Among the obstacles for the commercialization of the MoltenCarbonate Fuel Cell (MCFC), the dissolution of thestate-of-the-art lithiated NiO cathode is considered as aprimary lifetime limiting constraint. Development ofalternative cathode materials is considered as a main strategyfor solving the cathode dissolution problem. LiFeO2and LiCoO2had earlier been reported as the most promisingalternative materials; however, they could not satisfactorilysubstitute the lithiated NiO. On the other hand, ternarycompositions of LiFeO2, LiCoO2and NiO are expected to combine some desirableproperties of each component. The aim of this work was todevelop alternative cathode materials for MCFC in the LiFeO2-LiCoO2-NiO ternary system. It was carried out byinvestigating electronic conductivity of the materials, firstin the form of bulk pellets and then in ex-situ sinteredporous-gas-diffusion cathodes, and evaluating theirelectrochemical performance by short-time laboratory-scale celloperations. Materials in the LiFeO2-NiO binary system and five ternary sub-systems,each with a constant molar ratio of LiFeO2:NiO while varying LiCoO2content, were studied. Powders withcharacteristics appropriate for MCFC cathode fabrication couldbe obtained by the Pechini method. The particle size of LiFeO2-LiCoO2-NiO powders considerably depends on thecalcination temperature and the material composition. Theelectrical conductivity study reveals the ability of preparingLiFeO2-LiCoO2-NiO materials with adequate electricalconductivity for MCFC cathode application. A bimodal pore structure, appropriate for the MCFC cathode,could be achieved in sintered cathodes prepared usingporeformers and sub-micron size powder. Further, this studyindicates the nature of the compromise to be made between theelectrical conductivity, phase purity, pore structure andporosity in optimization of cathodes for MCFC application. Cellperformance comparable to that expected for the cathode in acommercial MCFC could be achieved with cathodes prepared from20 mole% LiFeO2- 20 mole% LiCoO2- 60 mole% NiO ternary composition. It shows aniR-corrected polarization of 62 mV and a iR-drop of 46 mV at acurrent density of 160 mAcm-2at 650 °C. Altogether, this study revealsthe possibility of preparing LiFeO2-LiCoO2-NiO cathode materials suitable for MCFCapplication. Keywords: molten carbonate fuel cell (MCFC), MCFC cathode,LiFeO2-LiCoO2-NiO ternary compositions, electrical conductivity,porous gas diffusion electrodes, polarization, electrochemicalperformance, post-cell characterization.

<p>Among the obstacles for the commercialization of the MoltenCarbonate Fuel Cell (MCFC), the dissolution of thestate-of-the-art lithiated NiO cathode is considered as aprimary lifetime limiting constraint. Development ofalternative cathode materials is considered as a main strategyfor solving the cathode dissolution problem. LiFeO₂and LiCoO₂had earlier been reported as the most promisingalternative materials; however, they could not satisfactorilysubstitute the lithiated NiO. On the other hand, ternarycompositions of LiFeO₂, LiCoO₂and NiO are expected to combine some desirableproperties of each component. The aim of this work was todevelop alternative cathode materials for MCFC in the LiFeO₂-LiCoO₂-NiO ternary system. It was carried out byinvestigating electronic conductivity of the materials, firstin the form of bulk pellets and then in ex-situ sinteredporous-gas-diffusion cathodes, and evaluating theirelectrochemical performance by short-time laboratory-scale celloperations.</p><p>Materials in the LiFeO₂-NiO binary system and five ternary sub-systems,each with a constant molar ratio of LiFeO₂:NiO while varying LiCoO₂content, were studied. Powders withcharacteristics appropriate for MCFC cathode fabrication couldbe obtained by the Pechini method. The particle size of LiFeO₂-LiCoO₂-NiO powders considerably depends on thecalcination temperature and the material composition. Theelectrical conductivity study reveals the ability of preparingLiFeO₂-LiCoO₂-NiO materials with adequate electricalconductivity for MCFC cathode application.