Theses and Dissertations at Montana State University (MSU)
Permanent URI for this collectionhttps://scholarworks.montana.edu/handle/1/733
Browse
2 results
Search Results
Item Investigation of aluminosilicate refractory for solid oxide fuel cell applications(Montana State University - Bozeman, College of Engineering, 2010) Gentile, Paul Steven; Chairperson, Graduate Committee: Stephen W. Sofie; Paolo R. Zafred and Stephen W. Sofie were co-authors of the article, 'Progress in understanding silica transport process and effects in solid oxide fuel cell performance' in the journal 'Proceedings of the ASME eight international fuel cell science, engineering & technology conference in Brooklyn, New York, USA'. Its abstract is contained within this thesis.; Stephen W. Sofie, Camas F. Key, and Richard J. Smith were co-authors of the article, 'Silicon volatility from alumina and aluminosilicates under solid oxide fuel cell operating conditions' in the journal 'International Journal of Applied Ceramic Technology' which is contained within this thesis.; Stephen W. Sofie was a co-author of the article, 'Investigation of aluminosilicate as a solid oxide fuel cell refractory' in the journal 'Journal of Power Sources' which is contained within this thesis.Stationary solid oxide fuel cells (SOFCs) have been demonstrated to provide clean and reliable electricity through electro-chemical conversion of various fuel sources (CH 4 and other light hydrocarbons). To become a competitive conversion technology the costs of SOFCs must be reduced to less than $400/kW. Aluminosilicate represents a potential low cost alternative to high purity alumina for SOFC refractory applications. The objectives of this investigation are to: (1) study changes of aluminosilicate chemistry and morphology under SOFC conditions, (2) identify volatile silicon species released by aluminosilicates, (3) identify the mechanisms of aluminosilicate vapor deposition on SOFC materials, and (4) determine the effects of aluminosilicate vapors on SOFC electrochemical performance. It is shown thermodynamically and empirically that low cost aluminosilicate refractory remains chemically and thermally unstable under SOFC operating conditions between 800°C and 1000°C. Energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) of the aluminosilicate bulk and surface identified increased concentrations of silicon at the surface after exposure to SOFC gases at 1000°C for 100 hours. The presence of water vapor accelerated surface diffusion of silicon, creating a more uniform distribution. Thermodynamic equilibrium modeling showed aluminosilicate remains stable in dry air, but the introduction of water vapor indicative of actual SOFC gas streams creates low temperature (<1000°C) silicon instability due to the release of Si(OH) 4 and SiO(OH) 2. Thermal gravimetric analysis and transpiration studies identified a discrete drop in the rate of silicon volatility before reaching steady state conditions after 100-200 hours. Electron microscopy observed the preferential deposition of vapors released from aluminosilicate on yttria stabilized zirconia (YSZ) over nickel. The adsorbent consisted of alumina rich clusters enclosed in an amorphous siliceous layer. Silicon penetrated the YSZ along grain boundaries, isolating grains in an insulating glassy phase. XPS did not detect spectra shifts or peak broadening associated with formation of new Si-Zr-Y-O phases. SOFC electrochemical performance testing at 800-1000°C attributed rapid degradation (0.1% per hour) of cells exposed to aluminosilicate vapors in the fuel stream predominately to ohmic polarization. EDS identified silicon concentrations above impurity levels at the electrolyte/active anode interface.Item Development of a novel high performance electrolyte supported solid oxide fuel cell(Montana State University - Bozeman, College of Engineering, 2007) Gentile, Paul Steven; Chairperson, Graduate Committee: Stephen W. SofieHigh power solid oxide fuel cell (SOFC) stacks are based on the planar design concept to yield high specific power densities. The key engineering challenges to planar stack reliability and robust operation is attaining low resistance interconnection of individual cells in series and hermetic sealing of interconnects. While stack design and contact paste development is paramount to address this issue, the basic design of the fuel cell introduces limitations. State-of-the-art anode supported cells (ASC) yield high power densities due to low ASR thin electrolytes, however, the asymmetrical design, anode/electrolyte CTE mismatch, and thick support anode yields undesirable cell camber and fuel transport issues. These deficiencies lead to poor interconnect contact, non-optimal sealing surfaces, and poor fuel utilization, which can mitigate the key benefit of the ASC. Conversely, the electrolyte supported cell (ESC) presents a host of advantages from ease of processing, large diameter scale-up potential, mechanical robustness, optimal seal contact surface, thin electrodes, and minimal cell curvature with the key obstacle arising from high cell ASR due to the thick structural electrolyte. MSU has developed a novel cell concept that merges the benefits of the ASC and ESC designs.