Understanding the Selectivity of the Oxygen Reduction Reaction at the Atomistic Level on Nitrogen-Doped Graphitic Carbon Materials

The oxygen reduction reaction (ORR) is of paramount interest, in the context of both alternative energy applications in fuel cells and for on-site hydrogen peroxide (H2O2) production in environmental remediation applications. Using theoretical and experimental methods, the mechanism involved in the ORR is studied on nitrogen-doped graphitic carbon materials. The two principal reaction pathways involved in the ORR are the four-electron pathway producing water (H2O), or the two-electron pathway leading to the formation of H2O2. An atomistic step by step ORR mechanism is proposed to understand the selectivity of the reaction toward the two principal pathways. The results show that graphitic N sites favor the two-electron pathway, in a similar way to three pyridinic N sites. Meanwhile, the one or two pyridinic N sites lead to the four-electron pathway. The calculations show the importance of dangling bonds and/or pentagonal C rings in selectivity toward the four-electron pathway. The results are consistent with recent reports on the importance of topological defects in graphitic carbon materials. The understanding of the ORR mechanism is very important for the design and development of novel ORR electrocatalysts to favor the required pathway, according to the application.