Synthesis And Performance Of La_1-xSr_xCo_1-yFe_yO_3-δ Cathodes For Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) is a kind of energy conversion device that convert chemical energy directly into electric power in a highly efficient way. The overall performance loss in SOFCs with thin electrolyte layer is dominated by the polarization on cathode due to the high activation energy and slow reaction kinetics for the O₂ reduction reaction. Therefore, the development of cathodes, especially for the intermediate temperature SOFC (IT-SOFC), with high electrocatalytic activity for oxygen reduction becomes a critical issue.La_1-xSr_xCo_1-yFe_yO_3-δ (LSCF) electrodes with mixed ionic and electronic conductivity (MIEC) are known for high activity for the O₂ reduction reaction in IT-SOFCs. Unfortunately, LSCF electrodes likely react with YSZ electrolyte to form a resistive phase at a temperature higher than 900℃and are thermally incompatible with YSZ electrolyte; therefore, Gd-doped CeO₂ (GDC) is often used as an interlayer when LSCF cathode is considered in combination with YSZ electrolyte.The dissertation makes a further study on LSCF. The investigation mainly includes four parts described as: synthesis of nano-crystalline cathode materials, optimization of LSCF cathode, and the performance of La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ+YSZ (LSCF+YSZ) nano-cathode prepared by impregnation, and the reaction kinetics for the O₂ reduction reaction.LSCF/YSZ composite cathodes of three kinds of micro-structure are prepared in this dissertation, and the electrochemical performance of the cathodes is studied. The major results obtained are described as follow:1) A modified polymer-assisted combustion synthesis method is developed for preparation of La_0.8Sr_0.2Co_0.5Fe_0.5O_3-δ nano-sized power by using organic additives (glucose and acrylamide) and metal nitrates. The number of the organic additives to the metal nitrates affects the combustion temperature and the particle size. The ratio of 4: 6: 1 for glucose: acrylamide : metal ions and the firing temperature at 700～800℃seems to be an appropriate choice for forming single LSCF phase.2) Cathode impregnated with LSCF shows the best performance among three kinds of composited cathodes, and the electrode polarization resistance for the O₂ reduction reaction is 0.218Ωcm² at 600℃and 0.047Ωcm² at 750℃, respectively.3) The electrode polarization resistance for the O₂ reduction reaction is 2.9⁰.09Ωcm² for Pd+LSCF cathode and 1.05～0.06Ωcm² for GDC+LSCF cathode, respectively, during the temperature range 600 to 750℃. The overpotential and polarization resistance on the Pd- or GDC-impregnated LSCF electrodes are lower than that on the LSCF cathode without impregnation. The electrochemical performance of LSCF cathode is enhanced by the impregnation of PdO_x or GDC nanoparticles which extend the active zone.4) The coefficient of thermal expansion (TEC) for LSCF+YSZ composites is approximate that of the YSZ backbone. The performance of LSCF+YSZ cathodes is affected by the micro-structure. Increasing the LSCF loading or rising the temperature is the effective way to enhance the electrochemical performance of cathode. The impedance curves of LSCF+YSZ composite cathodes can be good fitted by LR(QR)(QR)(X(RL)), (X=R,Q or O ) equivalent circuit.5) The stability of LSCF+YSZ cathodes is bad. Both the ohmic resistance and polarization resistance increase after a long time work when under current loading or at 750℃.6) The surfactant in the LSCF solution for impregnation has great effect on the microstructure of LSCF+YSZ cathodes, and further influence on the electrochemical performance.7) The low frequency loop appeared in the fourth quadrant on the impedance spectra of LSCF+YSZ cathode is under the influence of LSCF loading, oxygen partial pressure and DC bia. The apparent of the low frequency loop indicates that the most important limit condition of the O₂ reduction reaction on LSCF+YSZ composite cathode is the adsorption of oxygen species on the electrode surface.8) The rate-determining step (RDS) is transferred from the interface between electrode and electrolyte to the surface of the electrode as the increase of LSCF loading or temperatures. The process of RDS is listed as follow: charge transfer and ionic diffusion at the interface between electrode and electrolyte→oxygen atoms adsorbed at the electrochemical reaction site of the surface→oxygen molecular adsorbed at the surface of the electrode.