Für nanoskalige Füllpartikel, die stark zum Agglomerieren tendieren, ist das Ultraschalldispergieren mittels Sonotrode eine geeignete Methode, um Agglomerate hinreichend gut zu entbündeln. Um dabei die optimalen Ultraschallparameter ermitteln zu können, ist es nötig, das effektive Dispergiervolumen, in welchem die Agglomerate durch Kavitation aufgebrochen werden, zu kennen. Die hier vorgestellte CAD-basierte Methode zur Berechnung des effektiven Dispergiervolumens ist dabei deutlich weniger zeitintensiv als die bisher üblicherweise verwendete analytische Methode.

In the beginning of 21st century, like many nations across the world, Taiwan has worked hard in an attempt to establish a new economy. This new economy will be comprised of intellectual property and high-tech industries which high economic return, low pollution, and all other added values will truly bring another phase of prosperity to Taiwan. Ideally, the government hopes to pattern the successful model of Hsinchu Science Park to the new establishment of biotechnology-related science parks across the island. Therefore, the Southern Taiwan Science Park, Nankang Science Park, and Chupei Biomedical Science Park were set up. Meanwhile, the Cabinet¡]the Executive Yuan¡^also evaluated the existing bio-tech industries in terms of natural resources, academic support, and cluster effect through the Strategic Review Board (SRB). The SRB is composed of experts from industries, government officials, academicians, and researchers as well. After sophisticated discussion, SRB reached the following conclusions: 1. biotech science parks are essential to form the cluster effect to Taiwan¡¦s biotechnology. 2. The biotech science parks shall be established in regions across the island in order to integrate the resources from the northern and southern Taiwan . In June of 2002, the Cabinet has officially initiated the plan ¡§The Challenge 2008 ¡V Nation-level Development Plan¡¨ which included ¡§Two Trillion, Two Stars ¡¨ plan devised by the Ministry of Economic Affairs, and the bio-tech development is included in the ¡§Two-Two ¡¨ plan . At the moment, Taiwan¡¦s bio-tech annual output to 100.9 billion NT$, and will bring this number up to 250 billion NT$ by 2006. In addition, the government also aims to attract another 100 billion NT$ of investment in bio-tech by the establishment of another 500 bio-tech companies within the next ten years. Some Cabinet-level agencies, such as the Council for Economic Planning and Development, the National Science Council, and the Council of Agriculture also coordinated with the local governments to plan the establishment of bio-tech science parks. According to the government¡¦s ¡§Bio-tech Industry Promotion Programs (BIPP) ¡§, four bio-tech science parks have been planned : Nankang Bio-tech Park, Chupei Biomedical Park, Southern Taiwan Bio-tech Park, and Tunghwa Bio-tech Incubation Center. The success factors for those programs will hinge on the Taiwan¡¦s ability to conquer over the current economic slowdown, to promote and maintain Taiwan¡¦s existing economic competitiveness and growth. In fact, with Taiwan¡¦s economic power, the avid promotion of knowledge-based economy, the ambition to the bio-tech industry, and the avid support from the different fields, Taiwan has amassed the essential factors to create a successful bio-tech industry. However, to realize the dream of initiating biotechnology to become the next ¡§star industry¡¨ in Taiwan, the detail planning, the execution, and the integration of the government and private resources, it dearly deserves our close attention and supervision. The purpose of this study is to investigate the ¡§bio-tech cluster effect¡¨ with respect to other factors including¡¨ resource distribution¡¨, ¡§social interaction¡¨, ¡§cluster demographics¡¨, ¡§nuclear/peripheral development¡¨, ¡§cluster economics¡¨, ¡§non-profit marketing and sales¡¨ and so on. This study will also introduce some bio-tech science parks and clusters (e.g. Europe, U.S., Asia, and China), to present their current status and past experience, then compare with the planning and development of the bio-tech science parks in Taiwan. With theoretical analysis and factual experience, this study will analyze the advantage and restrictions of the development of bio-tech science parks. Additionally, this study will provide a few viewpoints from the foreign for the government and bio-tech industry in the building of the new star industry. This study pointed out, right at the juncture where the bio-tech sets for the next wave of industry, that we are faced with the shortage of talents, the high-level technology, and funds. As a result, the development of bio-tech industry in Taiwan has fallen short of expectation, and limited the scope of discussion of this study. As of now, the bio-tech industry in Taiwan has not created any significant bio-tech-related product, the funds for bio-tech-related research only amounts to 7 billion