1. FMNT, Riga, March 2010 Oxygen incorporation reaction into ABO 3 perovskites for energy applications Eugene A. Kotomin Institute for solid state physics @ UL, Riga and Max Planck Institute for Solid State Research, Stuttgart, Germany

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

3. 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

4. FMNT, Riga, March 2010 La 1-x Sr x MnO 3 (LSM) is one of basic cathode materials in SOFC Ni-ZrO 2 cermet Y 2 O 3 -stabilized ZrO 2 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

5. 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

6. 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

7. FMNT, Riga, March 2010 Limiting stage: possible reaction pathways of oxygen reduction and incorporation reaction LaMnO 3 – model material La 1-x Sr x MnO 3 – real cathode material BSCF type cathode- next talk E.Kotomin et al, PCCP 10, 4644 (2008)

8. FMNT, Riga, March 2010 Method Density Functional Theory Plane 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

9. FMNT, Riga, March 2010 Purpose of a study Atomistic/mechanistic details hardly detectable experimentally: -- Optimal sites for oxygen adsorption -- the energetics of O 2 dissociation, -- O and vacancy migration on the surface -- O penetration to cathode surface: what are the rate-determining reaction stages

10. FMNT, Riga, March 2010 Computational details VASP: GGA PW calculations atoms description: kinetic energy cutoff: 400 eV > E cutmax = 269.887 eV Monkhorst-Pack k-points sampling < 0.27 Å -1 ElementValence electrons Cutoff energy, eV Core radius, Å La5s26s25p65d15s26s25p65d1 219.2711.48 Mn3p63d64s13p63d64s1 269.8871.22 O2s22p42s22p4 250.0000.98

11. 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 1.182.542.74 7-, 8-plane slabs are sufficiently thick for surface processes modelling Charges on the two surface planes are not affected by slab stoichiometry

12. FMNT, Riga, March 2010 Oxygen adsorption sites

13. FMNT, Riga, March 2010 Molecular adsorption OrientationE ads (m) (O 2 ), eVDistances, ÅCharges b ), eSpin,  B O-O bond O-Mn s O(1)O(2)Mn s Mn c) O2O2 tilted-1.131.361.86 a) -0.29 a) -0.131.783.12D horizontal-0.891.421.85-0.35-0.301.773.05S 1.90 a) For O atom nearest to the surface, b) Atoms in O 2 molecule c) 3.80 µ B on a bare surface

14. FMNT, Riga, March 2010 O 2 molecule dissociation stable E ads = -1.04 eV 0.5 eV -3.4 eV TS = O 2 (superoxide) migration energy is estimated as 0.2 eV

15. FMNT, Riga, March 2010 Atomic oxygen adsorption a)The O-O dumbbell has an angle of 50° with the normal to the surface Site E ads (at) (O), eV E ads (m) (O), eV Distance from O ads, Å Charges, e Spin,  B OsOs Mn s OsOs O ads MnO -4.02 -1.072.55(4x)1.63-1.131.85-0.622.20S “bridge”-2.410.541.50 a) 1.87 (2x)-0.711.65-0.483.61S “hollow”-0.592.363.28(2x)----1.16-----0.32---T 3.18(2x)---(4x) Predominant adsorption site is atop surface Mn ion accompanied by large

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

17. FMNT, Riga, March 2010 Oxygen vacancy No.atomd, Å  q, e 1V o.. 2La0.17-0.01 3La0.220.00 4Mn0.22-0.21 5Mn0.19-0.20 6O0.32-0.03 7O0.32-0.02 Energy, eVbulksurface formation7.646.23 diffusion0.95 * 0.67 * Experimental E diff (SrTiO 3 ) = 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

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

19. 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 MnO 2 ).

20. FMNT, Riga, March 2010 Possible mechanisms of oxygen incorporation --The rate-determining step is encounter of adsorbed molecular oxygen (superoxide O 2 - or peroxide O 2 (2-) )with a surface oxygen vacancy --Both vacancy concentration and mobility are important for a fast oxygen Incorporation: BSCF>LSM>LMO.

21. FMNT, Riga, March 2010 Thermodynamics of the O adsorption at different temperatures and O 2 gas pressures LaO O (110) MnO 2 +O

22. FMNT, Riga, March 2010 Conclusions The (001) MnO 2 - terminated LaMnO 3 surface could play an important role in oxygen-related processes in SOFC. This surface permits dissociative O 2 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: O 2 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

23. 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).

24. 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

25. FMNT, Riga, March 2010 Thank You !

26. 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).

27. FMNT, Riga, March 2010