Positive regulation of active sites for oxygen evolution reactions by encapsulating NiFe2O4 nanoparticles in N-doped carbon nanotubes in situ to construct efficient bifunctional oxygen catalysts for rechargeable Zn–air batteries

Abstract

In this work, a bifunctional oxygen catalyst containing both spinel structure sites and nitrogen doping sites was prepared based on a directional regulation strategy by adding Ni(OH)₂ into a Fe³⁺–melamine precursor and then pyrolyzing the mixture. Melamine was pyrolyzed into N-doped carbon nanotubes, which afford oxygen reduction reaction (ORR) active sites, and meanwhile, the generated NiFe₂O₄ particles can act as active components for oxygen evolution reactions (OERs). The as-prepared catalyst named Fe–Me–Ni exhibits excellent bifunctional activities for ORRs with a half-wave potential of 0.841 V, and for OERs with an overpotential of 313 mV at 10 mA cm⁻². The Zn–air battery fabricated from Fe–Me–Ni can work at a discharge voltage of 1.23–1.28 V with a specific capacity of 798 mA h g_Zn⁻¹ at 10 mA cm⁻², outperforming those of the Pt/C + RuO₂ catalyst (1.17–1.24 V, 787 mA h g_Zn⁻¹). The battery evaluation results also indicate the remarkable rechargeability and long-term cycling stability of the catalyst Fe–Me–Ni in practical applications.