Mechanistic Investigation with Kinetic Parameters on Water Oxidation Catalyzed by Manganese Oxide Nanoparticle Film

Electrochemical water oxidation is a key counter reaction in obtaining value-added chemicals by reduction in aqueous solution. However, slow kinetics is a problem in this process, so the quantitative analysis of kinetic parameters is necessary to design film-type electrocatalysts. Although electrochemical impedance spectroscopy (EIS) has been proven to be a powerful tool in analyzing sparsely loaded catalysts on electrically conducting supporters, it turned out that film-type catalysts above 100 nm thickness are challenging to analyze with conventional models. Here, we propose a new transmission line model that was implemented with a Havriliak-Negami (H-N) capacitor and Warburg element. We successfully extracted meaningful kinetic parameters, such as the reaction rate constant at active sites and transport parameters across the film. We utilized this model to analyze monodisperse sub-10 nm partially oxidized MnO nanoparticles (p-MnO NPs) operating with superb activity under neutral pH. From this analysis, we revealed that protons are involved in transport on the surface of p-MnO NPs, explained the rationale for the optimum thickness, and correlated the reaction rate constant (22.1 s^-1 for a 300 nm-thick film at 1.35 V vs NHE) with the kinetic parameters obtained from electrokinetic analysis.