A numerical model of a solid oxide fuel cell electrode is presented. Using an already established algorithm for dropping spheres as a base, alterations are made to the algorithm to increase the realism of the model. Two changes are analyzed in detail: the ability to drop pore former particles, and the use of pre-agglomerated solid particles. These changes are characterized by their impact on mean pore size, tortuosity, and effective diffusivity. As pore former volume fraction is increased, so too are mean pore size and tortuosity. A higher mean pore size has a beneficial effect on effective diffusivity due to Knudsen effects, while a higher tortuosity has a detrimental effect on effective diffusivity. The impact of mean pore size and tortuosity on diffusivity generally balances and if the impact of porosity is ignored, pore former volume fraction does not greatly affect effective diffusivity. As pore former particle size is increased, mean pore size and tortuosity also increase. Similarly to before, the effects of mean pore size and tortuosity balance. However, effective diffusivity is shown to decrease slightly with an increasing pore former particle size, suggesting a change in tortuosity has greater impact on diffusivity than a change in mean pore size. For a domain constructed with pre-agglomerated particles, the tortuosity and mean pore size were both noticeably larger than when no pre-agglomerated particles are used. Effective diffusivity was only slightly higher for a domain constructed with pre-agglomerated particles than with no pre-agglomerated particles. It is also shown that the relationship of effective diffusivity with porosity for a domain constructed with pre-agglomerated particles does not fit the correlation proposed by Berson et al. [1] for low porosity structures. A secondary goal of this work is to examine pore size measurement techniques, and present a novel technique that allows the determination of a local pore size, and therefore, a local Knudsen number. Results from the local pore size technique do not match those of the random walk method and so although the novel technique may prove to be a good starting point, it is deemed not yet suitable for use. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2011-06-13 15:30:00.25

Electrochemical capacitors (ECs) have received a significant level of interest for use in the electric utility industry for a variety of potential applications. For example, ECs integrated with a power conversion system can be used to assist the electric utility by providing voltage support, power factor correction, active filtering, and reactive and active power support. A number of electric utility applications have been proposed but, to date, ECs have not been very well characterized for use in these applications. Consequently, there is a need to gain a better understanding of ECs when used in electric utility applications. ECs are attractive for utility applications because they have higher energy density than conventional capacitors and higher power density than batteries. ECs also have higher cycle life than batteries, which results in longer life spans. To better understand the system dynamics when ECs are used for utility applications requires suitable models that can be incorporated into the variety of software programs currently used to create dynamic simulations for the applications, programs such as PSPICE™, MATLAB Simulink™, and PSCAD™. To obtain a relevant simulation with predictive capability, the behavior of the EC on which the model is based must be well defined; this necessitates a thorough understanding of the electrical characteristics of these devices. This paper and the associated research focus on the use of the electrochemical impedance spectroscopy (EIS) to develop nonlinear equivalent circuit models to better understand and characterize symmetric ECs (SECs) for electric utility applications. It also focuses on the development of analytical solutions to better understand SEC efficiency and energy utilization. Representative static synchronous compensator (StatCom) systems, with and with out SECs, were simulated and discussed. The temperature effects on device ionic resistance and capacitance are covered as is the effect of temperature on maximum power transfer to a resistive load. Experimental data showed that the SEC's double-layer capacitance and ionic resistance are voltage dependent. Therefore a voltage-dependent RC network model was developed and validated and the results showed that this type of model mimicked the experimental SEC better than traditional electrical models. Analytical solutions were developed for the efficiency and energy utilization of an SEC. The analytical solutions are a function of operating voltages, constant current, and ionic resistance. The operating voltage method is an important factor in system design because the power conversion interface is typically limited by a voltage window and thus can determine the performance of SECs during charge and discharge. If the operating voltage window is not properly selected the current rating of the system can be reduced thus limiting the SECs performance. / Ph. D.

