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School of Materials Science and Engineering

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College of Engineering

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Investigations of gas/electrode interactions in solid oxide fuel cells using vibrational spectroscopy

(Georgia Institute of Technology, 2008-04-01) Abernathy, Harry W., III

The goal of current solid oxide fuel cell (SOFC) research is to design electrode materials and other system components that permit the fuel cell to be operated in the 400-700ºC range. Cell performance in this lower temperature range is limited by the oxygen reduction process at the SOFC cathode and by multiple contamination processes. The work presented demonstrates that Raman spectroscopy, a form of vibrational spectroscopy, can provide structural and compositional information complementary to that from traditional characterization methods. Initial experiments into the oxygen reduction mechanism on SOFC cathodes were unable to detect surface oxygen species on selected perovksite-based SOFC cathode materials. However, the Raman signal from the cathode surface was able to be enhanced by depositing silver or gold nanoparticles on the cathode, creating the so-called surface-enhanced Raman scattering (SERS) effect. The Raman sample chamber was also used to study two possible electrode contamination processes. First, the deposition of carbon on nickel and copper anodes was observed when exposed to different hydrocarbon fuel gases. Second, the poisoning of an SOFC cathode by chromium-containing vapor (usually generated by stainless steel SOFC system components) was monitored. Overall, Raman spectroscopy was shown to be useful in many areas crucial to the development of practical, cost-effective SOFCs. The techniques developed here could also be applied to other high temperature electrochemical and catalytic systems.
Investigations into the interactions between sulfur and anodes for solid oxide fuel cells

(Georgia Institute of Technology, 2008-03-05) Cheng, Zhe

Solid oxide fuel cells (SOFCs) are electrochemical devices based on solid oxide electrolytes that convert chemical energy in fuels directly into electricity via electrode reactions. SOFCs have the advantages of high energy efficiency and low emissions and hold the potential to be the power of the future, especially for small power generation systems (1-10 kW). Another unique advantage of SOFCs is the potential to directly utilize hydrocarbon fuels such as natural gas through internal reforming. However, all hydrocarbon fuels contain some sulfur compounds, which transform to hydrogen sulfide (H2S) in the reforming process and dramatically degrade the performance of the existing SOFCs. In this study, the interactions between sulfur contaminant (in the form of H2S) and the anodes for SOFCs were systematically investigated in order to gain a fundamental understanding of the mechanism of sulfur poisoning and ultimately to achieve rational design of sulfur-tolerant anodes. The sulfur poisoning behavior of the state-of-the-art Ni-YSZ cermet anodes was characterized using electrochemical measurements performed on button cells (of different structures) under various operating conditions, including H2S concentration, temperature, cell current density/terminal voltage, and cell structure. Also, the mechanisms of interactions between sulfur and the Ni-YSZ cermet anode were investigated using both ex situ and in situ characterization techniques such as Raman spectroscopy. Results suggest that the sulfur poisoning of Ni-YSZ cermet anodes at high temperatures in fuels with ppm-level H2S is due not to the formation of multi-layer conventional nickel sulfides but to the adsorption of sulfur on the nickel surface. In addition, new sulfur-tolerant anode materials were explored in this study. Thermodynamic principles were applied to predict the stability of candidate sulfur-tolerant anode materials and explain complex phenomena concerning the reactivity of candidate materials with hydrogen sulfide. The enhanced sulfur tolerance for some candidate anode materials such as (Gd2Ti1.4Mo0.6O7) is attributed to the transition of the surface from metal oxides to sulfides (i.e., MoS2), which enhances the catalytic activity and increases the number of reaction sites.
Discrete Numerical Simulations of Solid Oxide Fuel Cell Electrodes: Developing New Tools for Fundamental Investigation

(Georgia Institute of Technology, 2007-11-14) Mebane, David Spencer

A program of study has been established for the quantitative study of electrode reactions in solid oxide fuel cells. The initial focus of the program is the mixed conducting cathode material strontium-doped lanthanum manganate (LSM). A formalism was established treating reactions taking place at the gas-exposed surface of mixed conducting electrodes. This formalism was incorporated into a phenomenological model for oxygen reduction in LSM, which treats the phenomenon of sheet resistance. Patterned electrodes were designed that reduce the dimensionality of the appropriate model, and these electrodes were successfully fabricated using DC sputtering and photolithography. A new model for the bulk defect equilibrium in LSM was proposed and shown to be a better fit to nonstoichiometry data at low temperatures. The fitting was carried out with a particle swarm optimizer and a rigorous method for identification. It was shown that a model for the interface structure between LSM and yttria-stabilized zirconia (YSZ) that assumes free oxygen vacancies in YSZ does not accord with experimental observations. Cluster variation method (CVM) was adapted for analysis of the problem, and a new analytical method combining CVM and electrical contributions to the free energy was proposed.
Development of SOFC anodes resistant to sulfur poisoning and carbon deposition

