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

摘要

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_xO_(2-delta) (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_2 content; 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.