Study On The Preparation And Performance Of Doped Ceria-based Electrolytes For Intermediate Temperature Solid Oxide Fuel Cells

Solid oxide fuel cell (SOFC) is a highly efficient and clean energy conversion device. The research interest is to lower the operating temperature to intermediate range, in order to overcome the problem of technique and materials for the traditional high temperature SOFC (operated at about 1000℃), and further improve the performance and reduce the manufacturing cost. There are two main methods to realize the intermediate temperature SOFC. The first one is to develop some new electrolyte materials with high oxygen ionic conductivity at intermediate temperature, the potential candidates are doped ceria. The second one is to reduce the thickness of electrolyte membrance. So far, large amount of papers reported the research of doped ceria, but detailed investigation is still necessary. The current study investigated the influence of content of doped ion, synthesis, chemical composition, content of oxygen vacancies and morphology on the electrical conductivity of the doped ceria electrolytes. The main contents are as follows:1. Effect of the content of doped ion on the electrical properties of CeO₂A series of Ce_1-xSm_xO_<sub>2-δ (x= 0, 0.1 and 0.2) solid electrolytes were prepared by a citrate sol-gel method. Influence of content, density, grain size and concentration of oxygen vacancies on their electrical properties were investigated by X-ray diffraction (XRD), Raman spectroscopy, SEM and AC impedance spectroscopy. The XRD results confirmed the formation of Ce_1-xSm_xO_<sub>2-δ solid solution with uniform particle sizes. The Raman spectroscopic results show that a higher substitution of Sm into the ceria lattice could enhance the formation of oxygen vacancies. Under the same doped ion, grain size and density, the improvement of oxygen vacancies was beneficial to the migration of O^2-, which resulted in a higher electrical conductivity obtained on the Ce_1-xSm_xO_<sub>2-δ compared to that of the Ce_0.9Sm_0.1O_2-δ. 2. Effect of thermal treatment conditions on the electrical properties of CeO₂Ce_0.8Sm_0.2O_2-δ electrolytes were prepared by sol-gel methods with different thermal treatment conditions. It was found the particle size of the powders thermally treated in N₂ could be controlled less than 10 nm, due to the fact that carbons that enwrapped the precursors could inhibit their crystallization. At the same time, the nano-sized precursors could form larger nuclei at the initial stage of sintering, resulting in the formation of electrolyte with higher density and larger grain size. At the same consent of oxygen vacancies, the enhancement of electrical properties for the electrolyte was correlated to its microstructure obtained by the N₂ thermal treatment.3. Effect of co-doped on the electrical properties of CeO₂Ce_0.8Sm_0.2O_2-δ,Ce_0.8Sm_0.2O_2-δ and Ce_0.8Sm_0.1Nd_0.1O_2-δ samples were prepared by a citrate sol-gel method. Effects of microstructures and oxygen vacancies of the samples on their electrical properties were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), in situ Raman spectroscopy and AC impedance spectroscopy. SEM results indicated that larger grains were formed on the Ce_0.8Sm_0.2O_2-δ and Ce_0.8Sm_0.1Nd_0.1O_2-δ electrolytes compared to that on the Ce_0.8Sm_0.2O_2-δ. In situ Raman spectra suggested that the concentration of oxygen vacancies of the Ce_0.8Sm_0.1Nd_0.1O_2-δ sample was the highest while that of Ce_0.8Sm_0.2O_2-δ was the lowest. It was found that the difference in the electrical conductivity for these electrolytes was closely related to the microstructure and oxygen vacancies of the samples. The highest electrical conductivity obtained on the Ce_0.8Sm_0.1Nd_0.1O_2-δ sample was ascribed to its larger grain size and higher concentration of oxygen vacancies.4. Effect of sintering promoters on the electrical properties of CeO₂Ce_0.8Sm_0.2O_2-δ powder with Co₃O₄ was synthesized by a sol-gel method, and theeffect of Co₃O₄ addition on the sintering and conductivity properties was investigated. The results indicated that after being doped with small quantities of Co₃O₄, the sintering ability increased and densification was reached at 1200℃which was about 300℃lower than that of the Ce_0.8Sm_0.2O_2-δ. In the meantime, the electrical conducitivity of Ce_0.8Sm_0.1Co_0.1O_2-δ was improved compared with the Ce_0.8Sm_0.2O_2-δ.