Defect-Engineered Co₃ O₄ @Nitrogen-Deficient Graphitic Carbon Nitride as an Efficient Bifunctional Electrocatalyst for High-Performance Metal-Air Batteries

The ability to craft high-efficiency and non-precious bifunctional oxygen catalysts opens an enticing avenue for the real-world implementation of metal-air batteries (MABs). Herein, Co₃ O₄ encapsulated within nitrogen defect-rich g-C₃ N₄ (denoted Co₃ O₄ @ND-CN) as a bifunctional oxygen catalyst for MABs is prepared by graphitizing the zeolitic imidazolate framework (ZIF)-67@ND-CN. Co₃ O₄ @ND-CN possesses superb bifunctional catalytic performance, which facilitates the construction of high-performance MABs. Concretely, the rechargeable zinc-air battery based on Co₃ O₄ @ND-CN shows a superior round-trip efficiency of ≈60% with long-term durability (over 340 cycles), exceeding the battery with the state-of-the-art noble metals. The corresponding lithium-oxygen battery using Co₃ O₄ @ND-CN exhibits an excellent maximum discharge/charge capacity (9838.8/9657.6 mAh g^-1 ), an impressive discharge/charge overpotential (1.14 V/0.18 V), and outstanding cycling stability. Such compelling electrocatalytic processes and device performances of Co₃ O₄ @ND-CN originate from concurrent compositional (i.e., defect-engineering) and structural (i.e., wrinkled morphology with abundant porosity) elaboration as well as the well-defined synergy between Co₃ O₄ and ND-CN, which produce an advantageous surface electronic environment corroborated by theoretical modeling. By extension, a rich diversity of other metal oxides@ND-CN with adjustable defects, architecture, and enhanced activities may be rationally designed and crafted for both scientific research on catalytic properties and technological development in renewable energy conversion and storage systems.