Water Electrooxidation Kinetics Clarified by Time‐Resolved X‐Ray Absorption and Electrochemical Impedance Spectroscopy for a Bulk‐Active Cobalt Material

Abstract

Water oxidation, the oxygen evolution reaction (OER), is the anodic process in electrocatalytic production of hydrogen and further green fuels. Transition‐metal oxyhydroxides with bulk‐phase OER activity of the complete material or amorphized near‐surface regions are of prime application interest, but their basic electrochemical properties are insufficiently understood. Here the timescale of functional processes is clarified by time‐resolved X‐ray absorption spectroscopy and electrochemical impedance spectroscopy (EIS) for a thickness‐series of cobalt oxyhydroxides films (about 35–550 nm). At the outer material surface, an electric double‐layer is formed in microseconds followed by clearly cobalt‐centered redox‐state changes of the bulk material in the low millisecond domain and a slow chemical step of O2‐formation, within hundreds of milliseconds. Conceptually interesting, the electrode potential likely controls the OER rate indirectly by driving the catalyst material to an increasingly oxidized state which promotes the rate‐limiting chemical step. Rate constants are derived for redox chemistry and catalysis from EIS data of low‐thickness catalyst films; at higher thicknesses, catalyst‐internal charge transport limitations become increasingly relevant. Relations between electrochemically active surface area, double‐layer capacitance, and redox (pseudo‐)capacitance are discussed. These results can increase the power of EIS analyses and support knowledge‐guided optimization of a broader class of OER catalyst materials.