Thickness Dependence of Acidity and Microstructure in Nafion Films

Abstract

AbstractThin films of Nafion of various thickness viz. 6 and 1 µm, 280, 80 and 33 nm have been probed using the fluorescent molecule 2‐(2′‐pyridyl)benzimidazole (2PBI), at low as well as high hydration levels. The acidity of these films has been found to be less than that of Nafion 117 membrane at hydrated conditions. This has been ascribed to the high water uptake of the films compared to the membrane and the consequent decrease in the local concentration of protons. The fluorophore reports a significant decrease in the acidity of the film upon annealing them, for all values of thickness. Surprisingly, the acidity in annealed film is lesser than in nonannealed films in which H3O+ ions are exchanged with Na+ ions. Acid treatment of the annealed films causes a reversal of this effect, except for the film with thickness less than 80 nm. Thus, pockets inaccessible to proton appear to form in the ultrathin films. The effective proton concentration depends on the thickness of the films and decreases with decrease in thickness of the films. This may be due to the association of protons to sulfonate ions at lower thickness.