Investigating the synergistic effect of sulfur and nitrogen co-doping in graphene for enhanced CO2 reduction to CO: A computational study

Recently, N and S co-doped graphene showed excellent activity and selectivity for electrochemical CO2 reduction (CO2RR) to CO, much better than that of singly N doped counterpart, but the underlying mechanism of S promotional effect is still elusive. Here, by using density functional theory (DFT) calculation, N or S singly doped graphene and co-doped graphene for CO2RR to CO and competitive hydrogen evolution reaction (HER) are compared to reveal the synergy effect. The results show that singly S doped graphene exhibits negligible activity for CO production because of high formation energy of COOH intermediate, whereas graphene doped with pyridinic N and pyrrolic N show moderate energy barrier for CO2RR, but N atom would be easily passivated due to strong binding with H atom. In N and S co-doped graphene, N and S pair prefer to replace CC bond, but they are not bonded. In this configuration, the S atom can serve as active site, having even lower energy barrier for CO2RR than that of pyridinic N, meanwhile the S atom is not passivated by hydrogen atom. The electronic origin of enhanced activity for S atom is also presented.