Sandrine Lyonnard

Analysis of the small-angle intensity scattered by a porous and granular medium

Using X-ray scattering over a large range of scattering vectors, it is shown how to measure both the pore volume fraction and pore specific surface of an assembly of porous grains forming a powder. Depending on the presence or not of solvent in the inner pores and in the intergranular media, the scattered signal per unit volume of solid or per unit volume of grain are introduced, which allow a complete analysis even when the thickness of the layer and its compactness are unknown. The method is applied to three different systems presenting a well defined Porod regime at large scattering vector.

The kinetics of water sorption in Nafion membranes: a small-angle neutron scattering study

The optimization of the water management in proton exchange membrane fuel cells is a major issue for the large-scale development of this technology. In addition to the operating conditions, the membrane water sorption and transport processes obviously control the water management. The main objective of this work is to provide new experimental evidence based on the use of the small-angle neutron scattering (SANS) technique in order to allow a better understanding of water sorption processes. SANS spectra were recorded for membranes equilibrated with either water vapor or liquid. Sorption kinetics data were determined and the SANS spectra were analyzed using the method developed for extracting water concentration profiles across the membrane in operating fuel cells. The water concentration profiles across the membrane are completely flat, which indicates that the water diffusion within the membrane is not the limiting process. This result provides new insight into the numerous data published on these properties. For the first time, the swelling kinetics of a Nafion membrane immersed in liquid water is studied and a complete swelling is obtained in less than 1 min.

Fast Water Diffusion and Long-Term Polymer Reorganization during Nafion Membrane Hydration Evidenced by Time-Resolved Small-Angle Neutron Scattering

We report a small-angle neutron scattering study of liquid water sorption in Nafion membranes. The swelling of hydrophilic domains was measured on the nanoscale by combining in situ time-resolved and long-term static experiments, yielding kinetic curves recorded over an unprecedented time scale, from hundreds of milliseconds to several years. At low water content, typically below 5 water molecules per ionic group, a limited subdiffusive regime was observed and ascribed to nanoconfinement and local interactions between charged species and water molecules. Further ultrafast and thermally activated swelling due to massive liquid water sorption was observed and analyzed by using Ficks equation. The extracted mutual water diffusion coefficients are in good agreement with pulsed field gradient NMR self-diffusion coefficient values, evidencing a water diffusion-driven process due to concentration gradients within the Nafion membrane. Finally, after completion of the ultrafast regime, the kinetic swelling curves exhibit a remarkable long-term behavior scaling as the logarithm of time, showing that the polymer membrane can continuously accommodate additional water molecules upon hydration stress. The present nanoscale kinetics results provide insights into the vapor-versus-liquid sorption mechanisms, the nanostructure of Nafion, and the role of polymer reorganization modes, highlighting that the membrane can never reach a steady state.

Effects of Block Length and Membrane Processing Conditions on the Morphology and Properties of Perfluorosulfonated Poly(arylene ether sulfone) Multiblock Copolymer Membranes for PEMFC.

Perfluorosulfonated poly(arylene ether sulfone) multiblock copolymers have been shown to be promising as proton exchange membranes. The commonly used approach for preparation of the membrane is solvent casting; the properties of the resulting membranes are very dependent on the membrane processing conditions. In this paper, we study the effects of block length, selectivity of the solvent, and thermal treatment on the membrane properties such as morphology, water uptake, and ionic conductivity. DiMethylSulfOxide (DMSO), and DiMethylAcetamide (DMAc) were selected as casting solvents based on the Flory-Huggins parameter calculated by inversion gas chromatography (IGC). It was found that the solvent selectivity has a mild impact on the mean size of the ionic domains and the expansion upon swelling, while it dramatically affects the supramolecular ordering of the blocks. The membranes cast from DMSO exhibit more interconnected ionic clusters yielding higher conductivities and water uptake as compared to membranes cast from DMAc. A 10-fold increase in proton conductivity was achieved after thermal annealing of membranes at 150 °C, and the ionomers with longer block lengths show conductivities similar to Nafion at 80 °C and low relative humidity (30%).

Combining USAXS with osmotic pressure measurements: a tool for investigating the equation of states of diluted charged colloids

We have developed a cell allowing in situ measurement of the osmotic pressure of solutions under Ultra Small-Angle X-ray Scattering (USAXS) examination. The coupled set-up made of a Bonse/Hart camera and a deionisation close circuit allows the combined determination of the structure factor and the equation of state of colloidal suspensions. A precise control of ions concentration inside the reservoir allows to vary the ionic strength of the suspensions from the very pure case of colloidal crystals to sols with tunable screening of electrostatic repulsive interactions, We describe in detail the experimental set-ups and the data treatment methodology that we have been developing in the last years to allow detailed measurements of interactions and correlations between spherical particles. A review of our recent results obtained on the model system of negatively charged parabromostyrene latex particles forming solutions, crystals and microcrystals., is given as an illustration.

