Devido ao efeito estufa, a produção de hidrogênio a partir da reação de reforma do bioetanol tem se tornado um assunto de grande interesse em catálise heterogênea. Os catalisadores à base de Pt são empregados nos processos de purificação de H2 e também em eletrocatalisadores das células a combustível do tipo membrana polimérica (PEMFC). O hidrogênio obtido a partir da reforma do etanol contém como contaminante o acetaldeído e pequenas quantidades de CO. Assim, pode-se prever que muitas reações podem ocorrer na presença de catalisadores de Pt durante o processo de purificação do H2 e mesmo no próprio eletrocatalisador. Desta forma, este trabalho tem como objetivo descrever o comportamento do acetaldeído na presença de catalisadores de Pt. Para tanto foram preparados dois catalisadores, Pt/SiO2 e Pt/USY, contendo 1,5% de metal em ambos. Também foi estudado um eletrocatalisador (comercial) de Pt suportado em carvão (Pt/C). Os catalisadores foram caracterizados através das técnicas de análise textural, difração de raios X (DRX), quimissorção de H2, reação de desidrogenação do ciclohexano, espectroscopia no infravermelho de piridina adsorvida, dessorção a temperatura programada de n-butilamina (TPD de n-butilamina), dessorção a temperatura programada de CO2 (TPD-CO2), análise termogravimétrica, microscopia eletrônica de varredura (MEV) e espectroscopia de dispersão de energia (EDS). Os testes catalíticos foram realizados entre as temperaturas de 50 e 350 C em corrente contendo acetaldeído, H2 e N2. Foi observado que as propriedades ácido-básicas dos suportes promovem as reações de condensação com formação de éter etílico e acetato de etila. O acetaldeído em catalisadores de Pt sofre quebra das ligações C-C e C=O. A primeira ocorre em uma ampla faixa de temperaturas, enquanto a segunda apenas em temperaturas abaixo de 200 C. A quebra da ligação C-C produz metano e CO. Já a quebra da ligação C=O gera carbono residual nos catalisadores, assim como espécies oxigênio, que por sua vez são capazes de eliminar o CO da superfície dos catalisadores. Nota-se que o tipo de suporte utilizado influencia na distribuição de produtos, principalmente a baixas temperaturas. Além disso, constatou-se que a descarbonilação não é uma reação sensível à estrutura do catalisador. Verificou-se também a presença de resíduos sobre os catalisadores, possivelmente oriundos não somente da quebra da ligação C=O, mas também de reações de polimerização / Due to the greenhouse effect, hydrogen production from bioethanol reforming is a very important subject in heterogeneous catalysis research. Pt based catalysts are employed in H2 purification processes and also as electrocatalysts of PEM (Proton Exchange Membrane) fuel cells. Hydrogen obtained from ethanol reforming may contain acetaldehyde and small amounts of CO as contaminants. This very reactive aldehyde can interact with Pt based catalysts during purification process, and also with the electrocatalyst. Therefore, this work aims to study the acetaldehyde behavior in the presence of platinum based catalysts under hydrogen atmosphere. Two catalysts named Pt/SiO2 and Pt/USY were prepared, containing 1,5% of Pt. A commercial Pt eletrocatayst supported on carbon (Pt/C) was also studied. The catalysts were characterized by textural analysis, XRD, H2 chemisorption, cyclohexane dehydrogenation reaction, pyridine IR, n-butylamine TPD, CO2 TPD, TGA/DTG, SEM and EDS. The catalytic tests were carried out in a fixed bed reactor at temperature range of 50-350 C, under acetaldehyde, H2 and N2 flow. It was observed that the acid-basic supports properties promoted condensation reactions with the formation of ethylic ether and ethyl acetate. Once in contact with Pt based catalysts, acetaldehyde undergoes C-C and C=O bond scissions. The former occurs at a wide temperature range, whereas the latter occurs only at low temperatures (< 200 C). The C-C bond scission (decarbonylation) produces methane and CO. The C=O bond scission generates carbon residues on the catalyst, as well as oxygen species, which in turn eliminate CO from the catalytic surface. It was noticed that the type of support influences products distribution, mainly at low temperatures. The data also show that decarbonylation is not a structure-sensitive reaction. Residues were observed on Pt/USY which were generated not only from C=O bond rupture, but also from acetaldehyde polymerization