NT$, not even 1% of the U.S. With over 100 billion NT$ of venture capital funds, only less than 5% was invested in bio-technology. Unless the government increases its R&D fund and hasten its steps in the promotion of bio-tech start ups; otherwise, Taiwan will continue to lag behind in its bio-tech development. Another unexpected factor, which plays a major role in the development of bio-technology, is the instability of the policy. Bio-technology promotions and development are easily influenced by politicians and leaders of different political backers, and often stalling various plans. For example, an approved proposal with a budget at 3.9 billion NT$ was overturned by the reason of ¡§inappropriate budgeting plan¡¨ in April of 2003. The Cabinet¡¦s proposal consisted of recruiting new bio-tech companies, biotech experts, new technologies, and to purchase Nankang bio-technology building which will increase the substantiality of ¡§the Greater Taipei Biotechnology R&D Cluster¡¨, however, the Congress¡]the Legislative Yuan¡^considered the Chinese-American Economic and Social Development Fund was not suitable for such bio-tech development and promotion. This study also pointed out some certain aspects that are beneficial to the development of biotechnology in Taiwan, to express our support to the current bio-tech-related policies, we sincerely hope that bio-tech industries will has a fruitful growing in the near future. The previous success in manufacturing calculators, computers, and other electronics that once earned Taiwan the title of ¡§Island of Technology¡¨ will again bring the island a new title, ¡§Island of Biotechnology¡¨. One of the major suggestions of this study is to advise the bio-tech industries could give up their existing ¡§subjectivity¡¨ and replace this mindset with ¡§objectivity¡¨ where sharing of knowledge, critical discussion, and mutual interaction, will bring a ¡§cohesive effect¡¨ that will ultimately reflect the benefits of biotech cluster. Researchers, professors, scientists, and students from universities, research organizations, and incubation centers should bring their promising research to nearby science parks to find appropriate matching companies to give the further collaboration in research, co-develop, manufacture, market, and sale promising in biotech products. Another point worth mentioning here is that both the public and private sector should play the equal role and share the same responsibility. In other words, from the Ministry of Economic Affairs, National Science Council, Council of Agriculture to the local government, and the biotechnology science park should get involved the support to the bio-tech and avoid overlaps of resource allocation. The government should evenly distribute resources, encourage fair competitions and beneficial collaborations. Even more, state owned corporations such as Taiwan Sugar Corporation and ADImmune Corporation have jumped in the bio-tech band wagon and also participated in the formation of bio-tech science parks. Anyway, one of the most ideal propositions is to promote agricultural-biotech zone from the middle to southern Taiwan science parks, in order to take advantage of Taiwan¡¦s existing natural resources, the northern science parks should emphasize on R&D, drug manufacturing, medical device design and manufacturing, bio-tech service, and so on. For example, each bio-tech park should focus on a major area and its related-services such as flower industry should match well with fruits, and vegetables while water/ocean produces should be separated. This way, each bio-tech science park will distinguish itself and even cooperate with others, and each will have its own playground and market niche. In summary, combining local political/economic situations and overseas¡¦ bio-tech park experience, this study presents the following key points that may prove to be essential in the successful establishment of Taiwan¡¦s biotech science park and cluster development: 1. Regardless of which party is in charge, government should concentrate on the establishment of science parks; 2. The establishment of science parks should base on cluster style, absorbing all potential talents and encourage R&D seedlings; 3. Sufficient R&D fund, and develop core facilities to incubate up-coming technologies; 4. To establish good relationships and frequent interactions with surrounding universities and research institutes, to ensure the flow of outstanding talents; 5. Government should promote more establishment of science parks through the Association of Allied Industries in Science-Based Industrial Park, National Applied Research Laboratories, Industrial Technology Research Institute, and other non-profit organizations, and to make more reasonable development process and flexibility in terms of hardware planning; 6. To provide easy access to public transportation and highway networks and comfortable and quality living environment to science parks.

