| Solid Oxide Fuel Cell?SOFC?is an energy conversion device that could convert the chemical energy into electricity power efficiently and environmental friendly.At present,research on SOFC mainly focuses on modifying existing cathodes to improve their structural and chemical stability,developing new medium and low temperature cathode materials or new anode materials that could resist carbon deposition and sulfur poisoning.?La0.8Sr0.2?0.95MnO3-??LSM?cathode materials is a popular traditional SOFC cathode material with high electronic conductivity,remarkable oxygen reduction catalytic activity and structural stability.In order to improve the electrochemical performance of LSM at low or medium temperatures,it is often mixed with an electrolyte material?YSZ?such as an oxygen ion conductor to form an LSM-YSZ composite cathode.YSZ ceramics,which have high ionic conductivity,low electron conductivity and good mechanical strength,are also widely used as traditional electrolyte materials.Due to its good chemical compatibility and thermal expansion with LSM cathodes,the two materials are often mixed for research.However,studies have shown that the LSM-YSZ cathode/YSZ electrolyte interface,which should be in a stable state,undergoes elemental diffusion and morphological changes after the application of a polarization current through the interface at the operating temperature of the cell.In addition,the impurity element derived from the electrode is enriched along the boundary of the electrolyte,weakening the bonding force between the grains and causing rapid degradation of performance.It is important to explore the behavior of elements diffusion and morphological change in working process,which will improve the performance and long-term stability of SOFC.This paper uses LSM-YSZ/YSZ half-cell to investigate the effect of the current,current density,bias direction and Mn content in cathode on the diffusion behavior of the cathode element is studied.Further,the diffusion mechanism of Mn in the YSZ electrolyte is proposed.The changes in the cathode/electrolyte interface and morphology of electrolyte are then analyzed.The position and distribution of the elements in the cell,phase structure and morphology are systematically characterized by using EPMA,WDS and AFM.Finally,the effect of Mn element diffusion on the properties of YSZ electrolytes is investigated.The main contents and conclusions are summarized as follows:?1?Diffusion behavior of the cathode element with polarization current.During the sintering process,the Sr in the LSM-YSZ composite cathode will diffuse into the YSZ electrolyte.During the durability evaluation at 850°C and 900mAcm-2 for 850h,the Mn in the cathode will diffuse along the grain boundary of YSZ electrolyte and the diffusion depth is 23?m.The current promotes the diffusion of Mn element and the grain boundary of electrolyte is the way of diffusion.The increasing of the polarization time,the current density and the content of the Mn element in the cathode increase the diffusion depth of the Mn element.When the direction of the current is changed,the Mn element does not perform reverse diffusion.?2?Morphological changes of LSM-YSZ/YSZ interface and YSZ electrolyte with the polarization current.During the polarization with 900mAcm-2 current for several hours,contact pits of electrodes and electrolytes tend to be found at the cathode/electrolyte interface.The size of pits is drastically reduced and the depth is increased,which lead to a significant reduction of the TPB on the interface.The protruding YSZ particles emerges.The content and distribution of YSZ particles are affected by the content of YSZ in the cathode.When the content reaches 50 wt.%,the YSZ network is formed which severely resulted in the increase of impedance.The pits are the source of the diffusion of Sr and Mn elements.In addition,the diffusion along grain boundary affects the fracture form of YSZ and increases the ratio of intergranular fracture.?3?Effect of Mn element diffusion on mechanical properties of YSZ electrolyte.During the polarization of LSM-YSZ/YSZ cell at 850°C and 350mAcm-2 for 50h?125h and 250h,the breaking strength of YSZ electrolyte greatly decreases and the Vickers hardness increases.The hardness of YSZ electrolyte near the cathode side is higher than the other side.The reason is that the Mn diffusion along the grain boundary and the distribution of oxygen vacancies inside the electrolyte.The Mn between grain boundaries greatly weaken the bonding force between grains,resulting in the decrease of performance.After the test for 960h,the section of electrolyte appears a large-area transgranular fracture. |
PDF Full Text Request