Dans le cadre du projet collaboratif FUI "HyPlate", cette étude apporte une contribution originale dans la compréhension des mécanismes impliqués lors de l'assemblage d'une pile à combustible de type PEMFC, à l'aide de la mesure ou de la prédiction des déformations et des contraintes mécaniques dans le cœur de pile ou MEA. Basée sur une analyse multi-échelle, une approche duale expérimentale et prédictive a été menée, comprenant le développement non seulement de nouvelles méthodes de mesures expérimentales mais également un modèle numérique 3D, basé sur la méthode des éléments finis. Le couplage des différents résultats expérimentaux et simulés a permis de déterminer, dans un premier temps, le niveau de contraintes (ou de déformations) dans les différents composants du stack en fonction des conditions d'assemblage, et d'analyser, dans un second temps, l'influence de différents paramètres, tels que les propriétés des matériaux des plaques bipolaires, le nombre de cellules et leur position. Il a notamment été montré que la distribution des déformations dans le cœur de pile diminuait avec le nombre de cellules et au centre du stack. Des modifications sur le système de serrage de la pile ont alors été proposées afin d'améliorer l'homogénéité de l'état mécanique global. Des mesures de force in-situ ont alors permis de valider certaines propositions et d'analyser l'influence de deux modes de serrage, en contrainte ou déformation imposée, sur la performance électrochimique de la pile. / Within the context of the FUI project "HyPlate", this thesis bring an original contribution in the understanding of the mechanisms implied during the assembly of a Proton Exchange Membrane Fuel Cell (PEMFC), through the measurements and the predictions of the mechanical stresses and strains in the fuel cell core, also known as the MEA. Based on a multi-scale analysis, a double approach experimental and predictive was followed, including not only the development of new experimental measurement methods but also a 3D numerical model, based on the finite element method. The results gathered from these two studies allowed us to determine, in a first step, the stress (or strain) level within the different stack components according to the assembly conditions, and to analyze, in a second step, the influence of different parameters such as the bipolar plate mechanical properties, the number of cells and their position. It was notably shown that the MEA strain distribution decreased with the number of cells and at the center of the stack. Some modifications of the clamping system were proposed in order to enhance the global mechanical state homogeneity. In situ measurements of the clamping force allowed us to validate some propositions made and to analyze the influence of two clamping modes, implying a constant stress or strain, on the electrochemical performance of the fuel cell.

Ces travaux ont été dédiés à la synthèse et la caractérisation de nouveaux ionomères aromatiques à blocs pour les PEMFC. Les blocs hydrophiles sont constitués par des polysufones fonctionnalisés par des chaînes latérales alkylperfluorosulfoniques, les blocs hydrophobes sont des polysulfones partiellement fluorés. La synthèse du squelette polymère a été réalisée par de polycondensation, les fonctions ioniques ont été greffées par un couplage d'Ullmann. Trois ionomères de différentes capacités d'échange ionique ont été synthétisés en modulant les longueurs des blocs porteurs des fonctions alkylperflurosulfoniques. Ces ionomères ont été mis en œuvre sous forme de membranes par coulée-évaporation. L'impact du solvant d'élaboration et de la structure chimique des ionomères sur la morphologie et les propriétés intrinsèques des membranes ont été largement étudiés. Le solvant de mise en œuvre de la membrane a un effet spectaculaire sur l'organisation des chaînes polymères à l'échelle nanométrique. Les études par diffusion des neutrons aux petits angles montrent que la morphologie des membranes est dépendante de la longueur des blocs hydrophiles. Les propriétés thermomécaniques et les conductivités protoniques des membranes ionomères aromatiques sont supérieures au Nafion, au-delà de 60°C, ce qui les rend prometteuses pour l'application PEMFC opérant à plus de 100°C. / The purpose of this work was the synthesis and characterization of new aromatic ionomers for PEMFC. The ionomers are based on block copolymers containing hydrophilic blocks, functionalised with a perfluorinated acid, and hydrophobic blocks containing partially perfluorinated aromatic rings. The polymer main chain was performed by polycondensation reaction. The acidic functions were grafted onto the polymer in two steps: bromination and coupling Ullman reaction. Different copolymers with different lengths of hydrophilic block were synthetized. The membranes were obtained by casting, the impact of the solvent nature and Ionomer structure on the membrane morphology and properties was studied. The solvent has a strong impact on the membrane structuration at nanometric scale. By small angle neutrons scattering, we showed that the membrane morphology is depending on hydrophilic bloc length. The mechanical strengths and the conductivities of aromatic ionomer membranes are higher that the Nafion above 60°C that make them promising for PEMFC working at temperature higher than 100°C.

