Unveiling the Active Structure of Single Nickel Atom Catalysis: Critical Roles of Charge Capacity and Hydrogen Bonding.

Single nickel atom embedded in graphene is one of the most representative single atom catalysts, and has a high activity and selectivity for electrochemical CO2 reduction (CO2R) to CO. However, the catalytic origin, especially the coordination structure of Ni, remains highly puzzling, as previous density-functional-theory (DFT) calculations show all the possible structures should be inactive and/or non-selective. Here using ab-initio molecular dynamics (AIMD) and "slow-growth" sampling approach to evaluate the reaction kinetic barriers, we show that the charge capacity (of the site) and hydrogen bonding (with the intermediates), which were neglected/oversimplified in previous DFT calculations, play crucial roles, and including their effects can resolve the catalytic origin. Particularly, a high charge capacity allows the catalytic site to carry more charges than required for electrochemical step, lowering the electrochemical barrier; the hydrogen bonding promotes the reaction that produces polar intermediates, by stabilizing the intermediates and facilitating the H transfer from water, explaining the high selectivity for CO2R over hydrogen evolution reaction. Consequently, we find a hybrid coordination environment (with one nitrogen and three carbon atoms) for Ni atom is most active and selective for CO2R. Our work not only explains a long-standing puzzle for an important catalyst, but also highlights the crucial roles of charge capacity and hydrogen bonding, which can help elucidate the mechanisms of other heterogeneous electrocatalysts in aqueous solution and enable more effective catalyst design.