Annealing-Induced Surface Segregation and SrSO4 Formation on La0.6Sr0.4Co0.2Fe0.8O3-δ Cathodes

La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) perovskite oxide is one of the most important cathode materials being developed for intermediate-temperature solid oxide fuel cells (IT-SOFCs); however, it is prone to surface segregation leading to the formation of undesirable secondary phases and reduction of active surface sites. In this study, dense LSCF thin films having controlled surfaces were epitaxially grown using pulsed laser deposition (PLD) on gadolinia-doped ceria (GDC) films with (100), (110), and (111) orientations, and then subjected to long-term annealing at 800°C and 900°C in air for 1-3 months. Detailed microstructural characterizations revealed preferential surface segregation for LSCF thin films which are predominantly (110)-oriented, conversely this appears to be mostly inhibited for (100)-oriented (pseudocubic) LSCF films, which exhibited extensive pore formation on the surface. Compositional and structural analyses revealed that the surface-segregated crystallites are SrSO₄ in origin, which is attributed to the reaction of the segregated Sr cations from LSCF with SO₂ present in the air. Furthermore, SrSO₄ formation on LSCF surfaces was accompanied by drastic morphological changes as well as decomposition of the perovskite phase. These results have implications toward tailoring the performance of cathode surfaces by understanding the dependence of cation segregation on driving forces such as surface chemistry and microstructure.