Identifying the Active Site of N-Doped Graphene for Oxygen Reduction by Selective Chemical Modification

N-doped carbon materials are promising electro-catalysts for oxygen reduction reaction (ORR). However, the lack of knowledge in the nature of active sites limits the rational design of this type of catalysts. Although pyridinic N species are proposed to be active for ORR, little experimental evidence is provided to reveal the reactive sites. Herein, we developed a surface modification method to identify the ortho-carbon atom of the pyridinic ring as the reactive site for ORR on N-doped graphene. The pyridinic ring of N-doped graphene was selectively grafted by an acetyl group at pyridinic N and ortho-C atoms by electrophilic and radical substitution, respectively. The former remained most of ORR catalytic activity, while the latter lost its activity completely. DFT calculations confirm that O-2 can get adsorbed and reduced favorably on the ortho-C atom of the pyridinic ring. This study provides new insight into the nature of active sites and the ORR mechanism for N-doped carbon materials.