Boosting electrocatalytic activity for CO2 reduction on nitrogen-doped carbon catalysts by co-doping with phosphorus

Electrochemical reduction of CO2 (CERR) to value-added chemicals is an attractive strategy for greenhouse gas mitigation, and carbon recycles utilization. Conventional metal catalysts suffered from low durability and sluggish kinetics impede the practical application. On the other hand, doped carbon materials recently demonstrate superior catalytic performance in CERR, which shows the potential to diminish the problems of metal catalysts to some extent. Herein, we present the design and fabrication of nitrogen (N), phosphorus (P) co-doped metal-free carbon materials as an efficient and stable electrocatalyst for reduction of CO2 to CO, which exhibits an excellent performance with a high faradaic efficiency of 92% (−0.55 V vs. RHE) and up to 24 h stability. A series of characterizations including TEM and XPS verified that nitrogen and phosphorous are successfully incorporated into the carbon matrix. Moreover, the comparisons between co-doping and single doping catalysts reveal that co-doping can significantly increase CERR performance. The improved catalytic activity is attributed to the synergetic effects between nitrogen and phosphorous dopants, which effectively modulate properties of the active site. The density functional theory (DFT) calculations were also performed to understand the synergy effects of dopants. It is revealed that the phosphorous doping can significantly lower the Gibbs free energy of COOH* formation. Moreover, the introduction of the second dopants phosphorous can reduce the reaction barrier along the reaction path and cause polarization of density of states at the Fermi level. These changes can greatly enhance the activity of the catalysts. From a combined experimental and computational exploration, current work provides valuable insights into the reaction mechanism of CERR on N, P co-doped carbon catalysts, and the influence from synergy effects between dopants, which paves the way for the rational design of novel metal-free catalysts for CO2 electro-reduction.