Abstract
Partially substituted cerias are attractive materials for use as electrolytes in intermediate-temperature solid oxide fuel cells (SOFCs). Ceria doped with Sm or Gd has been found to have high ionic conductivities. However, there is interest in whether doping with multiple elements could lead to materials with higher ionic conductivities. The present study looks at the effects of co-doping Sr and Sm in ceria. A compositional series, Ce0.8+xSm0.2−2xSrxO2−δ (with x = 0–0.08), designed to have a constant oxygen vacancy concentration, was successfully prepared using the citrate–nitrate complexation method. A solubility limit of ~5 cation% Sr was found to impact material structure and conductivity. For phase-pure materials, with increasing Sr content, sinterability increased slightly and intrinsic conductivity decreased roughly linearly. The grain boundaries of phase-pure materials showed only a very small blocking effect, linked to the high-purity synthesis method employed, while at high %Sr, they became more blocking due to the presence of a SrCeO3 impurity. Grain capacitances were found to be 50–60 pF and grain boundary capacitances, 5–50 nF. The variation in the bulk capacitance with Sr content was small, and the variation in grain boundary capacitance could be explained by the variation in grain size. Slight deviations at high %Sr were attributed to the SrCeO3 impurity. In summary, in the absence of deleterious effects due to poor microstructure or impurities, such as Si, there is no improvement in conductivity on co-doping with Sr and Sm.
Highlights
Common commercial solid oxide fuel cells (SOFCs) are based on yttria-stabilised zirconia (YSZ) electrolytes and typically operate at 800–1000 ◦ C [6]
Oxygen vacancies are introduced into ceria electrolytes by doping with a lower valence species, typically trivalent lanthanide or divalent alkaline earth elements, and the conductivity of lanthanide-doped ceria is an order of magnitude higher than that of YSZ at 600 ◦ C [8]
A compositional series of ceria-based powders co-doped with varying Sr and Sm concentrations but with a constant oxygen vacancy concentration was successfully prepared concentrations but with a constant oxygen vacancy concentration was successfully preby the citrate–nitrate complexation method
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Explanations for the changes in total conductivity based on intrinsic properties have been proposed by Yeh et al, who state that Sr co-doping increases the radius of oxygen vacancies and so widens the oxygen-conducting channels [14], and Gao et al [23], who cite increased oxygen vacancy concentration and a suppression of vacancy ordering due to changes in configurational entropy, suggested by Yamamura et al [22]. This ordering effect is suggested by Jaiswal et al [25]. The extrinsic effects of microstructure were quantified by grain size analysis and minimised by the use of a citrate complexation synthesis method developed by the authors, which consistently produces fine, high-purity powders
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