One of the most promising electrolyte and cathode materials in SOFCs are respectively yttria-stabilized zirconia (YSZ) and strontium-substituted lanthanum based perovskite such as (La_0.8Sr_0.2)_0.98MnO₃ and LSCF (La_0.6Sr_0.4Co_0.2Fe_0.8O₃). The chemical and structural nature of the solid-state interface between the cathode and the electrolyte plays an important role in the cell performance, with respect to both long-term stability and efficiency. The formation of secondary phases such as La₂Zr₂O₇ (LZ) and SrZrO₃ (SZ) (1) at the interface with less favorable properties as well as interdiffusion of cations between the cell components will have a negative impact on the cell performance. It has been also reported that GDC (Gd_1-xCe_xO_2-y) interlayer effectively prevents the LZ and SZ formation between the YSZ and the strontium-substituted perovskite (2, 3).

In our previous study, effects of sintering temperature of LSCF cathode in the LSCF/GDC/YSZ SOFC on the SZ growth and the cell performance have been reported (4). In the present study, we have particularly investigated the early stage of SZ formation through transmission electron microscopy (TEM) in order to clarify the mechanism of SZ formation. The LSCF/GDC/YSZ half-cells were heat-treated at 1100 ℃ with various holding times for sintering LSCF. Cation interdiffusion at the LSCF/GDC/YSZ interfaces due to heat-treatment at 1100 ℃ has been also investigated by energy dispersive X-ray spectroscopy and electron energy loss spectroscopy (EELS). As a result of this, it was found that the SZ with tetragonal crystal structure is initially formed at both the surface of GDC particles and the GDC grain boundaries in the vicinity of GDC/YSZ interface. This SZ was grown to the layer with the thickness above 1.3 micron-meter after sintering for 16 h at 1100 ℃. From EELS analysis, we have confirmed the diffusion of La in SZ and that of Ce in YSZ close to the GDC/YSZ interface. Moreover, in situ TEM observation of the LSCF/GDC/YSZ was carried out during heating to 800 ℃ under an oxygen atmosphere of 3 mbar in an environmental TEM (ETEM). Segregation of the Sr-rich oxide was observed at the surface of GDC interlayer grains while the specimen was hold at 800 ℃.

This work was partially supported by the New Energy and Industrial Technology Development Organization (NEDO).

References

1. J. A. M. van Roosmalen, E. H. P. Cordfunke, Solid State Ionics, 52, 303 (1992).

2. W.-H. Kim, H.-S. Song, J. Moon, H.-W. Lee, Solid State Ionics, 177, 3211 (2006).

3. A. Mai, V. A. C. Haanappel, S. Uhlenbruck, F. Tietz, D. Stover, Solid State Ionics, 176, 1341 (2005).

4. S. Kanae, Y. Toyofuku, T. Kawabata, Y. Inoue, T. Daio, J. Matsuda, J.-T. Chou, Y. Shiratori, S. Taniguchi, K. Sasaki, ECS Transactions, 68 (1), 2463 (2015).