The MgO:TiO2 :ZrO2 ternary system was investigated as a possible novel anode material in a solid oxide fuel cell. Titanium-substituted yttria-stabilised zirconias have the necessary electrical conductivity properties for a ZrO2 -based fluorite electrode but problems have been encountered such as a decrease in unit-cell size upon reduction leading to mechanical failure. By incorporating magnesium into the titanium-stabilised zirconia structure, it was thought that the cubic-fluorite structure might be stabilised. A phase diagram study was made of the MgO:TiO2 : ZrO2 ternary system at 1500°C. Upon researching the literature phase diagram of the MgO:TiO2 :ZrO2 system, it was found that the authors had not studied the single-phase region in the ZrO2 -rich area extensively and did not use a consistent temperature for their analysis of samples. This has meant that the phase diagram has had to be reinvestigated. The results obtained at 1500°C are in disagreement with the previously published phase diagram. A large area bounded by single-phase cubic-fluorite was detected; however the central region of this domain contained both tetragonal and cubic-fluorite domains. This implies that for the central region of this phase area that the cubic- fluorite phase is not stable at 1500°C. Selected stabilised cubic-fluorite samples with ~ 10 atom% Mg were annealed at l000°C after preparation at 1500°C and it was found that due to the presence of other phases present at 1000°C, that the cubic-fluorite phase is thermodynamically unstable at lower temperatures. DTA analysis revealed that as the titanium content in the cubic-fluorite solid-solution increased, the phase transition from tetragonal phase (+ MgO) to cubic-fluorite phase decreased. These results were used to provide a basis for a temperature phase diagram showing the likely phase transitions that occur at a particular temperature range. The activation energy for conduction increased and ionic conductivity decreased with increasing titanium content in the solid solution, due to the effects of local distortions created by the smaller ionic radius of titanium when compared to zirconium.

No description available.

Two failure modes related to water management in Proton Exchange Membrane fuel cells (dehydration and flooding) were investigated using electrochemical impedance spectroscopy as a diagnosis tool. It was hypothesised that each failure mode corresponds to changes in the overall stack impedance that are observable in different frequency ranges. This hypothesis was corroborated experimentally. The experimental implementation required new testing hardware and techniques. A four-cell stack capable of delivering individually conditioned reactants to each cell was designed, built, tested, and characterised under a variety of operating conditions. This stack is the first reported prototype of its type. The stack was used to perform galvanostatic, impedance measurements in situ. The measurements were made at three different temperatures (62, 70 and 80°C), covering the current density range 0.1 to 1.0 A cm−2 , and the frequency range 0.1 to 4 × 105 Hz. The recorded data represent the first reported set of measurements covering these ranges. The failure modes were simulated on individual cells within the stack. The effects on individual cell and stack impedance were studied by measuring the changes in stack and cell impedances under flooding or dehydration conditions. Dehydration effects were measurable over a wide frequency range (0.5 to 105 Hz). In contrast, flooding effects were measurable in a narrower frequency range (0.5 to 102 Hz). Using these results, separate or concurrent impedance measurements in these frequency ranges (or narrow bands thereof) can be used to discern and identify the two failure modes quasi-instantaneously. Such detection was not possible with pre-existing, do techniques. The measured spectra were modelled by a simple equivalent circuit whose time constants corresponded to ideal (RC) and distributed (Warburg) components. The model was robust enough to fit all the measured spectra (for single cells and the stack), under normal and simulated-failure conditions. Approximate membrane conductivities were calculated using this model. The calculations yielded a range from 0.04 to 0.065 S cm−1 (under normal humidification), and conductivities that deviated from these nominal range under flooding or dehydrating conditions. The highest conductivity value (was ∼0.10 S cm−1) was measured under flooding conditions at j = 0.4 A cm−2. The lowest conductivity (∼0.02 S cm−1) corresponded to a dehydrated cell at j = 0.1 A cm−2. These values fall within the ranges of published data for modern proton exchange membranes. The phenomenological and numerical results reported in this work represent the first demonstration of these techniques on a PEMFC stack under real operating conditions. They are also the basis of ongoing research, development, and intellectual property protection. / Graduate

