Abstract: The degradation of intermediate temperature solid oxide fuel cell (ITSOFC) cathodes has been identified as a major issue limiting the development of ITSOFCs as high efficiency energy conversion devices. For ITSOFCs, metallic materials have become a preferential choice for the interconnect due to their low cost and 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 leading to rapid electrochemical performance degradation of the cathodes [1-4]. Despite the extensive research on the chromium deposition and poisoning processes, careful microstructural studies at multi-scale lengths are rare, which can provide valuable information for the fundamental understanding of the Cr poisoning mechanisms required for developing Cr tolerant cathode materials
In this presentation, I will summarize some of our recent results on the effect of Cr poisoning on (La0.6Sr0.4)0.95(Co0.2Fe0.8)O3-δ (LSCF6428), a particularly promising ITSOFC cathode material.
We examine the Cr poisoning mechanisms in LSCF materials by correlating the bulk electrochemical properties of the cell with their structural and chemical change at multi-scales down to the nanometer level. Cells with LSCF cathodes were prepared, and the effect of Cr poisoning on the electrochemical behavior of the cell was assessed by impedance spectroscopy. The change in nano/microstructure and chemistry due to poisoning were studied in parallel by a combination of several advanced electron microscopy techniques including focus ion beam (FIB) tomography, high resolution (scanning) transmission electron microscopy ((s)TEM) and analytical STEM. The systematic combination of bulk and high-resolution analysis on the same cells allows, for the first time, to directly correlate Cr induced performance degradation at multiscales, especially with subtle and localized structural/chemical changes of the cathode down to the atomic scale. Up to two orders of magnitude reduction in conductivity, oxygen surface exchange rate and diffusivity were observed in Cr poisoned LSCF6428 samples. These effects are associated with the formation of nanometer size SrCrO4; grain boundary segregation of Cr; enhanced B-site element exsolution (both Fe and Co); and reduction in the Fe valence, the latter two being related to Cr substitution in LSCF. The finding points to some new critical SOFC degradation mechanisms effective at the nanometer scale and below.
References
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