During recent years a lot of results on segregation of metal oxide components to the surfaces of perovskite electrodes for solid oxide cells (SOC) have been published. The electrodes are typically of the families (La_1-xSr_x)_sMO₃, M = Mn, Fe, Co, Ni with 0 < x < 1, and s is the so-called A/B ratio, usually 0.9 < s < 1.1. M may in principle be a mixture of all four metals, and the mentioned transition metals are only examples. Especially SrO segregations have been reported, but other of the components of the perovskite may also segregate to the surface as well as impurities in the raw materials. Finally, impurities from the reactant gases may adsorb to the surface of such electrodes (1 - 5).

However, it has been suggested that e.g. the SrO segregation is due to the vacuum based ex situ analytical tools such as low energy ion scattering (LEIS), x-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (ToF-SIMS). In general, the literature results are quite scattered, so the issue is still open for discussion.

The aim of this paper is to settle this discussion. First a brief summary of some main literature results are given, and next some results of our own on studies on popular commercial perovskite materials are presented and discussed. The methods used are Raman spectroscopy and XRD, which are not vacuum techniques, and ToF-SIMS and XPS, which are vacuum techniques.

The conclusion is that usually a perovskite surface will be covered with segregated material of as well components like SrO as impurities in the perovskite starting material. Furthermore, the available evidences point to the perovskite surfaces being very dynamic with relatively fast changes that are visible within periods of few hours with developments continuing for hundreds of hours.

Acknowledgement

We acknowledge financial support from Energinet.dk through the ForskEL programme Solid Oxide Fuel Cells for the Renewable Energy Transition contract no. 2014-1-12231, and by The Energy Technology Development and Demonstration Program (EUDP) at the Danish Energy Agency via project “Maturing SOEC”, contract no. 64015-0523.

References

1. K. Norrman, K.V. Hansen, T. Jacobsen, RSC Adv., 5, 87679 (2015).

2. F.S. Baumann, J. Fleig, M. Konuma, U. Starke, H.-U. Habermeier, J. Maier, J. Electrochem. Soc., 152, A2074 (2005).

3. J. Druce, H. Tellez, M. Burriel et al., Energy Environ. Sci., 7, 3593 (2014)

4. Z. Cai, Y. Kuru, J. W. Han, Yan Chen, Bilge Yildiz, J. Am. Chem. Soc., 133, 17696 (2011).

5. K.V. Hansen, K. Norrman, T. Jacobsen, Y. Wu, M.B. Mogensen, J. Electrochem. Soc., 162, F1165 (2015).