Solid oxide fuel cells (SOFCs) have attracted much interest as the most efficient electrochemical device to directly convert chemical energy to usable electrical energy. The porous Ni-YSZ anode known as the state-of-the-art cermet anode material is found to show serious degradation when using hydrocarbon as fuel due to carbon deposition, sulphur poisoning, and nickel sintering. In order to overcome these problems, doped strontium titanate has been investigated as a potential anode material due to its high electronic conductivity and stability in reducing atmosphere. In this work, A-site deficient strontium titanate co-doped with lanthanum and calcium, La₀.₂Sr₀.₂₅Ca₀.₄₅TiO₃ (LSCT[sub](A-)), was examined. Flat multilayer ceramics have been produced using the aqueous tape casting technique by controlling the sintering behaviour of LSCT[sub](A-), resulting in a 450µm thick porous LSCT[sub](A-) scaffold with a well adhered 40µm dense YSZ electrolyte. Impregnation of CeO₂ and Ni results in a maximum power density of 0.96Wcm⁻² at 800°C, higher than those of without impregnation (0.124Wcm⁻²) and with impregnation of Ni alone (0.37Wcm⁻²). The addition of catalysts into LSCT[sub](A-) anode significantly reduces the polarization resistance of the cells, suggesting an insufficient electrocatalytic activity of the LSCT[sub](A-) backbone for hydrogen oxidation, but LSCT[sub](A-) can provide the electronic conductivity required for anode. Later, the cells with the configuration of LSCT[sub](A-)/YSZ/LSCF-YSZ were prepared by the organic tape casting and impregnation techniques with only 300-m thick anode as support. The effects of metallic catalysts in the anode supports on the initial performance and stability in humidified hydrogen were discussed. The nickel and iron impregnated LSCT[sub](A-) cell exhibits a maximum powder density of 272mW/cm² at 700°C, much larger than 43mW/cm² for the cell without impregnation and 112mW/cm² for the cell with nickel impregnation. Simultaneously, the bimetal Ni-Fe impregnates have significantly reduced the degradation rates in humidified hydrogen (3% H₂O) at 700°C. The enhancement from impregnation of the bi-metal can possibly be the result of the presence of ionic conducting Wustite Fe₁₋ₓO that resides underneath the Ni-Fe metallic particles and better microstructure. Third, in order to improve the ionic conductivity of the anode support and increase the effective TPBs, ionic conducting ceria was impregnated into the LSCT[sub](A-) anode, along with the metallic catalysts. The CeO₂-LSCT[sub](A-) cell shows a poor performance upon operation in hydrogen atmosphere containing 3% H₂O; and with addition of metallic catalysts, the cell performance increases drastically by almost three-fold. However, the infiltrated Ni particles on the top of ceria layer cause the deposition of carbon filament leading to cell cracking when exposure to humidified methane (3% H₂O). No such behaviour was observed on the CeO₂-NiFe impregnated anode. The microstructure images of the impregnated anodes at different times during stability testing demonstrate that the grain growth of catalysts, the interaction between the anode backbone and infiltrates, and the spalling of the agglomerated catalysts are the main reasons for the performance degradation. Fourth, the YSZ-LSCT[sub](A-) composites including the YSZ contents of 5-80wt.% were investigated to determine the percolation threshold concentration of YSZ to achieve electronic and ionic conducting pathways when using the composite as SOFC anode backbone. The microstructure and dilatometric curves show that when the YSZ content is below 30%, the milled sample has a lower shrinkage than the unmilled one due to the blocking effect from the well distributed YSZ grains within LSCT[sub](A-) bulk. However, at the YSZ above 30% where two phases start to form the individual and interconnected bulk, the composites without ball milling process show a lower densification. The impact of YSZ concentration and ball milling process on the electrical properties of the composites reveals that the percolation threshold concentration is not only dependant on the actual concentration, but also related to the local arrangement of two phases. In Napier University, the electroless nickel-ceramic co-depositon process was investigated as a manufacturing technique for the anodes of planar SOFCs, which entails reduced costs and reduced high-temperature induced defects, compared with conventional fabrication techniques. The Ni-YSZ anodes prepared by the electroless co-deposition technique without the addition of surfactant adhere well to the YSZ electrolyte before and after testing at 800°C in humidified hydrogen. Ni-YSZ anodes co-deposited with pore-forming starch showed twice the maximum power density compared with those without the starch. It has therefore been demonstrated that a porous Ni-YSZ cermet structure was successfully manufactured by means of an electroless plating technique incorporating pore formers followed by firing at 450°C in air. Although the use of surfactant (CTAB) increases the plating thickness, it induces the formation of a Ni-rich layer on the electrolyte/anode interface, leading to the delamination of anode most likely due to the mismatched TECs with the adjacent YSZ electrolyte.

