Gérard Gebel

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.

3 In situ and operando determination of the water content distribution in proton conducting membranes for fuel cells: a critical review

Proton exchange membrane fuel cells have been recognized as a promising zero-emission power source for portable, mobile and stationary applications. The information of water content distribution in the different components of the cell during operation, particularly the proton conducting membrane, is a critical issue for the validation of mass transfer models, the definition of optimized operating conditions and the development of efficient systems with innovative designs for efficient water management. In order to fully understand the way a fuel cell performs, water transport and distribution have to be investigated in situ and operando. In this review, we critically examine the state-of-the-art of operando diagnostics sensitive to the membrane water content, particularly those techniques able (in principle) to give insights into water transport occurring along both the in- and through-plane directions. Particular attention is devoted to experimental results obtained across the membrane thickness i.e. to the determination of water concentration profiles originating from the water activity and electrical gradients occurring through the working fuel cell. Different operando techniques have been developed for this purpose, from the early 1990s up to the last few years: internal resistance measurements, magnetic resonance and neutron imaging, neutron and X-ray scattering, confocal μ-Raman spectroscopy. These techniques can be roughly separated as either direct (i.e. the water amount can be directly derived from the detected signal, avoiding sometimes arbitrary assumptions during data processing) but intrusive (i.e. they require significant modification of the fuel cell, compared to the current design and materials) or indirect but with a significantly lower intrusiveness. It appears that operando measurements of the membrane water distribution allow a unique picture of how the internal part of the fuel cell works, thus certainly contributing to the development of more effective cell designs and materials in the near future. Nevertheless, improvement in the fundamental understanding of the actual fuel cell requires further efforts to increase spatial and, more particularly, temporal resolution of current operando techniques. Also, the comparison of limitations arising from the basic principles of the different operando approaches suggests that ultimate progress will arise from the combination of complementary techniques for simultaneous measurements.

Inhomogeneous Transport in Model Hydrated Polymer Electrolyte Supported Ultrathin Films

The structure of polymer electrolyte membranes, e.g., Nafion, inside fuel cell catalyst layers has significant impact on the electrochemical activity and transport phenomena that determine cell performance. In those regions, Nafion can be found as an ultrathin film, coating the catalyst and the catalyst support surfaces. The impact of the hydrophilic/hydrophobic character of these surfaces on the structural formation of the films and, in turn, on transport properties has not been sufficiently explored yet. Here, we report classical molecular dynamics simulations of hydrated Nafion thin films in contact with unstructured supports, characterized by their global wetting properties only. We have investigated structure and transport in different regions of the film and found evidence of strongly heterogeneous behavior. We speculate about the implications of our work on experimental and technological activity.

Sulfonic and Phosphonic Acid and Bifunctional Organic–Inorganic Hybrid Membranes and Their Proton Conduction Properties

Hybrid organic-inorganic approaches are used for the synthesis of bifunctional proton exchange membrane fuel cell (PEMFC) membranes owing to their ability to combine the properties of a functionalized inorganic network and an organic thermostable polymer. We report the synthesis of both sulfonic and phosphonic acid functionalized mesostructured silica networks into a poly(vinylidenefluoride-co-hexafluoropropylene) (poly(VDF-co-HFP) copolymer. These membranes, containing different amounts of phosphonic acid and sulfonic acid groups, have been characterized using FTIR and NMR spectroscopy, SA-XRD, SAXS, and electrochemical techniques. The proton conductivity of the bifunctional hybrid membranes depends strongly on hydration, increasing by two orders of magnitude over the relative humidity (RH) range of 20 to 100%, up to a maximum of 0.031 S cm(-1) at 60 °C and 100% RH. This value is interesting as only half of the membrane conducts protons. This approach allows the synthesis of a porous SiO(2) network with two different functions, having -SO(3)H and -PO(3)H(2) embedded in a thermostable polymer matrix.