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

•Increase activity for ORR for N-GQDs of the N-GQDTac, N-GQDRac, N-GQDHac, N-GQDHzz.•N-GQDTzz and N-GQDRzz incative for ORR.•CN bond dissociation takes place in N-GQDTzz and N-GQDTac at O* adsorption.•Formation of new active sites after CN bond dissociation. 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. The drawing graphical abstract represents the volcano activity plot of oxygen reduction reaction (ORR), using as descriptor the free energy of the first reaction step (the O2 adsorption and protonation on the carbon active sites of the N-doped graphene quantum dots (N-GQDs), of various shapes: triangular - T, rhombohedral - R and hexagonal H, and terminations: zigzag (zz) and armchair (ac) – ∆GHOO*-∆GO2) vs. the theoretical onset overpotential (ηonset,th). The most active structures are placed close to the top of the volcano, namely triangular, hexagonal and rhombohedral ones, having armchair terminations, including the hexagonal one, with zigzag terminations. The most active sites are drawn with dark red atoms, closest neighbors to N-doping sites (blue).The reaction mechanism is an inset the red circle, indicating the HOO*/O*/HO* intermediates formation of on the active site (the red filled circle). [Display omitted]