Srikanth Gopalan

Electrochemical Performance of Solid Oxide Fuel Cells Manufactured by Single Step Co-firing Process

Anode-supported planar solid oxide fuel cells (SOFC) were fabricated by a single step co-firing process. The cells were composed of a Ni + yittria-stabilized zirconia (YSZ) anode, a YSZ electrolyte, an industrial Ca-doped LaMnO 3 (LCM) (or lab-made LCM) + YSZ cathode active layer, and an industrial LCM (or lab-made LCM) cathode current collector layer. The fabrication processes involved tape casting of the anode, screen printing of the electrolyte and the cathode, and one step co-firing of the green-state cells at 1300°C for 2 h. The performance of the cells was greatly improved by optimization of these materials and fabrication processes. The electrochemical performance tests of these cells showed that they could provide a stable power density of 0.2-1.0 W/cm 2 with hydrogen as fuel and air as oxidant while operating in the temperature range 700-900°C. The effects of various polarization losses including ohmic polarization, activation polarization, and concentration polarization were studied by impedance spectroscopy measurements and curve-fitting experimentally measured voltage vs current density traces into an appropriate model. Based on these measurements and curve fitting results, the relationships between cell performance and various polarization losses and their dependence on temperature and microstructure, were rationalized.

Analysis of Electrochemical Performance of SOFCs Using Polarization Modeling and Impedance Measurements

Anode-supported planar solid oxide fuel cells (SOFCs) were fabricated by a single-step cofiring process. It comprised of a porous Ni + yttria-stabilized zirconia (YSZ) anode support, a porous and fine-grained Ni + YSZ anode active layer, a dense YSZ electrolyte, a fine-grained porous Ca-doped LaMnO 3 (LCM) + YSZ composite cathode active layer, and a porous LCM cathode current collector layer. The cell was tested between 700 and 800°C with humidified hydrogen (97% H 2 + 3% H 2 O) as the fuel and air as the oxidant. The measured maximum power densities were 1.42 W/cm 2 at 800°C, 1.20 W/cm 2 at 750°C, and 0.87 W/cm 2 at 700°C. The cell was also tested at 800°C with various compositions of fuel and oxidant, and the cell parameters, which included the area specific ohmic resistance, exchange current densities, and anodic and cathodic limiting current densities, were obtained through polarization modeling. The polarization resistances of the cell were calculated using the cell parameters obtained from modeling, and compared with the corresponding values measured using impedance spectroscopy. The effect of cell operating conditions on various polarization losses and performance was analyzed in detail using the polarization modeling results.

Evaluation of Electrophoretically Deposited CuMn1.8O4 Spinel Coatings on Crofer 22 APU for Solid Oxide Fuel Cell Interconnects

Dense and well-adhered Cumn 1.8 O 4 spinel-oxide coatings were successfully deposited on Crofer 22 APU substrates by a cost-effective electrophoretic deposition technique. The coatings were effective in reducing the oxidation rate of the stainless steel substrates. A diffusion model has been developed that indicates that the coated stainless steel substrates exhibit paralinear oxidation kinetics. The effective diffusivities in the coating and in the thermally grown oxide scale were calculated to be 2 X 10 -15 and 2.5 X 10 -16 cm 2 /s, respectively, at 800°C and 5 X 10 -16 and 1.6 X 10 -17 cm 2 /s, respectively, at 750°C. The as-processed coated sample showed only 4.6 mΩ cm 2 of area specific resistance at 800°C.

Soft X-Ray Spectroscopic Study of Dense Strontium-Doped Lanthanum Manganite Cathodes for Solid Oxide Fuel Cell Applications

The evolution of the Mn charge state, chemical composition, and electronic structure of La{sub 0.8}Sr{sub 0.2}MnO{sub 3} (LSMO) cathodes during the catalytic activation of solid oxide fuel cell (SOFC) has been studies using X-ray spectroscopy of as-processed, exposed, and activated dense thin LSMO films. Comparison of O K-edge and Mn L{sub 3,2}-edge X-ray absorption spectra from the different stages of LSMO cathodes revealed that the largest change after the activation occurred in the Mn charge state with little change in the oxygen environment. Core-level X-ray photoemission spectroscopy and Mn L{sub 3} resonant photoemission spectroscopy studies of exposed and as-processed LSMO determined that the SOFC environment (800 C ambient pressure of O{sub 2}) alone results in La deficiency (severest near the surface with Sr doping >0.55) and a stronger Mn{sup 4+} contribution, leading to the increased insulating character of the cathode prior to activation. Meanwhile, O K-edge X-ray absorption measurements support Sr/La enrichment nearer the surface, along with the formation of mixed Sr{sub x}Mn{sub y}O{sub z} and/or passive MnO{sub x} and SrO species.

