Mise en évidcence de la dégradation du liant ionomère dans les électrodes de pile à combustible

The purpose of this study was to follow the behavior of ionomer binder after fuel cell operation. First, a series of techniques were used to investigate to characterize ionomer in electrode at solid state. Most of them were inefficient to study ionomer degradation. Only X-Ray Diffraction (XRD) pointed out a structural change of the binder in electrodes. Second, it has been decided to characterize ionomer in liquid state after water extraction. But first of all, a quantitative 19F NMR protocol composed of two to three steps, with a first step using a quantification through signal-to-noise ratio (S/N), was establish in order to quantify Nafion® and degradation products. Soxhlet extraction performed on electrodes allowed to detect a degradation product water-soluble. Finally, extraction with organic solvent (Dimethylacetamide) allowed to extract Nafion® and two acid: trifluoroacetic acid (TFA) and triflic acid (TFI) from electrodes. In conclusion, correlation between observation and literature allowed us to propose a degradation mechanism of ionomer in electrodes.Ce travail de thèse a pour but de suivre le comportement du liant ionomère après vieillissement en condition réelle d'utilisation de la pile. Dans un premier temps, diverses techniques de caractérisation en phase solide ont été utilisées afin d'étudier le ionomère présent dans les électrodes. La majeure partie de ces techniques se sont avérées insatisfaisantes pour le suivi du vieillissement du ionomère. Seule l'analyse par diffraction rayon X (DRX) a mis en avant un changement d'organisation structurale du ionomère dans les électrodes. Par la suite, nous avons choisi de caractériser le ionomère en solution après extraction par l'eau. Préalablement, un protocole de quantification en 2 à 3 étapes, dans lequel intervient une quantification via le rapport signal-sur-bruit (S/N), a été mis en place afin de quantifier le Nafion® et autres petites molécules fluorés. L'extraction Soxhlet a ensuite été réalisée sur les électrodes permettant de révéler la présence d'un produit de dégradation hydrosoluble après fonctionnement en pile, à la fois en cathode et en anode. Enfin, la macération des électrodes dans le diméthylacétamide (DMAc) a permis d'extraire le polymère Nafion® ainsi que deux acides : l'acide trifluoroacétique (TFA) et l'acide triflique (TFI). En conclusion, la corrélation de l'ensemble des observations nous a permis de proposer un mécanisme de dégradation du liant ionomère présent dans les électrodes

NMR contributions to the study of water transfer in proton exchange membranes for fuel cells

As programs to support efficient and sustainable energy sources are expanding, research into the potential applications of the hydrogen vector is accelerating. Proton exchange membrane fuel cells are electrochemical converters that transform the chemical energy of hydrogen into electrical energy. These devices are used today for low- and medium-power stationary applications and for mobility, in trains, cars, bicycles, etc. Proton exchange membrane fuel cells use a polymer membrane as the electrolyte. The role of the membrane is multiple: it must separate gases, be an electronic insulator and a very good ionic conductor. In addition, it must resist free-radical chemical attack and have good mechanical strength. Nafion-type perfluorinated membranes have all these properties: the fluorinated backbone is naturally hydrophobic, but the hydrophilic ionic groups give the material excellent water sorption properties. The water adsorbed in the structure is extremely mobile, acting as a transport medium for the protons generated at the anode. Although it has been studied for a long time and has been the subject of a large number of papers perfluorinated membranes are still the reference membranes today. This article reviews some contributions of Nuclear Magnetic Resonance methods in liquid state to the study of water properties in the structure of Nafion-type perfluorinated membranes

Evidence for degradation of ionomer binder in electrodes of fuel cell

Ce travail de thèse a pour but de suivre le comportement du liant ionomère après vieillissement en condition réelle d'utilisation de la pile. Dans un premier temps, diverses techniques de caractérisation en phase solide ont été utilisées afin d'étudier le ionomère présent dans les électrodes. La majeure partie de ces techniques se sont avérées insatisfaisantes pour le suivi du vieillissement du ionomère. Seule l'analyse par diffraction rayon X (DRX) a mis en avant un changement d'organisation structurale du ionomère dans les électrodes. Par la suite, nous avons choisi de caractériser le ionomère en solution après extraction par l'eau. Préalablement, un protocole de quantification en 2 à 3 étapes, dans lequel intervient une quantification via le rapport signal-sur-bruit (S/N), a été mis en place afin de quantifier le Nafion® et autres petites molécules fluorés. L'extraction Soxhlet a ensuite été réalisée sur les électrodes permettant de révéler la présence d'un produit de dégradation hydrosoluble après fonctionnement en pile, à la fois en cathode et en anode. Enfin, la macération des électrodes dans le diméthylacétamide (DMAc) a permis d'extraire le polymère Nafion® ainsi que deux acides : l'acide trifluoroacétique (TFA) et l'acide triflique (TFI). En conclusion, la corrélation de l'ensemble des observations nous a permis de proposer un mécanisme de dégradation du liant ionomère présent dans les électrodes.The purpose of this study was to follow the behavior of ionomer binder after fuel cell operation. First, a series of techniques were used to investigate to characterize ionomer in electrode at solid state. Most of them were inefficient to study ionomer degradation. Only X-Ray Diffraction (XRD) pointed out a structural change of the binder in electrodes. Second, it has been decided to characterize ionomer in liquid state after water extraction. But first of all, a quantitative 19F NMR protocol composed of two to three steps, with a first step using a quantification through signal-to-noise ratio (S/N), was establish in order to quantify Nafion® and degradation products. Soxhlet extraction performed on electrodes allowed to detect a degradation product water-soluble. Finally, extraction with organic solvent (Dimethylacetamide) allowed to extract Nafion® and two acid: trifluoroacetic acid (TFA) and triflic acid (TFI) from electrodes. In conclusion, correlation between observation and literature allowed us to propose a degradation mechanism of ionomer in electrodes

