Defect-Enriched Nitrogen Doped-Graphene Quantum Dots Engineered NiCo2S4 Nanoarray as High-Efficiency Bifunctional Catalyst for Flexible Zn-Air Battery

Flexible Zn-air batteries have recently emerged as one of the key energy storage systems of wearable/portable electronic devices, drawing enormous attention due to the high theoretical energy density, flat working voltage, low cost, and excellent safety. However, the majority of the previously reported flexible Zn-air batteries encounter problems such as sluggish oxygen reaction kinetics, inferior long-term durability, and poor flexibility induced by the rigid nature of the air cathode, all of which severely hinder their practical applications. Herein, a defect-enriched nitrogen doped-graphene quantum dots (N-GQDs) engineered 3D NiCo2S4 nanoarray is developed by a facile chemical sulfuration and subsequent electrophoretic deposition process. The as-fabricated N-GQDs/NiCo2S4 nanoarray grown on carbon cloth as a flexible air cathode exhibits superior electrocatalytic activities toward both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), outstanding cycle stability (200 h at 20 mA cm(-2)), and excellent mechanical flexibility (without observable decay under various bending angles). These impressive enhancements in electrocatalytic performance are mainly attributed to bifunctional active sites within the N-GQDs/NiCo2S4 catalyst and synergistic coupling effects between N-GQDs and NiCo2S4. Density functional theory analysis further reveals that stronger OOH* dissociation adsorption at the interface between N-GQDs and NiCo2S4 lowers the overpotential of both ORR and OER.