A água é essencial à manutenção da vida. No entanto, com as situações de estresse hídrico - disponibilidade hídrica inferior a 1.700 m per capita ao ano (FALKENMARK, 1989) - vivenciadas em diversos pontos do planeta, somadas ao acelerado crescimento da população mundial, os problemas relacionados ao uso da água tendem a aumentar. Neste contexto, a pegada hídrica (PH), que é um indicador de sustentabilidade ambiental, se torna uma importante ferramenta de gestão de recursos hídricos pois indica o consumo de água doce com base em seus usos. O presente trabalho objetiva mensurar a pegada hídrica em função das componentes industrial, doméstica e alimentar da população do bairro Rocinha, um aglomerado subnormal localizado no município do Rio de Janeiro. A pesquisa se deteve a um Estudo de Caso de 20 sub-bairros da comunidade. Sua abordagem foi quantitativa, contando com uma amostra de 203 domicílios, erro amostral de 7% e grau de confiança de 93%. Para tal, foi utilizada como ferramenta de cálculo o modelo Water Footprint Network do ano de 2005. Os resultados indicaram que, em média, a PH dos indivíduos que compõem a amostra é de 1715 m/ano per capita assim divididos: PH de consumo doméstico de água de 175 m/ano per capita (479 l/hab.dia); PH de produtos agrícolas igual a 1470 m/ano per capita, e PH de produtos industrializados de 70 m/ano per capita. Os resultados obtidos sugerem que os indivíduos da amostra com uma maior despesa mensal tendem a ter pegadas hídricas industrial e total também maiores. / Water is essential for sustaining life. However, the situations of hydric stress - less than 1.700 m per capita water availability per year (FALKENMARK, 1989) - experienced in many parts of the world, coupled with the rapid growth of the global population, the problems related to water use tend to increase. In this context, the water footprint (WF), which is an indicator of environmental sustainability becomes an important tool for water resource management indicates the consumption of fresh water based on their uses. This study aimed to measure the water footprint as a function of industrial components, domestic and feed of the population of the Rocinhas neighborhood, a cluster subnormal located in the municipality of Rio de Janeiro. The research was focused on a case study of 20 sub-districts of the community. Its approach was quantitative, with a sample of 203 households, sampling error of 7% and confidence level of 93%. For this purpose, was used as the tool for calculating the Water Footprint Network model year 2005. The results indicated that, on average, the WF of the individuals in the sample is 1715 m/ year per capita divided as follows: WF of domestic water consumption of 175 m/ year per capita (479 l/ inhab.day), WF agricultural products equal to 1470 m/ year per capita, and WF industrial products 70 m/ year per capita. The results suggest that individuals in the sample with a higher monthly expenditure tend to have industrial water footprints and overall greater.

Un nouveau procédé, basé sur une adaptation innovante du principe de protection cathodique des ouvrages métalliques en milieu marin, consiste à favoriser la précipitation d’un agglomérat calcomagnésien sur plusieurs centimètres d’épaisseur. En présence de sable et de coquillages, cet agglomérat pourrait permettre de stabiliser des zones littorales en cours d’érosion ou d’améliorer l’ancrage d’enrochements brise-lames. L’objectif de ce travail est d’étudier et de comprendre les mécanismes réactionnels de formation de cet agglomérat permettant une précipitation en volume loin de la cathode afin d’en optimiser la vitesse de formation ainsi que ses propriétés mécaniques. Premièrement, il a été montré dans ces travaux que le milieu électrolytique pouvait influer sur la croissance du dépôt, avec une possible inhibition de formation de carbonate de calcium pour les faibles potentiels. L’influence des produits de corrosion dans le mécanisme de protection cathodique a également été mise en évidence grâce à des polarisations effectuées dans NaCl, eau de mer artificielle et naturelle. Deuxièmement, des essais d’un mois en courant imposé ont été menés du laboratoire jusqu’au site naturel, afin de se rapprocher des conditions réelles. La densité de courant appliquée ainsi que les conditions hydrodynamiques sont des facteurs clefs étroitement liés l’un à l’autre, influant sur la masse et le rapport Ca/Mg. Ces travaux ont aussi permis de mettre en avant l’influence du temps de polarisation sur la composition du dépôt calcomagnésien, présentant une cinétique de croissance stable mais une fluctuation de composition après 15, 30 et 60 jours. Cette variation interne du rapport Ca/Mg, attribuée à un processus de « dissolution/précipitation » dépendant du gradient de pH, ainsi que la