La_0.6Sr_0.4CoO_3-δ electrode layers with three different microstructures were manufactured by screen-printing, spin-coating and infiltration into a porous Ce_0.9Gd_0.1O_1.95 backbone. Electrode performance was monitored at 700°C in 20% O₂ over periods of 1600 to 3860 hours by means of electrochemical impedance spectroscopy under open circuit conditions. Reference measurements were performed in dry atmospheres, where significant electrode activation was observed for cells with spin-coated and infiltrated electrodes. Subsequently, the relative humidity level in the surrounding atmosphere was set to 30% and further raised to 60%, thus simulating SOFC operation with ambient air without pre-drying. While no performance loss could be observed in dry atmospheres, significant degradation occurred in humid atmospheres with pronounced differences between degradation rates of half cells with different electrode microstructures. Post-test analyses by SEM and STEM were employed to identify the causes for the observed differences in degradation behavior. For screen-printed cells, the surface of the degraded electrodes was covered with small crystallites, probably consisting of SrO formed by Sr-segregation and surface precipitation, where humidity was found to be a crucial factor. For spin-coated and infiltrated electrodes, poisoning by impurities (Si, Cr, S) and particle coarsening were identified as potential causes.