Electrochemical Identification of Facet-Governing Redox Behavior and Catalytic Reactivity of Co(OH)₂ in the Oxygen Evolution Reaction

Traditional electrochemical redox assessments offer insights into material properties for charge storage and catalytic kinetics but often fail to link these to specific surfaces, obscuring the structure-performance relationship. Here, we reveal the facet-dependent electrochemical redox behaviors and their connection to oxygen evolution reaction (OER) catalysis using Co(OH)₂ nanosheets and nanorods as models. By correlating redox charge storage capacity and kinetics with distinct exposed surfaces, we uncover diffusion-controlled redox processes on the basal surface and non-diffusion-controlled behavior on the lateral surface and further utilize the distinct redox charging kinetics to differentiate the two. Integrated microscopic, spectroscopic, and electrochemical analyses reveal redox-active regions with a thickness of ∼12 Co atoms on the lateral surface and 85% Co atom equivalent extension on the basal surface, yielding benchmark redox charge capabilities of 3.04 mC cm^-2 for the lateral surface and 0.35 mC cm^-2 for the basal surface. By linking the surface-specific redox charges to catalytic performances, we find that the catalytic activity correlates with the lateral surface exclusively, corresponding well with previous demonstration of the catalytic reactivity of the lateral surface and the inertness of the basal surface. This work clarifies dynamic, heterogeneous, and anisotropic redox behaviors and offers a fast, reliable, and simplified method to probe the facet-specific redox and catalytically active surfaces electrochemically.