(Georgia Institute of Technology, 2007-11-14) Choi, Song Ho

The surface of a dense Ni-YSZ anode was modified with a thin-film coating of niobium oxide (Nb2O5) in order to understand the mechanism of sulfur tolerance and the behavior of carbon deposition. Results suggest that the niobium oxide was reduced to NbO2 under operating conditions, which has high electrical conductivity. The NbOx coated dense Ni-YSZ showed sulfur tolerance when exposed to 50 ppm H2S at 700°C over 12 h. Raman spectroscopy and XRD analysis suggest that different phases of NbSx formed on the surface. Further, the DOS (density of state) analysis of NbO2, NbS, and NbS2 indicates that niobium sulfides can be considered as active surface phases in the H2S containing fuels. It was demonstrated that carbon formation was also suppressed with niobium oxide coating on dense Ni-YSZ in humidified CH4 (3% H2O) at 850ºC. In particular, under active operating conditions, there was no observable surface carbon as revealed using Raman spectroscopy due probably to electrochemical oxidation of carbon. Stable performances of functional cells consisting of Pt/YSZ/Nb2O5 coated dense Ni-YSZ in the fuel were achieved; there was no observable degradation in performance due to carbon formation. The results suggest that a niobium oxide coating has prevented carbon from formation on the surface probably by electrochemically oxidation of carbon on niobium oxide coated Ni-YSZ. On the other hand, computational results suggest that, among the metals studied, Mo seems to be a good candidate for Ni surface modification. Ni-based anodes were modified with Mo using wet-impregnation techniques, and tested in 50 ppm H2S-contaminated fuels. It was found that the Ni-Mo/CeO2 anodes have better sulfur tolerance than Ni, showing a current transient with slow recovery rather than slow degradation in 50 ppm H2S balanced with H2 at 700°C.
Simulation and Characterization of Cathode Reactions in Solid Oxide Fuel Cells

(Georgia Institute of Technology, 2007-07-05) Williams, Robert Earl, Jr.

In this study, we have developed a dense La0.85Sr0.15MnO3-δ (LSM) Ce0.9Gd0.1O1.95 (GDC) composite electrode system for studying the surface modification of cathodes. The LSM and GDC grains in the composite were well defined and distinguished using energy dispersive x-ray (EDX) analysis. The specific three-phase boundary (TPB) length per unit electrode surface area was systematically controlled by adjusting the LSM to GDC volume ratio of the composite from 40% up to 70%. The TPB length for each tested sample was determined through stereological techniques and used to correlate the cell performance and degradation with the specific TPB length per unit surface area. An overlapping spheres percolation model was developed to estimate the activity of the TPB lines on the surface of the dense composite electrodes developed. The model suggested that the majority of the TPB lines would be active and the length of those lines maximized if the volume percent of the electrolyte material was kept in the range of 47 57%. Additionally, other insights into the processing conditions to maximize the amount of active TPB length were garnered from both the stereology calculations and the percolation simulations. Steady-state current voltage measurements as well as electrochemical impedance measurements on numerous samples under various environmental conditions were completed. The apparent activation energy for the reduction reaction was found to lie somewhere between 31 kJ/mol and 41 kJ/mol depending upon the experimental conditions. The exchange current density was found to vary with the partial pressure of oxygen differently over two separate regions. At relatively low partial pressures, i0 had an approximately dependence and at relatively high partial pressures, i0 had an approximately dependence. This led to the conclusion that a change in the rate limiting step occurs over this range. A method for deriving the electrochemical properties from proposed reaction mechanisms was also presented. State-space modeling was used as it is a robust approach to addressing these particular types of problems due to its relative ease of implementation and ability to efficiently handle large systems of differential algebraic equations. This method combined theoretical development with experimental results obtained previously to predict the electrochemical performance data. The simulations agreed well the experimental data and allowed for testing of operating conditions not easily reproducible in the lab (e.g. precise control and differentiation of low oxygen partial pressures).
Design, Fabrication and Characterization of Novel Planar Solid Oxide Fuel Cells

(Georgia Institute of Technology, 2007-02-27) Compson, Charles E.

