Today, vanadium redox flow batteries (VRFBs) are considered the most promising commercial large-scale hybrid power systems to efficiently store renewable energies. any are the advantages of using VRFBs: exclusion of the cross-contamination thanks to vanadium active electrochemical species at both electrodes, cathode, and anode; fast response to energy demand change; higher energy efficiency; low environmental impact; ambient temperature operations; long-life cycle; flexible design; low installation cost [1]. In VRFBs, membranes are one of the essential components since their morphological, structural, chemical, and physical properties affect the overall improved performance, durability, efficiency, and the overall total cost of this technology. Currently, Nafion membranes are widely used in the VFRBs thanks to the high proton conductivity and excellent chemical stability, although the high cost and the low battery performance, due to device deterioration for the high vanadium permeability, make it unsuitable for large-scale application and commercialization [2]. Among all possible alternatives to Nafion, membrane based on functionalized poly(ether ether ketone) (PEEK) containing sulfonic groups (SPEEK), have been reported to be suitable materials due to simple preparation and controllable sulfonation degree (DS), high stability and lower cost, high proton to vanadium ion selectivity [3]. In this contribution, SPEEK membranes with different DS and SiO-NH2 filler contents were prepared and characterized by physic-chemical and morphological analyses. VRFB tests performed using the optimized SPEEK membranes revealed a Coulombic efficiency of 95.8%, higher than that of commercial Nafion, used as reference. Finally, an in-house produced small-sized VRFB set-up made up of a 5- cell stack with optimized SPEEK membranes exhibited a good reproducibility and stability of the system with coulombic efficiency in the range 74-95%.