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ItemEnhancing air electrode performance of solid oxide cells by surface modification(Georgia Institute of Technology, 2022-04-15) Evans, ConorReversible solid oxide cells based on proton conductors (P-rSOCs) offer an efficient and clean option for energy storage and conversion. However, one issue holding back this renewable technology is the sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics that take place at the air electrode. The air electrode in a P-rSOC is also subject to harsh environments (e.g., high concentration of steam) that can cause degradation over time. Catalyst infiltration into the air electrode offers a possible solution to each of these issues. Several catalyst candidates were investigated using the state-of-the-art double perovskite air electrode material, PrBa0.8Ca0.2Co2O5+δ (PBCC), as the air electrode backbone. Symmetrical cells with catalyst coated PBCC electrodes were primarily used to screen catalyst solutions and isolate the air electrode performance. Electrochemical impedance spectroscopy (EIS) was utilized to characterize the electrochemical performance and the long-term stability of catalyst infiltrated symmetrical cells under various testing conditions containing either steam and/or Cr contaminants. Electrochemical performance of single cells with a catalyst coated PBCC electrode was measured in both the fuel cell mode and the electrolysis cell mode. X- ray diffraction (XRD), scanning electron microcopy (SEM), and Raman spectroscopy were used to characterize phase composition, electrode microstructure and morphology, as well as surface chemistry to gain better understanding of the air electrode degradation mechanism during testing. Several catalysts were screened and optimized via symmetrical cell tests, including LaNiO3, La2NiO4, BaCoO3, LaNi0.6Fe0.4O3, La2Ni0.6Fe0.4O4, and PrCoO3. Symmetrical cells infiltrated with a PrCoO3 catalyst demonstrated particularly excellent stability and electrochemical performance (with a polarization resistance as low as 0.147 Ω cm2 and minimal degradation over 500 hours) against various sources of Cr contaminations at steam concentrations as high as 30% at 600 °C. Single cells infiltrated with PrCoO3 exhibit a peak power density of 2.02 W cm-2 at 650°C in the fuel cell mode, a 35.5% increase in performance from the single cells without catalyst modification. When run in electrolysis mode these same infiltrated single cells demonstrate a current density of 3.22 A cm-2 at 650 °C, a 22.4% improvement from the performance of the cells without catalyst modification. The single cell based on a PrCoO3 infiltrated cathode was among the best performing P-rSOCs ever reported in literature
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ItemUnderstanding Degradation Mechanisms of Metallic Alloys in High-Temperature Molten Salt Environments(Georgia Institute of Technology, 2021-09-01) Hanson, Kasey L.Selection of structural alloys for harsh environments require understanding of material degradation mechanisms to enhance material lifespan, process efficiency, and overall safety. One category of harsh environments for metallic materials is molten salts where individual salts or mixtures of carbonates, sulfates, nitrates, halides etc. can induce significant material degradation at elevated temperatures and is commonly referred to as molten salt corrosion. This study separates molten salt corrosion into two different categories: oxygen bearing salts and non-oxygen bearing salts. Investigation of the material degradation mechanisms for each category is performed through the use of two different model systems. Metallic alloy selection and performance in the two categories of molten salt environments depends on whether a stable protective oxide can form on the alloy surface or not. The first category is an oxygen containing molten salt environment. The model system selected for this category was superheater tubes in biomass recovery boilers. Here, protective oxides can be fluxed due to the molten salt mixture: Na2SO4-Na2CO3-KCl-K2SO4. Moreover, the desired operating temperature range approaches this mixture’s first melting temperature, thereby accelerating corrosion not only from the molten salt mixture, but by oxidizing gaseous environments simultaneously. As a model system for the second category, eutectic salt mixture of KCl-MgCl2 was studied in the absence of any significant oxidizing gaseous environment. Material candidates for coolant loops in Molten Salt Reactors (MSRs) experience attack from molten salt by selective dissolution of active alloying elements. Thermodynamic calculations can be used to predict the extent to which alloys will suffer corrosive attack. The severity of attack can be controlled through the use of impurities in the molten chloride salt. Similarities and differences in the corrosion mechanisms for the two very different categories of molten salt environments is systematically studied in this project. Understanding of the various mechanisms of material degradation induced by molten salts can be used to better inform materials selection, engineering and design in the application of structural alloys.
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ItemCarbon Effects on Corrosion in Molten Fluoride Salt(Georgia Institute of Technology, 2020-02-18) Chan, Kevin JeremyThe fluoride salt-cooled high temperature reactor (FHR) is a Generation IV reactor concept that uses molten fluoride salt as the primary coolant to enable breakthrough improvements in economics and safety over the current generation of reactors. However, graphite fuel elements and other carbon components present large surface area to the coolant salt. They can degrade structural alloys by driving carburization or by forming metal carbides with corrosion products. A thorough understanding of the interactions between environmental carbon and structural alloys is necessary for the successful development and deployment of FHRs. Key knowledge gaps of carbon-alloy interactions in molten fluorides were addressed in this work. Simultaneous chromium depletion and carburization behavior of several alloys were studied in multi-duration exposure tests. Pre-carburization experiments evaluated the beneficial and detrimental effects of carburization related to corrosion. Most importantly, the mechanism of carbon transport was investigated in support of future designs of carburization control measures.