Uma variedade de suportes de óxidos metálicos em catalisadores foram sintetizados visando sua utilização na reforma a vapor do etanol para produção de uma mistura rica em hidrogênio para ser empregado nas células a combustível. Os catalisadores foram preparados pelos métodos da coprecipitação e geleificação interna, utilizando cobalto e níquel como metais ativos suportados em óxidos de alumínio, zircônio, lantânio e cério. Após preparados e calcinados a uma temperatura de 550ºC os sólidos foram caracterizados por diversas técnicas de análises tais como, difração de raios-X (DRX), espectroscopia de energia dispersiva (EDS), microscopia eletrônica de varredura (MEV), adsorção de nitrogênio (método B.E.T.), temperatura de redução programada em H2 (TRP-H2) e análise termogravimétrica. Os testes catalíticos foram realizados em um reator monolítico de quartzo onde foram variadas as condições termodinâmicas da reforma a vapor do etanol nas temperaturas de operação entre 500ºC e 800ºC. O gás de síntese obtido na reforma a vapor do etanol foi analisado on-line por um cromatógrafo a gás. O catalisador cobalto/níquel suportado em uma mistura de céria e lantânia (Co10% / Ni5% - CeO2La2O3) apresentou bom desempenho catalítico com seletividade em hidrogênio, alcançando uma concentração superior a 65%, quando comparado aos outros sistemas catalíticos como: Co10% / Ni5% - CeO2; Co10% / Ni5% - CeO2ZrO2; Co10% / Ni5% - ZrO2; Co10% / Ni5% - La2O3; Co10% / Ni5% - CeO2La2O3/K2%; Co10% / Ni5% - CeO2La2O3 / Na2%; Ni10% / Co5% - CeO2La2O3; Co-Al2O3 e Co-Al2O3CeO2. / A range of oxide-supported metal catalysts have been investigated for the steam reforming of ethanol for the production of hydrogen and subsequent application in fuel cells. The catalysts were synthesized by the co-precipitation and internal gelification methods using cobalt and nickel as active metals supported on aluminum, zirconium, lanthanum and cerium oxides. After prepared and calcined at 550 Cº the solids were fully characterized by different techniques such as X-rays diffraction(DRX), energy-dispersive X-ray spectroscopy (EDS), scanning electron microscopy, nitrogen adsorption (B.E.T), temperature-programmed reduction in H2 (TPR-H2) and thermogravimetric analysis. The catalytic tests were performed in a monolithic quartz reactor and submitted to different thermodynamic conditions of steam reforming of ethanol at temperatures varying from 500º C to 800 ºC. The product gas streams from the reactor were analyzed by an on-line gas cromatograph. The cobalt/nickel catalyst supported on a ceria-lanthania mixture (Co10% / Ni5% - CeO2La2O3) showed good catalytic performance in hydrogen selectivity reaching a concentration greater than 65%, when compared to other catalytic systems such as: Co10% / Ni5% - CeO2; Co10% / Ni5% - CeO2ZrO2; Co10% / Ni5% - ZrO2; Co10% / Ni5% - La2O3; Co10% / Ni5% - CeO2La2O3/K2%; Co10% / Ni5% - CeO2La2O3 / Na2%; Ni10% / Co5% - CeO2La2O3; Co-Al2O3 e Co-Al2O3CeO2.

