(255h) Surface Functionalization of SiO2 Nanoparticles in Nafion Nanocomposite Membranes for Use As Proton Exchange Membranes in Vanadium Redox Flow Batteries

Nafion nanocomposite membranes have received considerable attention for use as the proton conducting phase in vanadium redox flow batteries as a result of their ability to suppress vanadium ion crossover, an undesired event that occurs during battery operation, while maintaining a similar proton conductivity to that of the unmodified membrane. Although several studies have utilized nanoparticles with relatively simple surface chemistries, little has been done in the way of tailoring the surface functionalization of these nanoparticles to systematically control the interactions between Nafion and the nanoparticles. In this study, the effect of surface functionalization of silica nanoparticles on vanadium crossover in solution-cast Nafion nanocomposite membranes was investigated. Specifically, the permeability of vanadium ions across Nafion-SiO₂ membranes with sulfonic acid functionalized silica nanoparticles was measured using ultraviolet-visible spectroscopy. Different sulfonic acid supports were investigated, ranging from a relatively flexible, short saturated chain to a significantly less flexible chain containing aromatic groups. The nanoparticle loading and diameter were also varied in an attempt to better understand how nanoparticle-Nafion interactions act to reduce vanadium ion crossover in these nanocomposite films. In addition, the nanoparticle surface was grafted with polymers, e.g., polyvinylidene fluoride and polytetrafluoroethylene, to investigate the effect of surface chemistry on nanoparticle dispersion within the Nafion membrane. Results from this study indicate that the surface functionalization of the silica nanoparticles plays an important role in controlling the permeation of vanadium ions across these nanocomposite membranes.