Effect of yttrium ion on the space charge potential across grain boundaries regions of gadolinia-doped ceria electrolytes

In the present work, gadolinium-doped ceria-based powders were co-fired with additions of 1% (w/w) of SiO₂, and 5% (w/w) of Y₂O₃ to test the role of yttrium ion on improving the grain boundary conductivity across the grain boundary regions of low grade gadolinia-doped ceria (CGO) electrolytes. The samples were prepared by hot press at low temperature (1000 °C) to minimize bulk dissolution of yttrium in the CGO lattice. Structural characterization by XRD of the prepared ceramics confirms a CGO single phase material with the fluorite type structure. All the samples were characterized by impedance spectroscopy as a function of temperature in air, in order to de-convolute different microstructural contributions to the overall electrical behaviour. The results showed, as expected, that the presence of small amounts of impurity of silica reduces the total conductivity, when compared with pure CGO ceramic sample. The grain boundary resistance of these ceramics, under low operating temperatures, has a large effect on the total conductivity and is related, on one hand with the presence of a space charge layer created by the local segregation of trivalent rare earth elements, and the consequently depletion of oxygen vacancies, and on the other hand by the blocking effect of the silicon impurity. However, the obtained results show that addition of yttria increases total conductivity when compared with impure samples without yttria. This effect was related with the partial recover of specific grain boundary conductivity, suggesting a preferential location of Si and Y cations on grain boundaries. The space charge potential values, calculated using impedance data, provided an approach to the promoting effect of recovering grain boundary conductivity by the yttrium ion.