【作者】 刘毅辉；

【导师】 李箭；

【作者基本信息】 华中科技大学 ， 材料学， 2013， 博士

【摘要】 固体氧化物燃料电池（SOFC）是一种将化学能直接转化为电能的高效清洁能源转换装置。La_1-xSr_xCo_1-yFe_yO_3-δ（LSCF）是一种得到广泛研究的中温SOFC阴极材料，在600-800C范围内具有良好的氧离子/电子混合导电性。然而，在中温环境（600-800C）中，随着工作时间的延长，LSCF阴极性能会发生衰减，这已经成为限制其作为SOFC阴极材料得到广泛应用的关键问题。本文主要以LSCF及其复合阴极在工作环境中的性能稳定性为主要研究对象，研究导致性能衰减的各种因素，探索衰减机理，并且探讨提高LSCF阴极稳定性的措施和方法。本文研究了丝网印刷法制备的LSCF阴极在开路电压和电流极化条件下的稳定性，探索LSCF阴极发生衰减的本质原因；采用溶液浸渍法制备了LSCF-YSZ （Y₂O₃稳定的ZrO₂）复合阴极，重点研究了其在工作环境中的性能衰减机理，明确YSZ电解质与LSCF阴极的相互作用对阴极性能的影响；为了避免LSCF阴极与电解质在工作环境中发生固态反应，以GDC（Gd掺杂的CeO₂）为多孔骨架，采用溶液浸渍法制备出LSCF-GDC复合阴极，研究其长期稳定性，并提出抑制性能衰减、提高稳定性的措施；另外，以SDC（Sm掺杂的CeO₂）为骨架，采用溶液浸渍法制备出LSCF-SDC复合阴极，研究其在不同氧分压条件下的稳定性，探索阴极上的氧还原机制以及氧分压对性能衰减的影响。研究得到如下主要结果：（1）对于与YSZ电解质配合使用的传统LSCF阴极，在750C空气中测试500小时后，其欧姆阻抗和极化阻抗都显著增大。这主要是由阴极与电解质发生固态反应而形成SrZrO₃相，以及阴极内部结构发生聚合而使氧气传输变得困难导致的。电流极化会使LSCF阴极内部结构聚合更加严重，导致欧姆阻抗和极化阻抗的增大更加明显。在电流极化过程中，初始阶段欧姆阻抗和极化阻抗都会减小，阴极性能被活化。但是随着极化时间的延长，欧姆阻抗和极化阻抗呈增大趋势，阴极性能发生衰减。（2）对于溶液浸渍法制备的LSCF-YSZ阴极，在750C空气中测试500小时后，其欧姆阻抗和极化阻抗都增大，特别是极化阻抗显著增大。此类阴极性能发生衰减的主要原因是热作用使阴极内部LSCF颗粒在YSZ骨架表面发生平铺，引起了显著的极化损失。与不施加电流条件下相比，电流作用会导致LSCF-YSZ阴极发生更加严重的性能劣化，电流密度越大导致越严重的阴极性能衰减。在电流极化初期会出现明显的阴极性能活化过程。（3）对于溶液浸渍法制备的LSCF-GDC阴极，在750C空气中，LSCF颗粒长大不仅降低阴极的电化学活性、引起极化损失，而且破坏氧离子传输路径、引起欧姆损失，这是导致LSCF-GDC阴极性能衰减的主要原因。电流作用会加速LSCF-GDC阴极性能劣化，并且增大电流密度会促进LSCF颗粒长大，导致更加严重的性能衰减。LaNi_0.6Fe_0.4O₃（LNF）的引入能在保持阴极具有良好导电性的前提下，抑制LSCF颗粒长大，使LSCF-GDC阴极在工作环境中保持性能稳定。（4）对于溶液浸渍法制备的LSCF-SDC阴极，750C条件下的热作用导致LSCF颗粒的聚合长大以及阴极内部孔隙的减少，使该纳米结构阴极的性能劣化。在低氧分压条件下，由于氧气含量不充足，阴极表面SrO的形成受到抑制，使LSCF-SDC阴极性能衰减率降低。更多还原

