INTRODUÇÃOCélula a combustível é um dispositivo que converte eletroquimicamente combustíveis químicos em eletricidade; é, essencialmente, uma bateria que não para de fornecer corrente elétrica por causa da contínua alimentação externa de combustível. Em outras palavras, é uma bateria na qual os dois eletrodos não são consumidos durante a descarga, mas agem simplesmente como locais para a reação entre combustível e oxidante [1]. Células a combustível convertem energia química diretamente em energia elétrica com eficiência termodinâmica não limitada pelo ciclo de Carnot [2,3]. Essa vantagem das células a combustível depende, entretanto, de como os combustíveis que serão utilizados podem ser reformados para produzir hidrogênio e dióxido de carbono [4]. Toda célula a combustível é composta de uma seqüência de unidades, cada uma com quatro componentes: o eletrólito, o eletrodo para o ar (ar é o oxidante), o eletrodo para o combustível (o mais comum é o hidrogênio), e o interconector.Muitos tipos de células a combustível foram desenvolvidos, sendo as células classificadas geralmente de acordo com o tipo de eletrólito. Os cinco principais tipos são:1-célula a combustível de ácido fosfórico, operacional a 180 o C; 2-célula a combustível de membrana trocadora de prótons, ou célula a combustível de eletrólito de membrana polimérica, operacional na faixa de temperatura 60-80 o C; 3-célula a combustível de eletrólito alcalino, operacional a temperaturas relativamente baixas (80 o C). Tem sido usada no ônibus espacial como principal fonte de energia. Embora tenha operado confiável e eficientemente em missões espaciais por mais de 40 anos, não tem sido usada para outras finalidades, principalmente por causa do alto custo ResumoA partir da definição de células a combustível, é feita uma introdução sucinta dos tipos de células e dos materiais cerâmicos que são empregados em projeto e fabricação destes dispositivos geradores de energia elétrica. Tomando por base a ampla literatura científica disponível em publicações periódicas internacionais indexadas e arbitradas, bem como patentes, são relatados com detalhes os materiais cerâmicos com comportamento elétrico adequado para uso como eletrólitos, anodos, catodos, interconectores e selantes, que são os componentes básicos de células a combustível de óxidos sólidos. Por fim, é feita uma avaliação do estado da arte na pesquisa e desenvolvimento de materiais cerâmicos para uso em células a combustível de óxidos sólidos. Palavras-chave: célula a combustível, eletrólito sólido, anodo, catodo, interconector. Abstract Basic definitions of fuel cells and a brief introduction of different types of fuel cells
Self-assembled monolayers (SAMs) of N,N′bis(2-phosphonoethyl)-3,4,9,10-perylenediimide (PPDI), a perylene dye substituted with phosphonic acid groups, were deposited on indium tin oxide (ITO) substrates. Dye deposition was confirmed by UV−visible absorption spectroscopy and by electrochemical methods. Electrochemical characterization of the SAM was performed using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Two reversible redox waves were observed by CV for the PPDI monolayer, corresponding to E 1/2 = −0.49 V (radical anion formation) and E 1/2 = −0.90 V (dianion formation). The effect of applied bias on the EIS response was studied, comparing a region where PPDI was not reduced (applied bias = 0 V) with a region within the redox window of the imide (applied bias = −0.6 V). The EIS results were analyzed using either impedance (Nyquist and Bode) or capacitance (Cole−Cole) diagrams. Capacitance plots were shown to be by far more sensitive to study the faradaic activity of the SAM, allowing the determination of both the pseudocapacitance (C pc ) for charging the monolayer and the heterogeneous electron transfer rate constant (k et ) from the electrode to the SAM. A molecular coverage of 7 × 10 −11 mol/cm 2 was calculated for the SAM from the pseudocapacitance. A value of k et = 41 s −1 was obtained, consistent with literature data for similar systems.
