Investigation Of Micro Solid Oxide Fuel Cells For Portable Applications

Solid oxide fuel cells (SOFCs), which can convert chemical energy into electrical energy directly, are attracting more and more attentions because of their high-energy conversion efficiency, low pollution, and fuel flexibility. The portable or micro SOFC stack and diversity of fuel will be the key development orientation for SOFCs, which is of very great importance to reduce operational cost and increase utilization rate of Fossil Fuel. And now YSZ is still the most commonly used electrolyte for SOFCs due to its good chemical and thermal stability as well as pure oxygen-ion conduction in oxidizing and reducing atmosphere. The sintering temperature can be reduced and the electrical property can be increased when some impurities were added into YSZ, which is also very important for conservation of energy and developing the SOFC with high performance.Firstly a novel whole membrane components micro SOFC stack on solid porous support was developed in this thesis. And the feasibility of this design was demonstrated. Then we discussed the effect of the impurities on the ionic conductivity and sintering temperature for YSZ electrolyte. Finally we reformed the traditional Ni-base reforming catalyst in order to increase its stability and capability of the carbon tolerance for hydrocarbon fuel applications.Based on the principle of SOFC miniaturization, the single cells were made on the porous support by two membrane fabrication techniques, centrifugal casting and colloidal deposition respectively. The result shows that colloidal deposition technique is more useful to obtain the higher cell performance compared to the centrifugal casting. The Screen printing was employed to fabricate a pure LSM current collector layer on the LSM-YSZ cathode in order to reduce the interfacial contact resistance and the electrode polarization resistance. The result shows the cell performance was improved dramatically. On the basis of the success of the single cell, the micro SOFC stacks containing 2⁴ cells were made successfully and good output performance was obtained with the humidified hydrogen as the fuel. The results show, at 800℃, the OCV of the three-cell-stack and four-cell-stack are 2.7V and 3.85V and the maximal output power density are 601 mW/cm2 and 491 mW/cm2 respectively. The EIS results show that the dominant loss of the micro SOFC stacks are from the electrode polarization loss. So attempts to improve the cell performance should focus on seeking perfect electrode materials and optimizing the electrode microstructure.During the preparation process of SOFCs, because the sintering temperature of YSZ electrolyste is the highest compared to other components, it is hardly possible to cosinter the YSZ and cathode toghter, which lead to the long fabrication time and high power comsumption. To reduce the sintering temperature, some impurities such as transition metal oxide (Fe2O3) were added into YSZ as sintering promoters. In this thesis, pure YSZ and Fe doped (2mol% and 4mol%) YSZ composite electrolytes were prepared and sintered at 1200°C and 1400°C respectively by solid state method. The single SOFCs with the pure YSZ and Fe doped YSZ composite electrolyte have been assembled and measured utilizing the H₂. The research results show the structures of pure YSZ and YSZ doped with different percentage ofFe are cubic fluorite phase, which means that the crystal structure is not changed by Fe doping. But Fe dopant can increase the YSZ electrolyte density and performance of SOFC and affect its total conductivity.All the SOFCs with Fe-doped YSZ electrolytes show better performance and higher OCV than those with Fe-free electrolytes at same test temperature. It is the important development content for portable SOFCs to utilize the hydrocarbon fuel.The main issues associated with the utilization of hydrocarbon in SOFC are carbon formation on anode and low reaction rate. The internal reforming process can be considered as an inexpensive way for the production of synthesis gas (CO+H₂).It is efficient path to avoid coking and promote the reaction rate by utilizing the synthesis gas as fuel for SOFC. We successfully synthesized the Ni/Sn surface alloy catalyst supported on SLT support for the CH4 and C3H8 reforming reaction with CO₂ as well as steam reforming reaction. The research results show for low Sn concentrations, for example 1wt%Sn with respect to Ni, the Ni/Sn surface alloy is energetically preferred. We have also used XPS to characterize the catalysts. The quantified Sn and Ni XP spectra for Ni/Sn/SLT catalysts show that Sn preferentially segregates to the surface of the Ni/Sn alloy. We demonstrate that supported Ni/Sn alloy catalyst provides significantly higher reforming reactivity and resistance to deactivation via carbon deposition than supported monometallic Ni catalyst both in steam reforming of CH4 and CO₂ dry reforming. The synthesized catalysts, 1 wt% Sn/Ni alloy supported on SLT, were tested in the CO₂ reforming of methane with CH4/CO₂ ratio of 1 at 800℃for 48 hours. The alloy catalyst shows good stability for CO₂ reforming of CH4 during whole test period. The catalytic performance for Ni/Sn alloy is similar to the monometallic Ni .The formation of graphitic carbon was not observed. With increasing of CH4/CO₂ ratio, the activity of Ni/Sn decreases. A little carbon deposits can be detected when the stability test was carried out at CH4/CO₂ ratio of 60/40 for 26 hours. The activity of the alloy catalyst is sensitive to the Sn loading both for CO₂ reforming and steam reforming of CH4. The increased Sn loading leads to catalytic activity decrease. In addition, for the steam reforming reaction, steam to CH4 ratio (S/C) also influences the activity of Ni/Sn alloy catalyst. Increased S/C ratio can suppress the degradation of catalyst to some extent. A single SOFC with the Ni/Sn alloy catalyst have been assembled and measured utilizing the H₂ and mixture of CH4-CO₂ as fuel respectively at 800℃. Although the performance of the single cell using mixture of CH4-CO₂ is 50% lower than that using hydrogen, the stable performance was kept for 48 hours and no degradation was observed during test. Also no carbon deposit was detected on cell anode and alloy catalyst.