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 along with cost reduction. It is desirable to operate the PEMFC at uniform local current density and temperature distributions over the surface of the membrane electrode assembly (MEA). Non-uniform distributions of both current and temperature over the MEA could result in poor reactant and catalyst utilization as well as overall cell performance degradation. Local current distribution in the PEMFC electrodes are closely related to operating conditions, but it is also affected by the organization of the reactant flow arrangements in PEMFCs. Reactant depletion and water formation along the flow channel leads to current variation from the channel inlet to the exit, which leads to non-uniformity of local electrochemical reaction activity, and degradation of the cell performance. Flow arrangements between the anode and cathode streams, such as co-, counter- and cross- flow can exacerbate the effect of the non-uniformity considerably, producing complex current distribution patterns over the electrode surfaces. Thus, understanding of the local current density and its spatial characteristics, as well as the temperature distributions under different physical and operating conditions, is crucially important in order to develop optimum design and operational strategies. Despite the importance of the influence of the flow arrangement on the local current and temperature distributions under various operating conditions, few systematic studies have been conducted experimentally to investigate this effect. In this research, an experimental setup with special PEMFC test cells are designed and fabricated in-house, in order to conduct in-situ mapping of the local current and temperature distributions over the electrode surfaces. A segmented flow field plate and the printed circuit board (PCB) technique is used to measure the current distribution in a single PEMFC. In situ, nondestructive temperature measurements are conducted using thermocouples to determine the actual temperature distribution. Experimental studies have been conducted to investigate the effect of different flow arrangements between the anode and cathode (co-, counter-, and cross- flow) on the local current density distribution over the MEA surface. Furthermore, local current distribution has been characterized for PEMFCs under various operating conditions such as reactant stoichiometry ratios, reactant backpressure, cell temperature, cell potentials, and relative humidity for each one of the reactant flow arrangements. The dynamic characteristics of the local current in PEMFC under different operating conditions also have been studied. Temperature distributions along the parallel and serpentine flow channels in PEMFs under various operating conditions are also investigated. All independent tests are conducted to identify and optimize the key design and operational parameters for both local current and temperature distributions. It has been found that the local current density distribution is strongly affected by the flow arrangement between the anode and cathode streams and the key operating conditions. It has also been observed that the counter-flow arrangement generates the most uniform distribution for the current density, whereas the co-flow arrangement results in a considerable variation in the current density from the reactant gas stream inlet to the exit. Low stoichiometry ratio of hydrogen at the anode side has a predominant effect on the current distribution and cell performance. Further, it has been found that the dynamic characteristics and the degree of fluctuation of local current density inside PEMFC are strongly influenced by the crucial operating conditions. In-situ, nondestructive temperature measurements indicate that the temperature distribution inside the PEMFC is strongly sensitive to the cell’s current density. The temperature distribution inside the PEMFC seems to be virtually uniform at low current density, while the temperature variation increases up to 2 oC at the high current density. Finally, the present work contribution related to the local current and temperature distributions is required to understand the effect of each individual or even several operating parameters combined together on the local current and temperature distributions. This will help to develop an optimum design, which leads to enhancing the reliability and durability in operational PEMFCs.

<p>The production of chlorate, chlorine and sodium hydroxiderelies heavily on electrical energy, so that savings in thisarea are always a pertinent issue. This can be brought aboutthrough increased mass transfer of reacting species to therespective electrodes, and through increased catalytic activityand uniformity of current density distribution at theseelectrodes. This thesis will present studies involvingmathematical modelling and experimental investigations of theseprocesses. They will show the effect that hydrodynamicbehaviour has on the total current density and cell voltages,along with the effects on current density distributions andindividual overpotentials atthe respective electrodes.</p><p>Primary, secondary and psuedo-tertiary current densitydistribution models of a chlor-alkali anode are presented anddiscussed. It is shown that the secondary model presentsresults rather similar to the pseudo-tertiary model, when thecurrent density distribution is investigated, although thepotential distribution differs rather markedly. Furthermore, itis seen that an adequate description of the hydrodynamicsaround the anode is required if the potential distribution, andthereby the prevalence of side-reactions, is to be reasonablepredicted.</p><p>A rigorous tertiary current density distribution model ofthe chlorate cell is also presented, which takes into accountthe developing hydrodynamic behaviour along the height of thecell. This shows that an increased flowrate gives more uniformcurrent density distributions. This is due to the fact that theincreased vertical flowrate of electrolyte replenishes ioncontent at the electrode surfaces, thus reducing concentrationoverpotentials. Furthermore, results from the model lead to theconclusion that it is the hypochlorite ion that partakes in themajor oxygen producing side-reaction.</p><p>A real-scale cross-section of a segmented anode-cathode pairfrom a chlorate cell was designed and built in order to studythe current density distribution in industrial conditions.These experiments showed that increased flowrate brought aboutmore even current density distributions, reduced cell voltageand increased the total current density. An investigation ofthe hydrodynamic effects on the respective electrodeoverpotentials shows the anode reactions being more favoured byincreased flowrate. This leads to the conclusion that theuniform current density distribution, caused by increasedflowrate, occurs primarily through decreasing the concentrationoverpotential at the anode rather than by decreasing thebubble-induced ohmic drop at the cathode.</p><p>Finally, results from experiments investigating thebubble-induced free convection from a small electrochemicalcell are presented. These experiments show that Laser DopplerVelocimetry is the most effective instrument for investigatingthe velocity profiles in bubble-containing electrochemicalsystems. The results also show that the flow can transform fromlaminar to turbulent behaviour on both the vertical andhorizontal planes, in electrochemical systems where bubbles areevolved.</p>

