40 research outputs found
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A High Temperature Electrochemical Energy Storage System Based on Sodium Beta-Alumina Solid Electrolyte (Base)
This report summarizes the work done during the period September 1, 2005 and March 31, 2008. Work was conducted in the following areas: (1) Fabrication of sodium beta{double_prime} alumina solid electrolyte (BASE) using a vapor phase process. (2) Mechanistic studies on the conversion of {alpha}-alumina + zirconia into beta{double_prime}-alumina + zirconia by the vapor phase process. (3) Characterization of BASE by X-ray diffraction, SEM, and conductivity measurements. (4) Design, construction and electrochemical testing of a symmetric cell containing BASE as the electrolyte and NaCl + ZnCl{sub 2} as the electrodes. (5) Design, construction, and electrochemical evaluation of Na/BASE/ZnCl{sub 2} electrochemical cells. (6) Stability studies in ZnCl{sub 2}, SnCl{sub 2}, and SnI{sub 4} (7) Design, assembly and testing of planar stacks. (8) Investigation of the effect of porous surface layers on BASE on cell resistance. The conventional process for the fabrication of sodium ion conducting beta{double_prime}-alumina involves calcination of {alpha}-alumina + Na{sub 2}CO{sub 3} + LiNO{sub 3} at 1250 C, followed by sintering powder compacts in sealed containers (platinum or MgO) at {approx}1600 C. The novel vapor phase process involves first sintering a mixture of {alpha}-alumina + yttria-stabilized zirconia (YSZ) into a dense ceramic followed by exposure to soda vapor at {approx}1450 C to convert {alpha}-alumina into beta{double_prime}-alumina. The vapor phase process leads to a high strength BASE, which is also resistant to moisture attack, unlike BASE made by the conventional process. The PI is the lead inventor of the process. Discs and tubes of BASE were fabricated in the present work. In the conventional process, sintering of BASE is accomplished by a transient liquid phase mechanism wherein the liquid phase contains NaAlO{sub 2}. Some NaAlO{sub 2} continues to remain at grain boundaries; and is the root cause of its water sensitivity. In the vapor phase process, NaAlO{sub 2} is never formed. Conversion occurs by a coupled transport of Na{sup +} through BASE formed and of O{sup 2-} through YSZ to the reaction front. Transport to the reaction front is described in terms of a chemical diffusion coefficient of Na{sub 2}O. The conversion kinetics as a function of microstructure is under investigation. The mechanism of conversion is described in this report. A number of discs and tubes of BASE have been fabricated by the vapor phase process. The material was investigated by X-ray diffraction (XRD), optical microscopy and scanning electron microscopy (SEM), before and after conversion. Conductivity (which is almost exclusively due to sodium ion transport at the temperatures of interest) was measured. Conductivity was measured using sodium-sodium tests as well as by impedance spectroscopy. Various types of both planar and tubular electrochemical cells were assembled and tested. In some cases the objective was to determine if there was any interaction between the salt and BASE. The interaction of interest was mainly ion exchange (possible replacement of sodium ion by the salt cation). It was noted that Zn{sup 2+} did not replace Na+ over the conditions of interest. For this reason much of the work was conducted with ZnCl{sub 2} as the cathode salt. In the case of Sn-based, Sn{sup 2+} did ion exchange, but Sn{sup 4+} did not. This suggests that Sn{sup 4+} salts are viable candidates. These results and implications are discussed in the report. Cells made with Na as the anode and ZnCl{sub 2} as the cathode were successfully charged/discharged numerous times. The key advantages of the batteries under investigation here over the Na-S batteries are: (1) Steel wool can be used in the cathode compartment unlike Na-S batteries which require expensive graphite. (2) Planar cells can be constructed in addition to tubular, allowing for greater design flexibility and integration with other devices such as planar SOFC. (3) Comparable or higher open circuit voltage (OCV) than the Na-S battery. (4) Wider operating temperature range and higher temperature operation than the Na-S battery. (5) If a cell fails, it fails in the short circuit mode unlike Na-S batteries. Also, cells were successfully subjected to several freeze-thaw cycles. Finally, the feasibility of assembling a planar stack was explored. A two cell stack was assembled and tested. A five cell stack was assembled
