Pathways for oxygen incorporation in mixed conducting perovskites: A DFT-based Mechanistic analysis for (La, Sr)MnO_3-δ

An extensive set of DFT calculations on LaMnO3 slabs has been generated and used as a basis to identify the most probable reaction mechanism for oxygen incorporation into (La, Sr)MnO_3-δcathode materials. MnO ₂[001] is found to be the most stable surface termination under fuel cell operation conditions (high temperature, high pO2, cubic unit cell). Chemisorption leading to the formation of O^2-, 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. The variation of the reaction rate and preferred mechanism(s) with adsorbate and point defect concentrations is discussed.