Effects of Gd and Sr co-doping in CeO2 for electrolyte application in solid oxide fuel cell (SOFC)

Rare earth doped ceria is a key composition for electrolyte application in SOFC. The present trend is to investigate the co-doping approach in ceria either to improve further the electrical conductivity or sintering property. In this study, nano-structured Gd and Sr co-doped ceria (GSDC) powders have been prepared by gel combustion route. In all the compositions, total dopant cation concentration has been kept at 0.20 atom fraction having general formulae Ce0.8Gd0.2−xSrxO1.9−x/2. XRD patterns of GSDC powders calcined at 600 °C revealed the phase pure cubic fluorite structure with no Gd2O3 or SrO phase within detection limit. The combustion synthesized powders after sintering at 1350 °C resulted in pellets with density more than 96% of theoretical density. The total ionic conductivities have been evaluated from impedance data as a function of temperature in the range of 200-800 °C. The results indicate that ionic conductivity increases with small addition (~1 mol%) of SrO. At higher SrO concentration ionic conductivity decreases and beyond about 5 mol% SrO concentration the ionic conductivity becomes lower than that of CeO2 doped with only Gd2O3. The decrease in ionic conductivity at higher SrO concentration has been attributed to the increase in the enthalpy of oxygen vacancy association. The initial increase in conductivity could be due to scavenging of siliceous phase likely to be present in the grain boundary.