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In Situ Studies on the Catalytic and Electronic Properties of Multi-Dimensional Cathode

Description: The overall objective of this program was to provide the fundamental background to enable SECA Industrial Team Members to select and develop oxide cathodes for the operation of solid oxide fuel cells in the intermediate temperature (500-700 C). A number of experimental techniques were used to address the specific obstacles within the program. The work expanded our existing experimental techniques to the use of in situ diffraction and spectroscopic techniques that are sensitive to the oxygen and iron structural and chemical environment. Such measurements were coupled with the evaluation and determination of the catalytic properties of potential cathode oxides. The scope of the work was to prepare carefully controlled cathode samples of various compositions and microstructure and perform fundamental measurements of their physical properties. The measurement of physical properties of the cathode materials systems were used to correlate those properties to the overall effectiveness of the material as a constituent of an SOFC cell. The measurements techniques of neutron scattering and Moessbauer are relatively unique capabilities which have significance to the LSCF cathode system. The unique ability to make thin film samples can provide special samples for surface science efforts to understand cathodic catalytic activity. The data obtained from neutron diffraction, Moessbauer Spectroscopy, electrical measurements and modeling show that the oxygen vacancy content at 500 C in air for La{sub 0.8}Sr{sub 0.2}MnO{sub 3}, La{sub 0.6}Sr{sub 0.4}FeO{sub 3} and La{sub 0.6}Sr{sub 0.4}Fe{sub 0.8}Co{sub 0.2}O{sub 3-{delta}} are 0, 1% and 5% respectively. This is a significant finding that should relate to performance as a cathode, and gives criteria for the selection of new cathode materials.
Date: December 1, 2003
Creator: Anderson, Harlan U.
Partner: UNT Libraries Government Documents Department
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LOW TEMPERATURE CATHODE SUPPORTED ELECTROLYTES

Description: This project has three main goals: Thin Films Studies, Preparation of Graded Porous Substrates and Basic Electrical Characterization and Testing of Planar Single Cells. During this time period substantial progress has been made in developing low temperature deposition techniques to produce dense, nanocrystalline yttrium-stabilized zirconia films on both dense oxide and polymer substrates. Progress has been made in the preparation and characterization of thin electrolytes and porous LSM substrates. Both of these tasks are essentially on or ahead of schedule. In our proposal, we suggested that the ZrO{sub 2}/Sc system needed to be considered as a candidate as a thin electrolyte. This was because microcrystalline ZrO{sub 2}/Sc has a significantly higher ionic conductivity than YSZ, particularly at the lower temperatures. As a result, some 0.5 micron thick film of ZrO{sub 2}/16% Sc on an alumina substrate (grain size 20nm) was prepared and the electrical conductivity measured as a function of temperature and oxygen activity. The Sc doped ZrO{sub 2} certainly has a higher conductivity that either 20nm or 2400nm YSZ, however, electronic conductivity dominates the conductivity for oxygen activities below 10{sup -15}. Whereas for YSZ, electronic conductivity is not a problem until the oxygen activity decreases below 10{sup -25}. These initial results show that the ionic conductivity of 20nm YSZ and 20nm ZrO{sub 2}/16% Sc are essentially the same and the enhanced conductivity which is observed for Sc doping in microcrystalline specimens is not observed for the same composition when it is nanocrystalline. In addition they show that the electronic conductivity of Sc doped ZrO{sub 2} is at least two orders of magnitude higher than that observed for YSZ. The conclusion one reaches is that for 0.5 to 1 micron thick nanocrystalline films, Sc doping of ZrO{sub 2} has no benefits compared to YSZ. As a result, electrolyte films of …
Date: March 31, 2000
Creator: Anderson, Harlan U.
Partner: UNT Libraries Government Documents Department
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LOW TEMPERATURE CATHODE SUPPORTED ELECTROLYTES