Effect of Fuel Composition on Performance of Single-Step Cofired SOFCs

Anode-supported planar solid oxide fuel cells (SOFCs) were successfully fabricated employing a single-step cofiring process. The cells were comprised of a Ni + yttria-stabilized zirconia (YSZ) anode, a YSZ electrolyte, a Ca-doped LaMnO 3 (LCM) + YSZ cathode active layer, and an LCM cathode current collector layer. The fabrication process involved tape casting of the anode, screen printing of the electrolyte and the cathode, and single-step cofiring of the green-state cell in the temperature range of 1300-1330°C for 2 h. The maximum power densities were 1.50 W/cm 2 at 800°C, 1.20 W/cm 2 at 750°C, and 0.87 W/cm 2 at 700°C, with humidified hydrogen (97% H 2 -3% H 2 O) as fuel and air as oxidant. The experimentally measured voltage-current density (V-i) curves were fitted into a polarization model to obtain the area specific ohmic resistance, exchange current density (anodic and cathodic), anodic limiting current density, cathodic limiting current density, and effective binary diffusivity of hydrogen and water vapor in the anode as well as that of oxygen and nitrogen in the cathode. The cell was also tested at 800°C with various compositions of humidified hydrogen to simulate the effect of practical fuel utilization on the performance of single cells. The V-i curves obtained in various fuel compositions were successfully modeled by fitting only the exchange current density. Anodic and cathodic activation polarizations and the exchange current densities at various fuel compositions were determined. An analytical model describing H 2 -H 2 O reaction at the anode triple-phase boundaries was postulated based on the relationship between the anodic exchange current density and the hydrogen partial pressure in the fuel. The model predicted that the formation of water molecules from adsorbed hydrogen and hydroxyl radical was the rate-determining step in the anodic reaction.

Effect of Anode Active Layer on Performance of Single-Step Cofired Solid Oxide Fuel Cells

Anode-supported planar solid oxide fuel cells (SOFCs) with and without the anode active layer were fabricated by a single-step cofiring process. The cells were comprised of a porous Ni + yittria-stabilized zirconia (YSZ) anode support, a porous fine-grained Ni + YSZ anode active layer for some experiments, a dense YSZ electrolyte, a porous fine-grained Ca-doped LaMnO 3 (LCM) + YSZ cathode active layer, and a porous LCM cathode current collector layer. The fabrication process involved tape casting of the anode support followed by screen printing of the remaining component layers. The cells were then cofired at 1300°C for 2 h. Sintered cells were electrochemically tested between 700 and 800°C with air as oxidant and various compositions of humidified hydrogen as fuel to simulate the effect of fuel utilization on cell performance. Experimentally measured current density-voltage (I-V) characteristics of the cells were fitted into a polarization model, and the cell parameters, including the area-specific ohmic resistance, exchange current density, anodic limiting current density, cathodic limiting current density, effective binary diffusivity of hydrogen and water vapor in the anode, and that of oxygen and nitrogen in the cathode, were obtained. Evaluation of the electrochemical performance and the polarization modeling results indicated that the cell performance is dominated by the cathodic activation polarization at low fuel utilization condition. However, the cell performance loss due to the anodic activation polarization increases as the fuel utilization increases. The anode active layer significantly improves the cell performance at high fuel utilization by lowering the anodic activation polarization under H 2 O-rich fuel because the anode active layer has finer and less porous microstructures and increases the number of the reaction sites near the anode-electrolyte interface. Cell performance at high fuel utilization and the effect of the anode active layer on the cell performance were discussed in detail.

