Sulfonate and Ammonium-Grafted Poly(isatin triphenyl) Membranes for the Vanadium Redox Flow Battery

As one of the most critical components of the vanadium redox flow battery (VRFB), the ion exchange membrane directly influences the battery efficiency and cycle life. Herein, poly(isatin triphenyl) (PIT) containing a lactam structure and devoid of ether bonds is synthesized from isatin and p-terphenyl under superacid catalysis. To achieve the ion transport and investigate the impact of side chain end groups on the membrane performance, both cationic and anionic side chains are incorporated into PIT using glycidyl trimethylammonium chloride (GTA) and 1,3-propane sultone (PS) as the grafting reagents via ring-opening reactions. Thus, an anion exchange membrane (PIT-GTA) with quaternary ammonium and hydroxyl groups in the side chain and a cation exchange membrane (PIT-PS) with sulfonate groups in the side chain are obtained. Compared to PIT-PS and Nafion 115 membranes, the PIT-GTA membrane exhibits lower area resistance (AR, 0.22 Ω cm2) and reduced vanadium ion permeability (3.08 × 10−8 cm2 min–1), resulting in a significantly high ion selectivity value of 81.2 × 104 S min cm–3, which is approximately 270 times higher than that of Nafion 115 (0.3 × 104 S min cm–3). Additionally, the PIT-GTA membrane displays excellent chemical stability, with a mass loss of less than 10% after a 960 h test. Due to its superior restriction on vanadium ion migration, the VRFB assembled with PIT-GTA achieves a self-discharge duration of 289 h, significantly longer than that of Nafion 115 (86 h). In the current density range of 60–160 mA cm–2, the cell equipped with the PIT-GTA membrane demonstrates higher cell efficiencies compared to Nafion 115. Furthermore, the VRFB based on the PIT-GTA membrane exhibited excellent cycle stability and good discharge capacity retention over 1300 cycles of testing, indicating its enormous potential for application in the VRFB.