Co/Co₇Fe₃ heterostructures with controllable alloying degree on carbon spheres as bifunctional electrocatalyst for rechargeable zinc-air batteries

Exploring efficient and nonprecious metal electrocatalysts of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is crucial for developing rechargeable zinc-air batteries (ZABs). Herein, an alloying-degree control strategy was employed to fabricate nitrogen-doped carbon sphere (NCS) decorated with dual-phase Co/Co₇Fe₃ heterojunctions (CoFe@NCS). The phase composition of materials has been adjusted by controlling the alloying degree. The optimal CoFe_0.08@NCS electrocatalyst displays a half-wave potential of 0.80 V for ORR and an overpotential of 283 mV at 10 mA·cm⁻² for OER in an alkaline electrolyte. The intriguing bifunctional electrocatalytic activity and durability is attributed to the hierarchically porous structure and interfacial electron coupling of highly-active Co₇Fe₃ alloy and metallic Co species. When the CoFe_0.08@NCS material is used as air-cathode catalyst of rechargeable liquid-state zinc-air battery (ZAB), the device shows a high peak power-density (157 mW·cm⁻²) and maintains a stable voltage gap over 150 h, outperforming those of the benchmark (Pt/C+RuO₂)-based device. In particular, the as-fabricated solid-state flexible ZAB delivers a reliable compatibility under different bending conditions. Our work provides a promising strategy to develop metal/alloy-based electrocatalysts for the application in renewable energy conversion technologies.