The research into materials for use as cathode materials for solid oxide fuel cells (SOFC) is ongoing, with many different avenues being investigated. Copper based delafossites were studied for cathode side applications in SOFCs, as a novel and comparatively cheap material. The aim was to identify suitable materials with appropriate electrical conductivity, thermal, chemical and mechanical stability in air. Furthermore, understanding the behaviour of the delafossites during the thermal oxidation to spinel and copper oxide would be beneficial to further development of the materials. The structure and properties of the copper based delafossites CuFeO₂, CuAlO₂ and CuCrO₂ were studied, alongside several doped compositions for each parent composition. The electronic conductivity of the CuFeO₂ family was improved by doping fluorine into the structure, with 1 atomic % doping producing ~3.8 S cm⁻¹ at 800 °C. However, as reported in literature the structure is vulnerable to oxidation at higher temperatures. In contrast, CuAlO₂ was stable over the SOFC temperature range, and therefore had appropriate thermal expansion coefficients (TEC) of ~11 x 10⁻⁶ K⁻¹, but relatively low electronic conductivity. CuCrO₂ compositions had good overall TECs, but aliovalent doping of Mg²⁺ improved the conductivity to ~17.1 S cm⁻¹ at 800°C for 2.5 atomic % doped CuCrO₂. Neutron diffraction was utilised to study members of the solid solution CuFe₁₋ₓCrₓO₂ (x = 0, 0.25 and 0.5) during in-situ oxidation at high temperature. Points of positive scattering density were identified within the CuFeO₂ structure, which were attributed to the location of the intercalated oxygen ions before the transformation proceeded. Additionally, the cation distribution between the tetrahedral and octahedral sites within the developing spinel were characterised for x = 0, and partially for the x = 0.25 and 0.5 compositions using complimentary XRD patterns. Finally, magnesium doped CuCrO₂ delafossites were used in several different preliminary symmetrical cells for study using electrochemical impedance spectroscopy (EIS). Pure delafossite inks gave relatively large area specific resistance (ASR) values, 1.29 - 2.69 Ω cm² at 800 °C. It was attempted to improve upon these values through infiltration of CeO₂ and through change in microstructure using composite type inks, without much success. Inks using CuCr₀.₈Fe₀.₂O₂ were also tested as both a single phase electrode and as a composite type electrode. The pure delafossite electrode still had a large ASR value, (~33.4 Ω cm² at 800 °C) while composite electrodes obtained much more respectable ASR values ~0.75 Ω cm² at 800 °C.

Células a combustível de óxido sólido são sistemas capazes de gerar energia elétrica por meio da oxidação de moléculas hidrogenadas. Normalmente os sistemas planares e tubulares, são compostos por quatro constituintes bem definidos: cátodo, ânodo, eletrólito e selante. Este último componente é o foco do presente estudo, sendo que suas principais características são estabilidade química na temperatura de operação da célula, isolamento elétrico e coeficiente de expansão térmica compatível com os outros constituintes, além da viscosidade elevada e resistência química em atmosferas oxidantes e redutoras. Devido à geometria planar e de multicamadas da célula se optou por usar como selante vidros niobofosfatos. A selagem foi realizada a partir de dispersão de pó de vidro em álcool etílico, gerando uma solução viscosa que foi aplicada sobre o substrato. Posteriormente realizou se um tratamento térmico para a consolidação do selamento. Os vidros estudados foram denominados de Nb30, Nb37, Nb40 e Nb44, de acordo com o teor nominal de óxido de nióbio utilizado na composição. O objetivo desse trabalho foi caracterizar, a partir de precursores os selantes a base de vidros niobofosfatos para aplicar em células a combustível de óxido sólido do tipo planar. Foram feitos caracterizações dos pós dos vidros e de pastilhas cristalizadas para determinar os coeficientes de expansão térmica (CET), resistividade elétrica, difração de raio X e microscopia eletrônica de varredura (MEV), além de, caracterizar visualmente sua adesividade, molhabilidade, resistência mecânica em substratos de alumina e em conjunto com os componentes das SOFC, sendo também testados os selantes em operação nas unidades previamente formadas de SOFC (ciclos térmicos). / Solid oxide fuel cells (SOFC) are devices which generate d.c. power by the oxidation of hydrogen molecules. These devices can have a multilayer plane design containing a cathode, an anode, a solid electrolyte, and a sealing material. The sealing, which is the subject of this study, has to be chemically stable at relatively SOFC operational condition in oxi-redox atmospheres, electrical insulator, with a thermal expansion coefficient matching other components, and, in of glass, the viscosity must be relatively high. The aim of the present work is to characterize niobophosphate glasses which will be used as sealant precursors of Solid Oxide Fuel Cell with a plane design. Niobophosphate glasses, named Nb30, Nb37, Nb40, and Nb44 according to the niobium content, were investigated for this purpose. The sealing was performed by mixing glass powder with ethanol which was applied over the substrate. Later, a heat treatment was performed to consolidate the sealing. Glass powder and devitrified glass pellets were characterized by different techniques. The thermal expansion coefficient, electrical resistivity, and the X-rays diffraction pattern were determined for these materials. Scanning electron microscopy was also used to visualize the sealing/ substrate interface, and to evaluate the adhesiveness, wetability, apparent mechanical resistance in alumina substrates and in other SOFC components. The sealants were tested in SOFC, and also submitted to simulating thermal cycles.

