O envelhecimento dos eletrocatalisadores utilizados em cátodos de células a combustível de eletrólito polimérico (PEMFCs) é um dos principais fatores que restringem sua aplicação como conversores de energia em larga escala. Esse trabalho visa contribuir para o aprimoramento da estabilidade de nanopartículas de platina (NPs de Pt) por meio da modificação do suporte catalítico aos quais encontram-se impregnadas. Desse modo, foram realizadas sínteses de suportes catalíticos baseados em óxidos de molibdênio (MoO3 e MoO2) ancorados em carbono Vulcan® XC72-R, sendo os materiais produzidos caracterizados física, estrutural e eletroquimicamente antes e após a impregnação de NPs de Pt. Para investigar a estabilidade dos eletrocatalisadores, foi realizado um teste de degradação eletroquímico acelerado, o qual consistiu em aplicar os ciclos de potenciais entre 0,6 e 1,0 V vs. ERH por curto período de tempo. Os resultados mostraram que os métodos de síntese utilizados foram satisfatórios, levando a formação dos catalisadores com as proporções bem próximas das requeridas. O catalisador Pt/MoO3-C apresentou a maior atividade específica frente a reação de redução de oxigênio (RRO), atribuída a efeitos sinérgicos metal/suporte. Quando investigada a estabilidade dos materiais frente ao teste de degradação eletroquímico acelerado, observou-se que, a princípio, nenhum dos óxidos de molibdênio diminui a extensão da degradação da platina. Analisando-se as atividades específicas frente à RRO para cada catalisador antes e após o envelhecimento eletroquímico, foi observado que Pt/MoO2-C apresentou-se como o material mais estável dentre os demais. / The aging of Pt based electrocatalysts used in the polymer electrolyte fuel cell (PEMFC) cathodes is one of the main issues that restrict its wide application as energy converters. This work aims to contribute to the improvement of the stability of platinum nanoparticles (Pt NPs) by modification of the catalyst support at which they are impregnated. Thus, syntheses of catalyst supports based on molybdenum oxide (MoO3 and MoO2) anchored on Vulcan® XC72-R carbon were carried out and the produced materials were characterized physically, structurally and electrochemically prior and after impregnation of the Pt NPs. To investigate their stability, an electrochemical accelerated degradation test was performed, which consisted of applying a large number of short duration potential cycling steps between 0.6 and 1.0 V vs. RHE. The results showed that the synthetic methods used here were satisfactory, leading to the formation of catalysts with compositions near to those expected. The Pt/MoO3-C catalyst showed the highest specific activity toward the oxygen reduction reaction (ORR), and this was attributed to metal/support synergistic effects. When the stability against electrochemical accelerated degradation test of the materials was investigated, it was observed that, in principle, none of the molybdenum oxides really decreases the extent of platinum degradation. However, comparing the specific activities towards the ORR for each catalyst, before and after electrochemical aging, it is concluded that Pt/MoO2-C is the most stable material among all others.

A procedure is presented to analyze select geometric and effective properties of the porous transport layer (PTL) of the polymer electrolyte membrane fuel cell (PEMFC) in com- pressed and uncompressed states using micro-computed X-ray tomography (Micro CT). A method of compression using a novel device design was employed to mimic the non-homogeneous compression conditions found in functioning fuel cells. The process also features open source image processing and CFD analysis through the use of software packages Fiji and OpenFOAM (proprietary software is also used such as Matlab). Tomographic images of a PTL sample in different compressive states are first analyzed by measuring local porosity values in the through-plane and both in- plane directions. The objective of this study was to develop a method for imaging the PTL structure to show directionality within its properties using relatively inexpensive and non-destructional means. Three different PTL types were tested, one without any additives, one with Polytetrafluoroethylene (PTFE) and one with PTFE and a microporous layer (MPL). Non-homogeneous porosity was shown to exist with the highest and least variable porosity values obtained from the in-plane direction that was in-line with the direction of fibres. Porosity values compared well with values obtained from the literature. The profile of the PTL with MPL added was unattainable using this procedure as the resolution of the Micro CT was too low to resolve its pore space. The next stage involved the effective properties analysis which included effective electronic conductivity and effective diffusivity. It was found that the through-plane values for the effective electronic conductivity study were higher than expected. The ratio between through-plane and in-plane was found to be much higher than expected from literature. Lack of sufficient resolution of fibre contacts has been shown to play a role in this discrepancy. These contact problems were shown not too affect measurements of diffusivity in the pore phase. The in-plane direction parallel to the direction of fibres was found to have the highest values of effective transport properties. Effective diffusivity ratios of between 0.1 and 0.37 were found to be reasonable with the limited experimental evidence found in literature. The it was found that the Bruggeman relation for calculating diffusivity and percolation theory by Tomadakis and Sotirchos over predicted the values for diffusion within the PTL and it is suggested that these theories are not suitable for predicting diffusivity for this material. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2012-02-02 15:46:29.395

