Heterostructured CoFe/Co nanoalloys encapsulated in N-doped carbon as bifunctional oxygen-electrode catalysts for Zn-air batteries

Developing bifunctional oxygen electrocatalysts based on non-precious elements for the air electrodes of rechargeable Zn-air batteries (ZABs) remains a significant challenge. Herein, by adjusting the Co precursor content, we synthesized a bifunctional electrocatalyst (CoFe@NC-5) comprising CoFe/Co nanoalloys (~16 nm) encapsulated in N-doped carbon. The CoFe@NC-5 catalyst features the highest metal loading (37.24 wt% Co+Fe), uniform nanoalloy distribution, a unique encapsulated structure, and well-defined heterojunction interfaces between CoFe and Co phases. These characteristics endow CoFe@NC-5 with excellent bifunctional oxygen electrode activity, as evidenced by a low potential gap (ΔE) of 0.757 V between the half-wave potential for the oxygen reduction reaction (ORR, 0.84 V) and the potential for the oxygen evolution reaction (OER, 1.597 V) at 10 mA cm⁻². Density functional theory (DFT) calculations further reveal that the heterojunction interfaces in the CoFe/Co heterostructure of the CoFe@NC-5 catalyst significantly enhance interfacial electron accumulation and shift the d-band center closer to the Fermi level, thereby boosting its ORR and OER activities. Furthermore, a rechargeable ZAB assembled with CoFe@NC-5 as the air electrode exhibits a high power density of 363.7 mW cm⁻², a specific capacity of 785.8 mAh g_Zn⁻¹ at 50 mA cm⁻², and stable charge-discharge cycling for 330 hours.