First principles calculations of BSCF material for membrane applications

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First principles calculations of BSCF material
for membrane applications Eugene
Kotomin Laboratory of Theoretical Physics and
Computer Simulations of materials
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One of main priorities of our laboratory
New/more efficient Energy Sources and New
Materials for energy applications1. advanced
nuclear fuels for Generation IV reactors2. New
construction reactor (radiation resistant)
materials 3. solid oxide fuel cells 80
conversion of chemical energy into electricity4.
Ceramic membranes
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Development of new materials

• Large scale computer simulations of materials
• in close collaboration with state-of-the art
  experiments Max Planck Institute, Stuttgart
• Try-and-error approach does not work!
• Limitations of experiments
• Discrimination of processes (O vacancies
  migration) in the bulk and on surfaces,
• A role of different dopands and impurities
• Identification of adsorbates at low coverages

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General problem
Improvement of SOFC and membrane performance
requires -- better understanding of
Study and control of possible reaction pathways
of oxygen reduction and incorporation reaction

• Exciting and challenging multidisciplinary field
• Electrochemistry and materials chemistry,
• surface science of advanced oxides,
• , chemical kinetics,
• large-scale computer simulations

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Materials of interest magnetic perovskites

• LaMnO3 (LMO) model material
• La1-xSrxMnO3 (LSM) real cathode material
• Multi-component BSCF type cathodes
• These strongly correlated materials reveal
• numerous phenomena due to a combination
• of spin, orbit, lattice degrees of freedom
• -- 2 areas of applications low-T (spintronics)
• -- high T solid oxide fuel cells

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First experience with theoretical modelling of
LSM

• E.Kotomin et al, PCCP 10, 4644 (2008)
• R.Merkle et al, J. ECS Trans. 25, 2753 (2009)
• Yu.Mastrikov et al. J Phys Chem. C, 114, 3017
  2010.
• First BSCF paper submitted to Enenrgy and
  Environmental Science, 2010.
• Standard DFT (GGA) or DFT-HF hybrids ?
• calculations of large, low symmetry systems with
• defects and surfaces (up to 320 atoms per
  supercell)

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Method
Density Functional Theory Plane Wave basis set
4.6.19 08Dec03, Georg Kresse and Jürgen
Furthmüller Institut für Materialphysik, Universi
tät Wien
Generalised Gradient Approximation Perdew Wang 91
exchange-correlation functional Projector
Augmented Wave method Davidson algorithm for
electronic optimization Conjugate Gradient method
for structure relaxation Nudged Elastic Bands for
energy barriers estimation Bader charge analysis
(Prof. G. Henkelman and co-workers, Universiy of
Texas)
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Computational detailsVASP GGA PW calculations

• atoms description
• kinetic energy cutoff
• 400 eV gt Ecutmax 269.887 eV
• Monkhorst-Pack k-points sampling lt 0.27 Å-1

Element Valence electrons Cutoff energy, eV Core radius, Å
La 5s26s25p65d1 219.271 1.48
Mn 3p63d64s1 269.887 1.22
O 2s22p4 250.000 0.98
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Test calculations(PCCP 7, 2346 (2005)
Bulk calculations
Surface calculations
Orthorhombic (Pbnm)
(001) (110) (111)
a
b
strongly under-coordinated surface atoms strongly under-coordinated surface atoms strongly under-coordinated surface atoms strongly under-coordinated surface atoms
polar polar polar polar polar polar
/-1 e /-4 e /-4 e /-4 e /-4 e /-3 e
surface energy, eV/surface cell surface energy, eV/surface cell surface energy, eV/surface cell surface energy, eV/surface cell surface energy, eV/surface cell surface energy, eV/surface cell
1.18 1.18 1.18 2.54 2.74 2.74
c
Structure optimisation for the FM, A-, C-, G-AF
and non-magnetic states

• Cohesive energy, Structure, ionic charges
• practically (lt1) do not depend on
• the specific magnetic ordering
• In a good agreement with experimental data
• Non-magnetic state very unfavourable
• High covalency of the Mn-O bonding

7-, 8-plane slabs are sufficiently thick for
surface processes modelling Spin-polarized
calculations Charges on the two surface planes
are not affected by slab stoichiometry
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Preliminary resultsBa(0.5)Sr(0.5)Co(0.75)Fe(0.25
)O3-d

Ba
Co
Sr
Fe
O
Bulk and defect properties 40 atom supercells
(12.5) and 320 atoms (1.5)
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Test pure ABO3 perovskites

• Lattice constants (A, cubic phase)
• A B
• Co Fe
• Ba 3.96(--) 3.97(4.04)
• Sr 3.84(3.83) 3.85(3.85)
• IS, HS
• Pure BSCF a_o3.90-3.92 A (expt 3.98A)

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Effective (static) Bader atomic charges,e

• Ba, Sr 1.57e ? close to formal 2e
• Co 1.71e
• Fe 1.88e
• O -1.1 e
• Strong covalent contribution to the bonding

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Co-Vo-Fe vacancy
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Vacancy formation energies

• (Ba,Sr)CoO3 ca. 1 eV (LaCoO3 1.5 eV)
• (Ba,Sr)FeO3 ca. 2.4 eV (LaFeO3 1.2 eV)
• LMO 4.5-5 eV expt 3 eV
• STO 5.5-6 eV expt 5 eV
• Charge disproportionation effect
• 2Fe(3)Fe(2) Fe (4) is neglected in theory

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Charge redistribution around Vo
Red is electronic density deficiency, blue-
excess
Charge of a missing O2- ion is spread over
nearest Co and Fe ions
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Calculated lattice constants
Incorporation of vacancies improves agreement
with the experiment
Oxygen deficiency,
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Vacancy migration energy
0.46 eV
Co-Vo-Fe
Co-Vo-Co
Co-Vo-Fe
0.46 eV
0.52 eV
Co-Vo-Co
0.42 eV
For comparison LMO 0.9 eV
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Our ultimate goal--the mechanism of the
oxygenreduction in different materials LSCF?
under different conditions,--understanding of
the limiting reaction steps,--increase of O
reduction efficiency
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MilestonesAtomistic/mechanistic details
hardly detectable experimentally-- Optimal
sites for oxygen adsorption-- the energetics of
O2 dissociation,-- O and vacancy migration on
the surface -- O penetration to cathode surface
what are the rate-determining reaction
stages, O diffusion
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Mechanism of oxygen reduction M2 in LSM (Merkle
et al, J ECS Trans.2009)
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3 possible mechanisms of oxygen incorporation
--The rate-determining step is encounter of
adsorbed molecular oxygen (superoxide O2- or
peroxide O2 (2-) )with a surface oxygen
vacancy --Both vacancy concentration and
mobility are important for a fast
oxygen Incorporation
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3 possible mechanisms for oxygen reduction on LSM
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Thermodynamics of the O adsorption at different
temperatures and O2 gas pressures
LaO O (110) MnO2O
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Conclusions

• Standard ab initio computer codes are
• able to shed some additional light on
  cathode/surface reactions where expt tools are of
  a limited applicability
• We reproduce Vo low migration energies
• Lattice structure ? role of structural Vo
• low Vo formation energies
• To be used in the analysis of BSCF
  cathode/membrane performance