Transforming active sites in nickel-nitrogen-carbon catalysts for efficient electrochemical CO2 reduction to CO

Nitrogen-coordinated single-atom catalysts (SACs) catalyzed electrochemical reduction of CO2 (CO2RR) to CO has emerged as a promising strategy in the management of the global carbon cycle. Herein, we carried out density functional theory (DFT) calculations to investigate the role of possible Ni-N-x and Ni-C-4 coordinations in CO2RR catalysis. We discover that the free energy change for CO2RR is lowered with a decrease in Ni-N-x co-ordination number, with Ni-C-4 displaying the lowest overpotential for CO2RR. Using these findings, we develop an effective strategy to transform Ni-N-4 to Ni-C-4 active sites by removing N moieties within Ni embedded in a hollow nitrogen-doped carbon shell (Ni@NCH). We demonstrate an improvement in CO selectivity with this transformation of active sites and the optimized Ni@NCH-1000 catalyst is capable of converting CO2 to CO with high Faradaic efficiency for CO (FECO) of 96% and a current density (j) of -35 mA cm(-2) at an applied potential of -1 V vs Reversible Hydrogen Electrode (RHE). When adopted in a high-throughput gas diffusion electrolyzer, the newly-developed superhydrophobic catalyst is capable of maintaining CO selectivity >95% over a wide range of applied cell voltages from 2.4 V to 3 V with high current densities (similar to 100 mA cm(-2) at 3 V). Our insights and findings with active site transformation in Ni-N-C SACs can serve as guidelines for designing highly active SACs for large-scale CO2RR systems.