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In this thesis, ammonia (NH3) decomposition was assessed as a fuel processing technology for producing on-demand hydrogen (H2) for portable and distributed fuel cell applications. This study was motivated by the present lack of infrastructure to generate H2 for proton exchange membrane (PEM) fuel cells. An overview of past and recent worldwide research activities in the development of reactor technologies for portable and distributed hydrogen generation via NH3 decomposition was presented in Chapter 2. The objective was to uncover the principal challenges relating to the state-of-the-art in reactor technology and obtain a basis for future improvements. Several important aspects such as reactor design, operability, power generation capacity and efficiency (conversion and energy) were appraised for innovative reactor technologies vis-à-vis microreactors, monolithic reactors, membrane reactors, and electrochemical reactors (electrolyzers). It was observed that substantial research effort is required to progress the innovative reactors to commercialization on a wide basis. The use of integrated experimental-mathematical modelling approach (useful in attaining accurately optimized designs) was notably non-existent for all reactors throughout the surveyed openliterature. Microchannel reactors were however identified as a transformative reactor technology for producing on-demand H2 for PEM cell applications. Against this background, miniaturized H2 production in a stand-alone ammonia-fuelled microchannel reactor (reformer) washcoated with a commercial Ni-Pt/Al2O3 catalyst (ActiSorb® O6) was demonstrated successfully in Chapter 3. The reformer performance was evaluated by investigating the effect of reaction temperature (450–700 °C) and gas-hourly-space-velocity (6 520–32 600 Nml gcat -1 h-1) on key performance parameters including NH3 conversion, residual NH3 concentration, H2 production rate, and pressure drop. Particular attention was devoted to defining operating conditions that minimised residual NH3 in reformate gas, while producing H2 at a satisfactory rate. The reformer operated in a daily start-up and shut-down (DSS)-like mode for a total 750 h comprising of 125 cycles, all to mimic frequent intermittent operation envisaged for fuel cell systems. The reformer exhibited remarkable operation demonstrating 98.7% NH3 conversion at 32 600 Nml gcat -1 h-1 and 700 °C to generate an estimated fuel cell power output of 5.7 We and power density of 16 kWe L-1 (based on effective reactor volume). At the same time, reformer operation yielded low pressure drop (<10 Pa mm-1) for all conditions considered. Overall, the microchannel reformer performed sufficiently exceptional to warrant serious consideration in supplying H2 to low-power fuel cell systems. In Chapter 4, hydrogen production from the Ni-Pt-washcoated ammonia-fuelled microchannel reactor was mathematically simulated in a three-dimensional (3D) CFD model implemented via Comsol Multiphysics™. The objective was to obtain an understanding of reaction-coupled transport phenomena as well as a fundamental explanation of the observed microchannel reactor performance. The transport processes and reactor performance were elucidated in terms of velocity, temperature, and species concentration distributions, as well as local reaction rate and NH3 conversion profiles. The baseline case was first investigated to comprehend the behavior of the microchannel reactor, then microstructural design and operating parameters were methodically altered around the baseline conditions to explore the optimum values (case-study optimization). The modelling results revealed that an optimum NH3 space velocity (GHSV) of 65.2 Nl gcat -1 h-1 yields 99.1% NH3 conversion and a power density of 32 kWe L-1 at the highest operating temperature of 973 K. It was also shown that a 40-μm-thick porous washcoat was most desirable at these conditions. Finally, a low channel hydraulic diameter (225 μm) was observed to contribute to high NH3 conversion. Most importantly, mass transport limitations in the porouswashcoat and gas-phase were found to be negligible as depicted by the Damköhler and Fourier numbers, respectively. The experimental microchannel reactor produced 98.2% NH3 conversion and a power density of 30.8 kWe L-1 when tested at the optimum operating conditions established by the model. Good agreement with experimental data was observed, so the integrated experimental-modeling approach used here may well provide an incisive step toward the efficient design of ammonia-fuelled microchannel reformers. In Chapter 5, the prospect of producing H2 via ammonia (NH3) decomposition was evaluated in an experimental stand-alone microchannel reactor wash-coated