<p>In this study, the main objective was to investigate the tolerance of platinum based binary anode catalysts for CO poisoning from 10ppm up to1000ppm and to identify the<br /> best anode catalysts for PEMFCs that tolerates the CO fed with reformed hydrogen.</p>

<p>In this study, the main objective was to investigate the tolerance of platinum based binary anode catalysts for CO poisoning from 10ppm up to1000ppm and to identify the<br /> best anode catalysts for PEMFCs that tolerates the CO fed with reformed hydrogen.</p>

Magister Scientiae - MSc / In this study, the main objective was to investigate the tolerance of platinum based binary anode catalysts for CO poisoning from 10ppm up to1000ppm and to identify the best anode catalysts for PEMFCs that tolerates the CO fed with reformed hydrogen. / South Africa

Esta tese de doutorado aborda o desenvolvimento de sistemas microfluídicos eletroquímicos ultrassensíveis mediante a integração de eletrodos i) concêntricos e ii) nanoestruturados seletivos à detecção condutométrica sem contato acoplada capacitivamente (capacitively coupled contactless conductivity detection, C4D) e à amperometria, respectivamente. O uso dos eletrodos concêntricos, uma configuração inédita em microdispositivos na qual o filme fino metálico circunda todo o microcanal, se mostrou efetivo na melhora da detectabilidade da C4D para análises em fluxo de soluções padrão de LiClO4. O limite de detecção (LD) para esse sal foi igual a 343 pmol L-1, um valor aproximadamente quatro ordens de grandeza inferior àquele obtido com eletrodos planares. O microchip amperométrico nanoestruturado, por sua vez, consistiu de filmes de Au modificados com nanotubos de carbono de parede única (single-walled carbon nanotubes, SWCNTs) verticalmente alinhados e foi aplicado a padrões do neurotransmissor serotonina. A melhora na detectabilidade do método foi novamente apreciável; os valores de LD foram de 11,8 (Au liso) e 0,2 nmol L-1 (Au modificado com os SWCNTs verticalmente alinhados). Esse último é menor frente à grande maioria dos valores descritos na literatura, para os quais técnicas diversas foram empregadas, incluindo: i) potenciometria com eletrodos modificados (1,0 a 500 nmol L-1), ii) HPLC-MS (18,2 nmol L-1), iii) eletroforese capilar combinada com etapas de extração, empilhamento e préconcentração do analito (7,9 nmol L-1) e iv) sensor químico (200 nmol L-1). Finalmente, objetivando a fabricação de microchips de vidro, condutométricos e amperométricos, incorporando eletrodos concêntricos nanoestrurados, uma nova técnica de selagem foi desenvolvida. Essa técnica, designada como selagem adesiva de sacrifício, baseia-se no uso do resiste negativo SU-8 como camada intermediária de modo a permitir a vedação entre duas lâminas de vidro. Numa etapa posterior, a remoção seletiva do SU-8 sob o microcanal é realizada. Logo, canais microfluídicos com propriedades de superfície similares às do vidro foram obtidos. O protocolo experimental adotado é i) simples, ii) rápido, iii) não envolve níveis de pressão e temperatura elevados e iv) prescinde o uso de salas \"limpas\". Vedações com forças de adesão satisfatórias foram alcançadas, suportando pressões superiores a 4 MPa. / This PhD thesis reports the development of ultrasensitive electrochemical microfluidic systems by integrating concentric and nanostructured electrodes selective to capacitively coupled contactless conductivity detection (C4D) and amperometry, respectively. The use of the concentric electrodes, a new assembly in microdevices with thin films wrapping around the microchannel, showed to be effective towards improvement of the detectability in pressure-driven flow platforms incorporating C4D. The limit-of-detection (LOD) in flow analysis of LiClO4 solutions was 343 pmol L-1, ca. four orders of magnitude lower than to the levels obtained with planar electrodes alone. The nanostructured amperometric microchip, in turn, is related to integration of vertically aligned singlewalled carbon nanotubes (SWCNTs) over Au film. Such platform was applied to determination of serotonin standards. The nanomaterial influenced remarkably the sensitivity and detectability. Our system achieved a LOD of 0.2 nmol L-1, to the best of our knowledge one of the lowest values reported in the literature. Finally, in order to fabricate glass microdevices, conductometric and amperometric, with nanostructured concentric electrodes, we developed a new bonding method. This technique, called as sacrificial adhesive bonding, is based on SU-8 negative resist like intermediate layer so to allow the sealing between two glass slides. Next, the selective removal of the SU-8 under the microchannel is carried out. Thus, microfluidic channels presenting glass-like surface properties were achieved. The experimental protocol is simple and fast. In addition, neither high-pressure and elevated-temperature nor the use of \"clean\" rooms were not required. Bondings with satisfactory adhesion forces were obtained, supporting pressures above 4 MPa.

