Co-fired Solid Oxide Fuel Cells Research Articles

Development of a Novel Co-fired SOFC at Murata

Murata Manufacturing Co., Ltd. has been developing planar-type solid oxide fuel cells (SOFCs) based on new design concept. The cell is fabricated by a single-step co-firing process and consists of an electrolyte, electrodes, current collectors, gas-separators with manifolds, and gas-flow channels. The membrane electrode assembly (MEA) is sandwiched between ceramic gas-separators with via-hole electrodes providing electrical paths between cells. Murata’s new concept stack achieved volume power density higher than 0.5kW/L. By arranging gas manifolds in the central part of the cell, the temperature distribution in plane of the cell was moderated while keeping high volume power density. As a result, the stack operated stably at high efficiency. A long-term operation has been carried out at 750°C over 8000h under galvanostatic condition. The cell power degradation rate was less than 0.20%/kh after 2000h operation. Cr poisoning in the LSCF cathode after durability test was not observed. That is one reason of high durability of the cell.

Sulfur Poisoning of LSCF Cathode in Single Step Co-fired SOFC

Murata Manufacturing Co., Ltd. has been developing planar-type solid oxide fuel cells (SOFCs) based on new design concept. The cell is fabricated by a single-step co-firing process and consists of an electrolyte, electrodes, current collectors, gas-separators with manifolds, and gas-flow channels. The membrane electrode assembly (MEA) is sandwiched between ceramic gas-separators with via-hole electrodes providing electrical paths between cells. Removing metal interconnects around cathode in the cell enables reducing Cr vapor from metal and achieving high durability. A long-term operation has been carried out at 750°C over 8000h under galvanostatic condition. Single cell voltage degradation rate was less than 0.20%/kh after 2000h operation, and Cr poisoning in the LSCF cathode after long term operation over 8000h was not observed. From these results, the cell structure is effective to reduce Cr vapor in the cathode. But Sulfur distribution in the cathode was observed by analyzing in detail. Larger amount of Sulfur to form SrSO4 was distributed at the air channel on the upstream than that on the downstream. And furthermore, Co segregation was observed in LSCF near the GDC barrier layer at the air channel on the upstream. To study sulfur poisoning mechanism, SO2 poisoning test was carried out with various temperature and current density. Distribution of Sulfur and Co depended on operating temperature and current density.

Development of a Novel Co-fired SOFC at Murata

Murata Manufacturing Co., Ltd. has been developing planar type solid oxide fuel cells (SOFCs) based on a new design concept. The cell is fabricated by a single-step co-firing process (1). In this study, we report the results of a new type of cell with an improved cooling ability to remove heat of the electrochemical reactions. By arranging the gas manifolds in the central line of the cell, the temperature distribution in the plane of the cell was moderated while keeping a high volume power density. As a result, the stack operated stably and achieved a high power efficiency. A long-term operation has been carried out at 750ºC over 8000 h under galvanostatic conditions. The cell power degradation rate was less than 0.20 %/kh after 2000 h operation. Cr-poisoning in the LSCF cathode after the durability test was not observed, which led to the high durability of the cell.

Sulfur Poisoning of LSCF Cathode in Single Step Co-fired SOFC

Murata Manufacturing Co., Ltd. has been developing single step co-fired planar type solid oxide fuel cells (SOFCs). A long-term operation had been carried out at 750°C for over 8000 h under galvanostatic condition. The single cell voltage degradation rate was less than 0.20 %/kh after 2000 h operation, and Cr-poisoning in the LSCF cathode after 10000h operation was not observed. However sulfur distribution in the La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) cathode was observed by detailed analysis. To study the S- poisoning mechanism, SO2 poisoning test was carried out at various temperatures, current density and air utilization (Ua). As a result, it revealed that the distribution of sulfur depended on the operating temperature, current density and Ua.

Performance Analysis of Single Step Co-fired Solid Oxide Fuel Cells (SOFCs)

Anode-supported planar solid oxide fuel cell (SOFC) was fabricated by a single step co-firing process, and tested between 700~800oC with humidified hydrogen (97% H2+3% H2O) as fuel and air as oxidant. The measured maximum power densities were 1.42W/cm2 at 800oC, 1.20W/cm2 at 750oC, and 0.87W/cm2 at 700oC. The cell was also tested at 800oC with various compositions of 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.

