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

Abstract

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.

Mechanochemical Synthesis of N-Doped Graphite Nanoparticles: A Promising Material for Flow Vanadium Battery Applications

“…The mechanochemical or mechanochemically induced N-doping of carbon is a productive, simple, cheap, and, most importantly, environmentally acceptable method. − This method makes it possible to obtain carbon materials with a high degree of doping up to ∼30 atom % of nitrogen. Studies conducted by various research groups have shown that the materials obtained in this way are capable of exhibiting high functional characteristics in different fields, such as anodes for sodium- and lithium-ion batteries, , supercapacitor electrodes, electrocatalysts for oxygen reduction, − ,− among others.…”

“…Currently, only a limited number of publications are devoted to the mechanochemical doping of carbon materials with nitrogen atoms. Sources of nitrogen atoms during mechanochemical processing can be substances, such as melamine, ,,, urea, ammonium fluoride, ammonia, , and nitrogen gas. , In some cases, doping occurs directly in the mill, ,,,,, while in others, it is carried out during the subsequent processing of mechanochemically activated carbon materials. ,− From a practical perspective, doping carbon materials with nitrogen atoms directly in a mill in a nitrogen atmosphere is of undoubted interest due to the nontoxicity of the gas itself, its wide distribution, and the simplicity of the method. However, nitrogen gas is a very inert substance, and to achieve doping, it is necessary to carry out a prolonged mechanochemical treatment, which in some cases reaches several days. , …”