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)MnO_3-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^-, O₂^2-, 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 LaMnO₃MnO₂ (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