Metallic interconnects in intermediate temperature solid oxide fuel cells (IT-SOFC) stacks form Cr2O3 scales on their surface. Such oxide scales can be further oxidized to Cr6+ containing gaseous species that migrate and deposit at the cathode triple phase boundaries, causing significant degradation in the performance of the SOFCs. This phenomenon is termed as ‘Cr-poisoning’. A solution to this problem is the application of coatings on the interconnects that act as a diffusion barrier to Cr migration. Two different Cu/Mn spinel compositions, Cu1.3Mn1.7O4 and CuMn1.8O4, were studied as coating materials. Dense coatings were deposited on both flat plates and meshes by electrophoretic deposition (EPD) followed by subsequent thermo-mechanical or thermal densification steps. At room temperature, Cu1.3Mn1.7O4 coatings were found to have a mixture of CuO and spinel phases, while CuMn1.8O4 coatings were found to have a mixture of Mn3O4 and spinel phases. However, CuMn1.8O4 is a pure spinel phase between 750 °C and 850 °C. After densification processing and high temperature oxidation, a Cr2O3 layer was formed at the coating/alloy interface, which partially reacted with the spinel coatings to form a dense cubic spinel layer of the general composition (Cu,Mn,Cr)3-xO4. In addition, Cr-rich precipitates, formed in the dense layer close to coating/alloy interface. It is believed that these are Cr2O3 precipitates, formed when the solubility of Cr in the spinel phase is reached. Solubility experiments using powders showed that 1 mole of CuMn1.8O4 can effectively getter 1.83 moles of Cr2O3 at 800°C. Electrical conductivity of (Cu,Mn,Cr)3-xO4 was found to be at least two orders of magnitude higher than that of Cr2O3. The coatings acted as an effective Cr getter whose lifetime depends on the oxidation temperature, coating thickness, and the overall porosity in the coating. In-cell electrochemical testing showed that the CuMn1.8O4 coatings on Crofer 22 APU meshes performed significantly better than commercial Cu/Mn spinel coatings. The CuMn1.8O4 coatings gettered Cr effectively for 12 days at 800 ºC, leading to no performance loss of the cell due to Cr-poisoning. Significantly longer lifetime can be achieved at 750 ºC or lower, which is the target operational temperature regime of IT-SOFCs.

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

A series of cermet powders of composition NixCo(1-x)O-YSZ were synthesized for testing as cermet anode materials for SOFCs. The Co is found by powder XRD to become incorporated into the crystal lattice of the NiO, thus forming a true alloyed material. SEM and EDS results show two types of particles upon sintering to 1380oC: small, amorphous particles of YSZ and large, crystalline particles of nickel. The electrochemical oxidation of hydrogen on a cermet anode composed of Ni0.7Co0.3O-YSZ was investigated using a series of many button cells. Through EIS data, cyclic voltammetry data, the exchange current densities for these button cells were determined. Although a relatively large variation was found (expected to be due to microstructural variation) the average values for both methods of measurement is in good agreement in hydrogen. Following reduction in pure hydrogen, the fuel was changed to a mixture with high concentration of H2S. It was found that a concentration of 10 % H2S/H2 produced a sudden change in anode microstructure and resulted in loss of exchange current density. Lowering the amount of H2S in the initial fuel feed, which allowed for a more gradual microstructural change, allowed the cell to eventually function at concentrations in excess of 10 % H2S/H2. It was determined by OCV values in various concentrations of H2S/H2 that hydrogen is the predominant fuel of choice, even if H2S is available. Following electrochemical testing, slow cooling in a 10 % H2S/H2 mixture following produced metal sulfide spheres, as determined by SEM and EDS. Investigation in hydrocarbon, alcohol and biodiesel fuels was then undertaken to test the fuel variability of the given cermet anode material. Methane containing 10 % H2S was found to have increased exchange current density relative to poisoned hydrogen. Ethane and biodiesel experienced no increase in exchange current density, but a lengthening of the functional lifetime of the cell was observed, indicating reduced carbon poisoning. Methanol is a promising oxygen-containing SOFC fuel since it produced exchange current density values larger than hydrogen, and showed no evidence of coke formation by post-mortem SEM. Since oxygen-containing fuels are known to decompose in the gas phase at typical SOFC operating temperatures, the performance in a mixture of various CO/H2 fuels was then investigated. The Ni0.7Co0.3O-YSZ cermet anode gave higher exchange current density values for low ratio of CO/H2 fuels in the range 20/80 and 30/70 compared to pure H2. This is the first example of a Ni-based anode providing higher performance with a CO/H2 mixed fuel than for a pure H2 fuel. Finally, continuous running of a cell with fuel ratio 25/75 CO/H2 for 7 days produced exchange current density values, which were observed to increase significantly above the values for pure H2 during days 1-4 followed by deterioration below the value for hydrogen on subsequent days.

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