Identification of single-atom active sites in carbon-based cobalt catalysts during electrocatalytic hydrogen evolution

Monitoring atomic and electronic structure changes on active sites under realistic working conditions is crucial for the rational design of efficient electrocatalysts. Identification of the active structure during the alkaline hydrogen evolution reaction (HER), which is critical to industrial water-alkali electrolysers, remains elusive and is a field of intense research. Here, by virtue of operando X-ray absorption spectroscopy on a uniform cobalt single-site catalyst, we report the atomic-level identification of the dynamic structure of catalytically active sites under alkaline HER. Our results reveal the formation of a high-valence HO-Co-1-N-2 moiety by the binding between isolated Co-1-N-4 sites with electrolyte hydroxide, and further unravel the preferred water adsorption reaction intermediate H2O-(HO-Co-1-N-2). Theoretical simulations rationalize this structural evolution and demonstrate that the highly oxidized Co sites are responsible for the catalytic performance. These findings suggest the electrochemical susceptibility of active sites, providing a coordination-engineered strategy for the advance of single-site catalysis.