Anomalous, Multistage Liquid Water Diffusion and Ionomer Swelling Kinetics in Nafion and Nafion Nanocomposites

Abstract

The advent of commercially viable vanadium redox flow batteries has generated interest in improving upon existing membrane materials utilized in this grid-scale energy storage technology. Elucidating structure–transport relationships in perfluorosulfonated ionomer nanocomposites is of particular importance for the development of next-generation membranes, as there is a limited understanding of how nanoparticles alter transport in these membranes. In the present study, we attempt to resolve the impact of silica nanoparticles (SiNPs) on liquid water transport in Nafion–SiNP membranes. Specifically, liquid water sorption and ionomer swelling kinetics in a series of Nafion–SiNP membranes were evaluated using time-resolved attenuated total reflectance–Fourier transform infrared spectroscopy. Anomalous, multistage water uptake and swelling kinetics were observed for both Nafion and Nafion–SiNP membranes at all SiNP loadings. The first stage of the kinetic data was regressed to a diffusion–relaxation model, as water diffusion and resulting diffusion-induced polymer relaxation kinetics during this first stage were found to be highly coupled. Suppressed water transport and swelling kinetics were observed with the introduction of SiNPs, though this trend did not hold true for higher SiNP loadings. Thermal annealing of the membranes was also observed to impact the transport and swelling properties of the Nafion and Nafion–SiNP membranes, exhibiting a synergistic effect with the SiNPs at low nanoparticle loadings. Finally, the multistage water uptake mechanism in dry Nafion and Nafion–SiNP nanocomposites was understood to be governed by the time-dependent local water activity in the membrane. Overall, this study presents a clear framework that can be employed to characterize and tune aqueous transport through Nafion nanocomposite membranes.