Stable proton storage in α-MoO3 via a bifunctional molecular crowding electrolyte

Abstract

Layered molybdenum trioxide (α-MoO₃) storing protons exhibits a high theoretical capacity and extraordinary rate capability and can be used as a reliable anode material for aqueous proton batteries. But a severe dissolution issue during operation always results in rapid failure in battery performance. Herein, a bifunctional molecular crowding electrolyte was engineered to address this issue. The high-concentration inert polymer, polyethylene glycol 400, reduces the free water activity of the electrolyte and the low-concentration solute, trifluoromethanesulfonic acid, forms a proton-rich electrolyte environment under such a water-poor condition thanks to its intrinsic superacidity, both of which favour the reverse shift of the dissolution equilibrium, and with the latter enabling the electrolyte with high ion conductivity as well. Thus, the α-MoO₃ electrode in the above electrolyte shows a longer lifespan without sacrificing its rate capability. The analysis on charge storage mechanisms by X-ray powder diffraction technique demonstrates that α-MoO₃ undergoes bare proton-involved multistep two-phase and solid-solution reactions, where only the two-phase reaction between hydrogen molybdenum bronze II and III phases and the solid-solution reactions of themselves are reversible.