Electrical and Mechanical Properties of ZrO2-Y2O3-Al2O3 Composite Solid Electrolytes

Abstract

Strategic priorities in the field of hydrogen energy include the design of intermediate-temperature solid oxide fuel cells capable of highly efficient operation in the temperature range of 573–973 K. Consequently, attempts are being made to replace the widely applied cubic zirconia electrolyte with an electrolyte consisting of tetragonal zirconia. The rationale for this approach is that 3Y-TZP exhibits higher mechanical strength and higher electrical conductivity at temperatures below 973 K. The addition of Al2O3 in an amount that exceeds its solubility limit in 3Y-TZP has been found to result in increased electrical conductivity and improved mechanical properties. The aim of the study was to synthesize 3-YSZ powder via co-precipitation and use it to obtain composites with a 3Y-TZP matrix and 0.5 mol.% or 1.0 mol.% of Al2O3 inclusions. The correlation between these samples' electrical conductivity and resistance to brittle fracture and their phase composition and microstructure was investigated by means of X-ray diffractometry, scanning electron microscopy, electrochemical impedance spectroscopy and Vickers indentation tests. For comparison, the properties of composites with an 8-YSZ matrix and Al2O3 inclusions were also investigated. It was determined that the composite based on the 3Y-TZP matrix and containing 0.5 mol.% of Al2O3 inclusions can be considered a viable alternative for 8-YSZ electrolytes in IT-SOFC applications.

Effects of Aluminum Oxide Addition on Electrical and Mechanical Properties of 3 mol% Yttria-Stabilized Tetragonal Zirconia Electrolyte for IT-SOFCs

“…Alumina acts as an insulator, which is why higher concentrations of this compound affect activation energy in this way. The activation energy of the grain boundary conductivity was higher than the activation energy of the grain interior conductivity, which is consistent with the results presented in [31,63,67]. In the case of GS sinters, the activation energy values of grain interior and grain boundary conductivity were comparable regardless of the concentration of Al 2 O 3 inclusions.…”

“…The activation energy of total conductivity (Ea(tot)) was in the range of 0.79-0.82 eV for the GS sinters and in the range of 0.76-0.82 eV for the IS ones. Similar values have been reported in the literature for this type of ceramics [30,31,40,62,63]. GS-15 had the highest Ea(tot) value of the samples from the first group, while in the second group the highest value was found for IS-10.…”

“…Miyayama et al [28] and X. Guo [29] reported a slight decrease in the conductivity of both grain interiors and grain boundaries when the alumina content in zirconia was below the solubility limit. On the other hand, the authors of [30,31] determined the grain boundary and grain interior electrical conductivity for unmodified 3Y-TZP as well as 3Y-TZP samples containing 0.25, 0.5 and 1.0 mol% of alumina and found the that the sample with a 0.5 mol% alumina content exhibited the highest values of these two parameters. Certain other studies found no changes in the ionic conductivity of grain interiors in zirconia with a small number of alumina inclusions or a slight decrease with a marked improvement in grain boundary conductivity.…”

“…The latter leads to the so-called "scavenger effect" and Al 2 O 3 inclusions displacing the impurities to grain boundary triple junctions [19]. If a large amount of alumina is added to the 3Y-TZP material and as a result the solubility limit is exceeded by a considerable margin, there is a pronounced decrease in the ionic conductivity of both grain interiors and grain boundaries [28,31,32]. The effective medium theory can be used to explain this drop [33].…”