Sythesis And Characterization Of La_1-xSr_xFe_1-yMn_yO_3-δ For Intermediate Temperature Solid Oxide Fuel Cell (ITSOFC) Cathode Materials

Solid Oxide Fuel Cell (SOFC) is a clean, efficient energy generation technology that produces electricity by the direct electrochemical reaction between fuel and oxidant. Its high temperature operation, however, creates problems associated with the economic balance of plant equipment or materials. This has intensified the need for investigating the intermediate temperature Solid Oxide Fuel Cell (ITSOFC). A SOFC basically consists of three major components, a cathode, a anode, and an electrolyte. The cathode is one important component of a SOFC. The properties of the cathode play a significant role in determining the SOFC performance.It was reported that the La_1-xSr_xGa_1-yMg_yO_3-δ perovskite oxides as electrolyte exhibit high ion conductivity, and a high oxide ion conductivity stable over a wide range of oxygen pressures. Some perovskite oxides display good performance as cathode materials in SOFCs. Some Sr-substituted cobaltites can act as mixed oxide-ion and electronic conductors and afford excellent cathode performance. However, thermal expansion mismatch between perovskites oxides La_1-xSr_xGa_1-yMg_yO_3-δ and La_1-xSr_xGa_1-yMg_yO_3-δ was investigated. In view of economy, The price of Co is higher than Mn, and Fe is the cheapest. It is of great importance for SOFC to research new cathode materials. The mixed oxide-ion and electronic conductors La_1-xSr_xGa_1-yMg_yO_3-δ afford excellent cathode performance with a low TEC values. In this thesis, the synthesis process by there methods, influence of parameters on the structure and grain size of samples, such as the oxygen nonstoichiometry, electrical conductivity, cathodic polarization and chemical and thermal compatibility between La_1-xSr_xGa_1-yMg_yO_3-δ(LSFM) and La_1-xSr_xGa_1-yMg_yO_3-δ(LSGM), have been discussed. The major research results were as follows:1) Strontium-substituted lanthanum ferromanganites, La_1-xSr_xGa_1-yMg_yO_3-δ (x=0.1, 0.2, 0.3 and 0.4;y=0.1,0.2 and 0.3), for solid oxide fuel cell (SOFC) cathode materials have been synthesized by different methods: solid-state reaction method EDTA complexing sol-gel method and citrate method. The formed processes of perovskite structure were analyzed using thermogravimetric and thermal analysis(TG/DTA). It is demonstrate that the perovskite phase is formed after sintering at 1000℃, 650℃ and 600℃, respectively. X-ray power diffraction results being consist with that of TG/DTA analysis show a higher intensity of perovskite crystallization by citrate method.2) In prepared processes, influence of the organic/inorganic ratio, synthesis temperature, pH value of precursor and synthesis time etc on the structure and grain size of samples were discussed. The effects of different conditions were investigated by XRD and laser granularity analysis and the optimum conditions of the process are summarized that the organic/inorganic ratio is 1:1, the synthesis temperature is 800℃, the pH value of precursor is about 7 and sintering time is 2 hours.3) The oxygen nonstoichiometry of La_1-xSr_xFe_1-yMn_yO_3-δ perovskite oxides was measured by iodometric method. In conclusion, these perovskite oxides display the oxygen deficient composition. La_1-xSr_xFe_1-yMn_yO_3-δ, by increasing the number of Sr and Fe, increase the amount of oxygen vacancies. La_0.6Sr_0.4Fe_0.9Mn_0.1O_3-δ has the highest oxygen nonstoichiometry 0.23.4) The electrical conductivities of the La_1-xSr_xFe_1-yMn_yO_3-δ oxides sintered bars was measured as a function of temperature (250℃- 850℃) in air by a fourpoint dc method. It can be known that the electrical conductivity increase lineally with increasing temperature over the measurement temperature range. As the amount of substitution of Sr and Fe for increase, the electrical conductivity also increases, La_0.6Sr_0.4Fe_0.9Mn_0.1O_3-δ has the highest conductivity. It is 3.7 S cm^-1 at 250℃, 16.3 S cm^-1 at 600℃ and about 26.0 S cm^-1 at 850℃, respectively. The plot with better linear relationship of log σT versus 1/T for La_1-xSr_xFe_1-yMn_yO_3-δ suggests that the adiabatic mechanism prevail over the measurement temperature range. Activation energy for the electrical conductivity of La_0.6Sr_0.4Fe_0.9Mn_0.1O_3-δ is 0.223 eV, the lowest of all.5) The reduction profile (TPR) of LSFM shows two clear reduction stages, first at 430℃-650℃ and the second stage at about 810℃. Mn⁴⁺ reduces at a lower temperature, Mn⁴⁺→Mn³⁺ and at a high temperature, it is correspond to Mn³⁺ →Mn²⁺. Fe substitutes a part of Mn, the overlapping TPR bands region shift to lower temperature. These results suggest the generation of certain micro-structural defects form in the vicinity of Mn sites as a result of iron substitution. Thus, the diffusion of H₂ improved through the bulk and eventually facilitating the reduction.6) The composition of mixed power of LSFM and LSGM after sintering at 1250℃ for 40h exhibits single perovskite structure without any second phases by XRD. Based on thermodynamic analysis, it indicates that the composite is stable in air at high temperature. Therefore, it seems that it is chemically compatible between the pair LSFM/LSGM. The EDS results of cross sections of the cathode painted on electrolyte revealed that in LSFM the La/Sr/Fe/Mn ratio has no decreased or increased, as compared to the original value (0.6, 0.4, 0.9 and 0.1) and in LSGM the La/Sr/Ga/Mg ratio has no decreased or increased also, as compared to the original value (0.9,0.l,0.8and0.2). There is no any element interdiffusion until 1250℃.7) The fracture cross-sectional scanning electron microscope (SEM) image of a LSFM cathode painted on the LSGM electrolyte shows that the prepared LSFM cathode film is in good contact to the LSGM electrolyte substrate at high temperature, which shows LSFM have a better thermal expansion coefficient (TEC) match with the electrolyte LSGM and the LSGM electrolyte is very compact and the LSFM cathode film painted on the LSGM electrolyte is porous, which is good for permeation and reduction of oxygen.Therefore, the LSFM can be considered as a promising cathode material for SOFC applications with LSGM electrolyte for ITSOFC.