Single-chamber Solid Oxide Research Articles

SDC계 단실형 고체산화물 연료전지용 SDC-NiO 복합음극의 최적 환원 조건

We have determined an optimal reduction condition for NiO-based anode in single chamber solid oxide fuel cells that involve samaria-doped ceria (SDC) as an electrolyte. Optimal condition should not only induce sufficient reduction of NiO to Ni, but also prevent the reduction of SDC electrolyte in order to achieve high open circuit voltage (OCV) and power output. Thermodynamic consideration allowed us to determine the optimal anode reduction condition as 96%H₂-4%H₂O atmosphere at 250℃. This finding was in a good agreement with the experimental verifications by monitoring the conductivities of SDC and NiO under different reducing conditions.

Micro Single Chamber Solid Oxide Fuel Cell Characteristics at Various Operating Temperatures

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On the single chamber solid oxide fuel cells

Single chamber solid oxide fuel cells, SC-SOFCs, design and performance is discussed. It is shown that all of them operate on non-selective anodes. They operate on cathodes that become selective only under short residence times. As a result these cells are not functioning as true mixed reactant solid oxide fuel cells (MR-SOFC). The lack of selectivity is a serious draw back. True MR-SOFC can be constructed in ways that make them cheaper in fabrication, providing high power density, high fuel utilization and reduced explosion risk. The fact that SOFCs operate in a single cell is a necessary but not sufficient condition for the proper operation of MR-SOFCs. The selectivity looked for the anode and cathode is of a special kind. It selects between electrochemical and chemical reactions, preferring the first ones. Results reported here suggest that MR-SOFC can operate only up to ∼600 °C. At these relative low temperatures, materials other than oxides can also be considered for use. Future directions in the needed research in order to develop true MR-SOFCs are discussed.

From macro- to micro-single chamber solid oxide fuel cells

Single chamber solid oxide fuel cells (SC-SOFCs) with interdigitating electrodes were prepared and operated in CH 4/air mixtures. Both electrodes (Ni-Ce 0.8Gd 0.2O 1.9 cermet and Sm 0.5Sr 0.5CoO 3− δ perovskite) were placed on the same side of a Ce 0.8Gd 0.1O 1.95 electrolyte disc. The separating gap between the electrodes was varied from 1.2 to 0.27 mm and finally down to 10 μm. Screen-printing was used for the preparation of the cells with a gap in the millimetre range, whereas micromolding in capillaries (MIMIC) was used for the preparation of the micro-SC-SOFCs. The prepared micro-SC-SOFCs consisted of an array of 19 individual cells that were connected in parallel having 100 μm wide electrodes. An open circuit voltage of 0.65–0.75 V was measured in flowing mixtures of methane and air. The maximum power density of 17 mW cm −2 was limited by the ohmic resistance of the long conduction paths along the thin electrodes to the active sites of the individual cells. The feasibility of the micro-cell was demonstrated by comparing the performance with the performance of the cells having feature sizes in the millimetre range. The cell resistance of micro-SC-SOFCs may be significantly reduced when connecting the cells in series using interconnections between anode and cathodes of adjacent cells.

Single-Chamber SOFCs Using Dimethyl Ether and Ethanol

An anode-supported single-chamber solid oxide fuel cell (SC-SOFC), consisting of a electrolyte, a (SDC) cermet anode, and a cathode, was operated in a mixture feed of dimethyl ether (DME), ethanol or butane, and air at a furnace temperature of . This SC-SOFC showed comparatively poor performance for DME and ethanol fuels when compared to the performance for butane fuel, resulting from the relatively small difference in catalytic activity for DME and ethanol oxidation between the anode and the cathode. An effective improvement was achieved by attaching Ru/SDC/Ni and catalyst layers for DME and ethanol, respectively, on the anode surface. As a result, peak power densities of 64 and were obtained for DME and ethanol, respectively.

Single-Chamber Mini-Solid Oxide Fuel Cells Operated at a Lower Temperature

The performance characteristics of single chamber solid oxide fuel cell (SOFC) with micro-patterned electrode structures on the disc type electrolyte were investigated. We selected Sm0.2Ce0.8O2-x (samarium doped ceria, SDC) as an electrolyte, SDC-NiO cermat as an anode and SDC-Sm0.5Sr0.5CoO3(samarium strontium cobaltite) as a cathode for low operating temperature (500-600{degree sign}C). To fabricate complex micro-patterned electrodes on electrolyte, we used microfluidic lithography method. Before patterning the cathode, patterned NiO-SDC anode sintered at 1350{degree sign}C for 4 hrs, then SDC-SSC cathode was patterned and sintered at 1050{degree sign}C for 8 hrs, resulting in co- planar type single chamber SOFC in which interdigitated electrode has 100 μm line width with anode-to-cathode distance 100 μm. Reduction condition was proposed below 350{degree sign}C using humidified hydrogen gas. The shape of electrode and microstructure was observed by scanning electron microscope (SEM) and optical microscope. I-V characteristics of the SOFC were measured using a propane-air mixture gas by DC source measure unit.

