| Solid oxide fuel cells (SOFCs) have been attracting a great attention as a promising technique for electrical power generation due to its higher power density, high energy-conversion efficiency and extremely low pollution, in addition to flexibility in using hydrocarbon fuels. To date, yttria-stabilised zirconia (YSZ) has been mainly used as the solid oxide electrolyte for such fuel cells because of its nearly pure oxygen ionic conductivity in either oxidizing or reducing atmosphere, as well as good mechanical properties. However, high operating temperatures of over 900℃are required for this electrolyte, this condition increases the fabrication cost and accelerates degradation of the fuel-cell system. Therefore, it becomes increasingly important to reduce the operating temperature of the SOFC, and the development of intermediate temperature (500～700℃) SOFCs (IT-SOFCs) is the key to commercialization. Doped ceria-based electrolytes have been regarded as the most promising electrolytes materials for IT-SOFC because of higher oxide ionic conductivity and lower conductance activation energy than that of YSZ. However, ceria-based ceramics are dif cult to density below 1500℃, this makes them difficult for manufacturing ceria-based electrolytes, increases the cost and has bad mechanical properties. In order to improve the sintering properties of ceria-based electrolytes, adding some transition metal oxides (TMOS) as sintering aids to reduce the sintering temperature. The presence of SiO2 impurity is ubiquitous in precursor materials, SiO2 usually partial segregates in grain boundaries in sintering process and blocking the grain boundary mobility and the mass diffusion, increasing the grain boundary resistance, reducing the conductivity of electrolyte materials, the negative effect of this harmful siliceous phase needs to be mitigated even in relatively pure materials. Therefore, the reduction deleterious grain boundary effects arising from SiO2 impurity by adding the grain boundary scavenger, enhancing the total ionic conductivity, has become the focus of research in recent years. In this paper, we investigate the Nd2O3 doped CeO(NDC) system, MoO3 would be used an effective sintering aid and a grain boundary scavenger for NDC system. We systematically investigated the effects of sintering temperature and MoO3 doping concentration on the structures and electrical conductivities of high purity NDC and impurity NDC, we also got the appropriate sintering temperature and the optimal content of molybdenum oxide. The main research contents of this paper are as follows:The high purity Ceo.8Nd0.2O1.9+xMo03 (0.00≤x≤0.02) and the impurity (Ceo.8Nd0.2O1.9)1-x(Mo03)x+500ppmSi02 (0.00≤x≤0.05) were synthesized by citric-nitrate method.Their structures and ionic conductivities were characterized by X-ray Diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and electrochemical impedance spectroscopy (EIS). The results showed that:(1) The optimal content of MoO3 was 1mol% in the high purity NDC system. The total ionic conductivities,the grain conductivities and the grain boundary conductivities increased in the high purity MoO3 doped-NDC system, especially the grain boundary conductivity have increased greatly, at the same time decreased the sintering temperature by about 200℃, and enhanced densification obviously.(2) The optimal content of MoO3 was 3mol% in the impurity NDC system. The existence of SiO2 reduced the system densification and conductivity obviously. MoO3 doping improved densification,the total ionic conductivities,the grain conductivities and the grain boundary conductivities in impurity NDC system. This result suggests the addition of small amount of MoO3 could be used as sintering additive and grain boundary scavenger for the system of NDCSi at the same time. |
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