Anisotropic Proton Migration in Hexagonal Perovskite-Related Ba₅Er₂Al₂ZrO₁₃ Oxide

Hexagonal perovskite-related oxides have been of significant interest in recent years for their potential applications in electrochemical devices, particularly as solid electrolytes in fuel cells and electrolysis cells. The anisotropy of proton migration in these materials has been playing a critical role in their performance, but the underlying mechanisms and factors governing this anisotropy remain poorly understood. In this study, using the {0001}-plane preferentially oriented Ba5Er2Al2ZrO13 (BEAZ) hexagonal perovskite electrolyte-supported cell as a model system, we reveal the anisotropic characteristics of the proton conduction behavior. By controlling the orientation of the grains in the BEAZ thin film via the surface energy-driven secondary grain growth phenomenon, we demonstrate that proton migration in BEAZ is more favorable in the lateral direction than in the vertical direction. More importantly, density functional theory calculations and ab initio molecular dynamics simulations suggest that anisotropic proton migration in the lateral direction is preferred through the perovskite-like layer rather than the intrinsically oxygen-deficient layer. Our study demonstrates that an electrostatic neutral perovskite-like layer in hexagonal perovskite-related oxides should not be overlooked as a key parameter for achieving higher proton conduction kinetics.