Hydride-ion-conducting K₂NiF₄-type Ba-Li oxyhydride solid electrolyte

Hydrogen transport in solids, applied in electrochemical devices such as fuel cells and electrolysis cells, is key to sustainable energy societies. Although using proton (H⁺) conductors is an attractive choice, practical conductivity at intermediate temperatures (200-400 °C), which would be ideal for most energy and chemical conversion applications, remains a challenge. Alternatively, hydride ions (H^-), that is, monovalent anions with high polarizability, can be considered a promising charge carrier that facilitates fast ionic conduction in solids. Here, we report a K₂NiF₄-type Ba-Li oxyhydride with an appreciable amount of hydrogen vacancies that presents long-range order at room temperature. Increasing the temperature results in the disappearance of the vacancy ordering, triggering a high and essentially temperature-independent H^- conductivity of more than 0.01 S cm^-1 above 315 °C. Such a remarkable H^- conducting nature at intermediate temperatures is anticipated to be important for energy and chemical conversion devices.