Theses and Dissertations from UMD
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Item THE ROLES OF MATERIAL, SURFACE, & MICROSTRUCTURAL EFFECTS IN DEVELOPING CERAMICS FOR ENERGY APPLICATIONS(2022) Ostrovskiy, Yevgeniy; Wachsman, Eric; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Ceramics have a wide variety of applications for energy conversion and other industries because of their unique properties. Conduction of multiple charged species simultaneously enables their use as membranes, electrodes, and more. Perovskites especially, have highly tunable features, and can be modified through doping, surface coating, and microstructure. In this work, each of those approaches was used to improve and/or characterize ceramic components for either proton conducting membranes or solid oxide fuel cells (SOFCs). In the case of membranes, perovskites have limited electronic conductivity, which reduces their ability to permeate hydrogen. Through changing the dopants used in existing perovskite compositions, the electronic conductivity was improved dramatically allowing its use as an n-type conductor. This was achieved by using Pr as a dopant, which introduces electronic conductivity due its multivalent nature. It also has a favorable ionic radius for proton conduction, which is required for hydrogen permeation. In the case of fuel cells, both performance and stability need to be improved for their widespread adoption. The surface chemistry and physical properties of two major cathode materials were evaluated, La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) and Sm0.5Sr0.5CoO3 (SSC). Both are susceptible to the formation of unwanted secondary phases during operation as SOFC cathodes. By studying the surface chemistry of LSCF it was possible to better understand the mechanism of cathode degradation. In the case of LSCF, it was found that electrostatic forces that result from a chemical potential difference between the bulk and surface, promote the segregation of Sr cations from the bulk which is responsible for the degradation. However, the behavior of SSC was more difficult to determine. In SSC, cation segregation was far more dependent on the grain orientation than LSCF and therefore was more difficult to quantify, and the techniques used for improving stability in LSCF were unsuccessful when applied to SSC. Additionally thin films deposited through atomic layer deposition (ALD) were tested as a means of enhancing the performance of LSCF. Due to the challenges of using ALD on porous substrates, the role of variables in the deposition process that can be widely implemented were studied, with a focus on oxygen vacancies. It was found that the choice of oxidizer and the addition of an annealing step can dramatically improve the effectiveness of thing film electrode coatings. Although ALD may not be practical for modifying SOFC electrodes, these are process steps that can be easily implemented by other researchers to improve their existing approaches. Finally, the role of microstructure was addressed as well. Tuning the porosity of SOFC anodes is essential for large scale fabrication of fuel cells and improving their performance and reliability. A microstructure featuring a hierarchal porosity was able to improve the performance of SOFC anodes, especially at lower temperatures and fuel ratios. Improvements in microstructure will allow the fabrication of larger scale SOFCs that are more reliable and mechanically stronger, with minimized performance losses associated with using a thicker anode. The primary scientific merit of this research is demonstrated in the work on cathode degradation and coatings. There the focus was on using a methodology based on a fundamental material property, oxygen vacancies, which are essential to many applications of metal oxides. With this type of approach, it is possible to apply similar techniques to other areas of research involving metal oxides or thin films. The main engineering merit of this research is evaluating the relationship between microstructure, SOFC performance, and large SOFC production. Commercialization of SOFCs requires that they are as effective as possible outside of ideal conditions (pure fuel, high temperatures). Hierarchal porosity has been shown to improve performance under both conditions and can also be applied to cathodes.Item Performance and Enhancement of Solid Oxide Fuel Cell Electrodes Via Surface Modification(2020) Robinson, Ian Alexander; Wachsman, Eric D; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Solid oxide fuel cells (SOFCs) electrochemically convert chemical fuels to usable electricity with high efficiency and can operate on any oxidizable fuel. SOFCs fuel flexibility is accompanied by clean conversion by only converting the fuel to H2O and CO2 without the production of NOx. Additionally, the design of the device allows for a facile integration of carbon capture because the exhaust from the anode and cathode are already separated, allowing for a separated CO2 stream for carbon capture. Technical limitations have prohibited the commercial deployment of SOFCs at an impactful scale and the SOFC market is currently worth <$1 billion. The high operating temperature (T>800 °C) of SOFCs limits possible applications due to high degradation rates within cell components and a high balance of plant costs to use the requisitespecialized high temperature materials. The primary limitation to using to a lower temperature SOFC is the