Oxygen incorporation reaction into ABO 3 perovskites for energy applications Eugene A. Kotomin

1. 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

2. One of main priorities of our laboratory: New/More efficient Energy Sources and New Materialsfor 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 FMNT, Riga, March 2010

3. 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

4. Ni-ZrO2 cermet LSM Y2O3-stabilized ZrO2 La1-xSrxMnO3 (LSM) is one of basic cathode materials in SOFC • 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

5. 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

6. 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

7. Limiting stage: possible reaction pathways of oxygen reduction and incorporation reaction LaMnO3 – model material La1-xSrxMnO3 – real cathode material BSCF type cathode- next talk E.Kotomin et al, PCCP 10, 4644 (2008) FMNT, Riga, March 2010

8. Method Density Functional Theory Plane Wave basis set 4.6.19 08Dec03, Georg Kresse and Jürgen Furthmüller Institut für Materialphysik, Universität Wien 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 Badercharge analysis (Prof. G. Henkelmanand co-workers, Universiy of Texas) FMNT, Riga, March 2010

9. Purpose of a studyAtomistic/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

10. 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 FMNT, Riga, March 2010

11. La Mn O Test calculations Bulk calculations Surface calculations Orthorhombic (Pbnm) (001) (110) (111) a b c Structure optimisation for the FM,A-, C-, G-AF and non-magnetic states • 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 7-, 8-plane slabs are sufficiently thick for surface processes modelling Charges on the two surface planes are not affected by slab stoichiometry FMNT, Riga, March 2010

12. Oxygen adsorption sites FMNT, Riga, March 2010

13. Molecular adsorption 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

14. O2 molecule dissociation 0.5 eV -3.4 eV TS = stable Eads= -1.04 eV O2 (superoxide) migration energy is estimated as 0.2 eV FMNT, Riga, March 2010

15. Atomic oxygen adsorption a)The O-O dumbbell has an angle of 50° with the normal to the surface Predominant adsorption site is atop surface Mn ion accompanied by large FMNT, Riga, March 2010

16. Atomic oxygen diffusion along the [100] direction (Mn-O-Mn) Mn “bridge” O 0.40 eV 1.6 eV TS Migration energy is 2 eV: essentially immobile species FMNT, Riga, March 2010

17. Oxygen vacancy • segregation to the surface • low diffusion barrier *ExperimentalEdiff(SrTiO3) = 0.86 eV I. Denk, W. Munch, and J. Maier, Journal of the American Ceramic Society 78, 3265 (1995) FMNT, Riga, March 2010

18. Adsorbed O drop into vacancy No energy barrier detected FMNT, Riga, March 2010

19. 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

20. Possible mechanisms of oxygen incorporation --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 oxygen Incorporation: BSCF>LSM>LMO. FMNT, Riga, March 2010

21. Thermodynamics of the O adsorption at different temperatures and O2 gas pressures FMNT, Riga, March 2010

22. 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

23. 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

24. 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

25. Thank You ! FMNT, Riga, March 2010

26. 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

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