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

• The SPI/s-MoS2 composite membrane was first prepared by a blending method.
• The SPI/s-MoS2 membrane has low VO2+ permeability and high proton conductivity.
• Oxidative degradation is mitigated with new VRFB membrane design.
• The VRFB with SPI/s-MoS2 membrane has stable cycling charge–discharge performance.

We report the fabrication and properties of high performance and inexpensive composite membranes of sulfonated polyimide (SPI) and sulfonated molybdenum disulfide (s-MoS2) for the vanadium redox flow battery (VRFB) application. Scanning electron microscopy (SEM) images reveal the uniformity of SPI/s-MoS2 membrane. Both EDS and XRD results verify the existence of s-MoS2 in SPI/s-MoS2 membrane and show the interaction between SPI and s-MoS2. The physico-chemical properties of as-prepared SPI/s-MoS2 membrane, including water uptake and ion exchange capacity, etc. are evaluated and compared to those of SPI, SPI/MoS2 and Nafion 117 membranes, respectively. The SPI/s-MoS2 membrane possesses increased proton conductivity and reduced vanadium ion permeability than pure SPI membrane, and it has the highest proton selectivity (2.24×105 S min cm−3) among all membranes. The VRFB with SPI/s-MoS2 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-MoS2 membrane shows a longer discharge time (193 h above 0.8 V) than Nafion 117 (72 h). Moreover, the SPI/s-MoS2 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-MoS2 membrane is suitable for use in VRFB.