The mathematical analysis has been developed by using finite volume method, experimental data from literatures, and solving numerically to predict solid oxide fuel cell performances with different operating conditions and different material properties. The in-house program presents flow fields, temperature distributions, and performance predictions of typical solid oxide fuel cells operating at different temperatures, 1000 C, 800 C, 600 C, and 500 C, and different electrolyte materials, Yttria-Stabilized zirconia (YSZ) and Gadolinia-doped ceria (CGO). From performance predictions show that the performance of an anode-supported planar SOFC is better than that of an electrolyte-supported planar SOFC for the same material used, same electrode electrochemical considerations, and same operating conditions. The anode-supported solid oxide fuel cells can be used to give the high power density in the higher current density range than the electrolyte-supported solid oxide fuel cells. Even though the electrolyte-supported solid oxide fuel cells give the lower power density and can operate in the lower current density range but they can be used as a small power generator which is portable and provide low power. Furthermore, it is shown that the effect of the electrolyte materials plays important roles to the performance predictions. This should be noted that performance comparisons are obtained by using the same electrode materials. The YSZ-electrolyte solid oxide fuel cells in this work show higher performance than the CGO-electrolyte solid oxide fuel cells when SOFCs operate above 756 C. On the other hand, when CGO based SOFCs operate under 756 C, they shows higher performance than YSZ based SOFCs because the conductivity values of CGO are higher than that of YSZ temperatures lower than 756 C. Since the CGO conductivity in this work is high and the effects of different electrode materials, they can be implied that conductivity values of electrolyte and electrode materials have to be improved.

Reducing fabrication and operation costs while maintaining high performance is a major consideration for the design of a new generation of solid-state ionic devices such as fuel cells, batteries, and sensors. The objective of this research is to fabricate nanostructured materials for energy storage and conversion, particularly porous electrodes with nanostructured features for solid oxide fuel cells (SOFCs) and high surface area films for gas sensing using a combustion CVD process. This research started with the evaluation of the most important deposition parameters: deposition temperature, deposition time, precursor concentration, and substrate. With the optimum deposition parameters, highly porous and nanostructured electrodes for low-temperature SOFCs have been then fabricated. Further, nanostructured and functionally graded La0.8Sr0.2MnO2-La0.8SrCoO3-Gd0.1Ce0.9O2 composite cathodes were fabricated on YSZ electrolyte supports. Extremely low interfacial polarization resistances (i.e. 0.43 Wcm2 at 700¡ãC) and high power densities (i.e. 481 mW/cm2 at 800¡ãC) were generated at operating temperature range of 600¡ãC-850¡ãC. The original combustion CVD process is modified to directly employ solid ceramic powder instead of clear solution for fabrication of porous electrodes for solid oxide fuel cells. Solid particles of SOFC electrode materials suspended in an organic solvent were burned in a combustion flame, depositing a porous cathode on an anode supported electrolyte. Combustion CVD was also employed to fabricate highly porous and nanostructured SnO2 thin film gas sensors with Pt interdigitated electrodes. The as-prepared SnO2 gas sensors were tested for ethanol vapor sensing behavior in the temperature range of 200-500¡ãC and showed excellent sensitivity, selectivity, and speed of response. Moreover, several novel nanostructures were synthesized using a combustion CVD process, including SnO2 nanotubes with square-shaped or rectangular cross sections, well-aligned ZnO nanorods, and two-dimensional ZnO flakes. Solid-state gas sensors based on single piece of these nanostructures demonstrated superior gas sensing performances. These size-tunable nanostructures could be the building blocks of or a template for fabrication of functional devices. In summary, this research has developed new ways for fabrication of high-performance solid-state ionic devices and has helped generating fundamental understanding of the correlation between processing conditions, microstructure, and properties of the synthesized structures.

