Operando Spectroscopic Insights into the Key Role of Phase Engineering in Cobalt Diselenide for Overall Water Electrolysis

Identification of the true catalytically active species/sites holds the key to new catalyst design. In this study, combination of complementary operando surface/bulk sensitive spectroscopic techniques and density functional theory (DFT) calculations establish a clear picture of the structure-activity relations in marcasite- and pyrite-type CoSe2 for overall water splitting. Our results reveal that under acidic conditions marcasite CoSe2 undergoes slight surface corrosion, producing disordered [CoSe6] motifs with the active Se sites for catalyzing the hydrogen evolution reaction (HER). In contrast, during the alkaline HER, the marcasite CoSe2 initially interacts with the electrolyte to reconstruct an O-rich covered surface, which subsequently undergoes potential-driven restructuring to generate the metallic cobalt species as the true active species. Such dynamic changes of the active species/sites along with variations in pH values were not observed in pyrite CoSe2, either with or without heteroatom substitution, highlighting the significant importance of phase engineering in managing the HER kinetics. Further operando spectroelectrochemical monitoring demonstrates that the in situ generation of highly disordered Co4+ species is a common denominator of CoSe2 catalysts for the oxygen evolution reaction (OER). This study directly evidences the dynamic influence of local coordination geometries of the catalytic active centers on the underlying catalytic reaction kinetics.