Single-Chamber SOFC Operating with Hydrocarbon-Air and Hydrogen-Oxygen Gas Mixtures

A single-chamber solid oxide fuel cell (SC- SOFC), which is operated in a mixture of fuel and oxidant gas, provides several advantages over the conventional SOFC such as simplified cell structure (no sealing required). SC-SOFC allows using a variety of fuels without carbon deposition by selecting appropriate electrode materials and cell operating conditions. Operating conditions of single chamber SOFC was studied using hydrocarbon- air gas mixtures for a cell composed of NiO-YSZ/YSZ/LSCF-Ag. The cell performance and catalytic activity of the anode was measured at various gas flow rates. The results showed that the open-circuit voltage and the power density increased as the gas flow rate increased. A similar cell was tested in a double chamber configuration using hydrogen-air mixtures by controlling the hydrogen/air ratio at cathode and anode. Simulation of single chamber conditions in double chamber configurations allows distinguishing and better understanding of the electrode reactions in the presence of mixed gases. The effect of hydrogen-air mixtures as a fuel on the performance of anode and cathode materials in single-chamber and double- chamber SOFC configurations is presented.

Architectures and Performance of High-Voltage, Microscale Single-Chamber Solid Oxide Fuel Cell Stacks

In this study, we describe multi-cell SC-SOFC architectures with multiple different electrode/electrolyte systems. These SC-SOFCs are fabricated using thin-film techniques and patterned via photolithography. Micro SC-SOFCs with feature dimensions ranging from 5μm to 200μm are successfully constructed and repeatable. An OCV of approximately 380mV at 400{degree sign}C was found when using methane and air gases. OCV values were found to decrease with increasing temperature and a sign change in voltage was observed around 575{degree sign}C. Future studies will include the results from changing the sizes and spacing between electrodes to create a mechanistic study for SC-SOFCs to understand the rate limiting reactions and electrode selectivity. Additionally various SC-SOFC 'stacked' configurations of up to ten cells will be created. SC-SOFC variables will include changes in length, thickness, area and electrode/electrolyte material to observe the fuel cell stack performance as a result of the altered parameter.

Catalytic activity and performance of LSM cathode materials in single chamber SOFC

Abstract The catalytic activity of symmetrical LSM (La0.8Sr0.2MnO3) cells operated under single chamber solid oxide fuel cell (SC-SOFC) conditions was investigated for methane-to-oxygen ratios Rin between 1 and 2. The oxidation reactions over electrodes sintered at 1100 °C (LSM1100) and 1200 °C (LSM1200) were studied, and the effect of any combustion was followed through electrochemical impedance spectroscopy (EIS). The activity of the LSM1100 electrode increases with temperature. Above 700 °C, the conversion of the oxygen species may exceed 30%. As a consequence, oxygen depletion is occurring and a low frequency semicircle in the EIS spectra becomes predominant. An increase of the sintering temperature to 1200 °C leads to a decrease in the catalytic activity. A LSM1100 electrode deposited on a Julich half-cell proves to reach better performance at 600 °C than at 700 °C. On such complete cells, however, the catalytic combustion becomes much more complex than on a LSM cathode alone. We are thus proposing a comprehensive parameter, Rout, that is summarizing the processes inside the single chamber reactor.

Enhanced performance of a single-chamber solid oxide fuel cell with an SDC-impregnated cathode

The electrochemical performance of anode-supported single-chamber solid oxide fuel cells (SC-SOFCs) with and without SDC-impregnated cathodes was compared in a diluted methane–oxygen mixture. These cells were made of conventional materials including yttrium-stabilized zirconia (YSZ) thin film, a Ni + YSZ anode and a La 0.7Sr 0.3MnO 3 (LSM) cathode. Our results showed that the cell performance was greatly enhanced with the SDC-impregnated LSM cathode. At a furnace temperature of 750 °C, the maximum power density was as high as 404 mW cm −2 for a CH 4 to O 2 ratio of 2:1, which was 4.0 times higher than the cell with a pure LSM cathode (100 mW cm −2). The overall polarization resistance of the impregnated cell was 1.6 Ω cm 2, which was much smaller than that of the non-impregnated one (4.2 Ω cm 2). The impregnation introduced SDC nanoparticles greatly extended the electrochemical active zone and hence greatly improved the cell performance.

