Surface Oxide Ion Conduction of a - Axis Oriented Ce0.75Sm0.25O2-
δ
Thin Film Prepared on (200) YSZ Substrate
Authors: Morizane Ryota , Tabuchi Riku , Tabuchi Riku , Higuchi Tohru 
Published: December 2024

Fuel cells are attracting attention as a clean energy source because their only waste product is water. Solid oxide fuel cells (SOFC), which use oxide with oxygen ion conductivity as the electrolyte, can simplify the configuration of the power generation system and facilitate management, but they require a high operating temperature of 800 ℃ or higher [1]. Yttria-stabilized zirconia, which is currently in practical use as an electrolyte, exhibits high oxygen ion conductivity at temperatures above 800 ºC. Therefore, an electrolyte that exhibits high ion conductivity at low temperatures is required.

Fluorite-type CeO2-δ
oxides are good solid electrolyte candidates for solid oxide fuel cells due to their high oxygen ion conductivity in high-temperature regions above 800 ℃ because they take on a mixed valence state of Ce3+ and Ce4+ in the high temperature. Furthermore, it is known that doping CeO2 with trivalent dopants increases the amount of oxygen defects and improves ionic conductivity [2-4]. While we have reported the surface ionic conduction Sm-doped CeO2 thin films, the conductivity was not sufficiently high for practical applications [5].

In this study, in order to realize high surface oxide ion conductivity, we prepared a - Axis Oriented Sm-doped CeO2 (Ce0.75Sm0.25O2-

δ
: SDC) Thin Film without lattice distortion and investigated its surface oxide ion conduction.

The SDC thin films were prepared on YSZ (200) substrates by RF magnetron sputtering using ceramic targets. The flow rate of Ar gas controlled by a mass-flow controller was kept at approximately 2.81 sccm. The pressure of Ar gas was fixed at 3.5 × 10−3 Torr during the deposition. The prepared thin films were characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and A.C impedance method using a frequency response analyzer.

The prepared SDC thin films showed a 200 peak and a 400 peak on the YSZ substrates, indicating no change in lattice constants with the ceramics. Figure shows an Arrhenius plot of the conductivity of the As-deposited and Wet-annealed SDC thin films. As a reference, the Arrhenius plot of the conductivity of the YSZ substrate and ceramics is shown. The conductivity of the SDC thin film is much higher than that of the YSZ substrate and ceramics because the amount of oxygen vacancies increased more than those. The conductivity does not much change by wet annealing and depends on the film thickness. The conducting carrier above 200 °C is considered to be oxide ion. In this presentation, the author will show the details of the conductivity and discuss including XPS results as well as details of the conductivity.

References

[1] K. Eguchi, T. Setoguchi, T. Inoue and H. Arai, Solid State Ionics 52, 165-172 (1992).

[2] H. Yahiro, K. Eguchi, and H. Arai, Solid State Ionics 36, 71 (1989).

[3] Y. Nigara, K. Yashiro, T. Kawada, and J. Mizusaki, Solid State Ionics 145, 365 (2001).

[4] Y. Nigara, K. Yashiro, T. Kawada, and J. Mizusaki, Solid State Ionics 159, 135 (2003).

[5] G. Notake et al., Jpn. J. Appl. Phys. 61, SD1017 (2022).

Figure 1