Planar solid oxide fuel cells (SOFCs) were designed, fabricated and characterized in order to develop a (1) cost-effective method for fabrication of thin electrolyte layers, (2) hermetic sealing and (3) stable interconnects. Electrophoretic deposition (EPD) was discovered to be an excellent method for fabricating dense electrolyte layers of about 5m thick on porous non-conducting substrates. The EPD process was thoroughly studied from proof-of-concept to statistical reproducibility, deposition mechanism, modeling and process optimization. Deposition on non-conducting substrates was found to follow many of the same fundamental trends as that on conductive substrates except for the voltage efficiency and detailed charge transfer mechanism. Eventually, the process was optimized such that an SOFC was fabricated that achieved 1.1W/cm2 at 850C. Further, a novel sealless planar SOFC was designed that incorporates a hermetic interface between the electrolyte and interconnect similar to tubular and honeycomb designs. The hermetic interface successfully acted as a blocking electrode under DC polarization, indicating its potential to act as a sealant. Leakage rates across the interface were 0.027sccm at 750c, similar to polycrystalline mica seals. Through a process of tape casting and lamination, a two-cell stack without sealant was fabricated and achieved a power density of 75mW/cm2 at 750C. Finally, the degradation rate of silver and silver-based interconnects was studied under static and dual-atmosphere conditions. Corrosion of silver grain boundaries along with sublimation losses results in the formation of large pores, resulting in up to 30 of anode oxidation after 8hrs testing at 750c. Further stability studies indicated that silver-based interconnects would be better suited for applications at operating temperatures less than 650C.

Thesis (M.S.)--Engineering, Georgia Institute of Technology, 2004. / McDowell, David, Committee Chair; Neu, Richard, Committee Member; Lee, Jim, Committee Member; Cochran, Joe, Committee Member. Includes bibliographical references (leaves 159-162).

Thesis (M.S.)--West Virginia University, 2003. / Title from document title page. Document formatted into pages; contains xii, 100 p. : ill. (some col.). Vita. Includes abstract. Includes bibliographical references (p. 85-90).

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Solid oxide fuel cells (SOFCs) are electrochemical devices that can convert a variety of fuels directly into electricity. Their commercialization requires efficient operation of its components. The sluggish kinetics for the oxygen reduction reaction (ORR) at the SOFC cathode contributes to the loss in the fuel cell efficiency. In this work, the ORR was investigated for the strontium-doped lanthanum manganite cathode (LSM) and yttria-stabilized zirconia electrolyte (YSZ) system. A combined mathematical modelling and experimental framework was developed to estimate, for the first time, the kinetics of the elementary processes of the ORR for porous LSM cathodes. The kinetics of each process was then analyzed to identify the contribution to the cathode resistance. The steady state and impedance response for polarized and unpolarized LSM cathodes was collected over a temperature range between 750C and 850C and two different oxygen partial pressure (pO2) ranges: (i) between 0.0001atm and 0.001atm, where LSM is considered to be stoichiometric with respect to oxygen and (ii) between 0.01atm and 0.21atm, where LSM is considered to be superstoichiometric with respect to oxygen. A mathematical model was developed to analyze both the steady state and impedance data. Two pathways for the ORR were considered: one where oxygen is transported in the gas phase and one where oxygen is transported along the surface of the LSM cathode. Rate constants, transport coefficients and their respective activation energies were obtained for the adsorption/desorption, surface diffusion and charge transfer processes. The experimental results indicated different polarization behavior between low and high pO2. It is hypothesized that the concentration of cation vacancies on the LSM surface changes with both pO2 and extent of polarization and that cation vacancies on the LSM surface can promote the ORR. Modelling results at low pO2 suggested that the adsorption reaction was slow and that thermodynamic limitations resulting in low equilibrium oxygen surface coverage can play an important role at both low and high polarizations. Modelling in high pO2 was complicated by the nature of the LSM surface in these conditions and suggests an electrochemical reaction at the gas/LSM interface and the transport of charged adsorbed oxygen atoms. / Thesis (Ph.D, Chemical Engineering) -- Queen's University, 2009-12-31 11:53:23.535

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