(Invited) Effect of Atomic-Scale Surface Overcoat By Atomic Layer Deposition on Dopant Segregation of Perovskite Cathodes

Intense research has been performed to advance the solid oxide cell (SOC) technology because of their excellent efficiency for clean energy conversion and storage, but the lack of long-term durability, particularly of perovskite (ABO3)-based air electrodes, is currently the main technological bottleneck for wider commercial adoption. Long-term durability of SOC air electrodes has been mainly hampered by surface segregation of A-site dopant and morphological agglomeration. Since a deposition of an atomically thin oxide has proven to be highly effective in suppressing electrode agglomeration, a successful suppression of dopant segregation with the same approach will enhance the durability of air electrodes significantly by killing two birds with one stone. In the previous study [1], we demonstrated the effectiveness of an atomic scale overcoat in suppressing particle agglomeration. In this presentation, we demonstrate that even a few angstroms of metal oxide overcoat by atomic layer deposition (ALD) affects the behavior of Sr segregation in the underlying La0.8Sr0.2MnO3 (LSM) electrode significantly. By leveraging angle-resolved X-ray photoelectron spectroscopy (ARXPS), we provide a quantitative analysis of the relative atomic concentration on both the very surface and bulk of the electrode. Compared to bare LSM electrodes, those with 2 – 3 Å ALD overcoat show relatively high resistivity against Sr segregation when exposed to of 750 °C for 250 h. Particularly, an oxide overcoat with multi-valent cations (CeO2 and TiO2) tends to suppress Sr segregation effectively and even drive Sr species back into the bulk of LSM while an overcoat with single valent cations (ZrO2 and Y2O3) exhibits little effect on Sr segregation. It was further demonstrated that the segregation behavior is nicely aligned with the electrode performance; those coated with CeO2 and TiO2 showed better thermal stability in terms of polarization resistance. Based upon the time evolution of Sr content, cationic valence states, and oxygen defect concentration in the vicinity of surface, we conjecture that the amount of oxygen defects formed by the overcoat and the resulting chemical potential gradient at the interface between the overcoat underlying LSM surface plays a pivotal role in the Sr segregation behavior. The authors acknowledge the support from National Science Foundation (DMR 1753383) and NASA MIRO program (NNX15AQ01A). References: Li, H. et al. J. Mater. Chem. A, 8, 15927–15935, 2020.