Relation between Water Oxidation Activity and Coordination Environment of C,N-Coordinated Mononuclear Co Catalyst

Understanding the structure–activity relationship of an active site is of great significance toward the rational design of highly active catalysts. Herein, we present a combined experimental and theoretical study on water oxidation catalysis of mononuclear Co catalysts with CoN4Cl, CoCN3Cl, and CoC4Cl motifs incorporated into a graphene matrix. We found that the catalyst with the CoCN3Cl structure exhibits an overpotential of 359 mV at 10 mA/cm2 for the oxygen evolution reaction (OER), much lower than those of catalysts with CoC4Cl (396 mV) and CoN4Cl structure (>500 mV). By introducing the binding strength between the Co site and reaction intermediates (OH*, O* and OOH*) as the reaction descriptor, we revealed that the binding strength for Co═O* in these structures is getting stronger when N is replaced by the C atom, which plays a crucial role in the rate-determining step (RDS) and water oxidation performance. The Co site distinctively coordinated with the CN3Cl structure gives rise to the most suitable binding strength of Co═O* and consequently the highest OER performance (RDS: Co–OH* → Co═O*), much better than that coordinated by C4Cl with a strong binding strength (RDS: Co═O* → Co-OOH*) and N4Cl with a weak binding strength (RDS: Co–OH* → Co═O*). This work further demonstrates the importance of the coordination environment of the metal nucleus in catalysts for RDS manipulation and activity optimization in OER through modulating the binding strength between active site and reaction intermediates.