NiCo₂O₄ Nanorods Anchored on rGO as a Cathode Material for Flexible Hybrid Zn-Air Batteries

The growing energy demand calls for storage systems with high energy and power densities. Zn-air batteries (ZABs) offer great potential but are limited by sluggish oxygen reduction (ORR) and evolution (OER) kinetics at the cathode, necessitating efficient bifunctional electrocatalysts. In this study, NiCo₂O₄ nanorods anchored on reduced graphene oxide (rGO) are synthesized via a hydrothermal process and evaluated as cathode materials for ZABs. The NiCo₂O₄/rGO composite exhibited excellent catalytic performance with an ORR onset potential of 0.90 V and an overpotential (ΔE) of 0.67 V, attributed to the spinel structure and enhanced conductivity from the rGO matrix. The ZAB device showed a hybrid discharge mechanism with two plateaus: one from ORR and another sloping plateau from M─O─OH ↔ M─O redox reactions (M═Ni, Co), contributing to high energy and power densities. The NiCo₂O₄/rGO-based battery achieved a peak power density of 108 mW cm^{- 2}@135 mA cm^{- 2}, a specific capacity of 722 mAh g_Zn ^-1, and a cycle life of 270 h (810 cycles), outperforming the bare NiCo₂O₄ device. The enhanced performance stems from the synergistic interaction between NiCo₂O₄ and rGO, improving surface area, conductivity, and catalytic sites. These results demonstrate the potential of NiCo₂O₄/rGO as a bifunctional cathode for high-performance hybrid Zn-air batteries.