Theses and Dissertations at Montana State University (MSU)
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Item Operando optical and quantitative electrochemical studies of solid oxide fuel cell anode degradation and regeneration(Montana State University - Bozeman, College of Letters & Science, 2022) Pomeroy, Elias Deen; Chairperson, Graduate Committee: Robert Walker; This is a manuscript style paper that includes co-authored chapters.Solid Oxide Fuel Cells (SOFCs) are high temperature (600-1000 °C) devices that can generate electricity with extremely high efficiencies from a wide variety of fuels, including H 2, CH 4, Biogas, and crushed coal. Unfortunately, the SOFC anodes are highly sensitive to gas phase contaminants, including sulfur and carbon containing fuels. Sulfur is ubiquitous in all carbon containing fuels, with concentrations as low as a few parts per million to as high as 1% by mass. At all concentrations sulfur substantially decreases SOFC performance. Conventional models propose that sulfur decreases fuel cell performance by blocking anode active sites, preventing electrochemical reactions, and reducing surface area for heterogeneous catalysis. Carbon containing fuels can rapidly degrade SOFCs due to graphitic carbon formation. Graphite blocks active sites on the anode, causes damage within the anode microstructure, and removes electrocatalytic material via metal dusting. Studies presented in this work used operando optical techniques and quantitative electrochemistry to study degradation and remediation of SOFC anodes. First, since typical electrochemical techniques infer microstructural changes rather than directly measuring surface area, a traditional electrochemical technique, chronocoulometry (CC), was adapted to SOFCs for the first time to measure the electrochemically active area of the anode. This technique showed that active area is temperature dependent, and that sulfur participates in electrochemical reactions, decreasing performance with sluggish oxidation kinetics, rather than simply blocking active sites. Carbon monoxide, on the other hand, decreased the number of active sites, rather than participating in electrochemical reactions, either by blocking active sites or forming carbon. Then, a comparative study was undertaken of different methodologies of carbon remediation, comparing electrochemical oxidation, molecular oxygen, and steam as methods to remove graphite accumulated on SOFC anodes. This study found that with all methods, CO 2 played a key role in removing carbon, that both electrochemical oxidation and steam removed carbon more globally than oxygen, and that imaging the entire cell is critical for understanding the complex, spatially and temporally heterogeneous chemistry occurring across SOFC anodes. Finally, sulfur was employed to passivate SOFC anodes operating on dry methane, significantly reducing carbon formation with only slight decreases in electrochemical performance.Item Operando optical studies of next generation anode materials in high temperature solid oxide fuel cells(Montana State University - Bozeman, College of Letters & Science, 2020) Welander, Martha Maria; Chairperson, Graduate Committee: Robert Walker; Marley S. Zachariasen, Clay D. Hunt, Stephen W. Sofie and Robert A. Walker were co-authors of the article, 'Operando studies of redox resiliance in alt enhanced NIO-YSZ SOFC anodes' in the journal 'Journal of the electrochemical society' which is contained within this dissertation.; Marley S. Zachariasen, Stephen W. Sofie and Robert A. Walker were co-authors of the article, 'Enhancing Ni-YSZ anode resilience to environmental redox stress with aluminum titanate secondary phases' in the journal 'ACS applied energy materials' which is contained within this dissertation.; Marley S. Zachariasen, Stephen W. Sofie and Robert A. Walker were co-authors of the article, 'Mitigating carbon formation with Al 2TiO 5 enhanced solid oxide fuel cell anodes' in the journal 'The journal of physical chemistry C' which is contained within this dissertation.; Daniel B. Drasbaek, Marie L. Traulsem Bhaskar R. Sudireddy, Peter Holtappels and Robert A. Walker were co-authors of the article, 'What does carbon tolerant really mean? Operando vibrational studies of carbon accumulation on novel solid ocide fuel cell anodes prepared by infiltration' submitted to the journal 'RSC physical chemistry chemical physics' which is contained within this dissertation.; Disseration contains an article of which Martha Maria Welander is not the main author.Solid oxide fuel cells (SOFCs) are high temperature energy conversion devices capable of efficient and sustainable energy production. Because of the need to electrochemically reduce molecular oxygen and the relatively high activation energy required for oxide ions to diffuse through the dense, solid-state electrolyte, SOFCs typically operate at temperatures > or = 500 °C. High operating temperatures endow SOFCs with many advantages, including fuel flexibility and high conversion efficiencies, distinguishing them from other types of fuel cells. However, high temperatures also present challenges related to the stability of the electrode