</p><p>A bimodal pore structure, appropriate for the MCFC cathode,could be achieved in sintered cathodes prepared usingporeformers and sub-micron size powder. Further, this studyindicates the nature of the compromise to be made between theelectrical conductivity, phase purity, pore structure andporosity in optimization of cathodes for MCFC application. Cellperformance comparable to that expected for the cathode in acommercial MCFC could be achieved with cathodes prepared from20 mole% LiFeO₂- 20 mole% LiCoO₂- 60 mole% NiO ternary composition. It shows aniR-corrected polarization of 62 mV and a iR-drop of 46 mV at acurrent density of 160 mAcm^-2at 650 °C. Altogether, this study revealsthe possibility of preparing LiFeO₂-LiCoO₂-NiO cathode materials suitable for MCFCapplication.</p><p>Keywords: molten carbonate fuel cell (MCFC), MCFC cathode,LiFeO₂-LiCoO₂-NiO ternary compositions, electrical conductivity,porous gas diffusion electrodes, polarization, electrochemicalperformance, post-cell characterization.</p>

Submitted by Alexandre Sousa (alexandre.sousa@incqs.fiocruz.br) on 2016-05-11T13:46:20Z No. of bitstreams: 1 Thesis-1994-Jacob.pdf: 26119019 bytes, checksum: b8e19fd69a5cb720e5071df7ae1b03bb (MD5) / Approved for entry into archive by Alexandre Sousa (alexandre.sousa@incqs.fiocruz.br) on 2016-05-11T14:21:12Z (GMT) No. of bitstreams: 1 Thesis-1994-Jacob.pdf: 26119019 bytes, checksum: b8e19fd69a5cb720e5071df7ae1b03bb (MD5) / Made available in DSpace on 2016-05-11T14:21:12Z (GMT). No. of bitstreams: 1 Thesis-1994-Jacob.pdf: 26119019 bytes, checksum: b8e19fd69a5cb720e5071df7ae1b03bb (MD5) Previous issue date: 1994 / Fundação Oswaldo Cruz. Instituto Oswaldo Cruz. Departamento de Entomologia. Rio de Janeiro, RJ, Brasil. / A dual phase gas diffusion-FIA system containing a tubular PTFE-membrane was studied as a mean of producing gas samples for routine 15N/14N isotopic ratio mass spectrometry. The method is based on Rittenberg's reaction; the ammonium sample is injected into a liquid alkaline stream containing hypobromite and the N2 gas produced in the reaction diffuses across a PTFE-membrane into a helium carrier stream which carries it to the detector. Initially here, the use of a tubular microporous PTFE-membrane as a device for the preconcentration of samples in aqueous solutions was investigated. The performance of such a membrane was studied under a variety of operating conditions. A qualitative model of the membrane mechanism was developed based on the diffusion transport of vapour away from the contained liquid surface through the connected pore space. The dispersion undergone by the sample in the GD-FIA system containing this preconcentration unit was also studied and this FIA system was applied as a practical device for the determination and speciation of aluminium in a river water sample. The procedure for generating nitrogen gas involved optimisation of the system parameters including the oxidation reaction step and the production on-line of the chemicals used. The nitrogen gas was generated easily and rapidly, allowing a sample throughput capability of the order of 20 h-1. The system was applied to the determination of total nitrogen content in agricultural sample prepared by the Kjeldahl digestion. The method offered precision and accuracy comparable to those of the standard distillationtitration procedure. Isotope ratios were determined with good precision and means for obtaining accuracy comparable with established techniques were developed. It was also shown that the DPGD-FIA system can be readily adapted to enable different forms of nitrogen e. g. N02-, N03- and NH4+ to be determined.

