Water-mediated exsolution of nanoparticles in alkali metal-doped perovskite structured triple-conducting oxygen electrocatalysts for reversible cells

Abstract

Further development of oxygen catalysts with high electrocatalytic activity and hydration ability is crucial to improve the performance of electrochemical energy storage and conversion devices such as reversible fuel cell/electrolysis cells, because the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) kinetics often limit the performance of these cells. Particularly, significant research efforts are focused on triple conducting (H⁺/O²⁻/e⁻) oxide (TCO) catalysts, owing to their excellent performance in reversible ceramic cells. In this study, we propose a novel strategy to further enhance TCO electrocatalyst performance via exsolution. We demonstrate this approach by doping monovalent alkali metals with high basicity into BaCo_0.4Fe_0.4Zr_0.18Y_0.02O_3−δ, a well-known TCO, which enables the exsolution of barium oxides under humidified air conditions. The main objective of this strategy, which involves water-mediated exsolution coupled with doping of high basicity elements, is to facilitate high proton uptake and conduction pathways within the TCOs, boosting the ORR/OER activity. Our study confirms that BaO_x exsolved K_0.05Ba_0.95Co_0.4Fe_0.4Zr_0.18Y_0.02O_3−δ (KBCFZY) exhibits higher activity and stability for both ORR and OER compared to other perovskites, enabling reversible devices surpassing previously reported performance metrics, including >1.65 W cm⁻² peak power density in fuel cell (FC) mode and >6.5 A cm⁻² in electrolysis (EC) mode at 1.3 V at 650 °C (with >2.5 W cm⁻² and >10.5 A cm⁻² for these same FC and EC metrics at 700 °C). Density functional theory calculations suggest that the alkali-metal doping not only triggers the exsolution of catalytically active barium oxide nanoparticles, but also lowers the bulk oxygen vacancy formation energy and reduces the barrier for proton migration through the BCFZY structure, explaining the observed increases in activity and performance. These results provide new opportunities for the rational engineering of the exsolution process, facilitating the fast transfer of H⁺/O²⁻/e⁻ species for highly active TCO catalysts.