(470e) Impact of Sulfonation Degree and Nanoparticle Surface Chemistry on Ion Selectivity in Sulfonated Ionomer Nanocomposites

Sulfonated ionomer membranes have emerged as a promising proton exchange membrane (PEM) for vanadium redox flow batteries (VRFBs), due to their low cost, as well as their comparable mechanical properties and chemical resistance to that of the current benchmark material, Nafion. While membrane materials like sulfonated poly (ether ether ketone) (SPEEK)-based nanocomposite membranes have been thoroughly studied for use in fuel cells, important processing-performance property relationships investigations of SPEEK for their use in VRFBs remains insufficient. In this study, a series of SPEEK membranes were fabricated and the water and vanadium ion transport properties of the membranes were characterized. Specifically, the SPEEK nanocomposite membranes were prepared with degrees of sulfonation (DS) ranging from 30% to 80%, as well as nanoparticles (NPs) loadings ranging from 0 mass% to 10 mass% to elucidate how these design parameters affect the ion selectivity (i.e., proton conductivity divided by vanadium ion permeability) of this class of ionomer nanocomposites. The NPs (approximately 10 nm in diameter) were functionalized with both cationic (e.g., amine) and anionic (e.g., sulfonic acid) surfaces such that they interact electrostatically (both attractive and repulsive) with the fixed charges on the phenyl groups along the backbone of the SPEEK. The DS was observed to be well controlled by duration of the sulfonation reaction and was measured via both titration and ¹H NMR. The permeability of vanadium ions (specifically the vanadyl ion) through the ionomer nanocomposites was measured via ultraviolet-visible spectroscopy, where it was observed that a minimum DS was needed to observe any crossover of vanadyl ions, for both pristine SPEEK and SPEEK nanocomposites. Additionally, the through-plane proton conductivity of the membranes was measured using a custom-built 4-point conductivity test. Furthermore, the NP dispersion state was characterized by transmission electron microscopy. Results from this investigation establish important processing-performance property relationships for sulfonated ionomer nanocomposites, helping to facilitate the development of novel, better performing ionomer membranes for VRFBs.