Augmentation of grain boundary conductivity in Ca2+ doped ceria-carbonate-composite

Abstract

A systematic study including the optimization of dopant and salt contents in Ca2+ doped ceria (CDC)-sodium carbonate composite is presented. Ca2+ amount is optimized in CeO2 in terms of good solubility and high ionic conductivity. The optimized CDC sample with 5 mol% Ca2+ doping (5CDC) shows the best ionic conductivity with total conductivity of the order of 11 mS/cm at 800 °C and 0.65 mS/cm at 600 °C and nearly 80 times lower electronic conductivity. 5CDC was, therefore, used further to produce a ceria-carbonate-composite (CCC) electrolyte and the amount of Na2CO3 was optimized. XRD of the sintered samples showed no crystalline peak corresponding to the carbonate, indicating its amorphous structure. Furthermore, the amorphicity of the carbonate phase was confirmed by DSC and TEM analyses which reflect its presence as a percolating network surrounding crystalline CDC grains. A remarkable decrease in grain boundary resistance was noticed in Nyquist impedance plots. This enhances the total conductivity up to ∼25 times. Its electronic conductivity was found to be nearly 350 fold lower than the total conductivity confirming the dominating ionic conduction. The role of Na2O (from Na2CO3) behind this is also explained with the help of a new model/hypothesis.