3D hollow sphere CO₃O₄/MnO₂-CNTs: Its high-performance bi-functional cathode catalysis and application in rechargeable zinc-air battery

There has been a continuous need for high active, excellently durable and low-cost electrocatalysts for rechargeable zinc-air batteries. Among many low-cost metal based candidates, transition metal oxides with the CNTs composite have gained increasing attention. In this paper, the 3-D hollow sphere MnO₂ nanotube-supported Co₃O₄ nanoparticles and its carbon nanotubes hybrid material (Co₃O₄/MnO₂-CNTs) have been synthesized via a simple co-precipitation method combined with post-heat treatment. The morphology and composition of the catalysts are thoroughly analyzed through SEM, TEM, TEM-mapping, XRD, EDX and XPS. In comparison with the commercial 20% Pt/C, Co₃O₄/MnO₂, bare MnO₂ nanotubes and CNTs, the hybrid Co₃O₄/MnO₂-CNTs-350 exhibits perfect bi-functional catalytic activity toward oxygen reduction reaction and oxygen evolution reaction under alkaline condition (0.1 M KOH). Therefore, high cell performances are achieved which result in an appropriate open circuit voltage (∼1.47 V), a high discharge peak power density (340 mW cm⁻²) and a large specific capacity (775 mAh g⁻¹ at 10 mA cm⁻²) for the primary Zn-air battery, a small charge–discharge voltage gap and a high cycle-life (504 cycles at 10 mA cm⁻² with 10 min per cycle) for the rechargeable Zn-air battery. In particular, the simple synthesis method is suitable for a large-scale production of this bifunctional material due to a green, cost effective and readily available process.