Les piles à combustible sont une technologie prometteuse pour la génération d'énergie écologique dans diverses applications. Les piles à combustibles dans les applications de transport ont besoin de systèmes de refroidissement qui soient à la fois compacts, durables, non-conducteurs des électrons et qui travaillent à basse température. On choisit le pentane en<br />ébullition en débit forcé comme étant la méthode la plus prometteuse. Les écoulements sont en minicanal de 0.8 × 0.8 mm en laminaire et à faible flux thermique. Pour étudier les instabilités, on a fait varier la compressibilité du système en amont de l'évaporateur. Les écoulements ont été visualisés de manière directe en radiographie neutronique à l'ILL avec un petit canal non-isolé puis avec un autre canal plus massif dans des conditions thermiques<br />isolées. La visualisation neutronique montre deux régimes métastables : surchauffe liquide et ébullition. Les instabilités fortes de température des deux régimes sont corrélées avec le régime d'écoulement observé dans la visualisation. La boucle finale, ILULIAQ, a été conçue pour des tests en monocanal et en plaque. Les fuites thermiques ont été soigneusement<br />réduites et l'incertitude sur le flux total est moins de 0.1 W comparé au nominal de 1.9W de puissance utile au nominal. Les résultats montrent une surchauffe liquide, suivie d'ébullition stationnaire. Les éventuelles instabilités sont faibles et ne sont pas corrélées à la compressibilité en amont. La perte de pression et le coefficient de transfert de chaleur étaient suffisant pour le refroidissement d'une pile. L'ensemble des études menées montre que le refroidissement en pentane diphasique pourrait être adapté aux besoins spécifiques des piles à combustibles et donc de résoudre les problèmes actuels de refroidissement.

Proton exchange membrane fuel cells (PEMFCs) have attracted great attention due to their high power density, low-temperature operation and high energy conversion efficiency. However, the high cost of Pt catalysts and durability problems hinder their commercialization. So their cost must be lowered drastically and their durability must be extended. In an effort to overcome these problems, there have been intensive efforts to enhance the activity, durability and to lower the price of catalysts by alloying with other less expensive metals. In particular, the sluggish kinetics of ORR caused by Pt oxide at cathode and Pt catalyst degradation by electrochemical surface area (ECSA) loss have been a huge research area where a lot of researchers have paid lots of attention to solve. In this regard, the objective of this dissertation is to evaluate a series of Pt catalyst electrode surface electrochemical reactions on PEMFC electrode in order to help searching new catalysts and enhancing system design, assist in the search for new catalysts and improved system design by suggesting the developed mechanism of electrocatalyst activity and stability (durability). We have been focused on understanding the oxide-mediated dissolution of Pt by using electrochemical experiment methods such as RRDE, EQCN, SECM with a combination of ICP-MS and computational simulation with COMSOL Multiphysics. Firstly, in chapter 3, we showed the oxide-mediated Pt dissolution rate and the influence of hydrogen and cation underpotential deposition on Pt dissolution. In chapter 4, we revealed oxygen reduction reaction (ORR) plays a significant role in Pt oxide formation and reduction that influences the Pt catalyst dissolution, resulting in accelerated Pt dissolution rate at specific potential range. Finally, we found out the nature of mobile species generated during PtO₂ reduction process which have been disputed as Pt ion or other mobile species and fulfilled computational simulation for evaluation of SECM experiment in chapter 5. Based on these experiments and simulation, we were able to explain some mechanism of literature results that already were reported but have not been clearly explained so far. In terms of the purpose of this dissertation, the mechanism of oxide-mediated Pt dissolution, influence of ORR to Pt oxide formation/reduction and Pt dissolution, the nature of mobile species generated during PtO₂ reduction process, are sure to be very helpful in developing new catalysts and enhancing system design and suggested operating conditions. / text

Les piles à combustible constituent une alternative aux moteurs thermiques utilisés dans le cadre d'applications transport ou dans le cadre d'applications stationnaires. Cependant, il existe encore aujourd'hui des verrous technologiques limitant leur développement à l'échelle industrielle. Un des verrous importants est la détermination de leur état de santé en temps réel permettant un diagnostic voire un pronostic de leurs dysfonctionnements ou de leur temps de bon fonctionnement. Dans ce travail, deux approches différentes ont été abordées en vue de proposer des outils de diagnostic pour des empilements de taille industrielle : une approche électrochimique et une approche acoustique. Dans un premier temps, un modèle simple de spectroscopie d'impédance électrochimique et un algorithme d'identification des paramètres ont été développés dans l'optique de proposer un outil de diagnostic rapide et facile à implémenter. Dans un second temps, deux méthodes basées sur la propagation des ondes sonores (l'émission acoustique et les acousto-ultrasons) ont été testées et évaluées dans des conditions de fonctionnement optimales et dégradées.

