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Low-Temperature, Anode-Supported High Power Density Solid Oxide Fuel Cells With Nanostructured Electrodes

Description: 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}.
Date: June 21, 2001
Creator: Virkar, Anil V.
Partner: UNT Libraries Government Documents Department

Low-Temperature, Anode-Supported High Power Density Solid Oxide Fuel Cells With Nanostructured Electrodes

Description: Anode-supported solid oxide fuel cells with Ni + yttria-stabilized zirconia (YSZ) anode, YSZ-samaria-doped ceria (SDC) bi-layer electrolyte and Sr-doped LaCoO{sub 3} (LSC) + SDC cathode were fabricated. Fuel used consisted of H{sub 2} diluted with He, N{sub 2}, H{sub 2}O or CO{sub 2}, mixtures of H{sub 2} and CO, and mixtures of CO and CO{sub 2}. Cell performance was measured at 800 C with above-mentioned fuel gas mixtures and air as oxidant. For a given concentration of the diluent, the cell performance was higher with He as the diluent than with N{sub 2} as the diluent. Mass transport through porous Ni-YSZ anode for H{sub 2}-H{sub 2}O, CO-CO{sub 2} binary systems and H{sub 2}-H{sub 2}O-diluent gas ternary systems was analyzed using multicomponent gas diffusion theory. At high concentrations of the diluent, the maximum achievable current density was limited by the anodic concentration polarization. From this measured limiting current density, the corresponding effective gas diffusivity was estimated. Highest effective diffusivity was estimated for fuel gas mixtures containing H{sub 2}-H{sub 2}O-He mixtures ({approx}0.34 cm{sup 2}/s), and the lowest for CO-CO{sub 2} mixtures ({approx}0.07 cm{sup 2}/s). The lowest performance was observed with CO-CO{sub 2} mixture as a fuel, which in part was attributed to the lowest effective diffusivity of the fuels tested.
Date: September 26, 2001
Creator: Virkar, Anil V.
Partner: UNT Libraries Government Documents Department

Low-Temperature, Anode-Supported High Power Density Solid Oxide Fuel Cells With Nanostructured Electrodes

Description: Anode-supported cells comprising Ni + yttria-stabilized zirconia (YSZ) anode, thin ({approx}10 {micro}m) YSZ electrolyte, and composite cathodes containing a mixture of La{sub 0.8}Sr{sub 0.2}MnO{sub (3-{delta})} (LSM) and La{sub 0.9}Sr{sub 0.1}Ga{sub 0.8}Mg{sub 0.2}O{sub (3-{lambda})} (LSGM) were fabricated. The relative proportions of LSGM and LSM were varied between 30 wt.% LSGM + 70 wt.% LSM and 70 wt.% LSGM + 30 wt.% LSM, while the firing temperature was varied between 1000 and 1200 C. The cathode interlayer composition had a profound effect on cathode performance at 800 C with overpotentials ranging between 60 and 425 mV at 1.0 A/cm{sup 2} and exhibiting a minimum for 50 wt.% LSGM + 50 wt.% LSM. The cathodic overpotential decreased with increasing firing temperature of the composite interlayer in the range 1000 {le} T {le} 1150 C, and then increased dramatically for the interlayer fired at 1200 C. The cell with the optimized cathode interlayer of 50 wt.% LSM + 50 wt.% LSGM fired at 1150 C exhibited an area specific cell resistance of 0.18 {Omega}cm{sup 2} and a maximum power density of 1.4 W/cm{sup 2} at 800 C. Chemical analysis revealed that LSGM reacts with YSZ above 1000 C to form the pyrochlore phase, La{sub 2}Zr{sub 2}O{sub 7}. The formation of the pyrochlore phase at the interface between the LSGM/LSM composite cathode and the YSZ electrolyte limits the firing time and temperature of the cathode interlayer.
Date: March 26, 2002
Creator: Virkar, Anil V.
Partner: UNT Libraries Government Documents Department

Active Cathodes for Super-High Power Density Solid Oxide Fuel Cells Through Space Charge Effects

Description: 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.
Date: May 17, 2004
Creator: Virkar, Anil V.
Partner: UNT Libraries Government Documents Department

A Low-Cost Process for the Synthesis of Nanosize Yttria-Stabilized Zirconia (Ysz) by Molecular Decomposition

Description: This report summarizes the results of work done during the performance period on this project, between October 1, 2002 and December 31, 2003, with a three month no-cost extension. The principal objective of this work was to develop a low-cost process for the synthesis of sinterable, fine powder of YSZ. The process is based on molecular decomposition (MD) wherein very fine particles of YSZ are formed by: (1) Mixing raw materials in a powder form, (2) Synthesizing compound containing YSZ and a fugitive constituent by a conventional process, and (3) Selectively leaching (decomposing) the fugitive constituent, thus leaving behind insoluble YSZ of a very fine particle size. While there are many possible compounds, which can be used as precursors, the one selected for the present work was Y-doped Na{sub 2}ZrO{sub 3}, where the fugitive constituent is Na{sub 2}O. It can be readily demonstrated that the potential cost of the MD process for the synthesis of very fine (or nanosize) YSZ is considerably lower than the commonly used processes, namely chemical co-precipitation and combustion synthesis. Based on the materials cost alone, for a 100 kg batch, the cost of YSZ made by chemical co-precipitation is >$50/kg, while that of the MD process should be <$10/kg. Significant progress was made during the performance period on this project. The highlights of the progress are given here in a bullet form. (1) From the two selected precursors listed in Phase I proposal, namely Y-doped BaZrO{sub 3} and Y-doped Na{sub 2}ZrO{sub 3}, selection of Y-doped Na{sub 2}ZrO{sub 3} was made for the synthesis of nanosize (or fine) YSZ. This was based on the potential cost of the precursor, the need to use only water for leaching, and the short time required for the process. (2) For the synthesis of calcia-stabilized zirconia (CSZ), which has the ...
Date: May 6, 2004
Creator: Virkar, Anil V.
Partner: UNT Libraries Government Documents Department

