Enhanced electroreduction of CO₂ by Ni-N-C catalysts from the interplay between valency and local coordination symmetry

Many studies have focused on atomically dispersed metal-nitrogen-carbon (Me-N-C) catalysts owing to their unique chemistry and high catalytic activities. Me-N-C catalysts have active centers resembling metalloporphyrins; thus, being heterogeneous analogs of homogeneous catalysts, their catalytic characteristics can be described by organometallic principles. In this regard, the high electrochemical activity of Ni-N-C catalysts for carbon dioxide reduction reactions (CO₂RRs) is particularly difficult to understand because Ni²⁺ is a d⁸ species with a chemically inert axial site for intermediate binding in a square-planar ligand field. To resolve such a conundrum, we investigated the effects of different coordination geometries and Ni spin states on CO₂RR activities—both of which influence the chemical activity of the Ni center. We used the grand-canonical density functional theory (GC-DFT) and the occupation matrix control method to properly include a finite potential effect, and to control the oxidation state of the Ni center, respectively. We elucidated that the generation of Ni⁺ directly impacts the CO₂RR activity by providing strong intermediate binding energies to the Ni center, and a defective coordination environment is essential for stabilizing the Ni⁺ oxidation state. Our present study identifying governing factors for the high catalytic activity of Ni-N-C catalysts provides a design principle to develop high-performing catalysts for CO₂RR.