Impact of Sulfonation Degree and Nanoparticle Surface Chemistry on Ion Selectivity in Sulfonated Ionomer Nanocomposites*

In this study, a series of sulfonated poly(ether ether ketone) (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 60% to 80%, as well as nanoparticle (NPs) loadings ranging from 0 mass% to 10 mass% to elucidate how these design parameters affect the ion selectivity of these ionomer nanocomposites. The NPs were functionalized with both cationic and anionic surfaces such that they interact electrostatically 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 ¹H NMR. The permeability of vanadium through the ionomer nanocomposites was measured via ultraviolet-visible spectroscopy. Additionally, the through-plane proton conductivity of the membranes was measured. 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.