sluggish kinetics of the air electrode or cathode oxygen reduction reaction (ORR) at lower temperatures. This work increases the activity and durability of SOFC electrodes at lower temperatures by utilizing a facile, effective, low cost surface modification technique, defect engineering, and universal cathode scaffold design. Surface modification of SOFC cathodes also prevents the deactivation of the SOFC cathode typically caused by contaminant gasses like CO2 in Sr0.5Sm0.5CoO3-δ (SSC) cathodes. The surface modification technique also shows breakthroughs in the activity of SOFC cathodes SSC and La1-xSrxCo1-yFeyO3-δ (LSCF), allowing the SOFC to operate below 600 °C. The use of an engineered porous functional layer is shown to reduce the electronic leakage current in ceria-based electrolytes. This type of functional layer also increases the overall performance and durability of a SOFC at lower temperatures. Additionally, an approach was developed to deposit any desired cathode electrocatalyst on a universal scaffold to enable low-temperature operation and is compatible with existing cell components. 1 W/cm2 at 550 °C is achieved by utilizing the scaffold infiltration approach and demonstrates that high performance operations at low temperatures is achievable. Finally, the fuel flexibility of metal-supported solid oxide fuel cells (MS-SOFCs) was demonstrated to highlight their potential applications for carbon neutral transportation.Item LATTICE MODIFICATIONS ON SOLID STATE ELECTROLYTES FOR THE OPTIMIZATION OF ION TRANSPORT(2018) Jolley, Adam Garrett; Wachsman, Eric D; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A solid state electrolyte is one of the primary components of both a solid oxide fuel cells (SOFC) and an all-solid-state sodium battery. In both cases, the ionic conductivity of the electrolyte has a major impact on the performance of the electrochemical cell. For SOFCs, the conductivity of traditional electrolytes is not high enough for sufficient performance at intermediate and low temperature operation. Therefore, novel bismuth oxide compositions were developed to achieve higher conductivity. The conductivity of Bi2O3 was improved by reducing the total dopant concentration required to stabilize the highly conductive cubic phase. This strategy lead to the development of a Bi2O3 electrolyte (La7Zr3) with the highest oxygen ion conductivity to date. Unfortunately, at temperatures below 600°C the conductivity of the cubic phase was unstable. Therefore, rhombohedral bismuth oxide was investigated for low temperature SOFC operation due to its stability. For the first time, a dopant concentration less than 10% was used to stabilize the rhombohedral phase of Bi2O3. Furthermore, a novel phase diagram was constructed for the low dopant regime of the rhombohedral phase. Ultimately, the double doped bismuth oxide material (La5.1Y1.4) developed here was among the highest and most stable oxygen ion conductors below 600°C. Performance of an SOFC with a La5.1Y1.4 electrolyte verified that it is a promising material for low temperature SOFCs. A similar strategy of doping an electrolyte material to increase ionic conductivity was carried out on NASICON (Na3Zr2Si2PO12). NASICON is a promising electrolyte for room temperature sodium batteries, but traditionally it does not exhibit high enough conductivity to garner high performance. For the first time, the mechanism driving the phase transition in NASICON was determined and mapped out. Mitigation of the phase transition in the material was established to lower the activation energy barrier for sodium ion transport. Additionally, divalent cations were substituted into the NASICON lattice to generate an increase in sodium ion conductivity. Ultimately the phase and dopant concentration was optimized to deliver a material that is among the best sodium ion conducting ceramics to date (20% Zn-doped NASICON).Item In Operando Mechanistic Studies of Heterogeneous Electrocatalysis on Solid Oxide Electrochemical Cell Materials(2017) Geller, Aaron; Eichhorn, Bryan W; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation details the development and utilization of in operando protocols for observing electrochemical reactions on solid oxide electrochemical cells (SOCs) in order to better understand the fundamental chemistry governing their operation. Two key reactions in SOC processes are studied using ambient pressure X-ray photoelectron spectroscopy (AP-XPS), the oxygen reduction and evolution reactions (ORR and OER). Measurements made on lanthanum strontium manganite (La1-xSrxMnO3±∂, LSM), a standard electrode material show that the surface composition does not match the bulk stoichiometry. Sr extrudes onto the LSM surface in the form of SrO and greater Mn reduction is observed. These phenomena are further augmented by application of a cathodic bias (promoting ORR), while an anodic bias (promoting OER) results in the oxidation of Mn and no significant changes in Sr segregation. Surface potentials on the LSM are measured to locate regions of electrochemical activity when promoting ORR and OER. These measurements yield in operando spectroscopic evidence that all electrochemical activity occurs at the electrode/electrolyte interface and that LSM is more electrocatalytically active toward ORR than OER. We further compare surfaces between a pure LSM material and a composite of LSM and yttria-stabilized zirconia ((ZrO2)1-2x(Y2O3)x, YSZ) in different gaseous environments