Water confined in self-assembled ionic surfactant nano-structures

We present a coarse-grained model for ionic surfactants in explicit aqueous solutions, and study by computer simulation both the impact of water content on the morphology of the system, and the consequent effect of the formed interfaces on the structural features of the absorbed fluid. On increasing the hydration level under ambient conditions, the model exhibits a series of three distinct phases: lamellar, cylindrical and micellar. We characterize the different structures in terms of diffraction patterns and neutron scattering static structure factors. We demonstrate that the rate of variation of the nano-metric sizes of the self-assembled water domains shows peculiar changes in the different phases. We also analyse in depth the structure of the water/confining matrix interfaces, the implications of their tunable degree of curvature, and the properties of water molecules in different restricted environments. Finally, we compare our results with experimental data and their impact on a wide range of important scientific and technological domains, where the behavior of water at the interface with soft materials is crucial.

Water sub-diffusion in membranes for fuel cells.

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.

Controlling Microstructure–Transport Interplay in Highly Phase-Separated Perfluorosulfonated Aromatic Multiblock Ionomers via Molecular Architecture Design

Proton-conducting multiblock polysulfones bearing perfluorosulfonic acid side chains were designed to encode nanoscale phase-separation, well-defined hydrophilic/hydrophobic interfaces, and optimized transport properties. Herein, we show that the superacid side chains yield highly ordered morphologies that can be tailored by best compromising ion-exchange capacity and block lengths. The obtained microstructures were extensively characterized by small-angle neutron scattering (SANS) over an extended range of hydration. Peculiar swelling behaviors were evidenced at two different scales and attributed to the dilution of locally flat polymer particles. We evidence the direct correlation between the quality of interfaces, the topology and connectivity of ionic nanodomains, the block superstructure long-range organization, and the transport properties. In particular, we found that the proton conductivity linearly depends on the microscopic expansion of both ionic and block domains. These findings indicate that neat nanoscale phase-separation and block-induced long-range connectivity can be optimized by designing aromatic ionomers with controlled architectures to improve the performances of polymer electrolyte membranes.

Nanostructured multi-block copolymer single-ion conductors for safer high-performance lithium batteries

The greatest challenges towards the worldwide success of battery-powered electric vehicles revolve around the safety and energy density of the battery. Single-ion conducting polymer electrolytes address both challenges by replacing the flammable and unstable liquid electrolytes and enabling dendrite-free cycling of high-energy lithium metal anodes. To date, however, their commercial use has been hindered by insufficient ionic conductivities at ambient temperature (commonly not exceeding 10−6 S cm−1) and the limited electrochemical stability towards oxidation, in particular when incorporating ether-type building blocks, limiting their application to rather low-voltage cathode materials like LiFePO4. Here, we introduce ether-free, nanostructured multi-block copolymers as single-ion conducting electrolytes, providing high thermal stability and self-extinguishing properties and, if plasticized with ethylene carbonate, ionic conductivities exceeding 10−3 S cm−1 above 30 °C, i.e., approaching that of state-of-the-art liquid electrolytes. Moreover, these single-ion conducting ionomers present highly reversible lithium cycling for more than 1000 h and, as a result of their excellent electrochemical stability, highly stable cycling of Li[Ni1/3Co1/3Mn1/3]O2 cathodes. To the best of our knowledge, this is the first polymer electrolyte that presents such remarkable ionic conductivity and outstanding electrochemical stability towards both reduction and oxidation, thus, paving the way for advanced high-energy lithium metal batteries. Remarkably, the realization of well-defined continuous ionic domains appears to be the key to efficient charge transport through the electrolyte bulk and across the electrode/electrolyte interface, highlighting the importance of the self-assembling nanostructure. The latter is achieved by carefully (i) designing the copolymer structure, i.e., introducing alternating ionic blocks with a very regular distribution of weakly coordinating anions along the polymer chain and rigid blocks, which are completely immiscible with ethylene carbonate, and (ii) choosing the processing solvent, taking into account its interaction with the different copolymer blocks.

Sulfo-phenylated terphenylene copolymer membranes and ionomers.

The copolymerization of a prefunctionalized, tetrasulfonated oligophenylene monomer was investigated. The corresponding physical and electrochemical properties of the polymers were tuned by varying the ratio of hydrophobic to hydrophilic units within the polymers. Membranes prepared from these polymers possessed ion exchange capacities ranging from 1.86 to 3.50 meq g-1 and exhibited proton conductivities of up to 338 mS cm-1 (80 °C, 95 % relative humidity). Small-angle X-ray scattering and small-angle neutron scattering were used to elucidate the effect of the monomer ratios on the polymer morphology. The utility of these materials as low gas crossover, highly conductive membranes was demonstrated in fuel cell devices. Gas crossover currents through the membranes of as low as 4 % (0.16±0.03 mA cm-2 ) for a perfluorosulfonic acid reference membrane were demonstrated. As ionomers in the catalyst layer, the copolymers yielded highly active porous electrodes and overcame kinetic losses typically observed for hydrocarbon-based catalyst layers. Fully hydrocarbon, nonfluorous, solid polymer electrolyte fuel cells are demonstrated with peak power densities of 770 mW cm-2 with oxygen and 456 mW cm-2 with air.