Intermolecular Energy Gap-Induced Formation of High-Valent Cobalt Species in CoOOH Surface Layer on Cobalt Sulfides for Efficient Water Oxidation.

Transition metal-based electrocatalysts will undergo surface reconstruction to form active oxyhydroxide-based hybrids, which are regarded as the "true-catalysts" for the oxygen evolution reaction (OER). Much effort has been devoted to understanding the surface reconstruction, but little on identifying the origin of the enhanced performance derived from the substrate effect. Herein, we report the electrochemical synthesis of amorphous CoOOH layers on the surface of various cobalt sulfides (CoS α ), and identify that the reduced intermolecular energy gap (Δ inter ) between the valence band maximum (VBM) of CoOOH and the conduction band minimum (CBM) of CoS α can accelerate the formation of OER-active high-valent Co 4+ species. The combination of electrochemical and in situ spectroscopic approaches, including cyclic voltammetry (CV), operando electron paramagnetic resonance (EPR) and Raman, reveals that Co species in the CoOOH/Co 9 S 8 are more readily oxidized to CoO 2 /Co 9 S 8 than in CoOOH and other CoOOH/CoS α . This work provides a new design principle for transition metal-based OER electrocatalysts.