패턴된 전극을 가진 표면 전도형 단실형 고체산화물 연료전지의 제조

Co-planar type single chamber solid oxide fuel cell with patterned electrode on a surface of electrolyte has been fabricated by robodispensing method and microfluidic lithography. The cells were composed of NiO-GDC-Pd or NiO-SDC cermet anode, (La 0.7 Sr 0.3 ) 0.95 MnO₃ cathode, and yttria stablized zirconia electrolyte. The cell performance at 900℃ was investigated as a function of electrode geometries, such as anode-to-cathode distance, numbers of electrode pairs. Relationship between OCV and I-V characteristics at the optimized operation condition was also studied by DC source meter under the mixed gas condition of methane, air, and nitrogen. An increase of anode-facing-cathode area leads to lower OCV due to intermixing between product gases of anode and cathode, which in turn decreases the oxygen partial pressure difference.

Numerical Modeling of Single-Chamber SOFCs with Hydrocarbon Fuels

A two-dimensional numerical model of a single-chamber solid oxide fuel cell (SCFC) operating on hydrocarbon fuels is developed. The SCFC concept is a simplification of a conventional solid oxide fuel cell in which the anode and cathode are both exposed to the same premixed fuel-air mixture, and selective catalysts promote electrochemical oxidation of the fuel at the anode and simultaneous electrochemical oxygen reduction at the cathode. Optimization of SCFC stacks requires considering complex, coupled chemical and transport processes. The model accounts for the coupled effects of gas channel fluid flow, heat transfer, porous media transport, catalytic reforming-shifting chemistry, electrochemistry, and mixed ionic-electronic conductivity. It solves for the velocity, temperature, and species distributions in the gas, profiles of gaseous species and coverages of surface species within the porous electrodes, and the current density profile in an SCFC stack for a specified electrical bias. The model is general, and can be used to simulate any electrode processes for which kinetics are known or may be estimated. A detailed elementary mechanism is used to describe the reactions over the anode catalyst surface. Different design alternatives including yttria-stabilized zirconia vs electrolytes, the effects of mixed conductivity, and the optimal fuel-to-air ratio are explored.

Co-planar type single chamber solid oxide fuel cell with micro-patterned electrodes

A co-planar type single chamber solid oxide fuel cell (SC-SOFC) with linearly patterned electrode structures on the same surface as the electrolyte has been fabricated by robo-dispensing method. Paste materials of NiO-SDC-Pd cermet and (La0.7Sr0.3)0.95MnO3 (LSM) were selectively deposited onto a substrate of yttria stablized zirconia (YSZ) by extrusion through a syringe nozzle. The dispensed pastes were solidified upon solvent evaporation, and the anode and the cathode were sintered at 1350∘C for 2 h and 1200∘C for 1h, respectively. We have fabricated SC-SOFCs that have a single electrode pair with varying anode-to-cathode distances and interdigitated patterned electrodes with 2, 4, and 8 multiple pairs. The electrode microstructures of the resulting cells were examined by SEM. The electrochemical performance of the SC-SOFCs was also analyzed using impedance spectroscopy and a DC source meter.

High power-density single-chamber fuel cells operated on methane

Single-chamber solid oxide fuel cells (SC-SOFCs) incorporating thin-film Sm 0.15Ce 0.85O 1.925 (SDC) as the electrolyte, thick Ni + SDC as the (supporting) anode and SDC + BSCF (Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3− δ ) as the cathode were operated in a mixture of methane, oxygen and helium at furnace temperatures of 500–650 °C. Because of the exothermic nature of the oxidation reactions that occur at the anode, the cell temperature was as much as 150 °C greater than the furnace temperature. Overall, the open circuit voltage was only slightly sensitive to temperature and gas composition, varying from ∼0.70 to ∼0.78 V over the range of conditions explored. In contrast, the power density strongly increased with temperature and broadly peaked at a methane to oxygen ratio of ∼1:1. At a furnace temperature of 650 °C (cell temperature ∼790 °C), a peak power density of 760 mW cm −2 was attained using a mixed gas with methane, oxygen and helium flow rates of 87, 80 and 320 mL min −1 [STP], respectively. This level of power output is the highest reported in the literature for single chamber fuel cells and reflects the exceptionally high activity of the BSCF cathode for oxygen electro-reduction and its low activity for methane oxidation.

