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Co-doped samples of Ce 0.95−x Ca 0.05 Sr x O 1.95−x , where (x=0.00, 0.01, 0.02, and 0.03), have been prepared by auto-combustion method and characterized to explore their use as a solid electrolyte for intermediate temperature solid oxide fuel cells (IT-SOFCs). Crystal structure, microstructure, and ionic conductivity have been characterized by X-ray diffraction, scanning electron microscopy, and impedance spectroscopy, respectively. All the compositions have been found to be single phase. Results show that the samples co-doped with Ca and Sr exhibit higher ionic conductivity than the samples singly doped with Ca in the intermediate temperature range. Ce 0.93 Ca 0.05 Sr 0.02 O 2−δ exhibits maximum conductivity among all the compositions. This may be a potential candidate as a solid electrolyte for IT-SOFCs. Keywords Doped ceria electrolyte. Co-doping effect. Ionic conductivity. Solid oxide fuel cells distributed heat-power co-generation, an operation temperature in the range 500-700°C is highly desirable. This is in view of the cost effectiveness because inexpensive stainless steel may be used for this purpose. Doped ceria electrolytes have attracted great interest in recent years because of their potential as a solid electrolyte for intermediate temperature solid oxide fuel cells (IT-SOFCs) application [7]. In CeO 2 , temperature facilitates movement of oxygen ions through oxygen vacancies which are produced in the oxygen sublattice to neutralize the deficiency of charge created by lower valent dopant cations. Doped ceria oxides show much higher ionic conductivity at relatively low temperatures (500-700°C) as compared with yttria-stabilized zirconia. These have been extensively studied as the most promising electrolyte materials for IT-SOFCs. Among the various ceria compositions investigated so far [8-19], Gdand Sm-doped ceria (GDC and SDC) are considered as the most suitable low-temperature solid electrolytes for IT-SOFCs application. Both Gd 2 O 3 and Sm 2 O 3 , however, are very costly. Therefore, there is an increasing interest to develop new cost-effective ceria-based electrolytes. Ceria doped with alkaline earth oxides such as CaO [20, 21] and SrO [22, 23] has been studied extensively. Electrical conductivity of CaO-and SrO-doped ceria is much higher than that of undoped ceria. The highest conductivity was found in the composition Ce 0.90 Ca 0.10 O 1.90 by Shing et al. [24] which is ∼10 −3 S cm −1 at 600°C. Yamashita et al. [25] found that the composition Ce 0.90 Ca 0.10 O 1.90 has the highest conductivity which is ∼10 −2 S cm −1 at 600°C. Banerjee et al. [26] studied the electrical properties of Ce 1−x Ca x O 2−δ (0.05≤x≤0.20) samples prepared by a mixed fuel process followed by sintering at 1,250°C. They found that the composition Ce 0.80 Ca 0.20 O 1.80 exhibits the highest conductivity (1.29×10 −2 S cm −1) at 600°C. Compositions Ce 1−x Ca x O 2−δ with 0.05≤x≤0.20 have been prepared by auto-combustion method and characterized.

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