SAXS Investigation of the Effect of Freeze/Thaw Cycles on the Nanostructure of Nafion® Membranes

In this study, we performed small-angle X-ray scattering (SAXS) to investigate the structure of Nafion (R) membranes. The effect of freeze/thaw (F/T) cycles (from ambient temperature down to -40 degrees C) on the membrane nanostructure was considered for the first time. The SAXS measurements were taken for different samples: a commercial Nafion (R) 212 membrane swollen in water and methanol solution, and a water-swollen silica-modified membrane. The membrane structure parameters were obtained from the measured SAXS profiles using a model-dependent approach. It is shown that the average radius of water channels (R-wc) decreases during F/T cycles due to changes in the membrane structure as a result of ice formation in the pore volume after freezing. The use of water-methanol solution (methanol content of 20 vol.%) for the membrane soaking prevents changes in the membrane structure during F/T cycles compared to the water-swollen membrane. Modification of the membrane surface with silica (SiO2 content of 20 wt.%) led to a redistribution of water in the membrane volume and resulted in a decrease in R-wc. However, R-wc for the modified membrane did not decrease with the increasing number of F/T cycles due to the involvement of SiO2 in the sorption of membrane water and, therefore, the prevention of ice formation.

Automated summary

This study investigated the structure of Nafion (R) membranes using small-angle X-ray scattering (SAXS). The effect of freeze/thaw cycles on the membrane nanostructure was considered for the first time. It was found that the average radius of water channels decreased during freeze/thaw cycles, but could be prevented by soaking the membrane in a water-methanol solution or modifying the membrane with silica.