Oxygen Defect-Enriched Hierarchical NiCo₂O₄Hollow Rectangular Nanobars with Enhanced Bifunctional Oxygen Electrocatalysis for Efficient Rechargeable Zinc-Air Batteries

Developing efficient and low-cost bifunctional oxygen reduction/evolution reaction (ORR/OER) electrocatalysts is incredibly desired for the large-scale commercial application of zinc-air batteries. Hierarchical NiCo2O4 hollow rectangular nanobars (NiCo2O4 HRNBs) with ultrathin nanosheets were synthesized via the template-directed methodology, followed by sequential etching and annealing treatments. The NiCo2O4 HRNBs with advanced features of a hierarchical hollow nanoarchitecture and a high surface area expose more reaction sites and provide ample mass diffusion pathways. Meanwhile, the abundant and accessible oxygen vacancies on the surface of NiCo2O4 HRNBs can significantly accelerate the reaction kinetics of adsorbed oxygen-containing species. As a result, the as-obtained NiCo2O4 HRNBs deliver an outstanding bifunctional electrocatalytic performance, which generates a positive half-wave potential of 0.81 V for ORR (reaching Pt/C) and a current density of 10 mA cm-2 at a low potential of 1.59 V for OER (surpassing Pt/C) and robust stability in alkaline electrolytes. Expectedly, the rechargeable zinc-air batteries assembled by as-obtained NiCo2O4 HRNB cathodes demonstrate superior battery performance with a large specific capacity of 724.6 mA h gZn-1 at a current density of 10 mA cm-2, a high power density of 117.6 mW cm-2, and long-term cycle stability with a slight increase in polarization over an entire 200 h test, showing potential application in the field of new-generation rechargeable batteries.