First Principles Calculations of BSCF Material for Membrane Applications

1. First principles calculations of BSCF material for membrane applications Eugene Kotomin Laboratory of Theoretical Physics and Computer Simulations of materials NASA Meeting

2. 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 electricity4. Ceramic membranes NASA Meeting

3. 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 NASA Meeting

4. General problem Improvement of SOFC and membrane performance requires -- better understanding of Study and control of possible reaction pathways of oxygen reduction and incorporation reaction • Exciting and challenging multidisciplinary field: • Electrochemistry and materials chemistry, • surface science of advanced oxides, • , chemical kinetics, • large-scale computer simulations NASA Meeting

5. 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 NASA Meeting

6. 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) NASA Meeting

7. 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) NASA Meeting

8. 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 NASA Meeting

9. La Mn O Test calculations(PCCP 7, 2346 (2005) 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 Spin-polarized calculations Charges on the two surface planes are not affected by slab stoichiometry NASA Meeting

10. Preliminary results:Ba(0.5)Sr(0.5)Co(0.75)Fe(0.25)O3-δ Ba Co Sr Fe O Bulk and defect properties 40 atom supercells (12.5%) and 320 atoms (1.5%) NASA Meeting

11. 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) NASA Meeting

12. 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 NASA Meeting

13. Co-Vo-Fe vacancy NASA Meeting

14. 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 NASA Meeting

15. 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 NASA Meeting

16. Calculated lattice constants Incorporation of vacancies improves agreement with the experiment Oxygen deficiency, NASA Meeting

17. Vacancy migration energy 0.46 eV Co-Vo-Fe Co-Vo-Co 0.46 eV Co-Vo-Fe 0.52 eV Co-Vo-Co 0.42 eV For comparison: LMO 0.9 eV NASA Meeting

18. 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 NASA Meeting

19. 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 NASA Meeting

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

21. 3 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 NASA Meeting

22. 3 possible mechanisms for oxygen reduction on LSM NASA Meeting

23. Thermodynamics of the O adsorption at different temperatures and O2 gas pressures NASA Meeting

24. 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 NASA Meeting