Highly ion selective composite proton exchange membranes for vanadium redox flow batteries by the incorporation of UiO-66-NH2 threaded with ion conducting polymers

Proton exchange membranes (PEMs) with both high proton conductivity and low vanadium ion permeability are highly desirable for the vanadium redox flow batteries (VFBs) to achieve high performance. In this work, we prepared the UiO-66-NH2 threaded with polystyrene sulfonic acid (PSSA) (S-UiO) by in situ polymerization of the sulfonated monomer impregnated in the pores of UiO-66-NH2. The incorporation of S-UiO into the sulfonated poly(ether ether ketone) (SPEEK) increased the size of hydrophilic domains and the extent of phase separation for the SPEEK/S-UiO composite membranes, resulting in the significant enhancement of proton conductivity. Furthermore, S-UiO also acted as the barriers for the vanadium ion permeation. As a result, compared to that of the SPEEK control membrane, the SPEEK/S-UiO membrane with 15 wt% S-UiO exhibited 63% higher proton conductivity and 83% lower vanadium permeability. This led to much improved VFB performance for the composite membrane with an excellent EE of 83.9% under 120 mA/cm2, much higher than that of the SPEEK control membrane (77.3%).