Neste trabalho foi desenvolvido um método de preparação de eletrocatalisadores de Platina e Estanho suportados em carbono para a oxidação de etanol. Este método utiliza a decomposição térmica de precursores poliméricos, conhecido como método Pechini, para a produção de pós com tamanho de partículas da ordem de nanômetros. Foram preparados diversos catalisadores variando-se a proporção entre platina e estanho, e a proporção entre metal e carbono. Os eletrocatalisadores foram caracterizados por técnicas de Espectroscopia de Energia Dispersiva de raios X (EDX), Difração de raios X (DRX) e Microscopia Eletrônica de Transmissão (MET). Os resultados mostram que o método utilizado produziu catalisadores com composição nominal próxima da composição real, que os tamanhos de cristalitos estão na ordem de 5 nm e os tamanhos de partículas apresentados estão na ordem de 5 nm. As atividades eletrocatalíticas dos catalisadores foram investigadas utilizando-se técnicas eletroquímicas convencionais (voltametria cíclica e cronoamperometria) assim como através de testes em célula a combustível a etanol direto. Após os testes de cronoamperometria foram analisadas as concentrações de etanol e dos produtos formados através de Cromatografia Líquida de Alta Eficiência (CLAE). Os voltamogramas cíclicos na presença de etanol apresentaram correntes de oxidação de etanol próximos de 0,2 V vs. ERH e os resultados de cronoamperometria mostraram que há grandes diferenças de desempenho entres os eletrocatalisadores. Os resultados de eletrólise mostraram que o produto principal é o acetaldeido, e que a quebra da ligação C-C do etanol ocorreu em pequenas proporções. O melhor desempenho nos testes de célula a combustível foi encontrado utilizando-se PtSnO2/C com uma proporção de estanho de 10 % /platina 90 % (mol) e com uma porcentagem de 30 % (massa) de metal em relação ao carbono. A densidade de potência obtida neste caso foi de aproximadamente 72 mW cm-2. / In this work, a method for the preparation of eletrocatalysts containing platinum and tin supported on carbon was developed aiming at the ethanol oxidation. This method uses the thermal decomposition of polymeric precursors, known as the Pechini method, for the production of powders with particle size lying in the range of nanometers. Several catalysts were prepared by varying the platinum and tin ratio, as well as the metal and carbon proportion. The eletrocatalysts were characterized by Energy Dispersive X-ray Spectroscopy (EDX), X-ray diffraction (DRX) and Transmission Electron Microscopy (TEM). Our results showed that the method employed in the study produced catalysts with compositions close to the nominal one, with particle size in the order of 5 nm. The electrocatalytic activities of the catalysts were investigated by conventional electrochemistry techniques (cyclic voltammetry and chronoamperometry) and by tests in direct ethanol fuel cells. After the chronoamperometric tests, the ethanol and the products were analyzed by High Performance Liquid Chromatography (HPLC). Results obtained by cyclic voltammetry in the presence of ethanol showed that currents of ethanol oxidation at approximately 0.2 V vs. RHE were achieved and the chronoamperometry results gave evidence of large differences in the electroactivity of the eletrocatalysts. The electrolysis results showed that the main product was acetaldehyde, and that the break of the C-C bond of the ethanol occured in small proportions. The best result in the cell tests was found by using the catalyst PtSnO2/C with a tin / platinum molar ratio of 1:9 and 30% (mass) metal loading in relation to carbon. The power density obtained in this case was approximately 72 mW cm-2.

Fast oxide-ion and proton conductors are the subject of considerable research due to their technological applications in sensors, ceramic membranes and solid oxide fuel cells (SOFCs). This thesis describes the use of computer modelling techniques to study point defects, dopants and clustering effects in fluorite-and perovskitetype ion conductors with potential SOFC applications. Bi2O3 related phases are being developed with the objective of high oxide-ion conductivities at lower operating temperatures than 1000°C, as in current generation SOFC electrolytes. Doped Bi2O3 phases have shown promise as materials capable of accomplishing this goal. First, the Y-doped phase, Bi3YO6, has been investigated including the ordering of intrinsic vacancies. The defect and dopant characteristics of Bi3YO6 have been examined and show that a highly mobile oxygen sub-lattice exists in this material. A preliminary structural modelling study of a new Re-doped Bi2O3 phase was also undertaken. A comprehensive investigation of the proton-conducting perovskites BaZrO3, BaPrO3 and BaThO3 is then presented. Our results suggest that intrinsic atomic disorder in BaZrO3 and BaThO3 is unlikely, but reduction of Pr4+ in BaPrO3 is favourable. The water incorporation energy is found to be less exothermic for BaZrO3 than for BaPrO3 and BaThO3, but in all cases the results suggest that the proton concentration would decrease with increasing temperature, in accord with experimental data. The high binding energies for all the dopant-OH pair clusters in BaPrO3 and BaThO3 suggest strong proton trapping effects. Finally, a study of multiferroic BiFeO3 is presented, in which the defect, dopant and migration properties of this highly topical phase are investigated. The reduction process involving the formation of oxygen vacancies and Fe2+ is the most favourable redox process. In addition, the results suggest that oxide-ion migration is anisotropic within this system.

