Water reduction products catalyze the formation of a passivating layer that protects negative electrodes for batteries in aqueous superconcentrated electrolytes.

Recently, because of sustainability issues dictated by societal demands, more importance has been given to aqueous systems and especially to proton-based battery. However, the mechanisms behind the processes leading to energy storage in such systems are still not elucidated. Under this scope, our study is structured on the selection of a model electrode material, the protonic phase H x IrO 4 and the scrutiny of the interfacial processes through suitable analytical tools. Herein, we employed operando Electrochemical Quartz Crystal Microbalance (EQCM) combined with Electrochemical Impedance Spectroscopy (EIS) to provide new insights into the mechanism intervening at the Electrode-Electrolyte Interface. Firstly, we demonstrated that not only the surface or near surface but the whole particle participates in the cationic redox process. Secondly, we proved the contribution of the proton on the overall potential window together with the incorporation of water at low potentials solely. This is explained by the fact that water molecules permit a further insertion of protons in the material by shielding the proton charge but at the expense of the proton kinetic properties. These findings shed new light on the importance of water molecules in the ion-insertion mechanisms taking place at the Electrode-Electrolyte Interface of aqueous proton-based batteries. Overall, the present results further highlight the richness of the EQCM based methods for the battery field in offering mechanistic insights that are crucial for the understanding of interfaces and charge storage in insertion compounds.