Unlike polymer electrolyte fuel cells, solid-oxide fuel cells (SOFCs) have the potential to use a wide variety of fuels, including hydrocarbons and gasified coal or different types of ample carbonaceous solids. However, the conventional anode for an SOFC, a composite consisting of nickel and yttria-stabilized-zirconia (YSZ), is highly susceptible to carbon buildup (coking) and deactivation (poisoning) by contaminants commonly encountered in readily available fuels. Further, the low ionic conductivity of the electrolyte and the poor performance of the cathode at lower temperatures require SOFCs to operate at high temperatures (>800°C), thereby increasing costs and reduce system operation life. Thus, in order to make SOFCs fully fuel-flexible, cost-effective power systems, the issues of anode tolerance to coking and sulfur poisoning as well as the slow ionic conduction in the electrolyte and the sluggish kinetics at the cathode need to be addressed. In this thesis, a novel electrolyte was shown to have the highest ionic conductivity below 750°C of all known electrolyte materials for SOFCs applications, which allowed for fabrication of a thin-electrolyte cell with high power output at lower temperatures. The detailed electrochemical analyses of BZCYYb conductor revealed that the conductivities were sensitive to doping and partial pressure of oxygen, hydrogen, and water. When used in combination with Ni as a composite anode (Ni-BZCYYb), it was shown to provide excellent tolerance to coking and sulfur poisoning. Extensive investigations on surfaces of BZCYYb and Ni by Raman Spectroscopy and Scanning Auger Nanoprobe disclosed that its unique ability appears linked to the mixed conductor's enhanced catalytic activity for sulfur oxidation and hydrocarbon cracking/reforming, as well as enhanced multilayer water adsorption capability. In addition, the nanostructured oxide layers on Ni from dispersion of BZCYYb traces during high-temperature calcinations may effectively suppress the formation of carbon from dehydrogenation. Based on the fundamental understanding on surface properties, a new and simple modification strategy was developed to hinder the carbon-induced deactivation of the state-of-the-art Ni-YSZ anode. Compared to the complex Ni-BZCYYb anode, this modified Ni-YSZ anode could be readily adopted in the latest fuel cell systems based on YSZ electrolyte. The much-improved power output and tolerance to coking of the modified Ni-YSZ anode were attributed to the nanostructured BaO/Ni interfaces observed by synchrotron-based X-ray and advanced electron microscopy, which readily adsorbed water and facilitated water-mediated carbon removal reactions. Density functional theory (DFT) calculations predicted that the dissociated OH from H₂O on BaO reacted with C on Ni near the BaO/Ni interface to produce CO and H species, which were then electrochemically oxidized at the triple-phase boundaries of the anode. Also, some new insights into the sulfur poisoning behavior of the Ni-YSZ anode have been revealed. The so-called "second-stage poisoning" commonly reported in the literatures can be avoided by using a new sealant, indicating that this poisoning is unlikely the inherent electrochemical behavior of a Ni-YSZ anode but associated with other complications. Furthermore, a new composite cathode with simultaneous transport of proton, oxygen vacancies and electronic defects was developed for low-temperature SOFCs based on oxide proton conductors. Compared to the conventional oxygen ion-electron conducting cathode, this cathode is very active for oxygen reduction, extending the electrochemically active sites and significantly reducing the cathodic polarization resistance. Towards the end, these findings have great potential to dramatically improve the economical competitiveness and commercial viability of SOFCs that are driven by cost-effective and renewable fuels.