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 along with cost reduction. It is desirable to operate the PEMFC at uniform local current density and temperature distributions over the surface of the membrane electrode assembly (MEA). Non-uniform distributions of both current and temperature over the MEA could result in poor reactant and catalyst utilization as well as overall cell performance degradation. Local current distribution in the PEMFC electrodes are closely related to operating conditions, but it is also affected by the organization of the reactant flow arrangements in PEMFCs. Reactant depletion and water formation along the flow channel leads to current variation from the channel inlet to the exit, which leads to non-uniformity of local electrochemical reaction activity, and degradation of the cell performance. Flow arrangements between the anode and cathode streams, such as co-, counter- and cross- flow can exacerbate the effect of the non-uniformity considerably, producing complex current distribution patterns over the electrode surfaces. Thus, understanding of the local current density and its spatial characteristics, as well as the temperature distributions under different physical and operating conditions, is crucially important in order to develop optimum design and operational strategies. Despite the importance of the influence of the flow arrangement on the local current and temperature distributions under various operating conditions, few systematic studies have been conducted experimentally to investigate this effect. In this research, an experimental setup with special PEMFC test cells are designed and fabricated in-house, in order to conduct in-situ mapping of the local current and temperature distributions over the electrode surfaces. A segmented flow field plate and the printed circuit board (PCB) technique is used to measure the current distribution in a single PEMFC. In situ, nondestructive temperature measurements are conducted using thermocouples to determine the actual temperature distribution. Experimental studies have been conducted to investigate the effect of different flow arrangements between the anode and cathode (co-, counter-, and cross- flow) on the local current density distribution over the MEA surface. Furthermore, local current distribution has been characterized for PEMFCs under various operating conditions such as reactant stoichiometry ratios, reactant backpressure, cell temperature, cell potentials, and relative humidity for each one of the reactant flow arrangements. The dynamic characteristics of the local current in PEMFC under different operating conditions also have been studied. Temperature distributions along the parallel and serpentine flow channels in PEMFs under various operating conditions are also investigated. All independent tests are conducted to identify and optimize the key design and operational parameters for both local current and temperature distributions. It has been found that the local current density distribution is strongly affected by the flow arrangement between the anode and cathode streams and the key operating conditions. It has also been observed that the counter-flow arrangement generates the most uniform distribution for the current density, whereas the co-flow arrangement results in a considerable variation in the current density from the reactant gas stream inlet to the exit. Low stoichiometry ratio of hydrogen at the anode side has a predominant effect on the current distribution and cell performance. Further, it has been found that the dynamic characteristics and the degree of fluctuation of local current density inside PEMFC are strongly influenced by the crucial operating conditions. In-situ, nondestructive temperature measurements indicate that the temperature distribution inside the PEMFC is strongly sensitive to the cell’s current density. The temperature distribution inside the PEMFC seems to be virtually uniform at low current density, while the temperature variation increases up to 2 oC at the high current density. Finally, the present work contribution related to the local current and temperature distributions is required to understand the effect of each individual or even several operating parameters combined together on the local current and temperature distributions. This will help to develop an optimum design, which leads to enhancing the reliability and durability in operational PEMFCs.

