In situ reduction engineering of NiFe–NiFe2O4 heterojunctions for interfacial coupling toward efficient bifunctional zinc–air batteries

Abstract

Single-component catalysts face inherent challenges in striking a balance between the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) due to their sluggish kinetics and divergent reaction requirements. To overcome this limitation, we developed a facile in situ hydrogen reduction strategy to construct NiFe–NiFe₂O₄ heterostructures anchored on nitrogen-doped carbon spheres (NiFe–NiFe₂O₄@NC). The formation of the heterostructure induces pronounced interfacial charge transfer and a downward shift of the d-band center, thereby optimizing the adsorption energy of oxygen intermediates and significantly accelerating the kinetics of both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Benefiting from the interfacial synergy, NiFe–NiFe₂O₄@NC exhibits outstanding bifunctional activity, with a half-wave potential of 0.87 V for the ORR, an overpotential of only 323 mV at 10 mA cm⁻² for the OER, and a small potential gap (ΔE) of 0.68 V. Theoretical calculations further reveal that the heterostructure weakens the overly strong adsorption of *OH and *O, effectively lowering the energy barriers of the rate-determining steps. When employed as the air cathode in zinc–air batteries, NiFe–NiFe₂O₄@NC delivers a peak power density of 178.42 mW cm⁻² and stable cycling over 400 h, surpassing the commercial Pt/C + IrO₂ benchmark. These results highlight heterostructure engineering as an effective strategy to overcome the limitations of single-component catalysts and achieve efficient bifunctional oxygen electrocatalysis, providing valuable insights for the rational design of high-performance metal–air battery catalysts.