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.