In recent years, mixed ionic-electronic conducting (MIEC) materials such as La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) have been studied and developed as solid oxide fuel cell (SOFC) cathodes, due to their high activity for the oxygen reduction reaction at temperatures below 800°C.¹ However, a number of such MIEC materials that contain Sr, including LSCF, exhibit Sr surface segregation at elevated temperatures. In prior work done mainly on dense thin-film MIEC electrodes, Sr-rich surface phases (e.g., SrO) have been shown to hinder the oxygen surface exchange process.^2-5 A recent study, where the surface Sr coverage was quantified by a chemical etching method combined with 3-dimensional (3D) tomography, demonstrated increased amount of surface segregated Sr, along with increased polarization resistance, on LSCF porous electrodes after ageing at 800°C for 800 h.⁶

In this work, we present a study of degradation mechanisms of porous LSCF electrodes under thermal ageing. LSCF symmetric electrode cells with Gd_0.1Ce_0.9O_1.95 (GDC) electrolytes were maintained in ambient air at various temperatures ranging from 550°C to 850°C for different periods of time up to 1500 h, without current/polarization. Electrochemical impedance spectroscopy (EIS) measurements were taken once per day at 700°C, for cells aged at 700°C and 850°C. Increases in the polarization resistance with time by as much as 2 times were found for both ageing temperatures. The electrode morphological changes and Sr surface segregation were examined utilizing a combination of 3D tomography via focused ion beam-scanning electron microscopy (FIB-SEM) and chemical etching with inductively coupled plasma-optical emission spectrometry (ICP-OES) detection. The 3D imaging showed that the LSCF electrodes did not coarsen measurably over 700 h at 700°C or 1500 h at 850°C. On the other hand, the ICP-OES analysis detected an increased amount of water-soluble Sr with increasing ageing temperature and time, as shown in Fig. 1. Application of Adler-Lane-Steele (ALS) model using the measured impedance and microstructural parameters suggested that the increased polarization resistance resulted from a substantial decrease in both the surface exchange k_chem and oxygen diffusion coefficients D^*. The LSCF electrodes aged below 650°C showed no significant increase in surface Sr content compared to the as-prepared sample, indicating there may be a kinetic limitation on Sr surface segregation at lower temperatures.

1. Gao, Z., Mogni, L. V., Miller, E. C., Railsback, J. G. & Barnett, S. A. A perspective on low-temperature solid oxide fuel cells. Energ Environ Sci 9, 1602-1644 (2016).

2. Pan, Z. H., Liu, Q. L., Zhang, L., Zhang, X. W. & Chan, S. H. Effect of Sr Surface Segregation of La0.6Sr0.4Co0.2Fe0.8O3-delta Electrode on Its Electrochemical Performance in SOC. J Electrochem Soc 162, F1316-F1323 (2015).

3. Zhao, L., Drennan, J., Kong, C., Amarasinghe, S. & Jiang, S. P. Insight into surface segregation and chromium deposition on La0.6Sr0.4Co0.2Fe0.8O3-delta cathodes of solid oxide fuel cells. J Mater Chem A 2, 11114-11123 (2014).

4. Liu, Y. H. et al. Performance stability and degradation mechanism of La0.6Sr0.4Co0.2Fe0.8O3-delta cathodes under solid oxide fuel cells operation conditions. Int J Hydrogen Energ 39, 15868-15876 (2014).

5. Simner, S. P., Anderson, M. D., Engelhard, M. H. & Stevenson, J. W. Degradation mechanisms of La-Sr-Co-Fe-O3SOFC cathodes. Electrochem Solid St 9, A478-A481 (2006).

6. Wang, H. Q. et al. Mechanisms of Performance Degradation of (La,Sr)(Co,Fe)O3-delta Solid Oxide Fuel Cell Cathodes. J Electrochem Soc 163, F581-F585 (2016).