Impregnation of waterwheel supramolecules as proton carriers in Nafion-perfluorinated ionomer membranes

To improve proton conduction at elevated temperatures, in situ impregnation of Nafion membranes has been carried out by infusing Noria “waterwheel” supramolecules, containing numerous hydroxyl terminal groups, into the ionic domains of Nafion via swelling in mixed methanol and dimethylacetamide solutions. Fourier transform infrared (FTIR) spectroscopy study reveals that interspecies hydrogen bonding occurs between hydroxyl groups of Noria and sulfonate groups of Nafion, which has facilitated retaining the modifier molecules within the membrane. Water uptake experiments exhibit that the impregnation of Noria into Nafion ionic domains suppresses the membrane swelling. The ion exchange capacity also increases upon this impregnation. The proton conductivity is reduced at low operating temperatures relative to neat Nafion due to the loss of hydronium ion transport. However, the proton conductivity of the Noria-impregnated membrane shows 60 % improvement over that of neat Nafion at elevated temperatures of 115 °C. Of particular importance is that the Noria-impregnated membrane exhibits improved thermal, mechanical, and electrochemical stabilities with proton conductivity enhancement at elevated temperatures. Moreover, there is no noticeable difference in FTIR spectra before and after the proton fuel cell tests, indicating the improvement in the chemical stability of the Noria-impregnated membranes under the present proton fuel environment. It appears that these waterwheel supramolecules may have potential utility as high temperature electrolytes (or solid proton carriers) in proton fuel cells.