Tm(III)-doped d-Bi2O3 for solid - oxide fuel cells

Abstract

The oxide ion conductors have widely been investigated because of their application in many devices with high economical and ecological interests, such as solid oxide fuel cells (SOFC). -Bi2O3 polymorph possesses the highest known O2– ion conductivity, which is one to two orders of magnitude higher than that of stabilized zirconia at corresponding temperatures [1]. At the moment, the application of this high-temperature polymorph as an electrolyte in SOFC requires temperatures above 730 °C. However, the doping allows -Bi2O3 stabilization to room temperature and opens the possibility for construction of SOFC that will operate at intermediate temperatures (ca. 350 °C) [2]. As it is found that some lanthanides are suitable dopants [3], the possibility to stabilize O2– ion conductors related to the -Bi2O3 polymorph in the Bi2O3–Tm2O3 system was investigated. Two starting mixtures with compositions (Bi1–xTmx )2O3 (x = 0.04 and 0.20) were homogenized in an agate mortar, heat treated at 750 °C for 3 h and then slowly furnace cooled. The samples were characterized by XRPD, DTA and SEI techniques. Based on XRPD, the single-phase tetragonal β-Bi2O3 was identified in the sample with x = 0.04. Its unit cell parameters, a = 7.742(2) and c = 5.650(2) Å, well-correspond to those of undoped β-Bi2O3 [4]. On the other hand, the cubic -Bi2O3 phase was obtained in the sample with x = 0.20. Its unit cell parameter was greater than the value reported for Tm-doped -Bi2O3 sample with x = 0.25 [3] (5.5033(9) vs. 5.478 Å). Both values are smaller than reported for undoped -Bi2O3 [4]. This means that the unit cell parameter of cubic -Bi2O3 decreases as Tm-content increases and it is in accordance with Tm3+ and Bi3+ ionic radii [5]. For the sample with x = 0.04, cyclic DTA curves showed one reversible β-Bi2O3 ↔ -Bi2O3 transition with corresponding temperatures: on heating, 660 °C, and, on cooling, 600 °C. Surprisingly, no phase transitions were observed in the sample with x = 0.20 which indicates that the obtained -Bi2O3 is stable within the whole investigated interval, i.e., from room temperature to 1000 °C. Electrochemical impedance of -Bi2O3 phase was measured in the following temperature range: 300 – 800 °C. At higher temperatures (600 – 800 °C) the conductivities are similar (0.11 – 0.32 S cm –1 ), but with lowering temperature they rapidly decrease, and amount, for example, 2.1·10–5 S cm–1 at 300 °C. As a consequence, two activation energies are found: 0.45(4) eV (600 – 800 °C), and 1.33(2) eV (300 – 600 °C). References: [1] P. Shuk, H.-D. Wiemhöfer, U. Guth, W. Göpel, M. Greenblatt, Solid State Ionics 89 (1996) 179 [2] E. D. Wachsman, K.T. Lee, Science 334 (2011) 935 [3] H. T. Cahen, T. G. M. Van Den Belt, J. H. W. De Wit, G. H. J. Broers, Solid State Ionics 1 (1980) 411 [4] H. A. Harwig, Z. anorg. allg. Chem. 444 (1978) 151 [5] R. D. Shannon, Acta Cryst. A 32 (1976) 751