Uncovering the Structure of Nafion-SiO2 Hybrid Ionomer Membranes for Prospective Large-Scale Energy Storage Devices

Nafion nanocomposite membranes are attractive candidates for the ion conducting phase in energy storage devices such as vanadium redox flow batteries. Herein, vanadium crossover and Nafion-SiO2 nanostructure are quantified in a series of hybrid membranes created via solution-casting Nafion films with discrete SiO2 nanoparticles, as well as membranes created using an in situ silica sol-gel condensation process. The crossover of vanadium ions is suppressed in all Nafion membranes with the SiO2 inorganic phase when compared to unannealed, neat Nafion membranes. However, it is also observed that annealing the neat Nafion membranes is equally as effective at suppressing vanadium crossover. Small-angle neutron scattering measurements show that no significant changes to the Nafion structure occurred in membranes with discrete nanoparticles. In contrast, drastic changes in the scattering profiles of the Nafion-SiO2 membranes created via sol-gel chemistry are observed, where the SiO2 nanoclusters are determined to be on the order of 10 nm in diameter. These scattering length scales are verified through real space images using transmission electron microscopy. Insights from this investigation help elucidate the structure of the Nafion-SiO2 membranes and suggest that the current hypothesis by how vanadium crossover is reduced may not be fully accurate.