Preparation and Properties of Quaternary Ammonium Anion Exchange Membranes with Flexible Side Chains for the Vanadium Redox Flow Battery

In order to promote the conduction and stability of anion exchange membranes (AEMs) for the vanadium redox flow battery (VRFB), a kind of quaternary ammonium AEMs with flexible side chains (PAES-4N-xx, xx represents the proportion of monomers containing an amino structure) are prepared via aromatic nucleophilic polycondensation and grafting reactions. The ion exchange capacity (IEC) values of PAES-4N-xx are in the range of 1.58–2.10 mmol g–1, and their water uptake, swelling ratio, SO42– conductivity, and VO2+ permeability at 30 °C are in the range of 15–52%, 5–17%, 13.5–22.4 mS cm–1, and 8.6–43.3 cm2 min–1, respectively. In addition, PAES-4N-xx display good stability, and their retention ratio of IEC and conductivity reach more than 95% after soaking in a solution of 1.5 mol L–1 VO2+/3 mol L–1 H2SO4 at RT for 30 days. The typical battery assembled with PAES-4N-40 shows high efficiency; its Coulomb efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) are in the range of 93.5%–97.8%, 88.6%–70.5%, and 69.0%–84.9% at the fixed current density which is between 40 mA cm–2 and 200 mA cm–2, respectively. The battery also has good stability for 100 cycles of charging and discharging at a current density of 40 mA cm–2, and its retention of EE reaches 96.5%. The introduction of flexible side chains and dense cationic structures can effectively improve the performance of the VRFB.