Description: This project has three main goals: Thin Films Studies, Preparation of Graded Porous Substrates and Basic Electrical Characterization and testing of Planar Single Cells. During this time period substantial progress has been made in developing low temperature deposition techniques to produce dense, nanocrystalline yttrium-stabilized zirconia films on both dense oxide and polymer substrates. Microstructural changes in unsupported nanocrystalline yttrium stabilized zirconia (ZrO{sub 2}:16%Y, or YSZ) thin films were examined as a function of temperature and annealing time in order to determine the grain growth exponent and the mechanisms of pinhole formation. Grain growth and pinhole formation were measured using high resolution transmission electron microscopy (HRTEM), normal imaging mode transmission electron microscopy (TEM), electron diffraction, and energy dispersive X-ray microanalysis (EDS). Grain growth was found to vary with a time exponent of about one half before pinhole formation and about one third after. Pinhole formation in 70 nm thick films occurred at temperatures near 600 C, corresponding to a grain size of about 15 nm, or a grain size to film thickness ration of approximately 0.25. The deposition of films on porous substrates is hampered by the penetration of the polymer precursor solution into the substrate whose pores as > 0.2 {micro}m, therefore much attention has to be paid to the development of porous colloidal oxide films onto surfaces. Thus during this line period we have been studying these films. Optical properties have proven to be an excellent way to study the quality of these nanoporous films. The influence of porosity and densification on optical properties of films on sapphire substrates that were prepared from water colloidal suspensions of small ({approx}5nm) particles of ceria was investigated. The colloidal ceria films have initially very porous structure (porosity about 50%) and densification starts at about 600 C accompanied by grain growth. The concurrence …
Date: September 30, 2000
Creator: Anderson, Harlan U.; Huebner, Wayne & Kosacki, Igor
Partner: UNT Libraries Government Documents Department
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LOW TEMPERATURE CATHODE SUPPORTED ELECTROLYTES

Description: This project has three main goals: Thin Films Studies, Preparation of Graded Porous Substrates and Basic Electrical Characterization and testing of Planar Single Cells. In this portion of study we have focused on producing YSZ films on porous LSM substrates. When using the polymer precursor there are a number of obstacles to overcome in order to form dense electrolyte layers on porous substrates (cathode or anode). Probably the most difficult problems are: (1) Extreme penetration of the polymer into the substrate must be prevented. (2) Shrinkage cracking must be avoided. (3) Film thickness in the 1 to 5{micro}m range must be achieved. We have demonstrated that cracking due to shrinkage involved during the elimination of solvents and organic matter and densification of the remaining oxide is not a problem as long as the resulting oxide film is < {approx} 0.15 {micro}m in thickness. We have also shown that we can make thicker films by making multiple depositions if the substrate is smooth (roughness {le} 0.1 {micro}m) and contains no surface pores > 0.2 {micro}m. The penetration of the polymer into the porous substrate can be minimized by increasing the viscosity of the polymer and reducing the largest pore at the surface of the substrate to {le} 0.2 {micro}m. We have shown that this can be done, but we have also shown that it is difficult to make dense films that are defect free with areas > 1 cm{sup 2}. This is because of the roughness of the substrate and the difficulty in making a substrate which does not have surface voids > 0.2 {micro}m. Thus the process works well for dense, smooth substrates for films < 1 {micro}m thick, but is difficult to apply to rough, porous surfaces and to make film thickness > 1 {micro}m. As a result of …
Date: September 30, 2001
Creator: Anderson, Harlan U.; Huebner, Wayne & Kosacki, Igor
Partner: UNT Libraries Government Documents Department
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LOW TEMPERATURE CATHODE SUPPORTED ELECTROLYTES