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Electrically Conductive, Corrosion-Resistant Coatings Through Defect Chemistry for Metallic Interconnects
The principal objective of this work was to develop oxidation protective coatings for metallic interconnect based on a defect chemistry approach. It was reasoned that the effectiveness of a coating is dictated by oxygen permeation kinetics; the slower the permeation kinetics, the better the protection. All protective coating materials investigated to date are either perovskites or spinels containing metals exhibiting multiple valence states (Co, Fe, Mn, Cr, etc.). As a result, all of these oxides exhibit a reasonable level of electronic conductivity; typically at least about {approx}0.05 S/cm at 800 C. For a 5 micron coating, this equates to a maximum {approx}0.025 {Omega}cm{sup 2} area specific resistance due to the coating. This suggests that the coating should be based on oxygen ion conductivity (the lower the better) and not on electronic conductivity. Measurements of ionic conductivity of prospective coating materials were conducted using Hebb-Wagner method. It was demonstrated that special precautions need to be taken to measure oxygen ion conductivity in these materials with very low oxygen vacancy concentration. A model for oxidation under a protective coating is presented. Defect chemistry based approach was developed such that by suitably doping, oxygen vacancy concentration was suppressed, thus suppressing oxygen ion transport and increasing effectiveness of the coating. For the cathode side, the best coating material identified was LaMnO{sub 3} with Ti dopant on the Mn site (LTM). It was observed that LTM is more than 20 times as effective as Mn-containing spinels. On the anode side, LaCrO3 doped with Nb on the Cr site (LNC) was the material identified. Extensive oxidation kinetics studies were conducted on metallic alloy foils with coating {approx}1 micron in thickness. From these studies, it was projected that a 5 micron coating would be sufficient to ensure 40,000 h life
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Active Cathodes for Super-High Power Density Solid Oxide Fuel Cells Through Space Charge Effects, 2006, April 10
This report briefly summarizes the work done over the duration of the project, beginning October 1, 2003 and ending September 30, 2005. This project was on understanding cathode mechanisms in intermediate temperature solid oxide fuel cells (SOFC) and developing superior cathodes
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Functionally Optimized Ceramic Structures
The feasibility of using the Fused Deposition of Ceramics (FDC) process to rapidly fabricate
functional quality advanced ceramic components has been demonstrated multiple extrusion
heads enable the deposition of spatially engineered ceramic microstructures on the scale of 250
um. This unique capability of FDC allows components to be built with combinations of materials
and properties that are difficult or impossible to produce using conventional fabrication
processes. Some concepts will be presented, along with examples of multiple material laminates
produced using FDC. Strength data will be presented which demonstrates the performance
improvement possible using spatially engineered microstructures.Mechanical Engineerin
Development of a Novel Efficient Solid-Oxide Hybrid for Co-generation of Hydrogen and Electricity Using Nearby Resources for Local Application
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 limited resources. An inexpensive metallic interconnects fabrication process was developed in-house. BOP components were fabricated and evaluated under the forecasted operating conditions. Proof-of-concept demonstration of cogenerating hydrogen and electricity was performed, and demonstrated SOFEC operational stability over 360 hours with no significant degradation. Cost analysis was performed for providing an economic assessment of the cost of hydrogen production using the targeted hybrid technology, and for guiding future research and development
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Active Cathodes for Super-High Power Density Solid Oxide Fuel Cells Through Space Charge Effects Quarterly Report