O envelhecimento dos eletrocatalisadores utilizados em cátodos de células a combustível de eletrólito polimérico (PEMFCs) é um dos principais fatores que restringem sua aplicação como conversores de energia em larga escala. Esse trabalho visa contribuir para o aprimoramento da estabilidade de nanopartículas de platina (NPs de Pt) por meio da modificação do suporte catalítico aos quais encontram-se impregnadas. Desse modo, foram realizadas sínteses de suportes catalíticos baseados em óxidos de molibdênio (MoO3 e MoO2) ancorados em carbono Vulcan® XC72-R, sendo os materiais produzidos caracterizados física, estrutural e eletroquimicamente antes e após a impregnação de NPs de Pt. Para investigar a estabilidade dos eletrocatalisadores, foi realizado um teste de degradação eletroquímico acelerado, o qual consistiu em aplicar os ciclos de potenciais entre 0,6 e 1,0 V vs. ERH por curto período de tempo. Os resultados mostraram que os métodos de síntese utilizados foram satisfatórios, levando a formação dos catalisadores com as proporções bem próximas das requeridas. O catalisador Pt/MoO3-C apresentou a maior atividade específica frente a reação de redução de oxigênio (RRO), atribuída a efeitos sinérgicos metal/suporte. Quando investigada a estabilidade dos materiais frente ao teste de degradação eletroquímico acelerado, observou-se que, a princípio, nenhum dos óxidos de molibdênio diminui a extensão da degradação da platina. Analisando-se as atividades específicas frente à RRO para cada catalisador antes e após o envelhecimento eletroquímico, foi observado que Pt/MoO2-C apresentou-se como o material mais estável dentre os demais. / The aging of Pt based electrocatalysts used in the polymer electrolyte fuel cell (PEMFC) cathodes is one of the main issues that restrict its wide application as energy converters. This work aims to contribute to the improvement of the stability of platinum nanoparticles (Pt NPs) by modification of the catalyst support at which they are impregnated. Thus, syntheses of catalyst supports based on molybdenum oxide (MoO3 and MoO2) anchored on Vulcan® XC72-R carbon were carried out and the produced materials were characterized physically, structurally and electrochemically prior and after impregnation of the Pt NPs. To investigate their stability, an electrochemical accelerated degradation test was performed, which consisted of applying a large number of short duration potential cycling steps between 0.6 and 1.0 V vs. RHE. The results showed that the synthetic methods used here were satisfactory, leading to the formation of catalysts with compositions near to those expected. The Pt/MoO3-C catalyst showed the highest specific activity toward the oxygen reduction reaction (ORR), and this was attributed to metal/support synergistic effects. When the stability against electrochemical accelerated degradation test of the materials was investigated, it was observed that, in principle, none of the molybdenum oxides really decreases the extent of platinum degradation. However, comparing the specific activities towards the ORR for each catalyst, before and after electrochemical aging, it is concluded that Pt/MoO2-C is the most stable material among all others.

The gas diffusion layer (GDL) is one of the key components in a polymer electrolyte membrane (PEM) fuel cell. It performs several functions including the transport of reactant gases and product water to and from the catalyst layer, conduction of both electrons and heat produced in the catalyst layer, as well as mechanical support for the membrane. The overarching goal of this work is to thoroughly examine the GDL structure and properties for use in PEM fuel cells, and more specifically, to determine how to characterize the GDL experimentally ex-situ, to understand its performance in-situ, and to relate theory to performance through controlled experimentation. Thus, the impact of readily measured effective water vapor diffusivity on the performance of the GDL is investigated and shown to correlate to the wet limiting current density, as a surrogate of the oxygen diffusivity to which it is more directly related. The influence of microporous layer (MPL) design and construction on the fuel cell performance is studied and recommendations are made for optimal MPL designs for different operating conditions. A method for modifying the PTFE (Teflon) distribution within the GDL is proposed and the impact of distribution of PTFE in the GDL on fuel cell performance is studied. A method for characterizing the surface roughness of the GDL is developed and the impact of surface roughness on various ex-situ GDL properties is investigated. Finally, a detailed analysis of the physical structure and permeability of the GDL is provided and a theoretical model is proposed to predict both dry and wet gas flow within a GDL based on mercury intrusion porosimetry and porometry data. It is hoped that this work will contribute to an improved understanding of the functioning and structure of the GDL and hence advance PEM fuel cell technology.