with a commercial Cs-promoted Ru/Al2O3 catalyst (ACTA Hypermec 10010). The reactor performance was investigated under atmospheric pressure as a function of reaction temperature (723–873 K) and gas-hourly-space-velocity (65.2–326.1 Nl gcat -1 h-1). Ammonia conversion of 99.8% was demonstrated at 326.1 Nl gcat -1 h-1 and 873 K. The H2 produced at this operating condition was sufficient to yield an estimated fuel cell power output of 60 We and power density of 164 kWe L-1. Overall, the Ru-based microchannel reactor outperformed other NH3 microstructured reformers reported in literature including the Ni-based system used in Chapter 3. Furthermore, the microchannel reactor showed a superior performance against a fixed-bed tubular microreactor with the same Ru-based catalyst. Overall, the high H2 throughput exhibited may promote widespread use of the Ru-based micro-reaction system in high-power applications. Four peer-reviewed journal publications and six conference publications resulted from this work. / PhD (Chemical Engineering), North-West University, Potchefstroom Campus, 2015

In this thesis, ammonia (NH3) decomposition was assessed as a fuel processing technology for producing on-demand hydrogen (H2) for portable and distributed fuel cell applications. This study was motivated by the present lack of infrastructure to generate H2 for proton exchange membrane (PEM) fuel cells. An overview of past and recent worldwide research activities in the development of reactor technologies for portable and distributed hydrogen generation via NH3 decomposition was presented in Chapter 2. The objective was to uncover the principal challenges relating to the state-of-the-art in reactor technology and obtain a basis for future improvements. Several important aspects such as reactor design, operability, power generation capacity and efficiency (conversion and energy) were appraised for innovative reactor technologies vis-à-vis microreactors, monolithic reactors, membrane reactors, and electrochemical reactors (electrolyzers). It was observed that substantial research effort is required to progress the innovative reactors to commercialization on a wide basis. The use of integrated experimental-mathematical modelling approach (useful in attaining accurately optimized designs) was notably non-existent for all reactors throughout the surveyed openliterature. Microchannel reactors were however identified as a transformative reactor technology for producing on-demand H2 for PEM cell applications. Against this background, miniaturized H2 production in a stand-alone ammonia-fuelled microchannel reactor (reformer) washcoated with a commercial Ni-Pt/Al2O3 catalyst (ActiSorb® O6) was demonstrated successfully in Chapter 3. The reformer performance was evaluated by investigating the effect of reaction temperature (450–700 °C) and gas-hourly-space-velocity (6 520–32 600 Nml gcat -1 h-1) on key performance parameters including NH3 conversion, residual NH3 concentration, H2 production rate, and pressure drop. Particular attention was devoted to defining operating conditions that minimised residual NH3 in reformate gas, while producing H2 at a satisfactory rate. The reformer operated in a daily start-up and shut-down (DSS)-like mode for a total 750 h comprising of 125 cycles, all to mimic frequent intermittent operation envisaged for fuel cell systems. The reformer exhibited remarkable operation demonstrating 98.7% NH3 conversion at 32 600 Nml gcat -1 h-1 and 700 °C to generate an estimated fuel cell power output of 5.7 We and power density of 16 kWe L-1 (based on effective reactor volume). At the same time, reformer operation yielded low pressure drop (<10 Pa mm-1) for all conditions considered. Overall, the microchannel reformer performed sufficiently exceptional to warrant serious consideration in supplying H2 to low-power fuel cell systems. In Chapter 4, hydrogen production from the Ni-Pt-washcoated ammonia-fuelled microchannel reactor was mathematically simulated in a three-dimensional (3D) CFD model implemented via Comsol Multiphysics™. The objective was to obtain an understanding of reaction-coupled transport phenomena as well as a fundamental explanation of the observed microchannel reactor performance. The transport processes and reactor performance were elucidated in terms of velocity, temperature, and species concentration distributions, as well as local reaction rate and NH3 conversion profiles. The baseline case was first investigated to comprehend the behavior of the microchannel reactor, then microstructural design and operating parameters were methodically altered around the baseline conditions to explore the optimum values (case-study optimization). The modelling results revealed that an optimum NH3 space velocity (GHSV) of 