Ce travail a été réalisé dans le cadre du projet Systèmes Mécaniques Adaptatifs (SMA)du laboratoire mécatronique M3M de l'UTBM impliqué dans l'institut FCLAB de recherche sur les systèmes pile à combustible et du projet de l'équipe de recherche "Modélisation Multiphysique ", en cours de constitution, du département Sciences et Technologies de l'USEK. Les PEMFC font l'objet de nombreuses recherches pour augmenter leurs performances et diminuer leur coût mais la plupart des études se concentrent sur leurs aspects physicochimiques.Cette thèse par contre se propose de mettre en évidence l'influence, sur la production d'énergie, des sollicitations mécaniques statiques, dynamiques voire thermiques (serrages, vibrations, frottements, ...) comme phénomènes couplés relevant du domaine multiphysique (interactions fluide-structure, électrique ...). En premier, une analyse des différents paramètres de modèles dépendant des aspects mécaniques a été effectuée et les principaux paramètres à étudier dans le cadre de cette thèse ont été sélectionnés : porosité, perméabilité et coefficients de diffusion de la GDL, conductivité électrique du contact GDL/PB et volume des canaux après compression de la cellule. Ensuite, un modèle partiel de représentation mécanique de la GDL d'une PEMFC du côté cathode a été mis en oeuvre afin de déterminer la déformation de la GDL comprimée par une force répartie sur la PB. Sur la base des contraintes mécaniques calculées dans la GDL, les champs locaux de porosité, de perméabilité et de résistance électrique de contact GDL/PB sont obtenus. D'autre part, une modélisation 3D de type volumes finis pour l'étude de la pression du fluide à l'interface GDL/PB a été élaborée. L'analyse a permis de déterminer le champ local de pression d'oxygène sur l'interface GDL/PB du côté cathode. Les champs locaux de porosité et de perméabilité de la GDL, de résistance électrique de contact GDL/PB et de pression d'interface GDL/PB sont alors introduits dans le modèle multiphysique 2D d'une cellule de pile PEMFC. Une étude détaillée du comportement de la pile et de la modification de sa performance a pu être réalisée. Les résultats ont été présentés sous forme de courbes de polarisation et de densité de puissance. Finalement tous les résultats ont été rassemblés pour une analyse d'influence et de sensibilité afin d'identifier les paramètres qui auront le plus d'influence sur les variables simulées. Cette étude peut s'avérer un outil fort utile à la prise de décision concernant la géométrie de la dent des PB, la nature des PB, ...

Cette thèse réalisée dans le cadre des projets PIE CNRS AMELI-0Pt et AMEPlas et ANR AMADEUS a regroupé plusieurs entités autour de la thématique des piles à combustible : Dreux Agglomération puis l'Agence Innovation Made In Dreux (MID), le GREMI, le LACCO et initialement l'industriel MHS Equipment. L'objectif de ce travail est l'élaboration par voie plasma et l'optimisation d'électrodes de piles à combustible de type PEMFC et SAMFC dans le but d'obtenir de bonnes performances avec des charges de platine ultra faibles ou sans platine. Le projet a été organisé en quatre étapes : l'étude de la croissance simultanée de platine et de carbone co-pulvérisés par plasma, la dispersion optimale de quantités ultra faibles de catalyseur, le remplacement du platine par un alliage bimétallique à base de palladium, et le dépôt direct du catalyseur sur la membrane par plasma. En utilisant un faisceau synchrotron de rayons X (Synchrotron SOLEIL), en collaboration avec le CRMD, l'étude GISAXS des couches minces Pt-C co-pulvérisés a révélé l'organisation particulière du platine dans ce type de nanostructure. Ces couches minces Pt-C offrent d'excellentes performances (20 kW.gPt-1) avec des charges de platine ultra faibles. Des électrodes PdPt (5 %at Pt) faiblement chargées permettent d'atteindre de bonnes performances en PEMFC quasiment sans platine (12,5 kW.gPd-1 et 250 kW.gPt-1). L'étude de l'activité de catalyseurs PdAu vis-à-vis de l'oxydation du glycérol a révélé l'origine des effets synergiques du palladium et de l'or en milieu alcalin. Le dépôt plasma direct de platine associé ou non au dépôt de carbone sur membrane a été optimisé. Les performances obtenues avec des CCM (Catalyst Coated Membrane) plasma démontrent l'intérêt de ce type d'architecture.

