Elucidating the Impact of Silica Nanoparticles on Water Transport and Local Chain Relaxation Dynamics in Ionomer Nanocomposites

Nafion-silica nanocomposites (NSiNCs) have garnered interest as the proton exchange membrane in vanadium redox flow batteries as they help to combat the issue of high vanadium ion crossover, which results in a reduction of battery lifetime. To date, however, the mechanism of crossover reduction in these NSiNCs remains unclear. We expect the water-mediated ion transport to be coupled to the local chain relaxation dynamics and propose that silica nanoparticles (SiNPs) impact this interaction. To elucidate the relationship between water/ion transport and chain dynamics, we employ neutron spin echo (NSE) spectroscopy to quantify segmental relaxation dynamics and time-resolved attenuated total reflectance-Fourier transform infrared (tATR-FTIR) spectroscopy to evaluate the water sorption kinetics. Results from NSE suggest a decrease in segmental relaxation dynamics with introduction of SiNPs, while tATR-FTIR data indicate deviation from Fickian transport for all samples investigated. These sorption data were regressed to a diffusion-relaxation model to provide further insight into the effect of SiNPs on the viscoelastic properties of NSiNCs. Results from this investigation establish a basis for understanding the interplay between water/ion transport and chain dynamics in NSiNCs.