materials, accelerating cell degradation and limiting the development and integration of SOFCs into large scale power production strategies. These mechanisms are the result of fundamental changes in material properties that remain poorly described and difficult to predict. Studies presented in this work utilized operando Raman spectroscopy and electrochemical measurements to directly correlate material changes with changes in cell performance under various operating conditions. Research focused on developing and characterizing new electro-catalytic materials having improved conversion efficiencies and mechanical resilience to thermal and chemical stress. Because current state of the art SOFC Ni-YSZ cermet anodes are sensitive to oxidation, the first two studies investigated the effects of adding small amounts of Al 2TiO 5 to Ni-YSZ anodes and the impact of resulting secondary (2°) phases that formed on SOFC tolerance to electrochemical and environmental reduction and oxidation (redox) cycling. Results show that Al 2TiO 5 helps improve tolerance to both types of redox cycling by maintaining electrode-electrolyte connectivity and minimizing catalyst coarsening. The third study illustrates how this same dopant improved anode carbon tolerance when operating with hydrocarbon fuels. Because excessive carbon accumulation on SOFC anodes can lead to rapid cell failure, ways to improve carbon tolerance was further explored in the last two studies. These studies investigate the effect of decoupling the electro-catalytic and the electronically conductive phases of the anode under pure methane and biogas-surrogate environments. Collectively, the studies described in this dissertation provide insight into the materials-specific mechanisms responsible for limiting degradation of doped and functionally decoupled anodes to help guide the design of new SOFC electrode materials.Item Chlorine induced degradation of SOFCS operating on carbon containing fuels(Montana State University - Bozeman, College of Letters & Science, 2017) Reeping, Kyle Wyatt; Chairperson, Graduate Committee: Robert Walker; Robert A. Walker was a co-author of the article, 'In operando vibrational raman studies of chlorine contamination in solid oxide fuel cells' in the journal 'The journal of the Electrochemical Society' which is contained within this thesis.; John D. Kirtley, Jessie M. Bohn, Daniel A. Steinhurst, Jeffrey C. Owrutsky and Robert A. Walker were co-authors of the article, 'Chlorine-induced degradation in solid oxide fuel cells identified by optical methods' in the journal 'The journal of physical chemistry C' which is contained within this thesis.; Jessie, M. Bohn and Robert A. Walker were co-authors of the article, 'Chlorine-induced degradation in SOFCS operating with biogas' in the journal 'Sustainable energy and fuels' which is contained within this thesis.; Jessie, M. Bohn and Robert A. Walker were co-authors of the article, 'The palliative effect of H 2 on SOFCS operating on contaminated carbon containing fuels' submitted to the journal 'The journal of power sources' which is contained within this thesis.Chlorine present in green and synthetic fuels such as biogas and syngas can accelerate degradation of solid oxide fuel cell (SOFC) nickel-based anodes. Chlorine contamination has been studied in SOFCs where H 2 was the primary fuel but little attention has focused on deleterious, cooperative effects that result from Cl-contamination in predominantly carbon-containing fuels. Experiments described in this work examine degradation mechanisms in SOFCs with Ni-YSZ cermet anodes operating with a biogas surrogate and exposed to 110 ppm Cl (delivered either as CH 3Cl or HCl). Operando Raman spectroscopy is used to directly observe the the anode's catalytic activity as evidenced by observable carbon accumulation, and electrochemical impedance and voltammetry measurements report on overall cell performance. Studies performed at 650 °C and 700 °C show that Cl suppresses carbon accumulation and causes slow but steady cell degradation. Prolonged exposure to Cl results in and irreversible device failure. These results differ markedly from recent reports of Cl contamination in SOFCs operating independently with H 2 and CH 4.Item In operando spectroscopic studies of high temperature electrocatalysts used for energy conversion(Montana State University - Bozeman, College of Letters & Science, 2016) McIntyre, Melissa Dawn; Chairperson, Graduate Committee: Robert Walker; John D. Kirtley, David M. Halat, Kyle W. Reeping and Robert A. Walker were co-authors of the article, 'In situ spectroscopic studies of carbon formation in SOFCS operating with syn-gas' in the journal 'Electrochemical Society transactions' which is contained within this thesis.; Daniel M. Neuburger and Robert A. Walker were co-authors of the article, 'In operando raman spectroscopy studies of temperature dependent carbon accumulation on SOFCS operating with syn-gas' submitted to the journal 'The journal of the Electrochemical Society' which is contained within this thesis.; John D. Kirtley, Anand Singh, Shamiul Islam, Josephine M. Hill and Robert A. Walker were co-authors of the article, 