Arc plasma synthesis of sulfur-doped graphene flakes and impact of the doping concentration on oxygen reduction reaction

Abstract

The scalable and efficient production of heteroatom-doped graphene remains a significant challenge. The fast, process-efficient arc plasma gas-phase synthesis method had clear advantages for meeting this challenge. In this study, we employed a one-step, millisecond-scale rapid synthesis method to produce sulfur-doped graphene flakes using arc plasma. The plasma region's temperature was modulated by adjusting power. We generated sulfur-doped graphene flakes with varying doping concentrations. Experimental results showed that the average gas temperature rose with increasing input power. Rising temperature increased the sulfur concentration of sulfur-doped graphene flakes. Raman and XRD analyses revealed that sulfur introduction distorted the graphene lattice and generated additional defects. XPS analysis confirmed sulfur had been in the form of thiophene-S structures. Defects and heteroatoms promoted the catalytic reaction. Oxygen reduction reaction (ORR) catalytic tests were conducted on sulfur-doped graphene. The results demonstrated that the ORR activity of the sulfur-doped graphene catalyst was comparable to that of the Pt/C catalyst, exhibiting a four-electron transfer process in alkaline media. Additionally, the catalyst exhibited excellent methanol tolerance and durability. Density functional theory (DFT) calculations showed that Sulfur-doped graphene outperformed graphene, presumably because sulfur modified the spin density of adjacent carbons and reduced reaction energy barrier. This synthesis method offered a feasible solution for the large-scale production of low-cost oxygen reduction catalysts of carbon-based materials.