Chemical degradation of PFSA ionomer binder in PEMFC's catalyst layer

International audienc

Investigation of perfluorosulfonic acid ionomer solutions by 19 f NMR and DLS: Establishment of an accurate quantification protocol

International audienc

Towards a NMR-Based Method for Characterizing the Degradation of Nafion XL Membranes for PEMFC

International audiencePFSA-based reinforced membranes are used today as the benchmark material for the electrolyte in PEMFCs. Although greatly improved relatively to their unreinforced version, they still suffer from aging and degradation during fuel cell (FC) operation. In this study we first performed proton NMR to characterize the different water populations in the pristine Nafion XL reinforced membrane. Then we used proton and fluorine NMR, FTIR and sorption measurements in order to qualitatively observe the differences induced in the membrane’s chemical structure and properties by long term FC operation. Proton NMR is seen to be an adapted tool to quickly measure a signature that is correlated to the degradation state while FTIR can serve as a local probe of the chemical structure

Impact of the chemical degradation on transport properties in Nafion membrane after fuel cell operation

International audienc

Expertise des ionomères perfluorés à fonction acide sulfonique (PFSA) par spectroscopies Infrarouge et Raman

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Water transport in Nafion membranes under various conditions studied by NMR

International audienceIn a Proton Exchanging Membrane Fuel Cell (PEMFC), the membrane is at the heart of the assembly and is the place where electrochemical reactions happen. As water mobility governs these reactions, the Nafion polymer has been studied by NMR techniques (imaging, self diffusion…), but mostly outside a fuel cell. However, in an operating fuel cell, the membrane is exposed to multiple mechanicals constraints (hydration, swelling…). Therefore, we wanted to study the effect of such conditions on the water transport in the polymer.Three boundary conditions were investigated using different NMR techniques:-Stretching: Self diffusion experiments were performed on membranes under traction. Using a specially designed traction apparatus, we were able to measure diffusion anisotropy and to compare the results with experiments on membranes stretched outside the spectrometer. We demonstrated that this anisotropy is much higher in the case of in-situ traction in comparison with membranes stretched at high temperature before the experiment. We also developed a simple deformation model of the membrane in order to analyze the experimental results.-Compression: We used a home-made pressure chamber compatible with NMR experiments which allows us to apply on the membrane a pressure up to 140 bars. We then performed chemical shift, self diffusion and Single Point Imaging (SPI) experiments to see the effect of this pressure on the water transport. First results show that this pressure induces a decrease in water content and a slight reduction off the diffusion coefficient. Unlike stretching experiments, we were not able to observe any diffusion anisotropy.-Hydration and drying: We inserted the membrane inside a hydration cell inside the magnet. This device allows us to control the air humidity with the possibility to have different conditions on each side of the sample. We then performed SPI experiments across the membrane plane during hydration or drying phases. This allows us to visualize area in the membrane center with lower hydration and to extract drying and hydration kinetics constants from these experiments

Impact of a Compressive Stress on Water Sorption and Diffusion in Ionomer Membranes for Fuel Cells. A 1 H NMR Study in Vapor-Equilibrated Nafion

International audienceThe electrolyte membrane is exposed to high compression loads when operating in a proton exchange membrane fuel cell (PEMFC). It was reported in the literature that compression impacts the membrane properties when immersed in liquid water but little is known about the quantitative effects of stress on water sorption and its dynamical properties when the membrane is at equilibrium with a humid gas. In this work, we investigated the influence of a static normal stress on the properties of Nafion® by pulsed field gradient NMR (PFGNMR) with a NMR compatible compression device. The results demonstrated a reduction in water content due to compression at high relative humidity (≥ 90% RH) and much more moderate effects at low hydration (≤ 85% RH). The changes observed on the diffusion properties are more pronounced and related to both water loss and a reversible modification of the membrane structure effective at the micrometric scale