Preparation And Characterization Of Novel Electrolyte And Anode Materials For Intermediate Temperature Solid Oxide Fuel Cells

Solid Oxide Fuel Cell (SOFC) has attracted extensive attention over the past several decades due to its high-energy conversion efficiency, low pollution emission and high flexibility to various fuels. It is known as one of the green power devices in the 21^st century. A traditional SOFC employs zirconis-based ceramic, especially 8mol% yttria stabilized zirconia (YSZ) as electrolyte, because of the oxide ion conductivity and phase structure reliability. One of the disadvantages of ZrO₂-based ceramics is the high sintering temperature (≥1600℃) needed and the high cell operating temperature (≥800℃) based on this electrolyte, because of the relative low oxygen ion conductivity of YSZ below 800℃. To reduce the fabricating cost and to increase the ionic conductivity of zirconia based electrolyte at intermediate temperature (600℃-800℃). It is desirable to reduce the sintering temperature of ZrO₂-based electrolytes and to improve their ionic conductivity. In this dissertation, different rare earth oxides ( Samaria, lanthanum oxide and ceria) with yttria co-doped micron ceramic powders were prepared by the solid-state reaction method. The sinterability of the resulting powders and the microstructure and properties of sintered parts were investigated. The results showed that compacts of ZrO₂ co-doped with different rare earth oxides have different sintering shrinkage. Sni₂O₃ and Y₂O₃ co-doped ZrO₂ ceramics show highest sintered density. A density of 5.97g/cm³ was obtained for the compacts sintered at 1500℃.To further investigate to effects of co-doping, different amount of samaria(Sni₂O₃) with yttria(Y₂O₃) co-doped ZrO₂ ceramics were also prepared, and the influences of Sm₂O₃ doping amount on the phase composition, sinterability, mechanical and electrical conductivity of Sm₂O₃-Y₂O₃-ZrO₂(SY-ZrO₂) ceramics were studied. The results show that tetragonal SY-ZrO₂ ceramics can be obtained by adding proper amount of Sni₂O₃ and Y₂O₃ to ZrO₂- The SY-ZrO₂ ceramics shows a hardness of 90.5 (HRA) and a bending strength of 868MPa. Electrical conductivity of ZrO₂ can be improved obviously by doping Sm₂O₃. Conductivity of 0.02S/cm can be achieved at 800℃in air for 0.5mol% samaria and 3%molY₂O₃ co-doped ZrO₂ (0.5SY-ZrO₂).For a low-intermediate temperature SOFC, the interface between the electrode and electrolyte plays a more important role in determining its performances, because the interface resistance increases as the operate temperature decreases. Therefore, it is desirable to develop new electrodes with high electrochemical behaviors at low-intermediate temperature range(500-750℃). In this dissertation, Ce_0.8-xSm_0.2Ti_xO_2-y (x=0-0.15) powders were first prepared by a gel-cast method. Then NiO/Ce_0.7Sm_0.2Ti_0.1O_1.9 composites were prepared from the gel-casting Ce_0.7Sm_0.2Ti_0.1O_1.9 powders and basic nickel carbonate powders by the gel-cast method, which were subsequently reduced in H₂ to convert into Ni/Ce_0.7Sm_0.2Ti_0.1O_1.9 cermets. Microstructure and porosity of the NiO/Ce_0.7Sm_0.2Ti_0.1O_1.9 composites, as well as the Ni/Ce_0.7Sm_0.2Ti_0.1O_1.9 cermets were investigated and compared with those of NiO/Ce_0.7Sm_0.2Ti_0.1O_1.9 composites and Ni/Ce_0.7Sm_0.2Ti_0.1O_1.9 cermets prepared from mechanically mixed NiO and Ce_0.7Sm_0.2Ti_0.1O_1.9 powders by standard pressing-sintering process. It is shown that single-phase fluorite structure forms at a relatively low temperature when calcining the dried gel. The particle size of Ce_0.7Sm_0.2Ti_0.1O_1.9 powder increases with the doping count of TiO₂ in Ce_0.8-xSm_0.2Ti_xO_2-y powders. Impedance measurement results showed that the electrical conductivity decreases with increasing doped content of TiO₂ in Ce_0.8-XSm_0.2Ti_xO_2-y. NiO/Ce_0.7Sm_0.2Ti_0.1O_1.9 composites fabricated from the gel-casting Ce_0.7Sm_0.2Ti_0.1O_1.9 powders and Ni₂(OH)₂CO₃ by gel-cast method, and Ni/Ce_0.7Sm_0.2Ti_0.1O_1.9 cermets obtained through reduction NiO/Ce_0.7Sm_0.2Ti_0.1O_1.9 composites in H₂. Porosity of NiO/Ce_0.7Sm_0.2Ti_0.1O_1.9 composites and Ni/Ce_0.7Sm_0.2Ti_0.1O_1.9 cermets has more uniform and finer grain and pore size than those from the mechanically mixed NiO/Ce_0.7Sm_0.2Ti_0.1O_1.9 powder. Furthermore, Ni/Ce_0.7Sm_0.2Ti_0.1O_1.9 cermets prepared from the gel-cast NiO/Ce_0.7Sm_0.2Ti_0.1O_1.9 composite showed higher electrical conductivity than those of Ni/Ce_0.7Sm_0.2Ti_0.1O_1.9 cermets from the mechanically mixed NiO/Ce_0.7Sm_0.2Ti_0.1O_1.9 powder. At 773K in H₂, their conductivity is 648 S/cm and 430 S/cm, respectively. Due to the SOFC based on Ni/Ce_0.7Sm_0.2Ti_0.1O_1.9 anode prepared from gel-cast method shows higher open circuit voltage, 0.996V can be obtained at 500℃of SOFC based on Ni/Ce_0.7Sm_0.2Ti_0.1O_1.9 anode prepared from gel-cast method. The present test results have shown that the Ni/Ce_0.7Sm_0.2Ti_0.1O_1.9 cermets may be a potential anode materials for IT-SOFC.