Made available in DSpace on 2014-10-09T12:55:11Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:06:02Z (GMT). No. of bitstreams: 0 / A economia do hidrogênio apresenta um novo horizonte para suprir a crescente demanda global por energia. As células a combustível são os dispositivos que possibilitam a utilização do hidrogênio (H2) como um interessante vetor energético uma vez que ele não emite CO2 ao reagir com o oxigênio (O2), gerando H2O e energia elétrica. A produção do hidrogênio pode ocorrer de forma descentralizada e modulada, possibilitando atender às necessidades de comunidades que o sistema estabelecido de distribuição de energia não consegue chegar. Uma célula a combustível, é composta essencialmente por 3 elementos: o anodo, o catodo e o eletrólito. Existem vários materiais disponíveis para aplicação como eletrólito em células a combustível, mas o que mais se destaca é a zircônia estabilizada com óxido de ítrio (ZEI). Devido às suas características químicas e elétricas, a zircônia estabilizada é bastante pesquisada como eletrólito de células a combustível do tipo óxido sólido (CaCOS) e conformada por colagem de fitas (tape casting). A colagem de fitas é um dos meios mais baratos, simples e eficientes para conformar suspensões cerâmicas em lâminas finas, contribuindo para seu uso em células a combustível. Considerando esses aspectos, o objetivo deste trabalho foi estudar a viabilidade de conformação de fitas cerâmicas de zircônia estabilizada produzidas a partir da extração do minério nacional zirconita. É consenso na literatura que suspensões cerâmicas devem ser obtidas a partir de pós com baixa área superficial (geralmente entre 0,5 a 10m2/g). No presente trabalho, as fitas foram coladas a partir de pós com elevada área superficial (geralmente entre 40 a 80m2/g). O uso do potencial zeta é recomendado para avaliar a quantidade de polieletrólito que deve ser empregada em uma determinada suspensão. No entanto, para suspensões com elevada concentração de sólidos, faz-se necessário o uso da curva de fluxo do material, devido a compressão da dupla camada elétrica. Para os estudos de reologia, o pó cerâmico calcinado foi submetido a uma classificação, em um conjunto de peneiras ABNT com diferentes aberturas de malha e descartando a fração mássica retida em malha 60. Suspensões destes pós cerâmicos foram usadas para a determinação das curvas de fluxo, fato que possibilitou a conformação dos laminados cerâmicos. Para a colagem de fitas, é necessário o uso de plastificantes e ligantes. Dentre os disponíveis, o álcool polivinílico (PVA) foi usando como ligante e o 1,2,3 propano-triol (glicerina) como plastificante. Atuando em sinergismo e em proporção adequada (2,25:1) de glicerina em relação ao PVA, foi possível obter fitas cerâmicas com as características desejadas para aplicação em células a combustível. STUDY / Dissertação (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Made available in DSpace on 2014-10-09T12:27:27Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:06:46Z (GMT). No. of bitstreams: 0 / Células a combustível de óxido sólido são sistemas capazes de gerar energia elétrica por meio da oxidação de moléculas hidrogenadas. Normalmente os sistemas planares e tubulares, são compostos por quatro constituintes bem definidos: cátodo, ânodo, eletrólito e selante. Este último componente é o foco do presente estudo, sendo que suas principais características são estabilidade química na temperatura de operação da célula, isolamento elétrico e coeficiente de expansão térmica compatível com os outros constituintes, além da viscosidade elevada e resistência química em atmosferas oxidantes e redutoras. Devido à geometria planar e de multicamadas da célula se optou por usar como selante vidros niobofosfatos. A selagem foi realizada a partir de dispersão de pó de vidro em álcool etílico, gerando uma solução viscosa que foi aplicada sobre o substrato. Posteriormente realizou se um tratamento térmico para a consolidação do selamento. Os vidros estudados foram denominados de Nb30, Nb37, Nb40 e Nb44, de acordo com o teor nominal de óxido