Ionic cluster as template to confine the in-situ intergrowth of COFs in ion conductive membrane for vanadium redox flow battery

Covalent organic frameworks (COFs) are expected to break through the trade-off between proton conduction and vanadium ion permeation in ion conductive membranes, however, maximizing intrinsic benefits of the nanoscale COF materials in polymer-based membranes remains a challenge. Herein, the ionic cluster in the membrane acts as a template to create a unique morphology of COF/ionic cluster intergrown ion conductive channel. The ionic cluster template anchors tris (4-aminophenyl) amine (TAPA) monomer through multiple hydrogen bond interactions with sulfonic acid groups during the casting of the sulfonated polybenzimidazole (SPBI) membrane, inducing the intergrowth of TpTAPA-COF nanoparticles with ionic clusters. The intergrown COF/ionic cluster microphase significantly increases the size of hydrophilic ion conductive channel from 5-8 nm to 20–30 nm with only about COF content of 1 wt%, and establishes vanadium ion sieving networks through the 6 Å pores of TpTAPA-COF. This leads to a high ion selectivity of the SPBI/COF-1 % membrane (8.1 × 10 9  mS s cm −3 ), about 3.9 folds that of Nafion 212 (2.1 × 10 9  mS s cm −3 ). The corresponding vanadium redox flow battery (VRFB) shows a high energy efficiency of 82.3 % at 200 mA cm −2 and maintain stable cycling performance over 800 cycles without change in chemical structure. The design of COF/ionic cluster intergrown microstructure provides an effective strategy for incorporating COFs into polymer matrix.