Surface Ionic Conduction of Ce0.8Gd0.2O2-Δ Multilayer Thin Films with Large Amount of Oxygen Vacancies
Authors: Yamada Mariko , Tani Minami , Notake Go , Higuchi Tohru 
Published: December 2024

Gd-doped CeO2 (GDC) with high chemical stability is a good electrolyte material with high oxygen ion conduction above 600°C and surface proton conduction below 100°C in thin film form [1]. In particular, Ni-GDC cermet material is well known as anode electrode of solid oxide fuel cell (SOFC) operating at medium temperature region. Ni is suitable for the anode as it has the characteristics of being inexpensive and having high catalytic activity, which enhances the hydrogen oxidation reaction. Ni is characterized by its low cost, high catalytic activity, and enhanced hydrogen oxidation reaction. Mixtures of electronic conductors such as Ni and ionic conductors such as Yttria-stabilized zirconia (YSZ) and GDC are often used as anode materials because they expand the three-phase interface [2].

Based on these backgrounds, we have attempted to realize high conductivity in GDC (Ce0.90Gd0.10O2-δ) thin films, which contain a large amount of oxygen vacancies and lattice distortion. if the GDC films show high ionic conductivity above σ
T~10-1Scm-1K, the performance of SOFCs and EDLTs will be dramatically improved. performance of SOFC and EDLT will be dramatically improved. To realize this goal, thin films were prepared by sputtering method, the relationship between surface defect structure and ionic conductivity was investigated in detail, and electrodes were fabricated on the thin films with Pt and Ni thin films.

Based on these backgrounds, in this study, we tried to realize high conductivity by fabricating electrodes with Ni thin films on GDC (Ce0.90Gd0.10O2-
δ) with large amount of oxygen vacancies and lattice distortion. If the GDC thin films show high ionic conductivity above σ
T~10-1Scm-1K, the performance of SOFCs and EDLTs will improve dramatically. In order to realize this purpose, we fabricated thin films by the sputtering method and studied in detail the relationship between the surface defect structure and ionic conductivity.

The GDC thin films were prepared on Al2O3 (0001) substrates by RF magnetron sputtering using ceramic targets. The target size had a diameter of 47 mm and thickness of 3 mm. The RF power of the ceramic target was fixed at 30 W. The flow rate of Ar gas controlled by a mass-flow controller was kept at approximately 2.81 ccm. The pressure of Ar gas was fixed at 3.5 × 10−3 Torr during the deposition. The film thicknesses ~30 and ~110 nm to probe the changes in the lattice constant and the conductivity. The electrode film was fabricated on the GDC thin film using a similar method with Pt and Ni metal targets. Electrical conductivity was measured by the AC impedance method. The measurement range was 600 °C to 20 °C. The result when only Ar was filled at the time of measurement is called As-deposited, and the result when a 4:1 mixture of Ar and gases was filled at the time of measurement is called -annealed.

Arrhenius plots at low temperatures below 100 °C are shown in Fig. The thin films with Pt electrode are designated as Pt-GDC and those with Ni electrode as Ni-GDC. The PES spectra show that the OH- peak appears after Wet annealing. Therefore, it can be said that surface proton conduction by the Grotthuss mechanism is taking place [3]. In comparison for Pt-GDC, the O2 annealed condition resulted in higher conductivity than the As-deposited condition. This is thought to be due to the reduction of oxygen defects in the thin film by O2 annealing and the dominance of proton conduction. Ni-GDC resulted in lower conductivity than Pt-GDC under the two conditions. For Ni-GDC, there are reports that the peak power density varied with the voltage at the time of deposition [4], that ohmic resistance decreased by moderately thickening the anode and that the power density was highest at 800 nm thickness [4], and that the oxygen reduction reaction at the cathode was promoted by the grain boundaries and lattice defects report [5] that grain boundaries and lattice defects promote the oxygen reduction reaction at the cathode.

Reference

[1] M, Shirpor et al. Phys. Chem. Chem. Phys., 13 (2011) 937-940.

[2] I.D. Unachukwu et al. Journal of Power Sources
556 (2023) 232436

[3] D. Nishioka et al, Nanoscale Res. Lett. 15 (2020) 42-49

[4] S, Ryu et al. ACS Appl. Mater. Interfaces 2023, 15, 11845−11852

[5] Y, Li et, al, Scientific Reports
6, 22369 (2016)

Figure 1