Control of ion-transport channel structure of a metal-organic framework incorporated membrane for vanadium redox flow batteries

Abstract

Highly selective proton exchange membranes (PEMs) are essential for optimal performance in vanadium redox flow batteries (VRFBs). Achieving both high proton conductivity and low vanadium ion permeability simultaneously poses a significant challenge due to the intrinsic mechanism of ion transport within the membrane. In this study, a composite membrane series is developed by incorporating a Zr-based metal-organic framework (MOF) material, MIP-202, into sulfonated poly (ether-ether-ketone) (SPEEK) polymer. This integration restructures the ion-transport channels successfully, providing proton transfer paths but hindering vanadium crossover through size sieving effects. Additionally, it facilitates the formation of a hydrogen bonding network within hydrophilic domains and expands the ionic cluster sizes to enhance ion transfer. These combined effects significantly enhance membrane performance, resulting in a 76.5 % increase in ion selectivity. Detailed proton transfer processes within the polymer-MOF composite system are proposed based on experimental characterization and density functional theory (DFT) analysis to better understand the ion selectivity enhancement mechanism. The modification of the polymer membrane benefits the VRFB, demonstrating a Coulombic efficiency of 99.03 %, voltage efficiency of 81.99 %, and energy efficiency of 81.20 % at 120 mA cm⁻², while retaining 92.20 % capacity after 200 cycles, surpassing the performance of pristine SPEEK, Nafion 117 and Nafion 212 membranes.