Transmission Electron Microscopy Study of Cr Poisoning of Lscf Cathodes

(La,Sr)(Co,Fe)O_3-δ (LSCF) perovskite oxides have been considered to be a promising cathode material for SOFCs operating in the intermediate temperature range due to their significantly higher electrochemical activity for the O2 reduction reaction and higher oxygen ion conductivity than the conventional (La,Sr)MnO₃ (LSM) cathode [1,2]. In practice, metallic materials have become a preferential choice for the interconnect due to their excellent physical and chemical properties. However the presence of chromium in all commonly used metallic alloys has been found to cause poisoning of the cathode under operating conditions leading to rapid electrochemical performance degradation of the cathodes including LSCF [3–5].

Development of a Cr tolerant cathode material for increased long-term durability of SOFCs relies on a fundamental understanding of the mechanism of the chromium deposition and poisoning of the cathode materials. Extensive research on the chromium deposition and poisoning processes on SOFC cathodes has been carried out but careful microstructural studies especially on the nanometer to atomic scale are very limited. This is however a subject that can give valuable information on the detailed process of Cr incorporation and evolution in the cathode materials, providing an important insight for the mechanistic understanding of the Cr poisoning.  

LSCF/CGO/LSCF symmetrical cells were prepared by screen printing the LSCF cathode on Gd doped cerium oxide (CGO) electrolyte pallet, and the effect of Cr poisoning at 900 ˚C in air on the electrochemical behavior of the cell was assessed by impedance spectroscopy. The change in nano/microstructure and chemistry between cells with and without Cr poisoning were studied in parallel by TEM/STEM/EDX/EELS. To facilitate the TEM sample preparation using focus ion beam thinning (FIB), bulk samples containing the porous LSCF layer were firstly embedded in an epoxy resin.

Due to the peak overlap between the interested La and Cr and inherently poorer spatial resolution in EDX, EELS was employed with the aim for an improved accuracy in the analysis.  Figure 1a shows an example of EDX artefacts where the spectrum from an epoxy area in between LSCF grains exhibit the presence of the LSCF elements. On the other hand, EELS could suffer a higher detection limit compared to EDX due the generally poorer signal to background (S/B) ratio, which is typically worse when the sample thickness increases. Figure 1b shows an example of EELS spectrum, which fails to detect the presence of Co in the thicker area of a LSCF grain (t/λ ~ 0.86). It was found that all the expected elements (apart from Sr) in LSCF can be successfully detected by EELS when t/λ < ~ 0.7, and this criteria was kept during all the later EELS measurements.

Our results show that Cr is incorporated in LSCF in more complicated ways than a simple formation of SrCrO_x as suggested by previous studies. Cr was found to segregate in Sr rich phases that also contain other elements such as Fe and Co (Figure 2). In addition, Cr was incorporated in the LSCF perovskite structure and repelled sometimes the other B site elements from the lattice. As a result, Cr rich phases take the form of various compositions such as (La,Sr)(Fe,Co,Cr)O_x, (La,Sr)(Fe,Cr)O_x and (La,Sr)CrO_x (Figure 3). It is interesting to note that Cr appears to repel more readily Co than Fe. The Cr poisoning also promotes the formation of Co and Co-Fe rich phases that may contribute to the deficiency of Co in the LSCF grains (Figure 4). It can be seen that the area adjacent to the Fe-Co rich particles contains Cr and are deficient in Co with a composition close to that of (La,Sr)(Cr,Fe)O_x. Careful examination of Cr incorporation was also extended to LSCF grain boundaries with both structural and chemical analysis. The high-resolution microscopy results and their implications are discussed in relation to the degraded electrochemical properties of the LSCF cathode and the Cr poisoning mechanisms.

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[4]      L. Zhao, J. Drennan, C. Kong, S. Amarasinghe, S.P. Jiang, ECS Trans. 57 (2013) 599.

[5]      S.P. Jiang, X. Chen, Int. J. Hydrogen Energy 39 (2014) 505.