It is proposed that an integrated ammonia-PEM fuel cell could unlock the potential of ammonia to act as a high capacity chemical hydrogen storage vector and enable renewable energy to be delivered eectively to road transport applications. Catalysts are developed for low temperature ammonia decomposition with activity from 450 K (ruthenium and cesium on graphitised carbon nanotubes). Results strongly suggest that the cesium is present on the surface and close proximity to ruthenium nanoparticles and that it produces activity in ruthenium by donation of electrons. The activity of sustainable cobalt for ammonia decomposition is shown to be a function of particle size and is more active on microporous carbon supports compared to mesoporous ones. Unlike ruthenium, activity for cobalt was not inuenced by the degree of surface graphitic nature and cobalt supported on microporous carbon approached the activity of ruthenium on the same support. In accordance with the sustainable objectives of this thesis, a case-study on the sustainability of ammonia as a sustainable hydrogen storage vector was undertaken. In this scheme, hydrogen produced from renewable electricity by electrolysis is con- verted to ammonia by the Haber-Bosch process and then converted back to deliver pure hydrogen at the point of use. The energy eciency and carbon footprint fell short of targets set by the US Department of Energy and the UK Department for Transport, the biggest impact was the production of hydrogen by electrolysis and not the Haber-Bosch process which accounts for only 9 % of total energy losses. To assess the feasibility of the ammonia-PEM fuel cell, a conceptual design was un- dertaken to quantify the palladium membrane size and catalyst quantity required for a typical road transport vehicle. The predicted quantity of palladium was excessive and future work must consider improvements to membrane technology.

Proton exchange membrane (PEM) fuel cells are at the forefront among different types of fuel cells and are likely to be important power sources in the near future. PEM is a key component of the PEM fuel cells. The objective of this research is to investigate the fundamental aspects of PEM in terms of thermodynamics and proton transport in the membrane, so that the new proton conducting materials may be developed based on the detailed understanding. Since the proton conductivity increases dramatically with the amount of water in PEM, it is important to maintain a high humidification during the fuel cell operation. Therefore, the water uptake characteristics of the membrane are very important in developing fuel cell systems. Thermodynamic models are developed to describe sorption in proton-exchange membranes (PEMs), which can predict the complete isotherm as well as provide a plausible explanation for the long unresolved phenomenon termed Schroeder¡¯s paradox, namely the difference between the amounts sorbed from a liquid solvent versus from its saturated vapor. The sorption isotherm is a result of equilibrium established in the polymer-solvent system when the swelling pressure due to the uptake of solvent is balanced by the surface and elastic deformation pressures that restrain further stretching of the polymer network. The transport of protons in PEMs is intriguing. It requires knowledge of the PEM structure, water sorption thermodynamics in PEM, proton distribution in PEM, interactions between the protons and PEM, and proton transport in aqueous solution. Even proton conduction in water is anomalous that has received considerable attention for over a century because of its paramount importance in chemical, biological, and electrochemical systems. A pore transport model is proposed to describe proton diffusion at various hydration levels within Nafion¢ÃƒÂ§ by incorporating structural effect upon water uptake and various proton transport mechanisms, namely proton hopping on pore surface, Grotthuss diffusion in pore bulk, and ordinary mass diffusion of hydronium ions. A comprehensive random walk basis that relates the molecular details of proton transfer to the continuum diffusion coefficients has been applied to provide the transport details in the molecular scale within the pores of PEM. The proton conductivity in contact with water vapor is accurately predicted as a function of relative humidity without any fitted parameters. This theoretical model is quite insightful and provides design variables for developing high proton conducting PEMs. The proton transport model has been extended to the nanocomposite membranes being designed for higher temperature operation which are prepared via modification of polymer (host membrane) by the incorporation of inorganics such as SiO2 and ZrO2. The operation of fuel cells at high temperature provides many advantages, especially for CO poisoning. A proton transport model is proposed to describe proton diffusion in nanocomposite Nafion¢ÃƒÂ§/(ZrO2/SO42-) membranes. This model adequately accounts for the acidity, surface acid density, particle size, and the amount of loading of the inorganics. The higher proton conductivity of the composite membrane compared with that of Nafion is observed experimentally and also predicted by the model. Finally, some applications of PEM fuel cells are considered including direct methanol fuel cells, palladium barrier anode, and water electrolysis in regenerative fuel cells.

