Novel microporous sulfonated polyimide membranes with high energy efficiency under low ion exchange capacity for all vanadium flow battery

High proton conductivity and low vanadium permeation created great challenges in designing of highly efficient membranes for all vanadium redox flow battery. To overcome this trade-off phenomenon, a series of novel microporous sulfonated polyimides (SPI) with gradient sulfonic acid group concentrations (6FTMA-X) were prepared by a simple one-step polymerization. They demonstrated high molecular weight (Mn of 23 to 59 KDa), modest microporosity (SBET = 437.2 - 39.4 m2 g−1) and suitable interchain space (between 3.7 and 5.2 Å), which provided excellent ion sieving for proton and vanadium ions (2.4 and 6.0 Å). Consequently, although the ion exchange capacities (0.38 - 1.47 mmol g−1) of 6FTMA-Xs were low compared with the reported SPIs, they still achieved high energy efficiency (80.4%) under high current density (100 mA cm−2), which was comparable to Nafion 117 (N117, 81.7%) at the same current density. This was because of the acceptable proton conductivity and ultra-low vanadium permeation (0.59 - 3.56 × 10−7 cm2 min−1). We suppose the micropore inside the sulfonated polyimides behaviors as ion channel for proton transport and a vanadium barrier. All these results demonstrate that the 6FTMA-X membranes show great potential for vanadium redox flow battery applications. This polymer design strategy provided new insight for highly efficient membranes for VRFB applications.