Electronic synergism of pyridinic- and graphitic-nitrogen on N-doped carbons for the oxygen reduction reaction

Nitrogen-doped carbon materials (NCs) are extensively studied for the oxygen reduction reaction (ORR). However, the nature of active sites of pyridinic nitrogen (N-P) or graphitic nitrogen (N-G) on NCs is still under debate. Herein, we demonstrated that the ORR activity of NCs in alkaline media depended on the electronic synergism between N-P and N-G, rather than any single type of N. We measured the transferable electrons of NCs by absorption spectroscopy using 7'7'8'8-tetracyanoquinodimethane as an electron acceptor. The transferable electron amount of NCs is relevant to either N-P or N-G, leading to the [N-P]:[N-G] ratio as an electron transfer descriptor of NCs in a reverse volcano curve manner across nineteen NCs. The similar dependence of ORR activity on the [N-P]:[N-G] ratio of NCs was also discovered, demonstrating the synergistic effect of N-P and N-G. These results provide a new angle to understand the nature of ORR activity of NCs and optimize the ORR catalyst.