(A) study on electrolytes and cathodes for intermediate temperature SOFCs

Abstract

Significant efforts have been attempted to develop SOFCs operating at intermediate temperature region of 500-700℃ with high power density and sufficient durability. For the effective performance of the SOFC at the intermediate temperatures (IT-SOFC), it is necessary to develop a new solid oxide electrolyte, characterized by a sufficiently high ionic conductivity, and new cathode materials, working efficiently within this temperature range. In this study, the synthesis and characterization of electrolyte and cathode materials have been investigated to be used for IT-SOFCs. In chapter 3, the electrical properties correlated with microstructures of dense and porous three dimensional (3D) nanocrystalline $Ce_{0.9}Gd_{0.1}O_{1.95} $ (n-CGO) electrolytes prepared by the different sintering processes were investigated. The CGO pellets of dense-nano (DN-CGO), and porous-nano (PN-CGO) with grain size of 30 ~ 70 nm and their equivalent density pellets of dense coarse (DC-CGO) and porous coarse (PC-CGO) with larger grain size were prepared. The preparation of nanocrystalline CGO electrolytes with different grain sizes and densities was dependent on the different sintering processes, by controlled heating profiles, such as one step sintering and two steps sintering techniques. The electrical conductivity of CGO pellets was characterized using ac impedance spectroscopy (IS) as a function of temperature, oxygen partial pressure and grain size. The comparable electrical conductivity behavior for DN-CGO and DC-CGO may attribute to distant grain size level of DN-CGO from critical nano-grain size. On the other hand, a decreased electrical conductivity was observed in PN-CGO with grain size of 30 nm compared to its equivalent PC-CGO, including the porous conductivity correction, which confirms the existence of space charge region affecting the defect characteristics of nanocrystalline CGO. The electrical transport properties and microstructure parameters of the 3D nanostructure...