The thesis is focussed on the performance of the fuel cell and the design of the cell for operation with natural gas and renewable fuels, e.g. biogas or gasified biomass. The performance is one of the important issues for the development and commercialisation of fuel cell stacks. In order to operate fuel cell on renewable fuels, without preceding reforming of the fuel, a high temperature fuel cell is needed, i.e. a solid oxide fuel cell (SOFC) or a molten carbonate fuel cell (MCFC). At present, the latter fuel cell type is much more mature when regarding the technical aspects than is the solid oxide fuel cell. The German company MTU has up to date installed about thirty MCFC plants, mainly in Europe and the USA but also in Japan. Moreover the European Commission has decided that the use of renewable fuels must increase at the expense of fossil fuels. This decision is one step towards a smaller dependence on fossil energy sources and limited emissions of greenhouse gases. The objective of this work is to better understand the factors that influence the cell performance: to determine the kinetic parameters of the hydrogen oxidation and the carbon monoxide oxidation and to get more information about the reaction mechanism, even when dealing with gases of low hydrogen content. The latter is of special importance when operating the cells on biogas or gasified biomass. These fuels also contain higher concentrations of carbon monoxide and carbon dioxide. It was found that the hydrogen mechanism proposed by Jewulski and Suski describes the anode performance even at lower concentrations of hydrogen, i.e. gases corresponding to gasified biomass. Furthermore, the carbon monoxide reaction will only slightly influence the anode performance but if the rate of the shift reaction is small the influence of direct oxidation of carbon monoxide will increase. Experimental investigations have shown that mass transfer limitations in the gas phase exist. By mathematical modelling it was found that the current collector has a larger affect on the concentration gradients than the porous electrode. The concentrations gradients in the current collector are caused by the shift reaction that mainly takes place at the electrode. However, if the gas corresponds to equilibrium at the current collector the profiles will become almost uniform. Furthermore the influence of wetting properties, the pore structure and pore size distribution have also been investigated in this thesis. The outcome of this thesis may be used for electrode development and design, as well as for input to reliable cell and stack models for system simulations. / QC 20101008

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第20364号 / 工博第4301号 / 新制||工||1666(附属図書館) / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 邑瀬 邦明, 教授 杉村 博之, 教授 作花 哲夫 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

Les procédés électrochimiques présentent beaucoup d’avantages dans le domaine du traitement et de la récupération de matière d’effluents industriels. Cependant, dans le cas de solutions diluées en ions métalliques, les électrodes classiques sont fortement limitées par leur efficacité ainsi que par leur taille. Dès lors, les électrodes poreuses, de par leur surface spécifique importante et de par leur structure particulière qui améliore le transport de matière et donc l’efficacité de l’électrode, représentent une alternative très intéressante aux électrodes classiques. Parmi les électrodes poreuses, celles constituées de fibres métalliques semblent les plus prometteuses. L’objectif de ce travail est de donner les relations utiles pour dimensionner une cellule contenant ce type d’électrodes en vue du traitement d’effluents industriels contenant des ions métalliques. Les électrodes étudiées ont été caractérisées par différentes techniques : microscopie électronique, méthode électrochimique, mesure de la perte de charge, conductimétrie, porosimétrie,… Cette caractérisation a permis de connaître la porosité, les surfaces spécifiques (géométrique, dynamique et électrochimique) et la tortuosité des électrodes. Ensuite, le coefficient de transport de matière moyen a été étudié par une nouvelle méthode basée sur la mesure d’un rendement électrochimique. Cette méthode présente l’avantage de pouvoir travailler avec des vitesses de circulation de l’électrolyte compatibles avec celles utilisées industriellement. Pour cela, une cellule d’électrolyse à circulation forcée a été mise au point. Afin de comprendre comment la géométrie d’une électrode poreuse de ce type influence le transport de matière local et la densité de courant et donc l’efficacité de l’électrode, le transport de matière et la densité de courant locale ont été modélisés autour d’un cylindre (représentatif d’une fibre) et validés par des mesures expérimentales. La modélisation s’est ensuite étendue à un réseau de fibres cylindriques représentatif des électrodes poreuses étudiées. Cette