Current environmental and energy sustainability trends have instigated considerable interest in alternative energy technologies that exhibit reduced dependence on fossil fuels. The advantages of such a direction are two-fold: reduced greenhouse gas emissions (notably CO2) and improved energy sustainability. Fuel cells are recognized as a potential technology that achieves both of these goals. However, improvements to fuel cell power density and stability must be realized to make them economically competitive with traditional, fossil-based technologies. The work in this dissertation is largely focused on the use of analytical tools for the study of transport processes in three fuel cell systems toward improvement of fuel cell performance. Polymer electrolyte membrane fuel cells (PEMFCs) are fueled by hydrogen and oxygen to generate electrical current. Microbial fuel cells (MFCs) use bacteria to degrade carbon compounds, such as those found in wastewaters, and simultaneously generate an electric current. Enzyme fuel cells (EFCs) operate similarly to PEMFCs but replace precious metal catalysts, such as platinum, with biologically-derived enzymes. The use of enzymes also allows EFCs to utilize simple carbon compounds as fuel. The operation of all three fuel cell systems involves different modes of ion and electron transport and can be affected negatively by transport limitations. Electrochemical impedance spectroscopy (EIS) was used in this work to study the distribution of transport resistances in all three fuel cell systems. The results of EIS were used to better understand the transport resistances that limited fuel cell power output. By using this technique, experimental conditions (including operating conditions, construction, and materials) were identified to develop fuel cells with greater power output and longevity. In addition to EIS, neutron imaging was employed to quantify the distribution of water in PEMFCs and EFCs. Water content is an integral aspect of providing optimal power output from both fuel cell systems. Neutron imaging contributed to developing an explanation for the loss of water observed in an operating EFC despite conditions designed to mitigate water loss. The findings of this dissertation contribute to the improvement of fuel cell technology in an effort to make these energy devices more economically viable.

The ionomer-membrane interface in a membrane electrode assembly connects the catalyst and membrane and allows hydrated protons to move between the catalyst and membrane. The continuous operation of the polymer membrane electrolyte fuel cell at high temperature and/or in frequent freeze/thaw cycles leads to membrane degradation and delamination of the interface, which lower the proton conductivity. In this dissertation, we modeled the chemical degradation and proton conductivity of perfluorosulfonic acid (PFSA) ionomers by ab initio calculations and macroscopic modeling. All ab initio calculations were performed using Gaussian 03 suites of program by employing B3LYP/6-311++G** method/basis set. The macroscopic modeling involves nonequilibrium thermodynamics. The results show that PFSA membranes can degrade both via side-chain and backbone in the presence of hydroxyl radical. The energetics of homolytic bond cleavage show that the C–S bond in the side-chain is the weakest link and breaks exothermally in the presence of hydroxyl radical. The C–S bond in the membrane fragment radical can break at low activation energy. The side-chain degradation also leads to the split of the backbone into two parts. The backbone degradation starts with the reaction of –COOH impurities in the backbone with the hydroxyl radical, which has the lowest activation energy, and follows an “unzipping mechanism”. The reactions in this mechanism are exothermic. The channels in the interface were modeled as cylindrical pores and the anionic charges were fixed on the pore wall. The analytical expression of proton conductivity was derived from the evolution equations for mass and momentum of hydronium ions by using an order of magnitude analysis. The results show that the conductivity increases with increasing water content and pore radius. The conductivity usually increases on decreasing the separation distance between sulfonates on the length and decreases with decreasing sulfonates separation distance on the circumference. The conductivity of the two pores, one of the interface and the other of the membrane, is closer to the conductivity of the pore with the lowest conductivity and its magnitude depends on the relative radius and length of the pores.

