S-PEEK membranes optimized for Vanadium Redox Flow Battery: the effects of sulphonation degree and filler content on operative conditions and set-up configurations

Vanadium redox flow battery (VRFB) has attracted extensive attention as one of the most promising large-scale electrical energy storage system, thanks to the advantages of decoupled energy storage and power output, the high-energy efficiency, the fast response and ambient temperature operations, the flexible design and safety, the long cycling stabilities [1, 2]. Specifically, VRFB has an additional advantage in that it does not suffer from permanent self-discharge thanks to the use of the same element in both cell compartments. Therefore, VRFB possesses the proper technology maturity for market uptake in the next years and several VRFB systems have been already demonstrated all over the world [3-6]. Nevertheless, the commercialization of the VRFB is still hindered by the low stability and the high costs of some components such as vanadium precursors, electrolyte temperature management system in order to prevent precipitation of vanadium compounds, and mainly the membrane which is the key component to separate the catholyte and anolyte and to transport protons [7]. Therefore, to replace the widely used but expensive perfluorinated Nafion membranes, great efforts have been made to search alternative ones; among all candidates, sulfonated poly(ether ether ketone) (SPEEK)-based membrane seems to be the most suitable to replace Nafion membrane because of its lower cost, high proton to vanadium ion selectivity, if only the poor stability be solved [8]. In this contribution, S-PEEK membranes with different sulphonation degrees and SiO-NH2 filler contents were prepared and, after chemical and morphological characterizations [9], tested in labscale VRFB system by means of several consecutive charge-discharge cycles, and of Electrochemical Impedance Spectroscopy (EIS) measurements The performances of those prepared membranes were evaluated in terms of effectiveness, feasibility and sustainability, and the results were compared to those of commercial Nafion membrane, used as reference. The interplay between the chemical-physic, structural and electrochemical properties of S-PEEK membranes with different sulphonation degree and of the filler content grade was evaluated.