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

Abstract

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 × 105 S·min/cm3, which is 2.8 and 2.5 times higher than both SPI-60 (0.39 × 105 S·min/cm3) and commercial Nafion 212 (0.44 × 105 S·min/cm3) 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/cm2. Remarkably, the 500 cycles of BSPI-60 membrane at 140 mA/cm2 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.