Long side-chain quaternary ammonium group functionalized polybenzimidazole based anion exchange membranes and their applications

Abstract

In the present work, new anion exchange membranes (AEMs) are synthesized by using polybenzimidazole (PBI) as the matrix and (5-bromopentyl)-trimethylammonium bromide (BPTMA-Br) as the grafting reagent. The grafted long side-chain quaternary ammonium group endows the PBI membrane with a high ion exchange capacity (IEC) of 1.8 mmol g−1, and increased hydrophilicity and ionic conductivity. The PBI-48%BPTMA membrane shows a gradual decrease in conductivity when immersing in 1 M KOH at 80 °C, and maintains its 65.3% conductivity after 240 h measurement. Nevertheless, the PBI-48%BPTMA membrane displays a large potential in the vanadium redox flow battery (VRFB). The PBI-48%BPTMA membrane possesses a lower area resistance (0.43 Ω cm2) and vanadium ion permeability (6.9 × 10−7 cm2 min−1) than Nafion115 (0.61 Ω cm2 and 3.2 × 10−6 cm2 min−1). Consequently, the assembled VRFB shows higher energy efficiency of 82.7% at 80 mA cm−2 than Nafion115 (77.4%). Meanwhile, the single cell also exhibits almost unchanged performance within the 400 charge-discharge cycles at 100 mA cm−2, demonstrating superior stability of the PBI-BPTMA membrane in VRFB.