Theoretical insights into the activity of the N doped graphene quantum dots for oxygen reduction reaction as a function of the shape and edge terminations

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We have investigated the possible activity of the N-doped graphene quantum dots (N-GQD) toward the oxygen reduction reaction (ORR) using density functional theory (DFT) method. The effect of the shape (triangular (N-GQDT), rhombohedral (N-GQDR) and hexagonal (N-GQDH)), connected both with the zigzag (zz) and armchair (ac) edge terminations of the GQD and with the location of the N atom across the sheets are investigated. The activity of part of the studied N-GQDTac, GQDRac, N-GQDHac and N-GQDHzz structures improves significantly compared to the similar pristine structures. On the other hand, no matter on the location of the N in the N-GQDTzz and in the N-GQDRzz, their activity gets worsen, when compared with the activity of the pristine structures or with the activity of the other four investigated structures. N-GQDTac and N-GQDRac are the structures with the most possible locations of N, that generates active sites. CN bond breaking takes place in the triangular shapes (N-GQDTzz and N-GQDTac) when oxygen adsorption takes place. These bonds are less likely to regenerate and new reaction sites are created. The new reaction site created on the N-GQDTzz represents a possible active site compared to the site generated on the N-GQDTac, that is highly inactive.