In order to improve the performance of vanadium redox flow batteries (VRFBs), a series of anion exchange membranes (PAES-8mPip-x) containing multiple flexible side-chain piperidinium structures were selected for use as ionic membranes for VRFBs. The relationship between their battery performance and chemical structure was systematically investigated, and their oxidation stability and degradation mechanism in strong oxidizing solution were studied in detail. These membranes exhibited high SO4 2– conductivity as well as low area specific resistance due to the incorporation of densely flexible side-chain piperidinium. Their SO4 2– conductivity and area-specific resistance are in the range of 9.6–18.1 mS cm–1 and 0.19–0.34 Ω cm2 at 20 °C, respectively. The single battery assembled with the representative PAES-8mPip-0.25 displayed high efficiencies, and its coulombic efficiency, voltage efficiency, and energy efficiency were 94.78, 90.15, and 85.44% at a current density of 60 mA cm–2, respectively, which were higher than those of Nafion 212 (94.19, 89.08, and 83.90%) at the same testing condition. What is more, it was found that these AEMs had good stability in strong oxidizing solution. Although their polymer backbone was degraded to a certain extent in 1.5 mol L–1 VO2 +/3 mol L–1 H2SO4 at 40 °C, the flexible piperidinium in side chains did not undergo significant chemical degradation. Related research results provide a scientific basis for the design of high-performance anion membrane materials for VRFBs.

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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, SO 4 2− conductivity, and VO 2+ 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 cm 2 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 VO 2 + /3 mol L −1 H 2 SO 4 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.

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