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

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.