Preparation And Modification Of The Key Materials For Solid Oxide Fuel Cells Based On Doped Ceria Electrolytes

Solid oxide fuel cell (SOFC) is a promising technology with less emission of pollution for electric power generation in the 21st century. Because a SOFC based on YSZ electrolyte has to work at temperature higher than 800℃, its application is limited. To realize the commercialization of a practical SOFC, it is desirable to lower the operation temperature' of a SOFC to intermediate range(600⁸00℃). Two main methods can be employed to carry out it. The first one is to develop some new electrolyte materials with high oxygen ionic conductivity at intermediate temperature, the potential candidates are doped ceria (DCO) and some perovskite type compounds such as doped-LaGaO3. The second one is to reduce the thickness of electrolyte layer. This thesis deals with the preparation and characterization of the key materials for SOFC based on samarium doped ceria (SDC) electrolytes.SDC powder prepared by the conventional solid state reaction method usually needs to be sintered at temperature higher than 1600℃ because of its poor sintering activity. This not only consumes more energy, but also leads to interfacial reaction between the electrode and the electrolyte, and reduces the possibility of co-firing the electrolyte with other cell components. Recent study has shown that SDC powder with multielement dopants has relative high sinterability, and the sintering temperature can be lowered. In this experiment, CeO₂ powder co-doped with Al₂O₃ and Sm₂O₃ was prepared by a modified gel-casting process with organic aid. The properties of the powder, including its phase composition, particle size, as well as its sinterability, electrical conductivity were characterized. It was shown that fluorite phase can formed at a relative low temperature when Al₂O₃ was doped into SDC. Al₂O₃-SDC powder with fluorite structure can be obtain at 600℃ by calcined the dried gel. The Al₂O₃-SDC powder has a nanometer particle(40⁵0 nm) with narrow particle size distribution. Furthermore, the Al₂O₃-SDC powder showed high sinterablity, its compact can be sintered to relative density of 95% of the theoretical at temperature 1450℃.Although SDC electrolyte has high ionic conductivity, the poor strength limits its practical application. In this paper, effects of Al₂O₃ additions on theproperties of Al₂O₃-SDC materials were also studied. The results showed that for Al₂O₃-SDC samples sintered at a relatively low temperature of 1450℃ for 5h, the doping of AL₂O₃ also improved their mechanical properties while still keeping their high ionic conductivity. The bending strength of the AL₂O₃-SDC with 1% Al₂O₃ dopant was 109.5MPa, and the conductivity was 0.030S/cm.To reduce the operating temperature of a SOFC, high performance electrodes are also needed in intermediate-low temperature range. Studies have shown that Ni/SDC anodes have higher electrical conductivity than Ni/YSZ anodes. The catalytic activity and stability of Ni/DCO anodes are higher than Ni/YSZ anodes, and the interfacial reaction between the Ni/DCO anode and DCO electrolyte can also be reduced. Because properties of the cermet anodes depend greatly on the metal content in the anode, the microstructure of the anodes, which, in turn, depends greatly on the preparation process of the anode, it is necessary to modify the manufacture technology to obtain anode with ideal microstructure. In this experiment, SDC cladded with NiO powders were first prepared by a precipitation method. The properties of the powder such as their particle size, particle size distribution and sinterability were characterized. Then Ni/SDC anode cermets were prepared by die-pressing and sintering, followed by reduction in H₂. The properties of the Ni/SDC cermets were tested and compared with those of Ni/SDC samples prepared from the mechanically mixed NiO and SDC (NiO-SDC) powders. The results showed that Ni/SDC cermets prepared from the NiO cladded SDC powders have a microstructure with homogeneous distribution of Ni and SDC, and fine pores. They also have higher electrical conductivity and mechanical prope...