cooler Material Boosts Fuel Cells
SOFC cathode is hot stuff
Next generation of fuel cells
Tech Research News
R D Focus
Fuel Cell Works
49
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 didnt end because we ran out of
stones. -Anonymous
50
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

51
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).