modélisation a permis d’obtenir une relation générale liant les nombres de Sherwood, de Reynolds et de Schmidt à des nombres sans dimension caractérisant la géométrie du réseau de fibres. Cette relation donne des résultats concordants avec ceux obtenus expérimentalement pour les électrodes poreuses étudiées. Le volume utile d’une électrode poreuse dépend fortement des conditions expérimentales (concentration de l’électrolyte, vitesse de circulation, intensité du courant appliquée,…) et de la structure de l’électrode (porosité, surface spécifique,…). Ces paramètres influencent la distribution du potentiel et de la densité de courant dans l’électrode. Différents modèles de distribution sont comparés et appliqués aux électrodes poreuses étudiées. Cette distribution de courant influence le colmatage progressif de l’électrode poreuse en cours d’électrolyse. Il s’avère que l’électrode en contrôle diffusionnel (avec un rendement électrochimique faible) optimise la distribution du courant dans l’électrode et, de ce fait, ralenti son colmatage. De plus, travailler avec une solution diluée et une vitesse de circulation de l’électrolyte importante améliore la distribution du courant. Il en est de même si l’électrode poreuse présente une grande porosité et une faible surface spécifique. Ce travail aura donc permis de proposer des relations indispensables pour le dimensionnement d’une cellule à électrodes poreuses (constituées de fibres métalliques) ainsi que les conditions opératoires idéales dans le cas du traitement d’effluents industriels contenant des ions métalliques./ Electrochemical techniques offer many advantages for the prevention of pollution problems in the industrial processes. However, flat electrodes are not ideal to treat dilute solutions containing metallic ions. With their high specific surface and open structure, which enhance mass transfer, porous electrodes are a good alternative for the treatment this kind of effluent. Fibre materials are particularly well suited as material for the production of porous electrodes. The aim of this thesis is to study an electrochemical cell with a porous electrode in order to treat dilute metallic ions solutions and to provide dimensionless equations suited to scale-up the electrode for industrial application. The porous electrodes, used in this thesis, are made of a stainless steel fibre network. The main properties and characteristics of these electrodes are studied by means of several techniques : electron microscopy, electrochemical methods (voltammetry, limiting current density measurerment), conductivity measurement, porosimetry, pressure drop measurement,… The obtained parameters are : porosity, specific surfaces (geometric, dynamic and electrochemical), fibres' diameter, tortuosity and the geometric disposition of the fibres in the electrodes. Mass transfer inside the porous electrodes is studied experimentally by a new developed method, linked to the measurement of the faradic yield as a function of different electrolysis parameters. For these measurements, an experimental electrolysis cell with high electrolyte flow rate has been designed and builds. To understand how the geometry of the porous electrode influences the local and mean mass transfer coefficients and current densities, numerical studies and simulations have been performed. The first type of simulation deals with a single wire (representative of a fibre from the porous electrode). The second type of simulation deals with the integration of individual fibres in a fibre network. A correlation between dimensionless numbers such as Sherwood's, Reynolds' and Schmidt's numbers together with numbers characteristic of the electrode’s geometry has been established for Reynolds’s numbers ranging from 0,02 to 1,4. A good agreement between simulation and experimental measurements of mass transfer is observed. The real effective electrochemical volume of the porous electrode depends on experimental conditions (current, concentration, flow velocity…) and electrode’s geometry (porosity, specific surface,…). These parameters influence the potential and current distribution inside the porous electrode. Several models of current distribution are applied to these electrodes and the theoretical simulations are compared with experimental measures. As a result of these simulations, an electrode under diffusion control with a small faradic yield appears to be the best choice in order to homogenise the current density inside the porous electrodes. Dilute solutions, high flow velocity and electrodes with high porosity improve also the current density penetration inside the electrode. These observations are confirmed by an electrode’s plugging study. In conclusion, this thesis provides mathematical relationships to scale-up a cell with porous electrodes of metallic fibre, and provides guidelines to treat, in an efficient manner industrial effluents containing metallic ions.

