A theoretical study on the enhanced oxygen evolution performance of NiN₄-graphene by Ni nanoclusters

Isolated metal-coordinated nitrogen embedded carbon (M-N-C) materials are potential alternatives to noble catalysts for oxygen evolution reaction (OER), and the activity of metal centers can be further modulated by adjusting the coordination environment. Recently, experimental studies have shown that the aggregation of metal atoms into small clusters or particles is inevitable during the high temperature pyrolysis, while the influences of metal clusters on the OER activity of single metal atoms in M-N-C are unclear. Herein, taking Ni-based single atom as examples, the interaction characters of NiN4 doped graphene (NiN4-graphene) with different Ni clusters were studied. The modulation effects of Ni clusters to the NiN4-graphene were systematically investigated from the geometric configurations, electronic structures, and the OER activity of the Ni single atom. It was found that the OER performance of NiN4-graphene can be remarkably improved through the addition of Ni clusters, and the lowest overpotential of 0.43 V is achieved on NiN4-graphene with the modification of Ni-13 cluster, which is smaller than that of 0.69 V on NiN4-graphene. Electronic properties calculations showed that the charge transfer from Ni clusters to NiN4-graphene will alter the density of states of Ni single atom near the Fermi level, which promotes the charge transfer from NiN4-graphene to oxygen containing products and optimizes the adsorption strength of oxygen intermediate to close to the ideal adsorption free energy of 2.46 eV by enhancing the hybridization interaction between the O-p orbitals and the Ni-d(xz), Ni-d(yz) orbitals, and finally leading to an enhanced OER activity. The current findings highlight the important role of metal clusters on improving the catalytic performance of M-N-C materials, which benefits for the rational design of M-N-C catalysts with high catalytic activity.