L’hydrogène comme vecteur énergétique est une solution prometteuse pour réduire les émissions de gaz à effet de serre. En effet, l’hydrogène permet de stocker de grandes quantités d’énergie de façon totalement décarbonée. Pour favoriser l’utilisation à grande échelle de l’énergie hydrogène, il est essentiel de réduire le coût des piles à combustible et d‘augmenter leur durabilité et leurs performances. Les matériaux situés au coeur des piles à combustible ont un impact fort sur leurs performances et leur durabilité. Dans ce contexte, optimiser les matériaux est crucial. Nous développons dans cette thèse une démarche de modélisation des matériaux poreux des piles à combustible à membrane échangeuse de protons. Nous nous concentrons sur un matériau en particulier, celui intervenant dans les couches de diffusion des gaz (GDL). Les GDL ont de multiples fonctions, notamment de permettre en leur sein des transports simultanés de gaz, d’électrons, de chaleur et d’eau sous forme vapeur et liquide. Pour permettre ces transports, les GDL sont composées d’une phase fluide et d’une phase solide, elle-même constituée de plusieurs matériaux. La microstructure des GDL joue un rôle crucial sur les compromis entre les fonctions des GDL et l’efficacité des transports. Nous utilisons la tomographie aux rayons X pour imager la structure interne des GDL à l’échelle micrométrique, et développons des outils numériques pour simuler les transports sur les microstructures. Nous montrons que des simulations sur des images de grandes tailles sont réalisables en temps raisonnables. Nous validons les simulations de transports dans les GDL numériquement et expérimentalement. Le premier chapitre est consacré à la modélisation d’une expérience ex-situ d’injection d’eau dans les GDL. Nous développons un modèle réseau de pores extrait d’images tomographiques, pour simuler les écoulements d’eau dans les GDL en présence de forces capillaires. Nous validons les simulations réseaux de pores en utilisant des images tomographiques montrant l’eau liquide dans une GDL lors d’une expérience d’injection d’eau. Nous montrons que les courbes de pression capillaire peuvent être déterminées par simulations réseau de pores ou par simulations full morphology sur des images tomographiques. Le deuxième chapitre est consacré à la simulation des transports de gaz et d’électrons dans les GDL. Nous développons une méthode de simulation réseau de pores, consistant à décomposer l’image en régions de formes simples et à calibrer des modèles physiques sur ces régions. Cette approche à deux échelles est économe en temps de calcul. Nous comparons ces simulations à des simulations directes et à des formules analytiques. Une seconde partie concerne la comparaison des simulations directes à des mesures expérimentales. Nous montrons que les transports dans la phase fluide peuvent être déterminés avec fiabilité par simulation directe sur les images tomographiques, tandis que la simulation des transports dans la phase solide nécessite des informations non fournies par la tomographie aux rayons X. Le troisième chapitre est consacré à la modélisation de la condensation de l’eau dans les GDL. La vapeur d’eau produite par la réaction du dihydrogène avec le dioxygène traverse les GDL et condense dans les zones froides des GDL. Un modèle réseau de pores couplant diffusion de la vapeur d’eau, changement de phase et forces capillaires est développé. Nous étudions ce modèle sur des réseaux de pores générés virtuellement. Le dernier chapitre est consacré à l’étude de microstructures conçues virtuellement. Nous montrons qu’il est possible de produire virtuellement des microstructures proches de celles de matériaux réels, de chercher des microstructures optimales, et d’étudier des effets physiques par simulation sur matériaux virtuels. / Hydrogen as an energy carrier is a promising solution for reducing emissions of greenhouse gases. Indeed, hydrogen can be used to store large amounts of energy in a completely carbon-free way. To promote the widespread use of hydrogen energy, it is essential to reduce the cost of fuel cells and increase their durability and performance. The materials in the heart of fuel cells have a strong impact on their performance and durability. In this context, opti-mizing the materials is crucial. We develop in this thesis a modeling approach of porous materials in proton exchange membrane fuel cells. We focus on a specific material that takes part in the gas diffusion layers (GDL). The gas diffusion layers are crossed by gas, electron, heat and water fluxes. To allow such multiple transports, GDL are composed of a fluid phase and a solid phase, itself consisting of several materials. The microstructure of the GDL plays an essential role on the tradeoffs between transports. To model these tradeoffs, we use X-ray tomography to image the microstructure at micrometer scales, and develop digital tools to simulate the transport on tomographic images. We validate the simulations with experimental characterizations and tomographic images of GDL. Great care has been taken in the computer performance of the numerical tools, because tomographic images in three dimensions are a challenge because of the size of the data. The first chapter of this thesis is devoted to modeling of an ex-situ water injection experiment in a GDL. We develop a pore network model extracted from tomographic images, to simulate liquid water flows in GDL in the presence of ca-pillary forces. We validate pore