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 have been synthesized via Friedel-Craft alkylation and subsequent post-functionalization, and their chemical structure and morphology has been fully deconvoluted. The membranes, obtained by solution casting, have been characterized 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 the polymer 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 high proton conductivity. This result greatly impacted on flow battery self-discharge and on its capacity fading. The first increased from 2 to 7.5 days, while the latter decreased (over 25 cycles) from 8% to 5% for composite membranes with respect to Nafion 212.