Sulfonated polyimide/s-MoS2 composite membrane with high proton selectivity and good stability for vanadium redox flow battery

We report the fabrication and properties of high performance and inexpensive composite membranes of sulfonated polyimide (SPI) and sulfonated molybdenum disulfide (s-MoS 2 ) for the vanadium redox flow battery (VRFB) application. Scanning electron microscopy (SEM) images reveal the uniformity of SPI/s-MoS 2 membrane. Both EDS and XRD results verify the existence of s-MoS 2 in SPI/s-MoS 2 membrane and show the interaction between SPI and s-MoS 2 . The physico-chemical properties of as-prepared SPI/s-MoS 2 membrane, including water uptake and ion exchange capacity, etc. are evaluated and compared to those of SPI, SPI/MoS 2 and Nafion 117 membranes, respectively. The SPI/s-MoS 2 membrane possesses increased proton conductivity and reduced vanadium ion permeability than pure SPI membrane, and it has the highest proton selectivity (2.24×10 5   S   min   cm −3 ) among all membranes. The VRFB with SPI/s-MoS 2 membrane presents a higher coulombic efficiency ( CE ) and energy efficiency ( EE ) compared with Nafion 117 at the current density ranging from 20 to 80   mA   cm −2 . The SPI/s-MoS 2 membrane shows a longer discharge time (193   h above 0.8   V) than Nafion 117 (72   h). Moreover, the SPI/s-MoS 2 membrane exhibits stable operation performance up to 500 cycles with no significant decline in CE and EE . All experimental results confirm that the SPI/s-MoS 2 membrane is suitable for use in VRFB.