We investigated the ORR activity for the hexagonal nitrogen-doped graphene nanoclusters having zigzag or armchair edges, using first-principles calculations within the density functional theory. Electrocatalytic activities were evaluated on the basis of the computational hydrogen electrode model proposed by Nørskov et al.  In general, the ORR mainly proceeds in two pathways: For the two-electron pathway (2e-), oxygen molecule (O2) is reduced to hydrogen peroxide (H2O2), and for the direct four-electron pathway (4e-), the final product is water (H2O). H2O2 for the 2e- pathway might erode a carbon based electrocatalyst material itself, leading to low durability. Therefore, we also play up the selectivity for the 4e-pathway.
Fig.1(a) shows a model of the graphene nanocluster. The doping sites are denoted by blue circles with numbers. Numbers are allocated from the edge toward in-plane. A nitrogen atom is substituted in each doping site. Models are labeled as X-Y, where symbols X and Y denote doping sites of nitrogen atoms and reaction sites of ORR, respectively. Fig.1(b) shows the energy diagrams for the 4e- and the 2e- pathways for the model 5-6. The maximum potentials are 0.70 V and -0.42 V for the 4e- and the 2e- pathways, respectively. This means the 4e- pathway prefers to occur up to 0.70 V, and the 2e- pathway does not occur unless the backward voltage is applied. Therefore, the model 5-6 shows extremely high selectivity for the 4e- pathway. In almost all reaction sites, the maximum potentials for the 4e- pathway are higher than those for the 2e- pathway. It is noted that the high selectivity for the 4e- pathway is assured not just at the edge but inside the cluster. The effects of edge structures, cluster sizes, and doping configurations on the ORR activity will be discussed in the presentation
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