Synthesis and properties of branched sulfonated polyimides for membranes in vanadium redox flow battery application

Low-cost branched sulfonated polyimide (bSPI) membranes were prepared for vanadium redox flow battery (VRB) application. The degree of branching (DB) of membranes was controlled to be 0-12% through changing the mole ratio of branched non-sulfonated diamine to dianhydride. Both FT-IR and NMR spectra verified the successful preparation of bSPI membranes. The morphology was observed using SEM. Thermal and mechanical properties of bSPI-8 membrane were better than linear SPI. Chemical stability of bSPI membranes was higher than linear SPI. Both proton conductivity (2.97-4.53 × 10−2 S cm−1) and vanadium ion permeability (4.51-12.23 × 10−7 cm2 min−1) of bSPI membranes increase with the DB. The bSPI-8 membrane with the highest proton selectivity and superior stability shows higher coulombic efficiency (CE, 97-99%) and energy efficiency (EE, 67-80%) than Nafion™ 117 membrane (CE, 95-98%, and EE, 62-73% respectively) at 50-120 mA cm−2. Besides, the VRB with bSPI-8 membrane shows stable efficiency and good capacity retention during 800-cycle charge-discharge test. Therefore, the optimized bSPI-8 membrane is of great potential in VRB system because of its excellent battery performance and operational stability.