O desenvolvimento de sistemas de conversão de energia baseados na tecnologia de células a combustível tem demandado estudos de confiabilidade, uma vez que requisitos de durabilidade e custo passaram a ser fundamentais para a inserção desta tecnologia no mercado de energia. Neste trabalho foi proposta uma metodologia de análise de confiabilidade de células a combustível de membrana polimérica condutora de prótons (células a combustível do tipo PEM), tendo em vista a qualificação destes itens como protótipos de unidades comerciais. A metodologia proposta incluiu uma avaliação inicial qualitativa das possíveis falhas em células a combustível do tipo PEM, realizada pela aplicação de uma Análise de Modos de Falha e Efeitos, técnica conhecida pela sigla FMEA. Além disso, foi elaborado um plano de testes de vida para as unidades produzidas no IPEN CNEN/SP e foi efetuada a análise dos resultados usando-se métodos estatísticos para dados de confiabilidade. Assim, a parte experimental consistiu em produzir as células a combustível no laboratório e submetê-las aos testes de vida, nos quais foram simuladas condições reais de operação. Os dados amostrais foram analisados estatisticamente, gerando resultados importantes em relação às medidas de desempenho e durabilidade dos dispositivos em estudo. Por meio de uma análise não paramétrica, foi gerada uma estimativa da função de confiabilidade das células a combustível usando-se o estimador de Kaplan- Meier. Pela modelagem paramétrica, foi possível ajustar uma distribuição exponencial para o tempo de vida destes dispositivos, gerando uma estimativa de vida média de 1.094,58 horas, com intervalo de 95% de confiança de [533,03 horas; 2.836,13 horas]. Com relação ao desempenho, foi aplicada uma regressão linear simples aos dados de potencial elétrico ao longo do tempo, gerando um valor aproximado para a taxa de queda do potencial elétrico de 80 μV h-1. Cabe ressaltar que, a metodologia proposta neste estudo deverá ser integrada ao processo de desenvolvimento das células a combustível, para permitir o acompanhamento da melhoria de sua confiabilidade. / The development of power conversion systems based on fuel cells has been demanding reliability studies since the requirements associated to cost and durability of these products have become fundamental to their acceptance in the energy market. The reliability analysis proposed in this work aimed to qualify the proton exchange membrane (PEM) fuel cells produced at IPEN CNEN/SP as prototypes of commercial units. The methodology included an initial assessment of failures of the PEM fuel cells, which was carried out by the application of a Failure Mode and Effect Analysis (FMEA). Furthermore, the methodology involved the planning of life tests and the quantitative assessment based on statistical methods for reliability data. Thus, the experimental part of this work consisted of producing the fuel cells in the laboratory, and testing them under stable and dynamic operating conditions. The main results of the quantitative analysis were the estimates of the reliability function and mean time to failure of the PEM fuel cells. These estimates were obtained by means of two different approaches, a non parametric and a parametric one. In the non parametric analysis, the Kaplan-Meier estimator was used for the reliability function. Based on the parametric analysis, an exponential distribution was fit to data and the mean time to failure of the PEM fuel cells was estimated in 1.094,58 hours, with a 95% confidence interval of [533,03 hours; 2.836,13 hours]. Concerning the PEM fuel cells performance, a linear regression analysis of the observed data (voltage vs. time) was carried out and an approximate estimate for the voltage decay rate was calculated in 80 μV h-1. Finally, the methodology proposed in this work may be integrated into the development project of the PEM fuel cells so as to allow researchers to carry out a reliability growth analysis and monitor the improvements in these products.

