Abstract
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.
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