NiFe nanoparticles supported on N-doped graphene hollow spheres entangled with self-grown N-doped carbon nanotubes for liquid electrolyte/flexible all-solid-state rechargeable zinc–air batteries

Abstract

Exploring non-noble oxygen electrode catalysts for efficient and stable oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is of great importance for high-performance rechargeable zinc–air batteries. In this work, we reported for the first time that 2-methylimidazole (2-MIM) can effectively regulate the loading of transition metals nickel (Ni) and iron (Fe) on nitrogen-doped graphene hollow spheres (NGHS) entangled with N-doped carbon nanotubes (NCNTs). The introduction of 2-MIM not only effectively controls the size of loaded metal nanoparticles (NPs), but also plays a significant role in the formation of the metal alloy. In addition, the introduction of Ni is the key factor for the self-growth of NCNTs, and the introduction of Fe serves a key role in promoting the formation of metal nitrides and improving the bifunctional performance of the electrode materials. The NiFe₃@NGHS-NCNTs obtained after optimization of the preparation parameters exhibit excellent ORR/OER bifunctional electroactivity and long-term durability, which has obvious advantages over commercial 20% Pt/C + RuO₂ catalysts. Density functional theory (DFT) calculation demonstrated that the NiFe₃ alloy is strongly coupled with N-doped carbon (NC), endowing the C active site with moderate positive charge and valence state, thus improving the ORR and OER performance of the NiFe₃@NGHS-NCNTs. More importantly, a rechargeable liquid electrolyte Zn–air battery (ZAB) with NiFe₃@NGHS-NCNTs as the air cathode delivers a comparatively small charge–discharge voltage gap of 0.737 V at 10 mA cm⁻² and displays maximum power density of 126.54 mW cm⁻², which is higher than that of the commercial 20% Pt/C + RuO₂ catalyst (102.63 mW cm⁻²). The fabricated flexible all-solid-state (ASS) rechargeable ZAB with NiFe₃@NGHS-NCNTs exhibits a higher open-circuit voltage (OCV) (1.44 V) and peak power density (102.82 mW cm⁻²) than ASS-ZABs based on 20% Pt/C + RuO₂ (1.42 V, 35.35 mW cm⁻²).