Development of Anode-Supported Tubular SOFC Stack

Microtubular anode-supported solid oxide fuel cells (μt-SOFC) were created with a metallic coil embedded in the anode to act as current collector. The electrochemical performance was experimentally examined by comparing the power density (PD) of μt-SOFC with embedded coils of different turns per unit length and composition (nickel and palladium). It is shown that an increase in the turns per unit length results in a proportional current density increase and in a quadratic increment of PD. Additional performance improvement is found for the cell with palladium current collector due to the higher catalytic activity for hydrogen oxidation.

The performance of anode-supported tubular SOFC was investigated in terms of the thickness of anode support. Considering the geometry of single cell, it can be readily known that the concentration polarization across the anode- support is the most significant factor to determine the performance of the cell. For this purpose, we fabricated single cells having different anode-support thickness of 1mm and 2mm for both cells the cathode layer ((La0.85Sr0.15)0.98MnO3) and electrolyte layer (8mol%yttria-stabilized zirconia) were coated at the same condition. The cell performance was increased with decreasing anode-support wall thickness. The single cell having wall thickness of 1mm showed a power density of 0.42mW/cm2 at 0.7V and 700oC. From the analysis of impedance spectra, it was concluded that the total polarization of the anode supported tubular SOFC was mainly governed by the concentration polarization caused by the mass transport though the anode- support.

A corrugated mesoscale structure on electrode–electrolyte interface for enhancing cell performance in anode-supported SOFC

Objectives: In this paper, modeling and simulation of an anode supported axially symmetric tubular SOFC was performed. The nodal and current density variation of properties of SOFC were analyzed. The tubular SOFC was fed with 89% H2-11% water as fuel and high temperature air as the oxidant. The molar flow rates were adjusted so as to obtain the specified 85% fuel utilization&25% O2 utilization of air. The SOFC was operated at a reference of 1 std. atm. pressure and 1000oC temperature. Also, electrochemical analysis and computational fluid dynamics study of various components was done. Methods: The model developed was in congruence with experimental data available for commercial prototypic 2.2 cm diameter Siemens Westinghouse tubular SOFC 1.5 m axial length located at Pittsburgh. Findings: The nodal variation graphs depicted a decreasing concentration of reactants and increasing one for the products along the length of the Tubular SOFC. The losses encountered due to polarizations are found to decrease along the SOFC. The Nernst potential and polarization losses as expected increase with an increasing current density. The graph for variation of Voltage with a change in current density showed a decrease in voltage developed with increasing current density and was found to very closely approach the plot developed for Siemens Westinghouse tubular SOFC experimental data. The results although closely matches the experimental results are also helpful in designing of a practical Tubular SOFC. Applications: The modeled tubular SOFC can be further improved by variation of properties along the radial direction; any angular variation in properties for homogenous system can still be ignored.

Fabrication and operation of a 1 kW class anode-supported flat tubular SOFC stack

This work addresses the development of anode-supported solid oxide fuel cells using alternative low-cost techniques such as cold die pressing of the anode substrates and spray deposition of electrolyte layers. The new techniques have been developed up to the fabrication of disc cells with a diameter of 120 mm consisting of a co-sintered anode support and the yttrium-stabilized zirconia electrolyte layer. The concept was demonstrated in a three-cell stack according to the Sulzer HEXIS design and operated successfully in steam-reformed natural gas at around 800°C. The cell performance reached 430 mW/cm 2 . Electrical efficiencies of >60% at a fuel utilization of 90% represent excellent performance for the non-sealed HEXIS stack design.

The present paper refers to an innovative SOFC concept named EVOLVE after a running FP7 project which integrates advanced materials and an inventive anode current collector made from a NiCrAl preoxidized foam impregnated with a conductive perovskite. Even if the concept is the beneficial combination of Metal supported cell (MSC) and Anode supported cell (ASC) technologies, strong improvements in terms of reliability and durability are expected. In order to enhance anode efficiency, the 3D microstructure needs to be precisely described at the microscopic scale. ISTEC has produced LST-GDC active anode layers using screen printing process. Back-scattered electrons (BSE) SEM images that contain relevant microstructure information have been realized by Centre des Matériaux. All the original images have been enhanced by Centre de Morphologie Mathématique using a series of morphological operators to remove artefacts. Finally, a 3D pluri-Gaussian model has been generated from 2D images, and then computed to predict volume fraction of the 3 phases, 3D tortuosity, gas permeation and ionic conductivities.