Les piles à combustible sont des systèmes qui permettent de convertir directement de l'énergie chimique en énergie électrique. La structure physique d'une pile à combustible est composée d'une cathode et d'une anode poreuses séparées par un électrolyte dense. Les piles à combustible à oxydes solides (Solid Oxide Fuel Cell (SOFC))offrent une alternative intéressante pour la production d'énergie et une certaine polyvalence dans leur utilisation. Les recherches actuelles se focalisent sur l'abaissement de la température de fonctionnement de ce type de pile (500-700°C) pour augmenter leur durée de vie, diminuer les coûts de fabrication et les dégradations aux interfaces. Afin de compenser ces problèmes, la recherche tend vers des matériaux présentant de meilleures propriétés électrochimiques ou en modifiant la microstructure de la cathode pour améliorer le transfert de masse et le transfert de charge. La cathode est une couche très importante dans la pile SOFC car elle présente une résistance de la polarisation dont la réduction constitue un défi important à traiter. Dans une première partie de ce travail de thèse nous avons développé une méthode pour permettre d'améliorer les propriétés électrochimiques de cathodes de manganite de lanthane dopée au strontium (LSM). La seconde partie a été consacrée à l'élaboration et la caractérisation par spectroscopie d'impédance de demi-cellules symétriques de SOFC avec un matériau composite à base de LSM permettant d'améliorer les propriétés électrochimiques des électrodes à des températures comprises entre 600 °C - 700 °C. / Fuel cells are systems that convert chemical energy directly into electrical energy. The physical structure of a fuel cell is composed of a porous cathode and anode separated by a dense electrolyte. Solid Oxide Fuel Cells (SOFC) offer an alternative for power generation and versability in their use. Current research focuses on lowering the operating temperature of this type of fuel cell (500-700°C) to increase their life, reduce manufacturing costs and damageto the interfaces. In order to compensate these problems, research tends towards materials with better electrochemical properties or by modifying the microstructure of the cathode to improve mass transfer and charge transfer. The cathode is a very important layer in the SOFC stack because it has a polarization resistance whose reduction is a major challenge to deal with. In a first part of this thesis work we have developed a method to improve the electochemical properties of strontium doped lanthanum manganite (LSM) cathodes. The second part was devoted to the elaboration and caracterization by impedance spectroscopy of SOFC symmetric half-cells with a LSM-based composite material allowing to improve the electochemical properties of electrodes at temperatures between 600-700 °C.

