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School of Materials Science and Engineering

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College of Engineering

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A new family of proton conducting electrolytes with enhanced stability for reversible fuel cell operation: BaHfxCe0.8-xY0.1Yb0.1O3

(Georgia Institute of Technology, 2019-08-21) Murphy, Ryan Joe

Solid oxide fuel cell (SOFC) technology has the potential to be one of the most efficient energy conversion technologies and the same technology can be used to efficiently produce several chemical species such as hydrogen and syngas through reverse operation, known as solid oxide electrolysis cells (SOEC). However, the long-term performance of these systems is often limited by degradation of the electrolyte. In this study, a new family of proton conducing electrolyte materials, BaHfxCe0.8-xY0.1Yb0.1O3 (BHCYYb), have been developed, which demonstrate much improved stability while maintaining similar or higher conductivities than current state-of-the-art materials. The performance of the SOFCs based on these new electrolytes rivals that of the current best performance reported in literature, but with better durability. In addition, BHCYYb has been shown to possess higher stability through long term chemical stability and conductivity tests. Further, solid oxide cells based on BHCYYb have also been operated in the reverse mode, as SOECs for CO2-H2O co-electrolysis. Finally, a number of dopants have been introduced to the BaHfO3-based system in order to further improve the conductivity and stability.
Stability of double perovskite cathodes under high humidity for solid oxide fuel cells

(Georgia Institute of Technology, 2019-05-06) Liu, Yuchen

Solid 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.
Development and characterization of materials for intermediate temperature solid oxide fuel cell anodes

(Georgia Institute of Technology, 2019-04-30) Deglee, Ben

Solid Oxide Fuel Cells (SOFCs) are devices capable of directly converting chemical energy into electrical energy through high temperature electrochemical oxidation of fuels, but there remain serious obstacles before these devices can be fully implemented into the modern energy infrastructure. The operation of SOFCs with hydrocarbon fuels has the highest potential for commercial impact, but the activity of state-of-the-art materials toward these fuels is relatively low compared to hydrogen, and SOFCs can quickly degrade due to the deposition of solid carbon (coking). Lowering SOFC operating temperatures to less than 600 °C would expand the application of SOFCs while dramatically reducing system complexity and cost, but device performance at these temperatures remains prohibitively low. To address these obstacles, this work focuses on two key issues in SOFC technology development: improvement of SOFC materials and advancement of SOFC characterization techniques. First, a high performing SOFC was designed and demonstrated, uniquely suited for low temperature direct methane operation through the addition of an internal reforming catalyst layer. In situ spectroscopy was used extensively to evaluate the defect and surface structure of the reforming catalyst, directly relating the material structure to device performance. The second issue was addressed through the development of a novel testing platform for quantitative comparison of different anode surface coatings, as well as the design and fabrication of new operando equipment which increases the current testing capability of the SOFC community.
Towards rational design of solid oxide fuel cell electrodes through surface modification

(Georgia Institute of Technology, 2017-11-14) Doyle, Brian

Solid oxide fuel cells represent a scalable energy generation technology capable of operating at high efficiencies on multiple fuel sources. However, wide-spread implementation of SOFCs has been limited by the high degradation rate at current operating temperatures of 800-1000°C. Lowering the operating temperature to an intermediate range of 500-700°C will decrease the degradation phenomena, but will also decrease the catalytic activity of the electrodes. Modifying the surface of the electrodes is one method to increase the catalytic activity at these relatively low operating temperatures. This dissertation seeks to understand the role of surface modification on solid oxide fuel cell electrodes through conformal and non-conformal coatings. The first part of this dissertation demonstrates an asymmetric cell testing platform that is used to better understand the effects of conformal film deposition. Depositing a conformal thin film into a porous cathode is nontrivial and requires exhaustive optimization of either solution or gas phase deposition techniques. Even then, if the backbone material and the coating material aren’t very similar (in crystal structure, thermal expansion, etc), then the film will no longer be conformal after reaching SOFC operating temperatures. The asymmetric testing platform in this work was designed to focus on the effect of the thin film modification, which was accomplished by depositing a dense LSCF cathode on one side of an SDC electrolyte support with an accompanying porous LSCF counter electrode. Because of the high surface area of the counter electrode, the polarization resistances measured were dominated by the dense LSCF thin film. The planar dense film allows for precise control over the modification with conformal thin films via RF sputtering. The first part of the dissertation describes the fabrication and electrochemical characterization of this testing platform, which demonstrated the ORR activity was the dominant feature in the impedance spectra. The second part of the dissertation describes the surface modification with undoped ceria and samarium doped ceria. First, infiltration was used to modify the surface and it was seen that a change in morphology influenced the ORR activity. More specifically, for the undoped ceria, a more conformal morphology as opposed to a more dispersed, nanoisland morpohology lead to lower impedance for the ORR. Using the asymmetric testing platform and sputtered ceria, it was found that the thickness of the conformal ceria influenced the ORR. Thinner films showed an increase polarization resistance, while thicker films showed a decreased polarization resistance. The increase in polarization resistance for the thinner films was explained by an increase in vacancy concentration as demonstrated through comparison of the impedance behavior under bias to a doped ceria thin film. Second, it was found infiltration with samarium doped ceria decreased the polarization resistance. Interestingly, the performance increase was independent of the mol% of the samarium doped into ceria. This goes against the conventional thinking that increasing ionic conductivity (by increasing samarium mol %) will lead to increasing surface exchange properties. Thin film conformal deposition of 20SDC demonstrated an overall increase in polarization resistance with increasing resistance correlating to film thickness. These last two results suggest that the ionically conducting surface modification reduces the oxygen through a surface mediated process that requires high surface nanoparticles The third part of the dissertation describes the work using praseodymium doped ceria as the modification material to better understand the role of ionic and electronic conductivity in the ORR catalytic activity. Doping praseodymium into ceria increases both the ionic and electronic conductivity. Through infiltration, it was found that the optimal performance occurs at 50 mol% praseodymium in ceria even though 70 mol % exhibits higher electronic and ionic conductivities. Through XPS and TGA, it was found that amount of Ce3+ (i.e. reduced ceria) changes non-linearly with praseodymium dopant concentration. The 50 mol% doped ceria showed more Ce4+ available relative to the 30 and 70 mol% praseodymium concentration. Thus, it was found that oxygen ion vacancy concentration and electronic conductivity are not the only material properties relevant to increasing ORR activity. Instead, the results indicate a more nuanced view of oxygen reduction reaction and the correlation to bulk material properties. In the end, this work describes a platform for the characterization of conformal thin film surface modification and demonstrates the potential to increase material performance beyond bulk material properties. Importantly, this work has shown the nuanced performance enhancement beyond traditional correlations to ionic and electronic conductivity.
Exploring interfacial and nanoscale electrical effects in solid state ionic conductors for application in low temperature solid oxide fuel cells and solid state batteries

