Covalent organic framework in-situ grown on MXenes to improve ion selectivity of composite membranes for vanadium flow battery

Covalent organic frameworks (COFs) are highly porous crystalline polymers with well-defined pore structures, exhibiting great potential for the use of composite membranes to separate ions with different sizes. However, COFs are mostly insoluble and poorly dispersed in common solvents, making it very difficult to well disperse COFs in polymer matrix to prepare high-quality composite membranes. In this work, a MS hybrid was prepared by in-situ growing COFs on the surface of Ti₃C₂T_x MXenes, which helps improve the filler-matrix compatibility significantly. The synergetic effect of the two-dimensional structure of MXene nanosheets and the porosity of COF components in MS contributes to the improvement for the ion selectivity of the composite membranes (SPEEK/MS). The vanadium flow battery (VFB) loaded with the SPEEK/MS-10 membrane achieves the energy efficiency of 93 % at 40 mA cm⁻², as compared to 86 % for that with the SPEEK control membrane. Moreover, the improved cycling stability and capacity retention was achieved for the SPEEK/MS-10 based VFB. Therefore, this work provides a promising strategy to fabricate high-performance COF-based composite PEMs for the VFB applications.