Oxygen-vacancy-rich CoFe/CoFe2O4 embedded in N-doped hollow carbon spheres as a highly efficient bifunctional electrocatalyst for Zn–air batteries

Exploring transition metal-based bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is essential for implementing rechargeable Zn–air batteries (ZABs). Herein, we synthesized oxygen-vacancy-rich CoFe/CoFe2O4 embedded in N-doped hollow carbon spheres (Vo-CoFe/CoFe2O4@NC) through pyrolysis, carbonization, and partial reduction. The synergistic effect of introducing oxygen vacancies into CoFe2O4 and the well-defined heterointerfaces between the CoFe alloy and spinel-type CoFe2O4 moderately controlled the electronic structure and provided abundant active sites for the ORR and OER. Vo-CoFe/CoFe2O4@NC exhibited high ORR activity (half-wave potential: 0.858 V and Tafel slope: 56 mV dec–1) and delivered a low overpotential (360 mV at 10 mA cm−2) for the OER. Moreover, rechargeable ZABs using Vo-CoFe/CoFe2O4@NC as the air cathode revealed excellent open-circuit voltage (1.53 V), good power density (139.5 mW cm−2), and longer cycling durability than the state-of-the-art Pt/C-RuO2.