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

Abstract

This study reveals the transport behavior of lattice water during proton (de)insertion in the structure of the hexagonal WO₃·0.6H₂O electrode. By monitoring the mass evolution of this electrode material via electrochemical quartz crystal microbalance, we discovered (1) WO₃·0.6H₂O incorporates additional lattice water when immersing in the electrolyte at open circuit voltage and during initial cycling; (2) The reductive proton insertion in the WO₃ hydrate is a three-tier process, where in the first stage 0.25 H⁺ is inserted per formula unit of WO₃ 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 H₃O⁺ inserted per formula unit. Ex situ XRD reveals that protonation of the WO₃ hydrate causes consecutive anisotropic structural changes: it first contracts along the c-axis but later expands along the ab planes. Furthermore, WO₃·0.6H₂O exhibits impressive cycle life over 20 000 cycles, together with appreciable capacity and promising rate performance.