Influence of casting substrate on bulk morphology and vanadium ion transport in ionomer nanocomposites

Abstract

Perfluorosulfonic acid (PFSA) ionomer nanocomposites are a promising solution to address the poor ion selectivity of current membranes utilized in vanadium redox flow batteries. Herein, we investigate the impact of a casting substrate on the nanostructure and vanadium ion transport in bulk ionomer and ionomer nanocomposite membranes (i.e., films with thicknesses of ∼100 μm). Specifically, solution-cast ionomer nanocomposite membranes, containing either unfunctionalized (hydroxyl groups), amine-functionalized, or sulfonic acid-functionalized silica nanoparticles (SiNPs), were fabricated by casting on either a polished quartz or polytetrafluoroethylene (PTFE) substrates. Surprisingly, the choice of the casting substrate was seen to affect the bulk morphology of the PFSA ionomers, resulting in substrate-specific vanadium ion transport, where suppressed ion transport was observed for membranes cast on the polished quartz, when compared to their PTFE-cast counterparts. Additionally, the chemical composition of the substrate-adjacent surface was a function of both the substrate and the surface functionality of the SiNPs. Moreover, it was observed that both the chemical composition of the membrane surface and the substrate-induced changes to the bulk ionomer morphology governed vanadyl ion transport through the PFSA ionomers. Results from this work have direct implications for the design of next-generation ionomer nanocomposites, as the casting substrate used to fabricate these materials, and the orientation of these membranes inside the operating flow battery, can significantly influence transport of vanadium ions.