Polymer electrolyte fuel cells (PEFC) convert chemicalenergy into electrical energy with higher efficiency thaninternal combustion engines. They are particularly suited fortransportation applications or portable devices owing to theirhigh power density and low operating temperature. The latter ishowever detrimental to the kinetics of electrochemicalreactions and in particular to the reduction of oxygen at thecathode. The latter reaction requires enhancing by the verybest catalyst, today platinum. Even so, the cathode isresponsible for the main loss of voltage in the cell. Moreover,the scarce and expensive nature of platinum craves theoptimisation of its use. The purpose of this thesis was to better understand thefunctioning of the porous cathode in the PEFC. This wasachieved by developing physical models to predict the responseof the cathode to steady-state polarisation, currentinterruption (CI) and electrochemical impedance spectroscopy(EIS), and by comparing these results to experimental ones. Themodels account for the kinetics of the oxygen reduction as wellas for the transport of the reactants throughout the cathode,i.e. diffusion of gases and proton migration. The agglomeratestructure was assumed for the description of the internalstructure of the cathode. The electrochemical experiments wereperformed on electrodes having a surface of 0.5 cm2 using alaboratory fuel cell. The response of the cathode to various electrodecompositions, thickness, oxygen pressure and relative humiditywas experimentally investigated with steady-state polarisation,EIS and CI techniques. It is shown that a content in thecathode of 35-43 wt % of Nafion, the polymer electrolyte, gavethe best performance. Such cathodes display a doubling of theapparent Tafel slope at high current density. In this region,the current is proportional to the cathode thickness and to theoxygen pressure, which, according to the agglomerate model,corresponds to limitation by oxygen diffusion in theagglomerates. The same analysis was made using EIS. Moreover,experimental results showed that the Tafel slope increases fordecreasing relative humidity. For Nafion contents lower than 35wt %, the cathode becomes limited by proton migration too. ForNafion contents larger than 40 wt %, the cathode performance athigh current density decreases again owing to an additionalmass transport. The latter is believed to be oxygen diffusionthroughout the cathode. The activity for oxygen reduction ofcatalysts based on iron acetate adsorbed on a carbon powder andpyrolysed at 900°C in ammonia atmosphere was alsoinvestigated. It was shown that the choice of carbon has atremendous effect. The best catalysts were, on a weight basis,as active as platinum. <b>Keywords:</b>polymer electrolyte fuel cell, cathode, masstransport, porous electrode, modelling, agglomerate model,electrochemical impedance spectroscopy, current interrupt,transient techniques, non-noble catalysts

The proton exchange membrane fuel cell (PEMFC) is a potential electrochemicalpower device for vehicles, auxiliary power units and small-scale power plants. In themembrane electrode assembly (MEA), which is the core of the PEMFC single cell,oxygen in air and hydrogen electrochemically react on separate sides of a membraneand electrical energy is generated. The main challenges of the technology are associatedwith cost and lifetime. To meet these demands, firstly, the component expensesought to be reduced. Secondly, enabling system operation at elevated temperatures,i.e. up to 120 °C, would decrease the complexity of the system and subsequentlyresult in decreased system cost. These aspects and the demand for sufficientlifetime are the strong motives for development of new materials in the field.In this thesis, MEAs based on alternative materials are investigatedwith focus on hydrocarbon proton-conducting polymers, i.e. ionomers, and newcatalyst supports. The materials are evaluated by electrochemical methods, such ascyclic voltammetry, polarisation and impedance measurements; morphological studiesare also undertaken. The choice of ionomers, used in the porous electrodes andmembrane, is crucial in the development of high-performing stable MEAs for dynamicoperating conditions. The MEAs are optimised in terms of electrode compositionand preparation, as these parameters influence the electrode structure andthus the MEA performance. The successfully developed MEAs, based on the hydrocarbonionomer sulfonated polysulfone (sPSU), show promising fuel cell performancein a wide temperature range. Yet, these membranes induce mass-transportlimitations in the electrodes, resulting in deteriorated MEA performance. Further,the structure of the hydrated membranes is examined by nuclear magnetic resonancecryoporometry, revealing a relation between water domain size distributionand mechanical stability of the sPSU membranes. The sPSU electrodes possessproperties similar to those of the Nafion electrode, resulting in high fuel cell performancewhen combined with a high-performing membrane. Also, new catalystsupports are investigated; composite electrodes, in which deposition of platinum(Pt) onto titanium dioxide reduces the direct contact between Pt and carbon, showpromising performance and ex-situ stability. Use of graphitised carbon as catalystsupport improves the electrode stability as revealed by a fuel cell degradation study.The thesis reveals the importance of a precise MEA developmentstrategy, involving a broad methodology for investigating new materials both as integratedMEAs and as separate components. As the MEA components and processesinteract, a holistic approach is required to enable successful design of newMEAs and ultimately development of high-performing low-cost PEMFC systems. / QC 20100922

