Insights on the Proton Insertion Mechanism in the Electrode of Hexagonal Tungsten Oxide Hydrate.

This study reveals the transport behavior of lattice water during proton (de)insertion in the structure of the hexagonal WO·0.6HO electrode. By monitoring the mass evolution of this electrode material via electrochemical quartz crystal microbalance, we discovered (1) WO·0.6HO incorporates additional lattice water when immersing in the electrolyte at open circuit voltage and during initial cycling; (2) The reductive proton insertion in the WO hydrate is a three-tier process, where in the first stage 0.25 H is inserted per formula unit of WO while simultaneously 0.25 lattice water is expelled; then in the second stage 0.30 naked H is inserted, followed by the third stage with 0.17 HO inserted per formula unit. Ex situ XRD reveals that protonation of the WO hydrate causes consecutive anisotropic structural changes: it first contracts along the c-axis but later expands along the ab planes. Furthermore, WO·0.6HO exhibits impressive cycle life over 20 000 cycles, together with appreciable capacity and promising rate performance.