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第14163号 / 工博第2997号 / 新制||工||1445(附属図書館) / 26469 / UT51-2008-N480 / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 落合 庄治郎, 教授 河合 潤, 准教授 奥田 浩司 / 学位規則第4条第1項該当

La production d’hydrogène par électrolyse de l’eau PEM prendra une place importante dans le paysage énergétique pour le stockage des EnR. Le changement d’échelle nécessaire ne peut s’envisager que par une augmentation significative de la puissance nominale, passant essentiellement par l’accroissement de leur taille et de la densité de courant. Dans ces conditions, un fonctionnement optimal et une durée de vie suffisante ne pourront être obtenus que par l’homogénéisation de la répartition du courant à la surface des électrodes. Au cours de cette thèse, nous avons utilisé pour la première fois un outil de cartographie des distributions de courant et de température à la surface d’AME grande surface, issus d’un design industriel. Une carte de mesure S++® conçue sur mesure et adaptée à l’utilisation envisagée a été intégrée à une monocellule PEM de 250cm². Une caractérisation électro-mécanique de la cellule a mis en évidence le lien existant entre le champ de forces de compression mécanique et de la densité de courant. Nous montrons qu’une compression mécanique optimale n’est pas suffisante pour homogénéiser la distribution de courant : le design de cellule, et plus particulièrement la distribution des fluides, joue un rôle majeur dans l’inhomogénéité de la distribution de courant, récurrente entre le centre et la périphérie de la cellule. Nous soulignons la concentration des lignes de courant vers le centre de l’AME lors de tests dynamiques, conséquence d’un vieillissement spatialement différencié. Nous avons également développé une structure d’électrode permettant de ré-homogénéiser globalement la distribution de courant, ce qui permet un meilleur maintien des performances dans le temps. Nous avons également développé un modèle numérique de la couche catalytique permettant de mieux comprendre la répartition des lignes de courant en fonction des caractéristiques géométriques des collecteurs poreux. Nous mettons en lumière le rôle majeur des surtensions dans le pouvoir répartiteur de la couche active, qui est particulièrement faible côté cathodique. Nous préconisons de densifier la couche catalytique pour une meilleure répartition du courant et pour limiter les différenciations locales de vieillissement. L’ensemble des observations en mono cellule a été confirmé par des essais sur un stack commercial. / Hydrogen production from PEM water electrolysis will take a great place in the energy landscape for RES storage. This scale shift requires a significant increase of the nominal power, and therefore an increase in size and a gain in the current density. Optimal operation (in terms of efficiency and lifetime) can be obtained only if the distribution of current lines over the electrode surface is adequately homogeneous. In this thesis, we have used for the first time a specific tool for the in-situ mapping of current and temperature in a large surface area PEM single cell. A customized S++® measuring plate, adapted to our application, has been implemented in a 250cm² PEM single cell. Electromechanical characterization of the cell has put into evidence the link between the field of clamping force and the local current density. We have shown that an optimal mechanical compression is not sufficient to homogenize current distribution. We have demonstrated that the cell design, in particular the fluid distribution, plays a major role in current distribution inhomogeneities, which recurrently form between the center and the periphery of the cell. We have also shown that during dynamic operation, current lines tend to concentrate at the center of the cell as a consequence of spatially differentiated ageing. We have developed an electrode structure that facilitates the global re-homogenization of current lines and additionally shows an increased durability. In parallel, we have developed a numerical model to calculate the distribution of current lines within the thickness of catalytic layers as a function of the geometry of the PTL. We have found that overvoltages play a major role in current distribution, and that the cathode is prone to more heterogeneities. We propose to densify the catalyst layers for a better current repartition and a lesser differentiated ageing. Key findings from single cell tests have been confirmed on a commercial stack.

