Influence of different perfluorinated anion based Ionic liquids on the intrinsic properties of Nafion (R)
Résumé
This work investigates the impact of chemical nature of a Proton Conducting Ionic Liquid (PCIL) as a dopant on the intrinsic functional properties of Nafion membrane. The various PCILs utilized for this study were composed of the same cation i.e. triethylammonium but different anions based on alkylperfluorosulfonates with varying alkylperfluoro chain length between 1 to 8 carbon atoms. Depending on the chain length of the alkylperfluorosulfonate moiety of the PCIL as well as the final PCIL concentration, the resulting doped membranes exhibited very different morphology and functional properties. Small Angle Neutron Scattering (SANS) analysis clearly showed that the organization of the PCIL molecules in the Nafion-TEA membrane changes as a function of chain length of the alkylperfluorosulfonate moiety. At 20% wt of PCIL, contrarily to Cl doped membranes, which exhibit an important swelling of ionic domains, the C8 doped membranes present any evolution of SANS spectrum in comparison to that of pristine Nafion-TEA.
Introduction of Triethylammmonium Trifluorosulfonate (C1) molecules into the polymer matrix induces a decrease in the 13 relaxation temperature of Nafion-TEA. On the other hand, incorporation of Triethylammonium Perfluorinated Butane Sulfonate (C4) and Triethylammonium Perfluorinated Octane Sulfonate (C8) leads to a shift of this transition towards higher temperature. The ionic conductivities of membranes doped with Cl are very close to those based on C4 at similar lambda values. The incorporation of PCIL into Nafion-TEA matrix induces an improvement in the water sorption properties of the doped membranes. Yet significant deviation from additivity law is observed depending on the chemical nature of the PCIL. All PCIL doped membranes exhibit gas permeability coefficients analogous to the pristine reference membrane except when a crystalline PCIL phase is formed within the doped matrix especially at high PCIL concentration. An increase in the gas permeability is observed for such membranes, which is assigned to the presence of gas diffusion pathways around those crystalline PCIL domains.