Two-Phase Electrochemical Proton Transport and Storage in α-MoO3 for Proton Batteries

Diffusion-controlled charge storage and phase transitions of electrodes are typical indicators of sluggish kinetics in battery chemistries. However, fast rate capabilities are found in an alpha-MoO3 proton intercalation electrode that presents both features. Here, the unique topochemistry is shown to involve multiple ion-electrode interactions and proceeds via two key steps: hydronium adsorption on surfaces and proton insertion into bulk lattices. This triggers structure transitions from MoO3 to hydrogen molybdenum bronzes (HMBs). Following the first process, subsequent rearrangements proceed only among HMBs phases with high reversibility and kinetics, thus providing structural explanations to the fast rate capability. At electrode-electrolyte interfaces, hydronium is the active charge carrier that initiates charge transfer and surface hydration, which are accompanied by water adsorption/desorption with reduced polarization and enhanced kinetics. Further water activity is shown to induce material dissolution during function. These findings offer fundamental insights in proton chemistries, which may form the basis of future high rate and capacity energy storage.