Effect of Water on the Manifestation of the Reaction Selectivity of Nitrogen-Doped Graphene Nanoclusters toward Oxygen Reduction Reaction

We investigated the selectivity of N-doped graphene nanoclusters (N-GNCs) toward the oxygen reduction reaction (ORR) using first-principles calculations within the density functional theory. The results show that the maximum electrode potentials (U_Max) for the four-electron (4e^–) pathway are higher than those for the two-electron (2e^–) pathway at almost all of the reaction sites. Thus, the N-GNCs exhibit high selectivity for the 4e^– pathway, that is, the 4e^– reduction proceeds preferentially over the 2e^– reduction. Such high selectivity results in high durability of the catalyst because H₂O₂, which corrodes the electrocatalyst, is not generated. For the doping sites near the edge of the cluster, the value of U_Max greatly depends on the reaction sites. However, for the doping sites around the center of the cluster, the reaction-site dependence is hardly observed. The GNC with a nitrogen atom around the center of the cluster exhibits higher ORR catalytic capability compared with the GNC with a nitrogen atom in the vicinity of the edge. The results also reveal that the water molecule generated by the ORR enhances the selectivity toward the 4e^– pathway because the reaction intermediates are significantly stabilized by water.