networks simulations using tomographic images showing the liquid water in a GDL dur-ing a water injection experiment. We show that the capillary pressure curves can be determined reliably by pore net-work simulations or full morphology simulations on tomographic images. The second chapter is devoted to one-phase transport simulations in GDL. The first part of this chapter is devoted to the development of pore networks simulations for the diffusivity and the electrical conductivities of the GDL. We de-velop a two-scale simulation methodology, which consists of decomposing the image into elements having simple shapes, and to calibrate physical models on these elements. This method considers the effect of the microstructure on the physical transfers in an economical way, reducing the computing time. We compare the pore network simulations to direct simulation on microstructures and to analytical formulas. The second part is devoted to the comparison of transport simulations with experimental measurements. We show that the transports in the fluid phase can be deter-mined reliably by direct simulations on the tomographic images, while transports in the solid phase require additional information not provided by X-ray tomography. The third chapter is devoted to modeling of the condensation of water in the GDL. The steam produced by the reaction of the hydrogen with the oxygen passes through the GDL and condenses in the cold areas of the GDL. A pore network model coupling diffusion of steam, phase change and capillary forces is developed. We study this model on virtually generated pore networks. The last chapter is devoted to the study of virtually designed microstructures. Virtually exploring new materials designs has advantages over the experimental approach, in terms of speed, cost and control over the microstructures. We show that it is possible to virtually produce microstructures close to those of real materials, to seek optimal microstructures, and control the microstructure to better study some physical effects using simulation.

Biocélulas a combustível (BFCs) são definidas como dispositivos bioeletroquímicos que utilizam componentes biológicos, como enzimas ou microrganismos, para converter energia química em energia elétrica. Neste estudo, reporta-se o desenvolvimento de um bioeletrodo composto por fibras flexíveis de carbono (FFC) modificadas com a enzima álcool desidrogenase (ADH), o qual foi utilizado juntamente com a levedura Saccharomyces cerevisiae , sendo enzima e microrganismos usados como biocatalisadores cooperativos para bioprodução e oxidação de etanol. A glicose é oxidada pelas células de levedura sob condições anaeróbias, e o etanol formado pela fermentação alcóolica é, em seguida, oxidado a acetaldeído pela enzima ADH. A oxidação de etanol pela ADH resulta ainda, na redução da molécula de nicotinamida adenina dinucleotídeo, NAD+ a NADH. Posteriormente, o NADH formado nessa reação é eletroquimicamente oxidado a NAD+ na superfície do bioeletrodo de FFC baseado em ADH (FFC-ADH). Avaliou-se a influência da temperatura e do pH na bioeletrocatálise de etanol pela ADH e a melhor resposta obtida foi em 40 ºC e pH 8,5. Além disso, obteve-se uma excelente correlação linear entre os valores de concentração de etanol e densidade de corrente, indicando que a resposta bioeletrocatalítica da ADH é diretamente proporcional à concentração de etanol produzido a partir da fermentação. O conceito de que microrganismos e enzimas podem trabalhar cooperativamente para produzir uma nova classe de bioeletrodos, foi introduzido nesse trabalho. Por fim, demonstrou-se, que o bioeletrodo cooperativo pode ser aplicado com sucesso em uma BFC, utilizando o biocátodo de difusão de gás contendo a enzima bilirrubina oxidase (BOx) imobilizada em sua superfície. / Biofuel cells (BFCs) are defined as bioelectrochemical devices that use biological components, such as enzymes or microorganisms, to convert chemical energy into electric energy. In this study, we report the development of a bioelectrode composed of flexible carbon fibers (FCF) modified with the enzyme alcohol dehydrogenase (ADH) together with Saccharomyces cerevisiae yeast, being enzyme and microorganisms used as cooperative biocatalysts for bioproduction and oxidation of ethanol. Glucose is oxidized by the yeast cells in anaerobic conditions, and ethanol is produced through alcoholic fermentation and then it is oxidized to acetaldehyde by the ADH enzyme. The ethanol oxidation by ADH also results in the reduction of the nicotinamide adenine dinucleotide molecule, NAD+ to NADH. Subsequently, the NADH produced in this reaction is electrochemically oxidized to NAD+ on the surface of the FCF bioelectrode based on ADH (FCF-ADH). The influence of temperature and pH on the bioelectrocatalysis of ethanol was evaluated and the best performance was found at 40 ºC and pH 8.5. Additionally, the results demonstrated an excellent linear correlation between the ethanol concentration and the current generated, which indicates that the bioelectrocatalytic response of ADH is directly proportional to concentration of ethanol produced from the fermentation. The present study has introduced the concept that microorganisms and enzymes can work cooperatively to produce a new class of bioelectrodes. Finally, it has been demonstrated that the cooperative bioelectrode can be applied successfully to BFC using a gas-diffusion biocathode containing the bilirubin oxidase enzyme (BOx) immobilized on its surface.