Esta tese de doutorado aborda o desenvolvimento de sistemas microfluídicos eletroquímicos ultrassensíveis mediante a integração de eletrodos i) concêntricos e ii) nanoestruturados seletivos à detecção condutométrica sem contato acoplada capacitivamente (capacitively coupled contactless conductivity detection, C4D) e à amperometria, respectivamente. O uso dos eletrodos concêntricos, uma configuração inédita em microdispositivos na qual o filme fino metálico circunda todo o microcanal, se mostrou efetivo na melhora da detectabilidade da C4D para análises em fluxo de soluções padrão de LiClO4. O limite de detecção (LD) para esse sal foi igual a 343 pmol L-1, um valor aproximadamente quatro ordens de grandeza inferior àquele obtido com eletrodos planares. O microchip amperométrico nanoestruturado, por sua vez, consistiu de filmes de Au modificados com nanotubos de carbono de parede única (single-walled carbon nanotubes, SWCNTs) verticalmente alinhados e foi aplicado a padrões do neurotransmissor serotonina. A melhora na detectabilidade do método foi novamente apreciável; os valores de LD foram de 11,8 (Au liso) e 0,2 nmol L-1 (Au modificado com os SWCNTs verticalmente alinhados). Esse último é menor frente à grande maioria dos valores descritos na literatura, para os quais técnicas diversas foram empregadas, incluindo: i) potenciometria com eletrodos modificados (1,0 a 500 nmol L-1), ii) HPLC-MS (18,2 nmol L-1), iii) eletroforese capilar combinada com etapas de extração, empilhamento e préconcentração do analito (7,9 nmol L-1) e iv) sensor químico (200 nmol L-1). Finalmente, objetivando a fabricação de microchips de vidro, condutométricos e amperométricos, incorporando eletrodos concêntricos nanoestrurados, uma nova técnica de selagem foi desenvolvida. Essa técnica, designada como selagem adesiva de sacrifício, baseia-se no uso do resiste negativo SU-8 como camada intermediária de modo a permitir a vedação entre duas lâminas de vidro. Numa etapa posterior, a remoção seletiva do SU-8 sob o microcanal é realizada. Logo, canais microfluídicos com propriedades de superfície similares às do vidro foram obtidos. O protocolo experimental adotado é i) simples, ii) rápido, iii) não envolve níveis de pressão e temperatura elevados e iv) prescinde o uso de salas \"limpas\". Vedações com forças de adesão satisfatórias foram alcançadas, suportando pressões superiores a 4 MPa. / This PhD thesis reports the development of ultrasensitive electrochemical microfluidic systems by integrating concentric and nanostructured electrodes selective to capacitively coupled contactless conductivity detection (C4D) and amperometry, respectively. The use of the concentric electrodes, a new assembly in microdevices with thin films wrapping around the microchannel, showed to be effective towards improvement of the detectability in pressure-driven flow platforms incorporating C4D. The limit-of-detection (LOD) in flow analysis of LiClO4 solutions was 343 pmol L-1, ca. four orders of magnitude lower than to the levels obtained with planar electrodes alone. The nanostructured amperometric microchip, in turn, is related to integration of vertically aligned singlewalled carbon nanotubes (SWCNTs) over Au film. Such platform was applied to determination of serotonin standards. The nanomaterial influenced remarkably the sensitivity and detectability. Our system achieved a LOD of 0.2 nmol L-1, to the best of our knowledge one of the lowest values reported in the literature. Finally, in order to fabricate glass microdevices, conductometric and amperometric, with nanostructured concentric electrodes, we developed a new bonding method. This technique, called as sacrificial adhesive bonding, is based on SU-8 negative resist like intermediate layer so to allow the sealing between two glass slides. Next, the selective removal of the SU-8 under the microchannel is carried out. Thus, microfluidic channels presenting glass-like surface properties were achieved. The experimental protocol is simple and fast. In addition, neither high-pressure and elevated-temperature nor the use of \"clean\" rooms were not required. Bondings with satisfactory adhesion forces were obtained, supporting pressures above 4 MPa.

