Investigation of high temperature polymer electrolyte membrane fuel cells

PhD Thesisthe major issues limiting the introduction of polymer electrolyte membrane fuel cells (PEMFC) is the low temperature of operation which makes platinum-based anode catalysts susceptible to poisoning by trace amounts of CO, typically present in reformed fuel. In order to alleviate the problem of CO poisoning and improve the power density of the cell, operating at temperature above 100°C is preferred. Nafion® type perfluorosulphonated polymers have been typically used for PEMFC but cannot function at temperatures above 100°C. In addition, higher temperatures will enable more effective cooling of the cell stacks and provide a means for combined electrical and heat energy generation. The solution to improved PEMFCs technology is to develop a new polymer electrolyte membrane which exhibits stability and high conductivity in the absence of liquid water. A HighTemperature PEMFC based on a Phosphoric acid (H3P04) doped Polybenzimidazole poly[2,2- (m-phenylene)-5,5 bibenzimidazole] (PBI) membrane has been developed and demonstrated as an alternative to Nafion® for operation at temperatures up to 200°C. PBI membranes, when doped with phosphoric acid, do not rely on hydration for conductivity; a significantly lower water content of the membrane, compared to Nafion, is required for proton transport. The resulting system improvements include; high CO tolerance, simple thermal and water management, excellent oxidative and thermal stability, and good proton conductivity at elevated temperatures. Two issues associated with phosphoric acid in the PBI based fuel cell are the lower activity of the electrocatalysts and the potential loss of the acid into the fuel cell gas/vapour exhaust streams. The limited oxygen permeability and slow oxygen reduction kinetics in phosphoric acid is a major limitation for the performance ofPBI based PEMFCs. The kinetics of oxygen reduction in PBVH3P04 has been studied in electrochemical single electrode cells. Several Membrane Electrode Assemblies (MEAs) have been manufactured to allow optimisation of the electrode performance. Various electrochemical techniques such as chronoamperometry, polarisation curves and Frequency Response Analysis (FRA) were used to study and separate the effects of the various phenomena taking place at the electrode surface: IR losses, mass transport and kinetics. A new Electrode structure utilizing PTFE has been developed allowing higher oxygen permeability and therefore enhanced performance of 0.55 W cm-2 with oxygen and 0.27 W cm-2 with air (atm) at temperature as low as 120 ·C. The Platinum loading was reduced to 0.4 mgpt cm-2 at the cathode and 0.2 mgpt cm-2 at the anode. Further reduction of cathode platinum loading to 0.2 mgPI cm-2 was achieved without dramatic drop in the performance by utilising Pt based binary alloy catalyst (Pt-Co/C). A simplified thin film steady-state, isothermal, one dimensional model of a proton exchange membrane fuel cell (PEMFC), with a polybenzimidazole (PBD membrane, was developed. The electrode kinetics were represented by the Butler-Volmer equation, mass transport was described by the multi-component Stefan Maxwell equations and Fick's law, and the ionic and electronic resistances described by Ohm's law. The model incorporated the effects of temperature and pressure on the open circuit potential, the exchange current density and diffusion coefficients, together with the effect of water on the acid concentration and ionic conductivity. The polarisation curves predicted by the model were validated against experimental data for a PEMFC which included the effect of temperature and oxygen/air pressure on cell performance. An additional problem which faces the introduction ofPEMFC technology is that of supplying or storing hydrogen for cell operation, especially for vehicular applications. Consequently the use of alternative fuels such as methanol and ethanol is of interest, especially if this can be used directly in the fuel cell, without reformation to hydrogen. A limitation of the direct use of alcohol is the lower activity of oxidation in comparison to hydrogen, and hence to improve activity and power output higher temperatures of operation are preferable. The performance of a high temperature direct methanol fuel cell (DMFC) using PBI based electrode assemblies was investigated. The performance of the system was limited by poor methanol oxidation kinetics in a phosphoric acid environment and consequently power performance was inferior to that achieved with low temperature DMFCs based on Nafion membranes.European commission for their financial support through the FURIM project "Further Improvement and System Integration of High Temperature Polymer Electrolyte Fuel Cells" under frame work 6. FURIM partners: Technical University of Denmark, Volvo Technology Corporation, Norwegian University of Science and Technology, Danish Power Systems, Case Western Reserve University, University of Stuttgart, HyGear, Freudenber

Conception et fabrication d'un banc de mesure en vue de la caractérisation des traitements acoustiques pour les nacelles de turboréacteurs

