Non-Grotthuss proton diffusion mechanism in tungsten oxide dihydrate from first-principles calculations

Fast proton conduction mechanism is of key importance for achieving high performance in fuel cell membranes, batteries, supercapacitors, and electrochromic materials. Enhanced proton diffusion is often observed in hydrated materials where it is thought to occur via the famous Grotthuss mechanism through pathways formed by structural water. Using first-principles density-functional theory calculations, we demonstrate that proton diffusion in tungsten oxide dihydrate (WO3 center dot 2H(2)O), a known good proton conductor, takes place within the layers of corner-sharing WO6 octahedra without direct involvement of structural water. The calculated proton migration barrier in WO3 center dot 2H(2)O (0.42 eV) is in good agreement with the experimental value inferred from the temperature dependence of conductivity (0.36 eV). The preferred proton diffusion path in WO3 center dot 2H(2)O is essentially the same as in gamma-WO3, and we find an activation energy of 0.35 eV for the latter, which agrees well with the experimental values. In contrast to the small intercalation voltages calculated for WO3 and WO3 center dot 2H(2)O, we find that proton absorption in the monohydrate WO3 center dot H2O is energetically highly favorable, corresponding to voltages in excess of 1 eV in the dilute limit. However, strong proton-proton repulsion limits the equilibrium H content at zero voltage. We find a fast one-dimensional diffusion channel in WO3 center dot H2O with an activation energy of only 0.07 eV at dilute proton concentrations, but much higher barriers are expected at near-equilibrium concentrations due to strong repulsive interactions with other protons. Our results illustrate that low proton diffusion barriers and low insertion voltages both contribute to fast proton transport in bulk WO3 center dot 2H(2)O and gamma-WO3.