UC Berkeley

Understanding interfacial ion-transfer kinetics is central to advancing electrochemical processes associated with energy storage and catalysis. Here we investigate electrochemical proton-insertion kinetics at the electrode/electrolyte interface using dense crystalline WO3 films between ∼5 and 40 nm in thickness as a model system. Monoclinic WO3 is prepared on conductive F:SnO2 substrates via W-metal sputtering and calcination. Thin films offer a reasonably well-defined interface where confounding effects of ion and electron transport present in typical (nano)­porous electrodes are avoided. We develop a current-response model to decouple overlapping contributions of double-layer charging and electrochemical proton-insertion kinetics, enabling the quantification of kinetic parameters. Voltammetry, potential-step, and impedance spectroscopy experiments illustrate the influence of film crystallinity, thickness, and state of charge (SoC) on interfacial ion-transfer. Temperature-dependent measurements yield an activation energy of 29 kJ/mol for proton insertion/de-insertion, consistent with a molecular mechanism involving proton-transfer from hydronium at the WO3/electrolyte interface. This work establishes initial benchmark values for proton-insertion kinetics into solids and provides an experimental platform to study the interplay between interfacial structure and ion-transfer.