65.2 Nl gcat -1 h-1 yields 99.1% NH3 conversion and a power density of 32 kWe L-1 at the highest operating temperature of 973 K. It was also shown that a 40-μm-thick porous washcoat was most desirable at these conditions. Finally, a low channel hydraulic diameter (225 μm) was observed to contribute to high NH3 conversion. Most importantly, mass transport limitations in the porouswashcoat and gas-phase were found to be negligible as depicted by the Damköhler and Fourier numbers, respectively. The experimental microchannel reactor produced 98.2% NH3 conversion and a power density of 30.8 kWe L-1 when tested at the optimum operating conditions established by the model. Good agreement with experimental data was observed, so the integrated experimental-modeling approach used here may well provide an incisive step toward the efficient design of ammonia-fuelled microchannel reformers. In Chapter 5, the prospect of producing H2 via ammonia (NH3) decomposition was evaluated in an experimental stand-alone microchannel reactor wash-coated with a commercial Cs-promoted Ru/Al2O3 catalyst (ACTA Hypermec 10010). The reactor performance was investigated under atmospheric pressure as a function of reaction temperature (723–873 K) and gas-hourly-space-velocity (65.2–326.1 Nl gcat -1 h-1). Ammonia conversion of 99.8% was demonstrated at 326.1 Nl gcat -1 h-1 and 873 K. The H2 produced at this operating condition was sufficient to yield an estimated fuel cell power output of 60 We and power density of 164 kWe L-1. Overall, the Ru-based microchannel reactor outperformed other NH3 microstructured reformers reported in literature including the Ni-based system used in Chapter 3. Furthermore, the microchannel reactor showed a superior performance against a fixed-bed tubular microreactor with the same Ru-based catalyst. Overall, the high H2 throughput exhibited may promote widespread use of the Ru-based micro-reaction system in high-power applications. Four peer-reviewed journal publications and six conference publications resulted from this work. / PhD (Chemical Engineering), North-West University, Potchefstroom Campus, 2015

The Polymer Electrolyte Membrane (PEM) fuel cell has a great potential in leading the future energy generation due to its advantages of zero emissions, higher power density and efficiency. For a PEM fuel cell, the Membrane-Electrode Assembly (MEA) is the key component which consists of a membrane, two catalyst layers and two gas diffusion layers (GDL). The success of optimum PEM fuel cell power output relies on the mass transport to the electrode especially on the cathode side. The carbon based GDL is one of the most important components in the fuel cell since it has one of the basic roles of providing path ways for reactant gases transport to the catalyst layer as well as excess water removal. A detailed understanding and visualization of the GDL from micro-scale level is limited by traditional numerical tool such as CFD and experimental methods due to the complex geometry of the porous GDL structural. In order to take the actual geometry information of the porous GDL into consideration, the x-ray tomography technique is employed which is able to reconstructed the actual structure of the carbon paper or carbon cloth GDLs to three-dimensional digital binary image which can be read directly by the LB model to carry out the simulation. This research work contributes to develop the combined methodology of x-ray tomography based the three-dimensional single phase Lattice Boltzmann (LB) simulation. This newly developed methodology demonstrates its capacity of simulating the flow characteristics and transport phenomena in the porous media by dealing with collision of the particles at pore-scale. The results reveal the heterogeneous nature of the GDL structures which influence the transportation of the reactants in terms of physical parameters of the GDLs such as porosity, permeability and tortuosity. The compression effects on the carbon cloth GDLs have been investigated. The results show that the c applied compression pressure on the GDLs will have negative effects on average pore size, porosity as well as through-plane permeability. A compression pressure range is suggested by the results which gives optimum in-plane permeability to through-plane permeability. The compression effects on one-dimensional water and oxygen partial pressures in the main flow direction have been studied at low, medium and high current densities. It s been observed that the water and oxygen pressure drop across the GDL increase with increasing the compression pressure. Key Words: PEM fuel cell, GDL, LB simulation, SPSC, SPMC, x-ray tomography, carbon paper, carbon cloth, porosity, permeability, degree of anisotropy, tortuosity, flow transport.