Fuel cells are electrochemical devices that rely on the transport of reactants (oxygen and hydrogen) and products (water and heat). These transport processes are coupled with electrochemistry and further complicated by phase change, porous media (gas diffusion electrodes) and a complex geometry. This thesis presents a three dimensional, non-isothermal computational model of a proton exchange membrane fuel cell (PEMFC). The model was developed to improve fundamental understanding of transport phenomena in PEMFCs and to investigate the impact of various operation parameters on performance. The model, which was implemented into a Computational Fluid Dynamics code, accounts for all major transport phenomena, including: water and proton transport through the membrane; electrochemical reaction; transport of electrons; transport and phase change of water in the gas diffusion electrodes; temperature variation; diffusion of multi-component gas mixtures in the electrodes; pressure gradients; multi-component convective heat and mass transport in the gas flow channels. Simulations employing the single-phase version of the model are performed for a straight channel section of a complete cell including the anode and cathode flow channels. Base case simulations are presented and analyzed with a focus on the physical insight, and fundamental understanding afforded by the availability of detailed distributions of reactant concentrations, current densities, temperature and water fluxes. The results are consistent with available experimental observations and show that significant temperature gradients exist within the cell, with temperature differences of several degrees Kelvin within the membrane-electrode-assembly. The three-dimensional nature of the transport processes is particularly pronounced under the collector plates land area, and has a major impact on the current distribution and predicted limiting current density. A parametric study with the single-phase computational model is also presented to investigate the effect of various operating, geometric and material parameters, including temperature, pressure, stoichiometric flow ratio, porosity and thickness of the gas diffusion layers, and the ratio between the channel with and the land area. The two-phase version of the computational model is used for a domain including a cooling channel adjacent to the cell. Simulations are performed over a range of current densities. The analysis reveals a complex interplay between several competing phase change mechanisms in the gas diffusion electrodes. Results show that the liquid water saturation is below 0.1 inside both anode and cathode gas diffusion layers. For the anode side, saturation increases with increasing current density, whereas at the cathode side saturation reaches a maximum at an intermediate current density (≈ 1.1Amp/cm2) and decreases thereafter. The simulation show that a variety of flow regimes for liquid water and vapour are present at different locations in the cell, and these depend further on current density. The PEMFC model presented in this thesis has a number of novel features that enhance the physical realism of the simulations and provide insight, particularly in heat and water management. The model should serve as a good foundation for future development of a computationally based design and optimization method. / Graduate

This research focuses on new rapid prototype development techniques for Proton Exchange Membrane (PEM) fuel cell gas delivery plates. The study addresses several key issues in the design, analysis and manufacturing of fuel cell plates. The approach combines theoretical modeling, experimental study, physical plate making process, and virtual prototyping to form a new scheme for the rapid prototype development of fuel cell plates. The research extends the newly introduced screen-printing layer deposition manufacturing technique to complete the entire cycle of rapid plate development. A number of key issues on plate materials and the layer deposition process are addressed. The study has identified the cause of the problem with the poster-ink based screen-print ink material, explored various alternative composite ink materials, and narrowed down to the promising “conductive polymer ∼ epoxy ∼ graphite power” composite. A new, concurrent approach for developing new composite materials through various experiments and material tests has been introduced, and demonstrated through the development of one particular ink composite with promising results. In this research, the method for virtual prototyping fuel cell gas delivery plate using advanced CAD/CAE commercial software is introduced. The method allows a “virtual prototype” of the fuel cell plate to be constructed and the performance of the plate to be evaluated through various analyses as “virtual prototype tests.” These include the prediction of fuel cell performance through the CFD calculation on the average oxygen concentration; as well as the assessments of the maximum stress and undesirable temperature variation on the printed fuel cell plate through finite element analysis. Design optimization is conducted using the virtual prototypes to improve the design of the flow field and the plate. Three disciplinary models are simulated and their results are subject to disciplinary optimizations. Global design optimization is carried out using multiple objective optimization, combining the functional performance measures from three disciplinary models. This multi-disciplinary optimization integrates performance considerations of PEM fuel cell plate, and provides guidelines to the plate development. The research contributes to a new approach for the rapid development of fuel cell plates with a great potential to be applied to other mechanical parts. The study also extends the methodology of computational design and rapid prototyping. / Graduate

