Stabilization of a Zirconia System and Evaluation of Its Electrolyte Characteristics for a Fuel Cell: Based on Electrical and Mechanical Considerations

Abstract

The purpose of this study is to clarify the relationship between ionic conductivity and phase transformation of zirconia system codoped with scandium oxide Sc2O3 and ytterbium oxide Yb2O3. Aiming to achieve high ionic conductivity as well as high mechanical strength, the authors have also investigated the relationship between phase transformation and mechanical strength. The results have been discussed with respect to both the conductivity and the mechanical strength. The Sc- and Yb-codoped zirconia (ZrO2) used as samples in this study were prepared by a standard solid-state reaction. X-ray powder diffraction (XRD) method was used to determine the crystal structures of the sintered samples. To detect any phase change between room temperature and 1273K, thermal mechanical analysis (TMA) was conducted. To determine oxygen-ion conductivity in a temperature range from 873to1273K in air, impedance measurements were performed with alternating current (ac). Single-cell performance was confirmed under the condition of 26.2Pa partial hydrogen pressure. Finally, to measure bending strength, three-point bending tests were performed with a universal testing machine. The results of XRD and TMA showed that codoping of Sc2O3 and Yb2O3 into ZrO2 successfully stabilized the cubic phase when the average radius ratio of these two dopants in total was close to the ideal one for the eight-coordinate. The ac impedance measurement demonstrated that the cubic-phase stabilization achieved a high conductivity. Adequate amounts of dopants produced oxygen vacancies for high conductivity without complex defects: ZrO2 system doped with 1mol% of Yb2O3 and 8mol% of Sc2O3 showed the highest conductivity at 1273K and 0.30S∕cm. The bending strength decreased with increasing the content of doped Sc2O3 from 7mol%to11mol%, depending on the amount of the tetragonal phase, which contributes to strengthen materials. In the performance test, the ZrO2 system stabilized with doping 1mol%Yb2O3 and 8mol%Sc2O3 with thickness of 2.16mm showed maximum power density at 1273K, that is, 210mW∕cm2. From all the above tests, we recommend that, based on electrical and mechanical considerations, 1Yb8ScSZ is the present best option for an electrolyte material for a solid oxide fuel cell.