Abstract:

N-doped graphene has aroused much interest owing to its high activity and stability in oxygen reduction reaction (ORR) catalysis. However, the contribution of different types of N-doped graphene to ORR activity remains in dispute. Based on this issue, this paper conducts a comparative study of the ORR on graphitic N-doped graphene (GNG) and pyridinic N-doped grapheme (PNG). Band structure calculations show that the conductivity of GNG decreases as the nitrogen content increases; while that of PNG first increases to the highest at nitrogen content of 4.2% (atomic fraction), and then decreases. The conductivity of PNG is always higher than GNG when the doped nitrogen content is greater than 1.4%. Additionally, the free energy diagram of ORR shows that protonation of O2 is the potential-determining step among the whole ORR process, and the free energy change of this step on GNG is lower than on PNG, suggesting that GNG has higher ORR activity than PNG if their electron transport ability are the same. When the N content is lower than 2.8%, the conductivity difference between GNG and PNG is almost negligible, thus GNG with a higher capacity of O2 protonation exhibits better ORR activity than PNG. When the N content is greater than 2.8%, in this case, conductivity rather than free energy change will dominate, therefore the ORR on PNG will occur faster than on GNG because of its higher conductivity.

Key words: Oxygen reduction reaction, Nitrogen doped graphene, Pyridinic nitrogen, Graphitic nitrogen, Density functional theory