Polymer electrolyte fuel cells (PEFC) convert the chemically bound energy in a fuel, e.g. hydrogen, directly into electricity by an electrochemical process. Examples of future applications are energy conversion such as combined heat and power generation (CHP), zero emission vehicles (ZEV) and consumer electronics. One of the key components in the PEFC is the membrane / electrode assembly (MEA). Both the membrane and the electrodes consist of proton conducting polymers (ionomers). In the membrane, properties such as gas permeability, high proton conductivity and sufficient mechanical and chemical stability are of crucial importance. In the electrodes, the morphology and electrochemical characteristics are strongly affected by the ionomer content. The primary purpose of the present thesis was to develop experimental techniques and to use them to characterise proton conducting polymers and membranes for PEFC applications electrochemically at, or close to, fuel cell operating conditions. The work presented ranges from polymer synthesis to electrochemical characterisation of the MEA performance. The use of a sulfonated dendritic polymer as the acidic component in proton conducting membranes was demonstrated. Proton conducting membranes were prepared by chemical cross-linking or in conjunction with a basic functionalised polymer, PSU-pyridine, to produce acid-base blend membranes. In order to study gas permeability a new in-situ method based on cylindrical microelectrodes was developed. An advantage of this method is that the measurements can be carried out at close to real fuel cell operating conditions, at elevated temperature and a wide range of relative humidities. The durability testing of membranes for use in a polymer electrolyte fuel cell (PEFC) has been studied in situ by a combination of galvanostatic steady-state and electrochemical impedance measurements (EIS). Long-term experiments have been compared to fast ex situ testing in 3 % H2O2 solution. For the direct assessment of membrane degradation, micro-Raman spectroscopy and determination of ion exchange capacity (IEC) have been used. PVDF-based membranes, radiation grafted with styrene and sulfonated, were used as model membranes. The influence of ionomer content on the structure and electrochemical characteristics of Nafion-based PEFC cathodes was also demonstrated. The electrodes were thoroughly investigated using various materials and electrochemical characterisation techniques. Electrodes having medium Nafion contents (35&lt;x&lt;45 wt %) showed the best performance. The mass-transport limitation was essentially due to O2 diffusion in the agglomerates. The performance of cathodes with low Nafion content (&lt;30 wt %) is limited by poor kinetics owing to incomplete wetting of platinum (Pt) by Nafion, by proton migration throughout the cathode as well as by O2 diffusion in the agglomerates. At large Nafion content (&gt;45 wt %), the cathode becomes limited by diffusion of O2 both in the agglomerates and throughout the cathode. Furthermore, models for the membrane coupled with kinetics for the hydrogen electrode, including water concentration dependence, were developed. The models were experimentally validated using a new reference electrode approach. The membrane, as well as the hydrogen anode and cathode characteristics, was studied experimentally using steady-state measurements, current interrupt and EIS. Data obtained with the experiments were in good agreement with the modelled results. / QC 20101014

<p>Polymer electrolyte fuel cells (PEFC) convert chemicalenergy into electrical energy with higher efficiency thaninternal combustion engines. They are particularly suited fortransportation applications or portable devices owing to theirhigh power density and low operating temperature. The latter ishowever detrimental to the kinetics of electrochemicalreactions and in particular to the reduction of oxygen at thecathode. The latter reaction requires enhancing by the verybest catalyst, today platinum. Even so, the cathode isresponsible for the main loss of voltage in the cell. Moreover,the scarce and expensive nature of platinum craves theoptimisation of its use.</p><p>The purpose of this thesis was to better understand thefunctioning of the porous cathode in the PEFC. This wasachieved by developing physical models to predict the responseof the cathode to steady-state polarisation, currentinterruption (CI) and electrochemical impedance spectroscopy(EIS), and by comparing these results to experimental ones. Themodels account for the kinetics of the oxygen reduction as wellas for the transport of the reactants throughout the cathode,i.e. diffusion of gases and proton migration. The agglomeratestructure was assumed for the description of the internalstructure of the cathode. The electrochemical experiments wereperformed on electrodes having a surface of 0.5 cm2 using alaboratory fuel cell.