(Invited) Water and Proton Dynamics in Perfluorinated Surfactants and Membranes by QENS

Abstract

The proton transport in fuel cell ionomer membranes is intimately related to the dynamical properties of water molecules confined in the interconnected network of ionic domains. The confinement of a fluid at the nanoscale profoundly affects the molecular dynamics and results in important deviations with respect to the bulk. The rotational and translational motions can be uniquely investigated by means of Quasi Elastic Neutron Scattering experiments, especially in the case of protonated systems due to the high incoherent cross section of hydrogen. Insights into the nature of the motions, i.e. diffusive or confined, can be gained by analysing the scattering function S(Q,w) measured on the ps-ns time-scale, Q being the momentum transfer and w the energy transfer of the neutrons. We will present here a comprehensive QENS study of water/proton dynamics in PFSA membranes [1,2] and perfluorinated surfactants used as a model soft confining system [3.4]. The Gaussian model for localized translational diffusion [5] was used to obtain quantitative parameters as the relaxation times, confinement sizes and local/nanometric diffusion coefficients of protons and water molecules. The comparison between state-of-the art fuel cell membranes and self-assembled well-defined surfactant phases as host confining matrices will be used to highlight the subtle interplay between spatial confinement and local interactions, resulting in the presence of slow hydronium motions and faster water motions localized in nanometric droplets. Additionally, we will show that the QENS technique can be invaluably combined with PFG-NMR to obtain a multi-scale comprehensive description of the ionic diffusion and water behaviour [1,2]. These experimental results are also cross-fertilized by Molecular Dynamics simulations performed on the same materials using a coarse-grained backbone and explicit solvent molecules [6]. [1] J-C. Perrin, S. Lyonnard and F. Volino; Quasielastic neutron scattering study of water dynamics in hydrated nafion membranes, Journal of Physical Chemistry C, 111 (2007), 3393-3404.[2] S. Lyonnard and G. Gebel, Neutrons for Fuel Cells Membranes: structure, sorption and transport properties, European Physical Journal 213 (1) (2012), 195-211;S. Lyonnard, Structure and Transport Properties in Polymer Electrolyte Membranes Probed at Microscopic Scales, Springer-Verlag, New Energies, Ed. German Antonio Ferreira, 2013. [3] S. Lyonnard, Q. Berrod, B-A. Bruning, G. Gebel, A. Guillermo, H. Ftouni, J. Ollivier and B. Frick, 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., Eur. Phys. Journal Special Topics, 189 (1), 205-216 (2010). [4] Q. Berrod; S. Lyonnard ; A. Guillermo.;J.Ollivier.; B. Frick; G.Gebel, QENS investigation of proton confined motions in hydrated perfluorinated sulfonic membranes and self-assembled surfactants, The European Physical Journal 2014, in press. [5] F. Volino, J-C. Perrin and S. Lyonnard; Gaussian Model for Localized Translational Motion: Application to Incoherent Neutron Scattering, Journal of Physical Chemistry B, 110 (2006), 11217-11223. [6] S. Hanot, S. Lyonnard and S. Mossa, Water confined in self-assembled ionic surfactants nanostructures, under review (2014).