Hybrid inorganic-organic proton-conducting membranes based on SPEEK doped with WO3 nanoparticles for application in vanadium redox flow batteries

Highlights: • Hybrid [SPEEK/(WO₃)_x] membranes are prepared and investigated for application in VRFBs. • The transport properties of the [SPEEK/(WO₃)_x] membranes are strongly influenced by x. • At 30 °C the [H⁺/VO²⁺] ion selectivity of [SPEEK/(WO₃)_0.20] is 3.2 times higher than that of Nafion 212. • The VRFB mounting [SPEEK/(WO₃)_0.20] shows a lower capacity fade than that mounting Nafion 212. -- Abstract: The development of innovative proton-conducting membranes exhibiting an improved [H⁺/VO²⁺] ion selectivity in comparison with baseline perfluorinated ionomers (e.g., Nafion) is critical to prompt the large-scale implementation of vanadium redox flow batteries (VRFBs) characterized by a performance and cyclability compatible with the applications. This paper pursues this objective by reporting the preparation and characterization of a new family of hybrid inorganic-organic membranes, that are labeled "[SPEEK/(WO₃)_x]", consisting of a sulfonated poly (ether ether ketone) (SPEEK) matrix hosting between 0 and 23.6 wt% of WO₃ nanoparticles (NPs). The physicochemical characterization of the [SPEEK/(WO₃)_x] membranes elucidates the impact of the introduction of WO₃ NPs on the thermal, structural and transport properties of the SPEEK host. It is found that the [SPEEK/(WO₃)_0.20] membrane presents the highest [H⁺/VO²⁺] ion selectivity (2.1 × 10⁴ S min cm⁻³), that is more than three times higher than that of recast Nafion (6.5 × 10³ S min cm⁻³). [SPEEK/(WO₃)_0.20] is then mounted in a single cell VRFB, that is tested extensively under realistic operating conditions and demonstrates a coulombic efficiency and cyclabilty that are improved with respect to the corresponding figures of a VRFB incorporating a Nafion 212 membrane to provide the basis for a comparison. This outcome is in compliance with the result of the "ex-situ" investigations, and allows for the proposal of a structural model correlating the physicochemical properties of the [SPEEK/(WO₃)_x] membranes with the transport properties and the proton conductivity.