Fabrication And Characterization Of Doped Ceria-based Electrolyte Materials For Intermediate Temperature Solid Oxide Fuel Cells

Solid oxide fuel cell (SOFC) is an efficient, green energy conversion device. It can convert the fossil energy directly into electrical power without any thermal cycling, which has high electricity generating efficiency. Along with the increasing energy crisis and environmental pollution, the study of SOFCs has grabbed a lot of interests in energy and materials science. Due to the high operation temperature, about 1000 oC, SOFCs have met a lot of material and technical problems. Researchers have realized clearly that it is seriously important to reduce the operating temperature of SOFCs, which can be achieved by reducing the layer thickness of the electrolyte; optimizing the sintering and preparation of electrolyte; developing new electrolyte materials with high conductivities; and improving the electrode performance.This thesis focuses on low-temperature SOFCs. The optimization of electrolyte and electrode are both studied on doped ceria based electrolyte. Samaria doped ceria (SDC) is often used as the electrolyte for low-temperature SOFCs for its high conductivity at low temperature. The SDC powders are often used for preparing thin membranes of dense SDC electrolyte films using a dry-pressing technique, which is a simple and cost-effective method to fabricate ceramic membranes. The powders are also used in composite electrodes. It is therefore used as the inter-layer for intermediate-temperature SOFCs with zirconia electrolytes. In these applications, the powders are usually sintered at different temperatures to form the expected structures. Although the glycine-nitrate-derived powders have a lot of applications, which require different sintering temperatures, their sintering characteristics are not available till now. It is also known that the electrical properties will be greatly affected by sintering temperatures. In this work, the microstructures and conductivities of typical SDC ceramics derived from the powders were investigated regarding to the sintering temperatures. Single cell with SDC as electrolyte was fabricated. La1-xSrxMnO3 (LSM) was used as cathode. NiO/SDC was anode support. In this work, to develop low temperature SOFCs using LSM as the cathodes, both the cathodes and anodes were coated with SDC. High fuel cell performances were achieved, which are comparable to those with cobalt-based cathodes. Up to date, cell performance with both electrodes coated with doped ceria is not available in the literatures. Gadolinium and samarium are considered as the best matched dopants for ceria in terms of maximization of the ionic conductivity. The effect of Gd3+ and Ca2+ co-doped was then studied in this work. In chapter 1, the principle of SOFCs were briefly introduced. The electrolyte materials for SOFCs were reviewed. Based on the present development status and direction for SOFCs, the objectives and research content on the thesis work was thus presented.In chapter 2, SDC powders were prepared by GNP method. The characteristics were studied by thermal analysis, SEM, BET and AC impedance spectra. These would provide an experiment basis and a certain guiding for further study on this material. It was also helpful to optimize the performance of single cells and study on similar materials. The main achievements are summarized as follows:(1) Sm0.2Ce0.8O1.9 powders prepared by GNP method had a high surface area and good dispersion character. The total conductivity was 0.016 S cm-1 at 600 oC. With a sintering sintering of 1200 oC, the resulted pellet got low activation energy which was 0.66 eV. Sm0.2Ce0.8O1.9 electrolyte layer could achieve high density with co-firing of anode at 1200 oC. Sm0.2Ce0.8O1.9 was a good electrolyte material for low temperature SOFCs.(2) The lattice constant growed directly via sintering temperature. Combined to the XRD spectra and other researchers'results in Raman spectra, we concluded that the Ce4+ would be reduced to Ce3+ with high temperature sintering.(3) Single cells with SDC powders as the electrolyte were fabricated using a co-pressing and co-firing technique. The peak power density was achieved at 600 oC with humidified hydrogen as the fuel, which was 203 mW cm-2.(4)The values of grain interior and grain boundary were resolved from the total resistance in AC impedance spectra via ZSimpWin software and brick-layer model. The electrical properties were investigated respectively. Analysis results showed that: The total conductivity and the activation energy for SDC20 lied on a high level of SDC powders reported via different preparation methods. The effect of sintering temperature on the total conductivity was significant and the maximum conductivities were achieved with those sintered at 1300 oC for 5 h. High total conductivity should be associated with the small contribution of the grain. The migration enthalpy of the grain conductivity also increased with the increase of sintering temperature, which may attribute to the reduction of Ce4+ to Ce3+.In chapter 3, GdxCe1-xO1.9-δ(GDC) and GdxCa0.15-xCe0.85O1,9-δ(GCDC) were prepared using GNP method. The influence of doped element and content were studied. It proved a new idea for future study on co-doped ceria with high sintering property and conductivity. The results could also provide some reference for other co-doped systems. The results are as follows:(1) The highest conductivity of GDC prepared by GNP method was achieved with a composition of Gd0.15Ce0.85O1.9-δ. The total conductivity was 0.01375 S cm-1 at 600 oC. which was a little lower than that of SDC prepared by the same method. However, these GDC powders could still be a good choice of electrolyte material for low temperature SOFC.(2)The co-doping of calcium with gadolinium could improve the sintering properties of the powders. The conductivity could be improved to a certain extent. Nevertheless, excessively doping of calcium would raise the concentration of oxygen vacancy too much, thus the association of oxygen vancancies would increase directly and the conductivity would decrease.In chapter 4, low-temperature SOFCs based on LSM cathodes were developed by means of coating nanoscale samaria-doped ceria (SDC) onto the porous electrodes. The electrode activity of both the cathodes and anodes were significantly increased. The peak power density was 463 mW cm-2. The cell performances are comparable with those obtained with cobalt-based cathodes such as Sm0.5Sr0.5CoO3, and therefore encourage the development of low-temperature SOFCs with high reliability and durability. The effect of nano SDC particles on Au was also explored. It was found that the modification of nano SDC particles could effectively decrease the resistance of electrode and interface. Collaborating with the oxygen ion spilling over Au surface, the electrode property was highly improved.