A industrialização das células a combustível de eletrólito polimérico sólido ainda é um desafio, devido principalmente aos elevados preços dos catalisadores, da membrana, cinética lenta da reação de redução do oxigênio e estabilidade da célula. Neste trabalho a eficiência e a estabilidade de catalisadores nanoparticulados bimetálicos de Pt-Cr suportados em carbono de elevada área superficial foram estudadas para serem utilizados como cátodos em células a combustível de eletrólito polimérico sólido. A caracterização física destes materiais foi realizada através de técnicas como difração de raios-X (DRX), energia dispersiva de raios-X (EDX), absorção de raios-X (XAS), espectroscopia de fotoelétrons excitados por raios-X (XPS) e microscopia eletrônica de transmissão (TEM). Para os estudos eletroquímicos dos catalisadores foram realizados levantamentos das curvas de polarização na célula unitária utilizando a membrana de Nafion®115, alimentadas com H2 no ânodo e O2/ar no cátodo em diferentes temperaturas e pressões, também foi utilizado a voltametria cíclica e stripping CO para a determinação da área ativa dos catalisadores e finalmente foram realizados os estudos de estabilidade simulando uma degradação acelerada mediante ciclos voltamétricos. Os resultados das curvas de polarização dos catalisadores comercial e preparados pelo método de poliol indicaram que com o aumento da quantidade de cromo obtem-se um decréscimo do desempenho. O catalisador Pt3Cr preparado pelo método de impregnação apresentou o pior desempenho. Os catalisadores Pt3Cr comercial e preparado pelo método do poliol possuem tamanhos de partícula muito próximos e diferentes quantidades de Cr na forma de liga, sendo que cromo no catalisador sintetizado pelo método de poliol apresenta-se na forma oxidada. Os resultados da degradação acelerada para estes catalisadores mostraram que o catalisador sintetizado pelo método de poliol apresentou melhor desempenho e estabilidade. / The commercialization of fuel cells is still a challenge of this technology, principally due to high price of the catalysts and membrane, slow kinetic of oxygen reduction reaction and stability of the cell. In this work the efficiency and stability of the nanoparticulated bimetallic PtCr supported on carbon of high surface area catalysts were studied to be used in the cathodes of proton exchange membrane fuel cell. The physical characterization of these materials was carried out by various techniques, such as X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The catalysts were characterized electrochemically by measuring the polarization curves, which were recorded in a single cell, using Nafion®115 membrane with anode fed with H2 and cathode fed with O2/air. The gases used were saturated with mili-Q water at different temperatures and pressures. Cyclic voltammetry and CO stripping experiments were used to measure the active surface areas of the catalysts and finally the stability studies were conducted, simulating the accelerated degradation through the cyclic voltammetry. The polarization curves results of the catalysts prepared by poliol method and of commercial catalysts showed that with the increase in the quantity of chromium a decline in performance occurs. The Pt3Cr catalyst prepared by impregnation method presented a poor performance. The catalysts prepared by poliol and commercial catalyst have near the same particle sizes and different amount of alloyed Cr, such that the chromium in poliol catalys is present in oxides form. The accelerated degradation results showed that catalyst prepared by poliol method presented a better performance and stability.