répartition de ces éléments au sein du dépôt pourraient jouer un rôle majeur dans l’évolution de la tenue mécanique du dépôt au cours du temps. / A new method, based on an innovative adaptation of cathodic protection principle of metal structures in marine environment, is to promote the precipitation of a calcareous agglomerate several centimeters thick. In the presence of sand and shells, this cluster could stabilize coastal areas being eroded or improve anchorage of breakwater riprap. The objective of this work is to study and understand the reaction mechanisms allowing to create a voluminous calcareous deposit away from the cathode and to optimize deposition time as well as its mechanical properties. First, it was shown in this work that the electrolytic medium could affect the growth of the deposit, with a possible inhibition of formation of calcium carbonate fot the low potential. The influence of corrosion products in the cathodic protection mechanism was also highlighted through polarizations performed in NaCl, artificial and natural sea water. Second, one month test with impressed current were conducted from laboratory to natural site, in order to get closer of real conditions. Current density and hydrodynamic conditions are key factors closely bonded to each other, affecting the mass and composition of the Ca/Mg ratio. This work has also helped to highlight the influence of the polarization time on the composition, with stable growth kinetics but a composition’s evolution after 15, 30 and 60 days. This internal variation of the Ca/Mg ratio attributed to a process of « dissolution/precipitation » is pH dependent and the distribution of these elements within the deposit could play a major role in the evolution of the mechanical strength of deposit over time.

A água é essencial à manutenção da vida. No entanto, com as situações de estresse hídrico - disponibilidade hídrica inferior a 1.700 m per capita ao ano (FALKENMARK, 1989) - vivenciadas em diversos pontos do planeta, somadas ao acelerado crescimento da população mundial, os problemas relacionados ao uso da água tendem a aumentar. Neste contexto, a pegada hídrica (PH), que é um indicador de sustentabilidade ambiental, se torna uma importante ferramenta de gestão de recursos hídricos pois indica o consumo de água doce com base em seus usos. O presente trabalho objetiva mensurar a pegada hídrica em função das componentes industrial, doméstica e alimentar da população do bairro Rocinha, um aglomerado subnormal localizado no município do Rio de Janeiro. A pesquisa se deteve a um Estudo de Caso de 20 sub-bairros da comunidade. Sua abordagem foi quantitativa, contando com uma amostra de 203 domicílios, erro amostral de 7% e grau de confiança de 93%. Para tal, foi utilizada como ferramenta de cálculo o modelo Water Footprint Network do ano de 2005. Os resultados indicaram que, em média, a PH dos indivíduos que compõem a amostra é de 1715 m/ano per capita assim divididos: PH de consumo doméstico de água de 175 m/ano per capita (479 l/hab.dia); PH de produtos agrícolas igual a 1470 m/ano per capita, e PH de produtos industrializados de 70 m/ano per capita. Os resultados obtidos sugerem que os indivíduos da amostra com uma maior despesa mensal tendem a ter pegadas hídricas industrial e total também maiores. / Water is essential for sustaining life. However, the situations of hydric stress - less than 1.700 m per capita water availability per year (FALKENMARK, 1989) - experienced in many parts of the world, coupled with the rapid growth of the global population, the problems related to water use tend to increase. In this context, the water footprint (WF), which is an indicator of environmental sustainability becomes an important tool for water resource management indicates the consumption of fresh water based on their uses. This study aimed to measure the water footprint as a function of industrial components, domestic and feed of the population of the Rocinhas neighborhood, a cluster subnormal located in the municipality of Rio de Janeiro. The research was focused on a case study of 20 sub-districts of the community. Its approach was quantitative, with a sample of 203 households, sampling error of 7% and confidence level of 93%. For this purpose, was used as the tool for calculating the Water Footprint Network model year 2005. The results indicated that, on average, the WF of the individuals in the sample is 1715 m/ year per capita divided as follows: WF of domestic water consumption of 175 m/ year per capita (479 l/ inhab.day), WF agricultural products equal to 1470 m/ year per capita, and WF industrial products 70 m/ year per capita. The results suggest that individuals in the sample with a higher monthly expenditure tend to have industrial water footprints and overall greater.

