Enhancing CO2 reduction efficiency with axial oxygen coordinated Ni-N4 active sites on hierarchical pore N-doped carbon

The electrochemical reduction of carbon dioxide (CO2RR) is a promising strategy for achieving carbon neutralization. The Ni-N4 site is well known as the active site in metal single atoms on N-doped carbon catalysts, while its symmetric charge distribution nature is not favorable for electron transfer and then hindering the efficient CO2RR. Herein, we constructed a Ni SA/CNs single-atom catalyst. Notably, it features unique Ni-N4-O active sites, featuring one axial O atom and four planar N atoms, constituting a broken symmetrical electronic structure of Ni-N4 sites. Furthermore, hierarchical pore structures were obtained with the assistance of NaNO3 pore-forming agent during thermal treatment process, which promote electronic and mass transfer. And the resulting high specific surface area can host more Ni-N4-O active sites. These specialized active sites promote the key intermediate (*CO) adsorption/desorption and suppresses hydrogen evolution. Consequently, the Ni SA/CNs catalyst exhibits a high turnover frequency (TOF) value, reaching 34081 h−1 at -0.98 V vs. RHE. Additionally, it achieves an excellent CO Faradaic efficiency, exceeding 90%, over a wide potential range from -0.4 V to -1.0 V vs. RHE. This work not only offers a new method for the rational synthesize single-atom catalysts with unique Ni-N4-O active sites, but also provides in-depth insight into the origin of catalytic activity of porous carbon-base catalysts.