Recebido em 4/4/06; aceito em 16/3/07; publicado na web em 17/7/07 DIRECTIONS OF THE INDUSTRIAL DEVELOPMENT OF THE SOLID OXIDE FUEL CELLS TECHNOLOGY. This manuscript shows an overview of the solid oxide fuel cell (SOFC) technology based on industrial developments. The information presented has been collected mostly at conferences that the authors attended. It is observed that several companies have been pursuing the development of the SOFC technology. Significant advances in stability and power density have raised the economic interest in this technology recently. It is revealed that the SOFC materials are essentially the same ones that have been used in the past decades, and that the two most important designs of pre-commercial SOFC prototypes are the tubular and planar ones.Keywords: solid oxide fuel cell; industrial development; energy production. INTRODUÇÃOAs células a combustível de óxidos sólidos (CCOS, SOFC -"Solid Oxide Fuel Cell") são os dispositivos conhecidos mais eficientes para a conversão eletroquímica de um combustível em energia elétrica 1 . O funcionamento destes dispositivos baseia-se nos princípios eletroquímicos das células a combustível, onde a energia química de um combustível é convertida diretamente em energia elétrica, sem os limites impostos pelo ciclo de Carnot às má-quinas térmicas 2 . Nas CCOS as reações eletroquímicas de oxidação do combustível e de redução do oxidante ocorrem na interface gás (combustível ou oxidante) condutor eletrônico/condutor iônico, chamada de contorno de fase tripla ou tripla fase reacional. Uma célula unitária de óxidos sólidos consiste, essencialmente, de dois eletrodos porosos (catodo e anodo) separados por um eletrólito sólido denso. No anodo o combustível é oxidado, reagindo com os íons oxigênio provenientes do eletrólito, liberando elétrons e formando água. Os elétrons produzidos no anodo são transportados pelo circuito externo até o catodo onde o oxigênio é reduzido e os íons formados atravessam o eletrólito em direção ao anodo, completando a reação. O trabalho elétrico é realizado pelos elétrons do circuito externo. Na Figura 1 são apresentados o esquema de funcionamento e a reação global de uma CCOS.Considerando-se a geração de energia distribuída, a CCOS apresenta diversas vantagens em relação a outras tecnologias concorrentes, como motores a diesel e microturbinas a gás, ou mesmo outros tipos de células a combustível. Na Figura 2 é mostrado um diagrama comparativo de algumas propriedades de sistemas de geração (estacionária) de energia elétrica. Pode-se observar que as CCOS são os dispositivos que apresentam maiores eficiências e menores emissões de poluentes comparativamente às outras tecnologias. Entretanto, o principal fator que inibe a comercialização destes dispositivos ainda é o elevado custo da tecnologia.Entre as diferentes tecnologias de células a combustível, a CCOS destaca-se por ser o único dispositivo inteiramente no estado sóli-do. Outra importante característica que diferencia as CCOS das
Perovskite-type La0.8Sr0.2Co0.8Fe0.2O32d powders were prepared using a complex polymeric precursor method. Thermal analysis\ud was carried out on the perovskite precursor to investigate the oxide-phase formation. The structural phase of the powders was\ud determined by X-ray diffraction. These results showed that the decomposition of the precursors occurs in a two-step reaction and\ud temperatures higher that 1000°C are required for these decomposition reactions. For the electrochemical characterization,\ud La0.8Sr0.2Co0.8Fe0.2O32d electrodes were deposited by a wet spray technique on dense yttria-stabilized zirconia ~YSZ! layers. The\ud morphology of the deposited perovskite thick films (;50 mm) was investigated by field emission scanning electron microscopy\ud and showed a porous microstructure. Electrochemical impedance spectroscopy ~EIS! measurements were carried out under synthetic\ud air flux at temperatures ranging from 200-600°C in the 10 mHz-10 MHz frequency range showing an interfacial electrical\ud resistance related to the La0.8Sr0.2Co0.8Fe0.2O32d electrodes. EIS measurements were also performed in the same frequency range\ud at different oxygen partial pressures (1025-1 atm) at 600°C. At this temperature and frequencies below 0.1 MHz, the electrical\ud response to the applied signal of the electrode material is best fitted by two semicircles, which can be related to charge-transfer processes. The activation energy for the limiting step ~adsorption/desorption! was found to be 1.6 eV
