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CellsFuel 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)
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Contents The Problem of Energy Fuel Cell Technology Overview
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…
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The Problem of Energy The Problem The Solution Diminishing supply?
Resources in unfriendly locations? Environmental damage? The Solution Adequate domestic supply Environmentally benign Conveniently transported Conveniently used
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World Energy Consumption
(annual) Source: US Energy Information Agency 264 211 158 106 52 Exa Joules (1018) 8.4 6.7 5.0 3.3 1.7 Equivalent Power (TW, 1012) Quad = 10^15 Terra = 10^12 Joules, Btu = energy Power = energy*time 1999 totals: Q-Btu, 422 EJ, 13TW 2020 projections: 630 Q-Btu, 665 EJ, 21TW 90% fossil
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Fossil Fuel Supplies 400 yrs Rsv = Reserves (90%)
Source: US Energy Information Agency Rsv = Reserves (90%) Rsc = Resources (50%) Years remaining are if we switched entirely to that fuel (as the sole source) Source Reserves, yrs Resources, yrs Total, yrs Oil 10 – 35 Gas 7 – 40 Coal 32 270 300 400 yrs 56-77
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US Energy Imports/Exports: 1949-2004
Source: US Energy Information Agency Imports Exports 35 6 30 Total 5 25 Quad BTU Quad BTU Total 4 20 3 Coal 15 2 10 Petroleum 5 1 Petroleum 1950 1960 1970 1980 1990 2000 1950 1960 1970 1980 1990 2000 Net 65% of known petroleum reserves in Middle East 3% of reserves in USA, but 25% of world consumption 35 30 25 The International Energy Agency predicted a two-thirds increase in global- energy demand by 2030 that would prevent the world from breaking the oil dominance of the Middle East, which sits on 65% of known oil reserves. No prize for guessing which country is the most vulnerable. With 3% of the world's oil reserves, the USA consumes a quarter of global output. And its imports keep growing, from 35% in the 1970s to 54% today (year? around 2004) and a projected 64% by 2020 Quad BTU 20 1957: Net Importer 15 10 5 1950 1960 1970 1980 1990 2000
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Environmental Outlook
Global CO2 levels 2004: 378 ppm Projections: ppm by 2020 Anthropogenic Fossil fuel (75%) Land use (25%) Industrial Revolution Source: Oak Ridge National Laboratory
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Environmental Outlook
- 8 - 4 + 4 400 300 200 100 Thousands of years before present (Ky BP) T relative to present (°C) 500 600 700 800 CH4 (ppmv) -- CO2 -- CH4 -- T 325 275 250 225 175 CO2 CO2 in 2004: 378 ppmv Intergovernmental Panel on Climate Change, 2001; N. Oreskes, Science 306, 1686, 2004; D. A. Stainforth et al, Nature 433, 403, 2005
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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: years; CO2 build-up is cummulative Must make dramatic changes within next few years
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The Energy Solution Solar Wind Biomass Tide/Ocean Currents
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 -> lost in 100 yrs
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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
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A Sustainable Energy Cycle
H2O, CO2 C-free Source Solar or nuclear power plants Capture e- H2 ??? Hydrides? Liquid H2? Storage Batteries Hydrocarbon Delivery e- Utilization H2O + CO2 Fuel cell
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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
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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) * 75 kW = 100 hp *Can be as high as 80-90% with co-generation
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Fuel Cell: Principle of Operation
best of batteries, combustion engines conversion device, not energy source Anode Cathode e- H+ H2 O2 H2 2H+ + 2e- ½ O2 + 2H+ + 2e- H2O Electrolyte Overall: H2 + ½ O2 H2O
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Fuel Cell Performance H2 + ½ O2 H2O voltage losses power = I*V
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 1.2 0.8 cross-over theoretical voltage 1.0 slow reaction kinetics 0.6 Power [W / cm2] 0.8 Voltage [V] peak power 0.6 0.4 0.4 electrolyte resistance 0.2 0.2 slow mass diffusion 0.0 0.0 0.0 0.4 0.8 1.2 1.6 Current [A / cm2 ]
