| Solid Oxide Fuel Cell (SOFC) is the chemical-electrical energy conversion power device with high energy conversion efficiency, environmental friendliness and high flexibility with fuels. Reducing the operating temperature from high temperature of above1000℃to intermediate temperature (IT) range of600-800℃is the current development trend of SOFC Lowering the working temperature can improve the cell structural stability, lengthen the working lifetime and reduce its fabrication and running cost, however, it can also cause performance degradation of the component materials. In particular, due to high activation energy of oxygen reduction reaction occurring over the cathode, polarization resistance of the cathode increases fast with lowering temperatures, and therefore the cathode material becomes the key factor that determines the overall performance of the IT-SOFC. Development of cathode materials with excellent electrochemical performance at600-800℃is thus crucial for performance improvement and practical application of IT-SOFCs.Cobalt-based perovskite oxides 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 structures. In this thesis, the study focus has been put on tailoring of the components and structures of several cobalt-based perovskite oxides for property optimization. Systematic studies have been carried out with respect to their structural stabilities, high-temperature chemical compatibility, thermal expansion behaviors, electrical and electrochemical properties, as well as their relationship with components and structures of the materials. The main results are listed as below:1. Structure tailoring of A-site La3+/Ba2+cationic-disordering/ordering for the perovskite oxide (La, Ba, Co) O has been realized by changing temperature and atmosphere during the sol-gel synthesis process, and effects of the A-site cationic-ordering/disordering on structural stability and electrical and electrochemical properties have been studied. The results have demonstrated that:the cubic perovskite phase is characteristic of A-site La3+/Ba2cationic-disordering, while the double-layered perovskite phase is featured by cationic-ordering:both phases are structurally stable at850℃in the atmospheres of argon, air and oxygen, while the double-layered perovskite structure transforms to cubic structure when calcined at1050℃in air; With higher oxygen partial pressure in the atmospheres, both phases show higher oxygen contents, smaller cell volumes and increased electrical conductivities; Under the same conditions, however, conductivities of the double-layered perovskite phase are lower than the results of the cubic phase; EIS study on the GDC-based symmetric cells has shown that the double-layered perovskite oxide LaBaCo2O5+δ has lower area-specific resistances and smaller cathode reaction activation energy than the cubic counterpart La0.5Ba0.5CoO3-δ, which suggests that A-site La3+/Ba2+cationic-ordering can improve electrochemical performance of the (La, Ba, Co) O perovskite oxide;2. A comparative study has been made on electrochemical performance and electrode reaction mechanism of La0.5Ba0.5CoO3-δ and La0.5Ba0.5CoO3-δ-GDC (1:1weight ratio) composite cathode by using AC-impedance spectra. The results have indicated that, introduction of the ionic conductive GDC component improves electrochemical performance of the La0.5Ba0.5CoO3-δ-GDC composite cathode with lower area-specific resistance (ASRs) of0.115Ω·cm2at600℃than0.132Ω·cm2of the La0.5Ba0.5CoO3-δ single phase cathode; Such performance improvement is mainly ascribed to enhanced electrode reaction processes involving dissociation and reduction of the adsorbed oxygen molecules followed by incorporation into the lattice sites and oxygen ionic transfer across the cathode/electrolyte interface.3. Studies have been made on effects of A-site Ba2+-deficency on structure and properties of cubic perovskite oxide La0.5Ba0.5-xCoO3-δ (x=0.00-0.08). The results have demonstrated that, the Ba2+-deficiency content in La0.5B0.5-xCO is limited to x=0.05; with bigger x from0.000to0.050. its cell volume shrinks, thermal expansion coefficient (TEC) and electrical conductivity decrease gradually, while polarization resistance firstly decreases dramatically from x=0.00to0.025and then slightly increases at a higher Ba2+-deficiency (x=0.050); Among the studied samples, La0.5B0.475CO with the Ba2+-deficiency content x=0.025shows the best electrochemical performance with ASR values of0.080Ω·cm2and0.020Ω·cm2at600℃and700℃respectively, which are～60%lower than the results of the parent oxide La0.5B0.5CO (x=0) prepared under the same conditions: The maximum output power density of the anode-supported La0.5B0.475CO/GDC/Ni-GDC single cell is470-928mW·cm-2at temperatures of650-800℃.4. Studies have been made on effects of Ba2+-deficiency on structure, stability, electrical and electrochemical properties of double-layered perovskite oxide LaBa1-xCo2O5+δ (x0.00-0.15). The results have demonstrated that the Ba2+-deficiency content in LaBa1-xCo2O5can reach up to x=0.15; LaBa1-xCo2O5+δ has tetragonal layered-perovskite structure when calcined at850℃-1050℃in air but transforms into cubic perovskite structure at higher temperatures of1100-1150℃;With higher Ba2+-deficiency content, the conductivity of LaBa1-xCo2O5+δ decreases gradually, while the ASR value firstly dramatically decreases from x=0.00to x=0.10and then slightly increases at x=0.15; Among the samples studied, the LaBa0.9Co2O5+δ(x=0.10) oxide exhibits the best electrochemical performance; its ASR values at600℃and700℃are0.118Ω·cm2and0.023Ω·cm2respectively, which are～40%lower than the results of the parent oxide LaBaCo2O5+δ(x=0.00); Such electrochemical performance improvement is mainly ascribed to increased concentration of oxygen vacancy and realization of three-dimensional pathway for oxygen ionic diffusion in the LaBa1-xCo2O5+δ oxide due to introduction of Ba2+-deficency.5. Studies have been made on Ba2+-deficient layered-perovskite oxide PrBa1-xCo2O5+δ(x=0.00-0.10) with respect to structure, thermal expansion behavior, and electrical and electrochemical properties. The maximum Ba2+-deficiency content in PrBa1-xCo2O5+δ is x0.08; With higher Ba2+-deficiency content from x=0.00to x=0.08, cell volume of PrBa1-xCo2O5+δ shrinks, TEC value slightly decreases, while the conductivity firstly decreases then gradually increases from x=0.03to x=0.08; PrBa1-xCo2O5+δ is chemically stable with GDC electrolyte at1100℃in air; Polarization resistance of PrBa1-xCo2O5+δ cathode on the GDC electrolyte layer decreases greatly with higher Ba2+-deficiency content; Among the samples studied, the PrBa0.92Co2O5+δ oxide (x=0.08) exhibits the best electrochemical performance with the lowest ASR values; Its ASR value,0.093Ω·cm2,at600℃is～50%lower than the ASR of the parent oxide PrBaCo2O5+δ(x=0.00); The results have demonstrated that introduction of Ba2+deficiency is an effective way of improving electrocatalytic activity of the layered-perovskite oxides; however, due to the different A-site cations (Pr3+and La3+) in the PrBa1-xCo2O5+δ and LaBa1-xCo2O5+δ oxides, introduction of the Ba2+-deficiency has caused different results in some ways for these two oxides. |
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