Hydrogen produced by electrochemical water splitting is considered a promising energy carrier for future sustainable energy systems. However, the oxygen evolution reaction (OER) at the anode is the major kinetic bottleneck due to its complex multi-electron transfer mechanism. Among the various catalyst materials investigated for OER, transition-metal oxyhydroxides with bulk-phase catalytic activity have attracted considerable attention. Nevertheless, many aspects of their underlying electrochemical behavior and catalytic mechanism are still not fully clarified. In this study, cobalt-based oxyhydroxide electrocatalyst (CoCat) is employed as a model system for bulk-active catalysts for investigation of reaction kinetics. Electrochemical measurements are combined with operando spectroscopic techniques, including time-resolved X-ray absorption spectroscopy (XAS) and UV–Vis spectroscopy, in order to investigate the temporal evolution of oxidation-state changes and catalytic processes over a broad range of catalyst thicknesses and operating conditions. Additionally, electrochemical impedance spectroscopy (EIS) is employed to distinguish the characteristic timescales of double-layer charging, bulk redox transitions, and catalytic OER processes. Electric double-layer charging occurs on the microsecond timescale, followed by bulk cobalt oxidation-state transitions in the millisecond regime, whereas the catalytic OER process proceeds substantially more slowly. Correlation of time-domain measurements with impedance analysis enables assignment of equivalent circuit elements to physically meaningful electrochemical processes and provides a mechanistic interpretation of EIS parameters in terms of redox equilibration and catalytic turnover kinetics. Furthermore, temperature-dependent electrochemical measurements combined with operando XAS were performed to evaluate the activation energy parameters of the OER process and to investigate how the chemical state of the catalyst influences its catalytic activity. The analysis indicates that the Gibbs free energy of activation is determined predominantly by entropic contributions rather than solely by changes in the enthalpic energy barrier. Moreover, the results suggest that the applied potential does not directly lower the intrinsic activation barrier of the OER process, but instead controls the catalytic rate indirectly through changes in the oxidation state and population of reactive sites within the catalyst. The observed relationship between catalyst oxidation state, electrode potential, and catalytic current is described within an interaction-based framework, in which repulsive interactions between oxidized sites affect the thermodynamics of oxidation, while cooperative interactions enhance the reaction kinetics by reducing the activation energy. The temperature dependence of these interaction energies suggests a primarily entropic origin and highlights the balance between oxidation constraints and catalytic enhancement. Overall, this work establishes a comprehensive mechanistic picture, linking redox charge accumulation to changes in the chemical state of the catalyst, and ultimately to its catalytic activity, thereby providing new insight into the fundamental origin of potential- and temperature-dependent OER activity in bulk-active transition-metal oxyhydroxides.
