Study On Ceria-Based And Apatite-Type Lanthanum Silicate-Based Electrolytes

Solid oxide fuel cells (SOFCs) are novel energy conversion devices with high efficiency and low emission, which have been recognized as a green energy in the 21^st century. Especially, the electrolyte plays an important role in the performance of SOFC. The high operating temperature for the conventional Y stabilized zirconia (YSZ) electrolyte is too high (above 800oC), which can cause aging of electrolytes, interface reaction between electrolytes and electrodes, electrode sintering, choice of interconnect materials and so on. There remains a strong motivation to search for new, improved oxide-ion electrolyte, such as ceria based and apatite-type lanthanum silicate based electrolytes. Gd³⁺ or Sm³⁺ doped ceria (GDC or SDC) are extensively investigated due to their high conductivity in the intermediate temperature ranges (500⁷00oC); apatite-type lanthanum silicate based electrolytes seem to be particular interesting as a result of the negligible electric conduction in the wide range of oxygen partial pressures. Therefore, our research focuses on the optimization of electrical properties of both mentioned-above electrolytes.In the composite of MgO/GDC, the grain conductivity hardly changes due to the low solubility of Mg²⁺ in GDC lattice. However, it is found that the additions of MgO largely enhance the grain boundary conductivity, which can be explained by the optimization of space charge layer due to the segregation of MgO to the grain boundaries.In order to optimize the electrical properties of ceria co-doped with Sm³⁺ and Nd³⁺, the effects of co-doping, doping content and sintering conditions on grain and grain boundary conductivity were investigated in detail. It is found that Ce_0.9(Sm_0.5Nd_0.5)_0.1O_1.95 sintered at 1350¹400oC exhibits highest total conductivity with the value of 1.0×10^-2Scm^-1 at 550oC, which is also higher than that of conventional GDC or SDC prepared with the same method.The pure and high-dispersion nanopowders of apatite-type lanthanum silicate based electrolyte were successfully synthesized via co-precipitation method and subsequent acid washing process. The effects of sintering process, sintering temperature and doping at Si site were studied. Compared with the powders with the same composition synthesized by solid state reaction, the sintering temperature in our case are greatly reduced by around 300oC, and the conductivity is largely improved, which is also found to be higher than that of YSZ.In addition, the demand for small mobile energy storage devices and mobile power delivery systems has increased over the past few years with the development of portable electronics. Due to the high energy densities of the fuels used in SOFC, the exploitation of mico-SOFC has become a hot spot. Oriented GDC and YSZ nanofibers were synthesized via electrospinning, and their electrical properties were investigated for the first time. It is found that the conductivity of nanofibers is about 10^-1Scm^-1 at 450oC owing to the greatly enhancement of oxide ion conduction, which determines their great potential in the application of mico-SOFC.