Impact of Phase-Separated Janus-Type Formation on the Reversibility of Multicomponent Exsolved Nanoparticles from Complex Perovskites

Solid oxide electrochemical cells (SOCs) benefit from exsolution-based electrocatalyst design, where nanoparticles anchored in perovskites enhance stability and activity. Two of the most transformative features of this technology are the ability to engineer multielemental alloy nanoparticles for tailored catalysis and the potential for in situ catalyst regeneration through redox-driven redissolution. However, the fundamental mechanisms governing these processes in complex, multicomponent systems remain poorly understood. In this work, the simultaneous exsolution of Fe, Ni, Co, and Cu from the fuel electrode material Sr2Fe1.2Co0.1Ni0.1Cu0.1Mo0.5O6-δ was investigated using in situ powder neutron diffraction and synchrotron-based near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS), combined with advanced electron microscopy to capture morphological evolution. At 700 °C, Cu-rich nanoparticles dominate, consistent with Ellingham reducibility trends; however, higher temperatures favor the formation of Fe-enriched alloys, driven by the high availability of Fe cations. Conversely, prolonged reduction promotes the formation of phase-separated Janus-type nanoparticles, primarily due to Fe–Cu immiscibility. Interestingly, redox cycling tests revealed that nanoparticle composition dictates redissolution capacity. While homogeneous alloys exhibited total redissolution into the perovskite backbone and subsequent re-exsolution, Janus-type nanoparticles underwent irreversible transformation into pyramidal NiO nanoparticles via intermediate cubic mixed oxide structures during air exposure. These findings elucidate how temperature, time, and elemental composition govern exsolved nanoparticle chemistry, morphology, and regeneration, establishing design principles for inducing multimetal exsolution in complex oxides toward enhanced electrocatalytic performance in energy conversion technologies.