Effect of Mg and Sr co-doping on the electrical properties of ceria-based electrolyte materials for intermediate temperature solid oxide fuel cells Nandini Jaiswal a , Devendra Kumar a , Shail Upadhyay b , Om Parkash a,⇑ a Department of Ceramic Engineering, Indian Institute of Technology, Banaras Hindu University, Varanasi 221 005, India b Department of Applied Physics, Indian Institute of Technology, Banaras Hindu University, Varanasi 221 005, India article info Article history: Received 23 March 2013 Received in revised form 11 June 2013 Accepted 15 June 2013 Available online 27 June 2013 Keywords: Ceramics Chemical synthesis Ionic conduction Impedance spectroscopy Scanning electron microscope and X-ray diffraction abstract Attempts have been made to synthesize a few compositions in the system Ce 0.90 Mg 0.10x Sr x O 1.90 (x = 0.00, 0.02, 0.04 and 0.06) by citrate–nitrate auto-combustion method. XRD patterns reveal that all the samples have fluorite crystal structure similar to ceria. Microstructures of samples have been studied by scanning electron microscope. Ionic conductivity of singly doped and co-doped ceria has been investigated as a function of temperature by AC impedance spectroscopy in the temperature range 200–700 °C. Impedance plots show a significant decrease in grain boundary resistance after partial substitution of Sr in Mg-doped ceria in the intermediate temperature range. Composition with x = 0.04 shows the highest ionic conduc- tivity (2.0  10 2 S/cm at 700 °C) among all the samples studied. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Solid oxide electrolytes have received an increasing interest in recent years due to their excellent suitability as ionic conducting materials in solid oxide fuel cells (SOFCs). Currently yttria stabi- lized zirconia (YSZ) is widely used as an electrolyte due to its excel- lent chemical and mechanical stability. But it requires high operating temperature (1000 °C). High operating temperature can lead to complex material problems such as interfacial diffusion be- tween electrodes and electrolyte, mechanical stress due to differ- ent thermal expansion coefficients, long term stability and high cost of interconnects and other construction materials [1]. There- fore, development of new cost effective electrolytes of high ionic conductivity in the intermediate temperature range (600–800 °C) such as lanthanum gallate and doped ceria based electrolytes have been proposed [2,3]. Among these new materials, ceria doped with aliovalent cations such as rare earth and alkaline earth ions has been widely investigated as solid electrolytes for intermediate temperature solid oxide fuel cells (IT-SOFCs) [4–6]. Substitution of aliovalent cations in ceria leads to incorporation of oxygen vacancies for charge compensation. 2Ceo 2 ! M 2 O 3 2M 0 Ce þ 3O  O þ V€ O ð1Þ Ceo 2 ! MO M 00 Ce þ 2O  O þ V€ O ð2Þ where the symbols are used in accordance with Kroger Vink nota- tion of defects. M 0 Ce stands for a trivalent ion on Ce 4+ site, M 00 Ce stands for a divalent ion on Ce 4+ site V€ O is oxygen vacancy with doubly po- sitive effective charge and O  O represents that oxygen ion on O 2 site being neutral. Rare earth oxides have high solubility in ceria [7]. However, at high temperatures and low oxygen partial pressures, the electronic conductivity of doped ceria is considerably high. This is a problem for use of these materials as solid electrolyte for SOFC. Co-doped ceria appears to be a potential solution for this problem [8]. Gadolinium (Gd) and samarium (Sm) doped ceria have been widely used [9,10] because these have lattice parameter almost equal to un-doped ceria. Their substitution produces very small strain in the lattice which reduces activation energy for diffusion of oxide ions [11]. This has also been confirmed by atomistic com- puter simulation studies based on the binding energy between tri- valent cations and oxygen vacancies and the corresponding lattice relaxation energy [12]. Jadhav et al. [13] studied the Ce 1x Gd x O 2x/ 2 (x = 0.30) system which was synthesized by glycine–nitrate pro- cess and they found single phase formation in this system and the grain growth of sintered samples was observed to hindered with an increase in Gd content. Composition Ce 0.90 Gd 0.10 O 1.95 was syn- thesized using combustion technique by Jadhav et al. [14] and they found that the relative density of the sample was more than 90% at 1200 °C which was also confirmed by SEM analysis. The grain and grain boundary conductivity of a GDC (Ce 0.90 Gd 0.10 O 1.95 ) thin film 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.06.094 ⇑ Corresponding author. Tel.: +91 542 6701791; fax: +91 542 2368428. E-mail address: oprakash.cer@itbhu.ac.in (O. Parkash). Journal of Alloys and Compounds 577 (2013) 456–462 Contents lists available at SciVerse ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom