First principles calculations of BSCF material for membrane applications Eugene Kotomin

13.04.2010 NASA Meeting

First principles calculations of BSCF

material for membrane applications

Eugene KotominLaboratory of Theoretical Physics and

Computer Simulations of materials


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 electricity

4. Ceramic membranes


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


General problem

2( ) 4 2g e 2 cathode reaction: O O

Study and control of possible reaction pathways of oxygen reduction and incorporation reaction

Improvement of SOFC and membrane performance requires-- better understanding of

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


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


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)


Method

Density Functional TheoryPlane Wave basis set

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)

4.6.19 08Dec03, Georg Kresse and Jürgen Furthmüller

Institut für Materialphysik,Universität Wien


Computational detailsVASP: GGA PW calculations

• atoms description:

• kinetic energy cutoff:

400 eV > Ecutmax = 269.887 eV

• Monkhorst-Pack k-points sampling < 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


Test calculations(PCCP 7, 2346 (2005)

a b

c

La Mn O

• Cohesive energy, Structure, ionic charges practically (<1%) 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

Orthorhombic (Pbnm)

Structure optimisation for the FM, A-, C-, G-AF and non-magnetic states

Bulk calculations Surface calculations

(001) (110) (111)

strongly under-coordinated surface atoms

polar

+/-1 e +/-4 e +/-3 e

surface energy, eV/surface cell

1.18 2.54 2.74

7-, 8-plane slabs are sufficiently thickfor surface processes modellingSpin-polarized calculations Charges on the two surface planes are not affected by slab stoichiometry


Preliminary results:Ba(0.5)Sr(0.5)Co(0.75)Fe(0.25)O3-δ

• Ba

Sr

O

Co

Fe

Bulk and defect properties40 atom supercells (12.5%) and 320 atoms (1.5%)


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_o=3.90-3.92 A (expt 3.98A)


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


Co-Vo-Fe vacancy


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


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


Calculated lattice constants

Oxygen deficiency,

Incorporation of vacancies improvesagreement with the experiment


Vacancy migration energyCo-Vo-Fe Co-Vo-Co

Co-Vo-Fe

Co-Vo-Co 0.42 eV

0.46 eV

0.52 eV0.46 eV

For comparison: LMO 0.9 eV


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


Milestones:

Atomistic/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


Mechanism of oxygen reduction M2 in LSM (Merkle et al, J ECS Trans.2009)

O2

Mn

-1.1eV

-O O-

Mn O Mn+0.6eV

-O O-

Mn O Mn

-1.6eV VI

VII

Mn O Mn

O-

O

II

O Mn

+0.7eV

O-

Mn Mn

Mn O2- Mn

-1.5eV

VIIIVIII

O2 adsorption superoxide O2

-

dissociationwithout VO involved..

VO surface diffusion

..

incorporationof O- into VO

..


3 possible mechanisms of oxygen incorporation

-O O-

O Mn O

O-

Mn O- MnO-

Mn O Mn

Mn O Mn

O2

Mn Mn

O2

O-

Mn Mn Mn O2- Mn

O- O-

Mn O Mn

VO..

VO..

O22-

O-

--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 oxygenIncorporation


3 possible mechanisms for oxygen reduction on LSM

molecular adsorbate coverage surfa

ce ox

ygen

vaca

ncy

conc

entra

tion10-6

reac

tion

rate

per

uni

t cel

l and

sT = 1000 KpO2 = 1 bar

10-6

10-510-4

10-3

10-210-1

10-5

10-4

10-3

10-2

10-1

100

102

104

106

108

M2': diss. without VO..,

VO.. O- limiting

M1: adsorption directly into VO

..

M3: dissociation after O2

- VO.. encounter

M3: dissociation after O2

- VO.. encounter

M3'diss. after O2

- VO..,

VO.. O- limiting

M2:dissociation without VO

..

M2:dissociation without VO

..


Thermodynamics of the O adsorption at different temperatures and O2 gas

pressures

LaO O (110) MnO2+O


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

First principles calculations of BSCF material for membrane applications Eugene Kotomin

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