'Comparing in situ carbon tolerances of Sn-infiltrated and Bao-infiltrated Ni-YSZ cermet anodes in solid oxide fuel cells exposed to methane' in the journal 'The journal of physical chemistry C' which is contained within this thesis.; David R. Driscoll, Martha M. Welander, Josh B. Sinrud, Stephen W.Sofie, Robert A. Walker were co-authors of the article, 'In situ formation of multifunctional ceramics : mixed ion-electron conducting properties of zirconium titanium oxides' submitted to the journal 'The journal of materials chemistry A' which is contained within this thesis.; Thesis contains an article of which Melissa Dawn McIntyre is not the main author.Solid-state electrochemical cells are efficient energy conversion devices that can be used for clean energy production or for removing air pollutants from exhaust gas emitted by combustion processes. For example, solid oxide fuel cells generate electricity with low emissions from a variety of fuel sources; solid oxide electrolysis cells produce zero-emission H2 fuel; and solid-state DeNO x cells remove NO x gases from diesel exhaust. In order to maintain high conversion efficiencies, these systems typically operate at temperatures > or = 500°C. The high operating temperatures, however, accelerate chemical and mechanical cell degradation. To improve device durability, a mechanistic understanding of the surface chemistry occurring at the cell electrodes (anode and cathode) is critical in terms of refining cell design, material selection and operation protocols. The studies presented herein utilized in operando Raman spectroscopy coupled with electrochemical measurements to directly correlate molecular/material changes with device performance in solid oxide cells under various operating conditions. Because excessive carbon accumulation with carbon-based fuels destroys anodes, the first three studies investigated strategies for mitigating carbon accumulation on Ni cermet anodes. Results from the first two studies showed that low amounts of solid carbon stabilized the electrical output and improved performance of solid oxide fuel cells operating with syn-gas (H 2/CO fuel mixture). The third study revealed that infiltrating anodes with Sn or BaO suppressed carbon accumulation with CH 4 fuel and that H 2O was the most effective reforming agent facilitating carbon removal. The last two studies explored how secondary phases formed in traditional solid oxide cell materials doped with metal oxides improve electrochemical performance. Results from the fourth study suggest that the mixed ion-electron conducting Zr 5Ti 7O 24 secondary phase can expand the electrochemically active region and increase electrochemical activity in cermet electrodes. The final study of lanthanum strontium manganite cathodes infiltrated with BaO revealed the reversible decomposition/formation of a Ba 3Mn 2O 8 secondary phase under applied potentials and proposed mechanisms for the enhanced electrocatalytic oxygen reduction associated with this compound under polarizing conditions. Collectively, these studies demonstrate that mechanistic information obtained from molecular/material specific techniques coupled with electrochemical measurements can be used to help optimize materials and operating conditions in solid-state electrochemical cells.Item Mechanistic implications from 'in operando' optical and electrochemical studies of bio-related fuel chemistry in solid oxide fuel cells(Montana State University - Bozeman, College of Letters & Science, 2015) Kirtley, John David; Chairperson, Graduate Committee: Robert Walker; Bryan C. Eigenbrodt and Robert A. Walker were co-authors of the article, 'In situ optical studies of oxidation kinetics of NI/YSZ cermet anodes' in the journal 'ECS transactions' which is contained within this thesis.; David M. Halat, Melissa M. McIntyre, Bryan C. Eigenbrodt and Robert A. Walker were co-authors of the article, 'High temperature 'spectrochronopotentiometry': correlating electrochemical performance with in situ raman spectroscopy in solid oxide fuel cells' in the journal 'Analytical chemistry' which is contained within this thesis.; Melissa M. McIntyre, David M. Halat and Robert A. Walker were co-authors of the article, 'Insights into SOFC NI/YSZ anode degradation using in situ spectrochronopotentiometry' submitted to the journal 'ECS transactions' which is contained within this thesis.; Anand Singh, David Halat, Thomas Oswell, Josephine M. Hill and Robert A. Walker were co-authors of the article, 'In situ raman studies of carbon removal from high temperature NI-YSZ cermet anodes by gas phase reforming agents' submitted to the journal 'Journal of physical chemistry C' which is contained within this thesis.; Daniel A. Steinhurst, Jeffrey C. Owrutsky, Michael B. Pomfret and Robert A. Walker were co-authors of the article, 'In situ optical studies of methane and simulated biogas oxidation on high temperature solid oxide fuel cells' in the journal 'Physical chemistry chemical physics' which is contained within this thesis.; Daniel A. Steinhurst, Jeffrey C. Owrutsky, Michael B. Pomfret and Robert A. Walker were co-authors of the article, 'Towards a working mechanism