High proton conductivity of HxWO3 at intermediate temperatures: unlocking its application as a mixed ionic–electronic conductor

Abstract

Hydrogen tungsten bronzes (H_xWO₃), 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 °C)—remain unexplored. This study revealed the proton transport behavior of H_xWO₃, 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 H_xWO₃. Hydrogen incorporation into the sintered WO₃ pellet was found to occur preferentially near the surface, forming an approximately 500 μm-thick hydrogen-rich region. This region reached a composition of x = 0.24 and exhibited proton conductivity exceeding 10⁻¹ S cm⁻¹ at 275 °C—well above those of the 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 H_∼0.0001TiO₂, which exhibits mixed protonic–electronic conduction via hydrogen dissolution. The larger proton diffusion coefficient of H_0.24WO₃ suggests that large polaron formation enhances proton mobility. These findings unlock new functionality of H_xWO₃ as a MIEC in the intermediate-temperature range, paving the way for the development of next-generation hydrogen energy conversion systems.