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Page 1: SOFC perovskite- DFT work

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

Page 2: SOFC perovskite- DFT work

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)

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Page 3: SOFC perovskite- DFT work

Preferred structure of cathode materials

Cathode materials are Oxygen rich oxides

Perovskites represented by ABO3

A= Lanthanides/Group IIA

B= Transition metals

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Perovskites (SrCoO3)

For a good SOFC cathode

No phase transition

Ease of oxygen migration

Page 4: SOFC perovskite- DFT work

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.

Page 5: SOFC perovskite- DFT work

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

Page 6: SOFC perovskite- DFT work

Oxygen vacancies in BSCF

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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

Page 7: SOFC perovskite- DFT work

Activation energy of oxygen migration

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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

Page 8: SOFC perovskite- DFT work

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

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Page 9: SOFC perovskite- DFT work

DFT-simulation of BSCF

Self-Consistent field calculation

Plane wave basis set

PBE Functional

Vanderbilt Ultra-soft Pseudo potential

Marzari-Vanderbilt cold smearing

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Structural Optimization

BFGS algorithmPopulation 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/

Page 10: SOFC perovskite- DFT work

Spin state of transition metals

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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

Page 11: SOFC perovskite- DFT work

Co+4 spin state predicted – in

agreement with experiment

Experimental Results

Raman Spectroscopy

Theoretical Validation

Jahn-Teller Distortion

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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

Page 12: SOFC perovskite- DFT work

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

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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

Page 13: SOFC perovskite- DFT work

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

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There is no preferred cation ordering

Page 14: SOFC perovskite- DFT work

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……….

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Page 15: SOFC perovskite- DFT work

Vacancies prefer to cluster

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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%

Page 16: SOFC perovskite- DFT work

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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%

Page 17: SOFC perovskite- DFT work

Vacancy forms in square

B cations form tetrahedral geometry

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Boltzmann Factor for

Square vacancy : 47%

Linear vacancy : 9%

Page 18: SOFC perovskite- DFT work

Atomistic view of

Oxygen migration inside an perovskite

18Initial

Intermediate

Metal –O-Metal

450

Final

Page 19: SOFC perovskite- DFT work

Elementary step for vacancy diffusion

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0 2 4 6 8

En

ergy

(eV

)

NEB image number

83.

72.8

58.

42.8

28.4

14.4

8

Activation

Energy

Page 20: SOFC perovskite- DFT work

Activation energies for different

cation arrangements

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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

Page 21: SOFC perovskite- DFT work

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

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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.

Page 22: SOFC perovskite- DFT work

Acknowledgements

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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