Single step co-sintering is proposed as a method to minimise the time and cost of fabricating solid oxide fuel cells (SOFCs). Such a methodology is attractive but challenging due to the differing sintering behaviours and thermal mismatch of the constituent materials of the anode, cathode and electrolyte in solid oxide fuel cells. As a result it is likely that compromises are made for one layer with respect to optimising another. The single chamber solid oxide fuel cell (SC-SOFC) has not seen widespread adoption due to poor selectivity and fuel utilisation, but relaxed some of the stringent SOFC requirements such as sealing, and the need for a dense electrolyte layer. Thus, to initiate the study into single step co-sintering, the single chamber SOFC is earmarked as the first candidate. The effect of single step co-sintering on cell performance is also an attractive area to investigate. Therefore, in this study, a new co-sintering process (single step co-sintering) was applied to fabricate three different types (in terms of the supporting structure) of planar SC-SOFCSs (the anode, cathode and electrolyte supported planar cells) and anode supported wavy types of SC-SOFC in order to reduce fabrication cost and time owing to effective fabrication process. In addition, their performances were tested to establish functionality of the sintered specimens as working electrochemical cells as well as to investigate the maximum performance possible with these cells under single chamber conditions. Moreover, it is also aimed to improve the performance of SC-SOFCs by extending TPB (Triple phase boundary) via wavy type. This study presents a single step co-sintering manufacturing process of planar and wavy single chamber solid oxide fuel cells with porous multilayer structures, consisting of NiO-CGO, CGO and CGO-LSCF as anode, electrolyte and cathode respectively. Pressure of 2 MPa, with the temperature at 60˚C for 5 minutes, was deemed optimal for the hot pressing of these layers. The best result of sintering profile was obtained with heating rate of 1˚C min-1 to 500˚C, 2˚C min-1 to 900˚C and 1˚C min-1 to 1200˚C with 1 hour dwelling; the cooling rate was 3˚C min-1. Hence anode supported SC-SOFC (thickness: 200:40:40 µm, thickness ratio: 10:2:2, anode (A): electrolyte (E): cathode (C)) was fabricated via a single co-sintering process, albeit with curvature formation at edges. Its performance was investigated in methane-oxygen mixtures at a temperature of 600˚C. Maximum open circuit voltage (OCV) and power density of the anode supported planar cell were obtained as 0.69 V and 2.83 mW cm-2, respectively, at a fuel-oxygen ratio of 1. Subsequently, anode thickness was increased to 800 µm and electrolyte thickness was reduced 20 µm (thickness ratio of cell 40:1:2) to obtain curvature-free anode-supported SOFCs with the help of a porous alumina cover plate placed on the top of the cell. The highest power density and OCV obtained from this cell was 30.69 mW cm-2 and 0.71 V, respectively, at the same mix ratio. In addition, the maximum residual stresses between cathode end electrolyte layers of anode supported cells after sintering were investigated using the fluorescence spectroscopy technique. The total mean residual stresses along the x-direction of the final anode supported planar cell after sintering were measured to range from -488.688 MPa to -270.781 MPa. Determination of optimum thickness and thickness ratio of the cell with the defined ideal hot pressing and sintering conditions for single step co-sintering were carried out for cathode and electrolyte supported planar cells using similar fabrication processes. Their performance changes with thickness ratio were examined. The results show that the cathode and electrolyte supported planar cells can be obtained successfully via single step co-sintering technique with the help of alumina cover plates, as with the anode supported cell. In addition, an anode supported wavy SC-SOFC was fabricated via single step co-sintering and its performance was also investigated. The maximum power density and OCV from the final curvature free cathode supported planar cell (thickness: 60:20:800 µm, thickness ratio: 3:1:20, A:E:C) was measured to be 1.71 mW cm-2 and 0.20 V, respectively, at a fuel-oxygen ratio of 1.6. Likewise, the maximum OCV and power density were found to be 0.55 V and 29.39 mW cm-2, respectively, at a fuel-oxygen ratio of 2.6, for the final electrolyte supported curvature free planar cell (thickness: 60:300:40 µm, thickness ratio: 3:15:2, A:E:C). Furthermore, a maximum OCV of 0.43 V and power density of 29.7 mW cm-2 were found from the final anode supported wavy cell (thickness: 800:20:40 µm, thickness ratio: 40:1:2, A:E:C) at a fuel-oxygen ratio of 1. In essence, this study can be divided into five chapters. The first chapter addresses the overview of the research background, problem statement, aims and objective of this study as well as that of novelty and impact. In the second chapter, fundamental information is provided regarding SOFCs and SC-SOFCs in terms of working principles, main components including electrodes electrolytes, advantages and disadvantages, types, material used for each cell components, losses in the system, and so forth. Moreover, the second chapter also contains essential sintering information in order to understand how to approach sintering of ceramics or cermet to fabricate SC-SOFCs. The overall methodology of this study is explained in detail in the third chapter while experimental works are described in the chapter 4, chapter 5, chapter 6, chapter 7 and chapter 8. Chapter 5 also contains background for the fluorescence spectroscopy and a modelling technique for residual stress measurement between ceramic layers. The results of experiments with discussion session are also in the same chapter. The last chapter presents conclusions and the possible routes for future works of the study.