【Abstract】 Solid oxide fuel cell （SOFC） is a clean energy conversion device that convertschemical energy directly into electric power in a highly efficient way. La_1-xSr_xCo_1-yFe_yO_3-δ（LSCF） perovskite oxides are widely accepted as the cathode materials for SOFCs thatoperate in the intermediate temperature between600and800C due to their highelectronic and ionic conductivities. However, the electrochemical performance of LSCFcathodes is not stable with time under operation conditions and the performancedegradation becomes one of the most important issues hampering LSCF applications inSOFCs. In this dissertation, the stability of LSCF or LSCF composite cathodes ismeasured and reasons leading to performance degradation are investigated. Mechanismsof performance degradation are discussed and then methods of stability enhancement arepresented.The stability under the condition of open circuit or current polarization of LSCFcathodes prepared by screen-printing is measured to find the essential factors causingperformance degradation of cathodes. Performance degradation mechanisms ofimpregnated LSCF-YSZ composite cathodes are investigated in detail and effects ofinteraction between LSCF cathodes and YSZ electrolytes are revealed. To avoid the solidreaction between LSCF cathodes and electrolytes under operation conditions, GDC is usedas the scaffold in the preparation of LSCF-GDC composite cathodes by impregnation. Thestability of LSCF-GDC composite cathodes is measured and methods of stabilityenhancement for cathodes are explored. Additionally, the stability of LSCF-SDCcomposite cathodes prepared by impregnation is measured at different oxygen partialpressure. The mechanisms of oxygen reaction and effects of oxygen partial pressure onperformance degradation are interpreted.The major results obtained are described as follows:（1） After testing for500h at750C in air, both ohmic resistance and polarizationresistance of conventional LSCF cathodes increase drematiclly, which is caused by the formation of SrZrO3phases related to solid reaction between LSCF and YSZ, and theblock of oxygen transfer related to agglomeration of LSCF porous structure.Current polarization accelerates the agglomeration of LSCF porous structure and leadsto greater increase of both ohmic resistance and polarization resistance of LSCF cathodes.At the beginning stage of current polarization, both ohmic resistance and polarizationresistance decrease, indicating that the performance of cathodes is activated. Butprolonging current polarization time leads to the increase of both ohmic resistance andpolarization resistance and serious performance degradation.（2） After testing for500h at750C in air, both ohmic resistance and polarizationresistance of impregnated LSCF-YSZ cathodes increase. The polarization resistanceincreases dramatically compared to its origin value. It can be confirmed that plorizationlosses play a main role on the performance degradation of cathodes, which is caused bythe flattening of LSCF particles on the surface of YSZ scaffold.Comparing with LSCF-YSZ cathodes operating under open curcuite, currentpolarization leads to more serious performace degradation of cathodes. And higher currentdensity accelerates performace degradation. Obvious performance activation process isobserved at the beginning stage of current treatment.（3） During stability testing of LSCF-GDC cathodes at750C, the coarsening of LSCFparticles is the origin of the polarization losses, which is caused by the reduction of theelectrochemical activity, and the ohmic losses, which is caused by the damage of oxygenion diffusion paths.Current polarization leads to serious perforamance degradation of LSCF-GDCcathodes. Higher current density accelerates the coarsening of LSCF particles and thedeterioration process of cathodes.Under the operation condition, the introduction of LaNi0.6Fe0.4O3（LNF） phase intoLSCF-GDC cathodes can improve the performance stability as well as maintainingrelatively high conductivity and electrochemical activity of cathodes, due to thesuppression of the coarsening of LSCF particles.（4） For impregnated LSCF-SDC cathodes, thermal process at750C causes theagglomeration 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 degradationrate of LSCF-SDC cathodes, due to the suppression of the formation of SrO on thesurface of cathodes in the insufficient oxygen atmosphere.更多还原