This thesis presents a framework for a microstructural model of a catalyst layer in a proton exchange membrane (PEM) fuel cell. In this study, a stochastic model that uses individual carbon, platinum and ionomer particles as building blocks to construct a catalyst layer geometry, resulting in optimal porosity and material mass ratios has been employed. The construction rule set in this design is easily variable, enabling a wide range of catalyst layer geometries to be made. The generated catalyst layers were found to exhibit many of the features found in currently poduced catalyst layers. The resulting geometries were subsequently examined on the basis of electronic percolation, mean chord length and effective diffusivity of the pore phase. Catalyst layer percolation was found to be most effected by the number of carbon see particles used and the specified porosity. The mean chord lengths of all of the catalyst layer geometries produced Knudsen numbers ranging in order of magnitude between 0.1 and 10, thus indicating that gas diffusion within the catalyst layers lies in the transition regime between bulk and Knudsen diffusion. Calculated effective diffusivities within the pore space of the model were shown to be relatively insensitive to changes in the catalyst layer composition and construction rule set other then porosity, indicating that the pore size distribution does not significantly vary when the catalyst layer mass ratios vary. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2012-08-31 08:52:55.747

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Using glycerol from biodiesel production as a fuel in a microbial fuel cell (MFC) will generate electricity and valuable by-products from what is currently considered waste. This research aims to screen E. coli (W3110, TG1, DH5, BL21), P. freudenreichii (subspecies freudenreichii and shermanii), and mixed cultures enriched from compost (AR1, AR2, AR3) as anodic biocatalysts in a glycerol-oxidizing MFC. Anaerobic fermentation experiments were performed to determine the oxidative capacity of each catalyst towards glycerol. Using an optimized medium for each strain, the highest anaerobic glycerol conversion from each group was achieved by E. coli W3110 (4.1 g/L), P. freudenreichii ssp. shermanii (10 g/L), and AR2 (20 g/L). These cultures were then tested in an MFC system. All three catalysts exhibited exoelectrogenicity. The highest power density was achieved using P. freudenreichii ssp. shermanii (14.9 mW m-2), followed by AR2 (11.7 mW m-2), and finally E. coli W3110 (9.8 mW m-2).

The extensive amount of platinum required in order to facilitate the oxygen reduction reaction (ORR) occuring at the cathode of low temperature fuel cells provides cost limitations to the sustainable commercialization of this technology. The development of electrocatalyst materials with either reduced or eliminated platinum dependency is an urgent necessity. The present work investigates the application of nitrogen doped carbon nanotubes (N-CNTs) and their composites as electrocatalyst materials for the ORR. First, N-CNTs are investigated as platinum support materials for proton exchange membrane fuel cells. They were found to result in improved ORR activity in comparison with undoped CNT supported platinum, due to the enhanced catalyst-support interactions and electronic properties induced by nitrogen heteroatoms incorporated into the graphitic structure of CNTs. Second, N-CNTs synthesized from a variety of different precursor materials were investigated as ORR electrocatalysts in alkaline conditions. The influence of the precursor materials was illustrated with improved ORR activity and nitrogen concentration observed for N-CNTs synthesized with precursor materials containing higher nitrogen to carbon contents. Highly active N-CNTs based on ethylenediamine were fabricated into thin, free standing films for use as a stand-alone cathode catalyst layer in an alkaline anion exchange membrane fuel cell. Finally, metal-free N-CNTs were developed and demonstrated to provide promising ORR in the absence of any metal interactions.