Afin de rendre les piles à combustible de type PEMFC réellement compétitives, un certain nombre d'inconvénients liés à l'utilisation du Nafion® restent à contourner, en particulier sa mauvaise conductivité protonique à des températures supérieures à 80°C. Dans l'optique de pouvoir opérer à plus hautes températures (jusqu'à 120°C), le développement de membranes moins sensibles à l'eau s'avère donc déterminant. Les polymères à base de fonctions acide phosphonique sont considérés comme des candidats potentiels pour une intégration en tant que matériau électrolyte dans les PEMFC « hautes températures » (> 80°C) grâce à leur fort caractère amphotère qui leur confère une bonne conductivité protonique dans des conditions d'humidité réduites. Dans ce contexte, la majeure partie de ce travail de thèse concerne l'élaboration par polymérisation plasma (PECVD) de polymères à base de groupements acide phosphonique à partir du monoprécurseur diméthyl allyl phosphonate. Dans un premier temps, nous avons démontré la faisabilité d'élaborer par polymérisation plasma des polymères à base de fonctions acide phosphonique à partir d'un monoprécurseur. Nous avons confirmé par IRTF, EDX et XPS la présence des groupements acide phosphonique favorables au transport protonique et l'homogénéité de la composition chimique de la surface jusqu'au cœur du matériau plasma. Les matériaux plasma montrent une bonne stabilité thermique dans la gamme de température 80°C - 120°C. Ensuite, une optimisation des conditions de synthèse a été réalisée. Les plus importantes valeurs de vitesses de croissance (28 nm.min-1 sur plaquette de silicium, 22 nm.min-1 sur PTFE et 26 nm.min-1 sur Nafion®211), de CEI (4,65 meq.g-1) et de conductivité (0,08 mS.cm-1 à 90°C et 30% RH) sont celles de la membrane synthétisée à 60 W. Des mesures de perméabilité au méthanol, à l'éthanol et au glycérol ont été réalisées et montrent que les films plasma sont intrinsèquement 40 à 235 fois moins perméables au combustible que le Nafion®211 du fait de leur fort taux de réticulation. Les polymères ont été déposés en tant que liants sur des électrodes E-TEK® pour intégration en pile. Nous avons constaté que le liant phosphonique plasma possède une conductivité protonique suffisante pour permettre le transport des protons à l'interface membrane-électrodes. En parallèle, nous avons réalisé le traitement de surface par plasma d'une membrane phosphonique conventionnelle pour en améliorer la stabilité thermique et la rétention au combustible. Les analyses thermogravimétriques montrent une légère amélioration de la stabilité thermique suite au traitement de surface. Des tests de perméabilité au méthanol et à l'éthanol montrent que la membrane traitée par plasma est 2 à 4 fois moins perméable que la membrane vierge. Le traitement à 60 W conduit aux coefficients de diffusion les plus faibles (DMeOH = 9.10-12 m2.s-1 et DEtOH = 6.10-12 m2.s-1). Des tests en pile ont été effectués montrant de meilleures performances de la membrane traitée en comparaison de son homologue non traité. / The proton exchange membrane is a key component in the PEMFC-type fuel cell; it plays a decisive role as electrolyte medium for proton transport and barrier to avoid the direct contact between fuel and oxygen. The Nafion® is one of the most extensively studied proton exchange membrane for PEMFC applications. However, it has a number of drawbacks that need to be overcome, especially the poor performance at temperature above 80°C. That's why the development of effective and low cost membranes for fuel cell turned to be a challenge for the membrane community in the last years. Phosphonic acid derivatives are considered suitable candidates as ionomers for application in PEMFC at high temperature (> 80°C) thanks to their efficient proton transport properties under low humidity condition due to their amphoteric character.In this work, plasma polymers containing phosphonic acid groups have been successfully prepared using dimethyl allylphosphonate as a single precursor demonstrating the feasibility of plasma process for the manufacture of proton exchange membranes. Moreover, plasma polymers properties have been investigated as a function of the plasma conditions. The evolution of the films growth rate on three different supports as a function of the plasma discharge power is bimodal, with a maximum (close to 30 nm min-1 on Si) at 60 W. The chemical composition of plasma materials (investigated by FTIR, EDX and XPS) is quite homogeneous from the surface to the bulk; it is characterized by a wide variety of bond arrangements, in particular the presence of phosphonate and phosphonic acid groups which are above all concentrated in the plasma film synthesized at 60 W, characterized by the highest ion exchange capacity (4.65 meq g-1) and the highest proton conductivity (0.08 mS cm-1 at 90°C and 30% RH). TGA analysis has shown that phosphonic acid-based plasma polymers retain water and don't decompose up to 150 °C, which reveals a satisfying thermal stability for the fuel cell application. In terms of fuel retention, plasma films are intrinsically highly performing (methanol, ethanol and glycerol permeabilities being 40 to 235 lower than that of Nafion®211). The plasma films were deposited on fuel cell electrodes (E-TEK®) as binding agents. We have noticed that the phosphonic binder has a sufficient proton conductivity to allow proton transport at the electrode-membrane interface.A second part of this work concerns the surface treatment by plasma process of a conventional phosphonated membrane for improvement of thermal stability and fuel retention. TGA analysis has shown a slight improvement of the thermal stability for the treated membrane. Methanol and ethanol permeabilities tests show that the plasma-modified membrane is 2 to 4 times less permeable than the non-modified membrane. The treatment at 60 W shows the lowest fuel diffusion coefficients (DMeOH = 9.10-12 m2.s-1 and DEtOH = 6.10-12 m2.s-1). Fuel cell tests were realized showing better performance for the modified membrane compared to the non-modified one.