Orientador: Marcos Roberto de Vasconcelos Lanza / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-08-10T15:24:47Z (GMT). No. of bitstreams: 1 Rocha_RobsondaSilva_M.pdf: 1261455 bytes, checksum: df1d32d1e649b02714911e4465f4c58e (MD5) Previous issue date: 2007 / Resumo: Este projeto propõe o desenvolvimento e a otimização do tratamento eletroquímico de um efluente farmacêutico sintético contendo diclofenaco sódico. Neste trabalho foram executadas duas etapas, iniciando-se pelo estudo das reações redox do diclofenaco e, em seguida, pelo processo de degradação em um reator eletroquímico de bancada. No estudo eletroquímico do diclofenaco foram realizadas voltametrias hidrodinâmicas em meio aquoso (0,1 M de K2SO4) e meio não aquoso (N,N Dimetil Formamida, DMF, com 0,1 mol.L-1 de perclorato de sódio), no eletrodo de carbono vítreo e nas rotações de 0 rpm a 3000 rpm. O eletrodo de Carbono Vítreo em meio não aquoso apresentou as melhores respostas, foram observados dois picos de oxidação, -0,33 V vs. Ag/AgCl e 0,57 V vs. Ag/AgCl e um pico de redução em 0,73 V vs. Ag/AgCl. O estudo das voltametrias hidrodinâmicas mostrou, que as reações de oxi-redução do diclofenaco são influenciadas pela rotação do eletrodo de carbono vítreo. Na degradação do diclofenaco sódico foi utilizado um reator eletroquímico de bancada, como catodo utilizou-se um eletrodo de difusão gasosa e como anodo, um DSA-Cl2 ®, como eletrólito foi utilizado 1,0 L de K2SO4 0,1 M com 200 mg.L-1 de Diclofenaco, com uma vazão de 200 L.h-1, a pressão de O2 foi de 0,2 Bar e os ensaios no reator foram realizados com e sem 10 mM de FeSO4. Os resultados mostraram que o reator eletroquímico é eficiente na geração de H2O2 alcançando 350 mg.L-1 em duas horas de eletrólise sem a adição do fármaco. Os ensaios de degradação do diclofenaco utilizaram a oxidação química indireta, pelos radicais hidroxila formados a partir do H2O2 eletrogerado, e pela oxidação eletroquímica direta no anodo. Este processo se mostrou eficiente na degradação do diclofenaco, alcançando 99,2 % de redução da concentração inicial do fármaco e 27,4 % de redução da demanda química de oxigênio (DQO). Quando se utilizou eletro-Fenton, adição de FeSO4, como catalisador da formação de radicais hidroxila, a eficiência aumentou, a degradação do diclofenaco alcançou 99,4 % da concentração inicial e a diminuição da DQO chegou a 63,2 %. Os resultados mostraram que o processo de degradação utilizando reator eletroquímico é eficiente na degradação do fármaco e na diminuição da DQO / Abstract: This work proposes the development and the optimization of the electrochemical treatment of a synthetic effluent with sodium diclofenac. In this work two stages were executed the study of the redox reactions of the sodium diclofenac and, the process of this organic compound. Hydrodynamic voltammetry experiments were recorded to identity sodium diclofenaco redox reaction in a non-aqueous medium (DMF with 0,1 mol L-1 of NaClO4) and in aqueous medium (0.1 M of K2SO4). These experiments were performed using glassy carbon as working electrodes, at different rotations (0 up to 3000 rpm). The glassy carbon electrode in non aqueous medium presented the best answers, where observed two peaks of oxidation, at 0.33 V vs. Ag/AgCl and 0.57 V vs. Ag/AgCl, and a peak of reduction at 0.73 V vs. Ag/AgCl. The hydrodynamic voltammetry it experiments showed that the redox reactions of the sodium diclofenac are influenced by the rotation of the glassy carbon electrode. A flow electrochemical reactor was used for the sodium diclofenac degradation. It was used a gas diffusion electrode as cathode and DSA-Cl2 ® as anode. The electrolyte used was 1.0 L of 0.1 M K2SO4 with 200 mg L-1 of sodium diclofenac (flow rate: 200 L.h-1, pressure (O2): 0,2 Bar), with and without 10 mM of FeSO4. The performance was evaluated considering concentration decay of sodium diclofenac concentration (HPLC) and chemical oxygen demand (COD) as a function of the applied current and addiction of Fe(II) ions. The results showed that the electrochemical reactor was efficient in the generation of 350 mg L-1 of H2O2 after two hours of electrolysis without the addition of the organic compound. The sodium diclofenac degradation occurred by indirect chemical oxidation, for the hydroxyl radicals formation from H2O2 electrogenerated, and by direct electrochemical oxidation on anode. This process showed the efficiency in the degradation of sodium diclofenaco: 99,2 % of reduction of the initial concentration and 27,4 % of reduction of the chemical oxygen demand (COD). When electro-Fenton was used by addition of FeSO4 as catalyst hydroxyl radicals formation, the degradation efficiency increased. The of the drug degradation was 99,4 % of the initial concentration and the COD reduction was 63,2 %. The results showned that the degradation process using electrochemical reactor was efficient in the sodium diclofenac degradation and COD reduction / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Mecânica

Orientador: Prof. Dr. Mauro Coelho dos Santos / Dissertação (mestrado) - Universidade Federal do ABC. Programa de Pós-Graduação em Ciência e Tecnologia/Química, 2018. / Este trabalho visou sintetizar eletrocatalisadores baseados em nanobarras de céria (NR CeO2) suportadas em carbono Vulcan XC-72, em diferentes proporções mássicas. Caracterizar e avaliar suas eficiências, na reação de redução de oxigênio (RRO) tendo como objetivo o mecanismo 2-elétrons, ou seja, a eletrogeração de H2O2, pela técnica de eletrodo de disco-anel rotatório (RRDE). Além disso, realizaram-se as confecções de eletrodos de difusão gasosa (EDG) com o eletrocatalisador mais promissor tendo em vista a avaliação da capacidade de produção de H2O2, em comparação a um EDG de carbono Vulcan XC-72 aplicando-se em processos eletroquímicos oxidativos avançados (PEOAs) na degradação do paracetamol. Pelo estudo da RRO utilizando a técnica de RRDE, concluiu-se que houve uma grande melhora na eletrogeração de H2O2 suportando 1% de NR CeO2 no carbono Vulcan XC-72 devido ao efeito sinérgico entre baixa concentração de NR CeO2 e o carbono Vulcan XC-72. Esse eletrocatalisador mostrou um menor potencial de início da RRO, quando comparado ao do carbono Vulcan XC-72, com transferência de 2,1 elétrons na RRO e conversão de 95% do gás oxigênio (O2) em H2O2, enquanto o carbono Vulcan XC-72 foi capaz de converter apenas 54% de O2 em H2O2. O EDGCe confeccionado produziu 1463 mg L-1 de H2O2, enquanto o EDGV produziu apenas 490 mg L-1, em 2 horas de eletrólise no potencial -2,7 V vs. Ag/AgCl. O EDGCe foi aplicado em diferentes PEOAs visando a mineralização do paracetamol em uma célula eletroquímica de 350 mL, com taxas de mineralização de até 98,4% após 6 horas, o que indica ser um promissor EDG a ser utilizado em PEOAs. / This work aimed to synthesize electrocatalysts based on ceria nanorods (CeO2 NR) supported on Vulcan XC-72 carbon, in the different mass proportions. Characterize and evaluate its efficiencies, in the oxygen reduction reaction (ORR) aiming the 2-electron mechanism, ie, H2O2 electrogeneration, by the rotating ring-disc electrode (RRDE) technique. In addition, the gas diffusion electrodes (GDE) were made with the most promising electrocatalyst having as reference the evaluation of the H2O2 production capacity, compared to a Vulcan XC-72 carbon GDE applying in electrochemical advanced oxidative processes (EAOPs) in the degradation of paracetamol. By the ORR study using the RRDE technique, it was concluded that there was a great improvement in the H2O2 electrogeneration supporting 1% of CeO2 NR in the Vulcan XC-72 carbon due to the synergistic effect between low concentrations of NR CeO2 and Vulcan XC-72 carbon. This electrocatalyst showed a lower initial potential of ORR, when compared to of the Vulcan XC-72 carbon, with transfer of 2.1 electrons and conversion rate of 95% of the oxygen gas (O2) into H2O2, while the Vulcan XC-72 carbon was able to convert only 54% of O2 into H2O2. The prepared GDECe produced 1463 mg L-1 of H2O2, while GDEV produced only 490 mg L-1 of H2O2, in 2 hours of electrolysis in the potential -2.7 V vs. Ag/AgCl. The GDECe was applied in different EAOPs aiming at the mineralization of paracetamol in a 350 mL electrochemical cell, with mineralization rates up to 98.4% after 6 hours, indicating that it is a promising GDE to be used in EAOPs.

