Solid Oxide Fuel Cells: abinitio calculations of material properties and survey of contribution to w

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Solid Oxide Fuel Cells ab-initio calculations
of material properties and survey of contribution
to world energy supply

• Ashley Predith
• AMASS seminar
• October 15, 2003
• Anton Van der Ven, Gerbrand Ceder
• Department of Materials Science Engineering
• Massachusetts Institute of Technology
• Cambridge, Massachusetts

Alex Bogicevic, Chris Wolverton Scientific
Research Laboratories Ford Motor
Company Dearborn, Michigan
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Solid Oxide Fuel Cell

• nickel ZrO2 cerment
• yttria stabilized ZrO2
• LaMnO3 or
• (La,Sr)(Fe,Co)O3

Average operating temperature 850 ?C
Application stationary power generation for
apartment complexes, hospitals, schools
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Time line
t
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Outline

• Ab-initio calculations
• Cathode materials (La,Sr)(Fe,Co)O3-x
• Stability
• Radius size effects
• Electrolyte material (Zr,Y)O2-x
• Model structures of YSZ
• Cluster expansion
• Stable ground states
• Survey of contribution to world energy supply
• World energy supply, demand
• Feasibility of fuel cell adoption

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• SOFC cathode materials
• (La,Sr)(Fe,Co)O3-x

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Relative oxygen defect energies

• ?Hdefect formation
• Edefect cell Estoichiometric cell
• ½ Eoxygen binding

Oxygen defect energies define a measure of
stability of the material.
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Migration Enthalpies Perovskite ABO3

110 of cubic perovskite
J. Kilner and R. Brook, Solid State Ionics, 6,
237 (1982).
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Oxygen Ion Diffusion

• Experiments to models
• Ionic conductivity

?Hmigration Eactivated Estable
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Effect of A and B cation site radius
All migration enthalpy calculations model the
compounds as cubic perovskites.
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Electronic Structure in Perovskites ABO3
oxygen p metal d orbital overlap
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SrFeO3 density of states
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SrFeO3 band diagram
FeegO?2p FeegO?2p Fet2gO?2p Fet2gO?2p Fet2gO?
2p Fet2gO?2p O2p, O2p Fet2gO?2p Fet2gO?2p
Fet2gO?2p FeegO?2p FeegO?2p FeegO?2p
000 0 ½ 0 ½ ½ 0 000
½ ½ ½ 0 ½ 0
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SrFeO3 charge density

• Experimental vs. calculated lattice parameters
• 3.869 Å vs. 3.826 Å
• Charge density

O
Fe
Fe(t2g)
Fe(eg) O(?2p)
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• SOFC electrolyte material
• (Zr1-x,Yx)O2-0.5x

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Experimental YSZ Phase Diagram

• Yttria stabilizes cubic phase at room
  temperature.
• YO1.5 YZr ½VO??
• ? - 57.1 YO1.5 ordered phase (Zr3Y4O12)

C. Pascual, P. Duran, J Am Cer Soc 66 1 (1983)
23-27
V. Stubican, et. al. J Am Cer Soc 61 1-2 17-21
(1978).
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Cubic fluorite (Zr1-x,Yx)O2-(x/2)

• Ef(?1,?2,?N,?1,?2,?2N)

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Computational details
Trial structures 80,156 structures
Supercells contain up to 10 primitive fluorite
cells Structure energies Quantum
mechanical energy calculations DFT-GGA
4x4x4 Monkhorst Pack k-point convergence
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Determining ground state hull
Ground states are lowest energies on hull Common
tangent construction at compositions between
ordered states
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Cluster expansion
A parameterization of the energy of structures in
terms of variables describing their ionic
configurations.
Vi Effective Cluster Interactions energy
coefficients
?i Occupation Variables geometric clusters
of neighbors in cation sublattice, in anion
sublattice, and connecting the sublattices
Each structures occupation variables uniquely
identify it.
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Important sets of ?i?j
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Search for stable ground states
DFT energies
117 structures
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Search for stable ground states
DFT energies cluster expanded energies
117 structures rms error38 meV Vi,j,k(117
structures)
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Search for stable ground states
DFT energies, cluster expanded energies,
calculated trial energies
117 structures rms error38 meV Vi,j,k(117
structures)
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Search for stable ground states
DFT energies, cluster expanded energies,
calculated trial energies
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Search for stable ground states
Obtain Vj from cluster expansion of DFT energies
Calculate cluster expanded energy of trial
structures with known ?i,j
no
END!
Predict new stable structures?
yes

