Thermally stable proton-conducting oxy-hydroxides synthesized in concentrated water vapor

The diverse functionalities of (oxy-)hydroxides, such as electrocatalytic activity of transition-metal oxy-hydroxides in the oxygen evolution reaction (OER) and ion exchange capabilities of layered double hydroxides (LDHs), continue to attract significant interest. However, these compounds are typically synthesized in aqueous solutions at room temperature, under hydrothermal conditions, or in mild vapor atmospheres. Here, we present a novel (oxy-)hydroxide synthesis technique called “vapor hydroxidation,” which is conducted in a highly concentrated water vapor atmosphere at elevated temperatures. Structural analysis revealed the formation of a new oxy-hydroxide, [Ba₂O_x(OH)_y]_0.55InO₂, with a misfit-layered structure, characterized by alternating incommensurate barium hydroxide bilayers and indium oxide blocks. Unlike known (oxy-)hydroxides, this oxy-hydroxide displays exceptional thermal stability, retaining hydroxide ions within its crystal structure up to approximately 700 °C. These features suggest promising potential for high-temperature proton conductivity, as demonstrated by an electrical conductivity of 5 × 10⁻⁴ S cm⁻¹ at 500 °C even under dry argon. Our “vapor hydroxidation” method thus opens up a new avenue for the development of proton-functional materials with unconventional chemical and electronic characteristics.