Non-Grotthuss Proton Diffusion Mechanism in Tungsten Oxide Dihydrate from First-Principles Calculations

TL;DR

This study reveals that proton diffusion in tungsten oxide dihydrate occurs within WO6 layers without structural water involvement, challenging the traditional Grotthuss mechanism, and provides first-principles insights into its diffusion pathways and barriers.

Contribution

First-principles calculations show proton diffusion in WO3·2H2O occurs within WO6 layers, not via structural water, offering new understanding of proton conduction mechanisms.

Findings

Proton migration barrier in WO3·2H2O is 0.42 eV, matching experimental data.

Proton diffusion in γ-WO3 has an activation energy of 0.35 eV.

Proton absorption in WO3·H2O is energetically favorable but limited by proton-proton repulsion.

Abstract

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 (WO$_{3}$.2H$_{2}$O), a known good proton conductor, takes place within the layers of corner-sharing WO$_{6}$ octahedra without direct involvement of structural water. The calculated proton migration barrier in WO$_{3}$.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 WO$_{3}$.2H$_{2}$O is essentially the same as in $\gamma$-WO$_{3}$, 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 WO$_{3}$ and WO$_{3}$.2H$_{2}$O, we find that proton absorption in the monohydrate WO$_{3}$.H$_{2}$O 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 WO$_{3}$.H$_{2}$O 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 WO$_{3}$.2H$_{2}$O and $\gamma$-WO$_{3}$.