Planar solid oxide fuel cells (SOFCs) were designed, fabricated and characterized in order to develop a (1) cost-effective method for fabrication of thin electrolyte layers, (2) hermetic sealing and (3) stable interconnects. Electrophoretic deposition (EPD) was discovered to be an excellent method for fabricating dense electrolyte layers of about 5m thick on porous non-conducting substrates. The EPD process was thoroughly studied from proof-of-concept to statistical reproducibility, deposition mechanism, modeling and process optimization. Deposition on non-conducting substrates was found to follow many of the same fundamental trends as that on conductive substrates except for the voltage efficiency and detailed charge transfer mechanism. Eventually, the process was optimized such that an SOFC was fabricated that achieved 1.1W/cm2 at 850C. Further, a novel sealless planar SOFC was designed that incorporates a hermetic interface between the electrolyte and interconnect similar to tubular and honeycomb designs. The hermetic interface successfully acted as a blocking electrode under DC polarization, indicating its potential to act as a sealant. Leakage rates across the interface were 0.027sccm at 750c, similar to polycrystalline mica seals. Through a process of tape casting and lamination, a two-cell stack without sealant was fabricated and achieved a power density of 75mW/cm2 at 750C. Finally, the degradation rate of silver and silver-based interconnects was studied under static and dual-atmosphere conditions. Corrosion of silver grain boundaries along with sublimation losses results in the formation of large pores, resulting in up to 30 of anode oxidation after 8hrs testing at 750c. Further stability studies indicated that silver-based interconnects would be better suited for applications at operating temperatures less than 650C.
Doping and Defect Structure of Mixed-conducting Ceramics for Gas Separation

(Georgia Institute of Technology, 2006-11-21) Zuo, Chendong

My main objective is to gain a firm understanding of the correlation between the defect chemistry and the properties of Ba-based perovskite structure proton-conducting ceramics, especially B-site doped BaCeO3, so as to allow the engineering of these compounds with the desired properties for the application in devices; develop membranes of mixed protonic-electronic conductors suitable for hydrogen separation from gas mixtures; and further advance hydrogen separation technology by gaining fundamental understanding about electrochemical separation mechanism. BZCY proton conductors with various compositions have been synthesized and characterized. The absence of low-angle supercell reflections indicates a random B-site cation distribution. The substitution of Zr led to a decrease in cell volume and an enhanced structural stability against reactions with CO2. The total conductivity for BZCY pellets increased with temperature increased and decreased as the zirconium content increased at each fixed temperature. Dense Ni-BZCY composite membranes have been successfully fabricated for evaluating hydrogen permeability and stability. Doping Zirconium in the B-site only slightly reduced the hydrogen permeation at high temperatures, but dramatically increased the chemical stability in CO2- and H2O-containing gases. Among the compositions studied, the Ni-BZCY7 exhibited both highest H2 permeation rate and good chemistry stability, thus having potential for practical applications.
Synthesis and Characterization of Nanostructured Electrodes for Solid State Ionic Devices

(Georgia Institute of Technology, 2006-11-20) Zhang, Yuelan

The demands for advanced power sources with high energy efficiency, minimum environmental impact, and low cost have been the impetus for the development of a new generation of batteries and fuel cells. One of the key challenges in this effort is to develop and fabricate effective electrodes with desirable composition, microstructure and performance. This work focused on the design, fabrication, and characterization of nanostructured electrodes in an effort to minimize electrode polarization losses. Solid-state diffusion often limits the utilization and rate capability of electrode materials in a lithium-ion battery, especially at high charge/discharge rates. When the fluxes of Li+ insertion or extraction exceed the diffusion-limited rate of Li+ transport within the bulk phase of an electrode, concentration polarization occurs. Further, large volume changes associated with Li+ insertion or extraction could induce stresses in bulk electrodes, potentially leading to mechanical failure. Interconnected porous materials with high surface-to-volume ratio were designed to suppress the stress and promote mass transport. In this work, electrodes with these unique architectures for lithium ion batteries have been fabricated to improve the cycleability, rate capability and capacity retention. Cathodic interfacial polarization represents the predominant voltage loss in a low-temperature SOFC. For the first time, regular, homogeneous and bimodal porous MIEC electrodes were successfully fabricated using breath figure templating, which is self-assembly of the water droplets in polymer solution. The homogeneous macropores promoted rapid mass transport by decreasing the tortuosity. And mesoporous microstructure provided more surface areas for gas adsorption and more TPBs for the electrochemical reactions. Moreover, composite electrodes were developed with a modified sol-gel process for honeycomb SOFCs. The sol gel derived cathodes with fine grain size and large specific surface area, showed much lower interfacial polarization resistances than those prepared by other existing processing methods. Nanopetals of cerium hydroxycarbonate have been synthesized via a controlled hydrothermal process in a mixed water-ethanol medium. The formation of the cerium compound depends strongly on the composition of the precursors, and is attributed to the favored ethanol oxidation by Ce(IV) ions over Ce(IV) hydrolysis process. Raman studies showed that microflower CeO2 preferentially stabilizes O2 as a peroxide species on its surface for CO oxidation.
A Quantitative Determination of Electrode Kinetics using Micropatterned Electrodes