It is desirable to increase the operation temperature of proton exchange membrane fuel cells above 100oC due to fast electrode kinetics, high tolerance to fuel impurities and simple thermal and water management. In this study / the objective is to develop a high temperature proton exchange membrane fuel cell. Phosphoric acid doped polybenzimidazole membrane was chosen as the electrolyte material. Polybenzimidazole was synthesized with different molecular weights (18700-118500) by changing the synthesis conditions such as reaction time (18-24h) and temperature (185-200oC). The formation of polybenzimidazole was confirmed by FTIR, H-NMR and elemental analysis. The synthesized polymers were used to prepare homogeneous membranes which have good mechanical strength and high thermal stability. Phosphoric acid doped membranes were used to prepare membrane electrode assemblies. Dry hydrogen and oxygen gases were fed to the anode and cathode sides of the cell respectively, at a flow rate of 0.1 slpm for fuel cell tests. It was achieved to operate the single cell up to 160oC. The observed maximum power output was increased considerably from 0.015 W/cm2 to 0.061 W/cm2 at 150oC when the binder of the catalyst was changed from polybenzimidazole to polybenzimidazole and polyvinylidene fluoride mixture. The power outputs of 0.032 W/cm2 and 0.063 W/cm2 were obtained when the fuel cell operating temperatures changed as 125oC and 160oC respectively. The single cell test presents 0.035 W/cm2 and 0.070 W/cm2 with membrane thicknesses of 100 &micro / m and 70 &micro / m respectively. So it can be concluded that thinner membranes give better performances at higher temperatures.

A mathematical model for a proton exchange membrane fuel cell is the focus of this thesis. Modeling and simulations are carried out with an aim to understand the influence of operational and geometrical parameters on the inner reaction and performance of a proton exchange membrane fuel cell, and discuss the distributions of physical phenomena in membrane and catalyst layer. Than, the results of modeling are compared and analyzed with the experiments, and discuss the reasons of influences of the performance of PEMFC. The results show that activation overpotential is the major reason of influence of the performance at low current density (less than ), and diffusion and ohmic overpotential are substantially increased at high current density (great than ). The membrane of higher membrane conductivity and more thin, increasing pressure of cathode gas and use oxygen can enhance the performance of a PEMFC. The performance almost no influence for the catalyst layer over 0.3£gm. The catalyst layer thin and uniform can decrease coating of this layer. The results of modeling and experiments show that experiments have contact resistance between materials, and the performance slightly lower than performance of modeling, and the differences that at high current density great than low current density.