Active Cathodes for Super-High Power Density Solid Oxide Fuel Cells Through Space Charge Effects

Description: This report summarizes the work done during the fourth quarter of the project. Effort was directed in two areas, namely, continued 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, and its relationship to cathode polarization; and fabrication of samaria-doped ceria porous (SDC). The work on the model development involves calculation of the effect of space charge on transport through porous bodies. Three specific cases have been examined: (1) Space charge resistivity greater than the grain resistivity, (2) Space charge resistivity equal to the grain resistivity, and (3) Space charge resistivity lower than the grain resistivity. The model accounts for transport through three regions: the bulk of the grain, the space charge region, and the structural part of the grain boundary. The effect of neck size has been explicitly incorporated. In future work, the effective resistivity will be incorporated into the effective cathode polarization resistance. The results will then be compared with experiments.
Date: December 12, 2003
Creator: Virkar, Anil V.
Partner: UNT Libraries Government Documents Department

Active Cathodes for Super-High Power Density Solid Oxide Fuel Cells Through Space Charge Effects

Description: This report summarizes the work done during the fifth 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 space charge region. (2) Fabrication of porous samaria-doped ceria (SDC) and investigation of the effect of thermal treatment on its conductivity. The model developed accounts for transport through three regions: (a) Transport through the bulk of the grain, RI, which includes parallel transport through space charge region. (b) Transport through the space charge region adjacent to the neck (grain boundary), RII. (c) Transport through the structural part of the neck (grain boundary), RIII. The work on the model development involves calculation RI, RII, RIII, and the sum of these three terms, which is the total resistance, as a function of the grain radius ranging between 0.5 and 5 microns and as a function of the relative neck size, described in terms of the angle theta, ranging between 5 and 45{sup o}. Three values of resistivity of the space charge region were chosen; space charge resistivity greater than grain resistivity, equal to grain resistivity, and lower than grain resistivity. Experimental work was conducted on samaria (Sm{sub 2}O{sub 3})-doped ceria (SDC) samples of differing porosity levels, before and after thermal treatment at 1200 C. The conductivity in the annealed samples was lower, consistent with enhanced Debye length. This shows the important role of space charge on ionic transport, and its implications concerning cathode polarization.
Date: March 8, 2004
Creator: Virkar, Anil V.
Partner: UNT Libraries Government Documents Department

Cathodes for Low Temperature Sofc: Issues Concerning Interference From Inert Gas Adsorption and Charge Transfer

Description: 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 ...
Date: May 5, 2004
Creator: Virkar, Anil V.
Partner: UNT Libraries Government Documents Department

Active Cathodes for Super-High Power Density Solid Oxide Fuel Cells Through Space Charge Effects

Description: 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.
Date: November 3, 2003
Creator: Virkar, Anil V.
Partner: UNT Libraries Government Documents Department

Electrically Conductive, Corrosion-Resistant Coatings Through Defect Chemistry for Metallic Interconnects

Description: 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.
Date: December 31, 2006
Creator: Virkar, Anil V.
Partner: UNT Libraries Government Documents Department

ACTIVE CATHODES FOR SUPER-HIGH POWER DENSITY SOLID OXIDE FUEL CELLS THROUGH SPACE CHARGE EFFECTS

Description: 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.
Date: September 21, 2005
Creator: Virkar, Anil V.
Partner: UNT Libraries Government Documents Department

Active Cathodes for Super-High Power Density Solid Oxide Fuel Cells Through Space Charge Effects

Description: This report summarizes the work done during the second quarter of the project. Effort is directed in two areas: (1) The use of a novel method to achieve a given porosity level with high contiguity and thus conductivity. (2) Relate the measured conductivity to porosity and contiguity. The rationale for these experiments was to develop cathodes with high ionic conductivity, so that the effective polarization resistance will be concomitantly lowered.
Date: October 11, 2003
Creator: Virkar, Anil V.
Partner: UNT Libraries Government Documents Department

Modeling Degradation in Solid Oxide Electrolysis Cells

Description: Idaho National Laboratory has an ongoing project to generate hydrogen from steam using solid oxide electrolysis cells (SOECs). To accomplish this, technical and degradation issues associated with the SOECs will need to be addressed. This report covers various approaches being pursued to model degradation issues in SOECs. An electrochemical model for degradation of SOECs is presented. The model is based on concepts in local thermodynamic equilibrium in systems otherwise in global thermodynamic no equilibrium. It is shown that electronic conduction through the electrolyte, however small, must be taken into account for determining local oxygen chemical potential, , within the electrolyte. The within the electrolyte may lie out of bounds in relation to values at the electrodes in the electrolyzer mode. Under certain conditions, high pressures can develop in the electrolyte just near the oxygen electrode/electrolyte interface, leading to oxygen electrode delamination. These predictions are in accordance with the reported literature on the subject. Development of high pressures may be avoided by introducing some electronic conduction in the electrolyte. By combining equilibrium thermodynamics, no equilibrium (diffusion) modeling, and first-principles, atomic scale calculations were performed to understand the degradation mechanisms and provide practical recommendations on how to inhibit and/or completely mitigate them.
Date: September 1, 2010
Creator: Sohal, Manohar S.; Virkar, Anil V.; Rashkeev, Sergey N. & Glazoff, Michael V.
Partner: UNT Libraries Government Documents Department

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