Three cobalt-based oxides operating at the Co(III)/Co(II) redox couple have been investigated as potential cathode materials for the intermediate-temperature solid oxide fuel cells (IT-SOFCs). X-ray absorption spectroscopy measurements confirmed that both the oxygen-deficient perovskite Sr[subscript 0.7]Y[subscript 0.3]CoO[subscript 2.65-delta] (SYCO) and the double-perovskite Ba₂[Co][Bi[subscript x]Sc[subscript 0.2]Co[subscript 1.8-x]][subscript O6-delta] (x = 0.1 and 0.2) (BBSC) contain high-spin Co(III) in the bulk at room temperature and thus avoid the thermally driven spin-state crossover of the Co(III) ions usually observed in other cobalt-containing perovskite oxides. Electrochemical characterizations demonstrated that both cobalt oxides operating on the Co(III)/Co(II) redox couple are equally catalytically active for the oxygen reduction reaction as those operating on the Co(IV)/Co(III) redox couple. With an LSGM electrolyte-supported single test cell and NiO+GDC as anode, the maximum power densities Pmax at 800 ºC reach 927 and 1180 mW·cm⁻² for SYCO and BBSC cathodes, respectively. The oxygen-deficient perovskites Sr[subscript 1-x]R[subscript x]CoO[subscript 3-delta] (R = Eu-Ho, Y, x [approximately equal] 0.3) are identified as a new class of cathode materials for IT-SOFCs in this dissertation. On the other hand, the layered Ba2Co9O14 (BCO) containing the low-spin Co(III) at room temperature undergoes a thermally driven spin-state crossover, which has prevented it from being evaluated as the cathode of IT-SOFCs. This problem was overcome by fabrication of a 50-50 wt.% BCO + SDC (Sm[subscript 0.2]Ce[subscript 0.8]O[subscript 1.9]) composite cathode. The addition of SDC not only improved the adhesion to the electrolyte, but also enhanced the electrocatalytic activity for the oxygen reduction reaction. The composite cathode delivers a nearly stable P[subscript max] of ~450 mW·cm-2 at 800 °C in an LSGM electrolyte-supported single test cell. In addition, the electrochemical lithium intercalation process in the monoclinic Nb12O29 was studied with a Li/Nb₁₂O₂₉ half-cell, and the results showed that it can reversibly incorporate a relatively large amount of Li-ions in the voltage window of 2.5-1.0 V at a slow discharge/charge rate while retaining structural integrity. Compared with that of the bare Nb₁₂O₂₉, samples with carbon coating show an improved rate capability. The lithium insertion mechanism into Nb₁₂O₂₉ has also been discussed in terms of sites available to the lithium ions / text

Solid oxide fuel cells are a promising option for distributed stationary power generation that offers efficiencies ranging from 50% in stand-alone applications to greater than 80% in cogeneration. To advance SOFC technology for widespread market penetration, the SOFC should demonstrate improved cell lifetime and load-following capability. This work seeks to improve lifetime through dynamic analysis of critical lifetime variables and advanced control algorithms that permit load-following while remaining in a safe operating zone based on stress analysis. Control algorithms typically have addressed SOFC lifetime operability objectives using unconstrained, single-input-single-output control algorithms that minimize thermal transients. Existing SOFC controls research has not considered maximum radial thermal gradients or limits on absolute temperatures in the SOFC. In particular, as stress analysis demonstrates, the minimum cell temperature is the primary thermal stress driver in tubular SOFCs. This dissertation presents a dynamic, quasi-two-dimensional model for a high-temperature tubular SOFC combined with ejector and prereformer models. The model captures dynamics of critical thermal stress drivers and is used as the physical plant for closed-loop control simulations. A constrained, MIMO model predictive control algorithm is developed and applied to control the SOFC. Closed-loop control simulation results demonstrate effective load-following, constraint satisfaction for critical lifetime variables, and disturbance rejection. Nonlinear programming is applied to find the optimal SOFC size and steady-state operating conditions to minimize total system costs. / text

Solid oxide fuel cells are considered as one of the main alternatives for future sources of clean energy. To further improve their performance, theoretical methods able to describe the diffusion process in candidate electrolyte materials at finite temperatures are needed. The method of choice for simulating systems at finite temperature is molecular dynamics. However, if the forces are calculated directly from the Schrödinger equation (first-principles molecular dynamics) the computational expense is too high to allow long enough simulations to properly capture the diffusion process in most materials. This thesis introduces a method to deal with this problem using an external force field to speed up the diffusion process in the simulation. The method is applied to study the diffusion of oxygen ions in Sm-doped ceria, which has showed promise in its use as an electrolyte. Good agreement with experimental data is demonstrated, indicating high potential for future applications of the method.

