Dual Single-Atomic Ni-N4 and Fe-N4 Sites Constructing Janus Hollow Graphene for Selective Oxygen Electrocatalysis.

Nitrogen-coordinated metal single atoms in carbon have aroused extensive interest recently and have been growing as an active research frontier in a wide range of key renewable energy reactions and devices. Herein, a step-by-step self-assembly strategy is developed to allocate nickel (Ni) and iron (Fe) single atoms respectively on the inner and outer walls of graphene hollow nanospheres (GHSs), realizing separate-sided different single-atom functionalization of hollow graphene. The Ni or Fe single atom is demonstrated to be coordinated with four N atoms via the formation of a Ni-N4 or Fe-N4 planar configuration. The developed Ni-N4 /GHSs/Fe-N4 Janus material exhibits excellent bifunctional electrocatalytic performance, in which the outer Fe-N4 clusters dominantly contribute to high activity toward the oxygen reduction reaction (ORR), while the inner Ni-N4 clusters are responsible for excellent activity toward the oxygen evolution reaction (OER). Density functional theory calculations demonstrate the structures and reactivities of Fe-N4 and Ni-N4 for the ORR and OER. The Ni-N4 /GHSs/Fe-N4 endows a rechargeable Zn-air battery with excellent energy efficiency and cycling stability as an air-cathode, outperforming that of the benchmark Pt/C+RuO2 air-cathode. The current work paves a new avenue for precise control of single-atom sites on carbon surface for the high-performance and selective electrocatalysts.