Preparation And Properties Of La_0.6Sr_0.4Co_0.8Fe_0.2O_3-δ-Ce_0.8Sm_0.2O_2-δ Composite Cathode Materials

Perovskite-type complex oxides of La_0.6Sr_0.4Co_0.8Fe_0.2O_3-δ (LSCF) composition have superior mixed electronic-ionic conducting properties, which makes it promising candidate material for cathode of intermediate temperature solid oxide fuel cell (IT-SOFC). However, the excessively high thermal expansion coefficient (TEC) of the materials makes it unavailable for the material in matching with other IT-SOFC components. The overall properties of IT-SOFC cathode can be expected to be modified when using composite cathode by mixing the LSCF with a moderate amount of doped ceria electrolyte materials. In the present thesis, a carbonate co-precipitation method was adopted to synthesize Ce_0.8M_0.2O_2-δ (M=Gd, Sm, Pr) electrolyte materials. The synthesis and preparation, oxygen ionic conducting and thermal expansion properties have been investigated. Ce_0.8Sm_0.2O_2-δ (SDC) composition was ascertained to be preferred in preparing composite cathode materials. The effect of SDC content on the thermal expansion and mixed electronic-ionic conducting properties of La_0.6Sr_0.4Co_0.8Fe_0.2O_3-δ -Ce_0.8Sm_0.2O_2-δ, (LSCF-SDC) composite cathodes have been investigated, and the chemical compatibility between LSCF and SDC has also been examined.Ce_0.8M_0.2O_2-δ (M=Gd, Sm, Pr) fine powders have been synthesized by the carbonate co-precipitation method. TG-DSC measurement was employed to analyze the formation process of the fluorite structure as a function of temperature. The crystal structure and morphology of synthesized powders have been examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The preferred synthesis conditions, including the mole ratio of ammonium hydrogen carbonate to total metal cation content, reacting temperature, aging time and calcining temperature, were ascertained. The oxygen ionic conductivities of Ce_0.8M_0.2O_2-δ (M=Gd, Sm, Pr) ceramics were evaluated by AC impedance spectroscopy. The conductivity of Ce_0.8Pr_0.2O_2-δ was the highest in the three compounds because of it's electronic conduction and Ce_0.8Sm_0.2O_2-δ have higher oxygen ionic conducting properties than Ce_0.8Gd_0.2O_2-δ. So the Ce_0.8Sm_0.2O_2-δ was chosed to prepare composite cathode materials by mixing with LSCF.La_0.6Sr_0.4Co_0.8Fe_0.2O_3-δ fine powders have been synthesized by a glycine-nitrate process (GNP). The LSCF-SDC composite cathodes have been prepared with Ce_0.8Sm_0.2O_2-δ weight proportions of 30-60%. The thermal expansion properties of LSCF-SDC composite cathodes were measured using a thermal dilatometer method. The thermal expansion coefficient (TEC) ofcomposite cathodes generally decreases with increasing content ofCeo.8Smo.2O2.fi. The TEC value of 50%Lao.6Sro.4Coo.8Fe0.203.t-50%Ceo.8Srno.202-K composite cathode was 14.4X10"6K'' in the temperature range of 100⁷30°C, which can matched with CeO2-based electrolyte materials. The onset temperatures of slope change in the thermal expansion curves rises from 650°C for LSCF to 770 °C for 40%Lao.6Sro.4Coo.8Feo.203.6-60%Ceo.8Smo.202.f,. At a high SDC content, the thermal expansion curve tends to become a straight line.The electronic and ionic conductivities of LSCF-SDC composite cathode ceramics were evaluated by a DC four-terminal method and AC impedance spectroscopy using a two-terminal blocking electrode method, respectively. The increasing of Ceo.8Smo.2O2-5 content leads to a decrease in electrical conductivity and an increase in oxygen ionic conductivity. The electrical conductivity of 50%Lao.6Sro.4Coo.8Feo.203 e -5O%Ceo.8Smo.2O2- 5 composite cathode was above lOOS-cm'1 in the temperature range of 600-800 °C, which meets the least requirement in electrical conductivity for a SOFC cathode.The distribution of the two phases and the chemical compatibility between the LSCF and SDC in the composite cathodes were characterized by XRD, back-scattered SEM and X-ray energy-dispersive spectrometer (EDS). The LSCF-SDC ceramics show a homogeneous phase distribution. EDS result shows that inter-diffusion occurred between the LSCF and SDC. After sintering at 1200 for 12h, a small amount of SrCeO3 phase was detected in60%Lao.6Sro.4Coo.8Feo.203_?-40%Ce0.8Smo.202-iiand 50%Lao.6Sro.4Coo.gFeo.203.s-50%Ce0.8Smo.202. ? composite cathodes, meriting further examining the long-term chemical stability of the LSCF-SDC composite cathode materials.