Proton diffusion in hydrogen tungsten bronzes and beyond: A comprehensive review

Hydrogen tungsten bronzes (H x WO 3 ) have attracted renewed interest owing to their high hydrogen incorporation capacity and fast proton diffusion, which underpin their emerging applications beyond conventional chromic devices. However, the reported proton diffusion coefficients ( D H ) in H x WO 3 exhibit a striking scatter over several orders of magnitude, hindering a unified understanding of proton transport. Here, we provide a comprehensive overview of proton diffusion in H x WO 3 , with particular emphasis on the experimental methodologies used to evaluate the diffusion coefficients and the structural and electronic properties of WO 3 . We also summarize previous studies on the microscopic behavior of protons in electronically conductive H x WO 3 . Based on these findings, we discuss the factors governing fast proton diffusion in H x WO 3 . Proton diffusion in this system cannot be understood solely in terms of structural factors; rather, it is strongly influenced by the electronic carrier density and the degree of electron delocalization. These two factors have not been explicitly considered in conventional ion-conducting materials, highlighting the importance of an electronic-structure-based perspective. Such insights not only rationalize the unique proton transport properties of H x WO 3 but also provide a framework applicable to other hydrogen bronzes and mixed protonic-electronic conductors, thereby accelerating the development of hydrogen-related functional materials.