Mechanisms of the Oxygen Reduction Reaction on Defective Graphene-Supported Pt Nanoparticles from First-Principles

The mechanisms of the oxygen reduction reaction (ORR) on defective graphene-supported Pt-13 nanoparticles have been investigated to understand the effect of defective graphene support on the ORR and predict details of ORR pathways. We employed density functional theory (DFT) predictions using the projector-augmented wave (PAW) method within the generalized gradient approximation (GGA). Free energy diagrams for the ORR over supported and unsupported Pt-13 nanoparticles were constructed to provide the stability of possible intermediates in the electrochemical reaction pathways. We demonstrate that the defective graphene support may provide a balance in the binding of ORR intermediates on Pt-13 nanoparticles by tuning the relatively high reactivity of free Pt-13 nanoparticles that bind the ORR intermediates too strongly subsequently leading to slow kinetics. The defective graphene support lowers not only the activation energy for O-2 dissociation from 0.37 to 0.16 eV, but also the energy barrier of the rate-limiting step by reducing the stability of HO* species. We predict the ORR mechanisms via direct four-electron and series two-electron pathways. It has been determined that an activation free energy (0.16 eV) for O-2 dissociation from adsorbed O-2* at a bridge site on the supported Pt-13 nanoparticle into O* + O* species (i.e., the direct pathway) is lower than the free energy barrier (0.29 eV) for the formation of HOO* species from adsorbed O-2* at the corresponding atop site, indicating that the direct pathway may be preferred as the initial step of the ORR mechanism. Also, it has been observed that charge is transferred from the Pt-13 nanoparticle to both defective graphene and the ORR intermediate species.