Effect of manganese on the properties of the 8YSZ-electrolyte

Yttria-stabilised zirconia is the commonly used electrolyte in SOFC-applications. The maximum of ionic conductivity can be achieved by an addition of about 8 mol% yttria. Impurities are known to have an impact on the properties of the electrolyte. Their existence can be due to the impureness of the raw materials, whereas the high temperatures of the cell manufacturing and cell operation processes can also lead to a contamination of the electrolyte (due to interdiffusion processes). The present study examines the effect of small manganese oxide additions on the structural, electrical and chemical properties of the 8YSZ electrolyte. Different studies including XRD, SEM, impedance spectroscopy (IS), dc-conductivity (as a function of temperature and oxygen partial pressure respectively) and solid electrolyte coulometric (SEC) measurements were conducted, to get a comprehensive understanding of the changes in electrolyte properties. XRD measurements show a linear decline of the lattice parameter within the investigated composition range. That indicates the formation of a solid solution. EDX analyses confirm the results of the XRD measurements but show the formation of MnO(x) segregations in the pores of the 6 mol% Mn2O3 containing composition. DC-measurements reveal that the addition of Mn2O3 results in a drop of the ionic conductivity, which can be explained by the increased impact of defect interaction. IS-measurements (conducted at lower temperatures) show the drop of the grain and grain boundary conductivity caused by doping with small Mn2O3 amounts (< 4 mol% Mn2O3). The further increase of the Mn2O3 amount results in a drop of the activation energy due to an increased electronic conductivity. SEC measurements show an increased oxygen release at oxygen partial pressures above 10(exp -10) atm connected with a drop in ionic conductivity. This correlation is explained by the formation of additional oxygen vacancies due to changes in the valency of the Mn-cation.