Stability And Performance Degradation Mechanism Of La_1-xSr_xCo_1-yFe_yO_3-δ Cathodes For Intermediate Temperature Solid Oxide Fuel Cells

Solid oxide fuel cell (SOFC) is a clean energy conversion device that converts chemical energy directly into electric power in a highly efficient way. La1-xSrxCo1-yFeyO3-δ (LSCF) perovskite oxides are widely accepted as the cathode materials for SOFCs that operate in the intermediate temperature between600and800℃due to their high electronic and ionic conductivities. However, the electrochemical performance of LSCF cathodes is not stable with time under operation conditions and the performance degradation becomes one of the most important issues hampering LSCF applications in SOFCs. In this dissertation, the stability of LSCF or LSCF composite cathodes is measured and reasons leading to performance degradation are investigated. Mechanisms of performance degradation are discussed and then methods of stability enhancement are presented.The stability under the condition of open circuit or current polarization of LSCF cathodes prepared by screen-printing is measured to find the essential factors causing performance degradation of cathodes. Performance degradation mechanisms of impregnated LSCF-YSZ composite cathodes are investigated in detail and effects of interaction between LSCF cathodes and YSZ electrolytes are revealed. To avoid the solid reaction between LSCF cathodes and electrolytes under operation conditions, GDC is used as the scaffold in the preparation of LSCF-GDC composite cathodes by impregnation. The stability of LSCF-GDC composite cathodes is measured and methods of stability enhancement for cathodes are explored. Additionally, the stability of LSCF-SDC composite cathodes prepared by impregnation is measured at different oxygen partial pressure. The mechanisms of oxygen reaction and effects of oxygen partial pressure on performance degradation are interpreted.The major results obtained are described as follows:(1) After testing for500h at750℃in air, both ohmic resistance and polarization resistance of conventional LSCF cathodes increase drematiclly, which is caused by the formation of SrZrO3phases related to solid reaction between LSCF and YSZ, and the block of oxygen transfer related to agglomeration of LSCF porous structure.Current polarization accelerates the agglomeration of LSCF porous structure and leads to greater increase of both ohmic resistance and polarization resistance of LSCF cathodes. At the beginning stage of current polarization, both ohmic resistance and polarization resistance decrease, indicating that the performance of cathodes is activated. But prolonging current polarization time leads to the increase of both ohmic resistance and polarization resistance and serious performance degradation.(2) After testing for500h at750℃in air, both ohmic resistance and polarization resistance of impregnated LSCF-YSZ cathodes increase. The polarization resistance increases dramatically compared to its origin value. It can be confirmed that plorization losses play a main role on the performance degradation of cathodes, which is caused by the flattening of LSCF particles on the surface of YSZ scaffold.Comparing with LSCF-YSZ cathodes operating under open curcuite, current polarization leads to more serious performace degradation of cathodes. And higher current density accelerates performace degradation. Obvious performance activation process is observed at the beginning stage of current treatment.(3) During stability testing of LSCF-GDC cathodes at750℃, the coarsening of LSCF particles is the origin of the polarization losses, which is caused by the reduction of the electrochemical activity, and the ohmic losses, which is caused by the damage of oxygen ion diffusion paths.Current polarization leads to serious perforamance degradation of LSCF-GDC cathodes. Higher current density accelerates the coarsening of LSCF particles and the deterioration process of cathodes.Under the operation condition, the introduction of LaNi0.6Fe0.4O3(LNF) phase into LSCF-GDC cathodes can improve the performance stability as well as maintaining relatively high conductivity and electrochemical activity of cathodes, due to the suppression of the coarsening of LSCF particles.(4) For impregnated LSCF-SDC cathodes, thermal process at750℃causes the agglomeration of LSCF particles and the reduction of porosity of cathodes, which is responsible for the performance degradation.Lowering oxygen partial pressure results in the decrease of performance degradation rate of LSCF-SDC cathodes, due to the suppression of the formation of SrO on the surface of cathodes in the insufficient oxygen atmosphere.