O principal objetivo do presente trabalho foi compreender a dinâmica industrial do Aglomerado Urbano-Industrial de Londrina, considerando que no processo de reestruturação produtiva, as transformações vão se manifestar também nas cidades médias. Assim como o estado do Paraná o Aglomerado Urbano-Industrial de Londrina, formado pela cidade média de Londrina e seu entorno, a partir de 1992 passou a apresentar desenvolvimento industrial importante., porém acentuando o caráter desigual deste desenvolvimento. A dinâmica industrial recente apresenta dois processos distintos: o primeiro está relacionado ao desenvolvimento de setores industriais tradicionais na região, intensivos em mão-de-obra e diretamente relacionados a empresas de pequeno e médio porte, cuja origem do capital é local/regional, além dos ramos industriais diretamente vinculados a forte base agropecuária da região. O segundo está relacionado ao processo de concentração e centralização social e espacial da reprodução do capital, vinculando-se ao processo de transferências industriais, em geral da Região Metropolitana de São Paulo. Na maioria dos casos as empresas foram beneficiadas pelos incentivos fiscais do governo estadual, por intermédio do Programa Paraná Mais-Empregos e invariavelmente receberam incentivos municipais. De fundamental importância para a dinâmica industrial do Aglomerado foi a provisão das condições gerais de produção historicamente estabelecidas, que propiciaram uma intensa integração, quer entre os municípios que o compõe, quer intensificando e modificando as relações históricas estabelecidas com o estado de São Paulo e sua região metropolitana, pólo hegemônico industrial brasileiro. Isso nos leva a reforçar a hipótese inicial de nosso trabalho de que, as condições gerais de produção historicamente estabelecidas têm um peso fundamental no desenvolvimento regional e que, a atuação do Estado na provisão dessas condições tem um alcance social e regional amplo, enquanto os programas vinculados à guerra fiscal são essencialmente concentradores, social e regionalmente. / The main objective of this work is to understand the industrial dynamics of (Aglomerado Urbano-Industrial de Londrina), considering that in the process of productive reorganization, the transformations go to also disclose themselves in the medium cities. The Aglomerado Urbano-Industrial de Londrina is formed by the medium city of Londrina and its neighborhood. As well as the state of the Paraná, the Urban-Industrial Agglomerate of Londrina, since 1992 started to present an important industrial development, however accents this unequal character. The recent industrial dynamics presents two distinct processes: the first one is related to the development of traditional industrial sectors in the region, intensive in man power and directly related the companies of small and medium size, whose origin of the capital is local/regional, beyond the industrial branches directly tied the strong farming base of the region. The second is related to the both processes: concentration and social-spatial centralization of the capital reproduction, associated to the industrial transferences process, in general of the Region Metropolitan of São Paulo. Frequently, the companies had been benefited by the tax incentives of the state government, for intermediary of the Paraná Mais-Empregos Program and certainly they had received municipal incentives. Very important for the industrial dynamics of the Agglomerated was the provision of the general conditions of production historically established, that had favored an intense integration among these cities, or intensifying and modifying the historical relations established with the state of São Paulo and its metropolitan region, the hegemonic pole industrial of Brazil. This strengthen the initial hypothesis of our work, that the general conditions of production historically established have a great importance in the regional development and the performance of the State in providing these conditions has an ample social and regional reach, while the programs related to the fiscal war are essentially social and regional concentrative.

