Fluorine-Containing Branched Sulfonated Polyimide Membrane for Vanadium Redox Flow Battery Applications

A series of fluorine-containing branched sulfonated polyimide (Fb-SPI) membranes with different degrees of branching (0-12 %) were synthesized through polycondensation. The chemical structure of Fb-SPI-10 membrane was confirmed using ATR-FTIR and H-1 NMR spectroscopy. The physico-chemical properties of Fb-SPI membranes were systematically investigated and compared to linear SPI (l-SPI) and Nafion 117 membranes. The vanadium-ion permeabilities of Fb-SPI membranes (2.45-0.99x10(-7) cm(2) min(-1)) are much lower than that of Nafion 117 (17.1x10(-7) cm(2) min(-1)) membranes. Besides, the chemical stability of Fb-SPI membranes is superior to that of l-SPI membranes. During 500-time continuous cyclic charge-discharge measurements, the VRFB assembled with a Fb-SPI-10 membrane shows higher coulombic efficiency (98.4-99.7 %), higher or close energy efficiency (69.0-79.7 %) from 40 to 80 mA cm(-2), and higher or close capacity retention (52.8-100 %) from 40 to 70 mA cm(-2) compared with a device using a Nafion 117 membrane. All of these results demonstrate that the as-selected Fb-SPI-10 membrane has promising potential for VRFB applications.