As the demand for clean and renewable energy sources continues to grow, much attention has been given to solid oxide fuel cells (SOFCs) due to their efficiency and low operating temperature. However, the components of SOFCs must still be improved before commercialization can be reached. Of particular interest is the solid electrolyte, which conducts oxygen ions from the cathode to the anode. Samarium-doped ceria (SDC) is the electrolyte of choice in most SOFCs today, due mostly to its high ionic conductivity at low temperatures. However, the underlying principles that contribute to high ionic conductivity in doped ceria remain unknown, and so it is difficult to improve upon the design of SOFCs. This thesis focuses on identifying the atomistic interactions in SDC which contribute to its favourable performance in the fuel cell. Unfortunately, information as basic as the structure of SDC has not yet been found due to the difficulty in experimentally characterizing and computationally modelling the system. For instance, to evaluate 10.3% SDC, which is close to the 11.1% concentration used in fuel cells, one must investigate 194 trillion configurations, due to the numerous ways of arranging the Sm ions and oxygen vacancies in the simulation cell. As an exhaustive search method is clearly unfeasible, we develop a genetic algorithm (GA) to search the vast potential energy surface for the low-energy configurations, which will be most prevalent in the real material. With the GA, we investigate the structure of SDC for the first time at the DFT+U level of theory. Importantly, we find key differences in our results from prior calculations of this system which used less accurate methods, which demonstrate the importance of accurately modelling the system. Overall, our simulation results of the structure of SDCagree with experimental measurements. We identify the structural significance of defects in the doped ceria lattice which contribute to oxygen ion conductivity. Thus, the structure of SDC found in this work provides a basis for developing better solid electrolytes, which is of significant scientific and technological interest. Following the structure search, we perform an investigation of the electronic properties of SDC, to understand more about the material. Notably, we compare our calculated density of states plot to XPS measurements of pure and reduced SDC. This allows us to parameterize the Hubbard (U) term for Sm, which had not yet been done. Importantly, the DFT+U treatment of the Sm ions also allowed us to observe in our simulations the magnetization of SDC, which was found by experiment. Finally, we also study the SDC surface, with an emphasis on its structural similarities to the bulk. Knowledge of the surface structure is important to be able to understand how fuel oxidation occurs in the fuel cell, as many reaction mechanisms occur on the surface of this porous material. The groundwork for such mechanistic studies is provided in this thesis.

Improvements have been made to the fuel cell power module (FCPM) within the SOFC cogeneration simulation code developed under the umbrella of the International Energy Agencies Annex 42 project. The main objective of the improved model developed here is to increase generality and applicability, and to decrease reliance on empirical data. A semi-mechanistic model has been developed to represent a nominal 5 kW tubular SOFC stack power module. This model has been implemented into the building simulation software ESP-r. Results illustrating the FCPM system performance as it relates to the output power required by the building as well as results from a number of different operating scenarios are presented in this study. Actual tests performed on a 5 kW SOFC unit were used to validate the model. The simulation results are found to correlate well with experimental observations. The improved model also allows for manipulation of operational parameters such as fuel utilization, excess air ratio and temperature as well as physical parameters such as cell dimensions and materials. Results from sensitivity and parametric simulations are also presented to demonstrate the enhanced capabilities of the model.