which approximate standard operating conditions. The LSM/YSZ composite exhibits a larger concentration of surface oxygen vacancies in each environment allowing for greater oxygen reactivity. A method for measuring surface Co oxidation states with XPS is explored. In situ thermal redox studies on cathode material, lanthanum cobaltite (LaCoO3-∂), show a potential correlation between Co reduction and the Auger parameter. An in operando technique for monitoring SOCs with near infrared (NIR) imaging is presented. Ce oxidation states are tracked in an operating SOC using ceria (CeO2-x) electrodes in studies analogous with previous AP-XPS research. However, the NIR experiments take place in fully ambient conditions as opposed to the model, near ambient conditions used in the AP-XPS experiments. Ce reduction is observed within an electrochemically active region commensurate with that found with AP-XPS, simultaneously supporting the use of NIR imaging for in operando studies on these SOCs, and the model AP-XPS experiments previously conducted.Item DURABILITY AND OPTIMIZATION OF SOFC COMPOSITE CATHODES(2016) Painter, Albert Steven; Wachsman, Eric D; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The combination of the conventional cathode material, La0.8Sr0.2MnO3-𝛿 (LSM), and exceptional oxygen ion conducting material, (Er0.2Bi0.8)2O3 (ESB), has shown promise as a potential candidate for low temperature solid oxide fuel cell (LT-SOFC) cathodes. Though the initial performance of this composite is encouraging, the long-term stability of LSM-ESB has yet to be investigated. Here electrochemical impedance spectroscopy (EIS) was used to in situ monitor the durability of LSM-ESB at typical LT-SOFC operation temperatures. The degradation rate as a function of aging time was extracted based on the EIS data. Post analysis suggests that below 600 °C the order-disorder transition of ESB limits the performance due to a decrease in the oxygen incorporation rate. Above 600°C, the formation of secondary phases, identified as Mn-Bi-O, is the major performance degradation mechanism. Furthermore, the relative particle size of the LSM to ESB was optimized to minimize long-term degradation in cathode performance.Item Ceramic Materials Development for Intermediate Temperature Solid Oxide Fuel Cell (IT-SOFC)(2016) Pan, Ke-Ji; Wachsman, Eric D; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Solid oxide fuel cell (SOFC) is an electrochemical device that converts chemical energy into electric power with high efficiency. Traditional SOFC has its disadvantages, such as redox cycling instability and carbon deposition while using hydrocarbon fuels. It is because traditional SOFC uses Ni-cermet as anode. In order to solve these problems, ceramic anode is a good candidate to replace Ni. However, the conductivity of most ceramic anode materials are much lower than Ni metal, and it introduces high ohmic resistance. How to increase the conductivity is a hot topic in this research field. Based on our proposed mechanism, several types of ceramic materials have been developed. Vanadium doped perovskite, Sr1-x/2VxTi1-xO3 (SVT) and Sr0.2Na0.8Nb1-xVxO3 (SNNV), achieved the conductivity as high as 300 S*cm-1 in hydrogen, without any high temperature reduction. GDC electrolyte supported cell was fabricated with Sr0.2Na0.8Nb0.9V0.1O3 and the performance was measured in hydrogen and methane respectively. Due to vanadium’s intrinsic problems, the anode supported cell is not easy. Fe doped double perovskite Sr2CoMoO6 (SFCM) was also developed. By carefully doping Fe, the conductivity was improved over one magnitude, without any vigorous reducing conditions. SFCM anode supported cell was successfully fabricated with GDC as the electrolyte. By impregnating Ni-GDC nano particles into the anode, the cell can be operated at lower temperatures while having higher performance than the traditional Ni-cermet cells. Meanwhile, this SFCM anode supported SOFC has long term stability in the reformate containing methane. During the anode development, cathode improvement caused by a thin Co-GDC layer was observed. By adding this Co-GDC layer between the electrolyte and the cathode, the interfacial resistance decreases due to fast oxygen ion transport. This mechanism was confirmed via isotope exchange. This Co-GDC layer works with multiple kinds of cathodes and the modified cell’s performance is 3 times as the traditional Ni-GDC cell. With this new method, lowering the SOFC operation temperature is feasible.Item Solid Oxide Ionic Materials For Electrochemical Energy Conversion And Storage(2015) Ruth, Ashley Lidie; Wachsman, Eric D; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Solid state ionic materials can be utilized in components of both solid oxide fuel cells and lithium ion batteries. Solid oxide fuel cells (SOFCs) are devices used to convert chemical energy into useful electrical energy. The higher temperatures required to effectively conduct oxygen vacancies is a material limitation that prevents the implementation of this technology in today's society. Our group has developed the novel incorporation of a bilayer electrolyte utilizing the high conductivity properties of the cubic fluorite bismuth oxide material in the low temperature regime at 650 °C and below. This phase is stabilized by single and double doping of Er, Dy-W, Dy-Ce, and Dy-Gd chemistries in this study. Conductivity measurements through electrochemical impedance spectroscopy