Water management in a polymer electrolyte fuel cell is a critical issue in ensuring high cell performance. The water production, due to both electro-osmotic drag and the chemical reaction, in the cathode side of the fuel cell leads to liquid water formation in the gas diffusion layer and the reactant flow channel. If this water is allowed to accumulate in the fuel cell, the transport of reactants to the membrane assembly will be inhibited and cell performance will suffer. In order to maximize the potential performance of a fuel cell, understanding of the liquid water dynamics is required, and a two-phase flow numerical model has been for this purpose. If an accurate numerical model can be created the development cycle for new flow channel designs can be accelerated. The methodology adopted for the numerical simulation of dynamic two-phase flow is the volume of fluid (VOF) method. The major drawback of current VOF models is in the implementation of the three phase contact line. Current models use a constant static contact angle, which does not take into account real dynamics. This results in non-physical phenomena such as spherical and suspended droplets, instead of the experimentally observed attached semi-spherical droplets with the trailing edge of the droplet forming a tail. To remedy this shortcoming, the implementation of a dynamic contact angle relation is required. The relations used in the current work follows the Hoffman formulation where the dynamic contact angle is obtained as θD. A function of the capillary number, based on the contact line velocity, and of the equilibrium contact angle. The function was implemented within the commercial CFD framework of Fluent using user defined functions. The dynamic contact angle models were able to better predict the droplet dynamics, providing elongated droplet profiles. The dynamic contact angle model was also able to provide more realistic pressure profiles down the channel length. Parametric studies show the dramatic effects that air speed and static contact angle have upon the droplet dynamics. It was also observed that water injection velocity had a relatively small effect on the model. The dynamic contact angle model was found to be consistent with experimental work conducted in our laboratory in which the spinning motion of the fluid within the water droplet was observed [7]. The improved physical representation achieved with the new model results in more reliable simulations and provides a good foundation for the numerical modeling of fuel cell flow channels.

The work present in this thesis report introduces a novel non-destructive technique for experimentally measuring through thickness hygro-thermal strain of Nafion membranes though digital image correlation. An Optical Coherence Tomography (OCT) system was used to acquire images of a Nafion-TiO2 (titanium dioxide powder) composite membranes in a fuel cell like device. The proposed technique, commonly known as optical coherence elastography (OCE) makes use of the normalized correlation algorithm to calculate strain between two successive scans of different relative humidity step values. Different normalized correlation parameters were compared to measured results of PDMS-TiO2 phantoms in order to analyze accuracy. The effect of TiO2 on Nafion membranbes mechanical properties was further analysed by comparing the swelling behaviour of membranes with different concentrations. It has been found that Nafion undergoes approximately 25 – 30% more strain on the land section than on the channel section, regardless gas diffusion electrode (GDE) layer presence. Furthermore, it was shown that the overall strain on the material decrease by approximately 10% when GDE layers are present. Overall this work demonstrated how OCE is a viable technique for measuring through thickness strain distribution in Nafion composite membranes and has the potential to be implemented for non-destructive in situ measurements. / Graduate / 0548 / kellerv@uvic.ca

The polymer electrolyte membrane fuel cell (PEMFC) is an electrochemical device used for the production of power, which is a key for the transition towards green and renewable power delivery devices for mobile, stationary and back-up power applications. PEMFCs consume hydrogen and oxygen to produce power, water and heat. The transient start-up from sub-zero freezing temperature conditions is a problem for the successful, undamaged and unhindered operation. The generation and presence of water in the PEMFC stack in such an environment leads to the formation of ice that hinders the flow of gases, causes morphological changes in the membrane electrode assembly (MEA) leading to reversible and irreversible degradation of stack performance. Start-up performance is highly dependent on start-up operational conditions and procedures. The previous state of the stack will influence the ability to perform upon the next start-up and operation. Water generated during normal operation is vital and improves performance when properly managed. Liquid water present at shut-down can form ice and cause unwanted start-up effects. This phase change may cause damage to the MEA and gas diffusion media due to volume expansion. Removal of high water content at shutdown decreases proton conductivity which can delay start-up times. The United States Department of Energy (DOE) has established a set of criteria that will make fuel cell technology viable when attained. As specified by DOE, an 80 kWe fuel cell will be required by 2015 to reach 50% power in 30 seconds from start-up at an ambient temperature of -20°C. This work investigates freeze start-up in a multi-kilowatt stack approaching both shut-down conditioning and start-up operations to improve performance, moderate fuel cell damage and determine the limits of current stack technology. The investigation involved a Hydrogenics Corporation 5 kW 506 series fuel cell stack. The investigation is completed through conditioning the fuel cell start-up performance at various temperatures ranging from -5°C to below -20°C. The control of system start-up temperature is achieved with an environmental chamber that maintains the desired set point during dwell time and start-up. The supply gases for the experiment are conditioned at ambient stack temperature to create a realistic environment that could be experienced in colder weather climates. Temperature controls aim to maintain steady ambient temperatures during progressive start-up in order to best simulate ambient conditions. The control and operation of the fuel cell is maintained by the use of a fuel cell automated test station (FCATS™). FCATS supplies gas feeds, coolant medium and can control temperature and reactant humidity in reactants according to a prescribed procedure for continuous operation. The iv collection of data occurs by the same system recording cell voltage, temperatures, pressures, flow rates and current densities. A procedural start-up and characterization are conducted in order improve start-of performance and examine reactant flows, coolant activation time, stack conditioning and the effects by freezing temperatures. The resulting degradation is investigated by polarization curves and various ex-situ measurements. In this work, it was found that freeze start-up of a fuel cell stack can be aided and managed by conditioning the stack at shut-down and applying a procedure to successfully start-up and mitigate the damage that freezing can cause.

