Preparation of a one-dimensional hierarchical MnO@CNT@Co–N/C ternary nanostructure as a high-performance bifunctional electrocatalyst for rechargeable Zn–air batteries

Abstract

Developing high-performance bifunctional electrocatalysts for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is a significant challenge for the implementation of rechargeable Zn–air batteries. Herein, MnO₂ nanotubes (NTs) are prepared as both templates and oxidants to grow polypyrrole (PPy) nanotubes (NTs), on which a zeolite imidazole framework-67 (ZIF-67) is grown. Following that, a single calcination step transforms MnO₂, PPy and ZIF-67 into MnO nanoparticles, carbon nanotubes (CNTs) and Co–N doped carbon materials (Co–N/C), respectively to form a one-dimensional (1D) hierarchical ternary nanocomposite. In this composite, the CNTs encapsulate the MnO particles to effectively prevent their further agglomeration. The separated MnO particles possess a mixed valence of Mn^2+/4+ inside the CNTs, which can greatly facilitate electrolyte diffusion and electron transfer during the redox reactions. Furthermore, the Co–N/C and micro-CNTs formed on the CNT provide multiple catalytic active sites (Co-N_x, Co–O, and C–N moieties). At the optimized calcination temperature of 700 °C, MnO@CNT@Co–N/C exhibits excellent ORR/OER catalytic performance with a ΔE value of 0.81 V while maintaining structural and compositional stability. Remarkably, the rechargeable Zn–air battery fabricated with MnO@CNT@Co–N/C as the air electrode catalyst displays a higher peak power density (200.8 mW cm⁻²) and improved cyclability (300 h) at 5 mA cm⁻² compared to a precious metal commercial catalyst, indicating the potential application of this composite in energy storage and conversion technology.