Les piles à combustible sont des systèmes qui permettent de convertir directement de l'énergie chimique en énergie électrique. La structure physique d'une pile à combustible est composée d'une cathode et d'une anode poreuses séparées par un électrolyte dense. Les piles à combustible à oxydes solides (Solid Oxide Fuel Cell (SOFC))offrent une alternative intéressante pour la production d'énergie et une certaine polyvalence dans leur utilisation. Les recherches actuelles se focalisent sur l'abaissement de la température de fonctionnement de ce type de pile (500-700°C) pour augmenter leur durée de vie, diminuer les coûts de fabrication et les dégradations aux interfaces. Afin de compenser ces problèmes, la recherche tend vers des matériaux présentant de meilleures propriétés électrochimiques ou en modifiant la microstructure de la cathode pour améliorer le transfert de masse et le transfert de charge. La cathode est une couche très importante dans la pile SOFC car elle présente une résistance de la polarisation dont la réduction constitue un défi important à traiter. Dans une première partie de ce travail de thèse nous avons développé une méthode pour permettre d'améliorer les propriétés électrochimiques de cathodes de manganite de lanthane dopée au strontium (LSM). La seconde partie a été consacrée à l'élaboration et la caractérisation par spectroscopie d'impédance de demi-cellules symétriques de SOFC avec un matériau composite à base de LSM permettant d'améliorer les propriétés électrochimiques des électrodes à des températures comprises entre 600 °C - 700 °C. / Fuel cells are systems that convert chemical energy directly into electrical energy. The physical structure of a fuel cell is composed of a porous cathode and anode separated by a dense electrolyte. Solid Oxide Fuel Cells (SOFC) offer an alternative for power generation and versability in their use. Current research focuses on lowering the operating temperature of this type of fuel cell (500-700°C) to increase their life, reduce manufacturing costs and damageto the interfaces. In order to compensate these problems, research tends towards materials with better electrochemical properties or by modifying the microstructure of the cathode to improve mass transfer and charge transfer. The cathode is a very important layer in the SOFC stack because it has a polarization resistance whose reduction is a major challenge to deal with. In a first part of this thesis work we have developed a method to improve the electochemical properties of strontium doped lanthanum manganite (LSM) cathodes. The second part was devoted to the elaboration and caracterization by impedance spectroscopy of SOFC symmetric half-cells with a LSM-based composite material allowing to improve the electochemical properties of electrodes at temperatures between 600-700 °C.

Membranas à base de polímeros perfluorosulfonados, como a Nafion, vêm sendo extensivamente usadas como membrana de troca protônica em células a combustível (FC). O objetivo deste trabalho foi desenvolver membranas eletrólito à base de polindeno sulfonado (SPInd) e poli(fluoreto de vinilideno) (PVDF), para uso como membranas de troca protônica em condições semelhantes às da membrana Nafion. As membranas foram preparadas por casting em diferentes composições utilizando PVDF como reforço mecânico e PVDF sulfonado (SPVDF) como agente compatibilizante. Todas as membranas foram avaliadas por análise termogravimétrica, calorimetria exploratória diferencial, análise dinâmico mecânica, microscopia eletrônica de varredura, grau de inchamento, capacidade de troca iônica e espectroscopia de impedância eletroquímica. As membranas com características semelhantes à membrana Nafion foram avaliadas em protótipo de FC a 80 °C. A membrana SPInd50/PVDF e as membranas com agente compatibilizante apresentaram condutividades iônicas na ordem de 10-2 S/cm, comparáveis àquela da membrana Nafion. A membrana com melhor desempenho em protótipo de FC foi o SPInd/PVDFC12, preparado com 50% de SPInd, 47,5% de PVDF e 2,5% de SPVDF (p/p), cujos valores de potencial de circuito aberto e densidade de potência máximo foram de 1,02 V e 74,54 mW/cm2, respectivamente. Apesar da densidade de potência máxima ser inferior à da membrana Nafion (603 mW/cm2), a membrana SPInd/PVDFC12 apresenta potencial para uso como eletrólito em célula a combustível. / Perfluorosulfonic acid ionomer membranes, e.g. Nafion, have been extensively used as proton exchange membranes in fuel cells (FC) due to their high proton conductivity and good mechanical properties. The aim of this work was to develop electrolyte membranes based on sulfonated polyindene (SPInd) and poly(vinylidene fluoride) (PVDF) to be used in the same conditions as Nafion. Membranes were prepared by casting with different compositions using PVDF as mechanical reinforcement and sulfonated PVDF (SPVDF) as coupling agent. The produced membranes were evaluated by thermogravimetric analysis, differential scanning calorimetry, and dynamic mechanical analysis, scanning electron microscopy, water uptake, ion exchange capacity and electrochemical impedance spectroscopy. The membranes with similar results to Nafion, were evaluated in a FC prototype at 80 °C. The membrane SPInd50/PVDF and all the membranes with coupling agent had ionic conductivity in the order of 10-2 S/cm, comparable to the Nafion´s. The polyelectrolyte with the best performance was the SPInd/PVDFC12 which was prepared with 50 wt% SPInd, 47.5 wt% PVDF and 2.5 wt% SPVDF, that reached an open circuit voltage of 1.02 V and maximum power density of 74.54 mW/cm2. Even though Nafion´s maximum power density was higher (603 mW/cm2), the SPInd/PVDFC12 membrane showed potential to be used as electrolyte in fuel cells.

