Ab initio DFT+U analysis of oxygen transport in LaCoO3: the effect of Co3+ magnetic states

Abstract

Although solid oxide fuel cells (SOFCs) provide clean and efficient electricity generation, high operating temperatures (T > 800 °C) limit their widespread use. Lowering operating temperatures (600 °C < T < 800 °C) requires developing next-generation mixed ion-electron conducting (MIEC) cathodes that permit facile oxygen transport. One promising MIEC material, La_1−xSr_xCo_1−yFe_yO₃ (LSCF), can operate at intermediate temperatures, has a longer cell lifetime, and permits less expensive interconnect materials. However, the road to optimization of LSCF compositions for SOFC applications would benefit from fundamental, atomic-scale insight into how local chemical changes affect its oxygen ion conductivity. We provide this insight using ab initio density functional theory plus U (DFT+U) calculations to analyze the factors governing oxygen transport in the LSCF parent material LaCoO₃. We show that oxygen diffusion in LaCoO₃ depends strongly on the spin state of the Co³⁺ ions: in particular, low spin Co³⁺ promotes higher oxygen vacancy concentrations than other spin states. We also predict that different spin states of Co³⁺ significantly affect the oxygen ion migration barrier. Through electronic structure analysis, we uncover the fundamental details which govern oxygen diffusivity in LaCoO₃.