Ab initio study of graphitic-N and pyridinic-N doped graphene for catalytic oxygen reduction reactions

The nitrogen doped graphene has been reported to exhibit good catalytic activity for the oxygen reduction reaction (ORR) and to be a promising material to replace the use of platinum as the ORR catalyst in fuel cell application. Yet, the roles of the different doping types, e.g. the pyridinic-N and graphitic-N dopants, in mediating the ORR are still controversial. Such understanding is of great importance for the development of highly active ORR catalysts. In this work, we perform density functional theory (DFT) study of the ORR processes on the pyridinic-N and graphitic-N doped graphene. In the study of the doped graphene quantum dots, we found that the binding strength of O2 on the graphene correlates closely with electron donation from the graphene to the O2. The presence of the graphitic-N dopants is found to facilitate the O2 binding, and thus the ORR reaction, as each graphitic-N dopant introduces one extra electron to the graphene. We then extend our study to the graphitic-N doped extended graphene to examine the effects of the dopant percentage on the ORR process. As the graphitic-N dopant percentage increases, the difference between the energy of the lowest spin state and the energy of the second lowest spin state decreases, which favors the transfer of electrons to the O2 and thus the binding. On the other hand, according to the energy profiles of the ORR reaction, the ORR processes could stop at the unfavorable reduction of *O to *OH on the extended graphene. However, after the binding of a second O2, we identify two possible ORR pathways. One of them is the continued reduction of the *O in the presence of the newly adsorbed *O2. The other one involves the reduction of the newly adsorbed *O2 in the presence of the *O. Our findings contribute to the understanding of ORR processes on N-doped graphene and would help to the development of efficient ORR catalysts.