La croissance des unités de distribution des ressources énergétiques ainsi que l'intégration des nouvelles technologies dans la production et le stockage d'énergie, ont imposé un contrôle nouveau et de nouvelles stratégies opérationnelles. Grâce à sa capacité de stockage et étant considérée comme une énergie propre; la pile à combustible (Pac) est l'une des technologies les plus prometteuse en tant que source d'énergie stationnaire dans les réseaux micro et aussi dans les applications de transport. Par conséquent, deux sujets principaux sont abordés dans cet ouvrage, la conception et l'installation d'un banc d'essai complet instrumenté a membrane échangeuse de polymère (PEM) Pac et de conception et l'essai expérimental d'une nouvelle stratégie de contrôle dynamique d'échange de l'énergie pour les systèmes multi - source et multi - charge. Pour définir le test instruments banc exigences, un examen complet de modèle dynamique est donné dans la première partie. Dans la prochaine section seront inclues, les renseignements concernant la configuration de la conception et la mise en œuvre de banc d'essai de Pac, i.e., critères de spécification des instruments, acquisition, et affichage des données du système. Des résultats expérimentaux sont réalisés afin de démontrer les potentialités de l'installation. Dans le chapitre suivant, une nouvelle stratégie de contrôle dynamique de l'énergie d'échange (DSER) sera introduite et testée par simulation et expérimentalement dans un système à deux ports. Afin d'établir une comparaison et d'intégrer la DSER dans une application Pac, un système à trois ports - y compris un modèle statique de Pac - et deux différentes approches de contrôle, seront testés par simulation dans le cinquième chapitre. La thèse s’achèvera par quelques conclusions et quelques thèmes de recherche potentiels générés à partir de ce travail. / The growth of distributed energy resources together with the incorporation of new technologies in the generation and storage of energy are imposing new control and operational strategies. Due to its storage capability and that it is considered to be clean energy; fuel cell (FC) is one of the most promissory technologies as a stationary energy source in micro grids and also in transportation applications. Therefore, two main issues are addressed in this work; the conception, design, and setup of a fully instrumented test bench for proton exchange membrane (PEM) FC stacks and the design and experimental test of a new dynamic energy-exchange control strategy for multi source and multi load systems. To define the test bench instrument requirements, in the first part a complete dynamic model review is given. In the next section, relevant information regarding the setup of the FC test bench design and implementation is included, i.e., specification criteria of the instruments and acquisition and data display system. Some experimental results are performed in order to demonstrate the potentialities of the setup. In the following chapter, a new dynamic energy exchange control strategy (DSER) is introduced and tested in a two port system via simulation and experimentation. In order to establish a comparison and integrate the DSER in a FC application, in the fifth chapter a three port system – including a static model of FC – and two different control approaches, are tested via simulation. The thesis is closed with some concluding remarks and some potential research topics generated from this work.

