DFT-Based Analysis for Oxygen Incorporation Mechanism in Mixed Conducting Perovskites

E. Kotomin, Yu. A. Mastrikov*,R. Merkle, E. Heifets*

* Institute for Solid State Physics, University of Latvia, Riga

Based on an extensive set of DFT calculations on LaMnO3 slabs the most probable reaction mechanism for oxygen incorporation into (La, Sr)MnO3-δ cathode materials  is investigated. First, MnO2 (001) is identified as the most stable surface termination under fuel cell operation conditions (high temperature, high pO2, cubic unit cell). Chemisorption leading to the formation of O2-, O22-, and O- atop Mn is exothermic, but due to the negative adsorption entropy and electrostatic repulsion the levels of coverage of molecular oxygen adsorbates are low (in the few percent range). Under typical solid oxide fuel cell conditions, a mechanism in which the encounter of O- with a surface oxygen vacancy at the surface is rate-determining exhibits the fastest rate (see Fig.). The variation of the reaction rate and preferred mechanism(s) with adsorbate and point defect concentrations is discussed.

Energy (black) and free energy (red) profile for the most probable reaction pathway of oxygen incorporation on the LaMnOMnO2 (001) surface termination. The "bottleneck" (rate-determining step) is either the dissociation or the approach of an oxygen vacancy to adsorbed O- before the final incorporation step. From Kuklja et al (2013) with permission

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Publications:

M. M. Kuklja, E. A. Kotomin, R. Merkle, Yu. A. Mastrikov, and J. Maier: Combined theoretical and experimental analysis of processes determining cathode performance in solid oxide fuel cells Physical Chemistry Chemical Physics 15(15), 5443–5471 (2013). DOI: 10.1039/C3CP44363A

Yu. A. Mastrikov, R. Merkle, E. Heifets, E. A. Kotomin, and J. Maier: Pathways for Oxygen Incorporation in Mixed Conducting Perovskites: A DFT-Based Mechanistic Analysis for (La,Sr)MnO3-δ J. Phys. Chem. C 114(7), 3017-3027 (2010).  DOI:10.1021/jp909401g

L. Wang, R. Merkle, Yu.A. Mastrikov, E.A. Kotomin, and J. Maier, Oxygen exchange kinetics on solid oxide fuel cell cathode materials— general trends and their mechanistic interpretation. – J. Mater. Res., 27,  2000-2008 (2012).  DOI: 10.1557/jmr.2012.186

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