A hybridization cage-confinement pyrolysis strategy for ultrasmall Ni3Fe alloy coated with N-doped carbon nanotubes as bifunctional oxygen electrocatalysts for Zn–air batteries

Abstract

For rechargeable Zn–air batteries, it is still a great challenge to design a safe and efficient bifunctional oxygen catalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). In this paper, an ultrasmall Ni₃Fe alloy electrocatalyst embedded in nitrogen-doped carbon (denoted as Ni₃Fe-NCNTs-800) was obtained by pyrolysis of Ni-MOF hybridized with ferrocene and assisted by melamine in an argon atmosphere at 800 °C. Metal–organic framework (MOF) precursors serve as cages to exert synergistic effects with melamine to limit the aggregation of metal particles, which is a promising strategy for the preparation of advanced oxygen catalysts. Ni₃Fe-NCNTs-800 exhibits excellent electrochemical catalytic performance with an ORR (half-wave potential: 0.862 V) and OER (overpotential: 353 mV@10 mA cm⁻²) potential gap of 0.72 V. Density functional theory (DFT) calculations validate that the strong synergistic coupling of Ni₃Fe bimetal plays a key role in lowering the reaction barrier and promoting the reversible oxygen reaction. In addition, liquid Zn–air batteries assembled with Ni₃Fe-NCNTs-800 as the air cathode catalyst have ultra-high peak power density (211 mW cm⁻² at 240 mA cm⁻²), specific battery capacity (806 mA h g_Zn⁻¹), and robust cycle stability (no obvious decay after more than 1350 charge–discharge cycles), which are superior to commercial Pt/C + RuO₂ based Zn–air batteries. And three all-solid-state ZABs connected in series can successfully light a LED (∼2.2 V), demonstrating their great potential in portable devices.