Made available in DSpace on 2014-06-11T19:25:34Z (GMT). No. of bitstreams: 0 Previous issue date: 2011-08-12Bitstream added on 2014-06-13T19:12:28Z : No. of bitstreams: 1 gentil_r_me_araiq.pdf: 2192477 bytes, checksum: 6d17fc2600cc2ac67ecbe0a8f2b7cf59 (MD5) / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Nanopartículas de PtV e PtCr, de composição nominal atômica 50:50, foram preparadas utilizando o método do poliol modificado, com adição de trietilborohidreto de lítio como segundo agente redutor, e suportadas em pó de carbono. Os materiais obtidos foram caracterizados por difratometria de raios X, microscopia eletrônica de transmissão e espectroscopia de absorção de raios X dispersivos. Para o catalisador de PtV/C, os resultados mostraram que o método de síntese produziu nanopartículas com um grau de liga inferior ao nominal, tamanho de partícula pequeno (3,7 nm) e uma estreita distribuição de tamanho de partícula. Mesmo com um grau de liga em torno de 5%, o catalisador de PtV/C apresentou uma atividade para a reação de redução de oxigênio (RRO) superior ao catalisador comercial de Pt/C, sendo mais ativo tanto na ausência quanto na presença de metanol. Tratamentos térmicos em atmosfera de hidrogênio puro foram realizados em temperaturas de 150°, 200° e 300° C permitiram aumentar ligeiramente o grau de liga do catalisador de PtV/C, mas, em contrapartida, promoveram outras variações nas características físicas e eletrocatalíticas. Para elucidar as mudanças estruturais foi realizado um estudo acompanhando os tratamentos térmicos por difratometria de raios X. Os resultados deste estudo, combinados com as informações obtidas do diagrama de fases do sistema PtV, permitiram interpretar as variações nas propriedades físicas e catalíticas observadas para os materiais tratados. Para o catalisador de PtCr/C, os resultados obtidos mostraram que a rota de síntese produziu nanopartículas com um tamanho médio de cristalito similar ao do catalisador de PtV/C, mas o catalisador de PtCr/C apresentou uma incorporação de Cr a fase liga superior à incorporação de vanádio para o catalisador de PtV/C. A atividade frente... / PtV and PtCr nanoparticles of atomic nominal composition 50:50 were prepared by a modified polyol method with addition of lithium triethyl borohydride as second reducing agent, and supported on carbon powder. The materials thus prepared were characterized by X-ray diffraction, transmission electronic microscopy and dispersive X-ray absortion spectroscopy. For the PtV/C catalyst, results showed that the synthesis method adopted produced nanoparticles with an alloying degree inferior to the nominal composition, small average particle size (3.7 nm) and narrow size distribution. Despite the low V incorporation into the alloy phase (about 5%), the PtV/C catalyst exhibited a catalytic activity for the oxygen reduction reaction (ORR) higher than a commercial Pt/C sample, even in the presence of methanol. Heat treatments in pure hydrogen atmosphere carried out at 150°, 200° e 300° C, produced a slight increase in the degree of alloying and, at the same time, promoted other variations in physical and electrocatalytic properties. To elucidate the structural changes, a study of thermal treatments was made while collecting at the same time X-ray diffraction data. The results of this study, combined with the information extracted from the phase diagram of the PtV system, allow interpreting the observed changes in physical and catalytic properties for the thermally treated materials. For the PtCr/C catalyst, the results obtained showed that the synthesis method produced nanoparticles with average crystallite size similar to that observed for PtV/C, but with larger incorporation of Cr into the Pt lattice in comparison with the amount of V incorporated into the alloy in the PtV/C sample. The ORR activity of PtCr/C is slightly inferior to that of PtV/C while it showed a smaller loss of performance in the presence of methanol. A study of the effects of... (Complete abstract click electronic access below)

