High Proton Conductivity of HxWO3 at Intermediate Temperatures: Unlocking Its Application as a Mixed Ionic-Electronic Conductor

Abstract: Hydrogen tungsten bronzes (HxWO3), known for their mixed protonic-electronic conduction near room temperature, are extensively studied for electrochromic and gasochromic applications. However, their proton transport properties at elevated temperatures, particularly in the intermediate-temperature range (200-500 {\deg}C), remain unexplored. This study revealed the proton transport behavior of HxWO3, focusing on its potential as a proton-conducting mixed ionic-electronic conductor (MIEC) for intermediate-temperature electrochemical applications. By employing a proton-conducting phosphate glass as an electron-blocking electrode, we selectively measured the partial proton conductivity of sintered HxWO3. Hydrogen incorporation into the sintered WO3 pellet was found to occur preferentially near the surface, forming an approximately 500-micrometer-thick hydrogen-rich region. This region reached a composition of x = 0.24 and exhibited proton conductivity exceeding 10^-1 S/cm at 275 {\deg}C, well above those of state-of-the-art perovskite proton conductors. Impedance spectroscopy revealed distinct features of proton transport, including an isotope effect. The proton diffusion coefficient was 100-1000 times greater than that of H0.0001TiO2, which exhibits mixed protonic-electronic conduction via hydrogen dissolution. The larger proton diffusion coefficient of H0.24WO3 suggests that large polaron formation enhances proton mobility.These findings unlock new functionality of HxWO3 as a MIEC in the intermediate-temperature range, paving the way for the development of next-generation hydrogen energy conversion systems.