Understanding the Conductivity Degradation Mechanism of Er2O3-Stabilized Bi2O3

Abstract

Rare earth stabilized bismuth oxides have attracted much attention due to their high oxygen ionic conductivity. Among singly-doped stabilized bismuth oxides, the Er2O3-stabilized Bi2O3, which is generally abbreviated to ESB, is particularly known for its superior ionic conductivity to even other stabilized Bi2O3s, not to mention conventional oxygen ion conductors such as doped ceria and stabilized zirconia[1]. In spite of its remarkable ionic conductivity (≈ 0.3S/cm at 700℃), ESB has not been practically employed as a SOFC electrolyte. This is not only because ESB has thermodynamic instability under reduction conditions, but also because it experiences time-dependent conductivity degradation on extended annealing below 600℃. In this study, the crystallographic structure and conductivity of ESB were analyzed using X-ray diffraction and electrochemical impedance spectroscopy, respectively, during its prolonged annealing at 600℃ over 400h in order to investigate the mechanism of the time-dependent conductivity degradation of ESB. It was found that the initial cubic phase of ESB was mostly transformed to the rhombohedral phase within first 100h. At the same time, the discrepancy in time constant of the phase transformation(65h) and the conductivity degradation(52h) was observed. In order to elucidate this time constant discrepancy, the activation energies of low temperature domain before and after annealing were evaluated. The activation energy of as-sintered ESB was approximately 1.2eV, whereas that of the annealed ESB was approximately 1.01eV. Such a decrease in the activation energy can be attributed to order-disorder transition of the oxygen sub-lattice in ESB[2]. Therefore, the conductivity degradation of ESB is probably the combined effect of the phase transformation from cubic to rhombohedral as well as the order-disorder transition. Acknolwdge This research was supported by (1) Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2014R1A1A2057681), (2) the DGIST R&D Program of the Ministry of Science, ICT and Future Planning of Korea (1501- HRLA-01) and (3) the Global Frontier R&D on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science, ICT & Future Planning, Korea (2014M3A6A7074784).