Coaxially grafting conjugated microporous polymers containing single-atom cobalt catalysts to carbon nanotubes enhances sulfur cathode reaction kinetics

Abstract

Conjugated microporous polymers (CMPs) hold great potential for use in energy related applications due to their extended pi-conjugated structures, tunable pore sizes, and modular molecular functionalities. Herein, we report a novel composite material (labeled as Co-CMP-MWNTs) that consists of a CMP containing Co single-atom catalysts (Co SACs) and being coaxially grafted to multi-walled carbon nanotubes (MWNTs), and show that the material synergistically promotes the cathode reaction kinetics in lithium-sulfur (Li-S) batteries. The Co-CMPMWNTs are synthesized by coupling 2,4,6-tris(4-ethynylphenyl)-1,3,5-triazine to a dibromobipyridine-Co complex in the presence of bromopyrimidinyl-functionalized MWNTs. The composite features a conductive MWNT-based core and a CMP-based shell that contains nitrogen as well as Co. Cs-corrected high-resolution transmission electron microscopy and X-ray absorption near-edge structure (XANES) spectroscopy reveal that the Co species exist as single atoms. Additional XANES data coupled with density functional theory calculations elucidate the adsorption interactions formed between the Co SACs and various sulfur species as well as their electrocatalytic effects. Li-S cells prepared using Co-CMP-MWNTs as a cathode host material exhibit excellent performance in terms of specific capacity (1485 mA h g(-1) at 0.1 C), rate capability (602 mA h g-1 at 3 C), and cycling stability (510 mA h g(-1 )at 0.5 C after 1000 cycles, which corresponds to a capacity decay of 0.050% per cycle). Collectively, the results demonstrate that SACs can be prepared under benign conditions and used to enhance sulfur cathode reaction kinetics. The methodology described may be extended to enable the use of SACs in other contemporary energy conversion technologies.