fmnt, riga, march 2010 oxygen incorporation reaction into abo 3 perovskites for energy applications...
TRANSCRIPT
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
FMNT, Riga, March 2010
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
FMNT, Riga, March 2010
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
FMNT, Riga, March 2010
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
FMNT, Riga, March 2010
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
FMNT, Riga, March 2010
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
FMNT, Riga, March 2010
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)
FMNT, Riga, March 2010
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
FMNT, Riga, March 2010
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
FMNT, Riga, March 2010
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
FMNT, Riga, March 2010
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
FMNT, Riga, March 2010
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
FMNT, Riga, March 2010
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
FMNT, Riga, March 2010
O2 molecule dissociation
stableEads= -1.04 eV
0.5 eV
-3.4 eVTS
=O2 (superoxide) migration energy is estimated as 0.2 eV
FMNT, Riga, March 2010
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
FMNT, Riga, March 2010
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
FMNT, Riga, March 2010
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
FMNT, Riga, March 2010
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).
FMNT, Riga, March 2010
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.
FMNT, Riga, March 2010
Thermodynamics of the O adsorption at different temperatures and O2 gas pressures
LaO O (110) MnO2+O
FMNT, Riga, March 2010
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
FMNT, Riga, March 2010
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).
FMNT, Riga, March 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
FMNT, Riga, March 2010
• 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).