(709c) Segmental Dynamics and Water Transport in Nafion-SiO2 Hybrid Membranes | AIChE

(709c) Segmental Dynamics and Water Transport in Nafion-SiO2 Hybrid Membranes

Authors 

Balwani, A. - Presenter, Clemson University
Faraone, A., National Institute of Standards and Technology
Davis, E. M., Clemson University
Perfluorinated ionomers, specifically Nafion, are the most widely utilized polymer electrolyte membranes for vanadium redox flow battery (VRFB) applications due to their high proton conductivity and good chemical stability. However, Nafion suffers from high vanadium ion crossover, an undesired event that reduces energy efficiency and lifetime of the battery. Incorporation of silica (SiO2) nanoparticles into the Nafion membrane has proven to be effective in suppressing crossover of vanadium ions. However, the mechanism of this reduction in crossover is still unclear, making it impossible to rationally design cheaper and bettering performing membranes for VRFB applications. To understand the effect of SiO2 nanoparticles on both segmental dynamics and transport properties of the hybrid membranes, several spectroscopic techniques were employed to characterize hydrated Nafion-SiO2 hybrid films. Hybrid membranes were prepared by an in situ sol gel condensation process of SiO2 in Nafion 117 membranes, as well as by solution casting Nafion-SiO2 dispersions on Teflon substrates. Water transport was characterized by in situ time-resolved Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy, which provides molecular-level contrast between the diffusant and the polymer in real time during diffusion experiments, allowing for the simultaneous capture of both water transport and water-induced polymer relaxation (swelling). The diffusion of water through the hybrid membranes was found to be non-Fickian and the kinetic uptake data were regressed to a diffusion-relaxation model to obtain both water diffusivity and polymer relaxation dynamics. Further, the spectra were deconvoluted to resolve the morphological rearrangements within the membrane due to water sorption. In addition, the chain relaxation dynamics of the hybrid membranes, which is attributed to fluctuations on the length scale of inter-backbone spacing, were investigated using neutron spin-echo (NSE) spectroscopy. NSE data were collected at two temperatures, 298 K and 323 K, and these results were compared to unmodified Nafion with varying thermal treatments.