Made available in DSpace on 2014-10-09T12:35:07Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:00:26Z (GMT). No. of bitstreams: 0 / Dissertação (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Orientador: Hebe de Las Mercedes Villullas / Banca: Ernesto Chaves Pereira de Souza / Banca: Paulo Olivi / Resumo: Nanopartículas de PtV e PtCr, de composição nominal atômica 50:50, foram preparadas utilizando o método do poliol modificado, com adição de trietilborohidreto de lítio como segundo agente redutor, e suportadas em pó de carbono. Os materiais obtidos foram caracterizados por difratometria de raios X, microscopia eletrônica de transmissão e espectroscopia de absorção de raios X dispersivos. Para o catalisador de PtV/C, os resultados mostraram que o método de síntese produziu nanopartículas com um grau de liga inferior ao nominal, tamanho de partícula pequeno (3,7 nm) e uma estreita distribuição de tamanho de partícula. Mesmo com um grau de liga em torno de 5%, o catalisador de PtV/C apresentou uma atividade para a reação de redução de oxigênio (RRO) superior ao catalisador comercial de Pt/C, sendo mais ativo tanto na ausência quanto na presença de metanol. Tratamentos térmicos em atmosfera de hidrogênio puro foram realizados em temperaturas de 150°, 200° e 300° C permitiram aumentar ligeiramente o grau de liga do catalisador de PtV/C, mas, em contrapartida, promoveram outras variações nas características físicas e eletrocatalíticas. Para elucidar as mudanças estruturais foi realizado um estudo acompanhando os tratamentos térmicos por difratometria de raios X. Os resultados deste estudo, combinados com as informações obtidas do diagrama de fases do sistema PtV, permitiram interpretar as variações nas propriedades físicas e catalíticas observadas para os materiais tratados. Para o catalisador de PtCr/C, os resultados obtidos mostraram que a rota de síntese produziu nanopartículas com um tamanho médio de cristalito similar ao do catalisador de PtV/C, mas o catalisador de PtCr/C apresentou uma incorporação de Cr a fase liga superior à incorporação de vanádio para o catalisador de PtV/C. A atividade frente... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: PtV and PtCr nanoparticles of atomic nominal composition 50:50 were prepared by a modified polyol method with addition of lithium triethyl borohydride as second reducing agent, and supported on carbon powder. The materials thus prepared were characterized by X-ray diffraction, transmission electronic microscopy and dispersive X-ray absortion spectroscopy. For the PtV/C catalyst, results showed that the synthesis method adopted produced nanoparticles with an alloying degree inferior to the nominal composition, small average particle size (3.7 nm) and narrow size distribution. Despite the low V incorporation into the alloy phase (about 5%), the PtV/C catalyst exhibited a catalytic activity for the oxygen reduction reaction (ORR) higher than a commercial Pt/C sample, even in the presence of methanol. Heat treatments in pure hydrogen atmosphere carried out at 150°, 200° e 300° C, produced a slight increase in the degree of alloying and, at the same time, promoted other variations in physical and electrocatalytic properties. To elucidate the structural changes, a study of thermal treatments was made while collecting at the same time X-ray diffraction data. The results of this study, combined with the information extracted from the phase diagram of the PtV system, allow interpreting the observed changes in physical and catalytic properties for the thermally treated materials. For the PtCr/C catalyst, the results obtained showed that the synthesis method produced nanoparticles with average crystallite size similar to that observed for PtV/C, but with larger incorporation of Cr into the Pt lattice in comparison with the amount of V incorporated into the alloy in the PtV/C sample. The ORR activity of PtCr/C is slightly inferior to that of PtV/C while it showed a smaller loss of performance in the presence of methanol. A study of the effects of... (Complete abstract click electronic access below) / Mestre

Made available in DSpace on 2014-10-09T12:35:07Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:00:26Z (GMT). No. of bitstreams: 0 / As células a combustível do tipo PEM (Próton Exchange Membrane) alimentadas diretamente por hidrogênio são consideradas as mais promissoras para a geração de energia elétrica, entretanto o uso de hidrogênio como combustível nestas células apresenta ainda alguns inconvenientes operacionais e de infra-estrutura, o que dificulta o seu uso. Assim, nos últimos anos, uma célula a combustível que utiliza um álcool diretamente como combustível (DAFC - Direct Alcohol Fuel Cell) tem despertado bastante interesse, particularmente aquelas que são alimentadas pelos combustíveis metanol ou etanol, pois apresentam várias vantagens, como por exemplo, a não necessidade de estocar hidrogênio ou gerá-lo através da reforma de hidrocarbonetos.Porém, células que utilizam diretamente metanol como combustível, apresentam correntes relativamente baixas e a oxidação completa do etanol é dificultado pela quebra da ligação CC e também há a formação de intermediários fortemente adsorvidos no eletrocatalisador de platina, como o monóxido de carbono (COads), resultando em baixos potenciais operacionais na célula.Para minimizar o efeito causado pelos venenos catalíticos faz-se necessária a adição de outros metais na composição do eletrodo de Pt. Tais metais devem atuar na reação fornecendo sítios para a adsorção de espécies que contenham oxigênio (OH ou H2O), em potenciais inferiores ao potencial de adsorção de OH na Pt.Este trabalho apresenta estudos da reação de