This report summarizes the work done during the sixth quarter of the project. Effort was directed in three areas: (1) Further development of the model on the role of connectivity on ionic conductivity of porous bodies, including the role of grain boundaries and space charge region. (2) Calculation of the effect of space charge and morphology of porous bodies on the effective charge transfer resistance of porous composite cathodes. (3) The investigation of the three electrode system for the measurement of cathodic polarization using amperometric sensors
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Cathodes for Low Temperature Sofc: Issues Concerning Interference From Inert Gas Adsorption and Charge Transfer
This report summarizes the work done on the project over the duration of the project, from October 1, 2002 through December 31, 2003, which includes a three month no-cost extension. Effort was directed in the following areas: (1) Fabrication of Sr-doped LaCoO3 (LSC) dense and porous samples. (2) Design and construction of a conductivity relaxation apparatus for the estimation of surface exchange coefficient, k{sub chem}, which depends on adsorption, and oxygen chemical diffusion coefficient, {tilde D}{sub 0}, the parameters which are thought to describe the cathodic activation polarization (overall charge transfer) in mixed ionic electronic conducting (MIEC) cathodes. (3) The measurement of and K{sub chem} and {tilde D}{sub 0} on LSC by conductivity relaxation, as a function of temperature and oxygen partial pressure, p{sub O{sub 2}}. (4) Fabrication of YSZ electrolyte discs with patterned LSM and LSC electrodes with three-phase boundary (TPB) length, l{sub TPB}, varying between 50 and 1200 cm{sup -1}. (5) The measurement of charge transfer resistance, R{sub ct}, and estimation of the charge transfer resistivity, {rho}{sub ct}, as a function of temperature and p{sub O{sub 2}}, and the incorporation of the adsorption step in the analysis. (6) Preliminary cell tests with oxidants having different inert gas diluents; N{sub 2}, Ar, and CO{sub 2}. Dense samples of LSC of thickness as small as 150 microns were fabricated by sintering followed by grinding. Porous samples of LSC were also fabricated wherein the porosity was {approx}30%. Both samples were used in conductivity relaxation experiments. Analysis of data from the dense samples gives both and k{sub chem} and {tilde D}{sub 0}, while that of porous samples gives k{sub chem}. It was observed that at a given temperature, k{sub chem} increases with increasing p{sub O{sub 2}}, while the {tilde D}{sub 0} is essentially a constant. The dependence of k{sub chem} on p{sub O{sub 2}} is attributed to the adsorption step. It was also observed that the porous samples gave a more accurate measurement of k{sub chem}, as the data were not influenced by {tilde D}{sub 0}. By contrast, the results on dense samples were influenced by {tilde D}{sub 0}, especially at lower temperatures. It is thus concluded that the use of porous samples is preferred for the measurement of k{sub chem}. In the case of composite electrodes, such as LSM + YSZ, the relevant parameters are the {rho}{sub ct} (or R{sub ct}) and the ionic resistivity of YSZ {rho}{sub i}, where 1/{rho}{sub ct} is analogous to k{sub chem} and 1/{rho}{sub i} is analogous to {tilde D}{sub 0}. LSM patterned electrodes were deposited on YSZ discs using photomicrolithography. The R{sub ct} was measured as a function of temperature and p{sub O{sub 2}}using complex impedance techniques, on samples with l{sub TPB} varying between 50 and 1200 cm{sup -1}. The plot of 1/R{sub ct} vs. l{sub TPB} was linear, consistent with the occurrence of charge transfer at TPB. Also, the data plotted on the assumption of dissociative adsorption was consistent with the model. The significance of the role of adsorption is discussed. Similar results were observed with LSC, indicating a similar role of adsorption. In the case of LSC, however, a significant transport of oxygen also occurs through the dense part of the electrode. Preliminary work was conducted on the testing of button cells with mixtures of O{sub 2} + N{sub 2}, O{sub 2} + Ar, and O{sub 2} + CO{sub 2} as oxidants, wherein the p{sub O{sub 2}} was varied between {approx}0.05 and {approx}1.0 atm. As expected, the results showed that the higher the p{sub O{sub 2}}, the better was the performance. In pure oxygen, the maximum power density at 800 C was {approx}2.9 W/cm{sup 2}. However, in 5% O{sub 2}, it was {approx}0.6 W/cm{sup 2}. This difference is attributed to adsorption, indicating that both charge transfer and adsorption