Les piles à combustible à membrane électrolyte polymère (PEMFC) sont considérées depuis de nombreuses années comme une solution intéressante pour remplacer les moteurs thermiques. Cependant le coˆut élevé du catalyseur et sa faible stabilité limitent la viabilité économique des piles. Le platine pur déposé sur carbone est le catalyseur couramment plus utilisé à la cathode, cependant les nanoparticules d’alliages de type Pt-M (M = Co, Ni ou Fe) sont reconnues depuis peu comme une alternative remarquable en raison d’une meilleure activité et d’une plus grande stabilité au cours de la réaction de réduction de l’oxygène (ORR). Jusqu’à présent, les raisons fondamentales de cette amélioration significative n’ont pas été élucidées d’un point de vue cinétique et théorique. Par ailleurs, la simulation de la performance d’une pile PEM basée sur la loi empirique de Butler-Volmer ne permet pas de prédire de manière correcte ses propriétés cinétiques.Dans cette thèse, nous présentons une méthodologie théorique multi-échelles permettant de simuler le comportement transitoire d’une pile PEM par un modèle cinétique élémentaire. A l’échelle atomique, la théorie de la fonctionnelle de ladensité (DFT) a été utilisée pour modéliser et comprendre la formation de l’eau et du peroxyde d’hydrogène sur trois surfaces différentes de Pt3Ni(111) en comparaison avec la surface de référence Pt(111). Les calculs DFT ont montré que le taux de recouvrement des espèces hydroxyles en surface du catalyseur Pt(111) peut modifier le mécanisme réactionnel et que la plus grande activité catalytique des surfaces d’alliages Pt3Ni(111) est expliquée par la composition chimique en de surface, l’arrangement structural et le rˆole du second métal Ni sur les propri étés électroniques. Les énergies d’activation et les constantes de vitesse des étapes élémentaires du mécanisme réactionnel ont été déterminées et ont ensuite étéutilisées dans un modèle champ moyen décrivant le comportement du champ électrique et la distribution de charge à l’échelle nanométrique. Ces données ont été couplées avec des modèles décrivant la charge au niveau microscopique et les phénomènes de transport des réactifs jusqu’à la cathode. L’influence du choix du mécanisme élémentaire de l’ORR sur les prédictions des courbes de polarisation courant-tension a été déterminée et une comparaison avec les données expérimentales a été proposée pour valider le modèle. / Polymer Electrolyte Membrane Fuel Cell (PEMFC) is a possible solution for replacing the actual combustion engines. However the cost of the catalyst and its actual low stability are restricting their economical viability. Pt/C is the state-of-the-art cathode catalyst but Pt-M (M = Co, Ni or Fe) alloyed nanoparticles have been proposed as cheap, more stable and powerful, regarding the ORR activity, alternative material. Up to now the fundamental reasons of such improvement have not been elucidated from a kinetic point of view. In the other hand the standard simulation approaches of the PEMFC performance based on Butler-Volmer equations thus not describe correctly the kinetics of such systems. In this thesis we present a multiscale theoretical methodology to scale up ab initio calculated data into elementary kinetic model to simulate PEMFC transient behavior. Density Functional Theory calculations are carried out to understand the catalytic properties of three different Pt3Ni(111) alloy surfaces in comparison with Pt(111). As a result we show that the coverage of OH species may reverse the dominant ORR mechanism on pure Pt catalyst and that the reasons for higher ORR catalytic activity of the Pt3Ni alloys are related to the nature of the second metal, to its surface ability to be less oxidize and to an optimal structural arrangements. Using these properties we have built an elementary kinetic model and calculate the associated constant rate parameters. Then these parameters are implemented into a mean field interfacial model describing the behavior of the electric field and charge distribution at the nanoscale, which is in turn coupled with microscale and mesoscale level models describing the charge and reactants transport phenomena across the cathode. The impact of different ORR mechanisms on the calculated i-V curves is investigated, in comparison with experimental data.

