Role of transport polarization in electrocatalysis: A case study of the Ni-cluster/Graphene interface

Abstract

As cathodes, iron-series (Fe, Co, Ni) clusters supported by carbon materials exhibit outstanding electrocatalytic reduction activities in many electrocatalytic applications. To date, this general characteristic of iron-series clusters that should be related to the inherent attributes of these electrodes has not been fully understood from the perspectives of thermodynamics and electronic structure alone. Electron transport is a necessary process in electrocatalysis, and therefore, the study of the change of the electronic state in electron transport is beneficial for understanding this general characteristic of iron-series cluster catalysts. In this work, the electron transport properties, including the conductivity and transport spin-polarization at the Ni-cluster/graphene interface are carefully investigated as an example of carbon-supported iron-series electrodes. Using first-principles calculations within the framework of the nonequilibrium Green’s function density functional theory (NEGF-DFT), we reveal that the electronic transport states of the coupled Ni-cluster/graphene are strongly changed compared to those of their isolated Ni-cluster and graphene component. It is found that graphene dominates the overall conductivity of the interface, while the morphology of Ni-clusters controls the spin polarization efficiency. High spin polarization can lead to the self-excitation effect of the electrons that raises the energy of the electronic system, improves the thermodynamics of the reduction reaction and promotes catalytic activity. Our work hints that iron-series elements (Fe, Co, Ni) based electrodes may generally show transport polarization that is likely to give rise to a high electrocatalytic reduction activity and such transport polarizability can be used as a new factor in the further exploration and design for electrocatalytic materials.