Neste trabalho foi estudada a reação de metanação do CO em catalisador de Ru/C e sua aplicação em purificação de hidrogênio para utilização em célula a combustível. Tendo como meta a otimização da produção de metano em baixas temperaturas foram preparados materiais de RuxPd(1-x)/C, que também foram tratados termicamente em atmosfera úmida e redutora. Ao final do estudo com CO, também foi investigado o efeito da presença de CO2 no fluxo de hidrogênio, tal qual num sistema real utilizando-se gás de síntese. Os dados obtidos por espectroscopia de energia dispersiva de Raio-X (EDX) mostraram uma boa correspondência entre as composições teóricas e as obtidas experimentalmente dos catalisadores preparados neste trabalho. Analisando-se os difratogramas de Raio-X (DRX) foi possível observar a presença de fases metálicas principalmente de Ru, observando-se também o aumento da cristalinidade do material, promovido pelo tratamento térmico. Além disso, os tamanhos médios de cristalito (TMC) dos materiais foram obtidos com a equação de Scherrer e situaram-se em torno de 2,5 nm. As micrografias de TEM confirmaram o tamanho reduzido das partículas, mas apontaram para uma baixa dispersão dos materiais sobre o suporte, principalmente no caso do Ru/C. Com as medidas de XPS foram identificadas espécies de Ru metálico, Ru(IV), Ru(VII), RuOxHy e Ru.xH2O. O tratamento térmico somente afetou a população de espécies do material Ru/C, que passou a apresentar maior percentual de Ru metálico, em detrimento aos óxidos. Os catalisadores Ru/C e Ru7Pd3/C antes e após o tratamento térmico foram acomodados em filtro de linha gasosa (reator de leito fixo), sendo possível concluir que o tratamento térmico somente promoveu melhora no desempenho metanador para o catalisador Ru/C. Quando utilizados em camada difusora anódica este efeito foi ainda mais pronunciado, sendo observada a produção de CH4 em regime estacionário em Ru/C tratado termicamente quando a temperatura de metanação foi 85 °C. O material mais eficiente foi o de Ru/C após o tratamento térmico, sendo capaz inclusive de hidrogenar seletivamente o CO na presença de CO2. A melhora no desempenho da célula que utilizou este material foi mais notável a 105 °C, possibilitando a operação com 33 mV de sobrepotencial a 1 A.cm-2 quando alimentada com H2 contendo 75 ppm de CO. / In this work the CO methanation reaction over Ru/C catalyst and its practical applications for use as hydrogen purifier for low temperature fuel cell anodes were studied. Aiming at optimizing the low temperature methane production, two main procedures were proposed: The use of RuxPd(1-x)/C materials and its corresponding thermal treatment of the Ru/C catalyst under wet and reductive atmosphere. The effect of the presence of CO2 was then investigated under the same conditions as for CO in order to establish the catalysts selectivity for the CO methanation in the presence of CO2, as in a real system using syngas. EDX data showed a good agreement between the expected compositions and that actually obtained for catalysts prepared in this work. DRX spectra have indicated the presence of metallic phases, particularly of Ru, and the increase of the material crystallinity, promoted by the thermal treatment. Furthermore, the materials mean crystallite sizes were obtained by the use of the Scherer equation and they resulted around 2,5 nm for all samples. TEM micrographs confirmed the small particle size but also showed a poor material dispersion of the metals over the carbon support, especially in the case of Ru/C. XPS measurements have evidenced the presence of ruthenium species such as Ru(IV), Ru(VII), RuOxHy and Ru.xH2O in the prepared ruthenium material. The thermal treatment only affected the Ru species population, showing more metallic ruthenium and less oxides. The Ru/C and Ru7Pd3/C, prior and after the treatment, were accommodated inside a gas filter, and mass spectrometry investigations indicated that the thermal treatment only increased the methanation performance for the Ru/C catalyst. When used in anodic gas diffusion layer, this improvement was even more pronounced, and resulted in stationary methane production even at 85 °C. The most efficient material for CO methanation was Ru/C thermally treated, that promoted the CO methanation in a preferable way even when CO2 was present with percentages as high as 25%. The increase of the electrochemical performance of the fuel cell with this catalyst in the diffusion layer was better when at 105 °C.

