Title: Water sub-diffusion in membranes for fuel cells,
Abstract: We investigate the dynamics of water confined in soft ionic nano-assemblies,
an issue critical for a general understanding of the multi-scale
structure-function interplay in advanced materials. We focus in particular on
hydrated perfluoro-sulfonic acid compounds employed as electrolytes in fuel
cells. These materials form phase-separated morphologies that show outstanding
proton-conducting properties, directly related to the state and dynamics of the
absorbed water. We have quantified water motion and ion transport by combining
Quasi Elastic Neutron Scattering, Pulsed Field Gradient Nuclear Magnetic
Resonance, and Molecular Dynamics computer simulation. Effective water and ion
diffusion coefficients have been determined together with their variation upon
hydration at the relevant atomic, nanoscopic and macroscopic scales, providing
a complete picture of transport. We demonstrate that confinement at the
nanoscale and direct interaction with the charged interfaces produce anomalous
sub-diffusion, due to a heterogeneous space-dependent dynamics within the ionic
nanochannels. This is irrespective of the details of the chemistry of the
hydrophobic confining matrix, confirming the statistical significance of our
conclusions. Our findings turn out to indicate interesting connections and
possibilities of cross-fertilization with other domains, including biophysics.
They also establish fruitful correspondences with advanced topics in
statistical mechanics, resulting in new possibilities for the analysis of
Neutron scattering data.