The direct methanol fuel cell (DMFC) is considered to be a promising power source for portable electronic applications and transportation. At present there are several challenges that need to be addressed before the widespread commercialization of the DMFC technology can be implemented. The methanol electro oxidation reaction is sluggish, mainly due to the strong adsorption of the reaction intermediate carbon monoxide on platinum. Further, methanol crosses over to the cathode, which decreases the fuel utilization and causes cathode catalyst poisoning. Another issue is the accumulation of the reaction product CO₂ (g) in the anode, which increases the Ohmic resistance and blocks reactant mass transfer pathways. A novel anode configuration is proposed to address the aforementioned challenges. An extended reaction zone (thickness = ∼100-300 µm) is designed to facilitate the oxidation of methanol on sites that are not close to the membrane-electrode interface. Thus, the fuel concentration near the membrane may decrease significantly, which may mitigate adverse effects caused by methanol cross-over. The structure of the fibrous electrode, with its high void space, is believed to aid the disengagement of CO₂ gas. In this thesis the first objective was to deposit dispersed nanoparticle PtRu(Mo) catalysts onto graphite felt substrates by surfactant mediated electrodeposition. Experiments, in which the surfactant concentration, current density, time and temperature were varied, were conducted with the objective of increasing the active surface area and thus improving the reactivity of the electrodes with respect to methanol electro-oxidation. The three-dimensional electrodes were characterized with respect to their deposit morphology, surface area, composition and catalytic activity. The second objective of this work was to utilize the catalyzed electrodes as anodes for direct methanol fuel cell operation. The fuel cell performance was studied as a function of methanol concentration, flow rate and temperature by using a single cell with a geometric area of 5 cm². Increased power densities were obtained with an in-house prepared 3D PtRu anode compared to a conventional PtRu catalyst coated membrane. Coating graphite felt substrates with catalytically active nanoparticles and the utilization of these materials, is a new approach to improve the performance of direct fuel cells.

Thesis (M.S.)--West Virginia University, 2008. / Title from document title page. Document formatted into pages; contains vi, 86 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 82-84).

Solid Oxide Fuel Cells have attracted much attention over the past few decades due to their huge potential for clean power generation in stationary, portable and transport applications and our increasing need for sustainable energy resources. The purpose of this research is to develop an interconnect and cathode material for use in solid oxide fuel cells which demonstrates desired properties of high electrical conductivity, excellent chemical stability at high temperatures, desirable thermal expansion characteristics and which can be easily manufactured by sintering in conditions acceptable with other cell components. The present work was initiated to study the synthesis and properties of five different perovskite oxides comprising of Lanthanum in combination with different mol% of Chromium, Ferrum, Cobalt and Nickel. A polymer complexing route with slight modifications was used to prepare the precursor powders. The powder x-ray diffraction patterns at room temperature show that all samples were formed in single phase. The powders in the form of pellets were sintered at 1400°C. The temperature dependent resistivity data was measured and the conductivity data was calculated. This conductivity data have been fitted with the Arrhenius model for entire studied range of temperature (25-800°C) to calculate the activation energy. La based perovskite oxides were characterized using X-ray diffraction (XRD), and scanning electron microscopy (SEM). Electrical properties and microstructural studies show potential applications of the materials as interconnect and cathode for Solid Oxide Fuel Cell. The material which has the above desired properties was proposed and component modifications for tailoring such properties were shown for SOFCs and other similar applications.

