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SOFC perovskite- DFT work

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In this work, I am showing a faithful atomistic process of estimating the oxygen migration energetics within BSCF, oxygen migration energy exhibit a strong dependence on different local atomic structures of this doped perovskites. In addition, DFT calculations exhibit the reason of cubic phase stability of this doped perovskite in variable oxygen concentration.

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SOFC perovskite- DFT work

  1. 1. Atomistic simulations of a promising solid oxide fuel cell cathode materials Ba0.5Sr0.5Co0.8Fe0.2O3-δ  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
  2. 2. 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
  3. 3. 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
  4. 4. BSCF – new perovskite for SOFC Perovskites represented by ABO3, Ba0.5Sr0.5Co0.8Fe0.2O3-δ 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.
  5. 5. BSCF - no phase transition 5 Wang, 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
  6. 6. Oxygen vacancies in BSCF 6 Oxygen non-stoichiometry ″δ ″ 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
  7. 7. 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
  8. 8. 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
  9. 9. 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 algorithm Population Analysis  Löwdin 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/
  10. 10. 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
  11. 11. Co+4 spin state predicted – in agreement with experiment Experimental Results Raman Spectroscopy Theoretical Validation Jahn-Teller Distortion 11 O Co 3.849 Ǻ 3.849 Ǻ 3.652 Ǻ 3.581 Ǻ 3.901 Ǻ 3.581 3.849 3.849 3.9013.652
  12. 12. Lattice parameter predicted –in agreement with experiment Perovskites Calc (GPa) Expt (GPa) Calc Expt (Å) 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 Electron Relative 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
  13. 13. 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
  14. 14. 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
  15. 15. Vacancies prefer to cluster 15 One Vacancy The 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 Vacancies Co O O is more stable than Fe O O Boltzmann Factor 54%
  16. 16. 16 Three Vacancies 2 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%
  17. 17. Vacancy forms in square B cations form tetrahedral geometry 17 Boltzmann Factor for Square vacancy : 47% Linear vacancy : 9%
  18. 18. Atomistic view of Oxygen migration inside an perovskite 18 Initial Intermediate Metal –O-Metal 450 Final
  19. 19. Elementary step for vacancy diffusion 19 0 2 4 6 8 Energy(eV) NEB image number 83. 72.8 58. 42.8 28.4 14.4 8 Activation Energy
  20. 20. Activation energies for different cation arrangements 20 Ba Fe Migrating Vacancy Permanent Vacancy Ea 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
  21. 21. 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 (23–24), 1067-1073.
  22. 22. 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 (23–24), 1067-1073.  This work have been performed at Professor Artëm E. Masunov’s lab  Research Collaborators  Prof. Nina Orlovskaya  Prof. Ratan Guha  Prof. Jay Kapat  Prof. Ahmed Sleiti

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