Sr Surface Segregation on La0.6Sr0.4Co0.2Fe0.8O3-δ Porous Solid Oxide Fuel Cell Cathodes
Authors: Wang Hongqian , Barnett Scott A 
Published: February 2020

In recent years, mixed ionic-electronic conducting (MIEC) materials such as La0.6Sr0.4Co0.2Fe0.8O3-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 Gd0.1Ce0.9O1.95 (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 kchem
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.

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Figure 1