Membrane electrode assembly (MEA) is the foundation of the single cell as well as the core of the fuel cell when generating electricity. Its work efficiency is the key factor for single cell performance. This study aims to understand the variation between the conventional method and the decal method during the MEA process. By observing the microstructure morphology of electrode and the performance of single cell, as well as analyzing internal resistance and its stabilization, the advantages and disadvantages of MEA in the two methods is analyzed. The decal condition is 135¢XC, 15 kg/cm , 2.5 min at a high temperature (50¢XC 3M methanol), in air-breathing under atmosphere system. The maximum power density is approximately 22.5 mW/cm which is very close to the result of conventional method. The decal method is better than the conventional method particularly in regards to the high current density performance. It shows that there is an efficient influence of the decal method on the methanol mass transfer and it also improves its polarization and enlarges the current. If the single cell is operated in the high temperature, the fuel mass transfer can be advanced in the decal method and its performance can be raised. However, in the manufacturing process, more time has to be spent when producing the MEA. This experiment can be used as a reference on the single cell operation environment and manufacturing time for future studies.

A distinctive field in the coatings industry is the coating of porous media, with broad applications in paper, apparel, textile, electronics, bioengineering, filtration and energy sector. A primary industrial scale process that can be used to coat porous media in a fast and flexible manner is slot die extrusion. A major concern when coating porous media with a wetting fluid is fluid penetration into the substrate. Although some level of penetration is desirable to obtain specific material properties, inadequate or excessive fluid penetration can negatively affect the strength, functionality or performance of the resulting material. In spite of its apparent industrial importance, limited modeling and experimental work has been conducted to study fluid penetration into porous media during fabrication. The effects of processing parameters on the penetration depth, the effects of penetration on material quality, and the method to predict and control the penetration depth are not well understood. This dissertation is composed of two parts. Part I is an applied study for coating onto porous media. This part focuses on the first objective of this dissertation which is to elucidate clearly the feasibility, advantages and disadvantages of the direct coating method as a potential fabrication route for membrane electrode assembly (MEA). MEA samples are fabricated using both traditional and the direct coating methods. Then, the quality and performance of the MEA samples are examined. Experimental results in Part I demonstrate that it is feasible to fabricate MEAs using the direct coating method. However, Nafion® solution penetrates into the catalyst layer during the coating process and causes lower performance of fuel cells, which is the motivation for Part II of this thesis. The objective of Part II is to fundamentally understand the fluid penetration process and predict the penetration depth when directly coating porous media, using a comprehensive approach. A series of computational and analytical models are developed to predict the penetration depth for both Newtonian and non-Newtonian fluids with or without capillary pressure. Finally the accuracy of developed models are validated through experiments. The relative error between the predicted and experimentally measured penetration depth is generally lower than 20%.

Fuel cells are electrochemical energy devices that convert the chemical energy in a fuel into electrical energy. Although they are more efficient, clean, and reliable than fossil fuel combustion systems, they have not been widely adopted because of manufacturing challenges and high production cost. The most expensive component of a fuel cell is the membrane electrode assembly (MEA), which consists of an ionomer membrane coated with catalyst material. Best performing MEAs are currently fabricated by depositing and drying liquid catalyst ink on the membrane, however, this process is limited to individual preparation by hand due to the membrane’s rapid water absorption that leads to shape deformation and coating defects. This work models the swelling and drying phenomena of the membrane and coating during manufacturing, and then applies the results to develop and control a continuous coating line for the production of defect free fuel cell MEAs. A continuous coating line can reduce the costs and time needed to fabricate the MEA, incentivizing the commercialization and widespread adoption of fuel cells. Membrane swelling is a three-dimensional, transient, coupled mass transfer, heat transfer, and solid mechanics problem. Existing models describe the membrane’s behavior in operating conditions, but none predict the behavior during manufacturing. This work develops a novel physics-based model that describes the behavior of the membrane and coating in a continuous manufacturing scenario and incorporates effects that are missing from existing models. A model that can predict wrinkles, the most commonly observed defect during manufacturing, is presented. Simulation results from the above models are used to design and develop an improved continuous MEA coating process that includes pre-swelling and two-stage drying of the coated membrane. A prototype pilot-scale coating line to implement and test the improved coating process is designed and constructed. Finally, a Linear-Quadratic-Gaussian type controller is developed using the physics-based model of the manufacturing process to optimally control the temperature and humidity of the drying zones, and its effectiveness when implemented on the coating line is discussed. / text