Co₃O₄@NiCo₂O₄double-shelled nanocages with hierarchical hollow structure and oxygen vacancies as efficient bifunctional electrocatalysts for rechargeable Zn-air batteries

Highly efficient bifunctional oxygen electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are crucially important for the rechargeable Zn-air battery, a potential power source for applications in electric vehicles and grid-scale stationary storage systems. Herein, Co₃O₄@NiCo₂O₄double-shelled nanocages (Co₃O₄@NiCo₂O₄DSNCs) with hierarchical hollow structure and oxygen vacancies were designed and synthesizedviaannealing metal-organic frameworks. Co₃O₄@NiCo₂O₄DSNCs with large specific surface area and three-dimensional interconnected mesopores and cavity not only provide more reaction sites, but also offer an efficient transport environment for reactants. Moreover, oxygen vacancies on the surfaces improve the capture of oxygen species to enhance the reactivity of the catalyst. Consequently, Co₃O₄@NiCo₂O₄DSNCs displayed excellent bifunctional electrocatalytic performance, with a positive half-wave potential of 0.81 V (vs.reversible hydrogen electrode, RHE) for ORR (approaching the potential of commercial Pt/C catalyst) and a low potential of 1.65 V at 10 mA cm⁻²for OER (exceeding Pt/C). In a practical demonstration, the Zn-air battery using Co₃O₄@NiCo₂O₄DSNCs as the cathode delivered a satisfactory power density of 102.1 mW cm⁻², comparable to the Zn-air battery with a Pt/C cathode, and exhibited much longer cycling stability.