Facile synthesis of defect-rich nitrogen and sulfur Co-doped graphene quantum dots as metal-free electrocatalyst for the oxygen reduction reaction

Carbon-based metal-free nanomaterials have attracted immense and persistent attention as electrocatalysts for oxygen reduction reaction (ORR) in fuel cells, due to their high electrocatalytic activity, long-term stability and low cost. Here, a facile hydrothermal treatment in the presence of ammonium hydroxide (NH4OH) and sodium sulfide (Na2S), as precursors, has been developed to synthesize heteroatom-doped graphene quantum dots (GQDs). For comparison, undoped graphene quantum dots (GQDs), nitrogen doped graphene quantum dots (N-GQDs), sulfur doped graphene quantum dots (S-GQDs) and nitrogen and sulfur co-doped graphene quantum dots (N,S-GQDs) have been synthesized with the same method. The as-obtained N,S-GQDs possess a high N-doping content up to 9.36% and low S-doping content up to 0.78%, with a uniform size distribution similar to that of GQDs, N-GQDs and S-GQDs. Interestingly, the multi-types of NC or CSC bonding in metal free N,S-GQDs can significantly enhance the electrocatalytic activity for the oxygen reduction reaction (ORR), with the electron transfer number as good as 3.82, which verify the importance of our facile synthesis method to fabricate metal-free electrocatalysts for ORR and other electrochemical applications.