O objetivo desse trabalho foi investigar as relações entre a durabilidade e as diversas configurações dos componentes de uma célula a combustível do tipo PEM por meio de Testes de Durabilidade de Longa Duração. Foram comparados três tipos de geometria de fluxo, duas espessuras de membranas poliméricas e dois níveis de cargas de platina. Em diversos aspectos, a geometria de canais de fluxo do tipo serpentina se mostrou superior aos demais. Em relação às membranas, as do tipo Nafion 212 se mostraram bastante frágeis e suscetíveis ao crossover de H2, apesar de fornecerem maior potência elétrica que as membranas Nafion 115, as quais exibiram maior durabilidade. No que diz respeito à carga de platina nos eletrodos, verificou-se que os eletrodos preparados com 0,1 mg Pt cm-2 perderam, proporcionalmente, mais área eletroquimicamente ativa que aqueles preparados com 0,4 mg Pt cm-2, mas, ao mesmo tempo, apresentaram as menores taxas de perdas irreversíveis de desempenho. As análises por diversas técnicas eletroquímicas indicaram que os aumentos das resistências ôhmicas e de transporte de massas são os fatores que mais contribuem para as perdas irreversíveis de desempenho, enquanto que o aumento da resistência de transporte de cargas devido ao encharcamento dos eletrodos é o principal responsável pelas perdas reversíveis de desempenho. A proporção de ionômero na camada catalítica foi investigada e verificou-se que, apesar de facilitar para que ocorram perdas reversíveis de desempenho, a maior proporção de ionômero na camada catalítica contribuiu em mitigar a degradação do MEA. Por fim, observou-se que a qualidade do contato entre os eletrodos e a membrana tem grande contribuição na durabilidade das células a combustível do tipo PEM. / The aim of this work was to investigate the relations between durability and the several Proton Exchange Membrane Fuel Cell (PEMFC) setups via long-term durability tests. Comparisons were made with three types of flow field designs, two polymeric membranes thicknesses and two platinum loadings. In many aspects, the serpentine flow field design has presented better results than the others. Regarding the membranes, Nafion 212 has shown to be very fragile and susceptible to H2 crossover, although it provides more electrical power than the Nafion 115 membrane which exhibited better durability. Concerning the platinum loading, the electrodes prepared with 0.1 mg Pt cm-2 have lost proportionally more electrochemical surface area than the ones prepared with 0.4 mg Pt cm-2 but at the same time, the electrodes with the lowest platinum load presented lower irreversible performance loss rate. The analyses made by several electrochemical techniques have indicated that the raise of the ohmic and mass transport resistances are the factors that most contribute to the irreversible performance loss, meanwhile the charge transport resistance due to the electrodes flooding is the main responsible for the reversible performance loss. The proportion of ionomer in the catalytic layer was studied and it was possible to infer that the highest ionomer proportion contributes to mitigate the MEA degradation, although it facilitates the reversible performance loss occurrence. Finally, it was observed that the contact quality of the electrodes and the membrane has remarkable influence on the PEMFCs durability.

Single step co-sintering is proposed as a method to minimise the time and cost of fabricating solid oxide fuel cells (SOFCs). Such a methodology is attractive but challenging due to the differing sintering behaviours and thermal mismatch of the constituent materials of the anode, cathode and electrolyte in solid oxide fuel cells. As a result it is likely that compromises are made for one layer with respect to optimising another. The single chamber solid oxide fuel cell (SC-SOFC) has not seen widespread adoption due to poor selectivity and fuel utilisation, but relaxed some of the stringent SOFC requirements such as sealing, and the need for a dense electrolyte layer. Thus, to initiate the study into single step co-sintering, the single chamber SOFC is earmarked as the first candidate. The effect of single step co-sintering on cell performance is also an attractive area to investigate. Therefore, in this study, a new co-sintering process (single step co-sintering) was applied to fabricate three different types (in terms of the supporting structure) of planar SC-SOFCSs (the anode, cathode and electrolyte supported planar cells) and anode supported wavy types of SC-SOFC in order to reduce fabrication cost and time owing to effective fabrication process. In addition, their performances were tested to establish functionality of the sintered specimens as working electrochemical cells as well as to investigate the maximum performance possible with these cells under single chamber conditions. Moreover, it is also aimed to improve the performance of SC-SOFCs by extending TPB (Triple phase boundary) via wavy type. This study presents a single step co-sintering manufacturing process of planar and wavy single chamber solid oxide fuel cells with porous multilayer structures, consisting of NiO-CGO, CGO and CGO-LSCF as anode, electrolyte and cathode respectively. Pressure of 2 MPa, with the temperature at 60˚C for 5 minutes, was deemed optimal for the hot pressing of these layers. The best result of sintering profile was obtained with heating rate of 1˚C min-1 to 500˚C, 2˚C min-1 to 900˚C and 1˚C min-1 to 1200˚C with 1 hour dwelling; the cooling rate was 3˚C