Materials System for Intermediate-Temperature (600-800°C) SOFCs Based on Doped Lanthanum-Gallate Electrolyte

AC complex impedance spectroscopy studies were conducted between 600 and 800°C on symmetrical cells that employed strontium-and-magnesium-doped lanthanum gallate electrolyte, La 0 . 9 Sr 0 . 1 Ga 0 . 8 Mg 0 . 2 O 3 (LSGM). The objective of the study was to identify the materials system for fabrication and evaluation of intermediate-temperature (600-800°C) solid oxide fuel cells (SOFCs). The slurry-coated electrode materials had fine porosity to enhance catalytic activity. Cathode materials investigated include La 1 - x Sr x MnO 3 (LSM), LSCF (La 1 - x Sr x Co y Fe 1 - y O 3 ), a two-phase particulate composite consisting of LSM-doped-lanthanum gallate (LSGM), and LSCF-LSGM. The anode materials were Ni-Ce 0 . 8 5 Gd 0 . 1 5 O 2 (Ni-GDC) and Ni-Ce 0 . 6 La 0 . 4 O 2 (Ni-LDC) composites. Experiments conducted with the anode materials investigated the effect of having a barrier layer of GDC or LDC in between the LSGM electrolyte and the Ni-composite anode to prevent adverse reaction of the Ni with lanthanum in LSGM. For proper interpretation of the beneficial effects of the barrier layer, similar measurements were performed without the barrier layer. The ohmic and the polarization resistances of the system were obtained over time as a function of temperature (600-800°C), firing temperature, thickness, and the composition of the electrodes. The study revealed important details pertaining to the ohmic and polarization resistances of the electrode as they relate to stability and the charge-transfer reactions that occur in such electrode structures.

Hydrogen Production Using Solid Oxide Membrane Electrolyzer with Solid Carbon Reductant in Liquid Metal Anode

A laboratory-scale solid oxide membrane (SOM) steam electrolyzer that can potentially utilize the energy value of coal or any hydrocarbon reductant to produce high purity hydrogen has been fabricated and evaluated. The SOM electrolyzer consists of an oxygen-ion-conducting yttria-stabilized zirconia (YSZ) electrolyte with a Ni-YSZ cermet cathode coated on one side and liquid tin anode on the other side. Hydrogen production using the SOM electrolyzer was successfully demonstrated between 900 and 1000{sup o}C by feeding a steam-rich gas to the Ni-YSZ cermet cathode and solid carbon reductant into the liquid tin anode. It was confirmed that the energy required for hydrogen production can be effectively lowered by feeding a solid carbon reductant in the liquid tin anode. A polarization model for the SOM electrolyzer was developed. The experimental data obtained under different operating conditions were curve fitted into the model to identify the various polarization losses. Based on the results of this study, work needed toward increasing the electrochemical performance of the SOM electrolyzer is discussed.

Gd0.2Ce0.8O1.9-Y0.08Sr0.88Ti0.95Al0.05O3 + δ Composite Mixed Conductors for Hydrogen Separation

A new approach to separate hydrogen using oxygen ion and electron/hole mixed conductors has been proposed and is being currently investigated. In this approach, oxygen from steam diffuses across the membrane as ions through coupled transport with electrons/holes, resulting in conversion of the steam into hydrogen. Methane reformation by the transported oxygen occurs on the other side of the membrane. While doped lanthanum strontium cobalt iron oxides possess high mixed conductivity under relatively oxidizing conditions, they do not sustain these conductivities under relatively reducing conditions. Further, they are unstable under the relatively reducing conditions expected to prevail on both sides of the membrane in the current process. A new dual-phase composite mixed ionic and electronic conducting (MIEC) membrane, comprised of doped ceria, an excellent oxygen-ion conductor, and doped strontium titanate, an excellent electronic conductor, has been developed. This dual-phase composite is stable in very reducing conditions and suitable for application in the proposed hydrogen separation process. The physical and electrical characterization of the composite comprising doped ceria and doped strontium titanate has been investigated in this study.

Polarization study on doped lanthanum gallate electrolyte using impedance spectroscopy

Alternating current complex impedance spectroscopy studies were conducted on symmetrical cells of the type [gas, electrode/La1−xSrxGa1−yMgyO3 (LSGM) electrolyte/electrode, gas]. The electrode materials were slurry-coated on both sides of the LSGM electrolyte support. The electrodes selected for this investigation are candidate materials for solid oxide fuel cell (SOFC) electrodes. Cathode materials include La1−xSrxMnO3 (LSM), La1−xSrxCoyFe1−yO3 (LSCF), a two-phase particulate composite consisting of LSM and doped-lanthanum gallate (LSGM), and LSCF + LSGM. Pt metal electrodes were also used for the purpose of comparison. Anode material investigated was the Ni + Ce0.85Gd0.15O2 composite. The study revealed important details pertaining to the charge-transfer reactions that occur in such electrodes. The information obtained can be used to design electrodes for intermediate temperature SOFCs based on LSGM electrolytes.