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

Proton conductors have potential applications such as fuel cells, electrolysis cells, and sensors. These applications require new materials with high proton conductivity and high chemical stability at intermediate temperatures. Herein we report a series of new hexagonal perovskite-related oxides, Ba₅R₂Al₂SnO₁₃ (R = Gd, Dy, Ho, Y, Er, Tm, and Yb). Ba₅Er₂Al₂SnO₁₃ exhibited a high proton conductivity without chemical doping (e.g., 0.01 S cm^-1 at 303 °C), which is attributed to its high proton concentration and diffusion coefficient. The high diffusion coefficient of Ba₅Er₂Al₂SnO₁₃ can be attributed to the fast proton migration in the octahedral layers. The high proton concentration is attributed to full hydration in hydrated Ba₅Er₂Al₂SnO₁₃ and the large amount of intrinsic oxygen vacancies in the dry sample, as evidenced by both neutron diffraction and thermogravimetric analysis. Ba₅Er₂Al₂SnO₁₃ was found to exhibit high chemical stability under wet atmospheres of O₂, air, H₂, and CO₂. High proton conductivity and high chemical stability indicate that Ba₅Er₂Al₂SnO₁₃ is a superior proton conductor. Ba₅R₂Al₂SnO₁₃ (R = Gd, Dy, Ho, Y, Tm, and Yb) exhibited high electrical conductivity in wet N₂, suggesting that these materials also exhibit high proton conductivity. These findings will open new avenues for proton conductors. The high proton conductivity via full hydration and fast proton migration in octahedral layers in highly oxygen-deficient hexagonal perovskite-related materials would be an effective strategy for developing next-generation proton conductors.