• Verify energies with DFT

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Search for stable ground states
117 structures rms error38 meV Vi,j,k(117
structures)
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Search for stable ground states
Add 9 structures 126 structures rms error31
meV Vi,j,k(126 structures)
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Search for stable ground states
Add 28 structures 154 structures rms error 23
meV Vi,j,k(154 structures)
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YSZ cubic ground states
154 energies rms error 23 meV 4 stable,
ordered intermediate composition ground states on
cubic fluorite lattice
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57
29
33
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DFT energies with respect to MONOCLINIC ZrO2
Monoclinic ZrO2
NEW ground state at 33 .
Stable with respect to monoclinic.
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Predicted structure
Y2Zr4O11 33 YO1.5 structure
Double rows of yttria coupled to single vacancy
rows C2/m symmetry
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Predicted structure
Y2Zr4O11 33 YO1.5 structure
Zirconia face centered cubic Oxygen simple
cubic
Zr
Oxy
33
Predicted structure
Y and vacancy chains along cubic -1 1 2 Vacy
- vacy at 6nn within chain Y-Y at 3nn within
chain
vacy
Zr
Oxy
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Predicted structure
Y and vacancy chains along cubic -1 1 2 Vacy
- vacy at 6nn within chain Y-Y at 3nn within
chain
vacy
Y
Zr
Oxy
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Correlations of ordered structures
Y 1 oxy 1 Zr -1 vac -1
Structure information from correlations Probabili
ty deviation from random (probability of
vacancies in ordered structure at 1nn)
(probability of vacancies randomly being at 1nn
on a fluorite lattice at same composition)
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Vacancy ordering tendencies 1,2 nn
A. Bogicevic, C. Wolverton, et. al. Phys Rev B
64 014106
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Vacancy ordering tendencies 4, 6nn
A. Bogicevic, C. Wolverton, et. al. Phys Rev B
64 014106
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Vacancy ordering tendencies 3nn
A. Bogicevic, C. Wolverton, et. al. Phys Rev B
64 014106
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Vacancy ordering tendencies 5nn
A. Bogicevic, C. Wolverton, et. al. Phys Rev B
64 014106
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Vacancy ordering tendencies 7nn
A. Bogicevic, C. Wolverton, et. al. Phys Rev B
64 014106
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• Fuel cells and world energy supply

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Energy demand the changing world population

• World population
• Current 6 billion
• Estimated for 2025 8 billion
• Percent of population in urban areas
• Currently 47
• Estimated for 2025 58
• 1890 - 1990 49 of growth of world energy
  consumption came from increased population and 51
  from increased use per capita.
• Current world energy consumption 402 x 1018
  J/year

U.N. Population Division World Energy
Assessment. U.N. Development Programme, 2000
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Energy Intensity
Energy intensity energy consumption / GDP
Goldemberg Energy Policy. 2610 729 (1998).
Energy Information Assoc, DOE, 1998
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Energy sources fuel cells

• Coal (23), oil (35), natural gas (21), nuclear
  (6), renewables (wind, solar, hydro, geothermal,
  biomass 14)
• SOFC attributes
• Cogeneration of heat and electricity from natural
  gas/oil products. High efficiency (50-60)
• Decentralized power generation.
• Low NOx, SOx emissions
• 200 kW SOFC test plants in operation

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Future Energy Decisions

• Popular long term energy strategy is for a
  sustainable energy solution.
• Sustainable development meets the needs of the
  present generation without compromising the needs
  of future generations.
• Our Common Future United Nations World
  Commission on Environment and Development,1987

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Feasibility of fuel cell technology
environmental, political, economic factors

• Environmental effects CO2 gas by product
• Renewables are an alternative with different
  environmental impact
• Public funding
• In the U.S., NSF and DOE Basic Science research
  subject to Congressional and presidential
  approval.
• Competing technologies
• Natural gas infrastructure Europe, NA price
  spikes
• Combined cycle power generators

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Conclusions Acknowledgements

• Ab-initio Calculations
• LSCF formation energies follow experiment
• SrFeO3 shows some covalent bonding
• New YSZ 33 ground state stable with respect to
  monoclinic ZrO2.
• Y and vacancies have coupled ordering at 33 .
• SOFC World Energy
• Future energy demand will be high.
• Fuel cells can play a part in distributed power
  generation.
• In addition to technical considerations, a
  range of outside factors will influence the
  adoption or rejection of fuel cells.
• Acknowledgments
• National Science Foundation
• Ford Motor Company