Investigation of electrical conductivity as a function of dopant-ion radius in the systems Zr 0.75Ce 0.08M 0.17O 1.92 (M=Nd, Sm, Gd, Dy, Ho, Y, Er, Yb, Sc)

Abstract

Electrical conductivity versus dopant ionic radius studies in zirconia- and ceria-based, solid oxide fuel cell (SOFC) electrolyte systems have shown that oxygen-ion conductivity is highest when the host and dopant ions are similar in size [J. Am. Ceram. Soc. 48 (1965) 286; Solid State Ionics 37 (1989) 67; Solid State Ionics 5 (1981) 547]. Under these conditions, it is thought that the conduction paths within the crystal lattice become less distorted [Solid State Ionics 8 (1983) 201]. In this study, binary ZrO 2–M 2O 3 unit cells were expanded, via the partial substitution of Ce +4 for Zr +4 into the lattice, in an attempt to identify new, ternary, zirconia/ceria-based electrolyte systems with enhanced electrical conductivity. The compositions Zr 0.75Ce 0.08M 0.17O 1.92 (M=Nd, Sm, Gd, Dy, Ho, Y, Yb, Sc) were prepared using traditional solid state techniques. Bulk phase characterisation and precise lattice parameter measurements were performed with X-ray diffraction techniques. Four-probe DC conductivity measurements between 400 and 900 °C showed that the dopant-ion radius influenced electrical conductivity. The conductivity versus dopant-ion radius trends previously observed in zirconia-based, binary systems are clearly apparent in the ternary systems investigated in this study. The addition of ceria was found to have a negative influence on the electrical conductivity over the temperature range 400–900 °C. It is suggested that distortion of the oxygen-ion conduction path by the presence of the larger M +3 and Ce +4 species (relative to Zr +4) is the reason for the decreasing electrical conductivity as a function of increasing dopant size and ceria addition, respectively.