Nafion Nanocomposite Membranes for Use in Prospective Large-Scale Energy Storage Devices

Due to their ability to suppress the crossover of vanadium ions, there is growing interest in the use of Nafion nanocomposite membranes as the ion conducting phase in vanadium redox flow batteries. However, little is known about the mechanism by which these nanoparticles reduce the crossover of undesired vanadium ions, an issue that reduces battery lifetime and efficiency. Furthermore, there is a limited understanding of how the surface chemistry of embedded nanoparticles, and in turn the interaction between the Nafion matrix and the nanoparticles, affects vanadium crossover in these membranes. This investigation seeks to elucidate how the presence of silica (SiO₂) nanoparticles in Nafion impacts both water/ion transport and polymer chain relaxation dynamics since these membrane properties are intimately coupled. Specifically, liquid water transport and chain relaxation dynamics in hydrated Nafion nanocomposites were characterized using in situ time-resolved Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy and neutron spin echo (NSE) spectroscopy, respectively. Non-Fickian diffusion was observed for the Nafion films, and the water uptake data were regressed to a diffusion-relaxation model to obtain both water diffusivity and water-induced polymer relaxation time constants. In addition to studies on neat (non-functionalized) SiO₂ nanoparticles, water and vanadium ion permeability, measured via ultra-violet visible spectroscopy, in Nafion-SiO₂ membranes were investigated for a variety of nanoparticle surface chemistries. Results from these experiments indicate that the surface chemistry of the SiO₂ nanoparticles impacts the vanadium ion permeability of the Nafion nanocomposite membranes.