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Solid Oxide Fuel Cells (SOFCs) are considered suitable for alternative energy solutions due to advantages such as high efficiency, fuel flexibility, tolerance to impurities, and potential for combined cycle operations. One of the main operating constraints of SOFCs is fuel starvation, which can occur under fluctuating power demands. It leads to voltage loss and detrimental effects on cell integrity and longevity. In addition, reformer based SOFCs require sufficient steam for fuel reforming to avoid carbon deposition and catalyst degradation. Steam to carbon ratio (STCR) is an index indicating availability of the steam in the reformer. This work takes a holistic approach to address the aforementioned concerns in SOFCs, in an attempt to enhance applicability and adaptability of such systems. To this end, we revisit prior investigation on the invariant properties of SOFC systems, that led to prediction of fuel utilization U and STCR in the absence of intrusive and expensive sensing. This work provides further insight into the reasons behind certain SOFC variables being invariant with respect to operating conditions. The work extends the idea of invariant properties to different fuel and reformer types. In SOFCs, transient control is essential for U, especially if the fuel cell is to be operated in a dynamic load-following mode at high fuel utilization. In this research, we formulate a generalized abstraction of this transient control problem. We show that a multi-variable systems approach can be adopted to address this issue in both time and frequency domains, which leads to input shaping. Simulations show the effectiveness of the approach through good disturbance rejection. The work further integrates the aforementioned transient control research with system level control design for SOFC systems hybridized with storage elements. As opposed to earlier works where centralized robust controllers were of interest, here, separate controllers for the fuel cell and storage have been the primary emphasis. Thus, the proposed approach acts as a bridge between existing centralized controls for single fuel cells to decentralized control for power networks consisting of multiple elements. As a first attempt, decentralized control is demonstrated in a SOFC ultra-capacitor hybrid system. The challenge of this approach lies in the absence of direct and explicit communication between individual controllers. The controllers are designed based on a simple, yet effective principle of conservation of energy. Simulations as well as experimental results are presented to demonstrate the validity of these designs.

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<p>At solid oxide fuel cell (SOFC) operating temperatures (650^oC--800^oC), the chromia scale growth on the interconnect surface and chromium poisoning of cathode can lead to performance degradation of the whole cell. A spinel coating can be effective for blocking chromium outward diffusion to overcome this issue. In this thesis, two spinel-forming systems, Zn-Mn-O and Co-Cu-Mn-O were studied to identify a suitable coating.</p> <p>In-situ high temperature XRD was used to identify the phases in the Zn-Mn-O system between 600^oC and 1300^oC. The results showed that cubic spinel phase was stable only at high temperatures (above 1200^oC) and when the temperature decreases, the cubic phase tends to deform to tetragonal structure. In addition, the conductivity results showed low conductivities (below 3 S/cm) at SOFC operating temperature. Thus, the Zn-Mn-O system is not suitable for SOFC interconnect coatings.</p> <p>Another potential coating material analyzed was the Co-Cu-Mn-O system. This system exhibited promising conductivity values. Electrodeposition was used to apply Co-Cu-Mn-O coatings on both ferritic stainless steel and chromium-based alloy (Cr-5Fe) followed by oxidation in air at 800^oC. The spinel coating formed nicely on the stainless steel substrate. However, on the chromium plate, nitride formation, blistering and metal isolation were the common problems that occurred during the oxidation process. In order to improve the quality of coating on the chromium alloy, different heat treatments were explored, such as annealing in reducing atmosphere, oxidation in pure oxygen / mixed gas and decreasing the oxidation temperature. The objective of modifying the heat treatment was to produce adherent, dense coatings.</p> / Master of Applied Science (MASc)