Orientador: Antonio Carlos Dias Ângelo / Banca: Dayse Iara dos Santos / Banca: Luiz Henrique Dall'Antonia / O Programa de Pós-Graduação em Ciência e Tecnologia de Materiais, PosMat, tem caráter institucional e integra as atividades de pesquisa em materiais de diversos campi da Unesp / Resumo: Nos últimos anos a questão energética tem sido o foco principal de grande parte da comunidade cientifica, onde a mesma se empenha arduamente na busca por soluções alternativas aos sobrecarregados meios convencionais de obtenção de energia elétrica. Neste contexto, as células a combustível surgem como sistemas alternativos na geração de energia elétrica. Tais dispositivos convertem energia química em energia elétrica de forma direta, silenciosa e, principalmente, ecologicamente correta. Dentre os vários tipos de células a combustível, as células PEMFC (Próton Exchange Membrane Fuel Cell) se destacam das demais, pois podem utilizar vários combustíveis em sua configuração, além de operar em baixas temperaturas, sendo ideais para sistemas portáteis, veiculares e estacionários. Neste sentido, o presente trabalho se apresenta de forma a desenvolver materiais com elevada eletroatividade para serem implantados no compartimento anódico destes sistemas. Dentre os inúmeros tipos de materiais que podem ser utilizados nesta configuração de célula, os compostos intermetálicos de fase ordenada surgem como excelente alternativa, pois os mesmos apresentam elevada eletroatividade frente à oxidação de hidrogênio e álcoois de baixo peso molecular. Os compostos intermetálicos nanoparticulados PtSn, PtSb, PtSn/C e PtSb/C e a Pt/C foram sintetizados em baixa temperatura via processo poliol utilizando etileno glicol como solvente e agente redutor, e carbono vulcan XC-72 como suporte. Os eletrocatalisadores obtidos foram caracterizados por Difração de Raios - X (DRX), Microscopia Eletrônica de Varredura (MEV), Microscopia Eletrônica de Transmissão (TEM) e Espectroscopia de Energia Dispersiva por Raios - X (EDX). A técnica DRX mostrou que o produto final é predominantemente composto pelas fases intermetálicas ordenadas, coexistindo... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: In the last years the energy issue has been the main focus of much of the scientific community, where it strives hard in the search for alternative solutions to overburdened conventional ways of obtaining electric energy. In this context, fuel cells appear as alternative systems in the generation of electric energy. Such devices convert chemical energy into electrical energy in a direct, silent and mostly environmentally correct manner. Among the various configuration of fuel cells, the PEMFC (Proton Exchange Membrane Fuel Cell) one stand out from the others as they may fed by different fuels, operate at low temperatures and they are ideal for portable, veiculares and stationary uses. In this perspective, the present work present the results obtained from de search for electrode materials with high electroactivity to be implanted in the anodic compartment of PEMFC. Ordered intermetallics materials emerge as excellent electrode material, alternative to Platinum, and moreover they can be successfully used as methodological tool to understand electrocatalysis process. Furthermore, they exhibited high electroactivity toward the oxidation reaction of hydrogen and low molecular weight alcohols. The nanoparticles intermetallics compounds PtSn, PtSb, PtSn/C and PtSb/C and Pt/C were synthesized at low temperature by polyol process using ethylene glycol as a solvent and reducing agent, and carbon Vulcan XC-72 as support. The electrocatalysts were characterized by X-Ray Diffraction(XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscope (TEM) and Energy Dispersive X-ray Spectroscopy (EDX). XRD showed that the final product is predominantly composed by ordered intermetallics phases, coexisting a secondary phase of the platinum (cfc) in the materials PtSn/C, PtSb/C and PtSb... (Complete abstract click electronic access below) / Mestre