Cette thèse apporte des éléments de compréhension de la boucle basse fréquence des spectres d'impédance de PEMFC H2/air. Différentes expressions de l'impédance de transport de l'oxygène alternatives à l'élément de Warburg sont proposées. Elles prennent en compte des phénomènes de transport dans les directions perpendiculaire et parallèle à l'électrode qui sont habituellement négligés: convection à travers la GDL et le long du canal d'air, résistance protonique de la couche catalytique et appauvrissement en oxygène entre l'entrée et la sortie de la cellule. Une attention particulière est portée sur les oscillations de concentration induites par le signal de mesure qui se propagent le long du canal d'air. Ces différentes expressions de l'impédance de transport de l'oxygène sont utilisées dans un circuit électrique équivalent destiné à simuler l'impédance de la cellule. Une comparaison entre résultats expérimentaux et théoriques permet d'identifier les paramètres du circuit électrique. A partir de ces paramètres, il est possible d'analyser les mécanismes physiques et électro-chimiques qui se produisent dans la pile, ainsi que de tirer certaines conclusions sur les phénomènes de transport de l'oxygène dans les milieux poreux de la cathode. Pour cela, nous avons utilité des cellules segmentées et instrumentées conçues et fabriquées au laboratoire / The aim of this Ph.D thesis is to contribute to a better understanding of the low frequency loop in impedance spectra of H2/air fed PEMFC and to bring information about the main origin(s) of the oxygen transport impedance through the porous media of the cathode via locally resolved EIS. Different expressions of the oxygen transport impedance alternative to the one-dimensional finite Warburg element are proposed. They account for phenomena occurring in the directions perpendicular and parallel to the electrode plane that are not considered usually: convection through the GDL and along the channel, finite proton conduction in the catalyst layer, and oxygen depletion between the cathode inlet and outlet. A special interest is brought to the oxygen concentration oscillations induced by the AC measuring signal that propagate along the gas channel and to their impact on the local impedance downstream. These expressions of the oxygen transport impedance are used in an equivalent electrical circuit modeling the impedance of the whole cell. Experimental results are obtained with instrumented and segmented cells designed and built in our group. Their confrontation with numerical results allows to identify parameters characterizing the physical and electrochemical processes in the MEA

La pile à combustible (FC) est peu adaptée aux variations brusques de puissance rencontrées dans les applications transport. L’hybridation de la pile à un supercondensateur (SC) a alors été étudiée, puisque cet organe de stockage capacitif permet de gérer les transitoires de puissance. L’hybridation est directe/passive, permettant ainsi de réduire le volume, la masse et le coût du système. Initialement, la faisabilité et l’impact de la taille du SC sur la performance de la FC en mono-cellule ont été examinés numériquement. Cette modélisation montre que l’augmentation de la taille du SC renforce l’effet de lissage induit par l’utilisation du SC sur le courant de la FC. Il en résulte des variations lentes et une réduction des amplitudes de courant et de tension, une diminution du courant efficace de la FC, et donc des pertes électriques de celle-ci. L’hybridation de la FC, comparativement à son fonctionnement seule, permet en outre de réduire la surconsommation en H2 de près de 50 % dans les mêmes conditions opératoires. Ces résultats ont été validés par des essais expérimentaux réalisés en mono-cellule et 3-cellules de 100 cm2 hybridée ou non. Par la suite, toujours en utilisant le protocole de cyclage urbain (FC-DLC), la durabilité de la FC a été étudiée lors d’essais de longue durée. La durabilité de la FC, qu’elle soit hybridée ou non, est la même. L’hybridation n’améliore donc pas la durée de vie de la FC mais ne lui nuit pas non plus. Par la suite, afin d’encore réduire la surconsommation en H2 en longue durée cyclage, différentes stratégies ont été étudiées : diminution du débit minimum des gaz imposé par le cyclage et diminution du coefficient de surstœchiométrie en H2. Ces changements n’ont pas d’influence sur la durabilité de la pile hybridée et ont permis de réduire à 10 % la surconsommation en hydrogène. La