SOFC perovskite- DFT work

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Transcript of SOFC perovskite- DFT work

  • Atomistic simulations of a promising solid oxide fuel cell

    cathode materials Ba0.5Sr0.5Co0.8Fe0.2O3-

    Electronic Structure

    Phase Stability

    Oxygen Migration Energetics using Nudged Elastic Band (NEB)

    Shruba Gangopadhyay

    Email: shruba at gmail.com

    University of California, Davis

    IBM Research Almaden

    This work performed at University of Central Florida

    . 1

    If you are interested in my recent

    exciting (Li-air) battery related

    work please contact me directly

  • Solid Oxide Fuel Cell -SOFC

    Anode - Ceria/Nickel cermet

    Electrolyte - Gadolinia doped Ceria (CGO)

    Cathode - LSCF (a four component oxide based on

    La, Sr, Co, and Fe oxides)

    2

  • Preferred structure of cathode materials

    Cathode materials are Oxygen rich oxides

    Perovskites represented by ABO3

    A= Lanthanides/Group IIA

    B= Transition metals

    3

    Perovskites (SrCoO3)

    For a good SOFC cathode

    No phase transition

    Ease of oxygen migration

  • BSCF new perovskite for SOFC

    Perovskites represented by ABO3, Ba0.5Sr0.5Co0.8Fe0.2O3-

    A= Ba or Sr

    B= Fe or Co

    Constructed a model supercell 2x2x2 cell

    4Shao, Z. P.; Haile, S. M., Nature 2004, 431, (7005), 170-173.

  • BSCF - no phase transition

    5Wang, H. H.; Tablet, C.; Feldhoff, A.; Caro, H., Journal of Membrane Science 2005, 262, (1-

    2), 20-26.

    Dependence of lattice parameter

    w.r.t temperature

    Closest Analog SrCo0.8Fe0.2O3 shows vacancy ordering

  • Oxygen vacancies in BSCF

    6

    Oxygen non-stoichiometry

    Shao, Z. P.; Haile, S. M., Nature 2004, 431, (7005), 170-173.

    BSCF as a function of temperature at the

    oxygen partial pressures indicated

    We need to remove maximum

    Four oxygen from supercell

  • Activation energy of oxygen migration

    7

    D chem = Rate of diffusion

    Ea = Activation energy

    D0 = Temperature independent

    pre exponential factor depends

    on lattice vibrations and jump

    distance

    k = Boltzmann constant

    T = Temperature

    Self diffusion coefficient measures ease of oxygen mobility

  • Our goals

    Electronic Structure of BSCF

    Validation with Lattice Parameter

    Ground Spin state of Transition Metal (B)

    cations

    Stable Cation Arrangement

    Phase stability of BSCF

    Stable most vacancy position

    Activation Energy of Oxygen Migration

    8

  • DFT-simulation of BSCF

    Self-Consistent field calculation

    Plane wave basis set

    PBE Functional

    Vanderbilt Ultra-soft Pseudo potential

    Marzari-Vanderbilt cold smearing

    9

    Structural Optimization

    BFGS algorithmPopulation Analysis

    Lwdin population analysis

    Activation Energy for Oxygen Migration

    Symmetry Constarined Search and Nuged Elastic band(NEB)

    Quantum Espresso (Extensible Simulation Package for

    Research on Soft matter) )

    http://www.quantum-espresso.org/

    http://www.quantum-espresso.org/

  • Spin state of transition metals

    10

    2Co+4 4Co+4 6Co+4

    In BSCF

    Co +4 shows intermediate spin states

    -249.80

    -249.78

    -249.76

    -249.74

    -249.72

    -249.70

    -249.68

    -249.66

    -249.64

    7.00 7.20 7.40 7.60 7.80

    Intermediate

    High

    Low

    Low Intermediate High

  • Co+4 spin state predicted in

    agreement with experiment

    Experimental Results

    Raman Spectroscopy

    Theoretical Validation

    Jahn-Teller Distortion

    11

    O

    Co

    3.849

    3.849

    3.652

    3.581

    3.901

    3.581

    3.849

    3.849

    3.9013.652

  • Lattice parameter predicted in agreement

    with experiment

    Perovskites Calc

    (GPa)

    Expt

    (GPa)

    Calc Expt

    ()

    Bcalc Bexp acalc aexp

    BaTiO3 148.34 135 3.98 4.00

    SrTiO3 181.57 179 3.93 3.899

    SrFeO3 3.88 3.84

    SrCoO3 3.85 3.83

    Ba 0.5 Sr0.5 Co0.6Fe0.2O3 3.95 4.00

    12

    No of

    Unpaired

    ElectronRelative

    Energy

    Difference

    (kJ/mol)

