On the active sites for the oxygen reduction reaction catalyzed by graphene-based materials

At the heart of the mechanism and efficiency of oxygen reduction is the identification of active sites in graphene-based electrocatalysts ranging from carbon blacks to heat-treated phthalocyanines. Distinction between the transfer of two or four electrons points to O₂ dissociation as the essential mechanistic clue; here we examine this issue by exploiting the analogy with carbon oxidation, where production of CO vs. CO₂ has long been a crucial point. We compare our computational results with experimental evidence on the behavior of graphene as well as its N-, B- and transition-metal-doped counterparts. Electron transfer is revealed to occur readily through a carbene-type site upon oxygen surface rearrangement. Whether adsorbed O₂ dissociates depends on proton transfer occurring before or after the stabilization of a peroxy intermediate; this in turn depends on electron density distribution at and around the active site. A good correlation exists between spin density at the active site and O₂ adsorption energy.