FC non hybridée, quant à elle, a vu sa durabilité divisée par deux lors de la diminution des débits minimum et ne fonctionnait pas avec le coefficient de surstœchiométrie ramené à 1,1. Ensuite, les travaux ont été étendus à un stack FC de forte puissance (Système Ballard de 1,2 kW) hybridé à deux modules de SC de 165 F (Maxwell Technologies). En final, un système hybride de 34 kW (FC de 10 kW et SC de 566.67 F) a montré des performances suffisantes pour une application transport urbain et péri-urbain. De plus, comparativement à une pile de 34 kW 21 % d’hydrogène sont économisés et l'investissement des équipements peut être réduit de près 50 % / The fuel cell (FC) is poorly adapted to the sudden variations in power encountered in transport applications. The FC hybridization to a supercapacitor (SC) was then studied, since this capacitive storage device allows to manage the power transients. Hybridization is direct/passive, thus reducing the volume, mass and cost of the system. Initially, the feasibility and the impact of SC size on FC performance have been examined numerically. Theoretical investigations show that increasing the size of SC enhances the smoothing effect introduced by the supercapacitor on FC current. This results into slow variations and reduction in both current and voltage amplitudes, a decrease in the fuel cell’s effective current, and therefore in FC electrical losses. Hybridization, compared to its FC operation alone, still reduces hydrogen overconsumption by nearly 50 % under the same operating conditions. These results have been validated by experimental tests carried out on a 100 cm2 single FC and a 3 cell stack. Later, the durability of the FC system has been investigated through long term tests. These durability tests have been conducted on the 100 cm2 single FC test bench using urban cycling protocol (FC-DLC), for both hybridized and unhybridized FC system, with continuous evaluation of degradation extent and causes. These tests suggest no detrimental impact on durability of the FC. For these two operating modes, a progressive aging of the gas diffusion layer seems to appear. Subsequently, in order to further reduce the overconsumption of hydrogen in long-term FC-DLC cycling, different strategies were studied: reducing the minimum gas flow rate imposed by FC-DLC cycling from 0.2 to 0.05 A cm-2, and reducing the hydrogen overstoichiometry coefficient from 1.2 to 1.1. These changes have no influence on the durability of the hybrid cell and have reduced hydrogen overconsumption to 10 %. On the contrary, in case of the unhybridized FC, durability was halved as minimum flows were reduced and it did not work when the overstoichiometry reduced coefficient. Further, work has been extended to high power FC systems (1.2 kW FC system, hybridized with two modules of 165 F, SC module). Finally, the FC downsizing has been demonstrated from 34kW FC system to hybrid source system of 10kW FC hybridized with 566.64 F SC, presenting 21 % hydrogen saving and nearly 50 % net cost savings.

Das Fraunhofer IPT untersucht verschiedene Umformverfahren für das Formen metallischer Bipolarplatten wie das Streckziehen, Rubberforming und Hydroforming. Verschiedene Edelstähle wie 1.4301 und 1.4404 sowie Titanwerkstoffe werden dabei berücksichtigt. Durch Simulationen sowie experimentelle Untersuchungen konnte festgestellt werden, dass die auftretenden Normalkräfte und Scherspannungen im Kontakt zwischen Werkzeug und Blech beim Streckziehen größer sind als bei den beiden anderen Verfahren. Dadurch werden bei gleichen Spannungen im Material geringere Umformgrade erreicht. Hinsichtlich des Potenzials für das Upscaling der Produktion ist das Streckziehen im Folgeverbundwerkzeug aufgrund der einfachen Automatisierbarkeit und des Handlings bei geringen Zykluszeiten geeignet. / Fraunhofer IPT analyzes different technologies for the forming of metallic bipolar plates. Among them are stamping, rubberforming and hydroforming. Different materials like stainless steel (1.4301 and 1.4404) and titanium are considered. Numerical simulations and experimental validation show that contact pressure and shear stress in the contact between tool and sheet are larger for stamping processes. This leads to limited grades of deformation. Nevertheless, stamping in progressive die tools is a suitable forming technology for upscaling of the production of metallic bipolar plates as it has short cycle times and handling of the sheets can be automated easily.

