Identification of the active sites in sulfur-doped graphene for oxygen reduction reaction: The keyrole of dissociated O2 adsorption

Sulfur (S) doping in graphene is studied using first-principles calculations. The S dopants can assume various configurations such as graphitic-S, pyridinic-S, and pyrrolic-S, and prefer to locate at the graphene edges. Oxygen molecules (O-2) can be adsorbed on the S dopants at the edges, but with much larger adsorption energies than required for catalyzing the oxygen reduction reaction. We found an energetically favored three-pyridinic-S cluster in bulk graphene, on which the O-2 adsorption energies are much reduced and comparable to that on the Pt (111) surface. This is due to a confinement effect, which could be considered a key rule in doping graphene for potential applications.