Cette thèse porte sur la modélisation des piles à combustible à membrane d'échange de protons (PEMFC), en vue de leur surveillance et de leur diagnostic par spectroscopie d'impédance. La première partie du document présente le principe de fonctionnement de ces piles, ainsi que l'état de l'art de la modélisation et des méthodes de surveillance et diagnostic. Le modèle physique multi échelle particulièrement détaillé publié en 2005 par A.A. Franco sert de point de départ. Il est simplifié de façon à aboutir à un système d'équations aux dérivées partielles en une unique dimension spatiale. L'objectif principal de la seconde partie est l'analyse harmonique du fonctionnement de la pile. En s'inspirant de travaux classiques sur l'analyse géométrique de réseaux de réactions électrochimiques, un modèle réduit compatible avec la thermodynamique est obtenu. Cette classe de systèmes dynamiques permet de déterminer, pour un tel réseau, une formule analytique de l'impédance de l'anode et de la cathode d'une pile PEMFC. Un modèle complet de la pile est obtenu en connectant ces éléments à des éléments représentant la membrane, les couches diffuses et les couches de diffusion des gaz. Les modèles précédents supposent la pile représentée par une cellule unique et homogène. Afin de permettre d'en décrire les hétérogénéités spatiales, nous proposons finalement un résultat de modélisation réduite d'un réseau de cellules représentées par leur impédance. Ce modèle approxime l'impédance globale du réseau par une "cellule moyenne", connectée à deux cellules "série" et "parallèle" représentatives d'écart par rapport à la moyenne. / This PhD thesis focuses on reduced modeling of PEM fuel cell for supervision and diagnosis by impedance spectroscopy. The first part of the document presents the principle of the PEM fuel cell, as well as the state of the art of modeling and of the methods for supervision and diagnosis. The multiscale dynamic model published in 2005 by A.A. Franco is particularly detailed and serves as a starting point. It is simplified, in order to obtain a system of partial differential equations in a single spatial dimension. The second part is devoted to harmonic analysis of the PEM Fuel cell. Inspired by classical work on the geometric analysis of electrochemical reactions networks, a model compatible with thermodynamics is obtained. This class of dynamic systems allows establishing, for such a network, an analytical formula of the impedance of the anode and the cathode of the PEM fuel cell. A complete model of the cell is obtained by connecting these elements to the membrane, diffuse layers and gas diffusion layer. The previous models assumed the PEM Fuel cell represented by a single, homogeneous, cell. In order to describe the possible spatial heterogeneities, we finally propose a result of reduced modeling for the impedance of a cell network. This model approximates the overall impedance of the network by a "mean cell", connected to two cells, put in "serial" and "parallel", and representative of the deviations from the average.

