Solid-Gas Interactions of Nanoscaled La0.6Sr0.4CoO3‑δ and Their Impacts

Degradation of SOFC cathodes is the result of microstructural and/or chemical alterations taking place during operation [1,2]. The latter originate either from impurities or secondary phases that segregate or form at the electrochemical active solid/gas interface or from compositional changes of the cathode material itself. On the one hand, the cathode material can react with species from adjacent or nearby solid materials within the multi-layer cell setup. On the other hand, surrounding gas species or impurities can initiate the formation of secondary phases on the surface of the cathode material or the decomposition of the cathode material itself.

This contribution will focus on the solid/gas interaction between nanoscaled La_0.6Sr_0.4CoO_3‑δ (LSC) thin films and the surrounding gas atmosphere in terms of gas composition and partial pressures and discusses the resulting influence on the electrochemical properties. The thin-film cathodes were derived by a sol-gel process based on metal organic precursors and were directly deposited onto Ce_0.9Gd_0.1O_1.95electrolyte pellets [3].

            Differently designed experiments and thermodynamic calculations were performed in order to investigate the influence of water-vapor or carbon dioxide in the surrounding atmosphere and of the oxygen partial pressure on the cathode performance. The latter plays an important role during the cathode fabrication process. Low oxygen partial pressures occur during the thermal decomposition of the metal organic precursors of the cathode material, which lead to the formation of a beneficial hetero-interface of (La,Sr)₂CoO_4±δ and La_0.6Sr_0.4CoO_3-δ, as could be shown by thermodynamic calculations [4]. The influence of H₂O(g) and CO₂ on the electrochemical performance was investigated experimentally in a temperature range of 400 … 650 °C [5]. Both gases negatively affect the solid/gas electrochemistry of the cathode (cf. Figure 1). CO₂ leads to poisoning of the cathode, which is reversible at temperatures above 500 °C. At temperatures below 500 °C a regeneration of the cathodes could only be achieved by a treatment in pure oxygen. In contrast to this behavior, H₂O leads to an irreversible and continuously increasing degradation. A detailed analysis of electrochemical impedance spectroscopy data disclosed, which polarization processes are majorly affected.  CO₂ mainly affects the surface-exchange reaction, whereas H₂O – in the time frame of the test – impedes the oxygen ion diffusion within the cathode.

The results demonstrate the changeability of the solid-gas electrochemistry at mixed conducting cathodes. Based on these results operation strategies in terms of optimal operation temperature and required gas preprocessing (e.g. drying of oxidant gas) can be deduced, potentially decreasing the degradation during operation.

References

[1]   S. B. Adler, Chem. Rev., 104(10), p. 4791 (2004).

[2]   W. Lee, J. W. Han, Y. Chen, Z. Cai and B. Yildiz, J. Am. Chem. Soc., 135(21), p. 7909 (2013).

[3]   J. Hayd, L. Dieterle, U. Guntow, D. Gerthsen and E. Ivers-Tiffée, J. Power Sources, 196(17), p. 7263 (2011).

[4]   J. Hayd, H. Yokokawa and E. Ivers-Tiffée, J. Electrochem. Soc., 160(4), p. F351 (2013).

[5]   J. Hayd and E. Ivers-Tiffée, J. Electrochem. Soc., 160 (11), p. F1197 (2013).