Mechanism of Electrocatalytically Active Precious Metal (Ni, Pd, Pt, and Ru) Complexes in the Graphene Basal Plane for ORR Applications in Novel Fuel Cells

Metal-nitrogen-co-doped graphene has great potential for use as a high-efficiency catalyst in energy applications. In this paper, density functional theory (DFT) with the projector-augmented wave and generalized gradient approximation (PAW-GGA) method was used to explore the catalytic activity of M@Gra (M = Ni, Pd, Pt, and Ru) on different types of graphene for oxygen reduction reaction (ORR) applications. Both the direct hydrogenation and dissociative mechanism of O2 are used to evaluate the ORR performance, and the binding energy of the intermediates, rate-determining step, overpotential, and activation energy of metal-nitrogen-co-doped graphene are considered. The catalytic properties of 4Ru@Gra and 1Pt@Gra make them the best candidates for ORRs, as 4Ru@Gra and 1Pt@Gra exhibit a stronger interaction (ΔG*OH) with the nanosheets and excellent ORR catalytic performance compared to other compounds. Precious metals have a significant influence on reducing O2 and decreasing the reaction energy, and the strong interaction of *O may lead to a high overpotential for the ORR process. This demonstrates that these compounds can moderately bind with the ORR intermediates by tuning the relative free energy, resulting in the ORR intermediates binding neither too strongly nor too weakly, and this may lead to slow or fast kinetics. The 1Ni@Gra support has a higher activation energy for O2 dissociation of 0.74 eV as well as a small activation energy of 0.13 eV, and the rate-determining step is controlled by the binding of *OH. The ORR reduction pathway occurs via direct hydrogenation with four-electron reduction, and it was determined that the energy barrier was 0.35 eV for the *OOH form, which is lower than the energy barrier (0.74 eV) of the 2O* species produced from the O2 dissociation in 1Ni@Gra. This indicates that the direct hydrogenation pathway is preferred as the most favorable of the ORR mechanisms.