(Georgia Institute of Technology, 2016-03-16) Rainwater, Ben H.

High-performance solid-state energy storage and conversion devices are a vital technology component of the U.S. Department of Energy’s clean and renewable energy implementation strategy. Solid-state fuel cells are important technologies for efficient conversion of a wide variety of fuels to electricity, while development of solid-state batteries is critical for safe storage of electricity generated by clean technologies. Solid-state devices are pursued due to their inherent mechanical and chemical stability at high temperatures and under harsh conditions; ensuring long-lifetime, fuel flexibility and safe operation. However, large resistance to ionic transport in the solid state, especially at low operating temperatures, severely limits the performance of solid state devices. In this work, ionic transport properties at interfaces of solid-state electrolytes have been investigated as a route for developing high performance solid state energy conversion and storage devices. Interfacial effects including the space-charge effect, the strain effect, and the curvature effect alter ionic charge carrier mobility and/or concentration at interfaces in solid-state electrolytes, leading to dramatic changes in ionic conductivity. By harnessing these energetically-favorable effects at solid-state electrolyte interfaces and fabricating interface-rich electrolytes, the total conductivity of the solid electrolyte and performance of solid-state electrochemical devices can be greatly enhanced. Interfacial effects on oxygen, hydrogen and lithium ion transport in nanocrystalline bulk samples, heterostructured thin film samples, and powder samples at high pressure (35GPa) have been studied by structural and electrical characterization techniques. The work provides important insight into interfacial effects on ionic conductivity in solid-state electrolytes relevant to current solid-state fuel cell and battery development. The fundamental understanding of interfacial effects on ionic conductivity has been widened by this study and several conclusions from the work can be applied directly to enhance the performance of solid state energy conversion and storage devices.
Investigation into the surface chemistry of passivated carbon fiber/LiMn2O4 electrodes for lithium ion batteries

(Georgia Institute of Technology, 2016-02-29) Waller, Gordon Henry

Lithium-ion batteries are one of the most energy dense electrochemical energy storage systems available today and for the foreseeable future will be the dominant secondary battery type for applications needing large energy density and long operational lifetimes. Among the many varieties and applications of lithium-ion batteries, electrode design - and in particular the selection of active materials - is extremely influential in determining overall device specifications. Furthermore, the cathode plays a particularly important role in factors such as cell safety, lifetime, and cost. In applications which require low cost and high safety LiMn2O4 cathodes are an excellent choice; however, the well-known issue of rapid capacity fading has yet to be overcome. In this dissertation composite electrodes are formed by directly coating the LiMn2O4 active material onto carbon fiber current collectors. When tested as positive electrodes for lithium-ion batteries, these electrodes show comparable energy and power density to conventional tape-cast composites, but can be fabricated without the need for organic solvents, binders or metal foil current collectors. To reduce capacity loss from the LiMn¬2O4 active material ultrathin (<1 nm) coatings of aluminum oxide were deposited onto the surface of the LiMn2O4/carbon fiber composites using atomic layer deposition. Aluminum oxide coatings successfully improved capacity retention by over 100% and led to an unexpected increase in rate capability and total lithium diffusivity. To further investigate the mechanisms in which inert oxide coatings prevent capacity loss and influence cycling behavior, thin-film model electrodes were prepared and measurements of surface chemistry, crystal structure and electrochemical impedance were conducted.
Perspectives on Degradation in Solid Oxide Fuel Cells Using X-ray Spectroscopies and Scattering