La démontabilité des assemblages collés structuraux est une nouvelle problématique liée à l’augmentation de l’utilisation de la technologie du collage dans de nombreux secteurs industriels et de la prise en compte de l’éco-conception dans les processus industriels. Peu de travaux ont été menés à ce jour sur cette thématique qui représente un véritable verrou technologique pour la diffusion de la technologie d’assemblage par collage. Une des voies utilisées et détaillées dans la présente étude est basée sur l’incorporation, dans une formulation d’adhésif, d’un additif organique solide possédant un point de sublimation dans une gamme de température connue.Ainsi, l’étude de l’influence de la reformulation par incorporation de l’additif montre qu’en dépit de la réactivité entre cette molécule et certains composants de la formulation de l’adhésif, les propriétés de mise en œuvre et les caractéristiques thermo-mécaniques ne sont pas drastiquement modifiées et restent compatibles avec une utilisation industrielle.Le mécanisme de démontabilité est basé sur la diffusion du gaz libéré par la réaction de sublimation de l’additif jusqu’aux interfaces où les contraintes générées induisent une rupture interfaciale. Néanmoins, le désassemblage est grandement fonction de la nature de la surface des substrats assemblés. Une compétition a lieu entre le travail nécessaire pour faire croître les bulles à l’interface et le travail des forces d’adhésion.L’efficacité de ce procédé de démontabilité a été éprouvée dans le cadre de tests de validation des segments du tore d’un télescope. L’assemblage réalisé avec un adhésif incorporant un additif spécifique permet de transmettre d’importantes contraintes mécaniques puis après une phase d’activation, de se désassembler avec des contraintes résiduelles nulles / Structural adhesives are nowadays widely used in numerous industries like automotive, aerospace, avionics or microelectronics … for many reasons such as easy processing or weight and cost savings. A strong effort has been achieved so far to enhance the level of adherence in structural assemblies and in this particular case a new challenge appears: the easy dismantling of structural bonded joints. This innovative concept results from industrial constrains like maintenance or recycling needs.This work studies a process which offers a simple and efficient solution to the disassembling of structural bonds. Based on the use of specific additives activated by heating at a certain temperature, this new technology allows the drastic decrease of the bonding performance and allow the dismantling in a very short time. It fulfills the main characteristics required by this application, like no change in processing (implementation, curing conditions,…) and no or slight modification of the mechanical properties. The results of the influence of adding additive is evaluated.This process is based on the incorporation. of specific chemicals in the adhesive or primer formulations. These additives are selected according to specific properties like decomposition temperature. An innovative aspect of this technology lies in the localization of the dismantling. The first step of the additive action is migration from the bulk of the adhesive to the interface. In a second time, once at the interface, the decomposition gases generated by the additive (mainly steam and nitrogen) induce constrains and stresses. After a certain