Solid Oxide Fuel Cells (SOFCs) are devices that directly convert chemical energy into electrical energy, without proceeding through a Carnot combustion cycle. These devices are based on the usage of solid oxide electrolytes operating at relatively elevated temperatures. Two major hurdles must be overcome in order to decrease the operating temperatures of practical SOFCs. The first relates to reducing ohmic losses within solid electrolytes. The second relates to the need for developing high performance electrodes since electrolyte reaction rates at both anode and cathode are affected detrimentally as operating temperatures fall. This PhD project has focussed on addressing the first hurdle in two innovative ways: 1. the implementation of solid electrolytes with higher ionic conductivity than zirconia, 2. the development of very thin film electrolytes as thick as 5ƒÝm. Several thin films with novel electrode-electrolyte structures were fabricated and evaluated in order to demonstrate the viability of low temperature SOFC operations. Development of such thin films was innovative and challenging to achieve. The approach taken in this work involved fabricating a dense and thin gadolinia doped ceria (10GDC - Gd 10wt%, Ce 90wt%) oxide electrolyte. 10GDC is an electrolyte exhibiting higher conductivities than conventional materials during low temperature operations. A research contribution of this PhD was the demonstration of the deposition of 10GDC thin films using RF magnetron sputtering for the first time. 10GDC thin film electrolytes with thickness in a range between 0.1 to 5ƒÝm were fabricated on 10 yttrium stabilised zirconium (10YSZ) substrates by using a RF magnetron sputterer. The primary parameters controlling 10GDC thin film deposition using this method were explored in order to identify optimal conditions. The fabricated films were subsequently analysed for their morphology, composition and stoichiometry using a variety of methods, including Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray Spectrometry (EDS), optical microscopy, X-ray Photoelectron Spectroscopy (XPS), and X-ray Diffraction (XRD). A preliminary test was conducted in order to examine the function of 10GDC thin film electrolytes together with the cathode and anode substrates at intermediate temperatures (700oC). A complete planar single cell was designed and assembled for this purpose. However, when fully assembled and tested, the cell failed to generate any voltage or current. Consequently, the remainder of the PhD work was focused on systematically exploring the factors contributing to the assembled fuel cell failure. As fabrication failure analysis is seldom reported in the scientific literature, this analysis represents a significant scientific contribution. This analysis proceeded in a series of steps that involved several different methods, including SEM, red dye analysis, surface morphology and cross section analysis of the cell. It was found that pinholes and cracks were present during the fuel cell operating test. Cathode delamination was also found to have occurred during the test operation. This was determined to be due to thermal expansion mismatch between the cathode substrate and the 10GDC electrolyte thin film. A series of suggestions for future research are presented in the conclusion of this work.

