A. James Clark School of Engineering

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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.

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    TUNING THE STRUCTURE AND CHEMISTRY OF SOLID OXIDE FUEL CELL ELECTRODES FOR HIGH PERFORMANCE AND STABLE OPERATION
    (2021) Horlick, Samuel; Wachsman, Eric D; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Their reliability, fuel-flexibility, and high specific power make solid oxide fuel cells (SOFCs) promising next-generation power conversion devices. These advantages are theoretically attainable, but current material and structural limitations on the electrodes restrict the true potential of SOFCs on a cell level. Furthermore, ceramic processing challenges hinder the large-scale implementation of SOFCs. Here, SOFC electrodes are redesigned to develop the device closer to its theoretical potential. First, a fundamental investigation into the nature of exsolution materials provides a platform for controlling electrocatalyst properties such as: particle size, population, composition, and contact angle on host. Next, this knowledge is used to design a stable and active anode for the first ever exsolution-anode-supported SOFC and the practical limitations of this approach are identified to lead future research routes. In parallel to this study, a new method for synthesizing cheap, effective catalysts is developed to enable long-term SOFC operation with hydrocarbon fuel without sacrificing performance. Additionally, a systematic study identifies oxygen diffusion as the rate limiting step in the high current regime, and when this limitation is removed with improved system and electrode design, world-class power densities are achieved. Finally, a methodical investigation into ceramic processing of full-scale (5x5cm) SOFCs uncovers that cell flatness can be improved by optimizing green-tape compositions, sintering time/rate/temperatures, and top plate selection. Likewise, electrolyte quality depends on the top plate used in sintering and a light-weight YSZ-coated top plate gives the best balance between flatness and electrolyte quality.
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    ELECTROCHEMICAL COMPRESSION WITH ION EXCHANGE MEMBRANES FOR AIR CONDITIONING, REFRIGERATION AND OTHER RELATED APPLICATIONS
    (2017) Tao, Ye; Wang, Chunsheng; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The refrigeration industry in the US are facing two main challenges. First of all, the phase down of HFCs in the future would require industries to seek alternative refrigerants which do not contribute to global warming. Secondly, the mechanical compressor in the small scale cooling system with a large energy impact is reaching its limitation due to heat transfer and manufacturing tolerances. Therefore there is an urgent need to develop a highly efficient compression process that works with environmentally friendly refrigerants. And the electrochemical compressor is developed to meet these requirement based on the following reasons. First of all, the electrochemical compressor can achieve an isothermal compression efficiency of greater than 90%. It also operates without moving parts, lubrication and noise. Most importantly, the compressor works with environmentally friendly refrigerants. In this thesis, three distinct electrochemical compression processes were studied. The first study is focused on modeling a metal hydride heat pump driven by electrochemical hydrogen compressor. The performance of the cooling-generating desorption reactor, the heating-generating absorption reactor, as well as the whole system were demonstrated. The results showed the superior performance of electrochemical hydrogen compressor over mechanical compressor in the system with optimized operating condition and COP. The second study demonstrated the feasibility of electrochemical ammonia compression with hydrogen as a carrier gas. The reaction mechanisms and the compression principle were verified and the compression efficiency was measured to be greater than 90%. The technology can be applied to ammonia vapor compression refrigeration cycle and ammonia storage. The third study is about developing and studying the electrochemical CO2 compression process with oxygen as a carrier gas. The reaction mechanism was verified and compared for both Pt and CaRuO3 electro-catalysts. And the latter was selected due to better CO2 and O2 absorption. The technology can potentially be applied in carbon dioxide transcritical refrigeration cycle and carbon capture. In conclusion, the electrochemical compression is a promising technology with higher compression efficiency and would bring a revolutionary change to the compressor engineering industry and global refrigeration and air conditioning market. It can also be used in fuel storage and separation based on the selective properties of the ion exchange membrane.
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    Development and Validation of an NPSS Model of a Small Turbojet Engine
    (2017) Vannoy, Stephen; Cadou, Christopher P.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Recent studies have shown that integrated gas turbine engine (GT)/solid oxide fuel cell (SOFC) systems for combined propulsion and power on aircraft offer a promising method for more efficient onboard electrical power generation. However, it appears that nobody has actually attempted to construct a hybrid GT/SOFC prototype for combined propulsion and electrical power generation. This thesis contributes to this ambition by developing an experimentally validated thermodynamic model of a small gas turbine (~230 N thrust) platform for a bench-scale GT/SOFC system. The thermodynamic model is implemented in a NASA-developed software environment called Numerical Propulsion System Simulation (NPSS). An indoor test facility was constructed to measure the engine’s performance parameters: thrust, air flow rate, fuel flow rate, engine speed (RPM), and all axial stage stagnation temperatures and pressures. The NPSS model predictions are compared to the measured performance parameters for steady state engine operation.
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    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.
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    UNDERSTANDING DIRECT BOROHYDRIDE - HYDROGEN PEROXIDE FUEL CELL PERFORMANCE
    (2013) Stroman, Richard O'Neil; Jackson, Gregory S; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Direct borohydride fuel cells (DBFCs) generate electrical power by oxidizing aqueous BH4- at the anode and reducing an oxidizer, like aqueous H2O2 for an all-liquid fuel cell, at the cathode. Interest in DBFCs has grown due to high theoretical energy densities of the reactants, yet DBFC technology faces challenges such as side reactions and other processes that reduce cell efficiency and power generation. Relationships linking performance to cell design and operation will benefit from detailed and calibrated cell design models, and this study presents the development and calibration of a 2D, single-cell DBFC model that includes transport in reactant channels and complex charge transfer reactions at each electrode. Initial modeling was performed assuming ideal reactions without undesirable side reactions. Results were valuable for showing how design parameters impact ideal performance limits and DBFC cell voltage (efficiency). Model results showed that concentration boundary layers in the reactant flow channels limit power density and single-pass reactant utilization. Shallower channels and recirculation improve utilization, but at the expense of lower cell voltage and power per unit membrane area. Reactant coulombic efficiency grows with decreasing inlet reactant concentration, reactant flow rate and cell potential, as the relative reaction rates at each electrode shift to favor charge transfer reactions. To incorporate more realistic reaction mechanisms into the model, experiments in a single cell DBFC were performed to guide reaction mechanism selection by showing which processes were important to capture. Kinetic parameters for both electrochemical and critical heterogeneous reactions at each electrode were subsequently fitted to the measurements. Single-cell experiments showed that undesirable side reactions identified by gas production were reduced with lower reactant concentration and higher supporting electrolyte concentration and these results provided the basis for calibrating multi-step kinetic mechanism. Model results with the resulting calibrated mechanism showed that cell thermodynamic efficiency falls with cell voltage while coulombic utilization rises, yielding a maximum overall efficiency operating point. For this DBFC, maximum overall efficiency coincides with maximum power density, suggesting the existence of preferred operating point for a given geometry and operating conditions.