Novel ether-free membranes based on poly(p-terphenylene methylimidazole) for vanadium redox flow battery applications

The fabrication of low-cost and high-performance membranes is one of the most critical issues for vanadium redox flow batteries (VRFBs). Herein, a novel ether-free poly(p-terphenylene methylimidazole) (PTMIm) is synthesized by a facile one-pot Friedel-Crafts polyhydroxyalkylation of p-terphenyl and 1-methyl-2-imidazolecarboxaldehyde. The PTMIm has excellent organic solubility and good chemical stability towards high valence vanadium ions. Meanwhile, due to the present pendant imidazole groups, the PTMIm membrane exhibits a high sulfonic acid absorption capability, achieving a suitable area resistance (AR). In order to further enhance the performance of the PTMIm membrane, side-chain quaternization is carried out by using 1-bromopropane, 3-bromo-N,N,N-trimethylpropan-1-aminium bromide and 3-chloro-2-hydroxypropyltrimethyl ammonium chloride as quaternized reagents. Comparing with pure PTMIm membrane and propyl group grafted membrane (i.e. PTMIm-C3), terminal quaternary ammonium side chain grafted membranes of PTM-C3-QA and PTMIm-C3-OH-QA possess lower AR and better vanadium ion resistance simultaneously. Consequently, the ion selectivity value of the PTMIm-C3-QA membrane is 8.7 × 105 S min cm−3, which is approximately 130 times higher than that of Nafion 115 (6.6 × 103 S min cm−3), mainly resulting from its low AR (0.68 Ω cm2) and ultralow vanadium ion permeability (9.5 × 10−9 cm2 min−1). Meanwhile, both PTMIm and its grafted membranes exhibit excellent chemical stabilities within 400 h chemical stability test. Owing to the superior restriction on vanadium migration, the single cell with PTMIm-C3-QA displays an excellent self-discharge duration of 500 h, which is greatly longer than that of Nafion 115 (58 h). In the range of 40–160 mA cm−2, the PTMIm-C3-QA based cell expresses higher coulombic efficiencies (>99.5%) and larger energy efficiencies than the cell with Nafion 115. For the cycling test of VRFB during 500 cycles, the PTMIm-C3-QA based cell displays excellent cycle stability and good discharge capacity retention, indicating a great application potentiality of the PTMIm-C3-QA membrane for VRFBs.