Exsolution-driven surface engineering of Ruddlesden–Popper oxides for protonic ceramic air electrodes

Exsolution-driven surface reconstruction is an effective strategy for engineering catalytically active interfaces in layered oxides, yet its integration with defect-regulated proton transport in protonic ceramic air electrodes remains insufficiently understood. In this work, PrSr2NiCo1-xZrxO7-d (x=0, 0.05, 0.10) Ruddlesden–Popper oxides undergo thermally-induced NiO exsolution that drives a structural transformation from the n=2 toward the n=1 R-P configuration. Introducing Zr4+ on the B-site moderates exsolution through controlled lattice expansion, which enhances hydration, stabilizes oxygen sublattices, and improves proton transport. The optimized composition (x=0.05) develops a well-regulated exsolved surface with enhanced oxygen defect equilibria, resulting in a markedly enhanced low-temperature hydration tendency. These features promote proton uptake and strengthen the coupled electronic-protonic pathways that accelerate the proton-coupled oxygen reduction reaction (p-ORR) and superior hydration-assisted transport. Consequently, this composition exhibits a reduced activation barrier under humid air and delivers a peak power density of 0.995 W cm-2 in fuel cell mode and a remarkable current density of -2.22 A cm-2 in electrolysis mode at 650 °C, while maintaining stable operation for over 200 hours at 600 °C. In contrast, higher Zr content leads to SrZrO3 formation and reduced performance, while the undoped phase suffers uncontrolled NiO exsolution and elevated TEC. This study demonstrates a practical approach to tailoring Ruddlesden–Popper oxides with synergistic electronic, ionic, and protonic transport for high-performance protonic ceramic air electrodes.