Highly proton-conductive and stable sulfonated covalent organic framework hybrid membrane for vanadium redox flow battery

Covalent organic framework (COF), particularly sulfonated COF (SCOF) with high density -SO3H groups, demonstrates superior proton conductivity and excellent stability, rendering it an appealing candidate for acidbased redox flow batteries, including the vanadium redox flow battery. The fabrication of self-standing sulfonated COF membranes with high ion selectivity and stability is critical but remains a formidable challenge. In this study, we report a SCOF-based hybrid membrane by incorporating SCOF into the SPEEK matrix to create a pore structure that facilitates selective proton conduction. The high concentration of -SO3H groups and high hydrophilicity enhance the proton conduction. Meanwhile, the reduction in COF pore size caused by the polymer intrusion and the high density of -SO3H groups effectively prevents the permeation of vanadium ions. These factors contribute to an exceptionally low area resistance, highlighting the membrane's enhanced performance. The utilization of SCOF membrane in assembled cell yields high coulombic efficiency (97.1-99.5 %) and energy efficiency (92.5-77.5 %) at 20-200 mA cm-1. Noteworthy is the sustained stability of the SCOF/SPEEK membrane, enduring up to 200 cycles. These remarkable performances underscore the potential of SCOF/SPEEK as a highly ion-selective hybrid membrane for vanadium redox flow batteries.