This work is focused on a state-of-the-art tubular micro-solid oxide fuel cell (TSOFC), ~3 millimeters in diameter and ~300 microns thick, with Ni/YSZ and LSM/YSZ composite electrodes and a YSZ electrolyte. A 2D axi-symmetric, multi-scale CFD model is developed which includes the fluid flow, mass transfer, and heat transfer within the gas channels and the porous electrodes. The electrochemical reactions are modeled within the volume of the electrodes, enabling the model to account for the extent of the reaction zone. Thermodynamic expressions are developed to estimate the single-electrode reversible heat generation and the single-electrode electromotive force of a non-isothermal electrochemical cell. The isothermal, non-isothermal, and transient models are each validated against the experimental results, and consistent with the physical reality of the TSOFC. A novel approach is used to estimate the kinetic parameters, enabling the simulations to be used as a diagnostic tool. The model is used to gain a thorough insight about the TSOFC. The cathode electrochemical activity and the anode support ohmic loss are identified as the two major performance bottlenecks for this cell. Including radiation is found to be essential for a physically meaningful heat transfer model. The thermoelectric effects on the cell overall electromotive force is found to be negligible. It is found that the anode reaction is always endothermic, while the cathode reaction is always exothermic, and that the temperature gradients across the cell layers are less than 0.05C The cell transient response is found to be fast, and dominated by the thermal transients. Several physical properties used in the model are measured experimentally, indicating that that the correlations used in the literature are not always suitable, especially when new fabrication techniques are used. The conductivity of the anode support was measured to be several orders of magnitude lower than expected and very sensitive to temperature, which explains the lower than expected and occasionally degrading cell performance. A hypothesis is proposed to explain this phenomenon based on the thermal expansion effects which result in the formation and disruption of particle to particle contacts within the composite electrode. / Chemical Engineering

As the demand for green energy and fuel cells grows, more attention is drawn towards Solid Oxide Fuel Cells (SOFCs). Random and complex structure of composite electrodes and underlying electrochemical process has not been completely unveiled yet and further study is required to acquire more understanding. Modeling in this regard plays an important role as it pinpoints key parameters in optimum design of the cell without resorting to costly and uncertain experiments which might even lead to misinterpretations due to random nature of experimental data. The aim of this work is to develop a new rigorous model to study the structure performance relationship of (SOFC) composite electrodes. The work has been conducted in two phases, a two-dimensional continuous approach and three-dimensional discrete model. A new two-dimensional, geometrical model which captures the inhomogeneous nature of the location of electrochemical reactions based on random packing of electronic and ionic conducting particles has been developed. The results show that the concentration of oxygen inside the cathode in the two-dimensional model is not only a function of the electrode depth but also changes along the width of the electrode. Furthermore the effect of composition of the electrode on the length of three phase boundary (TPB) and total polarization resistance has been demonstrated. A parametric study of the effect of the conductivity of ionic conductor and diffusion coefficient on the performance of the electrode has been given. To make a more realistic analysis, a three-dimensional reconstruction of (SOFC) composite electrodes was developed to evaluate the performance and further investigate the effect of microstructure on the performance of electrodes. To enhance connectivity between particles and increase the length of TPB, sintering process is mimicked by enlarging particles to certain degree. Geometrical characteristics such as length of TBP and active contact area as well as porosity can easily be calculated using the current model. Electrochemical process is simulated using resistor-network model and complete Butler-Volmer equation is used to deal with charge-transfer process on TBP. The model shows that TPBs are not uniformly distributed across the electrode and location of TPBs as well as amount of electrochemical reaction is not homogeneous. Effects of particle size, electrode thickness, particle size ratio, electron and ion conductor conductivities and rate of electrochemical reaction on overall electrochemical performance of electrode are investigated. / Chemical Engineering

