Simple synthesis of flower-like NiCo₂O₄ as bifunctional electrocatalysts for methanol-assisted rechargeable Zn-air batteries

Aiming at the inevitable high charging potential problem of conventional Zn-air batteries (ZABs), biomass-assisted ZABs were proposed, which the oxygen evolution reaction (OER) in charging was replaced by biomass oxidation reaction. In this work, flower-like NiCo₂O₄ was synthesized by a simple chemical bath deposition method, which exhibited more outstanding performance for ORR and methanol oxidation reaction (MOR). Compared with Co₃O₄ and NiO, the NiCo₂O₄ air electrode with the mass loading of 2.0 mg cm⁻² displayed the highest current for ORR (−256.50 mA cm⁻²) and MOR (189.30 mA cm⁻²). Based on NiCo₂O₄ air electrode, the charging-discharging voltage gap of methanol-assisted ZABs was greatly reduced to 0.55 V from 0.80 V of ZABs. By analyzing the changes of air electrodes before and after the cycle, the main reasons for enhanced stability are maintained structure, avoided carbon oxidation, and maintained catalytic activity. Moreover, to further reduce the charging-discharging gap, the mass loading was increased to 4.0 mg cm⁻², charging-discharging voltage gap was reduced by 41 mV, which remained basically unchanged after cycling for 200 h.