Understanding and mitigating A-site surface enrichment in Ba-containing perovskites: a combined computational and experimental study of BaFeO3

Abstract

BaFeO3-Based perovskites are promising cathode materials for intermediate-temperature solid oxide fuel cells and protonic ceramic fuel cells due to their high electrocatalytic activity. However, during operation Ba has been observed to segregate towards the surface of these materials, deteriorating the activity. Herein, the surface of BaFeO3-based materials is studied using both density functional theory and strain-controlled epitaxial thin films. The results suggest that the surface Ba concentration can be controlled by tensile strain or by substituting Fe with a larger cation. Specifically, first-principles calculations suggest that the surface with low Ba content (i.e. (110)BaFeO), stabilizes when a biaxial tensile strain is applied, in agreement with the decreased surface Ba concentration observed in tensilely strained Ba0.95La0.05FeO3-delta thin films. As verified by simulations and experiments, substituting the Fe with the larger Zr cations also strains the material tensilely, thereby lowering Ba surface concentration. Further analysis suggests that stretching the Ba-O bond can strengthen the interaction between Ba and O and increase the energy required to form Ba vacancies in bulk, thereby reducing surface enrichment. Electrical conductivity relaxation and electrochemical impedance spectroscopy tests demonstrated that surface oxygen exchange kinetics is enhanced by introducing Zr into Ba0.95La0.05FeO3-delta. Zr-Substituted Ba0.95La0.05FeO3-delta is also shown to work effectively as a cathode for full solid oxide and proton ceramic fuel cells achieving high performance in the 600 to 850 degrees C range. This work explains how the surface composition of BaFeO3-based perovskites can be adjusted by lattice strain and cationic substitution, paving the way for the rational design of ceramic fuel cell cathode materials with enhanced performance.