Essa dissertação dedica-se ao estudo de instabilidades cinéticas em células a combustível de membrana trocadora de prótons (PEMFC, do inglês proton exchange membrane fuel cell). As PEMFC apresentam baixíssima perda por polarização quando operadas com H2. Contudo, quando o processo de produção de H2 se dá por reforma catalítica de hidrocarbonetos, CO está presente em níveis inaceitáveis para PEMFC equipada com ânodo de Pt/C. Dentre as propostas para superar esse problema, ligas bimetálicas de Pt têm se mostrado uma alternativa promissora para tornar a célula mais tolerante à CO. Além disso, é plausível que um comportamento dinâmico surja nesse tipo de sistema eletroquímico, devido à interação de fatores como transferência de massa, corrente, potencial do eletrodo e a presença de um veneno catalítico, nesse sistema o CO, que pode ser uma impureza do H2 ou um intermediário de reação (em células a combustível alimentadas diretamente com metanol, ácido fórmico ou etanol). Uma das motivações em se estudar tais instabilidades cinéticas é que uma célula a combustível operando em regime oscilatório pode resultar em um desempenho superior, uma vez que a limpeza auto-organizada da superfície previne que o ânodo seja completamente envenenado por CO. Nesse contexto, estudou-se a emergência de instabilidades cinéticas em PEMFC operando com ânodo de Pd-Pt/C ou Pd/C durante a oxidação de H2 e H2/CO, assim como em célula a combustível a metanol direto (DMFC, do inglês direct methanol fuel cell) com ânodo de Pt black. Os resultados indicaram que oscilações de potencial surgem na PEMFC durante a oxidação H2/CO sobre Pd-Pt/C assim como sobre Pd/C. Acoplando as medidas de potencial com espectrometria de massas on line na saída do ânodo, investigou-se o consumo de CO e a produção de CO2 durante as oscilações. Observou-se que as oscilações de potencial levam a variações na fração molar de CO e CO2. Adicionalmente, identificou-se oscilações de potencial em DMFC, fenômeno até então não relatado na literatura. / This dissertation deals with kinetic instabilities in proton exchange membrane fuel cells (PEMFC). PEMFCs show very small polarization losses when operating with pure H2. However, when the H2 production takes place by catalytic reforming of hydrocarbons, CO is present in the fuel stream at unacceptable levels for PEMFC equipped with a Pt/C anode. Among the possibilities to overcome this problem, bimetallic Pt alloys have proven to be a promising alternative to increase CO tolerance. Furthermore, it is plausible that a dynamic behavior emerge in such electrochemical system due to the interaction of factors like mass transfer, current, potential, and the presence of a catalyst poison, for this system CO which can be a H2 impurity or a reaction intermediate (in direct methanol/formic acid/ethanol fuel cells). One of the motivations for studying kinetic instabilities is that a fuel cell operating under oscillatory regime might result in higher performance, because the self-organized cleaning of the surface prevents the anode to be completely poisoned by CO. In this context, kinetic instabilities were studied in PEMFC operating with Pt-Pd/C or Pd/C anode during the oxidation of H2 and H2/CO mixture, as well as in direct methanol fuel cell (DMFC) with Pt black anode. It was observed the emergency of potential oscillations during the H2/CO oxidation on both catalysts, Pt-Pd/C and Pd/C. By coupling the potential measurements with on line Mass Spectrometry in the anode outlet it was investigated a variation in the concentration of CO and CO2 during oscillatory dynamic. It was found that the potential oscillations lead to variations in the molar fraction of CO and CO2. Additionally, it was observed potential oscillations in DMFC, phenomenon not previously reported in the literature.

Hydrogen fuel cells have the potential to dramatically reduce emissions from the energy sector, particularly when integrated into an automotive application. However, there are three main hurdles to the commercialisation of this promising technology; one of which is reliability. Cur- rent standards require an automotive fuel cell to last around 5000 h of operation (equivalent to around 150,000 miles), which has proven difficult to achieve to date. This hurdle can be overcome through in-depth reliability analysis including techniques such as Failure Mode and Effect Analysis (FMEA), Fault Tree Analysis (FTA) and Petri-net simulation. This research has found that the reliability field regarding hydrogen fuel cells is still in its infancy, and needs development, if the current standards are to be achieved. In this research, a detailed reliability study of a Polymer Electrolyte Membrane Fuel Cell (PEMFC) is undertaken. The results of which are a qualitative and quantitative analysis of a PEMFC. The FMEA and FTA are the most up to date assessments of failure in fuel cells developed using a comprehensive literature review and expert opinion. Advanced modelling of fuel cell degradation logic was developed using Petri-net modelling techniques. 20 failure modules were identfied that represented the interactions of all failure modes and operational parameters in a PEMFC. Petri-net simulation was used to overcome key pitfalls observed in FTA to provide a verfied degradation model of a PEMFC in an automotive application, undergoing a specific drive cycle, however any drive cycle can be input to this model. Overall results show that the modeled fuel cell's lifetime would reach 34 hours before falling below the industry standard degradation rate of more than 5%. The degradation model has the capability to simulate fuel cell degradation under any drive cycle and with any operating parameters. A fuel cell test rig was also developed that was used to verify the simulated degradation. The rig is capable of testing single cells or stacks from 0-470W power. The results from the verification experimentation agreed strongly with the degradation model, giving confidence in the accuracy of the developed Petri-net degradation model. This research contributes greatly to the field of reliability of PEMFCs through the most up-to-date and comprehensive FMEA and FTA presented. Additionally, a degradation model based upon Petri-nets is the first degradation model to encompass a 1D performance model to predict fuel cell life time under specific drive cycles.