Polymer electrolyte fuel cells (PEFC) convert chemicalenergy into electrical energy with higher efficiency thaninternal combustion engines. They are particularly suited fortransportation applications or portable devices owing to theirhigh power density and low operating temperature. The latter ishowever detrimental to the kinetics of electrochemicalreactions and in particular to the reduction of oxygen at thecathode. The latter reaction requires enhancing by the verybest catalyst, today platinum. Even so, the cathode isresponsible for the main loss of voltage in the cell. Moreover,the scarce and expensive nature of platinum craves theoptimisation of its use. The purpose of this thesis was to better understand thefunctioning of the porous cathode in the PEFC. This wasachieved by developing physical models to predict the responseof the cathode to steady-state polarisation, currentinterruption (CI) and electrochemical impedance spectroscopy(EIS), and by comparing these results to experimental ones. Themodels account for the kinetics of the oxygen reduction as wellas for the transport of the reactants throughout the cathode,i.e. diffusion of gases and proton migration. The agglomeratestructure was assumed for the description of the internalstructure of the cathode. The electrochemical experiments wereperformed on electrodes having a surface of 0.5 cm2 using alaboratory fuel cell. The response of the cathode to various electrodecompositions, thickness, oxygen pressure and relative humiditywas experimentally investigated with steady-state polarisation,EIS and CI techniques. It is shown that a content in thecathode of 35-43 wt % of Nafion, the polymer electrolyte, gavethe best performance. Such cathodes display a doubling of theapparent Tafel slope at high current density. In this region,the current is proportional to the cathode thickness and to theoxygen pressure, which, according to the agglomerate model,corresponds to limitation by oxygen diffusion in theagglomerates. The same analysis was made using EIS. Moreover,experimental results showed that the Tafel slope increases fordecreasing relative humidity. For Nafion contents lower than 35wt %, the cathode becomes limited by proton migration too. ForNafion contents larger than 40 wt %, the cathode performance athigh current density decreases again owing to an additionalmass transport. The latter is believed to be oxygen diffusionthroughout the cathode. The activity for oxygen reduction ofcatalysts based on iron acetate adsorbed on a carbon powder andpyrolysed at 900°C in ammonia atmosphere was alsoinvestigated. It was shown that the choice of carbon has atremendous effect. The best catalysts were, on a weight basis,as active as platinum. <b>Keywords:</b>polymer electrolyte fuel cell, cathode, masstransport, porous electrode, modelling, agglomerate model,electrochemical impedance spectroscopy, current interrupt,transient techniques, non-noble catalysts

The initial stage of liquid phase sintering, involving liquid penetration, agglomerate separation, particle spreading and growth has been investigated in experiments using tungsten heavy alloys. The particle composites used were produced by hot isostatic pressing (HIP) of pure powder mixtures of W-Ni-Fe-(Co). By using different HIP temperatures, volume fractions of tungsten, alloying elements like Cobalt and Sulphur or excluding Iron from the matrix, liquid penetration, agglomerate separation and particle growth conditions were affected. The investigations were performed mainly under microgravity (sounding rockets or parabolic trajectories by airplanes) but at high tungsten particle fractions, short sintering times or at infiltration of solid pure tungsten, they were performed at normal gravity. The liquid penetration of the tungsten agglomerates is explained by initial wetting under non-equilibrium conditions, due to the reaction between the liquid matrix and the particles, and a decrease of interfacial energy. The dissolving of tungsten gives a pressure drop in the penetrating liquid and a driving force for the liquid movement by a suggested parabolic penetration model. For cold worked tungsten, a penetration theory was proposed, where an internal stress release in the penetrated tungsten grains creates space for the advancing liquid. The spreading of the tungsten agglomerates is explained by an interagglomerate melt swelling due to a Kirkendall effect. The liquid matrix undergoes a volume increase since the diffusion rates of Ni-Fe are higher than for W and initial concentration gradients of W and Ni, Fe exists. The suggested model by Kirkendall are also used for an analysis of the interaction behaviour between solid particles and a solidification front and inclusion behaviour in iron base alloys during teeming and deoxidation. The average tungsten particles size decrease initially since part of the tungsten particles is dissolved when the non-equilibrium matrix phase is melting. When equilibrium is reached, the tungsten particles grow in accordance with the Ostwald ripening process by an approximately 1/3 power law. Larger particle fraction of particles showed a higher growth rate, due to shorter diffusion distances between the particles. Cobalt, Sulphur and absence of iron in the matrix were found to increase the growth rate of the tungsten particles due to a higher surface tension between the solid tungsten particles and the matrix melt. / QC 20100528

