Definable Active Center of the Nitrogen–Sulfur–Pentagon Carbon Structure for Oxygen Reduction Catalysis

Heteroatom-modified defective carbon materials have been intensively considered as promising oxygen reduction reaction (ORR) catalysts, especially with nitrogen (N)- and sulfur (S)- coordinated pentagonal carbon (C5) structures as highly active centers in acid medium. However, almost all the current methodologies cannot exclusively construct the proposed configuration of active centers; consequently, it is impossible to exactly identify the active center for a catalyst. Herein, we designed and synthesized a conjugated organic molecule with a well-defined N/S co-coordinated adjacent C5 (denoted as NS-C5) structure, demonstrating enhanced ORR performance than the individual pyrrole-based (N-C5) and thiophene-based (S-C5) counterparts. Theoretical calculations and in situ characterizations revealed that the N and S atoms co-optimize the local electronic microenvironment of the C-site in the C5 structure, inducing the low-energy barrier of OOH* intermediate formation, which is the determining step during the ORR process. Furthermore, by incorporating NS-C5 onto reduced graphene oxide (rGO), we investigated the effect of active site density on electrocatalytic activity. The results demonstrated that a higher density of catalytic sites is necessary for an enhanced reaction rate; however, the excess of active sites could actually result in a very slow increase in overall activity. This work provides valuable insights into the identification of a heteroatom-defective carbon structure for ORR, promoting the development of metal-free catalysts.