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ItemStability of double perovskite cathodes under high humidity for solid oxide fuel cells(Georgia Institute of Technology, 2019-05-06) Liu, YuchenSolid Oxide Fuel Cells (SOFCs) can directly convert a wide variety of fuels to electricity efficiently. They can also be run in reverse as Solid Oxide Electrolysis Cells (SOECs) to produce hydrogen (and carbon-containing fuels) from electrolysis of water (and carbon dioxide). However, the kinetics of oxygen reduction reaction (ORR) on the cathode is often hindered by various contaminants, which may react with the cathode to form insulating phases and degrade fuel cell performance. The stability and performance of the cathode in moisture is critical to the cell performance as SOFCs and SOECs. Several state-of-the-art cathode materials are investigated in a high moisture environment to uncover their performance and degradation mechanism. First, powders of electrode materials were analyzed for any degradation before and after long-term moisture exposure using XRD to probe the bulk and Raman Spectroscopy to probe the surface. SEM was also used to characterize any morphological changes during the exposure. Second, electrochemical impedance spectroscopy (EIS) was used to monitor the long-term performance of symmetric cells under various conditions. Finally, current-voltage relationships of symmetric cells were acquired under typical operating conditions for SOFCs and SOECs to determine the polarization resistance, stability and durability of the cathode materials.
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ItemElucidation of hydrogen oxidation kinetics on metal/proton conductor interface(Georgia Institute of Technology, 2013-05-13) Feng, ShiHigh temperature proton conducting perovskite oxides are very attractive materials for applications in electrochemical devices, such as solid oxide fuel cells (SOFCs) and hydrogen permeation membranes. A better understanding of the hydrogen oxidation mechanism over the metal/proton conductor interface, is critical for rational design to further enhance the performances of the applications. However, kinetic studies focused on the metal/proton system are limited, compared with the intensively studied metal/oxygen ion conductor system, e.g., Ni/YSZ (yttrium stabilized zirconia, Zr₁-ₓYₓO₂-δ). This work presents an elementary kinetic model developed to assess reaction pathway of hydrogen oxidation/reduction on metal/proton conductor interface. Individual rate expressions and overall hydrogen partial pressure dependencies of current density and polarization resistance were derived in different rate limiting cases. The model is testified by tailored experiments on Pt/BaZr₀.₁Ce₀.₇Y₀.₁Yb₀.₁O₃-δ (BZCYYb) interface using pattern electrodes. Comparison of electrochemical testing and the theoretical predictions indicates the dissociation of hydrogen is the rate-limiting step (RLS), instead of charge transfer, displaying behavior different from metal/oxygen ion conductor interfaces. The kinetic model presented in this thesis is validated by high quantitative agreement with experiments under various conditions. The discovery not only contributes to the fundamental understanding of the hydrogen oxidation kinetics over metal/proton conductors, but provides insights for rational design of hydrogen oxidation catalysts in a variety of electrochemical systems.
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ItemPreparation and characterization of vanadium oxides on carbon fiber paper as electrodes for pseudocapacitors(Georgia Institute of Technology, 2013-04-10) Cromer, Cynthia EcklesSupercapacitors are important electrochemical energy storage devices for microelectronic and telecommunication systems, electric cars, and smart grids. However, the energy densities of existing supercapacitors are still inadequate for many applications. Vanadium oxides have been studied as viable supercapacitor alternatives, with varying results. Methods are often complicated or time-consuming, and electrode fabrication often includes carbon powder and binder. The objective of this work was to study the effect of processing conditions on specific capacitance of supercapacitors based on vanadium oxides coated on carbon fiber papers. This study was conducted to form easily-fabricated compounds of vanadium oxides which could offer promise as pseudocapacitor material, and to nucleate these compounds directly onto inexpensive carbon fiber without binder. The incipient wetness impregnation technique was used to fabricate the electrodes. Electrochemical performance of the resulting electrodes was tested in a Swagelok-type electrochemical two-electrode cell, and the electrodes were characterized by XRD and SEM. Interesting nanofeatures were formed and the vanadium oxides exhibited pseudocapacitance at a respectable level.
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ItemElectrical properties of BaZr0.1Ce0.7Y0.1Yb0.1O3-δ and its application in intermediate temperature solid oxide fuel cells(Georgia Institute of Technology, 2012-07-06) Rainwater, Benjamin H.Conventional oxygen anion conducting yttria-stabilized zirconia (YSZ) based solid oxide fuel cells (SOFCs) operate at high temperatures (800oC-1000oC). SOFCs based on proton conducting ceramics, however, can operate at intermediate temperatures (450oC-750oC) due to low activation energy for protonic defect transport when compared to oxygen vacancy transport. Fuel cells that operate at intermediate temperatures ease the critical materials requirements of cell components and reduce system costs, which is necessary for large scale commercialization. BaCeO3-based perovskite materials are candidates for use as ion conductors in intermediate temperature SOFCs (IT-SOFCs) when doped with trivalent cations in the B-site. B-site doping forms oxygen vacancies which greatly increases the electrical conductivity of the material. The oxygen vacancies are consumed during the creation of protonic defects or electronic defects, depending on the atmosphere and temperature range. High performance IT-SOFCs based on the Y3+ and Yb3+ doped BaCeO3-based system, BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) have been recently reported. High conductivity in O2/H2O atmosphere was reported, however, a more basic understanding of the BZCYYb structure, electrical conductivity, and the portion of the charge carried by each charge carrier under fuel cell conditions is lacking. In this work, the BZCYYb material is fabricated by the solid state reaction method and the crystal structure at intermediate temperatures is studied using HT-XRD. The total conductivity of BZCYYb in H2/H2O, O2/H2O, and air atmospheres in the IT-SOFC temperature range is reported. The activation energy for transport at these conditions is determined from the conductivity data and the transference numbers of protonic defects, oxygen anion defects and electronic defects in the BZCYYb material are determined by the concentration cell - OCV method. BZCYYb is a mixed proton, oxygen anion, and electronic conductor at IT-SOFC temperature ranges (450oC - 750oC), in H2, O2, and H2O containing atmospheres. Ni-BZCYYb/BZCYYb/BZCYYb-LSCF fuel cells were constructed and peak power densities of ~1.2 W/cm2 were reported at 750oC after optimization of the Ni-BZCYYb anode porosity. Decreasing the Ni-BZCYYb anode porosity did not significantly affect the electrical conductivity of the anode, however the peak power densities of the IT-SOFCs based on the anode with less porosity, calculated from I-V curve data, showed dramatic improvement. The fuel cell with the lowest anode porosity demonstrated the highest performance. This finding is in stark contrast to the optimal anode porosity needed for high performance in YSZ-based, oxygen anion conducting SOFCs. Because of significant proton conduction in the BZCYYb material, fuel cell reaction products (water) form at the cathode side and less porosity is required on the anode side. The improvement in performance in the BZCYYb based IT-SOFC is attributed to the unique microstructure formed in the Ni-BZCYYb anode when no pore forming additives are used which may contribute to high electrocatalytic behavior for anode reactions. This work provides a basic understanding of the electrical properties of BZCYYb and clarifies the feasibility of using BZCYYb in each component of the IT-SOFC system as well as in other electrochemical devices. The high performance of the Ni-BZCYYb/BZCYYb/BZCYYb-LSCF IT-SOFC, due to low anode porosity, provides a new understanding for the rational development of high performance IT-SOFCs based on electrolytes with significant protonic conduction.