</p><p>The response of the cathode to various electrodecompositions, thickness, oxygen pressure and relative humiditywas experimentally investigated with steady-state polarisation,EIS and CI techniques. It is shown that a content in thecathode of 35-43 wt % of Nafion, the polymer electrolyte, gavethe best performance. Such cathodes display a doubling of theapparent Tafel slope at high current density. In this region,the current is proportional to the cathode thickness and to theoxygen pressure, which, according to the agglomerate model,corresponds to limitation by oxygen diffusion in theagglomerates. The same analysis was made using EIS. Moreover,experimental results showed that the Tafel slope increases fordecreasing relative humidity. For Nafion contents lower than 35wt %, the cathode becomes limited by proton migration too. ForNafion contents larger than 40 wt %, the cathode performance athigh current density decreases again owing to an additionalmass transport. The latter is believed to be oxygen diffusionthroughout the cathode. The activity for oxygen reduction ofcatalysts based on iron acetate adsorbed on a carbon powder andpyrolysed at 900°C in ammonia atmosphere was alsoinvestigated. It was shown that the choice of carbon has atremendous effect. The best catalysts were, on a weight basis,as active as platinum.</p><p><b>Keywords:</b>polymer electrolyte fuel cell, cathode, masstransport, porous electrode, modelling, agglomerate model,electrochemical impedance spectroscopy, current interrupt,transient techniques, non-noble catalysts</p>

<p>Polymer electrolyte fuel cells (PEFC) convert the chemically bound energy in a fuel, e.g. hydrogen, directly into electricity by an electrochemical process. Examples of future applications are energy conversion such as combined heat and power generation (CHP), zero emission vehicles (ZEV) and consumer electronics. One of the key components in the PEFC is the membrane / electrode assembly (MEA). Both the membrane and the electrodes consist of proton conducting polymers (ionomers). In the membrane, properties such as gas permeability, high proton conductivity and sufficient mechanical and chemical stability are of crucial importance. In the electrodes, the morphology and electrochemical characteristics are strongly affected by the ionomer content. The primary purpose of the present thesis was to develop experimental techniques and to use them to characterise proton conducting polymers and membranes for PEFC applications electrochemically at, or close to, fuel cell operating conditions. The work presented ranges from polymer synthesis to electrochemical characterisation of the MEA performance.</p><p>The use of a sulfonated dendritic polymer as the acidic component in proton conducting membranes was demonstrated. Proton conducting membranes were prepared by chemical cross-linking or in conjunction with a basic functionalised polymer, PSU-pyridine, to produce acid-base blend membranes. In order to study gas permeability a new in-situ method based on cylindrical microelectrodes was developed. An advantage of this method is that the measurements can be carried out at close to real fuel cell operating conditions, at elevated temperature and a wide range of relative humidities. The durability testing of membranes for use in a polymer electrolyte fuel cell (PEFC) has been studied in situ by a combination of galvanostatic steady-state and electrochemical impedance measurements (EIS). Long-term experiments have been compared to fast ex situ testing in 3 % H2O2 solution. For the direct assessment of membrane degradation, micro-Raman spectroscopy and determination of ion exchange capacity (IEC) have been used. PVDF-based membranes, radiation grafted with styrene and sulfonated, were used as model membranes. The influence of ionomer content on the structure and electrochemical characteristics of Nafion-based PEFC cathodes was also demonstrated. The electrodes were thoroughly investigated using various materials and electrochemical characterisation techniques. Electrodes having medium Nafion contents (35<x<45 wt %) showed the best performance. The mass-transport limitation was essentially due to O2 diffusion in the agglomerates. The performance of cathodes with low Nafion content (<30 wt %) is limited by poor kinetics owing to incomplete wetting of platinum (Pt) by Nafion, by proton migration throughout the cathode as well as by O2 diffusion in the agglomerates. At large Nafion content (>45 wt %), the cathode becomes limited by diffusion of O2 both in the agglomerates and throughout the cathode. Furthermore, models for the membrane coupled with kinetics for the hydrogen electrode, including water concentration dependence, were developed. The models were experimentally validated using a new reference electrode approach. The membrane, as well as the hydrogen anode and cathode characteristics, was studied experimentally using steady-state measurements, current interrupt and EIS. Data obtained with the experiments were in good agreement with the modelled results. Keywords: polymer electrolyte fuel cell, proton conducting membrane, porous electrode, gas permeability, degradation, water transport</p>