de nióbio utilizado na composição. O objetivo desse trabalho foi caracterizar, a partir de precursores os selantes a base de vidros niobofosfatos para aplicar em células a combustível de óxido sólido do tipo planar. Foram feitos caracterizações dos pós dos vidros e de pastilhas cristalizadas para determinar os coeficientes de expansão térmica (CET), resistividade elétrica, difração de raio X e microscopia eletrônica de varredura (MEV), além de, caracterizar visualmente sua adesividade, molhabilidade, resistência mecânica em substratos de alumina e em conjunto com os componentes das SOFC, sendo também testados os selantes em operação nas unidades previamente formadas de SOFC (ciclos térmicos). / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Made available in DSpace on 2014-10-09T12:34:56Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:07:28Z (GMT). No. of bitstreams: 0 / Os filmes micrométricos contendo óxido misto de lantânio, estrôncio, cobalto e ferro (La0,60Sr0,40)(Co0,20Fe0,80)O3-δ - LSCF, misturado com (Ce0,90Gd0,10)O1,95 - CGO, são relevantes para a utilização como camada funcional para o cátodo da Célula a Combustível de Óxido Sólido de Temperaturas Intermediárias. Estes filmes foram depositados no um substrato cerâmico e denso de CGO ou CGO sobre (ZrO2)0.92(Y2O3)0.08 - YSZ. O estudo deste cátodo é fundamental, pois é nele que ocorre a reação de redução do gás oxigênio, e o seu desempenho eletroquímico depende da interface destes dois materiais. Neste sentido, o presente trabalho contribui para a síntese dos particulados de LSCF para o processamento de filmes, utilizando a técnica de deposição com uso de aerógrafo e para sua conformação em camadas contendo porosidade com espessuras entre 30 e 50 μm. Inicialmente, os particulados de LSCF foram sintetizados pela técnica do citratos e de LSCFCGO obtidos por mistura mecânica, sendo caracterizados por DRX para a confirmação da formação da estrutura cristalina ortorrômbica para o LSCF e cúbica para CGO. Em seguida, foram preparadas suspensões orgânicas de LSCF, LSCFCGO e CGO que foram alimentadas por gravidade em um aerógrafo manual para deposição sobre substrato do eletrólito. Para a conformação dos substratos de CGO ou YSZ, utilizou-se prensa uniaxial e isostática, sinterização e retificação. Verificaram-se, pelas micrografias, que os substratos CGO e YSZ apresentaram densidades (> 92%) suficientes para serem utilizados como eletrólitos. Os filmes de LSCF e LSCFCGO apresentaram-se com porosidades adequadas (> 30%) e espessura total de aproximadamente 40 μm, com boa aderência ao eletrólito. A presença do cátodo compósito contendo eletrólito de CGO sobre YSZ possibilitou aumento de 25% no desempenho eletroquímico (2,50 Ω.cm2 para 650ºC) em decorrência da melhora na reação de redução do oxigênio na interface cátodo/eletrólito. / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Ce[subscript(x)]Zr[subscript(1-x)]O₂ mixed oxides of varying compositions were prepared by a sol-gel citrate complexion technique. In order to improve the catalytic activity of the oxides they were impregnated with copper using two different impregnation techniques. The bare oxides and copper impregnated samples were investigated using a range of Temperature Programmed (TP) techniques, in an attempt to establish their effectiveness as anode materials for solid oxide fuel cells (SOFCs) run on hydrocarbon fuels. In order to conduct the TP experiments a novel system was designed and constructed. The high Ce containing mixed oxides were shown to be reduced at lower temperature than high Zr content samples. TPRx experiments were employed to investigate which of the oxides was most prone to carbon deposition when reacted in methane, with the high ceria sample displaying the lowest level of carbon deposition. Lightoff experiments were undertaken to establish which oxide composition was the most active for methane oxidation. The activity of the oxides increased with ceria content up to 75 mole% (ZCe75), before decreasing for ZCe90. All the mixed oxides were shown to be more active for methane oxidation than CeO₂.