Le fonctionnement de la pile à combustible (PAC) de type PEM (à membrane polymère) est encore soumis à de nombreuses incertitudes, aux natures différentes, qui affectent ses performances électriques, sa fiabilité et sa durée de vie. L'objectif général de cette thèse est de proposer une méthode d'aide à l'évaluation de la fiabilité des PAC par la simulation ; la fiabilité étant vue ici comme la garantie d’accéder à un niveau de performance électrique donné dans les différentes conditions d’usage envisagées pour la PAC. La démarche proposée s’appuie sur un couplage physico-fiabiliste où la complexité des phénomènes physiques présents dans la pile est prise en compte par une modélisation de connaissance, dynamique, symbolique et acausale, développée dans l’environnement Modelica - Dymola. La modélisation retenue, monodimensionnelle, non isotherme inclut une représentation diphasique des écoulements fluidiques pour mieux retranscrire la complexité des échanges d’eau dans le coeur de la pile PEM. La modélisation permet aussi d’intégrer des incertitudes sur certains de ses paramètres physiques et semi-empiriques (classés en trois catégories : opératoires, intrinsèques et semi-empiriques) puis d’entreprendre, par des tirages de Monte-Carlo, la modélisation probabiliste des conséquences des incertitudes injectées sur la performance d’une PAC. Il est ainsi possible, par la suite, d’estimer la fiabilité d’une PAC par le calcul de la probabilité que la performance électrique reste supérieure à un seuil minimal à définir en fonction de l’application. Une analyse physico-fiabiliste détaillée a été menée en introduisant à titre d’exemple une incertitude sur la valeur de la porosité de la couche de diffusion cathodique d’une PAC de type PEM (coefficients de variation retenus : 1%, 5% et 10%). L’étude des conséquences de cette incertitude sur la tension et l’impédance d’une PAC a été menée en réalisant un plan d’expériences numériques et en mettant en oeuvre différents outils d’analyse statistique : graphes des effets, analyses de la variance, graphes des coefficients de variation des distributions en entrée et sortie du modèle déterministe. Dans cet exemple d’analyse et dans les conditions d’usages considérées, le taux de fiabilité prévisionnel (probabilité pour que la cellule de pile fournisse un minimum de tension de 0.68V) a été estimé à 91% avec un coefficient de variation d’entrée à 10%. / The Proton Exchange Membrane Fuel Cell (PEMFC) operation is subject to inherent uncertainty in various material, design and control parameters, which leads to performance variability and impacts the cell reliability. Some inaccuracies in the building process of the fuel cell (in the realization of the cell components and also during the assembly of the complete fuel cell stack), some fluctuations in the controls of the operating parameters (e.g. cell and gas temperatures, gas pressures, flows and relative humidity rates) affect the electrical performance of the cell (i.e. cell voltage) as well as its reliability and durability. For a given application, the selections of the different materials used in the various components of the electrochemical cell, the choices in the cell design (geometrical characteristics / sizes of the cell components) correspond to tradeoffs between maximal electrical performances, minimal fuel consumption, high lifespan and reliability targets, and minimal costs.In this PhD thesis, a novel method is proposed to help evaluating the reliability of a PEMFC stack. The aim is to guarantee a target level of electrical performance that can be considered as sufficient to meet any application requirements. The approach is based on the close coupling between physical modeling and statistical analysis of reliability. The complexity of the physical phenomena involved in the fuel cell is taken into account through the development of a dynamical, symbolic, acausal modeling tool including physical and semi-empirical parameters as well. The proposed knowledge PEMFC model is one-dimensional, non-isothermal and it includes a two-phase fluidic flow representation (each reactant is considered as a mix of gases and liquid water) in order to better take into account the complexity of the water management in the cell. The modeling is implemented using the MODELICA language and the DYMOLA software; one of the advantages of this simulation tool is that it allows an effective connection between multi-physical modeling and statistical treatments. In this perspective, the modeling is done with the aim of having as much relevant physical parameters as possible (classified in our work as operating, intrinsic, and semi-empirical parameters). The different effects of these parameters on the PEMFC electrical behavior can be observed and the performance sensitivity can be determined by considering some statistical distributions of input parameters, which is a step towards reliability analysis.A detailed physical and reliability analysis is conducted by introducing (as an example) an uncertainty rate in the porosity value of the cathodic Gas Diffusion Layer (coefficients of variance equal to 1%, 5% and 10%). The study of the uncertainty consequences on the cell voltage and electrical impedance is done through a design of numerical experiments and with the use of various statistical analysis tools, namely: graphs of the average effects, statistical sensitivity analyses (ANOVAs), graphs displaying the coefficients of variances linked with the statistical distributions observed in the inputs and outputs of the deterministic model. In this example of analysis and in the considered cell operating conditions, the provisional reliability rate (probability that the cell voltage is higher than 0.68V) is estimated to 91% with an input coefficient of variance equal to 10%.

