Preparation and properties of polybenzimidazole/quaternized poly(1-vinylimidazole) cross-linked blend membranes for vanadium redox flow battery applications

A series of novel cross-linked blend membranes have been successfully prepared from poly(2,2′-(p-oxydiphenylene)-5,5′-bibenzimidazole) (OPBI) and partially quaternized poly(1-vinylimidazole) (QPVI) at varied polymer weight ratios using 1,6-dibromohexane as the cross-linker. The resulting cross-linked blend membranes are ductile and transparent. Cross-linking causes slight increase in tensile strength and significant improvement in immobilization of the QPVI. The effect of the QPVI content on membrane properties such as mechanical properties, thermal stability, ion exchange capacity, water uptake, swelling ratio, and anion conductivity is investigated. The charge–discharge performance of the single cells assembled with the cross-linked blend membranes has also studied and compared with that of the one assembled with Nafion 117. The cross-linked blend membranes exhibit three to four orders lower VO2+ permeability than Nafion 117 leading to much slower self-discharge rate and significantly higher coulombic efficiency of the former. The single cells assembled with the cross-linked blend membranes show higher coulombic efficiency and energy efficiency at low current densities than the one assembled with Nafion 117. Furthermore, the single cell assembled with the cross-linked blend membrane OPBI/QPVI(3:7, weight ratio)-Cross-linked (CL) shows little decay in energy efficiency after 300 charge–discharge cycles test at 80 mA cm−2 indicating good durability of this membrane.