FMNT, Riga, March 2010 Oxygen incorporation reaction into ABO 3 perovskites for energy applications...

FMNT, Riga, March 2010

Oxygen incorporation reaction into ABO3 perovskites for energy

applications

Eugene A. Kotomin Institute for solid state physics @ UL, Riga

and Max Planck Institute for Solid State Research,

Stuttgart, Germany


One of main priorities of our laboratory: New/More efficient Energy Sources and New

Materialsfor energy applications

1. advanced nuclear fuels for Generation IV reactors

2. New construction reactor (radiation resistant) materials

3. solid oxide fuel cells: 80% conversion of

chemical energy into electricity


Close collaboration with many European

partners (Max Planck Institute, Stuttgart; Jülich Res. Center)

and EC FP7 Projects: EURATOM, NASA, F-Bridge

Interdisciplinary research including materials science, quantum chemistry, defect theory, Solid state physics, high performance computing


La1-xSrxMnO3 (LSM) is one of basic cathode materials in SOFC

Ni-ZrO2

cermet Y2O3-stabilized ZrO2

LSM

• Fuel-flexibility

• Efficiency up to 85%

• Output up to 2 MW…

• High T (800-1000 ºC)

• High cost!

• Metallic interconnects

• Intermediate T (600-700 ºC)

• NANOSTRUCTURED THIN FILMS


Why new materials?

• Complicated combination of properties:

• Efficient ionic-electronic conductors (ISSFIT conference at ISSP, June 2010)

• Efficient catalysers at low temperatures

• Low thermal expansion

• No interaction with impurities, electrolyte

• No degradation under extreme conditions


Solution

• Large scale computer simulations of materials

in close collaboration with state-of-the art experiments:

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


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

possible reaction pathways of oxygen reduction and incorporation reaction

LaMnO3 – model materialLa1-xSrxMnO3 – real cathode materialBSCF type cathode- next talk

E.Kotomin et al, PCCP 10, 4644 (2008)


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


Purpose of a study

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


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 electronsCutoff energy,

eVCore radius,

Å

La 5s26s25p65d1 219.271 1.48

Mn 3p63d64s1 269.887 1.22

O 2s22p4 250.000 0.98


Test calculations

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 modelling

Charges on the two surface planes are not affected by slab stoichiometry


Oxygen adsorption sites

2 2 2 22The (001) MnO - terminated surface

surface cell

(12.5% covarage)2(O )

2 2

(O)

Molecular adsorption atop Mn ion,

different orientations

(O ) (O )

Atomic adsorption at :

Mn ion

"bridge position"

O ion

"hollow position"

(O) (O) 2

(O)

adsads slab slab

at adsads slab slab

mads

E E E E

E E E E

E

2(O )(O)ads

slab slabE E E


Molecular adsorptionOrientation Eads

(m)(O2), eV Distances, Å Chargesb), e Spin, B

O-O bond

O-Mns O(1) O(2) Mns Mnc) O2

tilted -1.13 1.36 1.86a) -0.29a) -0.13 1.78 3.12 D

horizontal -0.89 1.42 1.85 -0.35 -0.30 1.77 3.05 S

1.90

a) For O atom nearest to the surface , b) Atoms in O2 molecule c) 3.80 µB on a bare surface


O2 molecule dissociation

stableEads= -1.04 eV

0.5 eV

-3.4 eVTS

=O2 (superoxide) migration energy is estimated as 0.2 eV


Atomic oxygen adsorption

a)The O-O dumbbell has an angle of 50° with the normal to the surface

SiteEads

(at)(O),

eV

Eads(m)(O),

eV

Distancefrom Oads, Å

Charges, e Spin, B

Os Mns Os Mns Oads Mn O

Mn -4.02 -1.07 2.55(4x) 1.63 -1.13 1.85 -0.62 2.20 S

“bridge” -2.41 0.54 1.50a) 1.87 (2x) -0.71 1.65 -0.48 3.61 S

“hollow” -0.59 2.36 3.28(2x) --- -1.16 ---- -0.32 --- T

3.18(2x) --- (4x)

Predominant adsorption site is atop surface Mn ion accompanied by large


Atomic oxygen diffusion

along the [100] direction (Mn-O-Mn) Mn “bridge” O

1.6 eV0.40 eV

TS

Migration energy is 2 eV: essentially immobile species


Oxygen vacancy

No. atom d, Å q, e

1 Vo

..