In this thesis, a Proton Exchange Membrane Fuel Cell is modeled with COMSOL Multiphysics software. A cross-section that is perpendicular to the flow direction is modeled in a 2-D, steady-state, one-phase and isothermal configuration. Anode, cathode and membrane are used as subdomains and serpentine flow channels define the flow field . The flow velocity is defined at the catalyst layers as boundary conditions with respect to the current density that is obtained by using an agglomerate approach at the catalyst layer with the help of fundamental electrochemical equations. Darcy&rsquo / s Law is used for modeling the porous media flow. To investigate the effects of species depletion along the flow channels, a different type of cross-section that is parallel to the flow direction is modeled by adding flow channels as a subdomain to the anode and cathode. Differently, Brinkman Equations are used to define flow in the porous electrodes and the free flow in the channels is modeled with Navier-Stokes equations. By running parallel-to-flow model, mass fractions of species at three different locations (the inlet, the center and the exit of the channel) are predicted for different cell po- tentials. These mass fractions are used as inputs to the perpendicular-to-flow model to obtain performance curves. Finally, by maintaining restricted amount of species by having a very low pressure difference along the channel to represent a single mid-cell of a fuel cell stack, a species depletion problem is detected. If the cell potential is decreased beyond a critical value, this phenomenon causes dead places at which the reaction does not take place. Therefore, at these dead places the current density goes to zero unexpectedly.

<p>Polymer electrolyte fuel cells (PEFC) convert chemicalenergy into electrical energy with higher efficiency thaninternal combustion engines. They are particularly suited fortransportation applications or portable devices owing to theirhigh power density and low operating temperature. The latter ishowever detrimental to the kinetics of electrochemicalreactions and in particular to the reduction of oxygen at thecathode. The latter reaction requires enhancing by the verybest catalyst, today platinum. Even so, the cathode isresponsible for the main loss of voltage in the cell. Moreover,the scarce and expensive nature of platinum craves theoptimisation of its use.</p><p>The purpose of this thesis was to better understand thefunctioning of the porous cathode in the PEFC. This wasachieved by developing physical models to predict the responseof the cathode to steady-state polarisation, currentinterruption (CI) and electrochemical impedance spectroscopy(EIS), and by comparing these results to experimental ones. Themodels account for the kinetics of the oxygen reduction as wellas for the transport of the reactants throughout the cathode,i.e. diffusion of gases and proton migration. The agglomeratestructure was assumed for the description of the internalstructure of the cathode. The electrochemical experiments wereperformed on electrodes having a surface of 0.5 cm2 using alaboratory fuel cell.</p><p>The response of the cathode to various electrodecompositions, thickness, oxygen pressure and relative humiditywas experimentally investigated with steady-state polarisation,EIS and CI techniques. It is shown that a content in thecathode of 35-43 wt % of Nafion, the polymer electrolyte, gavethe best performance. Such cathodes display a doubling of theapparent Tafel slope at high current density. In this region,the current is proportional to the cathode thickness and to theoxygen pressure, which, according to the agglomerate model,corresponds to limitation by oxygen diffusion in theagglomerates. The same analysis was made using EIS. Moreover,experimental results showed that the Tafel slope increases fordecreasing relative humidity. For Nafion contents lower than 35wt %, the cathode becomes limited by proton migration too. ForNafion contents larger than 40 wt %, the cathode performance athigh current density decreases again owing to an additionalmass transport. The latter is believed to be oxygen diffusionthroughout the cathode. The activity for oxygen reduction ofcatalysts based on iron acetate adsorbed on a carbon powder andpyrolysed at 900°C in ammonia atmosphere was alsoinvestigated. It was shown that the choice of carbon has atremendous effect. The best catalysts were, on a weight basis,as active as platinum.</p><p><b>Keywords:</b>polymer electrolyte fuel cell, cathode, masstransport, porous electrode, modelling, agglomerate model,electrochemical impedance spectroscopy, current interrupt,transient techniques, non-noble catalysts</p>