[pt] A crescente demanda energética verificada ao redor do mundo e a conscientização pública acerca dos efeitos deletérios do excesso de gases estufa na atmosfera vem colaborando para a articulação de compromissos de grande alcance em nome da adaptação das matrizes energéticas a formas ambiental e economicamente sustentáveis. A adesão à energias renováveis (como solar e eólica) e a descentralização da matriz energética por meio de tecnologias de geração distribuída (visando a melhoria da eficiência do uso da energia) são alguns dos movimentos mais relevantes realizados para fazer frente a essas demandas. Neste ínterim, o presente trabalho é dedicado à simulação numérica mediante o conceito 4E (Energy, Exergy, Environmental and Economic) de um sistema híbrido CHP (Combined Heat and Power) on-grid para atendimento de pequenas demandas residenciais ou industriais, tendo gás natural e energia solar como vetores energéticos preferenciais. O sistema inclui um reformador de gás natural para produção de gás de síntese rico em hidrogênio, uma célula a combustível com membrana de troca de prótons (PEM), painéis fotovoltaicos, baterias conectadas à rede elétrica por um inversor bidirecional, trocadores de calor e componentes auxiliares como compressores e boilers. Os componentes do sistema foram modelados separadamente com base em equações de conservação e seus modelos devidamente validados. Uma análise energética e exergética do reformador de gás natural foi conduzida mediante a metodologia de planejamento de experimentos a fim de avaliar a necessidade de considerar uma formulação complexa do combustível em vez de um substituto (metano puro). Posteriormente, estes modelos foram inseridos como módulos de uma rotina mais ampla destinada a simular o desempenho econômico do sistema integrado num intervalo de tempo de até 20 anos. Tal rotina, implementada no MATLAB, permite a flexibilização de critérios operacionais importantes como número de consumidores, configuração do sistema híbrido (armazenamento e participação de painéis fotovoltaicos), diferentes tipos de tarifa (convencional ou branca) e o possível uso de rejeito térmico para cogeração, enriquecendo o escopo de resultados obtidos. Paybacks entre 7 e 20 anos de operação do sistema foram alcançados para diferentes combinações dos parâmetros examinados considerando-se a adesão no ano de 2020, onde consumidores residenciais obtiveram resultados predominantemente melhores do que os industriais em virtude da demanda menos exigente dos primeiros. Foram também previstas reduções de até 50% no custo cumulativo total para consumidores residenciais referente a adesão ao sistema proposto por 20 anos, levando-se em conta a queda prevista nos custos de aquisição dos componentes para as próximas décadas. A avaliação do sistema em termos ambientais foi feita através da quantidade equivalente de CO2 por unidade de energia. Concluiu-se que a configuração completa, mesmo auxiliada por cogeração, supera a média de emissões da matriz energética brasileira (devido à alta participação das fontes renováveis nessa matriz), permanecendo, ainda assim, como uma opção melhor do que a combustão pura do gás natural, especialmente no que diz respeito ao atendimento de demanda térmica. / [en] The growing energy demand verified around the world and public awareness about the harmful effects of greenhouse gases excess in the atmosphere have been contributing to the articulation of far-reaching commitments in the name of adapt energy matrices to environmentally and economically sustainable ways. The adherence to renewable energy (such as solar and eolic) and descentralization of energy matrix through distributed generation technologies (aiming at the improvment of efficiency of energy use) are some of the more relevant movements done in order to deal with these demands. In the meantime, the present work is dedicated to numerical simulation using the 4E (Energy, Exergy, Environmental and Economic) concept of an on-grid hybrid CHP system to meet small residential or industrial demands, using natural gas and solar energy as preferred energy vectors. The system includes a natural gas reformer for the production of hydrogen-rich synthesis gas, a proton exchange membrane fuel cell (PEM), photovoltaic panels, batteries connected to the grid by a bidirectional inverter, heat exchanger and auxiliary componentes, such as compressors and boilers. The system components were modeled separately based on conservation equations and their models duly validated. An energy and exergy analysis of the natural gas reformer was conducted using design of experiment methodology in order to assess the necessity to consider a complex formulation of the fuel instead of a surrogate (pure methane). Subsequently, these models were inserted as modules of a broader routine designed to simulate the economic performance of the integrated system in a time interval of up to 20 years. This routine implemented in MATLAB allows for the flexibility of important operational criteria such as the number of consumers, configuration of the hybrid system (storage and participation of solar energy), different types of tariff (conventional or white) and the posible use of reject heat for cogeneration, enriching the scope of the results obtained. Paybacks between 7 and 20 years of system operation were achieved for different combinations of the examined parameters considering adherence in the year 2020, where residential consumers have predominantly obtained better results than industrial ones due to the less intense demand of the first ones. Reductions of up to 50% in the total cumulative cost related to adherence to the proposed system for 20 years for residential users were also foreseen, taking into account the expected drop in component acquisition costs over the next few decades. The evaluation of the system in environmental terms was assessed through equivalent amount of CO2 by energy unit. It was concluded that the complete configuration, even supported by cogeneration, exceeds the average of the brazilian energy matrix emissions (due to the high share of renewable sources in this matrix), nevertheless remaining as a better option than pure combustion of natural gas, specially for meeting thermal demand.