Description: This project has three main goals: Thin Films Studies, Preparation of Graded Porous Substrates and Basic Electrical Characterization and testing of Planar Single Cells. Substantial progress has been made on both characterizing thin films as well as developing methods to produce films on nanoporous substrates. The results of electrical conductivity measurements on ZrO{sub 2}:16%Sc nanocrystalline thin films under controlled oxygen partial pressure and temperature are presented. The experimental data have been interpreted using a defect model, which describes the interaction between Sc and oxygen vacancies resulting in the formation of donor - (Sc{sub Zr} - V{sub o} - e){sup x} and acceptor - (Sc{sub Zr}-h){sup x} levels. From this the electronic and ionic contribution to the electrical transport has been determined and correlated with the band structure. These results suggest that ZrO{sub 2}:16%Sc possesses higher electronic conductivity than ZrO{sub 2}:16%Y, which dominates the total conductivity in reducing atmospheres. This is an important result since it indicates that Sc-YSZ maybe useful in the anode regions of the cell. We have made important breakthroughs on depositing dense Ceria films on to porous LSM substrates. In previous studies we have found that in order to produce a surface which is smooth enough to coat with dense polymer precursor derived films, the required thickness of the colloidal film layer is determined by the maximum surface roughness. That is, if we wish to make 2 micron thick colloidal oxide layers, the roughness of the LSM surface can not exceed 2 microns. Currently, we are producing the composite CeO{sub 2}/LSM structures that can be coated with polymer precursor to produce 0.5 to 1.5 micron thickness dense YSZ films. In the next quarter, we will be testing SOFC's using these structures. YSZ/CeO{sub 2}/LSM composites have been formed by annealing at 800 C. Our studies show that the …
Date: March 31, 2001
Creator: Anderson, Harlan U.; Huebner, Wayne & Kosacki, Igor
Partner: UNT Libraries Government Documents Department
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LOW TEMPERATURE CATHODE SUPPORTED ELECTROLYTES

Description: This project has three main goals: Thin Films Studies, Preparation of Graded Porous Substrates and Basic Electrical Characterization and testing of Planar Single Cells. This period has continued to address the problem of making dense 1/2 to 5 {micro}m thick dense layers on porous substrates (the cathode LSM). Our current status is that we are making structures of 2-5 cm{sup 2} in area, which consist of either dense YSZ or CGO infiltrated into a 2-5 {micro}m thick 50% porous layer made of either nanoncrystalline CGO or YSZ powder. This composite structure coats a macroporous cathode or anode; which serves as the structural element of the bi-layer structure. These structures are being tested as SOFC elements. A number of structures have been evaluated both as symmetrical and as button cell configuration. Results of this testing indicates that the cathodes contribute the most to cell losses for temperatures below 750 C. In this investigation different cathode materials were studied using impedance spectroscopy of symmetric cells and IV characteristics of anode supported fuel cells. Cathode materials studied included La{sub 0.8}Sr{sub 0.2}Co{sub 0.2}Fe{sub 0.8}O{sub 3} (LSCF), La{sub 0.7}Sr{sub 0.2}MnO{sub 3} (LSM), Pr{sub 0.8}Sr{sub 0.2}Fe{sub 0.8}O{sub 3} (PSCF), Sm{sub 0.8}Sr{sub 0.2}Co{sub 0.2}Fe{sub 0.8}O{sub 3} (SSCF), and Yb{sub .8}Sr{sub 0.2}Co{sub 0.2}Fe{sub 0.8}O{sub 3} (SSCF). A new technique for filtering the Fourier transform of impedance data was used to increase the sensitivity of impedance analysis. By creating a filter specifically for impedance spectroscopy the resolution was increased. The filter was tailored to look for specific circuit elements like R//C, Warburg, or constant phase elements. As many as four peaks can be resolved using the filtering technique on symmetric cells. It may be possible to relate the different peaks to material parameters, like the oxygen exchange coefficient. The cathode grouped in order from lowest to highest ASR is …
Date: March 31, 2002
Creator: Anderson, Harlan U.; Dogan, Fatih & Petrovsky, Vladimir
Partner: UNT Libraries Government Documents Department
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LOW TEMPERATURE CATHODE SUPPORTED ELECTROLYTES

