Free-standing COF nanofiber in ion conductive membrane to improve efficiency of vanadium redox flow battery

The ion sieving effect of the porous two-dimensional covalent organic frameworks (COFs) in vanadium redox flow battery (VRFB) was greatly restricted by the brittleness and easy agglomeration of COFs in ion conductive membranes. Herein, the strategy of free-standing COF nanofiber as continuous ion conductive pathways is proposed to maximize the selective ion conduction of COFs. The free-standing TFP-TAPA electrospun COF nanofiber mat was fabricated by an in-situ growth-template etching method, densified with sulfonated polybenzimidazole (SPBI) and continuously dispersed in the membrane. Fibrillization gives rigid COF materials excellent flexibility to be freely bent, induces the orientated aggregation of sulfonic acid groups via hydrogen bonding for proton hopping, as well as the long-range vanadium ions repelling barrier via the sieving pores (6 Å) and Donnan effect of the imine groups in COF. The TFP-TAPA COF nanofiber continuous membrane shows 3 folds of H+/Vn+ selectivity over Nafion212, achieves excellent VRFB energy efficiency of about 80.5 % at 200 mA cm−2, which surpasses most state-of-the-art COFs based ion conductive membranes for VRFBs.