Interfacial Proton-Transfer Kinetics Using Model Tungsten Oxide Thin Films

Understanding interfacial ion-transfer kinetics is central in advancing electrochemical processes associated with energy storage and catalysis. Here we investigate 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 interfacial proton-transfer kinetics, enabling quantification of kinetic parameters. Voltammetry, potential-step, and impedance-spectroscopy experiments illustrate the influence of film crystallinity, thickness, and state of charge 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 benchmark values for proton-insertion kinetics into solids and provides a useful experimental platform to study the interplay between interfacial structure and ion transfer.