Polymer electrolyte membrane (PEM) fuel cell (FC) stacks require elastomeric gaskets for each cell to keep the reactant gases within their respective regions[1]. If any gasket degrades or fails, the reactant gases can leak or mix with each other directly during operation or standby, affecting the overall operation and performance of the FC. The elastomeric gaskets used as FC seals are exposed to a range of environmental conditions, and concurrently, subjected to mechanical compression between the bipolar plates forming the cell. The combination of mechanical stress and environmental exposure may result in degradation of the seal material[2] over a period of time. In order to address the durability and make reliability predictions, the long-term stability of the gaskets in FC assemblies is critical. The aim of this study is to investigate the performance of elastomeric seals in a simulated FC environment in the presence of mechanical stresses. The overall scope of the study includes mechanical and viscoelastic properties characterization, and lifetime durability predictions based on an accelerated characterization approach. With the help of finite element analysis software, ABAQUS, a fixture was designed to perform strain-based accelerated characterization of seal material in air, deionized (DI) water, 50v/50v ethylene glycol/water solution, and 0.1M sulfuric acid solution. Dogbone samples were strained to different levels in the custom fixture and submerged in liquid solutions at 90°C and in air at 90°C and 120°C. It was observed that mechanical properties such as tensile strength, strain to break, 100% modulus, crosslink density, and tensile set degrade due to aging and the extent of change (increase or decrease) depends significantly on the strain level on the specimen. Trouser tear tests were conducted on reinforced specimens in air and deionized water (DI) to evaluate the tear resistance of an elastomeric seal material intended for proton exchange membrane fuel cells. Plots relating the crack growth rate with tearing energy were obtained at various temperatures and provided significant insight into the rate and temperature dependence of the tearing strength of the seal material. Stick-slip crack propagation was observed at all temperatures and loading rates, although the behavior was suppressed significantly at low loading rates and high temperatures. Crack growth rate versus tearing energy data at different temperatures was shifted to construct a master curve and an estimate on the threshold value of tear energy was obtained which may be helpful in designing components where material tear is of concern. Strain energy release rate (SERR) value, calculated using the J-integral approach for a pre-existing crack in ABAQUS, was used to estimate the crack growth rate in a given seal cross-section to predict lifetime. In order to assess the viscoelastic behavior and to investigate the long term stress relaxation behavior of the seal material, compression stress relaxation (CSR) tests were performed on molded seals, called as SMORS, over a range of environmental conditions using a custom-designed fixture. The effect of temperature and environment was evident on material property changes and presented in terms of momentary properties and stress relaxation behavior. Various mechanisms involved in material degradation, chain scission and crosslinking, were suggested and insights were gained into how cure state and level of antidegradants in a material dictate the material behavior during the first phase of environmental exposure leading to change in material properties. Ring samples made of silicone were also tested using the fixture to obtain insight additional into material degradation due to aging. Results presented from testing on SMORS showed a lot more variation in data as compared to neat silicone rings due to the complexity involved in making SMORS. For understanding the deformation behavior of an elastomeric seal and its sealing performance, finite element characterization of seal cross-section was carried out on O-ring and SMORS cross-section. The effect of a seal's layout on distribution and magnitude of contact stresses and contact width was investigated for the O-ring and the information obtained thereby helped to analyze a complex assembly such as SMORS, where several interfaces and boundary conditions are involved. Stress/strain profiles were generated to visualize their concentration and distribution in the seal cross-section. Frictionless and rough interfacial conditions between seal material and platens were assumed and it was found that its effect on contact width and peak contact pressure was insignificant. Results obtained from FEA on SMORS were validated through comparison with contact mechanics approach and experimental data and it was found that Lindley's equation correlates well with experimental data whereas ABAQUS overestimates the load values at a given compression. Lindley's approach may be used to develop contact pressure profiles that may help estimate peak contact pressure at a given time so leaking can be avoided. / Ph. D.

Hydrogen fuel cells, and notably the polymer electrolyte fuel cell (PEFC), present an important opportunity to reduce greenhouse gas emissions within a range of sectors of society, particularly for transportation and portable products. Despite several decades of research and development, there exist three main hurdles to full commercialisation; namely infrastructure, costs, and durability. This thesis considers the latter of these. The lifetime target for an automotive fuel cell power plant is to survive 5000 hours of usage before significant performance loss; current demonstration projects have only accomplished half of this target, often due to PEFC stack component degradation. Health management techniques have been identified as an opportunity to overcome the durability limitations. By monitoring the PEFC for faulty operation, it is hoped that control actions can be made to restore or maintain performance, and achieve the desired lifetime durability. This thesis presents fault detection and diagnosis approaches with the goal of isolating a range of component degradation modes from within the PEFC construction. Fault detection is achieved through residual analysis against an electrochemical model of healthy stack condition. An expert knowledge-based diagnostic approach is developed for fault isolation. This analysis is enabled through fuzzy logic calculations, which allows for computational reasoning against linguistic terminology and expert understanding of degradation phenomena. An experimental test bench has been utilised to test the health management processes, and demonstrate functionality. Through different steady-state and dynamic loading conditions, including a simulation of automotive application, diagnosis results can be observed for PEFC degradation cases. This research contributes to the areas of reliability analysis and health management of PEFC fuel cells. Established PEFC models have been updated to represent more accurately an application PEFC. The fuzzy logic knowledge-based diagnostic is the greatest novel contribution, with no examples of this application in the literature.

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