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Membrane-Electrode Assembly (MEA)
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 electrolyte catalyst electrodes sealant Membrane-Electrode Assembly (MEA)
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Types differentiated by electrolyte, temperature of operation
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 [°F] PEM 90-110 [ ] AFC [ ] PAFC [ ] MCFC [ ] SOFC [ ] Fuel H2 + H2O H2 HC + CO Electrolyte Ion Nafion H3O+ KOH OH- H3PO4 H+ Na2CO3 CO32- Y-ZrO2 O2- Oxidant O2 O2 + H2O O2 + CO2 By-products: H2O, CO2
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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
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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? Toyota: hybrid SUV [90 kW PEM], in-house FC, 64mpg (gas equivalent), psi H2 tanks Honda: FC car [78 kW PEM], ballard, 170 mile range, 5000 psi H2 tanks (3.75 kg H2)
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Fuel Cell System Complexity
Membrane electrode assembly electrolyte catalyst electrodes sealant Electrolyte Stack System Taken from Krumpelt et al., Fuel Cell Seminar Abstracts
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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 New Catalysts
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
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New Electrolytes: Solid Acids
NSF & DOE Sponsored Program (past ONR support)
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“PEM” Fuel Cells Proton Exchange Membrane or Polymer Electrolyte Membrane SO3- + (H2O)nH+ (CF2)n 1 nm H(H2O)n+ Nafion (Dupont) saturate with H2O inverse micelle structure H(H2O)n+ ion transport H2O High conductivity Flexible, high strength Requires humidification & water management Operation below 90°C Permeable to methanol Kreuer, J Membr Sci 2 (2001) 185. Target: °C; examine inorganic H+ conductors
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½(Cs2SO4) + ½(H2SO4) CsHSO4
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
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Proton Transport Mechanism
Sulfate group reorientation 10-11 seconds S O Proton transfer 10-9 seconds
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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…
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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 Also volume change at transition
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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
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CsH2PO4 Dehydration CsH2PO4 CsH2-2xPO4-x CsH2PO4 CsPO3 + H2O
pressure detector Tc = 230 Toper = 250°C CsH2PO4 + H2O P(H2O) operation CsH2PO4 CsH2-2xPO4-x Use water partial pressure to suppress dehydration
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Conductivity of CsH2PO4 0.42 eV Humidified air p[H2O] = 0.4 atm 230 °C
![Conductivity of CsH2PO eV Humidified air p[H2O] = 0.4 atm 230 °C](http://slideplayer.com/slide/13330918/80/images/32/Conductivity+of+CsH2PO+eV+Humidified+air+p%5BH2O%5D+%3D+0.4+atm+230+%C2%B0C.jpg "Conductivity of CsH2PO eV Humidified air p[H2O] = 0.4 atm 230 °C")
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Proof of Principle H2O, H2 | Cell | O2, H2O Compared to polymers
T = 235ºC 50 mW/cm2 Compared to polymers High open circuit voltage Theoretical: 1.15V Measured: 1.00 V Polymers: V Power density Polymers: > 1 W/cm2 Platinum content Polymers: ~ 0.1 mg/cm2 Light blue text on edges 260m membrane; 18 mg Pt/cm2 D. A. Boysen, T. Uda, C. R.I. Chisholm and S. M. Haile, Science 303, (2004)
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Fuel Cell Longevity: Stable Performance
H2, H2O | cell | O2, H2O 260 m thick CsH2PO4 electrolyte T = 235ºC Current = 100 mA/cm2 CsH2PO4 – no degradation in 110 hr measurement CsHSO4 – functions for only ~ 30 mins (recoverable degradation)
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3rd Generation Fuel Cell
Fine CsH2PO4 2 m H2, H2O | cell | O2, H2O T = 248°C 8 mg Pt/cm2 Slurry deposit T. Uda & S.M. Haile, Electrochem & Solid State Lett. 8 (2005) A245-A246 10-40 m pores, ~40% porosity Open circuit voltage: V Peak power density: mW/cm2
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Impact The promise of protonics Solid Acids Show Promise...
