OH-Functionalized N-Doped Graphene Quantum Dots as an Efficient Metal-Free Catalysts for Oxygen Reduction Reaction in PEMFCs

Utilizing the density functional theory (DFT) method, we investigated the catalytic activity of N-doped graphene quantum dots (NGQDs) with nitrogen (N) atoms strategically doped at various active sites on the surface. We focused on exploring their efficiency in the 2e⁻ and 4e⁻ reduction pathways for oxygen reduction reaction (ORR). By introducing N-doping at the central benzene ring of carbon-based materials, we observed the formation of localized π-orbitals, significantly enhancing their electrocatalytic activity. In comparison to other reported catalysts, our N-doped GQD metal-free electrocatalyst displayed remarkable adsorption capability. Furthermore, we introduced the hydroxyl group (OH) into the functionalized N-doped GQDs, which further improved electrocatalytic performance. This enhancement was attributed to the decreased HOMO–LUMO energy gap and increased chemical reactivity. The calculated free energy (ΔG) values for each elementary reaction step in the 4e⁻ reduction pathway were highly favorable and indicated the feasibility of the process. Our findings indicate that N-doped GQDs exhibit exceptional activity for the ORR, positioning them as promising carbon-based metal-free electrocatalysts. Consequently, they hold significant potential as an alternative to noble metal-based catalysts in proton exchange membrane fuel cells (PEMFCs) and metal-air batteries.