A. James Clark School of Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/1654
The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item Oxygen Exchange Mechanisms on Solid Oxide Fuel Cell Cathodes in the Presence of Gas Phase Contaminants(2016) Pellegrinelli, Christopher; Wachsman, Eric D; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Polarization losses associated with the oxygen reduction reaction (ORR) at the cathode and degradation of cathode materials remain as hurdles for high performance solid oxide fuel cells (SOFC). Rates of degradation depend significantly on the operating temperature and gas conditions, such as the presence of unwanted oxygen-containing compounds, namely H2O and CO2. In this study ORR fundamentals for the common cathode materials, La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) and (La0.8Sr0.2)0.95MnO3±δ (LSM), as well as their composites with Gd0.10Ce0.90O1.95 (GDC) and (Y2O3)0.08(ZrO2)0.92 (YSZ), respectively, are evaluated as a function of operating environment. A combination of electrochemical impedance spectroscopy (EIS) and gas phase oxygen isotope exchange is used to probe the kinetics of heterogeneous gas-solid reactions and electrode performance under a wide range of conditions. The results suggest that CO2 and H2O actively participate in the ORR and that the level of participation and governing mechanisms are dependent on the specific conditions. It was found that CO2 adsorbs readily on the surface of LSCF and leads to significant performance loss, while the affect of CO2 on LSM, an arguably similar material, is minimal. We propose that intrinsic material properties, such as vacancy concentration, will alter the contaminant interactions significantly, leading to specific contaminant-material relationships. The ORR on LSM is compared to that on LSM-YSZ composite, where the triple phase boundary (TPB), gas-electrode-electrolyte interface, plays a vital role. Further, the role of water in both the single phase, and composite materials is explored. A new in operando isotope exchange technique that couples electrochemical polarization with gas phase isotopic transient and steady state results is proposed, and initial results discussed. The development of in operando experiments is crucial to gain a full understanding of electrodes under real operating conditions, where chemical species are being driven by an electrochemical potential. The results contribute to the understanding of gas-solid reaction kinetics on ion conducting catalysts, and provide a basis for future experimental investigations.Item Model Development for Gadolinia-doped Ceria-based Anodes in Solid Oxide Fuel Cells(2014) Wang, Lei; Jackson, Greg S; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Intermediate temperature (500 &ndash 700 °C) solid oxide fuel cells (IT&ndashSOFCs) with gadolinia&ndashdoped ceria (GDC) electrolytes have significant commercial potential due to reduced materials costs for seals and interconnect and improved performance with high oxide&ndashion conductivity at these temperatures. As an SOFC anode component in the reducing anode environments, GDC offers enhanced catalytic activity and tends to suppress carbon deposition in composite Ni/GDC anodes. The current study investigates relevant kinetics on GDC anodes for IT&ndashSOFC applications. Simultaneous electrochemical characterization and X&ndashray photoelectron spectroscopy of thin&ndashfilm Ni/GDC and Au/GDC electrochemical cells provide a basis for understanding pathways for H2 and CO electrochemical oxidation as well as H2O splitting on GDC and GDC composite electrodes. Differences in electrochemical performance of Ni/GDC and Au/GDC electrodes at temperatures below 650 °C reveal limitations of GDC surfaces in promoting electrooxidation under conditions of low polaron (electron) mobility. These results also suggest the role of the metal in promoting hydrogen spillover to facilitate change transfer reactions at the Ni/GDC interface. Variation in OH- concentration at the metal/GDC interface with operating temperature, effective oxygen partial pressure, and electric bias provides valuable insight into the nature of electrochemical and other heterogeneous reactions in IT&ndashSOFC anodes. A detailed kinetic model for the GDC surface reactions and Ni/GDC charge&ndashtransfer reactions of H2 oxidation and H2O electrolysis is developed based on electrochemical characterization and spectroscopic analysis of GDC surface electrochemistry. The thermodynamically consistent kinetic model is able to capture the observed chemical and electrochemical processes on the thin&ndashfilm Ni/GDC electrode. A full three&ndashdimensional IT&ndashSOFC stack model is developed with simplified kinetics to evaluate GDC&ndashbased anode performance with H2 and methane&ndashderived fuels. The stack model explores the effects of operating condition on performance of stacks with GDC electrolytes and Ni/GDC anodes. The parametric study results of stack model provide essential information for optimizing performance of IT&ndashSOFCs stack and guiding IT&ndashSOFC design. Temperature distribution in non&ndashisothermal model result suggests that internal CH4 reforming can be used as an effective thermal management strategy to maintain high current densities and cell voltages and to lower risk to thermo&ndashmechanical degradation.Item Kinetics and morphology of metallocene catalyzed syndiospecific polymerization of styrene in homogeneous and heterogeneous reaction systems(2008-11-21) Han, Joong Jin; Choi, Kyu Y; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Syndiotactic polystyrene (sPS) is a semicrystalline thermoplastic polymer with many advantageous properties such as excellent heat resistance with a high melting point of 270-272oC, strong chemical resistance against acids, bases, oils and water, and low dielectric constant. The relatively fast crystallization rate makes sPS a promising material for a large number of applications in the automotive, electrical and packaging industries. In this study, the kinetics of syndiospecific polymerization of styrene is investigated through experimentation and theoretical modeling using homogeneous and heterogeneous Cp*Ti(OCH3)3/MAO catalysts. During sPS slurry polymerization, the physical phase changes of reaction mixture occur. With an increase in total solid content, sPS slurry undergoes a series of physical changes from clear liquid to a wet cake or paste-like material. A detailed reaction kinetic model based on a two-site kinetic mechanism has been developed to predict the polymerization rate and polymer molecular weight distribution. The monomer partition effect is incorporated into kinetic models to account for the nonlinear dependence of polymerization rate on the bulk phase monomer concentration. Quite satisfactory agreement between the model simulation results and experimental data has been obtained. The morphological development of nascent sPS particles during the polymerization has also been investigated. Most notably, it was found that sPS particles grow with the nanofibrillar morphology with either homogeneous or silica-supported metallocene catalyst. The analysis of nascent morphology of sPS using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray (EDS/EDX) analysis, revealed that there is a strong correlation between the formation of sPS nanofibrillar structure and sPS crystallization. A mechanism for the growth of sPS particles is also proposed based on the experimental observations and analysis. Ultrahigh molecular weight sPS has also been synthesized in silica nanotube reactors (SNTRs) and the morphological characteristics of sPS produced in the nanotube reactors have been analyzed. A new mechanism is proposed for the formation and growth of sPS nanofibrils extruding out from the nanotube reactors. Also, a kinetic analysis is presented to interpret the observed molecular weight enhancement effect that is believed to be caused by the constrained reaction environment inside the nanotubes.