time, theses stresses are sufficient enough to overcome the adhesion forces and the adhesive debonds from the substrate, adhesive failure occurs. One logical consequence of this interfacial action is the use of a dismantling primer, which enables to improve the efficiency of the technology and allows additive savings.This work proposes mecanisms based on experimental results in order to explain what happens in the adhesive bulk and on the interfaces. Finally, an application of this process is described related to the use of debondable adhesive for structural coupling during ground tests in the frame of GAIA telescope. Its main structure is a multi segments brazed torus in silicon carbide. In order to test each segment, a structural and debondable epoxy adhesive was developed, allowing structural bonding at room temperature and easy dismantling with interfacial failure after thermal activation. After testing and dismantling, segments were reused for final application.

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

Cette thèse est consacrée à l'étude des propriétés des transports diphasiques au sein des couches de diffusions (Gas Diffusion Layer = GDL) des piles à combustible PEMFC (Proton Exchange Membrane Fuel Cells). La GDL est faite d'une structure fibreuse (dont l'épaisseur est de quelques centaines de micromètres) traitée généralement avec une matière hydrophobe. Des images numériques de la GDL réelle obtenues par tomographie X sont d'abord analysées afin d'étudier des propriétés telles que la porosité, la perméabilité, ou le tenseur de diffusion. L'écrasement de la GDL est ensuite simulé en utilisant un algorithme comprimant les fibres dans un plan transversal. Les transports diphasiques (invasion quasi statique d'eau liquide) sont modélisés dans des réseaux de pores, milieux représentatifs de l'espace poreux de la GDL, en relation avec le problème de la gestion de l'eau dans les piles PEMFC. Deux algorithmes d'invasion, dénommés algorithmes séquentiel et cinétique, sont développés et comparés pour analyser les distributions de phases au sein des GDL. Un point clé est que l'eau rentre dans la couche poreuse par divers points d'injection indépendants, conduisant à la possibilité de multiples points de percée. Des expériences sur un système microfluidique sont conduites pour valider les algorithmes utilisés. Une étude statistique est menée pour caractériser le nombre de points de percée, les profils de saturation, l'accès au gaz, le transport diffusif, de même que l'influence du piégeage et de la mouillabilité mixte. / This thesis is devoted to the study of transport properties and two-phase flow in the Gas Diffusion Layer (GDL) of Proton Exchange Membrane Fuel Cells (PEMFC). A GDL is a thin fibrous structure (a few hundreds μm thick) treated generally with a hydrophobic agent. Numerical images obtained from X-ray computed tomography X are first exploited to study properties such as the porosity, permeability and diffusion tensors of a real GDL microstructure. The effect of GDL compression is also investigated using an algorithm mimicking the compression in GDL through plane direction. Then two phase flow (quasi-static water invasion) is studied in relation with the water management problem in PEMFC, using a structured pore network representation of the pore space. Two invasion algorithms, referred to as the sequential and the kinetic algorithm respectively, are developed and compared to study the fluid distributions within the GDL. A key point is that water enters the porous layer through multiple independent inlet injection points, leading to the possibility of many breakthrough points. Experiments are conducted on a microfluidic device to validate the algorithms. A numerical statistical study is performed to characterize the breakthrough point statistics, saturation profiles, gas access, diffusion transport as well as the influence of trapping and mixed wettability.