Density Functional Theory Study of Oxygen Reduction on Graphene and Platinum Surfaces of Pt-Graphene Hybrids

Based on recent studies, an epitaxial monolayer of Pt on graphene yielded an enhanced catalytic activity with unprecedented stability resulting in an improved catalyst lifetime. We showed that the origin of this stability is due to strong Pt-C covalency and unique Pt and graphene surface morphologies compared to precedent physically binding metal-graphene surfaces or impurity-doped graphene catalysts. In this study, the oxygen reduction reaction (ORR) mechanisms on both surfaces are investigated using density functional theory for the first time. This two-dimensional catalyst possesses two reactive surfaces for ORR: one is the Pt surface and the other is the graphene surface. The graphene surface is found to have an undulated morphology from the sp(2)-sp(3) alternating hybridization of carbon atoms in graphene, which are generated due to the covalent bond formation with Pt atoms. The sp(2) region of graphene provides active reaction sites for the ORR, and all active sites on the Pt surface are characterized from the Pt-C(sp(3)) bonds. It is revealed that O-2 adsorption takes place via a dissociative chemisorption mechanism on both catalytic surfaces. Compared to the Pt surface, ORR on the graphene surface would be more facile due to a weaker O-2 binding energy and a lower overpotential. This study shows that a Pt-graphene hybrid system without defects or impurities on graphene is capable of exhibiting an electrochemical catalytic activity.