Electrocatalytic CO₂ reduction to CO enabled by Ni-N_x high activity sites formed by three-dimensional porous foams

Single-atom catalysts (SACs) are widely used in carbon dioxide reduction reaction (CO₂RR) due to their distinctive electronic configuration and coordination environment. However, designing catalysts with porous structures that provide more active sites is a challenge in practice. Herein, we utilized the porous network structure of melamine foam (MF) and high temperature calcination to increase the number of nickel atoms to promote the formation of Ni-N_x active sites, resulting in nitrogen-coordinated nickel SACs with high catalytic activity. The X-ray photoelectron spectroscopy test demonstrates that the optimal calcination temperature of 900 °C can achieve a Ni-N_x site content of 9.47 at.%, which is significantly higher than that of other temperature conditions. This confirms that the calcination temperature has a decisive regulatory effect on the nitrogen-doped configuration and the formation of active sites. Furthermore, MF facilitated efficient electron transfer, resulting in improved catalytic performance. The Faradaic efficiency of CO (FE_CO) could reach 91% at-0.66 V vs. reversible hydrogen electrode. In situ Infrared Spectroscopy for real-time monitoring has demonstrated a linear relationship between Ni-N_x and the reaction intermediate^* CO content, indicating that an elevated Ni-N_x content may facilitate the generation of more active sites and expedite the kinetic process of CO₂RR. The density functional theory calculations reveal that the Ni-N₂ coordination in the Ni-N_x surface is responsible for the superior catalytic activity of CO₂ RR due to the moderate adsorption strength of^* COOH with less negative U_L (-0.87 V). This experiment provides an innovative way to increase the active site content of SACs in CO₂RR.