Radiation synthesis of imidazolium ionic liquid grafted PVDF as the anion exchange membrane for vanadium redox flow batteries

Abstract

In this work, a series of novel anion exchange membranes (AEMs) were fabricated by grafting 1-vinyl-3-ethyl imidazolium tetrafluoroborate ([VEIm][BF₄]) onto polyvinylidene fluoride (PVDF-g-IL) and transferred them into membranes by a solution casting method. The charged imidazolium moieties were controllably grafted onto PVDF by adjusting the radiation dose and IL monomer concentration. The structure of the grafted samples was tested by FTIR, ¹H NMR, and SEM analyses. Besides, the PVDF-g-IL membranes were systematically evaluated in vanadium redox flow batteries (VRFB). The PVDF-g-IL membrane with a grafting yield of 25% had an ion conductivity of 9.05 mS cm⁻¹ due to the presence of the imidazolium ionic liquid. The PVDF-g-IL membrane exhibited a lower vanadium permeability of 0.98 × 10⁻⁷ cm min⁻¹ than that of Nafion117 (15.1 × 10⁻⁷ cm min⁻¹), which was attributed to the “Donnan” effect between the vanadium ions and imidazolium. The cell assembled with the membrane possessed a superior coulombic efficiency of 98.64%, good capacity retention and great cycling stability at a current density of 100 mA cm⁻². These results indicate that PVDF-g-IL-based AEMs are promising candidates for VRFB applications.