Parameterization of Water Electrooxidation Catalyzed by Metal Oxides Using Fourier Transformed Alternating Current...

Detection and quantification of redox transformations involved in water oxidation electrocatalysis is often not possible using conventional techniques. Herein, use of large amplitude Fourier transformed ac voltammetry and comprehensive analysis of the higher harmonics has enabled us to access the redox processes responsible for catalysis. An examination of the voltammetric data for water oxidation in borate buffered solutions (pH 9.2) at electrodes functionalized with systematically varied low loadings of cobalt (CoO_x), manganese (MnO_x), and nickel oxides (NiO_x) has been undertaken, and extensive experiment-simulation comparisons have been introduced for the first time. Analysis shows that a single redox process controls the rate of catalysis for Co and Mn oxides, while two electron transfer events contribute in the Ni case. We apply a "molecular catalysis" model that couples a redox transformation of a surface-confined species (effective reversible potential, E_eff⁰) to a catalytic reaction with a substrate in solution (pseudo-first-order rate constant, k₁^f), accounts for the important role of a Brønsted base, and mimics the experimental behavior. The analysis revealed that E_eff⁰ values for CoO_x, MnO_x, and NiO_x lie within the range 1.9-2.1 V vs reversible hydrogen electrode, and k₁^f varies from 2 × 10³ to 4 × 10⁴ s^-1. The k₁^f values are much higher than reported for any water electrooxidation catalyst before. The E_eff⁰ values provide a guide for in situ spectroscopic characterization of the active states involved in catalysis by metal oxides.