Actuellement, les turboréacteurs sont les principales sources de bruits d’un avion. Afin de supprimer cette source de bruit qui est très incommodante pour l’environnement, les ingénieurs ont conçu durant ces trente dernières années des matériaux permettant de réduire l’émission de bruit provenant des turboréacteurs. Ces matériaux communément appelés liners sont pour l’instant relativement efficaces. Afin de contrôler la qualité de fabrication des liners, l’instigateur du projet Pratt & Whitney Canada (PWC) veut caractériser ces traitements acoustiques en mesurant leur impédance acoustique en tenant compte des conditions réelles de champ acoustique et des conditions aérodynamiques. C’est ainsi qu’a pris naissance mon projet de maîtrise portant sur la conception et la fabrication d’un banc de mesure en vue de la caractérisation de ces liners. En premier lieu, une amélioration de la méthode de caractérisation géométrique et acoustique des liners au niveau de la nacelle du turboréacteur (in situ) a été apportée. En effet, une méthode de mesure par imagerie a été élaborée pour mesurer le taux de perforation et le diamètre de perforation d’un liner au sein même de la nacelle. D’autre part, les fuites acoustiques au sein du tube d’impédance in situ de PWC ont été éliminées. Par ailleurs, une méthode de mesure in situ de l’impédance des liners a été établie. En second lieu, un banc d’essai a été conçu et fabriqué pour mesurer, par méthode inverse, l’impédance des liners sous incidence rasante. Divers matériaux ont été testés afin de valider le banc. Les résultats expérimentaux sont bien proches des résultats numériques. Nous n’avons pas abouti à la partie considérant l’écoulement vu la limitation de puissance de la soufflerie du GAUS néanmoins le concept théorique et la méthode de mesure expérimentale restent bien valables dans le cas d’un écoulement. Nous avons choisi une soufflante pour générer l’écoulement dans le banc et cette partie sera traitée par un autre étudiant

Approche heuristique pour le placement des machines virtuelles dans un environnement infonuagique de grande taille

L’informatique en nuage est un nouveau paradigme qui fait référence à la mise en disposition des ressources d’une infrastructure informatique permettant de présenter un ensemble de services à la demande. Toutefois, un centre de données pourrait participer grandement dans l’augmentation de l’empreinte carbone si le procédé utilisé par le fournisseur de services ne présente pas une bonne gestion des ressources disponibles. Un fournisseur de services cloud pense principalement aux gains que ses data centers pourraient lui engendrer. Il devra donc trouver un moyen efficace pour que ses bénéfices ne soient jamais diminués. Si de plus, ce dernier pense à la réduction de l’empreinte carbone que pourraient générer ses data centers, il devra trouver un compromis entre les bénéfices qu’ils visent à atteindre et l’empreinte écologique tout en respectant l’accord de niveau de service avec ses clients. Un moyen pour satisfaire ces compromis serait de revoir la disposition des différents composants à l’intérieur du data center. Les machines virtuelles étant une des variables qui pourraient grandement participer dans la définition des stratégies pour la réduction de l’empreinte carbone, le fournisseur de services se voit dans l’obligation de trouver une solution efficace et peu coûteuse. Ainsi, trouver un bon placement des machines virtuelles dans un environnement infonuagique est une étape importante pour les fournisseurs de services afin d’améliorer l’efficacité énergétique de leurs centres de données. Les travaux antérieurs élaborés pour la résolution du problème de placement des machines virtuelles ne regroupent souvent pas toutes les informations concernant les composants dans un centre de données. En effet, chaque composant a sa propre consommation d’énergie et participe de près ou de loin à la définition de l’empreinte écologique d’un data center. Ce mémoire présente le modèle implémenté permettant de calculer l’empreinte carbone d’un centre de données. À travers ce modèle, nous minimisons la fonction objectif à partir des algorithmes de métaheuristique. Nous concevons un algorithme mémétique, une hybridation entre l’algorithme génétique que nous implémentons d’abord seul et l’algorithme de recherche tabou. Nous implémentons ensuite l’algorithme de recuit simulé.----------ABSTRACT: Cloud computing is a new paradigm that refers to the provision of resources of an IT infrastructure allowing to present a set of services on demand. However, in this context, a data center could play a significant role in increasing the carbon footprint, if the process used by the service provider does not adequately manage the available resources. The service provider will therefore need to find the best allocation of its data centers components to improve the energy efficiency. Since virtual machines are one of the components that could play an important role in the definition of strategies for reducing the carbon footprint, the service provider is intended to find an efficient and inexpensive solution. Hence, we can consider that finding an adequate placement of virtual machines in a cloud environment is an important step for service providers. To solve the virtual machines placement problem in the cloud, we need a mathematical model that allow us to compute the carbon footprint while considering all the data center' components. In this thesis, we present the model implemented to calculate the carbon footprint of a data center. Through this latter, we minimize the objective function based on the metaheuristic algorithms. We design a memetic algorithm, a hybridization between the genetic algorithm and the tabu search algorithm. Afterwards, we implement the simulated annealing algorithm. For the evaluation of these algorithms, we compare the results obtained in term of carbon footprint cost, as well as the runtime. All the results obtained will be compared with the iterated tabu search algorithm for various problem sizes. The experiments carried out in our work, demonstrate the effectiveness of the memetic algorithm to solve large-scale problems when trying to allocate virtual machines in data centers

THE VICTIM OF TRAFFICKING IN PERSONS IN TUNISIAN LAW WITH AN OVERVIEW OF THE QATARI LAW

It seems that trafficking in persons is a universal phenomenon, which spare no State. Tunisia is affected by this degrading crime as a country of origin, destination and transit. It is doing significant efforts in order to combat this appalling offense that reduces its victim to goods sold and bought for a miserable price. This study focuses on the efficiency of the victim’s protection which is a certain one, since it immunes the trafficked person from prosecution and provides her/him with different assistances, but it remains limited, as long as the trafficking actors can’t afford its implementation and the State doesn’t care about the vulnerability factors

