Modeling Oxygen Concentration Oscillation in the Gas channel of Polymer Electrolyte Fuel Cells: a Comparison between Numerical and Analytical Approaches

Considering the complexity of the electrical behavior (polarization curve and impedance spectrum) of PEFC and the lack of relevant models that could be used to identify the main transport parameters in Membrane Electrode Assemblies (MEA), a semi-analytical pseudo-bidimensionnal model is currently developed. Its main characteristics are first recalled. Then the results of a full numerical two-dimensional model are compared to those of the pseudo-bidimensionnal approach and analyzed. It is shown that oxygen diffusion inside the GDL can be considered as a 1D diffusion without any appreciable discrepancy and that the electrode can be modeled as a plane without major error either on impedance spectrum or polarization curve. Oxygen diffusion through the gas channel reveals an important discrepancy between volumetric and plane descriptions, leading to the necessity of a more complex model of the fluid flow in the gas channel

NMR study of the anisotropic transport properties of uniaxially stretched membranes for fuel cells

We used NMR techniques to probe the anisotropic properties of stretched Nafion®115membranes. The alignment of the polymeric structure under a uniaxial load is at the origin of a strong anisotropy of both the water self-diffusion coefficient and the proton conductivity. The determination of these two important membrane properties may lead to new fundamental information on the nature of the proton transport mechanisms in such oriented weaklycharged systems

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

Caractérisation de Milieux diffusants en écoulement par transport incohérent de lumière polarisée

In this thesis, we are interested in the general problem of characterizing concentrated dispersions. We develop an optical diffusing method which uses the incoherent polarized light transport. We propose an experimental setup to acquire the Mueller matrix (Matrix 4x4 2D). This matrix, composed by 16 images, represents all interactions between an electromagnetic wave and a diffusing medium. The experimental device is composed of a focused laser diode and a CCD camera to acquire the backscattered images. Four liquid crystal retarders are used to select quickly the appropriate polarization states of light that enter and leave the medium. To analyse polarisation effects, we develop a Monte Carlo simulation database using Mie theory. We show that knowing refractive indexes, Mueller matrices can be used to determine simultaneously and in situ an average particle size and particle volume fraction in dense turbid media. The measurement method is validated with oil in water emulsions and is used to study dynamically a complex coacervation mechanism. In the second part of the thesis, the anisotropic light transport is used to study the birefringence of turbid media or anisotropic suspensions. We measure dynamically the shear-induced global orientation in a clay suspension, a glass cylinders suspension and red blood cells suspensions. So, the static incoherent polarized light transport is a powerful method to characterize under flow various turbid medium in terms of particle size, concentration and objects orientation.Dans le cadre de cette thèse, nous nous intéressons au problème général de la caractérisation des dispersions concentrées. Nous développons une technique optique de diffusion basée sur le transport incohérent de lumière polarisée. Nous proposons un dispositif expérimental permettant l'acquisition de la matrice Mueller (Matrice 4x4 2D) représentant l'ensemble des interactions entre la lumière et un milieu diffusant. Le dispositif est constitué d'une source laser focalisée à la surface de l'échantillon et d'une caméra CCD permettant l'acquisition des images rétrodiffusées. Des lames à cristaux liquides sont utilisées pour sélectionner rapidement différents états de polarisation en entrée (source) et en sortie (acquisition). Parallèlement nous construisions puis analysons une base de données de simulations de Monte Carlo basée sur la théorie de Mie. Par confrontation des données expérimentales et numériques nous déterminons la taille moyenne des particules en suspension indépendamment de leur concentration. Le principe de mesure est validé sur des émulsions d'huile dans de l'eau puis appliqué à une étude d'un mécanisme de coacervation. Avec l'hypothèse que les propriétés optiques sont connues, nous effectuons une mesure simultanée et in situ de la taille des particules et de leur concentration. Une seconde partie de la thèse utilisera le transport anisotrope de lumière pour l'étude de suspensions biréfringentes ou anisotropes (Argile, bâtonnets de verre, suspensions sanguines) sous écoulement cisaillé. Pour chaque système, nous déterminons un axe d'orientation ou de déformation et nous quantifions l'anisotropie du système. Nous montrons donc que le transport stationnaire incohérent de lumière polarisée en milieu turbide est un outil permettant de caractériser des dispersions de nature variée en terme de taille, de concentration et d'orientation moyenne des objets dispersés sous écoulement