Electrode Performance of Nanostructured La1-aSraCo1-bFebO3-x on a Ce0.8Sm0.2O2 Electrolyte Prepared by Citrate-Nitrate Auto-Combustion

Nanostructured La1-aSraCo1-bFebO3-x (a=0.3-0.5; b=0-0.2) and Ce0.8Sm0.2O2 powders were successfully prepared by citrate- nitrate auto-combustion synthesis and were characterized by scanning electron microscopy and X-ray diffraction analysis. Electrochemical Impedance Spectroscopy measurements on symmetric cells were performed to evaluate the applicability of La1-aSraCo1-bFebO3-x as a cathode material for Single Chamber Solid Oxide Fuel Cells. Our results demonstrated that strontium has a positive effect while iron shows a slightly negative effect on the interfacial resistance. The effect of the electrode sintering temperature was investigated by varying the temperature from 850 {degree sign}C to 1250 {degree sign}C and the best sintering temperature was found to be 900 {degree sign}C. The best characteristics as cathode around the operating temperature of Single Chamber Fuel Cells were found for the composition with a=0.4 and the minimum iron doping; at 700{degree sign}C, an Area Specific Resistance of 0.13 ohm•cm2 was measured.

Thermodynamic Equilibrium of Single-Chamber SOFC Relevant Methane–Air Mixtures

Single-chamber solid oxide fuel cells (SC-SOFCs) rely on the selectivity of the anode and cathode materials for either the partial oxidation reaction of a hydrocarbon with oxygen from air (anode) or the oxygen reduction reaction (cathode). The observed open-circuit voltages (OCVs) of up to measured in SC-SOFCs suggest a very low oxygen partial pressure to be present at the anode side of the electrolyte while at the cathode it is more than 15 orders of magnitude higher but lower than in the gas phase. As lower boundary of the oxygen partial pressure in the anode the partial pressure of oxygen that is given by the equilibrium state of the gas mixture can be assembled. The initial nonequilibrium gas mixture of a fuel and air will react toward the equilibrium composition, which in turn is only a function of temperature and the initial gas composition. We present here the calculation of thermodynamic equilibrium for methane-air mixtures relevant in SC-SOFCs. Furthermore, we show that the calculation can describe qualitatively the observed OCVs and give guidelines to suitable gas mixtures for enabling high electric loads in SC-SOFCs.

Stable Single-Chamber Solid Oxide Fuel Cells Based on Doped Ceria Electrolytes and La0.5Sr0.5CoO3 as a New Cathode

Several cathodes working in hydrocarbons diluted atmospheres have been proposed for single-chamber solid oxide fuel cells (SOFCs) fabrication. But, few cathodes work well in propane. In this work, we propose a new stable cathode of La0.5Sr0.5CoO3 (LSCO) for the single-chamber SOFCs, which works well in diluted propane atmospheres. For this purpose, powders of Ce0.8Gd0.2O1.9 (CGO) for electrolytes and powders of (LSCO) for cathodes were prepared by sol-gel related techniques as pentadionate methods. These methods consist of preparing a solution from the acetates or pentadionates followed by a jellification. Then, the decomposition of the organic molecules is initiated, obtaining nanometric powders of the abovementioned compounds. In this way, we have obtained high purity powders of CGO and LSCO as confirmed by x-ray diffraction powder analysis. Microstructure was analyzed by SEM and TEM microscopy, the analysis of the powders obtained by sol-gel techniques shows a nanometric size grain (∼30nm) and reactivity for sintering was studied in detail. Very high-density electrolytes (∼95-99% of theoretical density) were obtained with better power density currents than in samples prepared by solid-state reaction samples. Uniaxial pressing ceria based electrolytes were prepared from these powders and characterized in one chamber reactor using propane as fuel and LSCO+CGO as cathode. Anodes were prepared from Ni+CGO mixtures. Different gas mixtures of propane and air were prepared and tested at different flow rates in order to optimize electrical properties. Stable single-chamber fuel cells working in diluted propane atmospheres could be prepared with these materials.

Development of Single Chamber Solid Oxide Fuel Cells with Coplanar Micro-Electrode Array

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Single Chamber Solid Oxide Fuel Cell with Micro-Patterned Interdigitated Electrodes

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Evaluation of Electrode Performances of Single-Chamber Solid Oxide Fuel Cells

The interfacial resistances of La1-xSrxCo1-yFeyO3-δ (denoted LSCF(10(1-x)/10x/10(1-y)/10y)) cathodes, and the catalytic activities of a Ni-Ce0.85Y0.15O1.925 (Ni-YDC) anode and an LSCF(2/8/8/2) cathode of a single-chamber solid oxide fuel cells (SOFCs) were investigated. LSCF cathodes with high oxide ion conductivities gave low interfacial resistances. LSCF cathodes with suitable thermal expansion coefficients formed favorable interfacial structures with a ceria-based electrolyte. Ni-YDC showed a higher conversion efficiency for CH4 and a lower selectivity for CO2 than LSCF(2/8/8/2). The single-chamber SOFC based on the Sm-doped ceria electrolyte with the Ni-YDC anode and LSCF(2/8/8/2) cathode showed a maximum power density of 186 mW/cm2 at 800°C.