A direct ethanol solid oxide fuel cell ͑SOFC͒ fabricated from conventional materials is described. The performance of Ni-cermet anodes with zirconia and ceria as the ceramic phase in SOFCs operating on hydrogen and ethanol is compared by means of current-voltage and impedance spectroscopy measurements in the 700-900°C temperature range. The experimental data indicate that the studied ethanol-fueled SOFCs have comparable performance. However, different ceramic matrix and metallic phase composition of cermets result in different behavior of the ethanol direct oxidation. Below 800°C, the performance of fuel cells is relatively more stable as compared to higher temperatures, but the catalytic activity for ethanol conversion of doped ceria is insufficient to prevent carbon formation. The experimental results provide evidence for the importance of an appropriate combination of fuel, operating conditions, and anode materials in designing SOFCs for direct operation using ethanol as fuel.Fuel cells have been increasingly accepted as efficient energy conversion systems with low environmental impact. In particular, solid oxide fuel cells ͑SOFCs͒ are potentially the most efficient and flexible fuel cells due to their ability to operate on various fuels at high temperatures. 1 The diverse array of fuels that can be fed to the anode of an SOFC include hydrogen, carbon monoxide, hydrocarbons, and alcohols. 2-4 The high operating temperature, which allows for either the direct oxidation or the internal reforming of these primary fuels, is one of the most attractive features of the SOFC. Direct use of available hydrocarbon or alcohol fuels without first reforming them to hydrogen will greatly decrease the complexity and cost of the fuel cell system.Among potential primary fuels, a great deal of attention has recently been given to ethanol as an efficient and low-cost renewable source for hydrogen production. The estimated losses for the conversion of the energy content in sugar ͑glucose͒ to ethanol and then reacting it to yield hydrogen is below 20%. 5 A rough estimate indicates that, in principle, a perfect fuel cell can convert the hydrogen generated by 1 mol of ethanol to 350 Wh of electricity at ϳ$0.04/kWh. 5 Ethanol has been already produced commercially in some countries and blended with gasoline for vehicular propulsion. Usually, it corresponds only to a small fraction of the fuel used in most countries, with the notable exception of Brazil, where bioethanol ͑ethanol derived from biomass, in this case, sugarcane͒ represents ϳ40% of the road transportation fuel. Cost reductions associated with the scaling up of the Brazilian biomass industry have made ethanol economically competitive. Brazil has a long tradition in alternative fuels and maintains a strategic advantage in the pursuit of independent and sustainable fuel provisions due to the huge availability of ethanol, which has been estimated to be ϳ17 billion liters per year. 6,7 Complete internal reforming of carbon-containing fuels in the anode of an SOFC has a number o...
Nafion β-relaxation was studied by detailed dielectric spectroscopy measurements carried out in a wide range of both temperature and relative humidity. Cast and extruded Nafion were compared in the H+, Na+, and Cs+ forms to evaluate contributions arising from membrane morphology and ionic interactions. The experimental data indicated that β-relaxation is associated with the motion of side chains of the ionomer. The unusual shift to lower frequencies of the β-relaxation was shown to be more pronounced for samples containing high water content and at temperatures above the α-relaxation, evidencing that the electrostatic repulsions between bare sulfonic groups reduce the side chain mobility. A pronounced increase of relaxation times was observed at relative humidity >60% suggesting a change of the morphology of the ionomer, in agreement with recent descriptions of the Nafion structure by small angle X-ray scattering analyzes.
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