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

Abstract

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-NH 2 ) nanohybrids, HMN, in which the formation of chemical bonding between HPW and MIL-101-NH 2 during the sintering process renders HMN water insolubility and prevents the HPW leakage in water. More importantly, HPW occupies the pores of MIL-101-NH 2 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 cm 2 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 cm 2 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%). • Water-insoluble HMN nanohybrids prepared by the sintering of HPW and MIL-101-NH 2 . • HMN incorporation greatly enhanced proton transport in the composite membranes. • Pore occupation of MIL-101-NH 2 by HPW slowed down the vanadium permeation. • SPEEK/HMN membranes broke the trade-off between proton conductivity and selectivity. • VFB performance was improved significantly using SPEEK/HMN-6 composite membrane.