    Boltzmann

    factor

    (%)

    Fe+4

    (d4)

    Co+4

    (d5)

    2 3 40 2

    4 3 0 98

    2 5 140 0

    4 5 404 0

  • A, B cations in BSCF are distributed

    randomly

    Fe1 Fe2 Ba1 Ba2 Ba3 Ba4

    E

    kJ/mol

    B

    Factor

    %

    1 5 10 12 14 16 0.00 18

    1 6 10 12 14 16 0.10 18

    1 8 10 12 14 16 0.76 13

    1 5 11 12 13 14 1.42 10

    1 6 11 12 13 14 1.41 10

    13

    There is no preferred cation ordering

  • Plan of action to determine preferred

    oxygen vacancy positions

    1. First take the lowest energy configuration with no

    oxygen vacancy

    2. Calculate the energetics of by removing one oxygen

    from nonequivalent sites in one oxygen less

    supercell

    3. Use the lowest energy configuration (obtained after

    removing one oxygen) as the starting configuration

    to determine second favorable vacancy positions

    4. Followed same way.

    14

  • Vacancies prefer to cluster

    15

    One VacancyThe most Stable for Co-x-Fe (not Co-x-Co Fe-x-Fe)

    Boltzmann Factor for this configuration 67%

    Cis cobalt coordination is the most Stable , Than Trans

    and cis Fe coordination

    Two VacanciesCo O

    O is more stable than

    Fe O

    O

    Boltzmann Factor 54%

  • 16

    Three Vacancies2 cis-octahedral one adjacent to Fe another Co

    Three vacancy forms in a plane of octahedral

    Vacancies form L shaped trimer

    Vacancies prefer to cluster

    Boltzmann Factor for this configuration 69%

  • Vacancy forms in square

    B cations form tetrahedral geometry

    17

    Boltzmann Factor for

    Square vacancy : 47%

    Linear vacancy : 9%

  • Atomistic view of

    Oxygen migration inside an perovskite

    18Initial

    Intermediate

    Metal O-Metal

    450

    Final

  • Elementary step for vacancy diffusion

    19

    0 2 4 6 8

    En

    ergy

    (eV

    )

    NEB image number

    83.

    72.8

    58.

    42.8

    28.4

    14.4

    8

    Activation

    Energy

  • Activation energies for different

    cation arrangements

    20

    Ba Fe Migrating

    Vacancy

    Permanent

    VacancyEa

    kJ/mol

    10 11 13 16 1 8 22 24 37.9

    10 12 14 16 1 8 35 36 52.3

    11 12 13 14 3 5 29 24 34.6

    11 12 13 14 3 5 38 39 20.2

    10 12 14 16 3 5 38 39 41.2

    10 12 14 16 3 5 38 39 42.6

    10 12 14 16 1 5 38 39 38.8

    10 12 14 16 1 5 36 35 23 24 49.2

    10 12 14 16 1 5 30 31 23 24 29 29.8

    10 12 14 16 1 5 35 40 24 29 36 18.5

    Experimental activation energy for

    Oxygen Migration 30-50 kJ/mol

    Energy from symmetry constrained

    Search

    Energy from NEB

  • Conclusions: BSCF

    Our DFT calculation shows experimental agreement

    Lattice Parameter of perovskites

    JT distortions

    Cations in stoichiometric BSCF is completely disorder

    Vacancy prefers to form L shaped trimer and square tetramer

    With removal of oxygen transition metals change octahedral to

    tetrahedrahal coordinations

    Oxygen migration energy(s) shows agreement with experiment

    Symmetry Constrained pathway and NEB calculations both shows

    similar energy value

    21

    Ref:

    S Gangopadhyay et.al. ACS Applied Materials & Interfaces 2009, 1 (7), 1512-1519

    S Gangopadhyay et.al. Solid State Ionics 2010, 181 (2324), 1067-1073.

  • Acknowledgements

    22

    Ref:

    S Gangopadhyay et.al. ACS Applied Materials & Interfaces 2009, 1 (7), 1512-1519

    S Gangopadhyay et.al. Solid State Ionics 2010, 181 (2324), 1067-1073.

    This work have been performed at Professor Artm E. Masunovs lab

    Research Collaborators

    Prof. Nina Orlovskaya

    Prof. Ratan Guha

    Prof. Jay Kapat

    Prof. Ahmed Sleiti