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

Hydrogen tungsten bronzes (H_xWO₃) 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_xWO₃exhibit 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_xWO₃, with particular emphasis on the experimental methodologies used to evaluate the diffusion coefficients and the structural and electronic properties of WO₃. We also summarize previous studies on the microscopic behavior of protons in electronically conductive H_xWO₃. Based on these findings, we discuss the factors governing fast proton diffusion in H_xWO₃. 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_xWO₃but also provide a framework applicable to other hydrogen bronzes and mixed protonic-electronic conductors, thereby accelerating the development of hydrogen-related functional materials.