(Georgia Institute of Technology, 2015-11-16) Lai, Samson Yuxiu

Solid oxide fuel cells (SOFCs) represent a major piece of a next-generation, renewable, clean energy economy and contribute to combating anthropogenic climate change by efficiently converting chemical energy into electrical energy through electrochemical reactions. However, despite adding significant chemical, mechanical, and microstructural complexity to push SOFC performance ever higher, cost and durability remain significant barriers to SOFC commercialization. Two of these issues are cathode stability in atmospheres containing carbon dioxide and water vapor and anode stability in fuel containing hydrogen sulfide. With regards to those aspects, state-of-the-art SOFC cathodes (La1-xSrxMnO3-δ and La1-xSrxCo1-yFeyO3-δ) and anodes (NiO and BaZr0.1Ce0.7Y0.1Yb0.1O3-δ) are studied to understand the interactions between contaminant and electrode. In this work, powerful in situ and operando x-ray spectroscopy and scattering experiments provide deep insight into the physiochemical phenomena that define the behavior of SOFC electrode materials. These studies demonstrate that proper combination of in situ and operando experiments, due partially to the powerful intensity and capabilities of synchrotron x-rays, can provide unique information that has never before been possible and is critical to gaining new perspectives and to better understand data where a single perspective may only lead to ambiguous conclusions. Such a multi-pronged characterization approach is vital to gaining a better understanding of complex SOFC materials and providing critical insights for rational design of next-generation SOFC electrode materials.
In situ characterization of electrochemical processes of solid oxide fuel cells

(Georgia Institute of Technology, 2014-08-18) Li, Xiaxi

Solid oxide fuel cells (SOFCs) represent a next generation energy source with high energy conversion efficiency, low pollutant emission, good flexibility with a wide variety of fuels, and excellent modularity suitable for distributed power generation. As an electrochemical energy conversion device, SOFC’s performance and reliability depend sensitively on the catalytic activity and stability of the electrode materials. To date, however, the development of electrode materials and microstructures is still based largely on trial-and-error methods because of inadequate understanding of the mechanisms of the electrode processes. Identifying key descriptors/properties of electrode materials or functional heterogeneous interfaces, especially under in situ conditions, may provide guidance to the design of electrode materials and microstructures. This thesis aims to gain insight into the electrochemical and catalytic processes occurring on the electrode surfaces using unique characterization tools with superior sensitivity, high spatial resolution, and excellent surface specificity applicable under in situ/operando conditions. Carbon deposition on nickel-based anodes is investigated with in situ Raman spectroscopy and SERS. Analysis shows a rapid nucleation of carbon deposition upon exposure to small amount of propane. Such nucleation process is sensitive to the presence of surface coating (e.g., GDC) and the concentration of steam. In particular, operando analysis of the Ni-YSZ boundary indicates special function of the interface for coking initiation and reformation. The coking-resistant catalysts (BaO, BZY, and BZCYYb) are systematically studied using in situ Raman spectroscopy, SERS, and EFM. In particular, time-resolved Raman analysis of the surface functional groups (-OH, -CO3, and adsorbed carbon) upon exposure to different gas atmospheres provides insight into the mechanisms related to carbon removal. The morphology and distribution of early stage carbon deposition are investigated with EFM, and the impact of BaO surface modification is evaluated. The surface species formed as a result of sulfur poisoning on nickel-based anode are examined with SERS. To identify the key factors responsible for sulfur tolerance, model cells with welldefined electrode-electrolyte interfaces are systematically studied. The Ni-BZCYYb interface exhibits superior sulfur tolerance. The oxygen reduction kinetics on LSCF, a typical cathode material of SOFC, is studied using model cells with patterned electrodes. The polarization behaviors of these micro- electrodes, as probed using a micro-probe impedance spectroscopy system, were correlated with the systematically varied geometries of the electrodes to identify the dominant paths for oxygen reduction under different electrode configurations. Effects of different catalyst modifications are also evaluated to gain insight into the mechanisms that enhance oxygen reduction activity. The causes of performance degradation of LSCF cathodes over long term operation are investigated using SERS. Spectral features are correlated with the formation of surface contamination upon the exposure to air containing Cr vapor, H2O, and CO2. Degradation in cathode performance occurs under normal operating conditions due to the poisoning effect of Cr from the interconnect between cells and the high operating temperature. The surface-modified LSCF cathode resists surface reactions with Cr vapor that impairs electrode performance, suggesting promising ways to mitigate performance degradation.
Elucidation of hydrogen oxidation kinetics on metal/proton conductor interface

(Georgia Institute of Technology, 2013-05-13) Feng, Shi

High 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.
Preparation and characterization of vanadium oxides on carbon fiber paper as electrodes for pseudocapacitors

(Georgia Institute of Technology, 2013-04-10) Cromer, Cynthia Eckles

Supercapacitors 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.