min-1. Hence anode supported SC-SOFC (thickness: 200:40:40 µm, thickness ratio: 10:2:2, anode (A): electrolyte (E): cathode (C)) was fabricated via a single co-sintering process, albeit with curvature formation at edges. Its performance was investigated in methane-oxygen mixtures at a temperature of 600˚C. Maximum open circuit voltage (OCV) and power density of the anode supported planar cell were obtained as 0.69 V and 2.83 mW cm-2, respectively, at a fuel-oxygen ratio of 1. Subsequently, anode thickness was increased to 800 µm and electrolyte thickness was reduced 20 µm (thickness ratio of cell 40:1:2) to obtain curvature-free anode-supported SOFCs with the help of a porous alumina cover plate placed on the top of the cell. The highest power density and OCV obtained from this cell was 30.69 mW cm-2 and 0.71 V, respectively, at the same mix ratio. In addition, the maximum residual stresses between cathode end electrolyte layers of anode supported cells after sintering were investigated using the fluorescence spectroscopy technique. The total mean residual stresses along the x-direction of the final anode supported planar cell after sintering were measured to range from -488.688 MPa to -270.781 MPa. Determination of optimum thickness and thickness ratio of the cell with the defined ideal hot pressing and sintering conditions for single step co-sintering were carried out for cathode and electrolyte supported planar cells using similar fabrication processes. Their performance changes with thickness ratio were examined. The results show that the cathode and electrolyte supported planar cells can be obtained successfully via single step co-sintering technique with the help of alumina cover plates, as with the anode supported cell. In addition, an anode supported wavy SC-SOFC was fabricated via single step co-sintering and its performance was also investigated. The maximum power density and OCV from the final curvature free cathode supported planar cell (thickness: 60:20:800 µm, thickness ratio: 3:1:20, A:E:C) was measured to be 1.71 mW cm-2 and 0.20 V, respectively, at a fuel-oxygen ratio of 1.6. Likewise, the maximum OCV and power density were found to be 0.55 V and 29.39 mW cm-2, respectively, at a fuel-oxygen ratio of 2.6, for the final electrolyte supported curvature free planar cell (thickness: 60:300:40 µm, thickness ratio: 3:15:2, A:E:C). Furthermore, a maximum OCV of 0.43 V and power density of 29.7 mW cm-2 were found from the final anode supported wavy cell (thickness: 800:20:40 µm, thickness ratio: 40:1:2, A:E:C) at a fuel-oxygen ratio of 1. In essence, this study can be divided into five chapters. The first chapter addresses the overview of the research background, problem statement, aims and objective of this study as well as that of novelty and impact. In the second chapter, fundamental information is provided regarding SOFCs and SC-SOFCs in terms of working principles, main components including electrodes electrolytes, advantages and disadvantages, types, material used for each cell components, losses in the system, and so forth. Moreover, the second chapter also contains essential sintering information in order to understand how to approach sintering of ceramics or cermet to fabricate SC-SOFCs. The overall methodology of this study is explained in detail in the third chapter while experimental works are described in the chapter 4, chapter 5, chapter 6, chapter 7 and chapter 8. Chapter 5 also contains background for the fluorescence spectroscopy and a modelling technique for residual stress measurement between ceramic layers. The results of experiments with discussion session are also in the same chapter. The last chapter presents conclusions and the possible routes for future works of the study.

Magister Scientiae - MSc / Fuel cells convert chemical energy from a fuel into electricity through chemical reaction with oxygen. This possesses some challenges like slow oxygen reduction reaction (ORR), overpotential, and methanol fuel cross over in a direct methanol fuel cell (DMFC). These challenges cause inefficiency and use of higher amounts of the expensive platinum catalyst.Several binary catalysts with better ORR activity have been reported. In this study we investigate the best catalyst with better ORR and MOR performances and lower over-potentials for PEMFC and DMFC applications by comparing the in-house catalysts (10%Pt/C, 20%Pt/C,30%Pt15%Ru/C, 40%Pt20%Ru/C, 30%PtCo/C, 20%Pt20%Cu/C and 20%PtSn/C) with the commercial platinum based catalysts (10%Pt/C, 20%Pt/C, 20%Pt10%Ru/C, 20%PtCo/C,20%PtCu/C and 20%PtSn/C) using the cyclic voltammetry and the rotating disk electrode to determine their oxygen reduction reaction and methanol tolerance. HRTEM and XRD techniques were used to determine their particle size, arrangement and the atomic composition. It was observed that the 20%Pt/C in-house catalyst gave the best ORR activity and higher methanol oxidation current peaks compared to others catalysts followed by 20%Pt10%Ru/C commercial catalyst. The 20%PtCo/C commercial, 30%PtCo/C in-house and 20%PtSn/C in-house catalysts were found to be the most methanol tolerant catalysts making them the best catalysts for ORR in DMFC. It was observed that the ORR activity of 20%PtCo/C commercial and 30%PtCo/C inhouse catalysts were enhanced when heat treated at 350 0C. From XRD and HRTEM studies, the particle sizes were between 2.72nm to 5.02nm with little agglomeration but after the heat treatment, the particles were nicely dispersed on the carbon support.

