| Solid Oxide Fuel Cell (SOFC) is a chemical-electrical energy conversion power device with high energy conversion efficiency, environmental friendliness and high flexibility with fuels. Lowering operating temperature of SOFC from high temperature of～1000℃to intermediate temperature (IT) range of600～800℃has become one of the main research goals in order to improve its structural stabilities, lengthen life-time and decrease the running cost. In this range of temperature, contribution of cathode to the total resistance of the cell becomes dominant due to high activation energy of oxygen reduction reaction taking place in the cathode. Thus, developing of new cathode materials with good electrical conductivity, high-electrocatalytic activity for oxygen reduction and fast oxygen transport kinetics at600～800℃is of great importance for IT-SOFCs.Cobalt-based perovskite oxides (ABO3) are important cathode materials of IT-SOFCs because of their high mixed ionic-electronic conductivities and high electrocatalytic activity for oxygen reduction reaction. Properties of the perovskite-typed cathode materials are closely related to their components and chemical defects. In this thesis, the study focus has been put on performance optimization for the cubic perovskite oxide Lao.5Bao.5CoO3.δ by introducing A-site cationic deficiencies with various deficiency cations (La3+and La3+-Ba2+) and deficiency contents. Systematic studies have been made on effects of A-site cationic deficiencies on phase structure, oxygen content, chemical defects, chemical stability and electrical and electrochemical properties. The main results are listed as below:Part I:Effects of La3+deficiency on structure and properties of Lao.5-x Bao.5CoO3-δ (x=0.025-0.075) oxides1. Cubic perovksite oxides of Lao.5-xBao.5CoO3-δ with various La3+deficiency contents of x=0.025-0.075have been synthesized with sol-gel method. The XRD results have demonstrated that the La3+deficiency hardly changes lattice structures of La0.5-xBa0.5CoO3-δ; Lao.5-xBa0.5CoCO3-δ oxides are chemical stable with GDC electrolyte material at temperatures of1050℃and below. Iodomatric titration results show that with highejr La3+deficiency content oxygen content of the sample decreases while concentration of oxygen vacancies increases;2. Electrical conductivities of La0.5-x Ba0.5CoO3-δ were measured by DC four-electrode method at various temperatures in air. The conductivities of all the samples meet requirement of IT-SOFCs for conductivity of a cathode. The conductivity decrease gradually with higher temperatures, while at the same temperature, it changes with the La3+deficiency content.3. Electrochemical performance of the Lao.5-xBao.5CoO3-δ cathode materials has been characterized by electrochemical impedance spectra measurement using symmetric cells of La0.5-xBa0.5CoO3-δ/GDC/La0.5-xBa0.5CoO3-δ at various temperatures in air. The results have indicated that introduction of La3+deficiency has greatly improved electrochemical performance of La0.5-xBao.5Co03-δ.4. La0.45Bao.5Co03-δoxide has shown the best electrochemical performance. ASR value is600℃0.071Ω·cm2and ASR value is700℃0.017Ω·m2which are76%and67%lower than the parent oxide without La3+deficiency. The excellent electrochemical performance has demonstrated that it is a new promising cathode material of IT-SOFCs.Part II Effects of La3+-Ba2+co-deficiency on structure and properties of (LaBa)0.5-yCoO3-δ (y=0.0125-0.0375) oxides1. Cubic perovksite oxides of (LaBa)o.5-yCoO3-δ with various La3+-Ba2+co-deficiency contents of y=0.0125-0.0375have been synthesized with sol-gel method. It has been found that the La3+-Ba2+co-deficiency hardly changes lattice structures of (LaBa)0.5-yCoO3_δ, while with higher La3+-Ba2+co-deficiency content, oxygen content and average valences of Con+ions gradually decrease and concentration of oxygen vacancies increases.2.(LaBa)o.5-yCoO3-δ oxides are chemical stable with GDC electrolyte material at temperatures of1050℃and below.3. Electrical conductivities of (LaBa)0.5-yCoO3-δ were measured at various temperatures in air. The conductivity values decrease gradually with higher temperatures, while at the same temperature, the value makes a down-up transition at y=0.025with the increasing La3+-Ba+co-deficiency content. The conductivities of all the samples meet the requirement of a cathode material of IT-SOFCs.4. The results of electrochemical impedance spectra measurement have demonstrated has greatly enhanced electrochemical reaction reactivities of (LaBa)0.5-yCoP3-δ cathodes characterized by the much lower area-specific resistances (ASRs). The improved electrochemical performance has been mainly caused by increase in concentration of oxygen vacancies;5. The La0.475Bao.475CoO3-δ oxide with La3+-Ba2+co-deficiency content of y=0.025shows the best electrochemical performance among the studied (LaBa)0.5-yCo03-δ oxides. Its ASR values at600℃,650℃,700℃and750℃are0.086Ω·cm2,0.038Ω·m2,0.018Ω·cm2and 0.010Ω·cm2respectively, which are much better than the required ASR value (≤0.15Ωcm2) for a cathode material. |
PDF Full Text Request