Low cost silicates with apatite-structure (general formula of apatite A10-xM6O26±δ, where A = rare earth or alkaline earth and M= Si, Ge, P, V..) have been proposed recently as promising solid electrolyte materials (oxygen ion conductors) for use at intermediate temperature solid oxide fuel cells (SOFCs). These materials exhibit sufficiently high ionic conductivity (e.g. ~ 0.01 S cm-1 at 700 oC), which is dominated by the interstitial site mechanism and can exceed that of yttria-stabilized-zirconia (YSZ), the solid electrolyte used in state-of-the-art SOFCs. The apatite structure is tolerant to extensive aliovalent doping, which has been applied for improving ionic conductivity. In this work are presented results concerning synthesis, conductivity and catalytic characterization of Fe- and/or Al-doped apatite type lanthanum silicates (ATLS) of the general formula La10-zSi6-x-yAlxFeyO26±δ as well as electrochemical characterization of interfaces of ATLS pellets with perovskite and Ni-based electrodes. The aim was to investigate the properties of these ATLS material, in particular as it concerns their potential use as SOFC components or as catalysts in oxidation reactions. The conductivity of pellets prepared from ATLS powders synthesized via four different methods and having different grain size was measured under air and at different temperatures in the range 600 -850 oC, aiming to identification of the effect of composition (doping), method of synthesis, grain size and pellet sintering conditions. For electrolytes of the same composition, those prepared via mechanochemical activation exhibited the highest conductivity, which was improved with increasing Al- and decreasing Fe-content. In state-of-the-art SOFCs perovskite electrodes are used as cathodes and Ni-based electrodes as anodes, thus electrochemical characterization of perovskite and Ni-based/ATLS interfaces was carried out. As it concerns perovskite/ATLS interfaces, the characterization focused on the study of the open circuit AC impedance characteristics of a La0.8Sr0.2Ni0.4Fe0.6O3-δ/La9.83Si5Al0.75Fe0.25O26±δ interface, at temperatures 600 to 800 oC and oxygen partial pressures ranging from 0.1 to 20 kPa. Under the aforementioned conditions, it was observed that the impedance characteristics of the interface were determined by at least two different processes, corresponding to two partially overlapping depressed arcs in the Nyquist plots. The polarization conductance of the interface was found to increase with increasing temperature as well as with increasing oxygen partial pressure, following a power law dependence. The electrochemical characterization of Ni-based electrodes/ATLS interfaces involved study of the electrochemical characteristics of NiO-apatite cermet electrodes as well as a Ni sputtered electrode interfaced with Al- or Fe-doped apatite electrolytes, under hydrogen atmospheres. The impedance characteristics of these electrodes were found to be determined by up to three different processes, their relative contribution depending on the electrode microstructure, Ni content (as it concerns the cermet electrodes), temperature, hydrogen partial pressure and applied overpotential. Aiming to investigation of potential catalytic properties of ATLS materials the catalytic activity for CO combustion of a series of ATLS powders was studied. For this purpose, two series of apatite-type lanthanum silicates La10-xSi6-y-zAlyFezO27-3x/2-(y+z)/2 (ATLS), undoped or doped with Al and/or Fe, were synthesized via sol-gel and modified dry sol-gel methods and tested as catalysts for CO combustion. The experiments revealed that the ATLS powders were catalytically active for CO combustion above approximately 300 oC, with light-off temperatures T50 (50% conversion of CO) ranging from 505 to 629 oC. The study focused on the effect on catalytic activity of the synthesis method and doping with Al and/or Fe. Non-doped ATLS with stoichiometric structure, namely La10Si6O27 prepared via the sol-gel method, exhibited the highest catalytic activity for CO oxidation among all tested compositions, the comparison being based on the measured catalytic rate (expressed per surface area of the catalyst) under practically differential conditions. Compared to La-Sr-Mn-O and La-Sr-Co-Fe-O perovskite powders, the tested ATLS powders exhibited lower catalytic activity for CO oxidation. / -

abstract: Fuel cells, particularly solid oxide fuel cells (SOFC), are important for the future of greener and more efficient energy sources. Although SOFCs have been in existence for over fifty years, they have not been deployed extensively because they need to be operated at a high temperature (∼1000 °C), are expensive, and have slow response to changes in energy demands. One important need for commercialization of SOFCs is a lowering of their operating temperature, which requires an electrolyte that can operate at lower temperatures. Doped ceria is one such candidate. For this dissertation work I have studied different types of doped ceria to understand the mechanism of oxygen vacancy diffusion through the bulk. Doped ceria is important because they have high ionic conductivities thus making them attractive candidates for the electrolytes of solid oxide fuel cells. In particular, I have studied how the ionic conductivities are improved in these doped materials by studying the oxygen-vacancy formations and migrations. In this dissertation I describe the application of density functional theory (DFT) and Kinetic Lattice Monte Carlo (KLMC) simulations to calculate the vacancy diffusion and ionic conductivities in doped ceria. The dopants used are praseodymium (Pr), gadolinium (Gd), and neodymium (Nd), all belonging to the lanthanide series. The activation energies for vacancy migration between different nearest neighbor (relative to the dopant) positions were calculated using the commercial DFT code VASP (Vienna Ab-initio Simulation Package). These activation energies were then used as inputs to the KLMC code that I co-developed. The KLMC code was run for different temperatures (673 K to 1073 K) and for different dopant concentrations (0 to 40%). These simulations have resulted in the prediction of dopant concentrations for maximum ionic conductivity at a given temperature. / Dissertation/Thesis / Ph.D. Materials Science and Engineering 2011

