First-principles calculations of the oxygen-diffusion mechanism in mixed Fe/Ti perovskites for solid-oxide fuel cells

The ionic conductivity of transition-metal (TM) oxide perovskites plays an essential role in their applications in solid-oxide fuel cells. The conductivity of oxygen ions can increase the reaction rate on the electrode surface and improve the performance of fuel-cell devices. Herein, the oxygen-diffusion mechanism in a series of Fe/Ti oxide perovskites, especially oxygen-vacancy formation and migration, was investigated using density functional theory calculations. By simulating various vacant sites, we found that the oxygen vacancy adjacent to the Fe cation has lower formation energy than that between two Ti cation sites. Analysis of differential charge densities demonstrated that electrons originating from oxygen vacancies are reorganized at the D-state of Fe through the weak covalency of TM-O bonds, resulting in the low formation energy of these vacancies. The energy profiles of oxygen migration around Fe and Ti cations were calculated in the rocksalt supercell of SrFeas(0.5)Ti(0.5)O(3) through the nudged-elastic-band method. The oxygen migration barrier around Ti cations was lower than that of Fe cations, which suggests that Ti cations also play an important role in oxygen diffusion in the perovskites. Overall, our study elucidated the oxygen-diffusion mechanism in the B-sites of mixed Fe/Ti oxide perovskites and provides a theoretical basis for the design of TM oxide perovskites with mixed ionic and electronic conductivity for solid-state fuel cells.