abstract: Proton exchange membrane fuel cells (PEMFCs) run on pure hydrogen and oxygen (or air), producing electricity, water, and some heat. This makes PEMFC an attractive option for clean power generation. PEMFCs also operate at low temperature which makes them quick to start up and easy to handle. PEMFCs have several important limitations which must be overcome before commercial viability can be achieved. Active areas of research into making them commercially viable include reducing the cost, size and weight of fuel cells while also increasing their durability and performance. A growing and important part of this research involves the computer modeling of fuel cells. High quality computer modeling and simulation of fuel cells can help speed up the discovery of optimized fuel cell components. Computer modeling can also help improve fundamental understanding of the mechanisms and reactions that take place within the fuel cell. The work presented in this thesis describes a procedure for utilizing computer modeling to create high quality fuel cell simulations using Ansys Fluent 12.1. Methods for creating computer aided design (CAD) models of fuel cells are discussed. Detailed simulation parameters are described and emphasis is placed on establishing convergence criteria which are essential for producing consistent results. A mesh sensitivity study of the catalyst and membrane layers is presented showing the importance of adhering to strictly defined convergence criteria. A study of iteration sensitivity of the simulation at low and high current densities is performed which demonstrates the variance in the rate of convergence and the absolute difference between solution values derived at low numbers of iterations and high numbers of iterations. / Dissertation/Thesis / M.S.Tech Chemistry 2011

abstract: With a recent shift to a more environmentally conscious society, low-carbon and non-carbon producing energy production methods are being investigated and applied all over the world. Of these methods, fuel cells show great potential for clean energy production. A fuel cell is an electrochemical energy conversion device which directly converts chemical energy into electrical energy. Proton exchange membrane fuel cells (PEMFCs) are a highly researched energy source for automotive and stationary power applications. In order to produce the power required to meet Department of Energy requirements, platinum (Pt) must be used as a catalyst material in PEMFCs. Platinum, however, is very expensive and extensive research is being conducted to develop ways to reduce the amount of platinum used in PEMFCs. In the current study, three catalyst synthesis techniques were investigated and evaluated on their effectiveness to produce platinum-on copper (Pt@Cu) core-shell nanocatalyst on multi-walled carbon nanotube (MWCNT) support material. These three methods were direct deposition method, two-phase surfactant method, and single-phase surfactant method, in which direct deposition did not use a surfactant for particle size control and the surfactant methods did. The catalyst materials synthesized were evaluated by visual inspection and fuel cell performance. Samples which produced high fuel cell power output were evaluated using transmission electron microscopy (TEM) imaging. After evaluation, it was concluded that the direct deposition technique was effective in synthesizing Pt@Cu core-shell nanocatalyst on MWCNTs support when a rinsing process was used before adding platinum. The peak power density achieved by the rinsed core-shell catalyst was 618 mW.cm-2 , 13 percent greater than that of commercial platinum-carbon (Pt/C) catalyst. Transmission electron microscopy imaging revealed the core-shell catalyst contained Pt shells and platinum-copper alloy cores. Rinsing with deionized (DI) water was shown to be a crucial step in core-shell catalyst deposition as it reduced the number of platinum colloids on the carbon nanotube surface. After evaluation, it was concluded that the two-phase surfactant and single-phase surfactant synthesis methods were not effective at producing core-shell nanocatalyst with the parameters investigated. / Dissertation/Thesis / M.S.Tech Technology 2012