A typical approach to the synthesis/design optimization of energy systems is to only use steady state operation and high efficiency (or low total life cycle cost) at full load as the basis for the synthesis/design. Transient operation as reflected by changes in power demand, shut-down, and start-up are left as secondary tasks to be solved by system and control engineers once the synthesis/design is fixed. However, transient regimes may happen quite often and the system response to them is a critical factor in determining the system's feasibility. Therefore, it is important to consider the system dynamics in the creative process of developing the system. A decomposition approach for dynamic optimization developed and applied to the synthesis/design and operation/control optimization of a solid oxide fuel cell (SOFC) based auxiliary power unit (APU) is the focus of this doctoral work. Called DILGO (Dynamic Iterative Local-Global Optimization), this approach allows for the decomposed optimization of the individual units (components, sub-systems or disciplines), while taking into account the intermediate products and feedbacks which couple all of the units which make up the overall system. The approach was developed to support and enhance current engineering synthesis/design practices by making possible dynamic modular concurrent system optimization. In addition, this approach produces improvements in the initial synthesis/design state at all stages of the process and allows any level of complexity in the unit's modeling. DILGO uses dynamic shadow price rates as a basis for measuring the interaction level between units. The dynamic shadow price rate is a representation of the unit's cost rate variation with respect to variations in the unit's coupling functions. The global convergence properties of DILGO are seen to be dependent on the mathematical behavior of the dynamic shadow price rate. The method converges to a "global" (system-level) optimum provided the dynamic shadow price rates are approximately constant or at least monotonic. This is likely to be the case in energy systems where the coupling functions, which represent intermediate products and feedbacks, tend to have a monotonic behavior with respect to the unit's local contribution to the system's overall objective function. Finally, DILGO is a physical decomposition used to solve system-level as well as unit-level optimization problems. The total system considered here is decomposed into three sub-systems as follows: stack sub-system (SS), fuel processing sub-system (FPS), and the work and air recovery sub-system (WRAS). Mixed discrete, continuous, and dynamic operational decision variables are considered. Detailed thermodynamic, kinetic, geometric, physical, and cost models for the dynamic system are formulated and implemented. All of the sub-systems are modeled using advanced state-of-the-art tools. DILGO is then applied to the dynamic synthesis/design and operation/control optimization of the SOFC based APU using the total life cycle cost as objective function. The entire problem has a total of 120 independent variables, some of which are integer valued and dynamic variables. The solution to the problem requires only 6 DILGO iterations. / Ph. D.

The metallic interconnect of a solid oxide fuel cell (SOFC) contains chromium in order to protect the metal from the corrosive environment in the fuel cell. Unfortunately, the chromium introduces chemical instability in the cathode as it migrates from the interconnect to the pores in the cathode. A model was developed previously in Asinari et al. [1] and Kasula et al [2] to model the flow of particles in a fuel cell electrode. To learn more about the migration of the chromium, the previous code is modified in this thesis work to include the effects of the chromium. The model uses Kinetic Theory to simulate the fuel cell at a mesoscopic scale. The discretized form of the Lattice Boltzmann equation is modified for enhanced performance and for use on a parallel processing system. With the new model, the migration of the chromium in the cathode and the performance degradation of the fuel cell are predicted. / Master of Science

Ammonia has emerged as an attractive potential hydrogen carrier due to its extremely high energy density (hydrogen density), ease of storage and transportation as a liquid, and carbon-free nature. Direct utilization of ammonia in high-temperature solid oxide fuel cells (SOFCs) has been demonstrated over the past decade. Concurrence of in situ endothermic ammonia decomposition and exothermic electrochemical hydrogen oxidation permit efficient heat integration. In this study, the experimental analyses of axial temperature and concentration profiles along the tubular SOFC (t-SOFC) fed directly with ammonia are performed to investigate the coupled ammonia decomposition and hydrogen oxidation reactions as well as the effect of polarization. Fast ammonia decomposition over the Ni catalyst is evident at the inlet of t-SOFC and complete ammonia conversion is confirmed above 600ºC. It is found that direct ammonia-fueled t-SOFC and an equivalent hydrogen-nitrogen fueled t-SOFC provide identical performances. With 100 SCCM of ammonia fuel feed, a maximum power of 12.2 W and fuel utilization of 81% are obtained at 800ºC in a t-SOFC with active area of 32 cm2. The temperature and concentration profiles validate that the efficient heat integration inside ammonia-fueled t-SOFC is feasible if t-SOFC is operated at the temperature of 700ºC and below. The 23-hour performance test and SEM-EDS images of the fresh and used Ni-YSZ cermet surfaces confirm uniform performance and good durability of ammonia t-SOFC.

