Perfluorinated surfactants as model charged systems for understanding the effect of confinement on proton transport and water mobility in fuel cell membranes. A study by QENS

摘要

We have investigated the dynamical properties of water confined in mesomorphous phases of perfluorinated sulfonic surfactants. These systems mimic the physico-chemical properties of the perfluorinated Nafion membranes which are used as electrolyte in fuel cells. As the surfactants offer the advantage to self-assemble in well defined organized phases (such as hexagonal and lamellar phases), they could be used as model charged systems to understand the structure-transport relationship in complex real materials. Indeed, the geometry as well as the typical confinement size can be easily controlled and tuned through water concentration and temperature. A QENS study of hexagonal and lamellar phases has been performed on both time-of-flight and backscattering spectrometers to cover a dynamic range from picoseconds to nanoseconds. Analysis of the data with localized translational diffusion models shows the existence of a strong confinement effect that depends on the geometry. Typical confinement sizes and diffusion coefficients can be extracted from the QENS analysis and compared to the Nafion membrane.