Caractérisation de Milieux diffusants en écoulement par transport incohérent de lumière polarisée

Dans le cadre de cette thèse, nous nous intéressons au problème général de la caractérisation des dispersions concentrées. Nous développons une technique optique de diffusion basée sur le transport incohérent de lumière polarisée. Nous proposons un dispositif expérimental permettant l acquisition de la matrice Mueller (Matrice 4x4 2D) représentant l ensemble des interactions entre la lumière et un milieu diffusant. Le dispositif est constitué d une source laser focalisée à la surface de l échantillon et d une caméra CCD permettant l acquisition des images rétrodiffusées. Des lames à cristaux liquides sont utilisées pour sélectionner rapidement différents états de polarisation en entrée (source) et en sortie (acquisition). Parallèlement nous construisions puis analysons une base de données de simulations de Monte Carlo basée sur la théorie de Mie. Par confrontation des données expérimentales et numériques nous déterminons la taille moyenne des particules en suspension indépendamment de leur concentration. Le principe de mesure est validé sur des émulsions d huile dans de l eau puis appliqué à une étude d un mécanisme de coacervation. Avec l hypothèse que les propriétés optiques sont connues, nous effectuons une mesure simultanée et in situ de la taille des particules et de leur concentration. Une seconde partie de la thèse utilisera le transport anisotrope de lumière pour l étude de suspensions biréfringentes ou anisotropes (Argile, bâtonnets de verre, suspensions sanguines) sous écoulement cisaillé. Pour chaque système, nous déterminons un axe d orientation ou de déformation et nous quantifions l anisotropie du système. Nous montrons donc que le transport stationnaire incohérent de lumière polarisée en milieu turbide est un outil permettant de caractériser des dispersions de nature variée en terme de taille, de concentration et d orientation moyenne des objets dispersés sous écoulement.In this thesis, we are interested in the general problem of characterizing concentrated dispersions. We develop an optical diffusing method which uses the incoherent polarized light transport. We propose an experimental setup to acquire the Mueller matrix (Matrix 4x4 2D). This matrix, composed by 16 images, represents all interactions between an electromagnetic wave and a diffusing medium. The experimental device is composed of a focused laser diode and a CCD camera to acquire the backscattered images. Four liquid crystal retarders are used to select quickly the appropriate polarization states of light that enter and leave the medium. To analyse polarisation effects, we develop a Monte Carlo simulation database using Mie theory. We show that knowing refractive indexes, Mueller matrices can be used to determine simultaneously and in situ an average particle size and particle volume fraction in dense turbid media. The measurement method is validated with oil in water emulsions and is used to study dynamically a complex coacervation mechanism. In the second part of the thesis, the anisotropic light transport is used to study the birefringence of turbid media or anisotropic suspensions. We measure dynamically the shear-induced global orientation in a clay suspension, a glass cylinders suspension and red blood cells suspensions. So, the static incoherent polarized light transport is a powerful method to characterize under flow various turbid medium in terms of particle size, concentration and objects orientation.NANCY1-SCD Sciences & Techniques (545782101) / SudocSudocFranceF

Size determination by use of two-dimensional Mueller matrices backscattered by optically thick random media

International audienceHere we are concerned with the systematic study of polarized light transport in thick, isotropic, homogeneous random media and of the associated inverse problem. An original spatial and intensity rescaling of the polarization transport allows one to account implicitly for the volume fraction. This parameter elimination permits a complete exploration, by means of Monte Carlo simulations of the dependence of polarized light transport on microscopic parameters. Analysis of the Mueller matrices obtained from the simulations show that additional correlations (with respect to scalar transport) are obtained between the microscopic parameters and the spatial distribution of specific elements of the Mueller matrix. As a consequence, using carefully chosen polarization states, one can determine an average particle size independently of the volume fraction of particles, with only the knowledge of the refractive-index ratio being required. This analysis is validated with experimental Mueller matrices obtained for emulsions of various size, concentration, and polydispersity

