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Abstract

Cobalt-based transition-metal oxides are promising candidates for the oxygen evolution reaction (OER). However, a complex interplay between the catalyst crystal structures and material morphologies as well as the surface reactions hampers a comprehensive understanding of the electrocatalytic OER at those materials. Here, we investigate the amount and reactivity of specific surface sites of a mesostructured cobalt oxide spinel powder by surface interrogation mode of scanning electrochemical microscopy (SI-SECM). The powder material was supplied in cavity microelectrodes and efficiently titrated with an Fe(II)-triethanolamine redox mediator generated at a gold microelectrode in an alkaline electrolyte. Thus, quantification of different surface sites was achieved, and their reactivity showed dependence on the cobalt oxidation state. Titration experiments after adjustable time delays with respect to the generation of the different surface sites indicated that these surface sites are active for the OER. Kinetic analysis revealed two pseudo-first-order decay constants that were related to fast and slow surface sites for the OER. Rate constants were determined for potentials where predominantly a mixed-valence Co^III/IV state might be present as the most active species. These results expand the great potential of the surface interrogation mode on studying the reaction kinetics of distinct surface sites for practically relevant powdered, nonprecious metal catalysts to address a highly relevant challenge in electrocatalysis.