In this work, imidazole group containing brominated poly(phenylene oxide) (Im-bPPO), in which imidazole functional groups are introduced to brominated poly(phenylene oxide), are synthesized and used as a low-cost, hydrocarbon-based anion exchange membrane for vanadium redox flow battery (VRFB). Although the imidazole group containing brominated poly(phenylene oxide) (Im-bPPO) membrane has a room temperature ionic conductivity of 0.02 S cm^-1, which is somewhat lower than that of Nafion 212 (0.08 S cm^-1), it shows three orders of magnitude lower VO²⁺ permeability of 1.75×10^-9 cm² min^-1 compared with Nafion 212 (1.41×10^-6 cm² min^-1), suggesting its benefit of easier rebalancing control during VRFB operation. In comparison with the single cells using Nafion 212, the Im-bPPO membrane-based cell has a higher coulombic efficiency (CE) because of its excellent rejection of vanadium ions. However, the voltage efficiency (VE) of the Im-bPPO-based cell is lower than that of Nafion 212-based cell due to the lower ionic conductivity of the Im-bPPO membrane. Also, the cycle life of the Im-bPPO-based cell is 15 times shorter than the Nafion 212-based cell because of the chemical deterioration by highly oxidative VO₂⁺ ions as observed for other hydrocarbon-based membranes. These results demonstrate the advantage of hydrocarbon-based anion exchange membrane in preventing vanadium crossover, but present a challenge in improving chemical stability for practical usage.

Keywords: Vanadium Redox Flow Battery; Anion Exchange Membrane; Imidazole; Polyphenylene Oxide (PPO)