Eletrocatalisadores baseados em Pd/C, PdCu/C, PdNi/C e PdAg/C foram produzidos pelo método de micro-ondas para serem utilizados como cátodo na célula a combustível alcalina na ausência e presença de álcool. Este método se mostrou bastante efetivo para a produção dos materiais, uma vez que as partículas apresentaram boa dispersão no suporte de carbono e produziram eletrocatalisadores com tamanho de partícula em torno de 3,5 nm, de acordo com as análises de DRX e MET. A partir das voltametrias cíclicas observa-se que para os eletrocatalisadores de PdCu/C e PdNi/C quanto maior a quantidade de Cu ou Ni, respectivamente, maior a área ativa do material estudado. Análises de disco anel rotatório foram realizadas nos eletrocatalisadores demonstrando que, independente da composição estudada, a quantidade produzida de peróxido foi de no máximo 4%. Estes dados corroboram com as inclinação das retas nas análises de Koutecky-Levich, uma vez que em ambos os casos a RRO ocorre via 4 elétrons. Análises de estabilidade dos materiais demonstraram que todos eles mantiveram ou melhoraram seu desempenho diante da RRO, quando se compara os dados obtidos antes e depois de 1000 ciclos voltamétricos. Testes de tolerância ao metanol e etanol foram realizados em meia célula com todas as composições de eletrocatalisadores produzidos. Na presença tanto de metanol quanto de etanol as composições atômica de 50:50, para todos os materiais estudados, foram as que demonstraram menor influência da presença do álcool durante a varredura linear da RRO. Nas medidas realizadas em célula unitária, com relação à tolerância ao metanol durante a RRO, o eletrocatalisador que demonstrou melhor desempenho foi o PdAg/C 70:30 enquanto que na presença de etanol o eletrocatalisador que demonstrou melhor desempenho foi o PdNi/C 70:30. / Pd/C, PdCu/C, PdNi/C and PdAg/C electrocatalysts were produced by microwave method to be used as cathode in alkaline fuel cell in the absence and presence of alcohol. This method showed to be effective for the materials production, the particles exhibited good dispersion in carbon support and it produced electrocatalysts with a particle size of about 3.5 nm, according to XRD and TEM analysis. In cyclic voltammetry is observed that PdCu/C and PdNi/C electrocatalysts has higher active area with higher amount of Cu and Ni, respectively. Rotating ring disk analysis in the electrocatalysts showed that the amount of peroxide produced was at most 4%. This data is similar to Koutecky-Levich analysis, once for both the ORR occurs via 4 electrons. Materials stability analysis showed that they kept or improve performance in ORR, comparing the data before and after 1000 voltammetric cycles. Tolerance tests in methanol and ethanol were performed in a half cell in all electrocatalysts compositions. In presence of methanol and ethanol the compositions 50:50, to all materials studied, showed less influence in the presence of alcohol in ORR linear scan. In alkaline fuel cell PdAg/C 70:30 showed better performance for ORR in presence of methanol and PdNi/C 70:30 showed better performance for ORR in ethanol presence.

