Beyond the Edge: Basal-Plane Defects as the Dominant Catalytic Sites in Sulfur-Doped Graphene

Sulfur-doped graphene (SG) has attracted considerable interest for energy conversion and storage applications. However, the relevant catalytic mechanisms remain obscure due to ongoing contentious debates regarding the location of dopant atoms. While theoretical studies often assume sulfur dopants preferentially reside at edge sites, experimental evidence consistently shows their homogeneous distribution throughout the carbon lattice. Here, we first demonstrated the thermal and dynamical instabilities of previously proposed models of S-bearing defects in the basal plane of SG, which were used to explain experimentally observed enhanced lithium adsorption and magnetism. We then presented new stable defect configurations in the graphene lattice that incorporate both sulfur dopants and inevitable oxygen-bearing functional groups, thereby explaining those experimental observations. These in-plane defect models provide an internally consistent explanation for the active sites and catalytic mechanisms of oxygen, nitrogen, and sulfur reduction reactions, and suggest that the catalytic performance of SG cannot be rationalized solely by edge-located sulfur dopants. In particular, several of the newly identified in-plane defects exhibit calculated activities comparable to, or in some cases exceeding, those of representative edge configurations. Our findings highlight a previously underappreciated role of basal-plane defects in sulfur-doped carbonaceous materials, encompassing both metal-free catalysts and graphene-based single-atom systems.