In-situ investigations of the chromium induced degradation of the oxygen exchange kinetics of the IT-SOFC cathode materials LSC and LSCF

The mixed ionic-electronic conducting transition metal perovskites (La,Sr)CoO3-δ and (La,Sr)(Co,Fe)O3-δ are among the most promising candidates for intermediate temperature solid oxide fuel cell cathodes. Due to the excellent catalytic activity of these materials for oxygen reduction, low area specific resistances can be obtained even at reduced temperatures of 500-700°C (1, 2). However, since the interconnects of SOFCs operating in this temperature range are usually based on metallic alloys (stainless steels) the stability of the cathode vs. Crpoisoning is a key issue. Especially the long-term decrease of the initial high performance due to the gasphase transport of volatile Cr-species and their reaction with the cathode is currently one of the main obstacles for the increase of life-time and reliability of SOFCs (3, 4). Nevertheless, long-term in-situ studies on the kinetic parameters for oxygen exchange vs. Cr-poisoning under defined conditions are scarce. In the present work, the long-term degradation of La0.6Sr0.4CoO3-δ (LSC) and La0.58Sr0.4Co0.2Fe0.8O3-δ (LSCF) is investigated without and with the presence of a Cr-source and taking into account the influence of the relative humidity of the test gas. Using the dc-conductivity relaxation technique (CR) the chemical surface exchange coefficient (kchem) and the chemical diffusion coefficient of oxygen (Dchem) are monitored in-situ as a function of time. X-ray photoelectron spectroscopy (XPS) is applied for the analysis of the relevant surface zones of fresh and degraded samples in order to gain further insight into the origins of the observed degradation.