Highly ion selective hydrocarbon-based membranes containing sulfonated hypercrosslinked polystyrene nanoparticles for vanadium redox flow batteries

Composite membranes based on sulfonated poly(phenylene sulfide sulfone) loaded with sulfonated hyper-crosslinked polystyrene nanoparticles are here proposed for redox flow battery applications. Nanoparticles havebeen synthesized via Friedel-Craft alkylation and subsequent post-functionalization, and their chemical structureand morphology has been fully deconvoluted. The membranes, obtained by solution casting, have been char-acterized ex-situ and in operando to shed light on their performances as compared to commercial Nafion 212.The use of organic fillers offers different advantages over inorganic ones, such as better compatibility with thepolymer matrix and easier tunability of the degree of sulfonation, and thus of the ion exchange capacity.Composite membranes displayed a significant lowering of the vanadium-ion crossover, yet maintaining highproton conductivity. This result greatly impacted on flow battery self-discharge and on its capacity fading. Thefirst increased from 2 to 7.5 days, while the latter decreased (over 25 cycles) from 8% to 5% for compositemembranes with respect to Nafion 212.