Intermediate-temperature ionic conductivity of ceria-based solid solutions as a function of gadolinia and silica contents

Abstract

Ceria-based solid solutions, especially Gd-doped CeO 2, have widely been used as electrolytes for use in intermediate-temperature fuel cells. In this study, three groups of Ce 1− x Gd x O 2− δ (0.05⩽ x⩽0.3) ceramics, with SiO 2 contents of 30, 200 and 3000 ppm, were prepared. This investigation was intended to demonstrate the effects of both gadolinia and silica contents on the ionic conductivities (especially the grain-boundary (GB) contribution). It was found that, with increasing SiO 2 content, the composition of maximum total conductivity shifted to high x values. For example, the composition of maximum total conductivity for the 30 ppm SiO 2 group was x=0.15. It shifted to x=0.2 and 0.25 as SiO 2 level increased to 200 and 3000 ppm, respectively. Meanwhile, the values of the maximum total conductivity were reduced significantly with increasing SiO 2 content. Although the three groups showed a similar variation trend in the GB conduction with increasing Gd content, the mechanisms governing this variation trend were different, depending on the SiO 2 content and the Gd content. The composition having the maximum GB conductivity (or the minimum activation energy for the GB conduction) could be taken as a critical value ( x C). Below this value ( x C), the GB conductivity was dominated by space-charge layers or/and resistive siliceous films, which was dependent on the SiO 2content; and above this value ( x C), the GB conductivity was determined by the segregation of undissolved Gd 2O 3 at the grain boundaries, which was irrespective of SiO 2 content.