Effect of SiO2 on the dynamics of proton conducting [Nafion/(SiO2)X] composite membranes: a solid-state 19F NMR study

Composite membranes, consisting of Nafion and inorganic oxide additives, are frequently discussed alternative materials to overcome the known low conductivity of pure Nafion at temperatures above 100 degrees C and at low relative humidity. It has been reported that under dry conditions, these membranes show enhanced water uptake and diffusion as compared to filler-free Nafion. This work focuses on the polymer mobility in Nafion/SiO2 composites and on the impact of the silica particles on the polymer dynamics. [Nafion/(SiO2) X] composite membranes (with X ranging from 0 to 15 wt%) in the dry and wet states were investigated by variable temperature solid-state F-19 NMR spectroscopy. F-19 T-1 and T-1 rho relaxation times, and NMR lineshapes (linewidths and spinning sideband intensities) were analyzed to get information about the polymer mobility. It is found that Nafion composite membranes, in general, possess a higher mobility as compared to recast Nafion which is in agreement with previous results from conductivity studies. These findings are attributed to the ability of the SiO2 particles to keep more water inside the composite membranes which also leads to a higher mobility of the polymer component. The results are further supported by the experimental F-19{H-1} CP/MAS NMR spectra. It is also shown that the structure of the composite membranes is more stable after dehydration, and possible condensation reactions are diminished in these membranes. In addition, the decrease in ionic exchange capacity after dehydration is less pronounced for the composite membranes as compared to filler-free Nafion.