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

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, MnO2nanotubes (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 Mn2+/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-Nx, 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 ΔEvalue 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−2compared to a precious metal commercial catalyst, indicating the potential application of this composite in energy storage and conversion technology.