Present results on intermediate temperature-operating solid oxide fuel cells (IT-SOFC) are mainly focused to obtain higher performance of single cell at lower operating temperature, especially with planar type. In order to develop a 1 kW-class SOFC system for Residential Power Generation (RPG) application that can be operated at intermediate temperatures, we have developed an anode-supported and planar type SOFC stack using cost-effective interconnects such as ferritic stainless steels. To improve electrical performance of the anode-supported SOFC, cells with alternative electrode or thin electrolyte layer of YSZ and ScSZ, respectively, were fabricated by slurry coating and their performances were investigated. Mixtures of NiO and 8YSZ powders were pressed into rectangular plates, which were pre-sintered at 1400°C for 1 h. Then the 8YSZ (8mol%Y2O3+ZrO2) or 10ScSZ (10mol%Sc2O3+1 mol%CeO2+ZrO2) electrolytes were coated by slurry coating method on the upper side of anode substrate and cofired at 1550 or 1500°C for an hour. Thereafter, composite cathode pastes of LSM (La0.7Sr0.2MnO3) or LSCF ((La0.6Sr0.4)(Co0.2Fe0.8)O3 ) were screen-printed on thin electrolytes and heat-treated at 1100°C for 2 h. The final size of anode-supported single cell sintered was about 5 × 5 cm2, and the thickness of the electrolyte and the cathode layer was about 20 μm and 30 μm, respectively. The I-V and AC impedance characteristics of these single cells were evaluated at intermediate temperature (650 ∼ 750°C) using hydrogen gas as a fuel. The maximum power density of anode-supported cell was 0.4 ∼ 1.7 W/cm2 at 750°C and 0.12 ∼ 0.55 W/cm2 at 650°C depending on the composition and microstructure of electrolyte/electrodes.

The anode-supported solid oxide fuel cells (SOFCs) with an yttria-stabilized zirconia (YSZ) electrolyte have been successfully prepared by the sequential steps of fabricating technique including tape casting, spin coating, and screen-printing. Two types of the SOFCs are manufactured and their cell performances are characterized via the detail analysis of the electrochemical impedance spectroscopy (EIS). The maximum power densities are 411 and for the high and low performance cells at , respectively. According to the EIS analysis, the electrochemical performance of the poor cell is dominated by the anodic non-charge-transfer resistance, which is mainly resulted by the gas diffusion polarization due to the low porosity of the anode. The high performance cell is controlled by the simultaneous contributions of the ohmic resistance , the anodic polarization , and the cathodic polarization at the operation temperature of . However, the will dominate the performance of the cell as the temperature is lowered down to .

A systematic study was initiated of anode-supported solid oxide fuel cells (SOFCs) with Nd2NiO4 cathodes. This type of cathode, a mixed conducting K2NiF4-type material, can be considered as a candidate for SOFC applications. In this study, the influence of (1) the presence of a Ce0.8Gd0.2O1.9 (CGO) interlayer between the electrolyte and the cathode, (2) sintering conditions, and (3) the grain size of the powder on the performance of SOFCs with Nd2NiO4 cathodes was investigated in more detail. Results from current density-voltage characteristics and permeation and gas diffusion measurements showed that the electrochemical performance was promising for SOFCs including a CGO interlayer and a Nd2NiO4 cathode (prepared with a powder with a d50 of 0.5A∕cm2 or 0.8μm) sintered at 1100°C or higher; current density at 800°C and 700mV between 1.2 and 1.3A∕cm2. The microstructure of the cathode was such that no gas diffusion problems occurred.

Anode supported tubular solid oxide fuel cells with Ni-YSZ anode tube, 8 mol% Y2O3-stabilized ZrO2 electrolyte, and Sr-doped LaSrMnO3 cathode have been investigated. The anode tube was fabricated using extrusion process, and the electrolyte layer and the cathode layer were coated onto the anode tube using slurry-dipping process and sintered by co-firing process. The electrolyte was coated as a function of the withdrawal speed in the range of 2 to 44 mm/sec, the number of dippings in the range of 1 to 5 times, and of the slurry viscosity. The thickness of the coated electrolyte layer was independent of withdrawal speed. On the other hand, as the slurry viscosity and the number of dippings increased, the thickness and gas tightness of the coated electrolyte layer increased. Performance characteristics of the single cell with effective area of 20 cm2 were evaluated between 650 and 900 °C in humidified hydrogen with 3 % water and air. The open circuit voltage was 1.1089 V and the performance was 700 mA/cm2 and 470 mW/cm2 at 800 °C.

