Bifunctional Cross-Linking Agents Enhance Anion Exchange Membrane Efficacy for Vanadium Redox Flow Batteries

A series of cross-linked fluorinated poly(aryl ether oxadiazole) membranes (FPAEOM) derivatized with imidazolium groups were prepared. Poly(N-vinylimidazole) (PVI) was used as the bifunctional cross-linking agent to (a) lower vanadium permeability, (b) enhance dimensional stability, and (c) concomitantly provide added ion exchange capacity in the resultant anion exchange membranes. At a molar ratio of PVI to FPAEOM of 1.5, the resultant membrane (FPAEOM-1.5 PVI) had an ion exchange capacity of 2.2 mequiv g^-1, a vanadium permeability of 6.8 × 10^-7 cm² min^-1, a water uptake of 68 wt %, and an ionic conductivity of 22.0 mS cm^-1, all at 25 °C. The FPAEOM-1.5 PVI membrane showed high chemical stability in highly acidic and oxidizing vanadium solution. Single cells prepared with the FPAEOM-1.5 PVI membrane exhibited a higher Coulombic efficiency (>92%) and energy efficiency (>86%) after 40 test cycles in vanadium redox flow battery.