Les piles à combustible à membrane échangeuse de protons (PEMFC) constituent une alternative aux moteurs thermiques utilisés dans le cadre d’applications transport ou dans le cadre d’applications stationnaires. Cependant, une large commercialisation des PEMFC dépend des progrès qui peuvent être réalisés pour améliorer leur fiabilité et leur durabilité. La PEMFC est sujette à plusieurs types de dégradations complexes et non entièrement maitrisées qui varient en fonction des conditions de fonctionnement. Cependant, il est admis qu’il est souhaitable de faire fonctionner la PEMFC à distributions de courant uniformes car des distributions de courant hétérogènes entraînent une mauvaise utilisation des réactifs et des catalyseurs, une diminution des performances globales et une possible dégradation des matériaux constitutifs du coeur de la pile. De nouvelles stratégies de diagnostic doivent donc être proposées en s’appuyant sur les distributions de courant. Mes travaux de recherche consistent à développer un nouvel outil de diagnostic s’appuyant sur une mesure du champ électromagnétique externe (non invasive) rayonné par la pile PEMFC. Le champ magnétique possède l’intérêt d’être corrélé à la distribution locale du courant circulant à l’intérieur de la pile, et permet d’avoir des informations sur les performances locales. Cette distribution est liée aux conditions opératoires de la pile. Il est alors possible, à partir d’une signature magnétique de remonter à une information locale et à la cause des distributions de courant non uniformes. Des bases (vecteurs) qui contiennent les données des champs magnétiques issues des 30 capteurs disposés autour de la PAC sont construites à partir de distributions de courant spécifiques. Ces bases constituent un espace de représentation du comportement anormal de la PEMFC et permettront de d’élaborer des signatures caractérisant les fonctionnements indésirables. Ainsi, deux méthodes ont été développées pour permettre : (i) d’extraire des paramètres pertinents sur la répartition de la densité de courant traduisant les performances locales de la PAC, (ii) de classifier les différents modes de fonctionnements indésirables. La première méthode consiste à générer des résidus vectoriels en comparant le comportement réel du système (caractérisé par un vecteur mesure) avec les bases générées. Des variables qualitatives ont été élaborées pour classifier les modes de fonctionnement indésirables de la pile. La deuxième méthode consiste à extraire des paramètres à partir de la projection du vecteur mesure dans la direction des bases. La classification est réalisée dans des espaces 2D. Une validation des deux méthodes proposées a été effectuée à partir de mesures expérimentales sur une PEMFC de taille industrielle (stack GENEPAC de 40 cellules construit par le CEA et PSA). La pertinence des paramètres extraits a été vérifiée en s’appuyant sur des distributions de courant mesurées directement. Les modes de fonctionnement indésirables prédéfinis permettent de localiser les paramètres opératoires ayant conduit à l’évolution de la distribution de courant. Les outils ainsi réalisés sont très facilement transposables à d’autres piles PEMFC. / Proton exchange membrane fuel cell (PEMFC) is a potential alternative energy conversion device for stationary and automotive applications. Wide commercialization of PEMFC depends on progress that can be achieved to enhance its reliability and durability. The PEM fuel cell is subject to several types of complex and not fully mastered degradations which vary with operating conditions. It is desirable to operate the PEMFC at uniform current distribution because non uniform current distribution over the MEA could result in poor reactant and catalyst utilization, overall cell performance degradation as well as corrosion processes inside the PEM fuel cell. Therefore, new diagnostic strategies must be proposed, including choice of information gathered on the system and the fuel cell operation representation. My research is to develop a new diagnostic tool based on a measure of the external electromagnetic field (non-invasive) radiated by the fuel cell. The magnetic field has the advantage of being correlated to the local distribution of the current flowing inside the fuel cell (a physical indicator to obtain information on local performance of a fuel cell); it is linked to the local operating conditions: relative humidity level, temperature etc. It is then possible, from a magnetic signature to trace local information. Baselines (vectors) which contain the magnetic fields data generated by specific current distribution are built to characterize the magnetic field generated by the undesirable operation of the fuel cell. Baselines constitute a representation space of abnormal system behavior. Two methods have been developed to enable: (i) to extract the relevant parameters on the distribution of the current density resulting from PEM fuel cell stack local performance, (ii) to classify different types of undesirables operations. The first method is to generate vector residuals by comparing the actual behavior of the system (characterized by a measurement vector) with the baselines generated. Qualitative variables were created to classify the undesirable modes of PEM fuel cell stack operation. The second method is to extract parameters from the projection of the vector in the direction of measurement baselines. The classification is performed in 2D space. Validation from experimental measurements of the two proposed methods has been carried out on a commercial scale PEMFC (GENEPAC stack of 40 cells built by the CEA and PSA). The relevance of the extracted parameters was verified based on current density distributions measured directly. The undesirable predefined operating modes were used to locate the operating conditions parameters that led to the evolution of the current density distribution. The tools are made easily transferable to other PEMFC stack.

