In situ growth of covalent organic framework on graphene oxide nanosheet enable proton-selective transport in flow battery membrane

Perfluorosulfonic acid (PFSA) is the most widely used membrane material for all-vanadium redox flow batteries (VRFB). However, severe vanadium ion permeation of PFSA membranes remains to be solved. In this work, a series of PFSA-based composite membranes are prepared by introducing two-dimensional continuous covalent organic framework (COF) to achieve enhanced performance. Two dimensional continuous Schiff base networktype COFs (GO/SNW) are synthesized by using the two-dimensional multi-functionalized surface of graphene oxide (GO) as a reaction site. Compared to the original form of agglomerated particles, two-dimensional continuous structure obtained a larger vanadium resistance area. Comparing to conventional PFSA membranes, the special pores of COF have a size sieving mechanism for selectively proton transfer, leading to simultaneously enhancement of proton conductivity and vanadium barrier property. VRFB assembled with PFSAGO/SNW-7 showed high Coulombic efficiency (CE: 97.47%-98.18 % VS PFSA: 80.73%-91.92 %) and energy efficiency (EE: 93.04%-85.71 % VS PFSA: 77.53%-81.78 %) at 40-120 mA cm-2. In addition, there is no significant decrease in the efficiency of battery assembled with PFSA-GO/SNW-7 after 800 cycle (than 650 h) tests. This work provides a new direction in the design of fillers for mixed matrix membranes.