Acid-base membranes of imidazole-based sulfonated polyimides for vanadium flow batteries

Acid-base sulfonated polyimides (SPIs) with different ratios of sulfonic groups and imidazole rings were synthesized by a one-step method for vanadium flow batteries (VFBs). The chemical structures of the SPI acid-base membranes were characterized by FTIR and 1H NMR spectroscopy. The various SPI acid-base membranes were tested in terms of their physicochemical properties, proton-to-VO2+ selectivity, and VFB single-cell performance. The proposed mechanism of proton-transport channel, formed by acid-base pairs in the interfacial zone, may enhance proton conductivity. The optimized SPI acid-base membrane with a 50% sulfonation degree (SPI50) showed lower VO2+ permeability (2.2 × 10−7 cm2 min−1) than that of the Nafion 115 membrane (20.2 × 10−7 cm2 min−1). In the rate performance (40–160 mA cm−2) test, coulombic efficiency (CE: 96.1–98.9%) and energy efficiency (EE: 74.3–90.1%) of the VFB with the SPI50 membrane were much higher than those of Nafion 115 membrane (CE: 88.7–94.3%, EE: 71.7–85.0%). Moreover, the VFB with the SPI50 acid-base membrane exhibited a stable battery performance over 400 charge-discharge cycles at 160 mA cm−2. The results demonstrate that the SPI50 acid-base membrane is a promising alternative for VFB applications.