2 La 0.17 -0.01

3 La 0.22 0.00

4 Mn 0.22 -0.21

5 Mn 0.19 -0.20

6 O 0.32 -0.03

7 O 0.32 -0.02

Energy, eV bulk surface

formation 7.64 6.23

diffusion 0.95* 0.67

*Experimental Ediff(SrTiO3) = 0.86 eV

I. Denk, W. Munch, and J. Maier, Journal of the American Ceramic Society 78, 3265 (1995)

• segregation to the surface• low diffusion barrier


Adsorbed O drop into vacancy

No energy barrier detected


LSM Modeling

Using our energy calculations and Vo estimate in LSM bulk R.De Souza, J.A.Kilner, Sol. St. Ionics, 106, 175 (1998),

we can consider different oxygen incorporation paths and thus determine the rate-determining step (next slide).

Our Ab initio HF-DFT claculations of the LSM atomic/electronic structure

S.Piskunov et al., Phys Rev B 76, 012410 (2007); 78, 121406 (2008) show:

-- considerable Sr segregation trend towards surface (0.5 eV)

-- half-metallic electronic structure instead of AFM semiconducting LMO (at low T)

-- Sr doping makes La(Sr)O termination favourable!!

Negative effect: No Vo segregation towards this surface (unlike MnO2).


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: BSCF>LSM>LMO.


Thermodynamics of the O adsorption at different temperatures and O2 gas pressures

LaO O (110) MnO2+O


Conclusions• The (001) MnO2- terminated LaMnO3 surface could play an

important role in oxygen-related processes in SOFC.

• This surface permits dissociative O2 adsorption with the energy gain of 2.2 eV per molecule

• Adsorbed oxygen atom has large diffusion energy of 2 eV unlike O vacancies (with the activation energy of 0.7-0.9 eV).

• Possible oxygen reduction mechanism: O2 molecule meets

one-by-one two O vacancies • More complicated cathode materials could be modeled (BSCF)

using the same approach but more refined hybrid functionals


Main relevant publications:1. R.A. Evarestov, E.A. Kotomin, Yu.A. Mastrikov, D. Gryaznov, E. Heifets, and J. Maier, Phys. Rev. B, 72, 214411 (2005).2. E.A. Kotomin, R.A. Evarestov, Yu.A. Mastrikov and J. Maier, Phys. Chem. Chem. Phys., 7, 2346 (2005).3. Yu. Zhukovskii, E.A. Kotomin, R.A. Evarestov, and D.E. Ellis, Int. J. Quant. Chem, 107, 2956 (2007) (review article on O vacancies in perovskites).4. E.A. Kotomin, Yu.A. Mastrikov, E. Heifets, and J.Maier, Phys. Chem. Chem. Phys. 10, 4644 (2008).5.Yu.A. Mastrikov, E. Heifets, E.A. Kotomin, and J.Maier, Surf. Sci. 603, 326 (2009).6. Yu. A. Mastrikov, R. Merkle, E. Heifets, E. A. Kotomin and J. Maier, J. Phys. Chem. C, 114, 3017–3027 (2010).


Thanks:

• R.Evarestov, St.Petersburg University

• R. Merkle, J. Maier, D.Gryaznov, Max Planck

Institute, Stuttgart

• Yu.Mastrikov, M.Kuklja, University of Maryland, USA

• E. Heifets, Caltech, Pasadena

• Yu. Zhukovskii, S. Piskunov, ISSP, Riga


Thank You !


• Y. Choi et al., Oxygen Reduction on LaMnO3-Based Cathode Materials in Solid Oxide Fuel Cells, Chem. Mater. 19, 1690 (2007).

• Y. Choi, M. C. Lin, and M. L. Liu, Computational study on the catalytic mechanism of oxygen reduction on La0.5Sr0.5MnO3 in solid oxide fuel cells, Angew Chem Int Edit 46, 7214 (2007).

• Y.Choi, M.E.Lynch, M. C. Lin, and M. L. Liu, Prediction of O2 dissocistion kinetics on LaMnO3 cathode materials. J.Phys. Chem. C 113, 7290 (2009).

FMNT, Riga, March 2010 Oxygen incorporation reaction into ABO 3 perovskites for energy applications...

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