Bilirrubina oxidase de Myrothecium verrucaria é uma multicobre oxidase capaz de reduzir O2 pela oxidação de fenóis, aminas aromáticas e polipirróis. Eletroquimicamente, essa reação de redução ocorre pela transferência de elétrons entre a enzima e um eletrodo. Nesta tese, foi investigada a eficiência da enzima como agente redutor de O2 na superfície de eletrodos modificados pela função orgânica naftil-2-carboxilato por acoplamento de diazônio. Essa modificação na superfície do eletrodo aumenta em até quatro vezes a atividade do filme catalítico em relação à obtida por eletrodos em que a adsorção da enzima foi feita de forma convencional, sem a modificação. Foram estudados os efeitos da temperatura sobre a atividade da enzima para a redução de O2, sendo observado um aumento linear da atividade do eletrodo com o aumento da temperatura até 30 °C, de tal forma que temperaturas mais altas proporcionaram o aumento da inativação natural das moléculas de enzima. Esse efeito de inativação foi confirmado pela diminuição da atividade com o tempo na presença de O2, por cronoamperometria, sendo a atividade interrompida pela inserção de argônio e retomada do mesmo ponto pela reinserção de O2, descartando a idéia da queda de corrente proveniente da dessorção de enzima. Foi estudado também o efeito do pH na máxima atividade da bilirrubina oxidase, conduzidos entre pH 5,0 e 8,0, e verificando-se que a máxima atividade da enzima foi obtida entre pH 5,5 e 6,0 e, além disso, verificou-se que a corrente catalítica em baixos valores de pH aumenta diretamente com o aumento do sobrepotencial aplicado. Porém, em altos valores de pH, a curva de redução toma a forma sigmoidal e passa a ser independente do sobrepotencial aplicado, sendo a reação governada por etapas químicas de transferência de prótons. O uso de eletrodos de disco rotatório possibilitou resolver parâmetros de Michaelis-Menten para a cinética do filme catalítico de forma mais precisa (a resposta de corrente é menos dependente do transporte de massa de reagentes) e esses dados foram obtidos dentro de um intervalo de pH importante para aplicações práticas. O sobrepotencial da reação de redução de O2 catalisada por bilirrubina oxidase foi comparado com o sobrepotencial obtido para a mesma reação catalisada por Platina eletrodepositada sobre a superfície de grafite pirolítico, onde foi observado um sobrepotencial de 140 mV para a catálise enzimática, demasiado menor que o valor de 415 mV obtidos para a Platina, sob as mesmas condições experimentais, em pH neutro. A metodologia proposta para a construção de um cátodo para aplicação em células a combustível enzimáticas e os subsequentes estudos possibilitaram uma investigação minuciosa para caracterizar a enzima bilirrubina oxidase como talvez o catalisador mais eficiente na redução eletroquímica de oxigênio molecular em células a combustível até o momento. / Bilirubin oxidase from Myrothecium verrucaria is a multicopper oxidase reducing O2 at the expenses of phenols, aromatic amines and polypyrrols oxidation. Electrochemically, this reduction reaction undergoes through the electron transfer between enzyme and electrode. In this thesis, the enzyme was investigated as an efficient O2 reducing agent on electrode surfaces modified by naphthil-2-carboxylate functionalities through diazonium coupling. This modification of the electrode surface increases the activity of the catalytic film up to four times comparing to that obtained by electrodes in which the enzyme molecules were adsorbed conventionally, without modification. It was studied the effect of temperature on O2 reduction, in which catalysis increased linearly with temperature up to 30 °C, and higher temperatures increased the natural enzyme inactivation. This inactivation was confirmed by the activity drop off with time in the presence of O2, by chronoamperometry, ceased out when argon was inserted into the cell and re-established from the same point when argon was purged out by insertion of O2. These results cast aside the idea of activity drop off caused by enzyme desorption. It was also investigated the pH effect on the maximum activity of bilirubin oxidase, carried out between pH 5.0 and 8.0, being the highest activity obtained at pH 5.5-6.0. Furthermore, it was observed that the catalytic current directly increases with applied overpotential, at low pH values, and the reduction wave shape becomes sigmoidal and independent on applied overpotential at high pH values. The reaction is then governed by chemical steps, as the proton transfer. The use of rotating-disc electrodes favored solving the Michaelis-Menten kinetics for the catalytic film in a much greater accuracy (the current response is much less dependent on reagent mass transport) and these data were obtained for pH interval important for practical applications. The overpotential for the O2 reduction reaction catalyzed by bilirubin oxidase was compared to the overpotential obtained by the same reaction catalyzed by Platinum electrodeposited onto a pyrolytic graphite electrode. An overpotential of only 140 mV was observed for the enzymatic catalysis, much lower compared to the 415 mV obtained for the Platinum electrode, under the same experimental conditions, at neutral pH. The proposed method for constructing a cathode for enzymatic fuel cell application and subsequent investigation described allowed an in-depth study of bilirubin oxidase characterization as perhaps the most efficient catalysts for the electrochemical reduction of molecular oxygen in fuel cells to date.