O dióxido de cério (CeO2), pode apresentar condutividade iônica e eletrônica (condutor misto) em temperaturas relativamente baixas (considerando a faixa de trabalho 1000°C de uma SOFC). Esta característica torna este material promissor para uso em células a combustível de óxido sólido (SOFC ou CCOS) assim como em catalisadores. Vale destacar que em altas temperaturas o dióxido de cério puro é um mau condutor iônico, contudo pode-se obter um aumento significativo com a substituição estrutural do íon cério (Ce+4) por outro íon metálico de menor valência (La+3 e Ca+2). O estudo proposto consiste em sintetizar óxido de cério dopado com lantânio e cálcio com características microestruturais e elétricas adequadas para uso em uma CCOS. Utilizando o método de síntese de combustão foi estudado a influência que o excesso de combustível (sacarose) pode proporcionar nas características finais dos pós cerâmicos. A caracterização dos pós foi realizada pelas técnicas de raios-X (DRX), área superficial especifica (BET), análise termogravimétrica (TGA), Microscopia Eletrônica de: Varredura (MEV) e Transmissão (MET), Microscopia de Calefação (MC) e por fim a análise elétrica por meio da Espectroscopia de Impedância Eletroquímica (EIE). Os principais resultados mostraram que a técnica de síntese por combustão é um método eficiente para obtenção de pós nanoparticulados, bem dispersos e com elevada homogeneidade. Observou-se ainda que a troca do tipo de dopante assim como o teor de combustível utilizado na síntese interfere diretamente nas propriedades microestruturais, físicas e elétricas dos compostos finais a base de céria dopada. As amostras apresentaram comportamento condutor em baixas temperaturas (500°C) o que viabiliza sua utilização como catalizadores e também em CCOS após tratamentos térmicos em atmosferas adequadas para aplicação como eletrodos ou eletrólitos. Os resultados também demonstram que a céria dopada com cálcio tem características que se torna viável a substituição do lantânio para uso em uma CCOS. / The cerium dioxide (CeO2) has ionic and electronic conductivity (mixed conductor) properties at relatively low temperatures (considering a working range of 1000°C for a SOFC). These characteristics make this material appropriate for use as anode in solid oxide fuel cells (SOFC or CCOS). It should be mentioned that pure cerium dioxide is a bad ionic conductor in high temperatures, but we have a significant increase with the structural substitution of the cerium ion (Ce+4) by another metal ion of lower valence (in its crystalline lattice). The proposed study consisted in the synthesis of ceria oxide with lanthanum and/ or calcium with microstructural and electrical characteristics, suitable for use in a CCOS. Using the combustion synthesis, the influence of excess of fuel (sucrose) on the final characteristics of the ceramic powder has been analyzed. The characterization of the powders was realized using X-ray (XRD), specific surface area (BET), Thermogravimetric Analysis (TGA), Scanning Electron Microscopy (SEM), Electron Microscope Transmission (TEM), Microscope Heating (HSM) and Electrochemical Impedance Spectroscopy (EIS). The main results showed that the combustion synthesis technique is an efficient method to obtain nanoparticulate and well dispersed powders with high homogeneity. It was observed that the exchange of the dopant type as well as the fuel content used in the synthesis interferes directly in the microstructural, physical and electrical properties of the final compounds of ceria doped. Therefore, the calcium doped ceria has interesting characteristics for use in a CCOS.

Magister Scientiae - MSc (Chemistry) / The anode catalyst is one of the important parts of the direct alcohol fuel cell (DAFC); it is responsible for the alcohol oxidation reaction (AOR) takes place at the anode side. Pd has been reported to have good alcohol oxidation reactions and good stability in alkaline solution. Better stability and activity has been reported for Pd alloyed catalysts when compared to Pd. Choosing a suitable alcohol also has an effect on the activity and stability of the catalyst. This study investigates the best catalyst with better AOR and the best stability and also looks at the better alcohol to use between glycerol and ethanol for the five in-house catalysts (20% Pd, PdNi, PdNiO, PdMn3O4 and PdMn3O4NiO on multi walled carbon nanotubes) using cyclic voltammetry (CV), linear sweep voltammetry (LSV), electrochemical impedance spectrometry (EIS) and chronoamperometry. HR-TEM and XRD techniques were used to determine the particle size and average particle size, respectively while EDS used to determine elemental composition and ICP was used to determine catalyst loading. It was observed from LSV that PdNiO was the most active catalyst for both ethanol and glycerol oxidation, and it was the most stable in ethanol while PdMn3O4 proved to be the most stable catalyst in glycerol observed using chronoamperometry. The best alcohol in this study was reported to be glycerol having given the highest current densities for all the inhouse catalysts compared to ethanol observed using LSV. From XRD and HR-TEM studies, particle sizes were in the range of 0.97 and 2.69 nm for XRD 3.44 and 7.20 nm for HR-TEM with a little agglomeration for PdMn3O4 and PdMn3O4NiO.