O óxido misto de Ba0,50Sr0,50Co0,80Fe0,20O3-&delta; (BSCF) apresenta propriedades funcionais para ser usado como material catódico de Células a Combustível de Óxido Sólido de Temperatura Intermediária (ITSOFCs). O BSCF, preparado a partir de nitratos, por reação em fase líquida, pelo método de complexação EDTA-Citratos em condições variadas de pH, foi calcinado para obtenção de particulados cristalinos e monofásicos, e posteriormente sinterizados. A caracterização dos particulados e corpos cerâmicos foi realizada utilizando as técnicas de Análise física, química e microestrutural, visando à avaliação do comportamento e das propriedades do composto para utilização como cátodo de ITSOFCs. O comportamento térmico dos particulados indicou estabilidade composicional após tratamento térmico acima de 850°C, a estequiometria real dos particulados ficou próxima da estequiometria teórica, a presença de carbono residual, após tratamento térmico acima de 800°C dos particulados, foi inferior a 0,5% em massa, foi encontrada presença de resíduos de carbonato nos particulados, com exceção daqueles obtidos em pH 4, os particulados com estrutura cristalina monofásica, de menor área de superfície específica e maior porosidade foram obtidos com tratamento térmico a 900°C por 5h. As cerâmicas apresentaram coeficiente de expansão térmica próximo dos eletrólitos de Céria dopada com Gadolínia (GDC) e Céria dopada com Samária (SDC), comumente utilizados em ITSOFCs, densificação e porosidades adequadas, quando sinterizadas a 1000°C por 1h. A condutividade elétrica foi maior para as cerâmicas, sinterizadas a 1000°C por 1h, originadas dos particulados obtidos em pH 6 de síntese, calcinados a 900°C por 5h. / The mixed oxide of Ba0,50Sr0,50Co0,80Fe0,20O3-&delta; (BSCF) present functional properties to be used as cathode material of Intermediate Temperature Solid Oxide Fuel Cell (ITSOFC). The BSCF, prepared from nitrates by liquid phase reaction with complexing method EDTA-Citrates, under conditions of varying pH, was calcined to obtain a particulate crystalline single-phase and, subsequently sintered. The characterization of particulate and ceramic bodies was performed using the physical, chemical and microstructure analysis, in order to evaluate the behavior and properties of the compound for use as a cathode for ITSOFCs. The thermal behavior of particulate indicated compositional stability after heat treatment above 850°C, the actual stoichiometry of the particles was close to theoretical stoichiometry, the presence of residual carbon after thermal treatment above 800°C of particulates was less than 0.5% in weight, was found residues of carbonate in particulate, except those obtained at pH 4, the particulates with monophasic crystalline structure, with lower specific surface area and greater porosity were obtained with thermal treatment at 900°C for 5h. The ceramics exhibited thermal expansion coefficient close to the electrolyte Gadolinia doped Ceria (GDC) and Samaria doped Ceria (SDC), commonly used in ITSOFCs, porosity and adequate densification when sintered at 1000°C for 1h. The electrical conductivity was higher for ceramics sintered at 1000°C for 1h, originating from particulates obtained at pH 6 synthesis, calcined at 900°C for 5h.

Sistemas de células a combustível alimentadas por etanol (DEFC - Direct Ethanol Fuel Cell) são vistos como candidatos para ajudar preencher a lacuna entre a demanda e oferta de energia elétrica. Entretanto, as DEFC ainda não apresentam desempenho tão elevado, que incentivem a comercialização. Um dos principais esforços para elevar o desempenho é obter catalisadores anódicos mais eletroativos e estáveis para reação de oxidação de etanol (ROE). Neste trabalho, visou-se obter e avaliar catalisadores Pt-Sn-Terras raras (La, Ce, Pr e Eu) para ROE. Os catalisadores foram sintetizados a partir de algumas modificações do método do poliol; caracterizados fisicamente, por técnicas de EDX, ICP, ATG, DRX, TEM, XPS e XAS; e caracterizados eletroquimicamente em células de três eletrodos e em células unitárias. O método de síntese promoveu tamanhos médios de partícula entre 3,0 a 4,5 nm para catalisadores PtSnTR/C (TR: La, Ce, Pr e Eu), um dos menores valores reportados para esse tipo de material. Por DRX, foi possível verificar que a síntese promoveu um certo grau de liga Pt-Sn. Por XPS, confirmou-se a presença de Pt e Sn metálicos, óxidos de estanho e de platina, e terras raras em forma de óxidos mistos. Por