Investigation of branched sulfonated polyimide membranes containing self-synthesized trianhydride monomer for vanadium flow battery via a combined theoretical-experimental strategy

In this work, a new branched trianhydride monomer 1,3,5-tris(4-naphthyloxy-1,8-diacid) phthalic anhydride is established for improving the chemical/dimensional stability and proton conduction of the branched sulfonated polyimide (BSPI) membrane for application in vanadium flow battery (VFB). Compared with linear SPI-60 membrane containing conventional dianhydride monomer 1,4,5,8-naphthalenetetra-carboxylic dianhydride, BSPI-60 membrane exhibits remarkable resistance to vanadium ions , proton conduction and structural stability . Besides, the challenge of simultaneously improving vanadium ions resistance and proton conductance is tackled. The proton selectivity of BSPI-60 membrane achieves 1.11 × 10 5  S·min/cm 3 , which is 2.8 and 2.5 times higher than both SPI-60 (0.39 × 10 5  S·min/cm 3 ) and commercial Nafion 212 (0.44 × 10 5  S·min/cm 3 ) membranes, respectively. At the same time, BSPI-60 membrane exhibits higher coulomb efficiencies (97.2 %–99.3 %) and energy efficiencies (85.6 %–69.5 %) compared those of SPI-60 and Nafion 212 membranes at 100–300 mA/cm 2 . Remarkably, the 500 cycles of BSPI-60 membrane at 140 mA/cm 2 are also stably executed. The internal reasons for enhanced chemical/dimensional stabilities and proton conduction of BSPI-60 membrane are clarified from theoretical calculations with mean square displacement value, fractional free volume and natural bond orbital charge via density functional theory and molecular dynamics simulation . Our study encompasses not only the synthesis of a novel branched trianhydride monomer but also the development of a BSPI membrane with a unique molecular structure specifically designed for VFB applications.