Performance Analysis of Single Step Co-fired Solid Oxide Fuel Cells (SOFCs)

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Effect of Anode Active Layer on Performance of Single Step Co-fired Solid Oxide Fuel Cells (SOFCs)

Anode-supported planar solid oxide fuel cells (SOFCs) with and without the anode active layer were fabricated by a single step co-firing 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 LaMnO3 (LCM) + YSZ cathode active layer, and a porous LCM cathode current collector layer. Fabricated cells were electrochemically tested between 700~800oC with air as oxidant and various compositions of humidified hydrogen as fuel to simulate the effect of fuel utilization on cell performance. The electrochemical performances of the cells were analyzed using a polarization model, and it indicated that the anode active layer significantly improves the cell performance at high fuel utilization by lowering the anodic activation polarization.

Electrical Performance of Calcium Doped Lanthanum Ferrite for use in Single-Step Co-Fired Solid Oxide Fuel Cells (SOFCs)

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Effect of Anode Active Layer on Performance of Single Step Co-fired Solid Oxide Fuel Cells (SOFC's) at High Fuel Utilizations

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Electrical Performance of Calcium doped Lanthanum Ferrite for use in Single-Step Co-fired Solid Oxide Fuel Cells (SOFCs)

AbstractIn the single-step SOFC co-firing process YSZ electrolytes with sintering aid densify at a temperature of ˜1300°C. Electrodes employed in the single-step co-fired SOFC must therefore sinter with the right microstructure at ˜1300°C. Calcium-doped lanthanum ferrite, La0.8Ca0.2FeO3±δ (LCF-20) was identified in earlier studies as a possible stable cathode material for the single-step co-fired SOFC. LCF-20 is also expected to be a more stable cathode material than LSCF (strontium and cobalt doped lanthanum ferrite). Four-probe conductivity tests yielded ˜93 S/cm at 800°C and showed an increase in conductivity as pO2 increases, characteristic of p-type conduction. LCF has a higher electrical conductivity compared to LSCF and LCM+YSZ cathode materials. Oxygen ion conductivity of LCF-20 obtained from permeability measurements is higher than that of YSZ and LSF-20. Therefore LCF has excellent mixed conducting properties to serve as a catalytically active cathode material for co-fired solid oxide fuel cells operating at intermediate temperatures. Electrochemical Impedance Spectroscopy (EIS) measurements were made on symmetrical cells fabricated with YSZ electrolyte and the electrode materials. A gadolium doped ceria (GDC) barrier layer was employed to prevent LCF/YSZ reaction. Comparison of LCF/GDC/YSZ/GDC/LCF EIS data to LCM+YSZ/YSZ/LCM+YSZ EIS data gathered using identical test conditions and electrode microstructures shows that LCF has a measured polarization resistance (Rp) of approximately half that seen in LCM+YSZ. Variations in cathode thickness and porosity show the best performance with a cathode of a critical thickness and finer porosity. Slight stoichiometric deviations in LCF result in the formation of a Ca-Fe-O liquid phase during electrode sintering at around 1220 C. The liquid phase migrates into and through the GDC layer. EDX line scans show the second phase to be rich in Ca and Fe. Thicker GDC layers seem to prevent the liquid phase from reaching the electrolyte/GDC interface. Structural analysis with TEM will be performed. The properties and the effect of the Ca-Fe-O phase on the cathodic performance of the cell are being investigated; however, preliminary results indicate that minor amounts of the Ca-Fe-O phase will not interfere with the electrochemical performance of the LCF cathode. The high temperature instability in LCF has been observed in other studies, but has not been studied specifically in the literature.

Anode Polarization Effects in Single Step Co-fired Solid Oxide Fuel Cells

SOFCs were fabricated employing a single step co-firing process and electrochemically characterized in the temperature range of 700-800oC with humidified hydrogen (3% H2O) and air. Voltage-current density characteristics (V-i) at 800oC in hydrogen-water vapor mixtures with compositions ranging from 3% to 73% water vapor have been obtained to simulate the effect of practical fuel utilization on the performance of single cells. The V-i plots obtained in hydrogen containing 3% water vapor were curve-fittd with a model to obtain the ohmic resistance of the cell, the exchange current density and the cathodic and anodic limiting current densities. The V-i curves obtained in other fuel compositions were successfully modeled by varying only the exchange current density and a physical interpretation of the anodic reaction was proposed.

