Ionic covalent organic polymer (iCOP) composite membranes with enhanced efficiency for iron-chromium redox flow battery

The utilization of intermittent clean energy requires high efficient energy storage technologies to minimize energy losses during charge-discharge processes. In this work, ionic covalent organic polymer (iCOP) composite membranes are presented to promote the battery efficiencies of iron-chromium redox flow battery (ICRFB). iCOP powder was synthesized by interfacial polymerization method and the resulting composite membrane possessed superior physicochemical membrane. Sulfonic acid groups in the synthesized iCOP facilitated the proton transportation of the prepared membranes. A hydrogen bond network formed between perfluorinated PEM and iCOP further promoted the proton transportation of the membrane. Meanwhile, the positively charged amino groups under acidic conditions reduced the permeation of Fe 3+ and Cr 3+ ions due to the Donnan effect. Specifically, proton conductivity of iCOP-8 membrane reached 215 mS cm −1 at 80 °C. Moreover, iCOP-8 demonstrated the lowest permeation rates for both Fe 3+ (1.44 × 10 −7  cm s −1 ) and Cr 3+ (0.47 × 10 −7  cm s −1 ) among all membranes herein. In terms of the ICRFB efficiencies, a Coulombic efficiency of 97.66 % and energy efficiency of 87.11 % was achieved at a current density of 100 mA cm −2 . The battery also operated stably for over 200 cycles without significant performance degradation (CE: >97.01 %, EE: >86.01 %).