Anisotropic Proton Migration in Hexagonal Perovskite-Related Ba₅Er₂Al₂ZrO₁₃ Oxide

Abstract

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 Ba 5 Er 2 Al 2 ZrO 13 (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.

High Proton Conduction in the Octahedral Layers of Fully Hydrated Hexagonal Perovskite-Related Oxides

“…At 200 °C in wet atmospheres, the bulk conductivity of BEAS was higher than that of Ba 5 Er 2 Al 2 ZrO 13−5y/2 (OH) 5y (BEAZ). 33 The higher bulk conductivity of BEAS compared to BEAZ is attributed to the higher proton concentration of Ba S22), which corresponds to the higher occupancy at the O4 site of hydrated BEAS compared to hydrated BEAZ. It is likely that the higher proton concentration in BEAS compared to BEAZ is attributed to the higher ability to incorporate the extra oxygen atoms O4 due to hydration into the h′ layer of BEAS compared to BEAZ.…”

“…Murakami et al 28 proposed the water uptake leading to the formation of protons in the h′ layers, which undergo two-dimensional proton diffusion. Later, Youn et al 33 ) have the same crystal structure as Ba 5 Er 2 Al 2 ZrO 13 (10H polytype, hexagonal crystal system, space group P6 3 /mmc) and also exhibit significant proton conductivity. 26,28,30,32,33 In general, a high proton concentration y is required for high proton conductivity.…”

“…Later, Youn et al 33 ) have the same crystal structure as Ba 5 Er 2 Al 2 ZrO 13 (10H polytype, hexagonal crystal system, space group P6 3 /mmc) and also exhibit significant proton conductivity. 26,28,30,32,33 In general, a high proton concentration y is required for high proton conductivity. Here, the y values of hydrated perovskitetype BaBO 3−δ and hexagonal perovskite-related oxide Ba Ba 5 A 2 Al 2 ZrO 13 materials are not high due to partial hydration.…”

High proton conduction by full hydration in highly oxygen deficient perovskite

“…51 For example, several hexagonal perovskite-related oxides such as Ba 2 LuAlO 5 have two-dimensional intrinsically oxygen deficient layers and high proton conductivities. 13,41,43–47,50 Very recently, we have reported quite high proton conduction (over 10 mS cm −1 above 320 °C) and high chemical stability in the cubic perovskite-type BaSc 0.8 Mo 0.2 O 2.8 . 18 The cubic BaSc 0.8 Mo 0.2 O 2.8 perovskite is stabilized by Mo 6+ donor-doping into the parent material BaScO 2.5 with three-dimensionally disordered intrinsic oxygen vacancies v in BaScO 2.5 v 0.5 .…”

“…Recently, many oxides with “intrinsic oxygen vacancies” have attracted much attention due to high proton and oxide-ion conductivities. 6,13,18,36–50 Here, the “intrinsic oxygen vacancies” v are the oxygen vacancies in a parent material ( e.g. , v in high temperature cubic Bi 2 O 3 v ).…”