S. M. Haile, D. A. Boysen, C. R. I. Chisholm and R. B. Merle, “Solid Acids as Fuel Cell Electrolytes,” Nature 410, (2001). The promise of protonics Solid Acids Show Promise... Some Like It Medium Hot Nature: News & Views Physics Today Online Science Now Magazine
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Fuel Cell Stack
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‘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 – 300°C Electrolyte: 240 – 280°C Steam reforming: endothermic Fuel cell rxns: exothermic Integrated design Incorporate alcohol reforming catalyst in anode chamber
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‘Direct’ Methanol Fuel Cells
With reformer Without reformer T = 260 °C; 47 m membrane T = 240 °C; 34 m membrane hydrogen hydrogen reformate 1%CO 24%CO2 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
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‘Direct’ Alcohol Fuel Cells
With reformer T = 260 °C; 47 m membrane ethanol 36 vol% Vodka 80 proof Ethanol power ~ 40% H2 power
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New Cathodes for Solid Oxide Fuel Cells
NSF & DOE Sponsored Program (past DARPA support)
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State-of-the-Art SOFCs
Component Materials cathode (air electrode) (La,Sr)MnO3 electrolyte 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 Goal: 500 – 800 °C Challenges Slower kinetics Electrolyte resistance Poor anode activity Poor cathode activity
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Solid Oxide Fuel Cell Cathodes
(Ba0.5Sr0.5)(Co0.8Fe0.2)O2.3 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 almost 1 in 4 vacant ‘triple-point’ path electrode bulk path O2 O2 Oad Oad cathode 2e- cathode Oad 2e- O2- O2- electrolyte O2- electrolyte
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Cathode Electrocatalysis
+ 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 -
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Cathode Electrocatalysis
Oad slow 2e- fast O2- 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 cm2 (LaSr)(CoFe)O3: 48 cm2 0.5 – 0.6 cm2
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Cell Fabrication 0.71 cm2 Electrolyte surface 1.3 cm Anode supported
Sinter, 1350oC 5h Dual dry press NiO + SDC (Ce0.85Sm0.15O2) SDC NiO + SDC 600oC 5h, 15%H2 Spray cathode Calcine, 950oC 5h, inert gas cathode electrolyte anode Porous anode 0.71 cm2 Anode: 700 m Cathode: 20 m Electrolyte ~ 20m Electrolyte surface 2 m Well-suited to electrode studies because one can make many bilayers with the same general morphology, and then try out different electrodes for the third layer. 1.3 cm
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Fuel Cell Power Output H2, 3% H2O | fuel cell | Air
> 1 W/cm2 at 600°C!!! The final fuel cells had the following properties: diameter of 1.33 cm, anode thickness of 0.7 mm, anode porosity of ~46 %, electrolyte thickness of ~20 m, cathode thickness of m, and cathode porosity of ~ 30%. Comparison: literature cathode material 350 mW/cm2 at 600°C
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Impact Tech Research News R & D Focus Fuel Cell Works
Z. Shao and S. M. Haile, “A High Performance Cathode for the Next Generation Solid-Oxide Fuel Cells,” Nature 431, (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
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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
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Acknowledgments The people The agencies
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
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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, (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, (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, (2003). S. M. Haile, D. A. Boysen, C. R. I. Chisholm and R. B. Merle, “Solid Acids as Fuel Cell Electrolytes,” Nature 410, (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, (2005). Z. Shao and S. M. Haile, “A High Performance Cathode for the Next Generation Solid-Oxide Fuel Cells,” Nature 431, (2004). S. M. Haile, “Fuel Cell Materials and Components,” (invited) Acta. Met. 51, (2003). S. M. Haile, “Materials for Fuel Cells,” (invited) Materials Today 18, (2003).
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