Dynamics and control of active sites in cobalt diselenide catalysts for overall water electrolysis

Identification of the true catalytically active species/sites is crucial for catalyst design. Herein, we combine complementary operando surface/bulk sensitive spectroscopic techniques and density functional theory calculations to establish clear structure-activity relations for marcasite- and pyrite-type CoSe2 toward overall water splitting. Our results reveal that under acidic conditions marcasite CoSe2 undergoes slight surface corrosion, producing disordered Se-Co-Se moieties for catalyzing the hydrogen evolution reaction (HER). In contrast, during the alkaline HER, marcasite CoSe2 undergoes potential-driven restructuring from the initial reconstructed O-rich covered surface into the generation of metallic Se-Co-Co-Se moieties as the true active species. Such dynamic changes of the active species/sites along with variations in pH values are not observed for either pristine or heteroatom-substituted pyrite CoSe2. Further operando spectroelectrochemical monitoring demonstrates that the in situ formation of highly disordered Co(IV) species is a common denominator of chalcogenide catalysts for the oxygen evolution reaction. This study illustrates the dynamic influence of local coordination geometries of the catalytically active centers on the underlying catalytic reaction kinetics. Identifying the real catalytic sites is key to better water-splitting catalysts. Here, the authors report that marcasite CoSe2 reshapes into distinct Se-Co-Se or Se-Co-Co-Se motifs as pH shifts, while disordered Co(IV) drives oxygen evolution, revealing geometry-governed kinetics.