Les travaux précédents trouvés dans la littérature ont mis l'importance sur la pile à combustible PEM ou électrolyseur PEM. Certains articles ont étudié également la pile à combustible réversible et le système d'alimentation en hydrogène par énergie solaire en intégrant à la fois la pile à combustible et électrolyseur. Contrairement à un « Unitised regenerative fuel cell (URFC)», notre conception a un compartiment individuel pour chaque système de PEM-Fuel Cell et d'electrolyseur-PEM et nommé Quasi - URFC. Grâce à ce nouveau concept, l'objectif principal est de réduire le coût de la pile à combustible régénératrice (RFC) en minimisant le rapport de surface superficielle géométrique du catalyseur de l'assemblage membrane électrodes (AME) des deux modes dans la cellule. D'ailleurs, nous visons également à construire un RFC plus compact, léger et portable par rapport à une pile à combustible ou l'électrolyseur classique. Ce travail de recherche est divisé en trois parties : la modélisation et simulation numérique, l'assemblage du prototype et le travail d'expérimentation. Quant à la partie de modélisation, un modèle physique multi-2D a été développé dans le but d'analyser les performances d'une pile à combustible à régénérée à trois-compartiments, qui se compose d'une piles à combustible et d'électrolyseur. Ce modèle numérique est basée sur la résolution des équations de conservation de masse, du momentum, des espèces et du courant électrique en utilisant une approche par éléments finis sur des grilles 2D . Les simulations permettent le calcul de la vitesse, de la concentration de gaz, la densité de courant et les distributions de potentiels en mode pile à combustible et en mode d'électrolyse, ainsi nous aider à prédire le comportement de quasi - RFC. En outre, l'assemblage du premier prototype du nouveau concept de pile à combustible à combustible régénérée a été achevée et testée au cours des trois années d'études dans le cadre d'une thèse. Les résultats expérimentaux de la 3 Compartiments R-PEMFC ont été prometteurs dans les deux modes, soit en mode piles à combustible et soit en mode d'électrolyseur. Ces résultats valideront ensuite les résultats de la simulation, obtenus auparavant par la modélisation. / The past works found in the literature have focused on either PEM fuel cell or electrolyzer-PEM. Some of the papers even studied the unitised reversible regenerative fuel cell (URFC) and the solar power hydrogen system by integrating both fuel cell and electrolyzer. Unlike the URFC, our design has an individual compartment for each PEMFC and E-PEM systems and named Quasi-URFC. With this new concept, the main objective is to reduce the cost of regenerative fuel cell (RFC) by minimizing the ratio of the catalyst’s geometric surface area of the membrane electrode assembly (MEA) of both cell modes. Apart from that, we also aim to build a compact, light and portable RFC.This research work is divided into three parts: the modeling, assembly of the prototype and the experimentation work. As for the modeling part, a 2D multi-physics model has been developed in order to analyze the performance of a three chamber-regenerative fuel cell, which consists of both fuel cell and electrolyzer systems. This numerical model is based on solving conservation equations of mass, momentum, species and electric current by using a finite-element approach on 2D grids. Simulations allow the calculation of velocity, gas concentration, current density and potential's distributions in fuel cell mode and electrolysis mode, thus help us to predict the behavior of Quasi-RFC. Besides that, the assembly of the first prototype of the new concept of regenerative fuel cell has been completed and tested during the three years of PhD studies. The experimental results of the Three-Chamber RFC are promising in both fuel cell and electrolyzer modes and validate the simulation results that previously obtained by modeling.

