Density Functional Theory Study of the Relationship between N-Dopants and Vacancy Defects on Graphene Quantum Dots for Oxygen Reduction Electrocatalysis

Graphene quantum dots (GQDs), as novel zero-dimensional nanomaterials, have demonstrated exceptional electrocatalytic activities, primarily due to quantum confinement and edge effects. However, the relative contributions of heteroatom doping versus defect induction to catalytic activity as well as their interplay remain topics of ongoing research. In this study, four different vacancy defects were introduced into three distinct N-doped GQDs (graphitic-N, pyridinic-N, and pyrrolic-N) to investigate the synergistic effects of N-doping and defect induction on the oxygen reduction reaction (ORR) catalytic performance of the GQDs. The structural configurations of these defect-/heteroatom coengineered GQDs and their structure–activity relationships were analyzed using density functional theory methods at the B3LYP/6-311G(d,p) level. In the ORR’s four-electron pathway, the formation of the *OOH intermediate was identified as a decisive step. The study revealed that the synergistic effect of vacancy defects and nitrogen doping significantly reduced the adsorption energy of the *OOH intermediate in pristine GQDs. For example, the original GQDs exhibit an adsorption energy of 6.32 eV with an overpotential of 2.63 V for the *OOH intermediate. Notably, introducing graphitic-N with a divacancy defect reduced the adsorption energy to 4.76 eV and the overpotential to 1.07 V. Furthermore, the introduction of defects was found to induce curvature in the sp2-hybridized carbon atom sheets, thereby altering their electrocatalytic activities. These findings highlight the crucial role of N-dopants and vacancy defects in lowering the activation energy barrier for oxygen dissociation, thereby enhancing the ORR performance and advancing the application of carbon-based nanomaterials in renewable energy conversion and storage devices.