Les procédés électrochimiques présentent beaucoup d’avantages dans le domaine du traitement et de la récupération de matière d’effluents industriels. Cependant, dans le cas de solutions diluées en ions métalliques, les électrodes classiques sont fortement limitées par leur efficacité ainsi que par leur taille. Dès lors, les électrodes poreuses, de par leur surface spécifique importante et de par leur structure particulière qui améliore le transport de matière et donc l’efficacité de l’électrode, représentent une alternative très intéressante aux électrodes classiques. Parmi les électrodes poreuses, celles constituées de fibres métalliques semblent les plus prometteuses. L’objectif de ce travail est de donner les relations utiles pour dimensionner une cellule contenant ce type d’électrodes en vue du traitement d’effluents industriels contenant des ions métalliques. Les électrodes étudiées ont été caractérisées par différentes techniques : microscopie électronique, méthode électrochimique, mesure de la perte de charge, conductimétrie, porosimétrie,… Cette caractérisation a permis de connaître la porosité, les surfaces spécifiques (géométrique, dynamique et électrochimique) et la tortuosité des électrodes. Ensuite, le coefficient de transport de matière moyen a été étudié par une nouvelle méthode basée sur la mesure d’un rendement électrochimique. Cette méthode présente l’avantage de pouvoir travailler avec des vitesses de circulation de l’électrolyte compatibles avec celles utilisées industriellement. Pour cela, une cellule d’électrolyse à circulation forcée a été mise au point. Afin de comprendre comment la géométrie d’une électrode poreuse de ce type influence le transport de matière local et la densité de courant et donc l’efficacité de l’électrode, le transport de matière et la densité de courant locale ont été modélisés autour d’un cylindre (représentatif d’une fibre) et validés par des mesures expérimentales. La modélisation s’est ensuite étendue à un réseau de fibres cylindriques représentatif des électrodes poreuses étudiées. Cette modélisation a permis d’obtenir une relation générale liant les nombres de Sherwood, de Reynolds et de Schmidt à des nombres sans dimension caractérisant la géométrie du réseau de fibres. Cette relation donne des résultats concordants avec ceux obtenus expérimentalement pour les électrodes poreuses étudiées. Le volume utile d’une électrode poreuse dépend fortement des conditions expérimentales (concentration de l’électrolyte, vitesse de circulation, intensité du courant appliquée,…) et de la structure de l’électrode (porosité, surface spécifique,…). Ces paramètres influencent la distribution du potentiel et de la densité de courant dans l’électrode. Différents modèles de distribution sont comparés et appliqués aux électrodes poreuses étudiées. Cette distribution de courant influence le colmatage progressif de l’électrode poreuse en cours d’électrolyse. Il s’avère que l’électrode en contrôle diffusionnel (avec un rendement électrochimique faible) optimise la distribution du courant dans l’électrode et, de ce fait, ralenti son colmatage. De plus, travailler avec une solution diluée et une vitesse de circulation de l’électrolyte importante améliore la distribution du courant. Il en est de même si l’électrode poreuse présente une grande porosité et une faible surface spécifique. Ce travail aura donc permis de proposer des relations indispensables pour le dimensionnement d’une cellule à électrodes poreuses (constituées de fibres métalliques) ainsi que les conditions opératoires idéales dans le cas du traitement d’effluents industriels contenant des ions métalliques./ Electrochemical techniques offer many advantages for the prevention of pollution problems in the industrial processes. However, flat electrodes are not ideal to treat dilute solutions containing metallic ions. With their high specific surface and open structure, which enhance mass transfer, porous electrodes are a good alternative for the treatment this kind of effluent. Fibre materials are particularly well suited as material for the production of porous electrodes. The aim of this thesis is to study an electrochemical cell with a porous electrode in order to treat dilute metallic ions solutions and to provide dimensionless equations suited to scale-up the electrode for industrial application. The porous electrodes, used in this thesis, are made of a stainless steel fibre network. The main properties and characteristics of these electrodes are studied by means of several techniques : electron microscopy, electrochemical methods (voltammetry, limiting current density measurerment), conductivity measurement, porosimetry, pressure drop measurement,… The obtained parameters are : porosity, specific surfaces (geometric, dynamic and electrochemical), fibres' diameter, tortuosity and the geometric disposition of the fibres in the electrodes. Mass transfer inside the porous electrodes is studied experimentally by a new developed method, linked to the measurement of the faradic yield as a function of different electrolysis parameters. For these measurements, an experimental electrolysis cell with high electrolyte flow rate has been designed and builds. To understand how the geometry of the porous electrode influences the local and mean mass transfer coefficients and current densities, numerical studies and simulations have been performed. The first type of simulation deals with a single wire (representative of a fibre from the porous electrode). The second type of simulation deals with the integration of individual fibres in a fibre network. A correlation between dimensionless numbers such as Sherwood's, Reynolds' and Schmidt's numbers together with numbers characteristic of the electrode’s geometry has been established for Reynolds’s numbers ranging from 0,02 to 1,4. A good agreement between simulation and experimental measurements of mass transfer is observed. The real effective electrochemical volume of the porous electrode depends on experimental conditions (current, concentration, flow velocity…) and electrode’s geometry (porosity, specific surface,…). These parameters influence the potential and current distribution inside the porous electrode. Several models of current distribution are applied to these electrodes and the theoretical simulations are compared with experimental measures. As a result of these simulations, an electrode under diffusion control with a small faradic yield appears to be the best choice in order to homogenise the current density inside the porous electrodes. Dilute solutions, high flow velocity and electrodes with high porosity improve also the current density penetration inside the electrode. These observations are confirmed by an electrode’s plugging study. In conclusion, this thesis provides mathematical relationships to scale-up a cell with porous electrodes of metallic fibre, and provides guidelines to treat, in an efficient manner industrial effluents containing metallic ions.

