Mechanochemical Synthesis of Nitrogen-Doped and Sulfur-Doped Multilayer Graphene for Use in Bifunctional Oxygen Electrodes

Nitrogen-doped and sulfur-doped mechanochemically synthesized multilayer graphene (N-doped and S-doped MSMG) were prepared by planetary ball-milling, and they were used in bifunctional gas diffusion electrodes (GDEs) for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Graphite, melamine, and elemental sulfur were used as raw materials. The surface area-normalized linear sweep voltammograms revealed that the N-doped and S-doped MSMG have higher intrinsic ORR/OER activity than the undoped MSMG. When the MSMG samples were used in GDEs, the N-doped and S-doped MSMG showed higher OER activity but lower ORR activity than the undoped MSMG. We analyzed the relationship between the specific surface area, intrinsic ORR/OER activity, and ORR/OER activity of GDEs and found that both the intrinsic ORR activity and surface area are important in the fabrication of GDEs with high ORR activity and that the intrinsic OER activity rather than the surface area is important in the fabrication of GDEs with high OER activity. The GDE fabricated from the S-doped MSMG showed the highest ORR/OER bifunctional activity among the MSMG-based GDEs, and its ORR/OER bifunctional activity was higher than the GDEs fabricated from other materials, such as reduced graphene oxide and electroconductive oxides.