Single transition metal atoms on nitrogen-doped carbon for CO2 electrocatalytic reduction: CO production or further CO reduction?

Electrochemical CO2 reduction reaction (CO2RR) is an effective strategy for CO2 conversion and clean fuel generation. Diverse nitrogen-doped carbon-supported transition metal (M-N-C) have been proved as favorable catalysis materials for CO2RR, especially for CO production. Herein, density functional theory is performed to investigate the CO2RR mechanisms over M-N-x-C with twelve types of transition metal centers. Fe-, Co-, Ni-, Cu-, Rh-, Pd-, Ir-, and Pt-N-x embedded on graphene with double vacancies are relatively stable structures contributing to electrocatalysis operation. The adsorption energies of COOH and CO, which are the reaction intermediates during CO2RR, are linearly related and can be used as a descriptor for CO production. Moderate adsorption strengths of CO and COOH are favorable for CO generation with low limiting potential. Co-N-4, Ni-N-1, Pd-N-1, Pt-N-1, and Rh-N-4 are the most active sites for CO2RR by thermodynamics analysis. However, the discussion on competition between CO2RR and hydrogen evolution reaction (HER) indicates that these sites show low selectivity. Considering both CO2RR selectivity and activity, Ni-N-4 site is proved to be the most effective site for CO production among all the M-N-x sites. It is also noted that Fe-N-4 is more favorable to further reduction towards *CO with the products of CH3OH and CH4.