XAS, observou-se que os metais adicionados promoveram um preenchimento da banda 5d da Pt. As caracterizações eletroquímicas indicaram maior densidade de corrente na ROE para os materiais ternários PtSnTR/C 75:20:05 do que os catalisadores Pt/C, e que PtSn/C 60:40, na maioria dos casos. Os testes de envelhecimento acelerado indicaram que, mesmo com a alteração dos perfis voltamétricos, os catalisadores ternários apresentaram menores perdas na atividade que os catalisadores Pt/C e PtSn/C. Ao caracterizar os produtos da oxidação dos catalisadores por FTIR e HPLC, observou-se que os produtos majoritários foram acetaldeído e ácido acético; e apenas uma pequena formação de CO2 foi detectada . A adição de terras raras, em baixas concentrações, aumentou a atividade eletrocatalítica dos catalisadores Pt/C e PtSn/C provavelmente por disponibilizar espécies oxidadas a baixos potenciais, favorecendo a oxidação de intermediários da ROE, e contribuiu para a estabilidade dos catalisadores PtSn/C. / Direct ethanol fuel cell (DEFC) systems are seen as candidates to fill the gap between demand and supply of electric electricity. However, the DEFC\'s performance does not encourage its large commercialization yet. One of the main efforts to increase the performance is obtaining more electroactive and stable anodic catalysts for the ethanol oxidation reaction (EOR). In this work, Pt-Sn-Rare Earth catalysts (La, Ce, Pr and Eu) were obtained and evaluated for EOR. The catalysts were synthesized using polyol method with some modifications; physically characterized by XDS, ICP, TGA, XRD, TEM, XPS and XAS techniques; the electrochemical characterization involved tests on three electrodes cell and on unit cell systems. The synthesis method promoted mean particle sizes between 3.0 and 4.5 nm for PtSnRE/C catalysts (RE: La, Ce, Pr and Eu), one of the smallest values reported for this type of material. By XRD, it was observed some degree of Pt-Sn alloy. The presence of Pt and Sn metallic, tin and platinum oxides, and rare earths in the form of mixed oxides were confirmed by XPS. The added metals promoted a fill of the Pt 5d band, as observed by XAS. Electrochemical characterizations indicated higher current density in the EOR for the ternary materials than Pt/C catalysts and PtSn/C 60:40, in most of cases. The accelerated aging tests indicated that, even with voltammetric profiles changes, the ternary catalysts had lower losses in the activity than the Pt/C and PtSn/C catalysts. By FTIR and HPLC, it was observed that the main products were acetaldehyde and acetic acid; and small concentration of CO2 was detected. The low addition of rare earths increased the electrocatalytic activity of Pt/C and PtSn/C catalysts probably because they promoted more oxidized species at low potentials, favoring the intermediates oxidation of the EOR, and contributed to PtSn/C catalysts stability.

Neste trabalho, investigou-se a atividade catalítica de materiais a base de Ni/YSZ modificados com Rh ou Ru a 0,5%, 1% e 3%, para a reforma a vapor de etanol (RVE) e seus desempenhos como eletrocatalisadores em células a combustível de óxido sólido (SOFCs - Solid Oxide Fuel Cell). Os catalisadores foram preparados pelo método Pechini e de impregnação. A caracterização estrutural foi realizada utilizando-se as técnicas de Energia Dispersiva de Raios X, Difratometria de Raios X, Redução à Temperatura Programada, Fisissorção de Nitrogênio, Microscopia Eletrônica de Varredura e Análise Elementar. Os testes catalíticos foram realizados a 700 e 900 &deg;C, em uma linha de reação acoplada a um cromatógrafo a gás para o monitoramento dos produtos reacionais gasosos. Os produtos líquidos resultantes da RVE foram analisados por Cromatografia Líquida de Alta Eficiência (CLAE). O objetivo principal foi correlacionar a estrutura e a composição destes materiais com a produção de H2, distribuição de outros produtos reacionais e formação de depósitos de carbono. Os resultados obtidos mostraram que a incorporação de Rh ou Ru no catalisador de Ni/YSZ não resultou em mudanças significativas na estrutura e atividade catalítica, porém promoveu uma diminuição na quantidade de carbono formado, sendo mais expressiva para o caso da adição de Rh. O aumento da temperatura de reação de 700 &deg;C para 900 &deg;C resultou em um aumento da seletividade dos catalisadores para os produtos gasosos e diminuição da formação de coque. O estudo em uma célula unitária de SOFC foi conduzido utilizando-se