Highly selective proton exchange membranes for vanadium redox flow batteries enabled by the incorporation of water-insoluble phosphotungstic acid-metal organic framework nanohybrids

By utilizing functionalized nanoparticles, composite proton exchange membranes (PEMs) have the potential to break the trade-off between proton conductivity and ion selectivity to achieve high performance in the vanadium redox flow batteries (VFBs). In this work, we prepared the phosphotungstic acid (HPW)-metal organic framework (MIL-101-NH2) nanohybrids, HMN, in which the formation of chemical bonding between HPW and MIL-101-NH2 during the sintering process renders HMN water insolubility and prevents the HPW leakage in water. More importantly, HPW occupies the pores of MIL-101-NH2 to block the permeation of vanadium ions and creates additional continuous ordered pathways for proton transport, allowing the improvement of proton conductivity and the reduction of the vanadium permeability simultaneously for the sulfonated poly(ether ether ketone) (SPEEK) composite membranes filled with HMN. The SPEEK composite membrane with 6 wt% HMN (SPEEK/HMN-6) exhibits proton conductivity of 0.070 S cm−1 and vanadium permeability of 1.4 × 10−7 cm2 min−1 at 25 °C, 63% higher and 80% lower than those of the SPEEK control membrane (0.043 S cm−1 and 6.9 × 10−7 cm2 min−1), respectively. The VFB assembled with the SPEEK/HMN-6 composite membrane demonstrated an energy efficiency of 82.1% at 120 mA cm−2, much higher than that with the SPEEK control membrane (76.9%).