A series of cermet powders of composition NixCo(1-x)O-YSZ were synthesized for testing as cermet anode materials for SOFCs. The Co is found by powder XRD to become incorporated into the crystal lattice of the NiO, thus forming a true alloyed material. SEM and EDS results show two types of particles upon sintering to 1380oC: small, amorphous particles of YSZ and large, crystalline particles of nickel. The electrochemical oxidation of hydrogen on a cermet anode composed of Ni0.7Co0.3O-YSZ was investigated using a series of many button cells. Through EIS data, cyclic voltammetry data, the exchange current densities for these button cells were determined. Although a relatively large variation was found (expected to be due to microstructural variation) the average values for both methods of measurement is in good agreement in hydrogen. Following reduction in pure hydrogen, the fuel was changed to a mixture with high concentration of H2S. It was found that a concentration of 10 % H2S/H2 produced a sudden change in anode microstructure and resulted in loss of exchange current density. Lowering the amount of H2S in the initial fuel feed, which allowed for a more gradual microstructural change, allowed the cell to eventually function at concentrations in excess of 10 % H2S/H2. It was determined by OCV values in various concentrations of H2S/H2 that hydrogen is the predominant fuel of choice, even if H2S is available. Following electrochemical testing, slow cooling in a 10 % H2S/H2 mixture following produced metal sulfide spheres, as determined by SEM and EDS. Investigation in hydrocarbon, alcohol and biodiesel fuels was then undertaken to test the fuel variability of the given cermet anode material. Methane containing 10 % H2S was found to have increased exchange current density relative to poisoned hydrogen. Ethane and biodiesel experienced no increase in exchange current density, but a lengthening of the functional lifetime of the cell was observed, indicating reduced carbon poisoning. Methanol is a promising oxygen-containing SOFC fuel since it produced exchange current density values larger than hydrogen, and showed no evidence of coke formation by post-mortem SEM. Since oxygen-containing fuels are known to decompose in the gas phase at typical SOFC operating temperatures, the performance in a mixture of various CO/H2 fuels was then investigated. The Ni0.7Co0.3O-YSZ cermet anode gave higher exchange current density values for low ratio of CO/H2 fuels in the range 20/80 and 30/70 compared to pure H2. This is the first example of a Ni-based anode providing higher performance with a CO/H2 mixed fuel than for a pure H2 fuel. Finally, continuous running of a cell with fuel ratio 25/75 CO/H2 for 7 days produced exchange current density values, which were observed to increase significantly above the values for pure H2 during days 1-4 followed by deterioration below the value for hydrogen on subsequent days.

Ceramics oxides with mixed ionic and electronic conductivity have received a lot of attention due to their wide range of applications in solid oxide fuel cells, interconnects, gas sensors, and ion transport membranes. However, owing to harsh operating conditions, the choice of proper materials and engineering their properties are still challenging. Perovskite and fluorite structures are two promising structures for ceramic membrane applications. The objective of this research is to explore the stability of lanthanum chromite-based perovskite ((La,Sr)(Cr,Fe)O3-δ) as single phases and dual-phase composites with fluorite phases under fabrication and operating conditions of Oxygen Transport Membranes (OTM). The current research has been categorized into two sections: structural and chemical stability of perovskite phases and dual-phase perovskite/fluorite composites. Also, investigation on both categories has been conducted with two separate approaches: experimental examinations and computational Thermodynamic. In the computational part, independent methods have been considered for the single-phase perovskite and dual-phase perovskite/fluorite composites. In the experimental section, the bulk chemical stability of the dual-phase samples has been examined under controlled oxygen partial pressure p(O2) atmospheres at 1400ᵒC for 10 hours with slow and fast cooling rates. Besides, the phase stability of the perovskite structures as a single-phase has been also examined under OTM fabrication conditions. The results present new phenomena in the chemical stabilities of the materials. They include formations of liquid phases, Sr-segregation, and perovskite phase separations. The correlations between compositions/ temperature/ p(O2) and secondary phases have been investigated to improve the chemical stability and extend the lifetime of the materials. The findings in this thesis enhance the knowledge about the chemical stabilities of OTMs and help to develop more reliable materials for ceramic-based OTMs.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第22773号 / 工博第4772号 / 新制||工||1746(附属図書館) / 京都大学大学院工学研究科航空宇宙工学専攻 / (主査)教授 岩井 裕, 教授 吉田 英生, 教授 江口 浩一 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

