Superprotonic Conductivity in Hexagonal and Tetragonal Cesium Hydroxide Hydrate

Solid-state proton conductors with high conductivity at intermediate temperatures (100–300 °C) have recently gained attention due to their potential for a wide range of new electrochemical applications. The proton conductivity of cesium hydroxide hydrate (CsOH^.H₂O) is reported on, a superprotonic conductor whose ionic conductivity is hitherto unreported. Via a thermal treatment, the hexagonal and tetragonal CsOH^.H₂O phases are prepared and the effect of the degree of hydration (H₂O content) on the structure and proton conductivity is investigated. It is shown that the temperature in the thermal treatment has an enormous influence on the structure and water content, and consequently, the proton transport. The conductivity of the hexagonal and tetragonal phases exceeds 3 × 10³ S cm⁻¹ at 30 °C and 10⁻² S cm⁻¹ at 120 °C, making them potentially suitable for both low- and intermediate-temperature electrochemical applications. ¹H NMR spin-lattice relaxation rate measurements reveal fast dynamic processes with rates reaching the GHz regime, resulting in diffusion coefficients as high as 1.3 × 10⁻¹¹ m² s⁻¹ at 60 °C, in good agreement with the proton conductivity results derived from impedance spectroscopy. These findings will inspire the development of novel inorganic solid proton conductors based on hydroxide hydrates.