Ionic conductivity in multiply substituted ceria-based electrolytes

Cerias, appropriately doped with trivalent rare earth ions, have high oxide ion conductivity and are attractive SOFC (solid oxide fuel cell) electrolytes. Here, seven compositions of Ce_0.8Sm_xGd_yNd_zO_1.9 (where x, y and z = 0.2, 0.1, 0.0667 or 0 and x + y + z = 0.2) are synthesised using a low temperature method in order to determine the effect of multiple doping on microstructure and conductivity. Analysis using scanning and transmission electron microscopy, inductively coupled plasma mass spectrometry, X-ray diffraction and impedance spectroscopy is carried out. Crystallite sizes are determined in the powders and relative densities and grain size distributions were obtained in sintered pellets. Total, bulk and grain boundary conductivities are obtained using impedance spectroscopy and corresponding activation energies and enthalpies of ion migration and defect association are calculated. The highest total conductivity observed at 600 °C is 1.80 Sm⁻¹ for Ce_0.8Sm_0.1Gd_0.1O_1.9 and an enhancement effect on conductivity for this combination of co-dopants between 300 °C and 700 °C relative to the singly doped compounds - Ce_0.8Sm_0.2O_1.9 and Ce_0.8Gd_0.2O_1.9 - is seen. This has interesting implications for their application as SOFC electrolytes, especially at intermediate temperatures.