| Summary: | 博士 === 元智大學 === 機械工程學系 === 105 === Anode–supported planar solid oxide fuel cells (SOFC) have received much attention, owing to their good power density at intermediate temperatures. The main advantages of power production through SOFCs are the high conversion efficiency, the absence of combustion, and fuel flexibility which allows a variety of fuels, including those derived from renewable sources, to be used. However, there are several challenges concerning the SOFC research. Little is known about the key technology of MEA (membrane electrode assembly) fabrication, sealing material, and flow fuel distribution. Therefore, this research work focused three important aspects of development and fabrication of the membrane electrode assembly (MEA), investigation of sealing material properties, design and fabrication of SOFC devices (single cell and cells stack).
First, Anode–supported cells are prepared by a sequence of hot pressing and co–sintering processes for solid oxide fuel cell (SOFC) applications. Commercially available porous anode tape (NiO/YSZ = 50 wt%/50 wt%), anode tape (NiO/YSZ = 30 wt%/70 wt%), and YSZ are used as the anode substrate, anode functional layer, and electrolyte layer, respectively. After hot pressing, the stacked layers are then sintered at different temperatures (1250 °C, 1350 °C, 1400 °C, 1450 °C) for 5 h in air. Different compressive loads are applied during the sintering process. A (La, Sr)MnO3 (LSM) paste is coated on the post–sintered anode–supported electrolyte surface as the cathode, and sintered at different temperatures (1100 °C, 1150 °C, 1200 °C and 1250 °C) for 2 h in air to generate anode–supported cells with dimensions of 60 #westeur024# 60 mm² (active reaction area of 50 #westeur024# 50 mm² ). SEM is used to investigate the anode structure of the anode–supported cells. In addition, confocal laser scanning microscopy is used to investigate the roughness of the cathode surfaces. At sintering temperatures of 1400 °C and 1450 °C, there is significant grain growth in the anode. Furthermore, the surface of the cathode is smoother at a firing temperature of 1200 °C. It is also found that the optimal compressive load of 1742 Pa led to a flatness of 168 µm/6 cm and a deformation of 0.72%. The open circuit voltage and power density of the anode–supported cell at 750 °C were 1.0 V and 178 mW cm-2, respectively.
Secondly, this thesis presents a post–experimental analysis of variously sealants–mica paper, flexible mica paper, and thermiculite 866–used in high–temperature SOFC operation. The sealants are exposed to air or hydrogen at 600 °C to 1000 °C for 100 h. The goal of this work is to investigate the thermal expansion properties (thickness expansion, coefficients of thermal expansion CTE and porosity), mechanical stability, and leakage during midterm operation. After the sealants fired at 1000 °C, their relative thicknesses increased to around 0.98, 1.01, and 0.568 mm, respectively. The coefficients of thermal expansion CTEs (600–1000 °C) for mica paper and flexible mica paper were calculated to be from 8.0#westeur024#10-4 K-1 to 10.0#westeur024#10-4 K-1, the CTEs of thermiculte 866 were around 1.0#westeur024#10-4 K-1. The relative porosity of thermiculite 866, as determined through the density method, changed from 15.4% to 28.7% for temperatures from 600 °C to 1000 °C, respectively. Scanning electron microscopy is used to investigate the structure of thermiculite 866. It is tested for leakage using hydrogen from 500 to 3000 cc min-1 at 25 °C and 800 °C. The leakage rates at 3000 cc min-1 are 4.06% and 8.4% at 25 °C and 800 °C, respectively.
Thirdly, a design of planar solid oxide fuel cells (SOFCs) based on a modified button cell was improved. A planar SOFC apparatus with a 6 cm #westeur024# 6 cm anode–supported single cell based on a modified button cell was designed for operation in both regular cell and button cell configurations. The apparatus was assembled with a regular cell with an active area of 5 cm #westeur024# 5 cm and with a button cell with an active area of 2.54 cm2. The fabricated cells could be used as regular cells or button cells with a variety of cell sizes. They consist of fuel and oxidant chambers that allow heat conversion and thus increase the temperatures of the fuel inlet and oxidant inlet streams. The chambers reduce the cooling effect, thereby improving the cell performance. In addition, another objective of this study was the design of a suitable heat exchanger. Since a high operating temperature of the SOFC improves its cell performance, the system temperature, which is due to the furnace, must be maintained sufficiently high to enable SOFC operation. In this study, a wire–in–tube–type heat exchanger was designed and fabricated. This device was installed in front of the fuel inlet of the SOFC apparatus to allow heat conversion processes to increase the temperature of the fuel inlet stream and to reduce the cooling effect and thus improve the cell performance. Our results indicate that the fuel outlet temperature of the system with the heat exchanger is much higher than that of the system without the heat exchanger. Thus, the performance of former system is greater than that of the latter system. This difference means that the former system increases the hydrogen inlet temperature by enhancing heat conversion.
Finally, an innovative two–dimensional (2D) solid oxide fuel cell (SOFC) stack based on a horizontal structure was developed. Planar anode–supported cells were utilized and integrated into a 2D SOFC stack. The stack was designed to be operated in parallel as a large single cell and in series as a stack. When the cells were connected in parallel, the 2D stack functioned as a large single cell that generates a high current. When the cells were connected in series, the stack generated a high voltage. This 2D SOFC stack thus functioned as a large single cell and as a stack. Test results are reported to show the feasibility of such a 2D design as a single cell and as a two–cell stack.
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