A novel double branched sulfonated polyimide membrane with ultra-high proton selectivity for vanadium redox flow battery

The development of a vanadium redox flow battery (VRFB) urgently requires proton conductive membranes with high proton selectivity. Herein, we synthesized branched anhydride monomer 1,3,5-tris(3,4-anhydride phenoxy)benzene using a three-step method. Furthermore, we prepared successfully novel double branched sulfonated polyimide (dbSPI) membranes with ultra-low vanadium ion permeability (0.4–2.5×10−8 cm2 min−1), one or two orders of magnitude lower than that of the Nafion 212 membrane (7.25×10−7 cm2 min−1). Among all membranes, the dbSPI-50 membrane with the highest proton selectivity (23.25×105 S min cm−3), approximately 68 times higher than that of the Nafion 212 membrane, was chosen for assembling of a VRFB single cell. Because of its ultra-low vanadium ion permeability, the dbSPI-50 membrane exhibited much longer self-discharge time (97 h) and superior capacity retention (82.6–91.3%) compared with the Nafion 212 membrane. Moreover, both coulomb efficiency (99.6–98.5%) and energy efficiency (69.3–80.1%) of the dbSPI-50 membrane are higher than those of the Nafion 212 membrane after 300 times charge-discharge tests at a current density from 200 to 100 mA cm−2. All these results demonstrate that the optimized dbSPI-50 membrane is a promising candidate for applications in VRFBs.