(255j) Relaxation Dynamics and Water Transport in Nafion-SiO2 Nanocomposite Membranes

Perfluorinated ionomers, Nafion in particular, are the most widely utilized proton exchange membranes in vanadium redox flow battery applications due to its 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. While the introduction of SiO₂ nanoparticles into the Nafion membrane has proven to be effective at suppressing crossover of vanadium ions, little is understood about the mechanism by which this reduction in crossover occurs. In this study, the relaxation dynamics of Nafion-SiO₂ nanocomposite membranes, in the presence of water, was investigated using a number of spectroscopic techniques. Water transport experiments in these hybrid membranes were carried out using in situ time-resolved Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy. Unlike typical gravimetric based techniques, time-resolved FTIR-ATR spectroscopy 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 nanocomposite membranes was found to be non-Fickian and the kinetic uptake data was modeled using a diffusion-relaxation model. In addition, the chain relaxation dynamics of the nanocomposite membranes was investigated using neutron spin-echo (NSE) spectroscopy, a technique that is used to study slow relaxational processes that occur in polymers. Scattering data for all membranes were attained at q â?? 1.2 Ã?^-1, meaning that the measured dynamics of the nanocomposite membranes can be attributed to fluctuations on the length scale of the backbone of the Nafion. NSE spectroscopy data was collected at two temperatures, 298 K and 348 K, and the results were compared to unmodified Nafion with varying thermal treatments.