eletro-oxidação destes álcoois, nos meios ácido e alcalino, sobre os eletrocatalisadores Pt/C, PtAu/C e PtAuBi/C, utilizando o método da redução via feixe de elétrons. Os eletrocatalisadores PtAuBi/C foram preparados com diferentes composições atômicas a fim de se avaliar o efeito da adição de bismuto. Os experimentos foram caracterizados por voltametria cíclica e cronoamperometria, utilizando a técnica do eletrodo de camada fina porosa, obtendo informações em relação às atividades dos catalisadores, perfis eletroquímicos e suas estabilidades em relação ao tempo de operação. Os eletrodepósitos foram examinados usando análise de energia dispersiva de raios-X (EDX) e microscopia eletrônica de varredura (MEV) a fim de determinar a composição de fases, o tamanho e a distribuição das nanopartículas metálicas no suporte. Os resultados eletroquímicos mostraram para oxidação eletroquímica de metanol, no meio alcalino, que o catalisador de PtAu/C apresentou melhor atividade eletrocatalítica e, no meio ácido, o catalisador Pt/C foi mais efetivo com relação às demais formulações preparadas e os eletrocatalisadores PtAuBi/C apresentaram-se pouco efetivos. No caso da oxidação do etanol, os dados eletroquímicos mostraram que, no meio ácido os catalisadores PtAu e Pt/C possuem comportamentos similares e os catalisadores PtAuBi/C demonstram baixa atividade. No meio alcalino, o sistema PtAuBi/C obteve melhor desempenho em relação aos demais catalisadores, obtendo maiores valores de correntes à baixos potenciais. / Dissertação (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Magister Scientiae - MSc / In this work a modified polyol method was developed to synthesize in-house catalysts. The method was modified for maximum delivery of product and proved to be quick and efficient as well as cost effective. The series of IH catalysts were characterized using techniques such as UV-vis and FT-IR spectroscopy, TEM, XRD, ICP and CV. The polyol method developed effectively reduced and deposited Pt nanoparticles onto different carbon supports. Functionalization of some of the supports was also successfully carried out through acid oxidative treatment which introduced carboxylic acid and hydroxyl groups onto the surface of the supporting material, which was supported by FT-IR which demonstrated that there was a relative increase in absorbance of functionalities viz., carboxylic acid (1270 cm-1) and hydroxyl groups on the surface of the acid treated MWCNT’s-F compared to the untreated MWCNT’s. Addition of a sedimentation promoter proved to increase the amount of metal deposition on the support thereby improving the loading dramatically. It was also found that addition of a specific amount of water in the polyol method allowed one to control the particle growth during the deposition phase on the various carbon supports investigated namely; XC-72 carbon, MWCNT’s and MWCNT’s-F. The in-house catalysts synthesized namely; Pt/C-IH, Pt/MWCNT’s-IH and Pt/MWCNT’s-F-IH all displayed narrow particle size distributions with average mean particle sizes of 2-5 nm, 2-6 nm and 3-6 nm respectively which was in good agreement with particle size measurements obtained from XRD using the Scherrer formula. All measured CV’s obtained for the series of IH catalysts prepared by this protocol were comparable with the commercial catalyst. The IH catalysts displayed the characteristic XRD peaks associated with Pt on carbon supports in acidic media. The ORR measurements for Pt/MWCNT’s-F-IH (functionalised) proved to be slightly superior (0.058 A/cm2) compared to that of the commercial catalyst (0.047 A/cm2) at a potential of 0.3V. This was attributed to the fact that the addition of water effectively controlled particle growth and deposition onto the support, and the fact that MWCNT’s-F offered a larger surface area and with the functionalized surface offering anchorage sites for Pt nanoparticles through carboxylic and hydroxyl functional groups. Further attempts to modify the developed polyol method using NaOAc as an electrostatic stabilizer to control the growth of Pt nanoparticles were made. It was found that although the stabilizer employed effectively stabilized Pt nanoparticles supported on XC-72 carbon, in our hands we were not able to produce the same results in the case of Pt supported on the MWCNT’s. TEM images revealed that Pt/CNaOAc displayed narrow particle size distribution with a mean particle size of 2-5 nm whereas particle agglomeration was observed for Pt/MWCNT’s-NaOAc and Pt/MWCNT’s-F-NaOAc with a broader particle size distribution and average mean particle sizes of 2-7 and 2-8 nm in diameter respectively. The CV’s obtained for these modified IH catalysts were in the main comparable to that of the commercial catalyst. However ORR activities of the IH Pt/C-NaOAc catalysts when carefully compared to the commercial catalyst revealed that it was indeed slightly superior (0.064 A/cm2). This is mainly attributed to the narrow particle size and even distribution of particles on the carbon support and resembled the best particle required to provide maximum activity in the ORR as a consequence of the facial kinetics involved. / South Africa