Fundamentals of Proton-Insertion Coupled Electron Transfer (PICET) in Metal Oxides for Aqueous Batteries

The need for sustainable and low-cost energy storage for grid-scale applications motivates research into new types of aqueous rechargeable batteries. Within this context, proton-insertion coupled electron transfer (PICET) into transition metal oxide electrodes presents an attractive charge storage mechanism that utilizes non-metallic charge carriers in high conductivity electrolytes. The purpose of this Review is to present the fundamentals of PICET into transition metal oxides from neutral or acidic electrolytes as it relates to aqueous rechargeable batteries. We elaborate on the electrochemical features of PICET and the choice of transition metal oxide hosts, the electrochemical thermodynamics and kinetics of PICET, and the properties of hydrogenated transition metal oxides. A particular emphasis is placed on the composition and properties of the aqueous electrolyte used for PICET, which influences the kinetics and thermodynamics of the charge storage mechanism. Finally, we conclude with a discussion of the practical considerations and outlook for proton-based aqueous batteries utilizing transition metal oxide electrodes.