Synthesis And Characterization Of Sm_0.5-xBa_xSr_0.5Co_0.8Fe_0.2O_3-δ As Cathode Material For Intermediate-temperature Solid Oxide Fuel Cell And Electrode Processes

Energy crisis and environmental Pollutions are Problems that all country is now facing for the sustainable development. Fuel cells, which have been seen as a keystone for the future energy economy, have received considerable attention for their high energy conversion efficiency and bw impact to environment as a mean of generating electricity. Among the fuel cell comunity, the solid oxide fuel cell (SOFCs) is a currently hot topic. However, the traditional SOFCs work at high temperatures, leading to many problems, such as electrode sintering, diffusion at interface and difficulty in preparation of seals and interconnect.The current trend in SOFC developments is the reduction of their working temperatures. The lower operating temperature leads to poor oxygen reduction reaction kinetics with the conventional cathode material, La1-xSrxMnO3. As a result, there has been huge interest in the development of new cathode materials with improved performance for potential use in future SOFCs. To address this issue, this thesis focuses on a series of new cathode materials and improvement of cathode microstructure.First, the influence of Ba2+ ions on high temperature properties and performance of the provskite Sm0.5-xBaxSr0.Co0.8Fe0.2O3-δ(SBSCF) oxides is investigated systematically. The cell volume, thermal expansion coefficient (TEC) of the cathode materials increase with increasing of Ba2+ content. And the electrical conductivity, area specific resistant decrease with increasing of Ba2+ content. These results suggest that catalytic activity for the oxygen reduction reaction in SOFC is enhanced by Ba2+ substituting for Sn3+ while the thermal expansion coefficient increase. Kinetics of oxygen reduction reaction (ORR) on porous Sm0.5-xBaxSr0.5C0.08Fe0.2O3-δ cathode is investigated by analysis of impedance spectra. Both medium-frequency and low-frequency ASR change in the range of 0.37Ωcm-2 to 0.25Ωcm-2 with an oxygen partial pressure (pO) and increase with the increasing of temperature. These results suggest whole electrode process is a mixing process involving oxygen reduction reaction related to atomic oxygen and oxygen ions conduction step together with total charge-transfer step.A composite cathode SBSCF/SDC is used to optimize performance. The AC impedance and thermal expansion coefficient of the SBSCF/SDC composite are measured in order to evaluate the potential for cathode application.The thermal expansion coefficient decreases with increasing of SDC content in the composite and is about 14.95×10-6K-1 for the sample with 50% weight ratio SDC. The cathodic polarization of composite cathodes with SDC shows that a weight ratio between Sm0.3Ba0.2Sr0.5C00.8Fe0.2O3-δ and SDC of 1:1 gives the lowest area specific resistance (ASR) of 0.098Ωcm2. The maximum power densities of an anode supported cell with SBSCF (50%wt) composite cathode are 1050mW cm-2 at 650℃ and 600mW cm-2 at 600℃, respectively.The benefit of compositionally grading a cathode functional layer (CFL) for SOFCs is explored. Cells are prepared wherein a linearly compositionally graded cathode functional layer painted between electrolyte and cathode current collecting regions. The electrochemical performance is analyzed using an AC impedance spectroscopy. The minimum polarization resistance (Rp) value is obtained for a cell with 50%SBSCF+50%SDC-70%SBSCF +30%SDC configuration. And the peak power densities of the compositionally graded CFL cell are 1170mWcm-2 at 650℃ and 840mWcm-2 at 600℃, respectively. The results show that this approach is a viable to produce SOFC functional layers with unique composition and interfacial properties.In this thesis, the MIEC Sm0.5Sr0.5C00.8Fe0.203-δ was investigated systematically by using of optimization composition and micro structure of electrode for development of new SOFC cathode materials for intermediate-temperature SOFC. The influence of the Ba2+ ions on the high temperature properties and electrochemical performance of the provskite Sm0.5Sr0.5C00.8Fe0.2O3-δ oxides is investigated systematically. Meanwhile, the composite graded cathode was adoped for enhancing the thermal expansion match with electrolyte and the performance.The materials mentioned above showed the good performance in intermediate-temperature SOFCs, which are promising cathode materials.