Sulfonated polyimide membranes with different non-sulfonated diamines for vanadium redox battery applications

A series of vanadium ion blocking sulfonated polyimide (SPI) membranes with various non-sulfonated diamines were prepared for vanadium redox battery (VRB) applications. FT-IR spectrum verifies the successful synthesis of SPIs, FSEM images illustrate that the membrane surface facing the negative electrode is more intact than that facing the positive electrode. The properties of SPI membranes such as water uptake, ion exchange capacity, thermal stability, proton conductivity and vanadium ion permeability can be adjusted by the species and the structure of non-sulfonated diamine. All SPI membranes show over one or two order of magnitude lower permeability of vanadium ion (0.48 - 2.36 × 10−7 cm2 min−1) than currently used Nafion 117 membrane (17.10 × 10−7 cm2 min−1). Vanadium redox static batteries with SPI membranes exhibit lower self-discharge rate, higher coulombic efficiency (97% - 99%) compared with Nafion 117 (94%) at 30 mA cm−2. Meanwhile, as-prepared SPI membranes are durable in 0.1 mol L−1 VO2+ + 3.0 mol L−1 H2SO4 solutions at 40 °C. All results show that SPI membranes are promising proton conductive membranes for VRB applications, among which the SPI (BAPP) has the best VRB performance.