Caractérisation de l'écoulement diphasique dans les canaux des plaques bipolaires des piles à combustible à membrane

L'objectif de cette étude est d'examiner les écoulements diphasiques liquide/gaz dans les canaux des plaques bipolaires des piles à combustible afin de comprendre et de trouver des solutions au problème d'engorgement. L'influence de la section du canal et du matériau utilisé sur les pertes de charge (PDC) et sur la structure de l'écoulement d'eau liquide est plus particulièrement étudiée dans une expérience hors pile. Les mesures des PDC ont permis de mettre en évidence des séquences de bouchages et de débouchages, la fréquence de ces dernières augmentant avec le débit. Par ailleurs il est montré que le rapport des PDC diphasiques moyennées par les PDC en air sec décroit avec le débit et ceci indépendamment de la dimension du canal et qu'il est d'autant plus faible que le revêtement est hydrophile. Enfin différents régimes d'écoulements diphasiques (stratifié et de gouttes) ont pu être mis en évidence dans les différentes zones du canal et un modèle pour chacun d'eux a été établi, confirmant les résultats expérimentauxThis work focuses on the gas-liquid flows in the cathode plate, with the objective to observe their patterns, to understand their behavior, to estimate the pressure drops (PD) and eventually, to reduce clogging and its possible consequences in term of oxygen starvation downstream. A special emphasis is put on the effect of the channel section (typically between 0.5 and 1 mm ) and on the surface properties of the flow field plate materials. The experiments are performed ex-situ. The PD is measured locally along the channel as well as globally between the inlet and outlet, which put forward the existence of clogging/unclogging sequences. The characteristic frequency of these sequences increases with the air flow rate. The results show that the ratio of PD in two-phase flow to PD in dry flow decreases with the air flow rate while it does not seem to depend on the channel size (within the tested range). Moreover this ratio is lower with hydrophilic coating. Finally two main flow patterns (slug and annular flow) have been observed depending on the distance from the inlet and they have been simulatedNANCY-INPL-Bib. électronique (545479901) / SudocMETZ-SCD (574632105) / SudocSudocFranceF

The influence of the microscopic characteristics of a random medium on incoherent light transport

25 pagesInternational audienceIn this paper the influence of the microscopic characteristics of a random medium on non polarized, incoherent steady light transport (ISLT) is investigated. After close examination of current diffusion models, the source term in those models is modified, allowing a complete modelling of experimental and simulated radial dependance of backscattered and transmitted intensities for media thicknesses larger than the transport length. The new model only presents an additional source with respect to the elementary point source model. Thanks to more than 200 Monte-Carlo simulations, this parameter is correlated to the backscattering part of the Mie phase function. Incoherent Steady Light Transport measurements on two industrial emulsions at various volume fractions validate experimentally this correlation. This establishes a complete link between the microscopic characteristic of the random medium (size, optical indexes and volume fraction) and its macroscopic description in terms of diffusion and source parameters, openning new potential applications of the ISLT technique to, for example, the evaluation of the particles interaction potential in concentrated suspension

Method of Characterizing the Anisotropy of a Scattering Medium and Device for Implementing such a Method

The invention relates to a method of characterizing a scattering medium. According to the invention, the processing on the electromagnetic radiation scattered by the scattering medium is carried out for an unpolarized signal. In this way, only the anisotropic incoherent transport of radiation induced by the scattering medium is obtained in the characterization according to the invention. According to the invention, the data representative of the angular variation of the first image representing the unpolarized scattered radiation is representative of the purely isotropic part of the scattering. Having obtained this purely isotropic part, it is then possible, according to the invention, to calculate a second image representative of the non-isotropic part of the scattering. This non-isotropic part represents the anisotropic transport of radiation induced by the medium at the moment of scattering