Available online at www.sciencedirect.com Materials Chemistry and Physics 108 (2008) 102–108 Y 2 Zr 2 O 7 (YZ)-pyrochlore based oxide as an electrolyte material for intermediate temperature solid oxide fuel cells (ITSOFCs)—Influence of Mn addition on YZ M. Kumar a , I. Arul Raj b,∗ , R. Pattabiraman b a National Metallurgical Laboratory, CSIR Madras Complex, Chennai, India b Central Electrochemical Research Institute, Karaikudi, India Received 20 March 2007; received in revised form 7 September 2007; accepted 14 September 2007 Abstract Compositions in the pyrochlore system Y 2 Zr 2 O 7 (YZ) and Y 2 Zr 2-x Mn x O 7-δ (YZM) (where x = 0.025, 0.05, 0.075 and 0.10) were examined as possible alternatives to stabilised zirconia solid oxide electrolyte in intermediate temperature solid oxide fuel cells (ITSOFC). Such materials were prepared by glycine–nitrate combustion process. The prepared compounds were characterised by X-Ray Diffraction and Thermal analysis. Circular pellets were fabricated and annealed at different temperatures ranging from 1000 to 1400 ◦ C. The sintering behaviour of YZ and YZM were investigated to obtain information on the densification factor, relative percentage shrinkage/expansion in volume after heat treatment and apparent porosity value. The small doping level of Mn (≤10.0 wt%) resulted in increased conductivity values. The component diffusion coefficients and mobility of ions are calculated from its conductivity value. D comp and μ i are found in the range of 10 -8 cm 2 s -1 and 10 -7 cm 2 V -1 s -1 , respectively. The key features, which make the YZ and YZM systems attractive as a fuel cell electrolyte, are discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: Yttrium zirconate pyrochlore oxides; Combustion method; Sintering studies; Scanning electron microscope; Conductivity measurements 1. Introduction Yttria-stabilised zirconia (YSZ) is generally used as an elec- trolyte material for solid oxide fuel cell (SOFC) because of its good mechanical strength and high stability under oxidiz- ing and reducing atmosphere at high temperatures [1,2]. The YSZ electrolyte usually has to be operated at about 1000 ◦ C, where the ionic conductivity reaches the required high level. However, such high temperatures lead to a high fabrication cost and degradation of cell components. To reduce the operating temperature of the SOFC system, much attention has been paid recently to investigate alternate electrolyte materials such as par- tially substituted lanthanum gallate, ceria and also pyrochlore type oxide materials (Y 2 Zr 2 O 7 , yttrium zirconate (YZ)). Rare earths based pyrochore oxides exhibiting high ion conductivity have been reported [3–12]. It has also been reported that at low doping levels, the addition of Mn/Fe or Co enhances the con- ductivity of the material [13–15]. It is desirable in this context to ∗ Corresponding author. Tel.: +91 4565 227552; fax: +91 4565 227779. E-mail address: iarulraj@rediffmail.com (I.A. Raj). determine whether higher doping levels might yield yttrium zir- conate based compositions exhibiting adequate conductivity for use as materials for components in ITSOFCs [16]. The present study is under taken to determine the effect of partially substi- tuting Mn/Fe or Co on Zr-site in Y 2 Zr 2 O 7 in comparison with Y 2 Zr 2 O 7 . 2. Experimental The yttrium zirconate and Mn-doped yttrium zirconate were prepared by combustion synthesis process using glycine as a fuel [17]. The combustion method involves the combustion of saturated aqueous solution containing sto- ichiometric quantities of the corresponding nitrates (oxidizers) and glycine (fuel). The appropriate quantities of the precursor nitrate salts (if oxide/acetate salts were taken, they were converted as nitrates by treating them with nitric acid) were calculated according to the concepts of propellant chemistry [18,19]. The oxidizer and fuel ratio was calculated based on oxidizing and fuel valen- cies of the reactants keeping O/F = 1 as reported [20,21]. The aqueous redox solution containing metal nitrates and glycine when introduced into a muf- fle furnace preheated to 823 K, boils, froths, ignites and catches fire (at a high temperature 1373 K). At this temperature the metal nitrates decompose to metal oxides and oxides of nitrogen and hence act as oxidizer for further combustion, which leads to voluminous, foamy combustion residue in less than 5 min. The as-synthesised powders were calcined at 1073 K for 6 h in air 0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.09.010