Effect of Chromium on La0.6Sr0.4Co0.2Fe0.8O3-δ Solid Oxide Fuel Cell Cathodes.

The transfer of chromium from metallic components into the cathode of a solid oxide fuel cell (SOFC) can severely degrade its performance and is known as 'chromium poisoning'. In this study we investigate the relationship between the amount of chromium introduced into La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ, LSCF (6428), cathodes and their electrochemical performance, and clarify further the poisoning mechanism. LSCF cathodes were screen printed onto Ce_0.9Gd_0.1O_1.95 electrolyte pellets and infiltrated with Cr(NO₃)3 solutions to different Cr levels up to 2 wt%. Electrochemical impedance spectroscopy at 500--800°C showed that even very low levels of Cr give a significant increase in cathode polarization resistance, which increases with Cr concentration. The impedance response was analyzed using the model of Adler, Lane and Steele to extract oxygen self-diffusion (D_O) and surface exchange (k_O) parameters for the LSCF. The results show that Cr reduces both D_O and k_O, the latter being the more affected. However, the activation energies for polarization resistance, D_O and k_O are not significantly affected by Cr content. This indicates that the Cr poisoning mechanism involves the de-activation of sites for oxygen exchange on the LSCF surface and that the cathode's residual activity is by means of remaining active sites. [ABSTRACT FROM AUTHOR]