Redox flow batteries (RFB) often operate at extreme pH conditions and may require cooling to prevent high temperatures. The stability of the battery membranes at these extreme pH-values at high temperatures is still largely unknown. In this paper, a systematic screening of the performance and stability of nine commercial membranes at pH 14 and pH ≤ 0 with temperatures up to 80 °C is conducted in an organic aqueous RFB. Swelling, area resistance, diffusion crossover, battery performance and membrane stability after 40–80 °C temperature treatment are shown, after which a recommendation is made for different user scenarios. The Aquivion E98-05 membrane performed best for both the Tiron/2,7-AQDS battery and the DHPS/Fe(CN)₆ battery at 40 mA/cm², with stable results after 1 week of storage at 80 °C. At 80 mA/cm², E-620-PE performed best in the DHPS/Fe(CN)₆ battery, while Sx-050DK performed best in the Tiron/2,7-AQDS battery.

In composite membranes, non-woven substrates are often included to offer higher mechanical strength. The use of non-wovens is currently limited in electrochemical applications, apart from lab-made electrospun non-woven membranes. In this manuscript, three commercial non-wovens are compared to test their potential use in acid-based electrochemical applications, for instance redox flow batteries, and are also compared to a woven fabric substrate. The three non-wovens are found to have variable suitability in terms of the stability of solvents used in further membrane processing. However, all are deemed limiting due to their relatively high area resistance (0.37–1.47 ohm.cm²). In comparison, free-standing and selective commercial ion exchange membranes have area resistances around 0.08–0.27 ohm.cm². More open substrate backings such as a woven structure are recommended instead to allow for lower resistance of the resulting composites.

Redox flow batteries (RFBs) are an interesting option for long-term energy storage. A well-performing membrane sits at the heart of the electrochemical battery cell and should effectively mitigate crossover of active species while minimizing resistance. However, current commercial membranes are rather expensive and demonstrate sub-optimal performance, leading to an extensive search for alternatives. Research on membranes for RFBs has long been dominated by dense ion-exchange membranes and porous membranes, both potentially with fillers. In recent years, increased interest in alternative morphologies such as thin-film composites (TFCs) has ignited new research directions. TFCs consist of a thin dense layer on top of a porous support, aiming to merge the advantages of both. Traditionally, TFCs are made using polyamide top layers. In this paper, a novel chemistry is developed with increased chemical stability for RFBs. Poly(vinylbenzyl chloride) is crosslinked interfacially with a diamine, demonstrating for the first time the potential of support-mediated interfacial crosslinking with two immiscible solvents. Optimization of the support, amine crosslinker, reaction time and synthesis procedure allowed a shift of the trade-off between vanadium crossover and proton transport, highlighting the opportunities for this promising TFC chemistry.

Porous oxidized sulfonated poly(arylene sulphide sulfone) (O-SPASS) membranes with different degrees of sulfonation were fabricated by non-solvent-induced phase separation and evaluated for the first time as separators in all-vanadium redox flow batteries (VRFBs) The degree of sulfonation was shown to control both the density of fixed ionic groups and the phase inversion kinetics, leading to distinct membrane morphologies and performances. In contrast to the conventional expectation that higher sulfonation favours water uptake and ion transport, herein the membrane with the lowest sulfonation degree, O-SPASS-10, exhibited the highest water uptake owing to its more open and interconnected sponge-like structure. This porous morphology promoted proton transport, while a dense selective surface layer limited vanadium crossover. As a result, O-SPASS-10 achieved the best conductivity/selectivity balance among the investigated membranes, with an ionic conductivity of 61 mS·cm-1 in 0.5 M H2SO4 and a vanadium permeability of 2.1×10-12 m²·s-1. The membranes retained high hydrated stiffness and preserved good selectivity towards vanadium ions after strong oxidative ageing in acidic vanadium (V) solution. Tested in a VRFB at 100 mA·cm-2, O-SPASS-10 delivered a stable coulombic efficiency close to 98% and competitive voltage and energy efficiencies compared with Nafion 115, remaining over 67 % after 80 cycles. This work highlights controlled phase inversion as a powerful approach to tune the morphology of sulfonated aromatic membranes and partially overcome the classical trade-off in VRFB applications.