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.