Abstract text (incl. figure legends and references)
Q. Tran1, H. Türk1, F. Schmidt1, T. Götsch1, A. Hammud1, D. Abou-Ras2, D. Ivanov1, E. Stotz1, L. de Haart3, I. Vinke3, R. Eichel3, K. Reuter1, R. Schlögl1, A. Knop-Gericke1, C. Scheurer1, T. Lunkenbein1

1Fritz Haber Institute of the Max Planck Society, Berlin, 14195 Germany

2Dep. Structure and Dynamics of Energy Materials, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, Berlin, 14109 Germany

3Fundamental Electrochemistry (IEK-9), Institute of Energy and Climate Research, Forschungszentrum Jülich GmbH, Jülich, 52428 Germany

High temperature solid oxide electrolysis cells (SOECs) are a promising technology for intermittent energy conversion and storage. However, their wider adoption for commercial applications is still limited by the rapid degradation of the cells even under steady state conditions. This calls for an in-depth understanding of their structure-performance relationship during operation, especially at the chemically and kinetically active regions between the electrolyte, electrode, and gas phase. A recent study has also confirmed the existence of a so-called complexion region at such an interface (1, 2).

In this study, we aim to elucidate the atomic scale structure and chemistry at the boundaries between the electrolyte and electrode using Cs-corrected scanning transmission electron microscopy (STEM). For this purpose, we investigated a half cell consisting of LSM (La_0.8Sr_0.2)_0.95MnO_3-δ) || LSM + YSZ composite, which represents the air electrode of the electrolysers, screen printed on top of a dense 8YSZ electrolyte (ZrO₂ with 8 mol% Y₂O₃) after thermal treatment in air at 1173 K for 150 h without any biasing operations.

Our Electron Energy Loss Spectroscopy (EELS) analysis reveals the enrichment of divalent Mn²⁺ not only at the LSM/YSZ interface, but also at the nearby YSZ grain boundaries, which is in contrast to the dominant Mn³⁺/Mn^(2+,3+) within the bulk LSM (Fig. 1). Such a high level of Mn²⁺ reflects an increased concentration of oxygen vacancies and may promote electron transfer for oxygen gas evolution and transport at these interfaces. With this result, we show that thermal annealing independently plays an important role for such an elemental segregation, in contrast to another report of cathodic polarisation being the main driving factor (3).

Moreover, with atomic-resolution imaging, numerous nano-scale domains with a characteristic two-fold ordered structure can be found within the YSZ grains in contact with the LSM (Fig. 2). Their origin is likely related to the formation of a solid solution of La³⁺ and Mn²⁺ within the YSZ structure, assisted by the long-range diffusion of these ions along the grain boundaries. This requires further insights.

In conclusion, our study offers atomic insights into the local structures at the electrolyte/electrode interface boundaries. The result is of scientific and technological importance toward the design of more efficient and durable cells.

References

(1) Türk, H. et al., Adv. Mater. Interfaces 2021, 8, 2100967.

(2) Türk, H. et al., ChemCatChem, https://doi.org/10.1002/cctc.202200300.

(3) Backhaus-Ricoult, M., Solid State Ion. 2006, 177, 2195-2200.

Fig. 1: a) HAADF images at the YSZ/LSM interface, b), c) Sequential spectra along the line scan indicated in (a) using energy dispersive spectroscopy EDS and EELS, respectively.

Fig. 2: Colour-enhanced high angle annular dark field (HAADF) image at the YSZ grain boundaries which shows the presence of nano-scale ordered structure domains.