Cette thèse contribue à l'identification des mécanismes de dégradation qui ont lieu durant les phases de démarrage et d'arrêt des Piles à Combustible à Membrane Échangeuse de Proton. Dans un premier temps, des démarrages et arrêts individuels sont étudiés au moyen d'une cellule équipée de collecteurs de courants segmentés. Les courants internes qui sont produits durant ces opérations peuvent ainsi être mesurés. La mesure du dioxide de carbone dans les gaz d'échappement de la cathode révèle qu'une partie des courants internes correspond à de l'oxydation du carbone. Une autre part provient des réactions (réversibles ou non) d'oxydoréduction impliquant du platine. L'hétérogénéité des dégradations subies par la pile entre l'entrée et la sortie de la cathode est mise en évidence lors de protocoles de vieillissement répétant des démarrages et arrêts. Des analyses post-mortem révèlent un autre niveau d'hétérogénéité, qui concerne également le carbone, entre les dents et les canaux. De ces expériences, il ressort également que les dégradations sont plus importantes lorsque les gaz sont injectés à faible vitesse dans le compartiment anodique mais aussi quand de l'air est utilisé à la place de l'azote pour arrêter la pile. L'influence des caractéristiques de la MEA sur l'intensité des dégradations est aussi étudié. Un chargement en platine élevé à l'anode ou des électrodes avec des surfaces de carbone actif élevées accélèrent la chute des performances électriques. Au contraire accroitre le chargement en platine à la cathode limite ces pertes. Enfin, des simulations numériques des phases de démarrage complètent les résultats expérimentaux. L'oxydation réversible du platine est notamment identifiée comme étant responsable d'une part importante des courants internes / This works contributes to the identification of the various degradation mechanisms in Polymer Electrolyte Membrane Fuel Cell during start-up and shut-down operations. Single start-ups and shut-downs are first analysed using a cell with segmented cathode current collectors. Thus, internal currents which occur during these operations can be measured. Carbon dioxide measured in the cathode exhaust gas reveals that they result partially from carbon oxidation. Another contribution is the reversible or non reversible redox reactions involving platinum. The heterogeneity of the non reversible platinum oxidation between the inlet and outlet of the cathode is evidenced by the in-situ monitoring of the Electrochemical Surface Area during long-term start-up and shut-down aging protocols. Post-mortem analysis reveals another level of heterogeneity, which concerns also carbon oxidation, between land and channel. From these experiments, it appears also that degradations are more important when gases are injected with a low velocity in the anode compartment and when air is used instead of nitrogen to flush the anode compartment during shut-down. The influence of the MEA characteristics on the extent of the degradation observed during these aging protocols is also analyzed. High platinum loading in the anode and high surface carbon electrodes accelerate the drop of the electrical performances, while increasing the cathode platinum loading limits their decay. Finally, numerical simulations of start-ups complete the experimental results. Reversible platinum oxidation was found to be one of the main contribution to the internal currents

In a previous study, we reported that polymer electrolyte fuel cells prepared by direct membrane deposition (DMD) produced power densities in excess of 4 W/cm2. In this study, the underlying origins that give rise to these high power densities are investigated and reported. The membranes of high power, DMD-fabricated fuel cells are relatively thin (12 μm) compared to typical benchmark, commercially available membranes. Electrochemical impedance spectroscopy, at high current densities (2.2 A/cm2) reveals that mass transport resistance was half that of reference, catalyst-coated-membranes (CCM). This is attributed to an improved oxygen supply in the cathode catalyst layer by way of a reduced propensity of flooding, and which is facilitated by an enhancement in the back diffusion of water from cathode to anode through the thin directly deposited membrane. DMD-fabricated membrane-electrode-assemblies possess 50% reduction in ionic resistance (15 mΩcm2) compared to conventional CCMs, with contributions of 9 mΩcm2 for the membrane resistance and 6 mΩcm2 for the contact resistance of the membrane and catalyst layer ionomer. The improved mass transport is responsible for 90% of the increase in power density of the DMD fuel cell, while the reduced ionic resistance accounts for a 10% of the improvement.