Role of iron addition on grain boundary conductivity of pure and samarium doped cerium oxide

Abstract

The present paper reports the effect of iron doping (0.5, 1.5 mol%) on the densification and electrical properties of cerium oxide (CeO₂) and 20 mol% samarium-doped cerium oxide (SDC) electrolytes for intermediate temperature solid oxide fuel cell (ITSOFC) applications. A single-step solution combustion method was used for doping and the resultant powder was compacted into green pellets and subsequently sintered at 1200 °C. X-ray diffraction (XRD) studies indicated the presence of a cubic fluorite CeO₂ structure without the formation of a secondary phase and the stoichiometry was confirmed by X-ray fluorescence spectroscopy. In the as-compacted green pellets, the XRD peak position shifted to lower or higher angles depending on the ionic radii of the dopants due to lattice level mixing. Addition of iron resulted in smaller crystallite sizes (<11 nm) in the case of the green pellet, while an opposite trend was observed (>40 nm) after sintering. Densification was found to be higher (95%) in iron-doped samples than in bare samples (<90%) due to viscous flow sintering. Upon sintering the calculated strain value showed a lower value due to the segregation of iron from the lattice. Raman spectroscopic studies indicate that sintering marginally modifies the oxygen vacancy concentration in the SDC system, and found it to be higher than in CeO₂. Addition of iron into the SDC improved the grain boundary conductivity 1.8 fold, but only a minor change was noticed for CeO₂. The activation energy for the grain boundary conductivity was found to be lower for 1.5 mol% (1.06 eV) iron-doped SDC than for pure SDC (1.24 eV). Our results indicate that lattice level mixing of iron in SDC improves the density at relatively lower sintering temperatures and scavenges the grain boundary impurities, thereby increasing the grain boundary conductivity.