champion (Bi0.88Dy0.08Gd0.04)2O3 as the most suitable electrolyte for future testing in SOFCs. Using the bilayer system in button type cells, the layer thickness ratio is optimized for highest open circuit voltage. Using neutron diffraction was used to better understand the activation energy change in conductivity in DWSB due to phase transformation that masked oxygen ordering at lower temperatures. Stabilized bismuth oxides are incorporated into a suitable composite cathode via an in-situ nano-scale mixing with La0.8Sr0.2MnO3-δ, improving the oxygen reduction reaction kinetics. Utilizing lessons from ceramic materials synthesis in SOFCs, cathode materials for Li-ion batteries were synthesized. In previous work, LixMn2O4-yClz spinel demonstrated enhanced charge potential and discharge potential while maintaining reversibility. However the original method for synthesis was extremely cumbersome. Using the simple glycine-nitrate reaction, we could fabricate an operating button cell starting from raw powders in less than 8 hours. X-ray diffraction and x-ray fluorescence confirm spinel structure and maintenance of chlorine through ignition respectively. In demonstrating favorable charge/discharge performance and cyclability, we considered the benefits of B-site doping of the spinel. For the first time LixMn2-wFewO4-yClz was also easily synthesized and tested for more than 250 charge-discharge cycles with 98% capacity retention. Similarly, Ni is introduced to the LixMn2O4-yClz spinel in order to take advantage of the intrinsic redox couple of Ni2+/Ni4+ at 4.7V and demonstrate reversibility from 5.0 V to 2.0 V.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 Investigation of Low Temperature Solid Oxide Fuel Cells for Air-Independent UUV Applications(2012) Moton, Jennie Mariko; Jackson, Gregory S; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Unmanned underwater vehicles (UUVs) will benefit greatly from high energy density (> 500 Wh/L) power systems utilizing high-energy-density fuels and air-independent oxidizers. Current battery-based systems have limited energy densities (< 400 Wh/L), which motivate development of alternative power systems such as solid oxide fuel cells (SOFCs). SOFC-based power systems have the potential to achieve the required UUV energy densities, and the current study explores how SOFCs based on gadolinia-doped ceria (GDC) electrolytes with operating temperatures of 650°C and lower may operate in the unique environments of a promising UUV power plant. The plant would contain a H2O2 decomposition reactor to supply humidified O2 to the SOFC cathode and exothermic aluminum/H2O combustor to provide heated humidified H2 fuel to the anode. To characterize low-temperature SOFC performance with these unique O2 and H2 source, SOFC button cells based on nickel/GDC (Gd0.1Ce0.9O1.95) anodes, GDC electrolytes, and lanthanum strontium cobalt ferrite (La0.6Sr0.4Co0.2Fe0.8O3- or LSCF)/GDC cathodes were fabricated and tested for performance and stability with humidity on both the anode and the cathode. Cells were also tested with various reactant concentrations of H2 and O2 to simulate gas depletion down the channel of an SOFC stack. Results showed that anode performance depended primarily on fuel concentration and less on the concentration of the associated increase in product H2O. O2 depletion with humidified cathode flows also caused significant loss in cell current density at a given voltage. With the humidified flows in either the anode or cathode, stability tests of the button cells at 650 °C showed stable voltage is maintained at low operating current (0.17 A/cm2) at up to 50 % by mole H2O, but at higher current densities (0.34 A/cm2), irreversible voltage degradation occurred at rates of 0.8 - 3.7 mV/hour depending on exposure time. From these button cell results, estimated average current densities over the length of a low-temperature SOFC stack were estimated and used to size a UUV power system based on Al/H2O oxidation for fuel and H2O2 decomposition for O2. The resulting system design suggested that energy densities above 300 Wh/L may be achieved at neutral buoyancy with seawater if the cell is operated at high reactant utilizations in the SOFC stack for missions longer than 20 hours.Item Thermal integration of tubular solid oxide fuel cell with catalytic partial oxidation reactor and anode exhaust combustor for small power application(2010) Maxey, Christopher; Jackson, Gregory S; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In the current study, a system configuration of a tubular SOFC with a catalytic partial oxidation (CPOx) reactor and an anode exhaust catalytic combustor is explored to test the feasibility of such a system. A system level model was developed to more fully assess system design and operability issues. For the SOFC, a detailed 1-D SOFC determines local current production and is combined with down-the-channel flow models for the SOFC as well as the catalytic combustor/heat exchanger, and CPOx reactor. System model results showed that variations in fuel flow and air to fuel ratio have large impacts on temperature distribution and power out, with lower fuel flows and air-to-fuel ratios providing higher SOFC power densities (~0.64 W/cm2) at high efficiencies (~45%). The system model also shows that external heat loss greatly reduces system power and efficiency but lower air-to-fuel ratios can offset associated temperature and associate performance losses.