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.