A planar anode-supported electrolyte was fabricated using a tape casting method that involved a single step cofiring process. A standard NiO∕8YSZ cermet anode, 8mol% YSZ electrolyte, and a lanthanum strontium manganite cathode were used for the solid oxide fuel cell unit cell. A pressurized cofiring technique allows the creation of a thin layer of dense electrolyte about 10μm without warpage. The open circuit voltage of the unit cell indicated negligible fuel leakage through the electrolyte film due to the dense and crack-free electrolyte layer. An electrochemical test of the unit cell showed a maximum power density up to 0.173W∕cm2 at 900°C. Approximated electrochemical properties, e.g., activation energy, Ohmic resistance, and exchange current density, indicated that the cell performance was significantly influenced by the electrode properties of the unit cell.

Biogas‐fed Solid Oxide Fuel Cell (SOFC) systems can be considered as interesting integrated systems in the framework of distributed power generation. In particular, bio‐methane and bio‐hydrogen produced from anaerobic digestion of organic wastes represent renewable carbon‐neutral fuels for high efficiency electrochemical generators.With such non‐conventional mixtures fed to the anode of the SOFC, the interest lies in understanding the multi‐physics phenomena there occurring and optimizing the geometric and operation parameters of the SOFC, while avoiding operating and fuel conditions that can lead to or accelerate degradation processes.In this study, an anode‐supported (Ni‐YSZ) tubular SOFC was considered; the tubular geometry enables a relatively easy separation of the air and fuel reactants and it allows one to evaluate the temperature field of the fuel gas inside the tube, which is strictly related to the electrochemical and heterogeneous chemical reactions occurring within the anode volume.The experiments have been designed to analyze the behavior of the cell under different load and fuel utilization (FU) conditions, providing efficiency maps for both fuels.The experimental results were used to validate a multi‐physics model of the tubular cell. The model showed to be in good agreement with the experimental data, and was used to study the sensitive of some selected geometrical parameters modification over the cell performances.

The performance characteristics of high-performance Ni-YSZ (yttria-stabilized zirconia) anode-supported solid oxide fuel cells (SOFCs) are examined under typical fuel cell operating conditions over a period of ∼3000 h when hydrogen contaminated with different concentrations of H2S was used as the fuel. Analyses reveal some new insights into the sulfur poisoning behavior of a Ni-YSZ anode. First, an initial drop in power output upon exposure to H2S-contaminated fuels is more dramatic in an anode-supported cell (with small cell resistance) than in electrolyte-supported cell (with large cell resistance), creating an illusion that a high-performance cell appears to be less sulfur tolerant than a low-performance cell. Second, the transition time for this rapid performance drop to cease is much longer for an anode-supported cell (with a thicker anode) than an electrolyte-supported cell (with a thinner anode). Third, the degree of sulfur poisoning (or the cell resistance increase due to sulfur poisoning) diminishes with operating cell current density, suggesting that the water produced at the active sites on anode surface from electro-oxidation of hydrogen (or oxygen ions) may promote the oxidation and removal of adsorbed sulfur. This effect becomes less pronounced at higher concentration of H2S, but still significant at ∼1 ppm of H2S. Fourth, the subsequent slow degradation in performance after the rapid performance drop upon initial exposure to H2S can be avoided, indicating that the previously reported slow degradation is unlikely the inherent behavior of a Ni-YSZ anode, but associated with other complications. This further implies that sulfur poisoning may be reversible and the performance drop could be fully recovered when the fuel is switched back to clean hydrogen.

In recent years, anode-supported solid oxide fuel cell (SOFC) technology gains greater attention. Kinetic and thermodynamic parameters play a pivotal role in influencing the overall efficiency and functionality of SOFCs. In present study, 3D model of a single cell anode-supported SOFC is developed using ANSYS fluent. Kinetic and thermodynamic parameters are optimized by comparison with experimental data. Optimized parameters are subsequently used to investigate the impact of pressure and fuel utilization on the overpotential. Simulation results indicate that optimized parameters give rise to an increased fuel consumption and hence water formation. This can be explained by the fact that optimized parameters give rise to an increased rate of reaction. Optimized parameters lead to a reduction in cell overpotential thereby increasing current and power densities at elevated pressure. Increased fuel utilization correlates with a decrease in power and current densities.