Cationic group distribution and elemental composition are two key factors determining the conductivity and stability of anion exchange membranes (AEMs) for vanadium redox flow batteries (VRFBs). Herein, fluorinated tetra-dimethylaminomethyl-poly(fluorenyl ether)s (TAPFE)s were designed as the polymer precursors, which were reacted with 6-bromo-N,N,N-trimethylhexan-1-aminium bromide to introduce di-quaternary ammonium (DQA) containing side chains. The resultant DQA-TAPFEs with a rigid fluorinated backbone and flexible multi-cationic side chains exhibited distinct micro-phase separation as probed by small-angle X-ray scattering (SAXS) and atomic force microscopy (AFM). DQA-TAPFE-20 with an ion exchange capacity (IEC) of 1.55 mmol g-1 exhibited a SO42- conductivity of 10.1 mS cm-1 at room temperature, much higher than that of a control AEM with an identical backbone but spaced out cationic groups, which had a similar IEC of 1.60 mmol g-1 but a SO42- conductivity of only 3.2 mS cm-1. Due to the Donnan repulsion effect, the DQA-TAPFEs exhibited significantly lower VO2+ permeability than Nafion 212. The VRFB assembled with DQA-TAPFE-20 achieved an energy efficiency of 80.4% at 80 mA cm-1 and a capacity retention rate of 82.9% after the 50th cycling test, both higher than those of the VRFB assembled with Nafion 212 and other AEMs in the literature. Therefore, the rationally designed DQA-TAPFEs are promising candidates for VRFB applications.
Keywords: anion exchange membrane; conductivity; flow battery; morphology; stability.