The excessive presence of liquid water in a gas diffusion layer (GDL) hinders the access of reactant gases to the active sites of the catalyst layer leading to decreased performance of a polymer electrolyte membrane fuel cell (PEMFC). Therefore, GDLs are usually treated with a hydrophobic agent to render their fibres more hydrophobic in order to facilitate gas transport and water removal. Numerous studies have been conducted to investigate water transport in PEMFCs in recent years; however, the behaviour of liquid water in a GDL at a pore-level is poorly understood. Macroscopic models fail to incorporate the influence of the structural morphology of GDLs on liquid water transport behaviour. Experimental methods are not conducive towards a good understanding at a microscopic level because of the diminutive size of the GDLs porous structure. Alternatively, the Lattice Boltzmann (LB) method has gathered interest as it is found to be particularly useful in fluid flow simulations in porous media due to its capability to incorporate the complex boundaries of actual GDL structures. To date, most studies on fluid transport in GDLs integrated artificial structures generated by stochastic simulation techniques to the LB models. The stochastic-based model, however, does not represent closely the microscopic features of the actual GDL as manufactured. In addition, comparison of liquid water transport behaviour in different GDL structures using the LB method is rare since only a single GDL material has been utilised in most of those studies. This thesis aims to develop our understanding of liquid water transport behaviour in GDLs with morphologically different structures under varying wettability conditions based on the LB method and the X-ray computed tomography (XCT) technique. GDLs with paper and felt structures were reconstructed into 3D digital volumetric models via the XCT process. The digital models were then incorporated into a LB solver to model water saturation distribution through the GDL domains. The GDL wettability was also altered so that the effect on liquid water behaviour in the GDL could be examined. This project is divided into three main sections. In the sensitivity analysis, the effect of image resolution on gas permeability through the X-ray reconstructed GDL was carried out using a single-phase LB model. It was found that the resolution variation could significantly affect the resulting gas permeability in both principal and off-principal directions, as well as computational time. An optimum resolution, however, exists at 2.72 μm/pixel, which consumed 400 times less computational time with less than 8% difference in the resulting permeability compared to the base resolution. This study also served as a guideline for selecting a resolution for generating the XCT images of the GDLs which were utilised in the following studies. In the structure analysis, the structures of the paper and felt GDLs were generated using the XCT and the key properties of each GDL, including thickness, porosity, permeability and tortuosity, were characterised. The thickness and the through-plane porosity distributions of each GDL were examined based on the tomography images. The resulting local through-plane porosity distributions were then used to calculate through-plane permeability and tortuosity distributions using an analytical model available in the literature. This study revealed the heterogeneity of the GDLs and how the heterogeneous nature of the GDL structures affects others properties of the GDLs. In this study, the absolute through-plane permeability and tortuosity of the X-ray-reconstructed GDL samples were also characterised using the single-phase LB model. The results from the two models were then compared and validated against data in the literature. In the water transport analysis, the two-phase LB model was employed to examine the effects of GDL structures on the behaviour of liquid water in the GDLs, including invasion patterns, saturation distribution and breakthrough behaviour under varying GDL wettability conditions. It was found that wettability was responsible for invasion patterns and water saturation levels whilst the GDL structure was mostly responsible for breakthrough occurrence and saturation distribution. It was observed that water travelled with stable displacement saturating all pores in hydrophilic GDLs, while it travelled with capillary fingering causing decreased saturation in hydrophobic GDLs, about 50% in the highly hydrophobic cases. The GDL structure was found to play a key role in breakthrough behaviour in the hydrophilic GDL as it was seen that the through-plane fibres in the felt structure and the through-plane binders in the paper structure encouraged water removal from the GDL in the thickness direction. Conversely, the GDL structure was found to have negligible influence on breakthrough in the hydrophobic GDL. Each GDL structure, however, contributed to a distinct difference in water distribution in the GDL with hydrophobic wettability. The work presented in this thesis contributes to the understanding of liquid water transport behaviour in the GDLs under the combined effects of the GDL structures and wettability conditions, which is essential for the development of effective PEMFC water management and the design of future GDL materials.