(449a) Density Functional Theory Study of Enhanced One Dimensional Mobility of Oxygen On Strained LaCoO3 (001) Surface

Amongst the various types of fuel cells, the solid oxide fuel cell (SOFC) is particularly attractive given its fuel flexibility and high conversion efficiencies. The slow rate of oxygen reduction at the cathode is, however, the main barrier to achieve higher power output in SOFCs at intermediate temperatures (500°C-700°C). Mechanisms by which lattice strain alters the oxygen reduction reaction (ORR) kinetics are important to understand in order to increase the ORR activity of SOFC cathodes. Here we assess the mechanistic and quantitative effects of strain on oxygen diffusion on the LaCoO₃(LCO)(001) surfaceusing density functional theory (DFT) calculations. Planar tensile strain reduces the migration barrier of oxygen vacancy anisotropically on the LCO(001) surface, and increases that of the adsorbed atomic oxygen. The reduction in the migration barrier of oxygen vacancy becomes increasingly anisotropic with increasing strain, inducing an enhanced mobility along the [1-10] direction and a suppressed mobility along the [110] direction. The reduction of the energy barrier of migration along the [1-10] is due to the increase in the space around Co that the oxygen traverses with a curved path. The increase in the unstability of oxygen vacancy formation at every other surface lattice oxygen row in the [1-10] direction due to the increasing octahedral distortions with increasing planar tensile strain inhibits the migration of oxygen vacancy along the [110] direction. The increasing energy barrier for the migration of adsorbed oxygen atom is due to the weakening hybridization of the in-plane Co-O bonds, which consequently enhances the atomic oxygen adsorption on the surface. From a qualitative estimate, the significantly lower energy barrier for oxygen vacancy diffusion is expected to dominate the other degrading factors and actually accelerate the ORR kinetics on LCO(001) up to 3% strain. Furthermore, the insights obtained here are useful for designing strategies to control the desired anisotropic and uni-directional oxygen transport along strained hetero-interfaces.