Construction And Characterization Of Micro-and Nano-Meter Scale Composite Cathodes For SOFC

Since the degradation problems resulted from long term operation and the drop of lifetime can be effectively relieved at intermediate temperatures, the intermediate temperature solid oxide fuel cell (IT-SOFC) is now the main research direction of the SOFC field. However, as the operation temperature decreases, the catalytic activity of the cathode decreases and the polarization resistance increases, which is the key problem limiting the development of IT-SOFCs. In order to solve the problem, optimum microstructure design is an effective way to improve the electrochemical performance of the cathodes at intermediate temperatures. On the one hand, the high specific surface area and catalytic activity of the nanomaterial can significantly enlarge the triple-phase boundary and lower the polarization resistance of the SOFC cathode, improving its electrochemical performance. On the other hand, fast gas phase transportation can be achieved by optimizing the pore structure and pore size distribution of the cathode membrane.In this study, three-dimensionally ordered, one-dimensional nanotube and nanorod structured and honeycomb porous La0.8Sr0.2MnO₃/Zr0.84Y0.16O₂(LSM/YSZ) composite cathodes are prepared by the colloidal crystal, polycarbonate and water-droplet templating method, respectively. Scanning electron microscope (SEM) and transmission electron microscope (TEM) are used to observe the microstructure and morphology of the cathodes. The influence of experimental conditions on the microstructure of the cathodes is investigated and the optimized conditions are therefore obtained. The electrocatalysis performance of the cathodes prepared is characterized by electrochemical impedance spectroscopy (EIS). The relationship between cathode microstructure and the electrochmical performance is studied and the oxygen reduction reaction kinetics is also investigated.Monodispersed polystyrene (PS) colloidal spheres of 600nm in diameter are synthesized by the emulsifier-free emulsion polymerization method and PS colloidal crystals are assembled by the vertical deposition method. Three-dimensionally ordered macroporous (3-DOM) LSM/YSZ membranes are prepared by LSM/YSZ mixed sol impregnation. With the help of the fast-firing technique, 3-DOM LSM/YSZ composite cathodes are finally fabricated. Through raising the heating and cooling rates and shortening the dwell time, good contact between cathode and electrolyte is obtained by the fast-firing method while the nanostructure is maintained. The influence of calcination temperature and dwell time of the fast-firing process on the 3-DOM structure, and thereby on the cathode polarization resistance is studied. The polarization resistance of the 3-DOM LSM/YSZ composite cathode fast-firing at 1000℃for 15min are 0.71?·cm² and 0.57?·cm² at 650℃and 700℃, respectively. Compared with the LSM/YSZ composite cathode with conventional structure, the polarization resistance of the 3-DOM cathode is significantly decreased at intermediate temperatures.Monodispersed SiO₂ spheres of 330nm in diameter are synthesized by the St?ber method and SiO₂ colloidal crystals are assembled by the vertical deposition method. Non-close-packed SiO₂ colloidal crystals are fabricated by 1010℃calcination and 1wt% HF etching. Finally, three-dimensionally (3D) ordered LSM/YSZ composite cathodes are obtained by impregnation. After firing at 1000℃for 2h, the nano-network composed of 35nm thick hollow spheres remains stable, indicating that the 3D ordered LSM/YSZ composite cathode shows very high thermal stability. The electrochemical behavior of the 3D ordered composite cathode is different from the LSM/YSZ composite cathode showing conventional structure, but is similar to the mixed electronically-ionically conductive materials. Nanometer scale LSM and YSZ particles and the continuous network of the 3D ordered cathode are the main reasons for the divergence.LSM/YSZ composite nanotubes of different diameters are prepared by the pore-wetting technique, using polycarbonate templates with different pore diameters. Nanotube and nanorod structured LSM/YSZ composite cathodes sintered on the YSZ electrolyte are prepared by the fast-firing method. The influence of the heating and cooling rates and the diameter of the nanotubes on the microstructure, and thereby on the electrochemical performance of the LSM/YSZ composite cathodes are studied by SEM and EIS. The nanotube structured LSM/YSZ composite cathode prepared with the heating and cooling rate of 200℃·min?1 shows lower ohmic and polarization resistance than that of the nanorod structured cathode. For the nanotube structured LSM/YSZ composite cathodes with different diameters, the cathode prepared using the 400nm template shows the lowest polarization resistance. At 700℃, 750℃, 800℃and 850℃, the polarization resistance are 0.56Ω·cm², 0.41Ω·cm², 0.28Ω·cm² and 0.18Ω·cm², respectively, much lower than the values for the LSM/YSZ cathode showing conventional microstructure.Honeycomb porous LSM/YSZ composite cathodes containing ordered micro-scale pores, which facilitate the gas phase transportation, are prepared by the water-droplet templating method. The effects of ambient temperature, relative humidity, thickness of the slurry, concentration of the polymer and the LSM/YSZ powder on the pore size and pore distribution are observed by SEM. It is indicated that the pores templated by the water-droplets should be in suitable size, in order to decrease the polarization resistance of the honeycomb porous composite cathode. The LSM/YSZ composite cathode which is prepared at 35℃and relative humidity of 70%⁷5% shows the lowest polarization resistance. Through EIS analysis, the honeycomb porous structure designed by the water-droplet templating method is effective in decreasing the low-frequency polarization resistance concerning diffusion processes at 650℃and 700℃.