needs to be addressed in order to improve cathode performance at lower temperatures and under high oxidant utilization (in low p{sub O{sub 2}} atmospheres). Data at low current densities was analyzed to deduce the polarization resistance, R{sub p}, and its dependence on p{sub O{sub 2}}. Although preliminary, the results show differences in R{sub p}, which are attributed to possible differences in adsorption characteristics. Specifically, it was observed that the polarization resistance was lower in O{sub 2} + CO{sub 2} atmospheres compared to O{sub 2} + N{sub 2} and O{sub 2} + Ar atmospheres. This suggests that CO{sub 2} has a lower tendency for adsorption compared to the other two diluents
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Active Cathodes for Super-High Power Density Solid Oxide Fuel Cells Through Space Charge Effects
This report summarizes the work done during the third quarter of the project. Effort was directed in two areas: (1) Further development of the model on the role of connectivity on ionic conductivity of porous bodies, including the role of grain boundaries, and its relationship to cathode polarization. Included indirectly through the grain boundary effect is the effect of space charge. (2) Synthesis of LSC + SDC composite cathode powders by combustion synthesis. (3) Fabrication and testing of anode-supported single cells made using synthesized LSC + ScDC composite cathodes
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ACTIVE CATHODES FOR SUPER-HIGH POWER DENSITY SOLID OXIDE FUEL CELLS THROUGH SPACE CHARGE EFFECTS
This report summarizes the work done during the eleventh quarter of the project. Conductivity relaxation experiments were conducted on porous La{sub 0.5}Sr{sub 0.5}CoO{sub (3-{delta})} (LSC50) samples over a temperature range from 350 to 750 C, and over an oxygen partial pressure, p{sub O{sub 2}}, switch between 0.04 and 0.06 atm in order to determine the surface exchange coefficient, k{sub chem}. The normalized conductivity data could be fitted to a first order kinetic equation. The time constant decreased with decreasing temperature between {approx}750 and {approx}450 C, but sharply increased with decreasing temperature between 450 and 350 C. The corresponding k{sub chem} was estimated using three models: (a) A porous body model wherein it is assumed that the kinetics of surface exchange is the slowest. (b) Solution to the diffusion equation assuming the particles can be approximated as spheres. (c) Solution to the diffusion equation assuming the particles can be approximated as cylinders. The values of k{sub chem} obtained from the three models were in good agreement. In all cases, it was observed that k{sub chem} increases with decreasing temperature between 750 and 450 C, but below 450 C, it sharply decreases with further decrease in temperature
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Low-Temperature, Anode-Supported High Power Density Solid Oxide Fuel Cells With Nanostructured Electrodes
A simple, approximate analysis of the effect of differing cathode and anode areas on the measurement of cell performance on anode-supported solid oxide fuel cells, wherein the cathode area is smaller than the anode area, is presented. It is shown that the effect of cathode area on cathode polarization, on electrolyte contribution, and on anode resistance, as normalized on the basis of the cathode area, is negligible. There is a small but measurable effect on anode polarization, which results from concentration polarization. Effectively, it is the result of a greater amount of fuel transported to the anode/electrolyte interface in cases wherein the anode area is larger than the cathode area. Experiments were performed on cells made with differing cathode areas and geometries. Cathodic and anodic overpotentials measured using reference electrodes, and the measured ohmic area specific resistances by current interruption, were in good agreement with expectations based on the analysis presented. At 800 C, the maximum power density measured with a cathode area of {approx}1.1 cm{sup 2} was {approx}1.65 W/cm{sup 2} compared to {approx}1.45 W/cm{sup 2} for cathode area of {approx}2 cm{sup 2}, for anode thickness of {approx}1.3 mm, with hydrogen as the fuel and air as the oxidant. At 750 C, the measured maximum power densities were {approx}1.3 W/cm{sup 2} for the cell with cathode area {approx}1.1 cm{sup 2}, and {approx}1.25 W/cm{sup 2} for the cell with cathode area {approx}2 cm{sup 2}