The Solid oxide membrane (SOM) electrolysis is an innovative green technology that produces technologically important metals directly from their respective oxides. A yttria-stabilized zirconia (YSZ) tube, closed at one end is employed to separate the molten salt containing dissolved metal oxides from the anode inside the YSZ tube. When the applied electric potential between the cathode in the molten salt and the anode exceeds the dissociation potential of the desired metal oxides, oxygen ions in the molten salt migrate through the YSZ membrane and are oxidized at the anode while the dissolved metal cations in the flux are reduced to the desired metal at the cathode. Compared with existing metal production processes, the SOM process has many advantages such as one unit operation, less energy consumption, lower capital costs and zero carbon emission. Successful implementation of the SOM electrolysis process would provide a way to mitigate the negative environmental impact of the metal industry. Successful demonstration of producing ytterbium (Yb) and silicon (Si) directly from their respective oxides utilizing the SOM electrolysis process is presented in this dissertation. During the SOM electrolysis process, Yb2O3 was reduced to Yb metal on an inert cathode. The melting point of the supporting electrolyte (LiF-YbF3-Yb2O3) was determined by differential thermal analysis (DTA). Static stability testing confirmed that the YSZ tube was stable with the flux at operating temperature. Yb metal deposit on the cathode was confirmed by scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS). During the SOM electrolysis process for silicon production, a fluoride based flux based on BaF2, MgF2, and YF3 was engineered to serve as the liquid electrolyte for dissolving silicon dioxide. YSZ tube was used to separate the molten salt from an anode current collector in the liquid silver. Liquid tin was chosen as cathode to dissolve the reduced silicon during SOM electrolysis. After electrolysis, upon cooling, silicon crystals precipitated out from the Si-Sn liquid alloy. The presence of high-purity silicon crystals in the liquid tin cathode was confirmed by SEM/EDS. The fluoride based flux was also optimized to improve YSZ membrane stability for long-term use.

This work represents the integration of a hybrid system based on Micro Gas Turbine system available at the division of Heat and Power Technology at KTH and Solid Oxide Fuel Cell.  The MGT available is an externally fired recuperated and the SOFC is of planar type. Before the integration, these two different candidates of environmentally friendly power generation systems are discussed separately. The advantages and performances of the two separate systems are presented. The operation conditions as pressure and temperature are fixed at different stations based on the previous experiments. Keeping the parameters constant a reduction of fuel to the combustor could be achieved. Finally, layout of the hybrid system diagram is suggested and orientation of a computer designed layout is also presented. An efficiency of 65% from SOFC has been achieved and reductions of the fuel by more than 50% to the MGT are noteworthy.

This thesis focuses on developing an appropriate cathode material throughnanotechnology as a key component for a promising alternative of renewable energygenerating systems, Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFC).Aiming at a working cathode material for IT-SOFC, a recently reported capable oxideperovskite material has been synthesized through two different chemical methods.BaxSr1-xCoyFe1-yO3−δ (BSCF) with y =0.8 and x =0.2 was fabricated in nanocrystallineform by a novel chemical alloying approach, co-precipitation- as well as conventionalsol-gel method to produce oxide perovskites. The thermal properties, phase constituents,microstructure and elemental analysis of the samples were characterized by TG-DSC,XRD, SEM and EDS techniques respectively. Thermodynamic modeling has beenperformed using a KTH-developed software (Medusa) and Spark Plasma Sintering (SPS)has been used to obtain pellets of BSCF, preserving the nanostructure and generatingquite dense pellets for electrical conductivity measurements.The results show that the powders synthesized by solution co-precipitation have cubicperovskite-type structure with a high homogeneity and uniform distribution and meanparticle size of 50-90 nm range, while sol-gel powders are not easy to form a pure phaseand mostly the process ends up with large particle containing two or three phases.Finer resultant powder compared to sol-gel technique and earlier research works onBSCF has been achieved in this project using oxalate co-precipitation method. Topreserve nanoscaled features of BSCF powder which possess a significant increase ofelectrical conductivity due to decrease the electrical resistivity of grain boundaries, forthe sample synthesized through co-precipitation, ~92% dense pellet sintered by SPS atV1080 °C and under 50 MPa pressure and its electrical conductivity has been measuredfrom room temperature to 900 °C.Specific conductivity values were precisely measured and the maximum of 63 S.cm-1 at430 °C in air and 25 S.cm-1 at 375°C in N2 correspondingly are two times higher thanconventional BSCF implying a high pledge for nano-BSCF as a strong candidate ascathode material in IT-SOFC.