(Georgia Institute of Technology, 2006-04-11) Koep, Erik Kenneth

Interfacial polarization resistances limit the performance of many thin film solid-state devices, especially at low temperatures. To improve performance, a fundamental understanding of the electrode kinetics that govern interfacial reaction rates must be developed. The goal of this work is to determine site-specific reaction mechanisms and the relative significance of various reactions in order to quantify optimum structural parameters within the cathode microstructure. Key parameters include the length of triple phase boundary (TPB), the quantity of exposed electrolyte/electrode surface, and the ratio of electrolyte to electrode material. These parameters, when studied in a specific system, can be incorporated into broader models, which will encompass the specific conductivity of each component to develop an optimized three-dimensional network. The emphasis of this work is the systematic control and manipulation of potential cathodic reaction sites in order to develop an understanding of the relative importance of specific reaction sites. Since the physical dimensions of reaction sites are relatively small, an approach has been developed that utilizes micro-fabrication (similar to that used in integrated-circuit fabrication) to produce small and highly controlled microstructures. Investigations were made into the nature and reactivity of Triple Phase Boundaries (hereafter TPB) through the use of patterned platinum electrodes since only the TPBs are active in these electrodes. After the processing details of micro-fabrication were established for the platinum electrodes, patterned Mixed-Ionic/Electronic Conducting (MIEC) electrodes were fabricated and studied using impedance spectroscopy to determine the contributions from the MIEC surface versus the TPB. Systematically changing the geometry of the MIEC electrodes (thickness and line width) allowed for the determination of the effect of ambipolar transport within the MIEC on the activity of MIEC surfaces versus the TPB. This information is critical to rational design of functionally graded electrodes (with optimal particle size, shape, porosity and conductivity). In addition to experimental studies, representative patterned electrode samples were made available for collaborative studies with surface scientists at other institutions to provide additional techniques (such as Raman Spectroscopy) on the carefully designed and controlled cathode surfaces.
Fabrication of Nanostructured Electrodes and Interfaces Using Combustion CVD

(Georgia Institute of Technology, 2005-08-25) Liu, Ying

Reducing fabrication and operation costs while maintaining high performance is a major consideration for the design of a new generation of solid-state ionic devices such as fuel cells, batteries, and sensors. The objective of this research is to fabricate nanostructured materials for energy storage and conversion, particularly porous electrodes with nanostructured features for solid oxide fuel cells (SOFCs) and high surface area films for gas sensing using a combustion CVD process. This research started with the evaluation of the most important deposition parameters: deposition temperature, deposition time, precursor concentration, and substrate. With the optimum deposition parameters, highly porous and nanostructured electrodes for low-temperature SOFCs have been then fabricated. Further, nanostructured and functionally graded La0.8Sr0.2MnO2-La0.8SrCoO3-Gd0.1Ce0.9O2 composite cathodes were fabricated on YSZ electrolyte supports. Extremely low interfacial polarization resistances (i.e. 0.43 Wcm2 at 700¡ãC) and high power densities (i.e. 481 mW/cm2 at 800¡ãC) were generated at operating temperature range of 600¡ãC-850¡ãC. The original combustion CVD process is modified to directly employ solid ceramic powder instead of clear solution for fabrication of porous electrodes for solid oxide fuel cells. Solid particles of SOFC electrode materials suspended in an organic solvent were burned in a combustion flame, depositing a porous cathode on an anode supported electrolyte. Combustion CVD was also employed to fabricate highly porous and nanostructured SnO2 thin film gas sensors with Pt interdigitated electrodes. The as-prepared SnO2 gas sensors were tested for ethanol vapor sensing behavior in the temperature range of 200-500¡ãC and showed excellent sensitivity, selectivity, and speed of response. Moreover, several novel nanostructures were synthesized using a combustion CVD process, including SnO2 nanotubes with square-shaped or rectangular cross sections, well-aligned ZnO nanorods, and two-dimensional ZnO flakes. Solid-state gas sensors based on single piece of these nanostructures demonstrated superior gas sensing performances. These size-tunable nanostructures could be the building blocks of or a template for fabrication of functional devices. In summary, this research has developed new ways for fabrication of high-performance solid-state ionic devices and has helped generating fundamental understanding of the correlation between processing conditions, microstructure, and properties of the synthesized structures.