UC Merced

Solid oxide cells (SOCs) have emerged as a promising electrochemical energy conversion and storage device, especially for large-scale applications, due to their high efficiency, fuel flexibility, and reversibility between fuel cell and electrolysis modes. However, the high operational temperature poses durability challenges, and lowering the operating temperature significantly reduces the catalytic activity, particularly in the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) occurring in air electrodes. La-based perovskites with an A-site dopant (La1-xA'xBO3) are commonly employed as the air electrode owing to their high electrochemical kinetics and decent chemical stability in oxidizing environment. In this dissertation, I demonstrate that an atomically thin oxide overcoat on perovskite-based air electrodes can simultaneously address two key degradation processes: agglomeration and dopant segregation, while enhancing the electrode surface kinetics for both ORR and OER. The overcoat provides mechanical stabilization and suppresses the agglomeration of the perovskite nanoparticles, while simultaneously limiting A-site dopant segregation toward the electrode surface.Firstly, I show that an atomic layer deposition (ALD) overcoat facilitates the ORR kinetics and improves cell durability. By employing an angstrom-level CeO2 and Y2O3 overcoat via ALD on a ceria nanodot-decorated LaNi0.6Fe0.4O3- (LNF) air electrode, we demonstrate that the angstrom-level metal oxide overcoat is highly effective in suppressing the nanodot agglomeration and enhancing the ORR kinetics. Secondly, I demonstrate that an ALD overcoat of a specific type of oxides suppresses A-site dopant segregation effectively. I apply a ~ 2 Å thick metal oxide ALD coating (ZrO2, Y2O3, CeO2 and TiO2) on a La0.8Sr0.2MnO3- (LSM) air electrode and quantify the Sr segregation behavior and ORR kinetics for 250 h at 750 ºC. Results reveal that a coating of metal oxide with multi-valent cations lead to a suppression of surface segregation or even de-segregation of Sr from the electrode surface. The oxygen vacancies formed on the perovskite surface by the ALD treatment are identified as the key to controlling Sr segregation. Lastly, I demonstrate that an ALD overcoat with metallic Ru catalyst (7.5-20 Å in nominal thickness) enhances both ORR and OER kinetics and preserves cell durability under electrolysis mode when applied on a conventional La0.6Sr0.4Co0.2Fe0.8O3- (LSCF) air electrode via plasma-enhanced ALD (PE-ALD). The Ru catalyst reacts with surface-segregated Sr species, forming a secondary perovskite phase that suppresses further Sr segregation and improves cell stability. These findings highlight the potential of surface engineering to effectively enhance the performance and durability of both solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs).