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 (NP) or graphitic nitrogen (NG) on NCs is still under debate. Herein, we demonstrated that the ORR activity of NCs in alkaline media depended on the electronic synergism between NP and NG, 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 NP or NG, leading to the NP : NG 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 NP : NG ratio of NCs was also discovered, demonstrating the synergistic effect of NP and NG. These results provide a new angle to understand the nature of ORR activity of NCs and optimize the ORR catalyst.