Description: This report represents a summary of the work carried out on this project which started October 1999 and ended March 2003. A list of the publications resulting from the work are contained in Appendix A. The most significant achievements are: (1) Dense nanocrystalline zirconia and ceria films were obtained at temperatures < 400 C. (2) Nanocrystalline films of both ceria and zirconia were characterized. (3) We showed that under anodic conditions 0.5 to 1 micron thick nanocrystalline films of Sc doped zirconia have sufficient electronic conductivity to prevent them from being useful as an electrolyte. (4) We have developed a process by which dense 0.5 to 5 micron thick dense films of either YSZ or ceria can be deposited on sintered porous substrates which serve as either the cathode or anode at temperatures as low as 400 C. (5) The program has provided the research to produce two PhD thesis for students, one is now working in the solid oxide fuel cell field. (6) The results of the research have resulted in 69 papers published, 3 papers submitted or being prepared for publication, 50 oral presentations and 3 patent disclosures.
Date: March 31, 2003
Creator: Anderson, Harlan U.; Dogan, Fatih & Petrovsky, Vladimir
Partner: UNT Libraries Government Documents Department
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Microporous and Thin Film Membranes for Solid Oxide Fuel

Description: One of the major limitations to the commercialization of solid oxide fuel cells is the expense of fabricating the required (anode/electrolyte/cathode) cells (currently planar or tubular). The current technology being employed by most of the producers of solid oxide fuel cells is essentially all the same in that it involves standard ceramic processing to make composite structures (either electrolyte or electrode supported). Each investigator/program has their own individual processing steps to produce the cells, but in the end, they all make these cells in a very similar manner. As a result only limited progress has been made towards cost reductions over the last 2-3 decades. It is our contention that major changes in the processing of cells need to be made before SOFCs can become economically feasible.
Date: February 1, 2007
Creator: Anderson, Harlan U.
Partner: UNT Libraries Government Documents Department
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Development of a Novel Efficient Solid-Oxide Hybrid for Co-generation of Hydrogen and Electricity Using Nearby Resources for Local Application

Description: Developing safe, reliable, cost-effective, and efficient hydrogen-electricity co-generation systems is an important step in the quest for national energy security and minimized reliance on foreign oil. This project aimed to, through materials research, develop a cost-effective advanced technology cogenerating hydrogen and electricity directly from distributed natural gas and/or coal-derived fuels. This advanced technology was built upon a novel hybrid module composed of solid-oxide fuel-assisted electrolysis cells (SOFECs) and solid-oxide fuel cells (SOFCs), both of which were in planar, anode-supported designs. A SOFEC is an electrochemical device, in which an oxidizable fuel and steam are fed to the anode and cathode, respectively. Steam on the cathode is split into oxygen ions that are transported through an oxygen ion-conducting electrolyte (i.e. YSZ) to oxidize the anode fuel. The dissociated hydrogen and residual steam are exhausted from the SOFEC cathode and then separated by condensation of the steam to produce pure hydrogen. The rationale was that in such an approach fuel provides a chemical potential replacing the external power conventionally used to drive electrolysis cells (i.e. solid oxide electrolysis cells). A SOFC is similar to the SOFEC by replacing cathode steam with air for power generation. To fulfill the cogeneration objective, a hybrid module comprising reversible SOFEC stacks and SOFC stacks was designed that planar SOFECs and SOFCs were manifolded in such a way that the anodes of both the SOFCs and the SOFECs were fed the same fuel, (i.e. natural gas or coal-derived fuel). Hydrogen was produced by SOFECs and electricity was generated by SOFCs within the same hybrid system. A stand-alone 5 kW system comprising three SOFEC-SOFC hybrid modules and three dedicated SOFC stacks, balance-of-plant components (including a tailgas-fired steam generator and tailgas-fired process heaters), and electronic controls was designed, though an overall integrated system assembly was not completed because of …
Date: June 30, 2009
Creator: Tao, Greg, G.; Virkar, Anil, V.; Bandopadhyay, Sukumar; Thangamani, Nithyanantham; Anderson, Harlan, U. & Brow, Richard, K.
Partner: UNT Libraries Government Documents Department
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