Processing and conduction behavior of nanocrystalline Gd-doped and rare earth co-doped ceria electrolytes

Gadolinia (Gd2O3) doped ceria (GDC) and GDC co-doped with Eu2O3 (Eu-GDC) and Nd2O3 (Nd-GDC) were processed in a single step by citrate-nitrate solution combustion synthesis and a detailed impedance spectroscopy analysis was carried out to understand the conduction behavior. X-ray diffraction (XRD) and transmission electron microscopy (TEM) confirmed the nanocrystallinity in the as-prepared powders for all compositions. Dense sintered pellets were obtained by conventional sintering at lower temperatures compared to microcrystalline ceria. The Arrhenius plots of bulk conductivity showed two different slopes indicating presence of different conduction mechanisms at lower and higher temperatures. At lower temperatures, the conduction mode was found to be ionic and it changed to mixed ionic and electronic conduction at higher temperatures. The mixed conductivity was confirmed by the half-tear drop shape of the impedance plots. The grain boundary conductivity, on the other hand, exhibited ionic character and was influenced by the space-charge layer and defect chemistry. The conductivity was improved in Nd-GDC while Eu-GDC showed similar conductivity compared to GDC. (C) 2016 Elsevier Ltd. All rights reserved.