Os materiais à base de céria (CeO2) e zircônia (ZrO2) estão presentes em diversas aplicações tecnológicas, destacando-se como anodo de células de combustível de óxido sólido (SOFC) e em catálise, tanto para a produção de hidrogênio, como na automotiva (Three-Way Catalysis). A solução sólida ZrxCe1-xO2- é de especial interesse, pois apresenta melhor estabilidade térmica e maior capacidade de armazenamento de oxigênio (OSC), quando comparada com os óxidos não dopados. Os materiais mesoporosos (poros de 2 a 50 nm) possuem elevada área superficial e permeabilidade a gases, características estas importantes para o desempenho das SOFCs e dos processos de catálise. Neste trabalho, zircônia-céria (Zr0,1Ce0,9O2-) mesoporosa foi sintetizada pelo processo sol-gel, utilizando, como precursores, os cloretos inorgânicos (ZrCl4 e CeCl3.7H2O), o copolímero em bloco P123 (PEO20PPO70PEO20) como direcionador de estrutura e o TIPB (tri-isopropil-benzeno) como agente dilatador. A solução passou por tratamento hidrotérmico durante 48h a 80°C, com posterior calcinação a 400°C para a remoção do polímero, resultando no óxido cristalizado. Na análise foram utilizadas as técnicas: difração de raios X em alto ângulo (XRD), espalhamento de raios X a baixo ângulo (SAXS), isotermas de adsorção de nitrogênio (NAI) e microscopia eletrônica de varredura e transmissão (SEM e TEM). Os resultados mostraram que o material possui elevada área superficial (110m2/g), mesoporos de várias dimensões, atingindo valores médios em torno de 30 nm, fase majoritariamente cúbica Fm3m e, em menor proporção, tetragonal P42/nmc. As micrografias revelaram que o óxido está totalmente nano-cristalizado, com os poros tipo fendas e uma mesoporosidade secundária com distribuição de tamanhos menor e mais estreita. Quatro amostras foram sintetizadas com diferentes razões em massa TIPB/P123 (0, 1, 2 e 4), de forma que foi possível verificar um aumento na dimensão dos poros devido à inclusão do dilatador. As demais propriedades estruturais e morfológicas mantiveram-se inalteradas entre todas as amostras, mesmo com diferentes quantidades de TIPB. / The ceria (CeO2) and zirconia (ZrO2) based materials are present in several technological applications, mainly as Solid Oxide Fuel Cells (SOFC) anodes and catalysts, for hydrogen production and automotive converter (Three-Way Catalysis). The solid solution ZrxCe1-xO2- has attracted special attention, since it shows better thermal stability and higher oxygen storage capacity (OSC), if compared to the non-doped oxides. The mesoporous materials (pores of 2 to 50 nm) show high surface area and gas permeability, important properties for SOFCs and catalysts efficiency. In this work, mesoporous ceria-zirconia (Zr0,1Ce0,9O2-) was synthesized by a sol-gel route using inorganic chlorides (ZrCl4 e CeCl3.7H2O) as precursors, block copolymer P123 (PEO20PPO70PEO20) as template and TIPB (tri-isopropyl-benzene) as swelling agent. The solution was submitted to hydrothermal treatment for 48h at 80°C and calcined at 400°C to remove the template, resulting in the crystallized oxide. The characterization was performed by X-ray diffraction at high angles (XRD), small angle X-ray scattering (SAXS), nitrogen adsorption isotherms (NAI) and transmission and scanning electron microscopy (TEM and SEM). The results showed that the material has high surface area (110m2/g), a wide pore size distribution with mean values around 30 nm, predominant cubic phase Fm3m and, in less quantity, tetragonal P42/nmc. The micrographs revealed that the oxide is totally nano-crystallized, having pores with slit shape and a secondary smaller mesoporosity with a narrow size distribution. Four samples were produced with different TIPB/P123 mass rate (0, 1, 2, 4), therefore was possible to verify the pore size expansion due to the swelling addition. The structural and morphological properties remained unchanged, even with different quantities of TIPB.