platina no cátodo e 3%Rh/40%Ni/YSZ(P) no ânodo, em uma célula operando com H2 e ar a 900 &deg;C. Embora as curvas de polarização tenham apresentado baixas densidades de potência, os resultados mostraram que o material de 3%Rh/40%Ni/YSZ(P) foi ativo para a produção e eletro-oxidação de H2 em condições reais de operação das SOFCs. Além disso, mostrou-se que é possível investigar a atividade de eletrocatalisadores de ânodos de SOFC para a reforma de etanol em linhas de reação comumente utilizadas em estudos de catálise heterogênea. / In this work, it was investigated the electrocatalytic activity of Ni/YSZ promoted with Rh or Ru (0.5 wt%, 1.0 wt% and 3.0 wt% content) for the Ethanol Steam Reforming (ESR) reaction, and their performance as electrocatalysts in Solid Oxide Fuel Cells (SOFCs). The catalysts were prepared by the Pechini and Impregnation methods. The material characterization was carried out by Energy Dispersive X-ray (EDX), X-ray Diffraction (DRX), Temperature Programmed Reduction (TPR-H2), N2 physisorption, Scanning Electron Microscopy (SEM), and Elemental Analysis. The catalytic tests were performed at 700 and 900 &deg;C in a reaction system coupled to a gas chromatograph in order to monitor the gaseous products. The liquid products were analyzed by High Performance Liquid Chromatography (HPLC). The structure and composition of these catalysts were correlated to the H2 formation, with the distribution of other parallel reaction products, including the carbon deposition. The obtained results showed that the incorporation of Rh or Ru does not change significantly the structure and catalytic activity, but it decreases the carbon deposits, being more significant for the addition of Rh. The increase of the reaction temperature from 700 &deg;C to 900 &deg;C increased the gaseous products selectivities and decreased the carbon deposition. The study in SOFC unit cells were conducted using platinum and 3%Rh/40%Ni/YSZ(P) in the cathode and anode, respectively. The SOFC operated with H2 and air, and 900 &deg;C. Although the polarization curves have presented low power densities, the obtained results showed that the 3%Rh/40%Ni/YSZ(P) electrocatalyst was active for the H2 production and eletro-oxidation in the SOFC real operation conditions. Furthermore, the results have demonstrated that it is possible to investigate SOFC electrocatalysts activity for the ethanol steam reforming in reaction lines commonly utilized in heterogeneous catalysis studies.

Fuel cells are predicted to be the power delivery devices of the future. They have many advantages such as the wide fuel selection, high energy density, high efficiency and an inherent safety which explains the immense interest in this power source. The need for advanced designs has been limited by the lack of understanding of the transport processes inside the fuel cell stack. The reactant gases undergo many processes in a fuel cell that cannot be observed. Some of these processes include convective and diffusional mass transport through various types of materials, phase change and chemical reaction. In order to optimize these variables, an accurate mathematical model can provide a valuable tool to gain insight into the processes that are occurring. The goal of this dissertation is to develop a mathematical model for polymer electrolyte-based fuel cells to help contribute to a better understanding of fuel cell mass, heat and charge transport phenomena, to ultimately design more efficient fuel cells. The model is a two-phase, transient mathematical model created with MATLAB. The model was created by using each fuel cell layer as a control volume. In addition, each fuel cell layer was further divided into the number of nodes that the user inputs into the model. Transient heat and mass transfer equations were created for each node. The catalyst layers were modeled using porous electrode equations and the Butler-Volmer equation. The membrane model used Fick's law of diffusion and a set of empirical relations for water uptake and conductivity. Additional work performed for this dissertation includes a mathematical model for predicting bolt torque, and the design and fabrication of four fuel cell stacks ranging in size from macro to micro scale for model validation. The work performed in this dissertation will help improve the designs of polymer electrolyte fuel cells, and other polymer membrane-based fuel cells (such as direct methanol fuel cells) in the future.