This thesis is concerned with the thermodynamic analyses of ion and proton-conducting solid oxide fuel cells (SOFC) fed with ammonia and urea as fuels. A multi-level approach was used to determine the feasibility and the performance of the fuel cells. First, the cell-level thermodynamics were examined to capture the effect of various operating parameters on the cell voltage under open-circuit conditions. Second, electrochemical studies were conducted to characterize the cell-level performance under closed-circuit conditions. Third, the fuel cells were individually integrated in a combined-cycle power generation system and parametric studies were performed to assess the overall performance as well as the thermal and exergy efficiencies. The findings of this study showed that the overall performance and efficiency of the ammonia fed SOFC is superior in comparison to that of the urea fed counterpart. In particular, the ammonia fed system combined with proton-conducting SOFC achieved a thermal efficiency as high as 85% and exergy efficiency as high as 75%. The respective efficiencies of the ammonia fed system combined with ion-conducting SOFC were lower by 5-10%. However, the urea fed system combined with ion or proton-conducting SOFC demonstrated much lower performance and efficiencies due to higher thermodynamic irreversibilities. / UOIT

The reduction of manufacturing costs and the facilitation of direct-oxidation of hydrocarbon fuels have been identified as means of promoting the commercialization of the solid oxide fuel cell, a technology that offers both environmental and fuel conservation benefits compared to conventional energy conversion technologies. This research was conducted with the aim of realizing the production of direct-oxidation anodes using atmospheric plasma spraying, which has been identified as a fabrication technique that has the potential to reduce the manufacturing costs of solid oxide fuel cells. This thesis details the rationale behind the selection of the anode compositions (BaO-Ni-YSZ, CeO2-Ni-YSZ, and Sn-Ni-YSZ) and the specifics of the specialized fabrication strategy (SPS-SPPS) that was devised with the aim of realizing microstructures similar to those where the secondary phases (BaO, CeO2, and Sn) coat the surfaces of the primary Ni and YSZ phases. Results of XRD, SEM and EDS analyses are presented.

The reduction of manufacturing costs and the facilitation of direct-oxidation of hydrocarbon fuels have been identified as means of promoting the commercialization of the solid oxide fuel cell, a technology that offers both environmental and fuel conservation benefits compared to conventional energy conversion technologies. This research was conducted with the aim of realizing the production of direct-oxidation anodes using atmospheric plasma spraying, which has been identified as a fabrication technique that has the potential to reduce the manufacturing costs of solid oxide fuel cells. This thesis details the rationale behind the selection of the anode compositions (BaO-Ni-YSZ, CeO2-Ni-YSZ, and Sn-Ni-YSZ) and the specifics of the specialized fabrication strategy (SPS-SPPS) that was devised with the aim of realizing microstructures similar to those where the secondary phases (BaO, CeO2, and Sn) coat the surfaces of the primary Ni and YSZ phases. Results of XRD, SEM and EDS analyses are presented.

The Solid Oxide Fuel Cell (SOFC) is a promising technology for electricity generation. It converts the chemical energy of the fuel gas directly to electricity energy and therefore, very high electrical efficiencies can be achieved. The high operating temperature of the SOFC also provides excellent possibilities for cogeneration applications. In addition to producing power very efficiently, the SOFC has the potential to concentrate CO₂ with a minimum of an overall efficiency loss. Concentration of CO₂ is a desirable feature of a power generation process so that the CO₂ may be subsequently sequestered thus preventing its contribution to global warming. The primary purpose of this research project was to investigate the role of the SOFC technology in power generation processes and explore its potential for CO₂ capture in power plants. <br /><br /> This thesis introduces an AspenPlus^TM SOFC stack model based on the natural gas feed tubular internal reforming SOFC technology. It was developed utilizing existing AspenPlus^TM functions and unit operation models. This SOFC model is able to provide detailed thermodynamic and parametric analysis of the SOFC operation and can easily be extended to study the entire process consisting of the SOFC stack and balance of plant. <br /><br /> Various SOFC-based power generation cycles were studied in this thesis. Various options for concentrating CO₂ in these power generation systems were also investigated and discussed in detail. All the processes simulations were implemented in AspenPlus^TM extending from the developed natural gas feed tubular SOFC stack model. The study shows that the SOFC technology has a promising future not only in generating electricity in high efficiency but also in facilitating CO₂ concentration, but the cost of the proposed processes still need be reduced so SOFCs can become a technical as well as economic feasible solution for power generation.