Water cluster characteristics of fuel cell gas diffusion layers with artificial microporous layer crack dilation

The formation of discrete water clusters in polymer electrolyte fuel cell gas diffusion layers (GDL) can lead to increased resistance for oxygen transport in the catalyst layer. This study investigates the effect of MPL crack propagation on the water cluster development in a X-ray computed tomography (CT) microstructure using the volume-of-fluid method (VoF). The VoF calculation was compared to operando CT data by voxel matching, obtaining a maximum 88 % accuracy. Using 3D contact angle extraction, the local scale heterogeneous wettability in the GDL was investigated. In a simulation study, MPL cracks were created as the boundary sources for water and the effect of increasing the area fraction covered by cracks on the water distribution in the GDL was investigated. The increased cracking, created larger discrete water clusters in the GDL with greater connectivity, due to in-plane coalescence. The in-plane movement leads to coalescence of clusters, forming fewer, larger clusters at later times close to breakthrough to the channel. This phenomena is shown by the decrease in water cluster density (n mm−2) from 10 to 5. This immobile water impacts the distribution of oxygen at the catalyst layer (10 % local difference) and therefore the current density distribution

Anode partial flooding modelling of proton exchange membrane fuel cells:model development and validation

A two-dimensional along-the-channel CFD (computational fluid dynamic) model, coupled with a two-phase flow model of liquid water and gas transport for a PEM (proton exchange membrane) fuel cell is described. The model considers non-isothermal operation and thus the non-uniform temperature distribution in the cell structure. Water phase-transfer between the vapour, liquid water and dissolved phase is modelled with the combinational transport mechanism through the membrane. Liquid water saturation is simulated inside the electrodes and channels at both the anode and cathode sides. Three types of models are compared for the HOR (hydrogen oxidation reaction) and ORR (oxygen reduction reaction) in catalyst layers, including Butler–Volmer (B–V), liquid water saturation corrected B–V and agglomerate mechanisms. Temperature changes in MEA (membrane electrode assembly) and channels due to electrochemical reaction, ohmic resistance and water phase-transfer are analysed as a function of current density. Nonlinear relations of liquid water saturations with respect to current densities at both the anode and cathode are regressed. At low and high current densities, liquid water saturation at the anode linearly increases as a consequence of the linear increase of liquid water saturation at the cathode. In contrast, exponential relation is found to be more accurate at medium current densities

Alterations in Metabolic Profile Occur in Normal-Weight and Obese Men during the Ramadan Fast Despite No Changes in Anthropometry

We examined the variations in eating behavior, appetite ratings, satiety efficiency, energy expenditure, anthropometric and metabolic profile markers prior to, during as well as 1 and 4 months after Ramadan in normal-weight and obese men. Anthropometric, energy expenditure (indirect calorimetry and accelerometry), metabolic (fasting blood sample), appetite (visual analogue scales), and eating behavior (Three-Factor Eating Questionnaire) measurements were performed in 10 normal-weight (age: 25.2 ± 4.7 years; BMI: 24.4 ± 1.9 kg/m2) and 10 obese (age: 27.0 ± 4.5 years; BMI: 34.8 ± 3.7 kg/m2) men. The satiety quotient (SQ) was calculated 180 minutes after breakfast consumption. All anthropometric variables, as well as resting and total energy expenditure, were greater in obese compared to normal-weight participants (P = 0.02–0.0001). Similarly, obese participants had greater triglycerides, insulin, and homeostatic model assessment-insulin resistance concentrations (P = 0.02–0.002). Greater apolipoprotein B, glucose, total cholesterol, and low-density lipoprotein concentrations were noted during Ramadan (P = 0.04–0.0001). Dietary restraint scores were also greater during Ramadan (P=0.0001). No differences in anthropometry, other metabolic profile markers, energy expenditure, appetite ratings, and SQ were noted across sessions. Lastly, changes in anthropometric measurements correlated with delta metabolic profile markers, as well as changes in disinhibition eating behavior trait and dietary restraint scores. The Ramadan fast led to increases in certain metabolic profile markers despite no changes in appetite and anthropometry

The role of tungsten oxide in enhancing the carbon monoxide tolerance of platinum-based hydrogen oxidation catalysts

Significant reductions in total cost of ownership can be realized by engineering PEM fuel cells to run on low-purity hydrogen. One of the main drawbacks of low-purity hydrogen fuels is the carbon monoxide fraction, which poisons platinum electrocatalysts and reduces the power output below useful levels. Platinum-Tungsten oxide catalyst systems have previously shown high levels of CO tolerance during both ex situ and in situ investigations. In this work, we explore the mechanism of enhanced tolerance using in situ electrochemical attenuated total reflection-infrared (ATR-IR) and Raman spectroscopy methods and investigate, using a mixture of Pt/C and WO3 powders, the role of the WV/WVI redox couple in the oxidation of adsorbed CO