of fuel oxidation in SOFCS: in situ optical studies of simulated biogas and methane' submitted to the journal 'Journal of physical chemistry C' which is contained within this thesis.Solid oxide fuel cells using bio-renewable fuels promise efficient, sustainable, and clean electricity production, and are becoming more attractive sources of electrical power as global consumption of non-renewables accelerates. The excellent efficiencies of solid oxide fuel cells as solid state electrochemical devices arise mainly from the direct conversion of chemical to electrical energy--through oxygen reduction at the cathode, oxide diffusion through the electrolyte, and fuel oxidation at the anode. Some of these processes possess high activation energies, requiring high operational temperatures (generally > 650 °C). These conditions can hasten deleterious carbon accumulation and anode deterioration. These high operating temperatures also pose significant challenges in directly observing chemical reactions responsible for electrochemical oxidation and materials degradation. Yet, these observations are needed to understand fundamental mechanisms responsible for these processes--an understanding necessary to improve the performance, durability and versatility of SOFCs, especially as these devices are required to operate with complex fuels and fuel mixtures. In this work, solid oxide fuel cells constructed with traditional Ni/yttrium stabilized zirconia ceramic-metallic (or cermet) anodes are studied in operando and in situ with several novel optical techniques (Raman vibrational spectroscopy, near infrared thermal imaging and fourier-transform infrared emission spectroscopy) and electrochemical measurements that provide vital insights into mechanisms surrounding bio-related fuel electrochemistry. The first study demonstrated that Ni oxidation is slower than reduction at the anode. The second study quantified electrochemically accessible anode carbon accumulation under methane fuel, while detailing deleterious mechanisms and microstructural changes that accompany cell polarization in the absence of gas phase fuels. The third study explored the kinetics of carbon removal from Ni-based anodes by CO 2, H 2O, and O 2 and detailed mechanistically why they may be different. The fourth study revealed mechanisms associated with carbon formation and current generation from biogas and methane as a function of operational condition. Collectively, these studies have begun to provide the direct, molecularly specific information necessary to empirically evaluate mechanistic descriptions of biorelated fuel electrochemistry and anode degradation.Item Spatial distribution of carbon accumulation on solid oxide fuel cells operating under methane(Montana State University - Bozeman, College of Letters & Science, 2015) Neuburger, Daniel Miller; Chairperson, Graduate Committee: Robert WalkerWith the looming threat of climate change, new technologies that can reduce the use of fossil fuels are desirable. One such technology is the solid oxide fuel cell (SOFC). SOFCs convert chemical energy directly into electrical energy, so they can be much more efficient than traditional sources of electrical power generation. SOFCs utilize an oxide-ion conducting electrolyte to generate electricity by reducing molecular oxygen at the cathode and oxidizing fuel at the anode. High temperatures (<650 °C) are required to catalyze oxide conduction through the electrolyte. High temperatures enable SOFCs to use a variety of fuels, but these same conditions also accelerate unfavorable reactions that can cause anode degradation. Research described in this dissertation examined the effects of carbon accumulation in SOFC anodes on overall device performance, and the spatial variation in anode degradation. Fuel was introduced to the fuel cell using a single fuel inlet. This method of fuel introduction created spatial variance in factors such as fuel concentration, and this variance was expected to affect carbon accumulation on the anode. Carbon accumulation is a major contributor to anode degradation through a variety of mechanisms including mass transport obstruction, anode delamination from the electrolyte, and metal dusting, a process describing anode disintegration induced by carbon growth disrupting the anode's electronically conducting network. The amount of carbon accumulation on the surface of the anode was analyzed in operando using Raman spectroscopy. Spectroscopic results showed that more carbon accumulated on the anode further from the fuel inlet than closer to it. In situ electrochemical measurements coupled with post mortem visual and FEM analysis suggested that severe anode degradation occurred early in an experiment, affecting spectroscopic results. We infer that the highest amount of carbon accumulation occurs close to the fuel inlet causing fast and severe anode degradation, decreasing the amount of carbon that can accumulate in further trials when the majority of the spectroscopic data was collected. These results suggest that spectroscopic results should be analyzed with experimental factors such as anode location and trial sequence specifically in mind.