Graphene Defects Trap Atomic Ni Species for Hydrogen and Oxygen Evolution Reactions

Downsizing the catalyst to atomic scale provides an effective way to maximize the atom efficiency and enhance activity for electrocatalysis. Here, we report a concept whereby graphene defects trap atomic Ni species (aNi) inside to form an integrity (aNi@defect). X-ray adsorption characterization and density-functional-theory calculation revealed that the diverse defects in graphene can induce different local electronic densities of state (DOSs) of aNi, which suggests that aNi@defect serves as an active site for unique electrocatalytic reactions. As examples, aNi@G585 is responsible for the oxygen evolution reaction (OER), and aNi@G5775 activates the hydrogen evolution reaction (HER). The derived catalyst exhibits exceptionally good activity for both HER and OER, e.g., an overpotential of 70 mV at 10 mA/cm² for HER (analogous to the commercial Pt/C) and 270 mV at 10 mA/cm² for OER (much superior to that of Ir oxide). Recently, for maximizing atom efficiency, atomically dispersed metal species (aMs) have sparked new interest in heterogeneous catalysis. However, our understanding of aMs is still in its infancy. Therefore, it is imperative to get in-depth insight on the unique interactions between atomic metal species and carbon. We report a concept whereby the graphene defects trap atomic Ni species (aNi) inside to form an integrity (aNi@defect) as the active site. This leads to aNi@Di-vacancy and aNi@5775 coordination, which can significantly tune the local electronic structures, leading to enhanced electrocatalytic performance of hydrogen and oxygen evolution. An integrated coordination structure composed of atomic Ni trapped in graphene defects is directly identified by probe-corrected TEM. The tuned electronic structure is considered to be the origin of enhanced oxygen evolution reaction and hydrogen evolution reaction.