The development of both experimental and diagnostic equipment to assist with simulating the mechanical effects of cold X-ray deposition is covered by this work. This thesis reviews the various experimental techniques suitable for conducting the electromagnetic launch of flyer plates and the chosen technique is developed into a fully functional experimental facility. The development of a bespoke 1-dimensional computer model is also described in the text. A novel current distribution measurement technique is also fully described. This new diagnostic approach will allow the variation of the current across the width of a large conductor to be easily determined which is something not previously demonstrated.

With the growth of renewable energy usage and energy storage adoption in recent decades, people have started to reevaluate the possible roles of dc systems in current and future electrical systems. The dc voltage distribution has been applied in various applications, such as data centers and aircraft industry, for high efficiency and power density. However, for some applications such as subsea gas and oil fields, and ocean observatory systems, the dc current distribution is preferred over dc voltage distribution for its low cost and robustness against cable faults. Design and control of dc power distribution systems for different applications is an emerging research area with complex technical challenges. This dissertation solves the technical challenges in analysis, design, modeling, control and protection of series resonant converters (SRCs) for dc current distribution applications. An optimum design that has high efficiency, high reliability, and minimum required control efforts for the SRC with constant input current has been achieved and demonstrated by applying the analysis and design procedures developed in this dissertation. The modeling and analysis presented in this dissertation represents an operating condition that has not been studied in the literature and could be easily extended to other resonant converter topologies. Explicit analytical expressions have been provided for all key transfer functions, including input impedance and control-to-output, offering valuable resources to design feed-back regulation and to evaluate system stability. Based on the control strategies and control design presented in this dissertation, stable and reliable operation of dc current distribution systems with long distance cable has been achieved and demonstrated. The proposed analysis, design procedure, stability evaluation, control strategy and protection techniques in this dissertation can be applied to a wide range of similar scenarios as well, which greatly increases their value.

In this thesis, two types of handset antenna design respectively for dual-band GSM850/1900 operation and seven¡Vband LTE/WWAN operation are presented. The proposed antennas can achieve decreased antenna size and enhanced operating bandwidth by shaping the system ground plane of the handset. The seven-band LTE/WWAN antenna is printed on the system circuit board with good radiation characteristics and occupies a small size of 24.5 x 10 mm2. At first, we propose a half-loop antenna structure which integrates its facing edge of the system ground plane as the resonant path of the antenna. In this case, the required length of the half-loop antenna structure is only about a half compared with the traditional loop antenna. Next, to enhance the operating bandwidth of the antenna, a parasitic shorted monopole strip is added and a shaped system circuit board is used. The shaped system circuit board can integrate the battery with the circuit board to help reduce the thickness of handset. Also, much enhanced operating bandwidth for the lower band can be obtained, owing to the enhanced surface current excitation in the system ground plane resulting from properly shaping the system circuit board. Effects of the user¡¦s head and hand on the proposed antenna are studied, and the simulated SAR and HAC results are also analyzed for bio-compatibility issue.

In this dissertation, novel printed multiband antennas for slim mobile communication devices are presented. The design techniques on achieving decreased antenna size and enhanced operating bandwidth in different shaped system ground planes with the antennas printed thereon are proposed. At first, a WWAN antenna integrated with a U-shaped ground plane is presented. Two techniques are applied to achieve WWAN operation with a small antenna size. The first technique is to embed load a proper chip inductor at a proper position in the shorted strip monopole of the antenna. The second one is to use a band-stop matching circuit disposed on the main circuit board of the handset. For the second and third antenna designs, owing to the C-shaped ground plane, much enhanced operating bandwidth for their lower band can be obtained. The second antenna design can cover the LTE/WWAN eight-band operation without increasing the size of the antenna by using a reconfigurable circuit element. The third antenna design integrated with a battery element and a metal midplate for practical application to help reduce the thickness and enhance the robustness of handset. The antenna can cover seven-band LTE/WWAN operation with good radiation characteristics and occupy a small size of 35 x 8 mm2. Finally, the simulated SAR and HAC results are analyzed for bio-compatibility consideration.