Refractory Cathode Investigation for Single-Step Co-fired Solid Oxide Fuel Cells

A single-step co-firing process for fabricating planar solid oxide fuel cells (SOFCs) requires refractory electrodes to prevent excessive sintering of the electrode while facilitating full-density sintering of the electrolyte. Single cell current-potential curves and impedance measurements indicate that the majority of the performance losses occur in the cathode and are due to activation polarization. A-site deficient calcium and cerium doped lanthanum ferrite cathode powders were synthesized and investigated as possible refractory cathode materials with low activation polarization losses. Four-probe conductivity measurements indicated that all compositions were suitable as cathodes. However, reactivity with YSZ reduced the conductivity by as much as two orders of magnitude. Future experiments will investigate the applicability of a doped- ceria barrier layer to prevent reaction of lanthanum ferrite cathodes with YSZ electrolyte.

High Performance Low Cost Co-Fired Solid Oxide Fuel Cells

Anode-supported planar SOFCs were fabricated by a single step co-firing process. The cells comprised of Ni+yttria-stabilized zirconia (YSZ) anode, YSZ electrolyte, (La,Ca)MnO3 (LCM)+YSZ cathode active layer, and an LCM cathode current collector. The fabrication process involved tape casting of the anode, screen printing of the electrolyte and the cathode, and single step co-firing between 1300-1330{degree sign}C. Cells were tested between 700-800oC with humidified hydrogen and air. Modeling of cell- test results showed that most of the polarization losses were due to ohmic losses associated with the electrodes, activation polarization loss in the cathode, and concentration polarization loss in the anode. The ohmic loss was lowered by improving electrical contact, activation polarization was reduced by improving microstructure of the cathode active layer and the anode concentration polarization loss was lowered by increasing anode porosity. With all these improvements maximum power density of 1.5W/cm2 was achieved at 800oC with humidified H2(g) and air.

High Performance Single Step Co-Fired Solid Oxide Fuel Cells

AbstractAnode-supported planar solid oxide fuel cell (SOFC) was successfully fabricated by a single step co-firing process. The cell comprised of a Ni + yittria-stabilized zirconia (YSZ) anode, a YSZ electrolyte, a Ca-doped LaMnO3 (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 co-firing of the green-state cells at 1300°C for 2 hours. The maximum power densities were 1.50W/cm2 at 800°C, and 0.87W/cm2 at 700°C with humidified hydrogen (97% H2-3% H2O) as fuel and air as oxidant. The experimentally measured I-V curves were fitted into a polarization model to obtain cell parameters including the area specific ohmic resistance, exchange current density, effective binary diffusivity of hydrogen and water vapor in the anode, and that of oxygen and nitrogen in the cathode. The cell was also tested at 800°C with various compositions of humidified hydrogen to evaluate the cell performance at high fuel utilization, and the maximum power densities were 1.30W/cm2 with 75.7% H2-24.3% H2O, 0.94W/cm2 with 49.4% H2-50.6% H2O, and 0.49W/cm2 with 27.3% H2-70.7% H2O.

Refractory Cathode Investigation for Single-Step Co-Fired Solid Oxide Fuel Cells (SOFCs)

AbstractA single step co-firing process for fabricating solid oxide fuel cells (SOFCs) requires refractory electrodes to prevent excessive sintering of the electrode while facilitating full-density sintering of the electrolyte. Single cell current-potential curves and impedance measurements indicate that the majority of the performance losses occur in the cathode and are due to activation polarization. A-site deficient calcium and cerium doped lanthanum ferrite cathode powders were synthesized and investigated as possible refractory cathode materials with low activation polarization losses. Four-probe conductivity measurements indicated that all compositions were suitable as cathodes. However, reactivity with YSZ reduced the conductivity by as much as two orders of magnitude, too low for use as a cathode. These refractory cathode compositions could be effective if a suitable barrier layer is applied to prevent reaction with YSZ. Experiments will investigate the applicability of a doped-ceria barrier layer to prevent reaction between the lanthanum ferrite cathode layer and the YSZ electrolyte layer.