Effect of Electrostatic Interactions Between Nafion and Functionalized Nanoparticles on Ionomer Morphology and Nanoparticle Dispersion

Ionomer nanocomposites have emerged as a promising replacement to traditional polymer electrolyte membranes for technologies like vanadium redox flow batteries as they curtail undesired vanadium ion crossover while maintaining high proton conductivity. However, the mechanism by which the introduction of silica nanoparticles (SiNPs) acts to increase ion selectivity remains elusive. Presently, SiNP surface chemistry was systematically altered to determine the relationship between SiNP dispersion, membrane morphology, and vanadium ion transport, via transmission electron microscopy, small-angle neutron scattering (SANS), and ultraviolet-visible spectroscopy, respectively. Results indicate that the SiNP surface chemistry plays a vital role in controlling their dispersion and resultant ionomer morphology, where changes in SANS data were observed as a function of both surface chemistry and SiNPs loading in the ionomer membrane. Further, it was observed that the SiNP dispersion state had a direct impact on the vanadium ion permeation: SiNP aggregation reduced vanadium ion crossover as compared to well-dispersed particles with the same end functionality. Additionally, the overall NP surface charge is a key factor in controlling vanadium ion permeation through these composite membranes.