Keywords: in situ exsolution, nanocatalysts, nanofibers, electron microscopy, solid oxide cells In energy conversion and storage technologies, such as Solid Oxide Cells (SOCs), stable and catalytically active materials are highly needed to reduce the operating temperature and ensure long service life. Nanofibrous materials may provide an effective solution to these requirements. Nanofibers are regarded as promising electrode materials, providing continuous porosity that enhances the transport of reactants to the electrode, along with a high specific surface area [1–2]. Improved catalytic activity and stability can be achieved by in situ exsolution process. In situ exsolution is a segregation of selected metals to the surface and forming metallic nanoparticles. This process involves diffusion, reduction, nucleation, and growth, and is driven primarily by oxygen vacancy formation. Oxygen vacancies not only drive exsolution but also increase ionic conductivity by creating fast oxygen-ion diffusion paths. Exsolved nanoparticles anchored in the parent oxide show improved thermal stability and high resistance against sintering due to strong metal-support interaction. Compared to conventional wet impregnation methods, exsolution offers better nanoparticle distribution, size control, and durability. Reversible exsolution, where nanoparticles can be dissolved and exsolved depending on redox conditions can provide additional benefits. Reversibility of the process may mitigate agglomeration and carbon deposition when SOCs are powered by cheaper and more available hydrocarbons. Therefore, nanofiber electrodes with in situ exsolved nanoparticles are considered highly beneficial for the application in SOC electrodes [1–3]. In this work, the in situ exsolution process was comprehensively investigated by electron microscopy studies, including TEM, SEM, and EDX, along with electron and X-ray diffraction techniques. The materials were synthesized by sol-gel, and electrospinning methods. It was found that metallic nanocatalysts are exsolved in situ on the surface of the Fe-doped Ln 0.9 Ba 0.9 Mn 1.8 (Co/Ni) 0.2 O 6−δ (Ln= Nd, Sm) double perovskites. Co, Ni, and Fe are capable of being reduced to their atomic state under the operating conditions of the fuel electrode in SOCs. Microscopic analysis revealed that nanoparticles are partially anchored in the perovskite oxide (see Fig. 1a). This enhances the stability of the mixed ionic-electronic conductors and improves resistance to carbon poisoning when the fuel cell is powered by cheaper, and more available hydrocarbons. The presence of the perovskite structure was proven by the HR-TEM, with all the observed spots in the Fast Fourier Transform FTT pattern assigned to the P 4/ nmm space group. The FTT performed on the selected areas of the TEM images reveled that exsolved from perovskite nanocatalysts crystallize in Fm-3m space group. These results are consistent with those from the XRD data. The measurements of interplanar distances in the FTT images allowed phase confirmation and determination of the crystals’ orientation. EDX mapping and point measurements confirmed that the nanocatalysts are enriched with Co, Ni, and Fe (see Fig. 1b), forming a Co–Ni–Fe alloy. During the oxidation process in air, metal nanoparticles are partially dissolved into the perovskite structure, while the remaining metals on the surface form an “empty core–shell” structure, increasing the specific surface area of the nanocatalysts. It was also found that the morphology of the material plays a key role in the in situ exsolution process. Nanofibers obtained via the electrospinning method enable the exsolution of nanocatalysts with smaller diameters (around 30 nm) and up to 20 times higher surface nanoparticle density compared to powder compounds. A reversible in situ exsolution process enables the formation of stable nanocatalysts with a reduced tendency toward sintering compared to traditional methods such as impregnation or deposition. This enhances stability, contributes to the efficient and long-term operation of Solid Oxide Cells. The favorable properties of the studied materials were confirmed in SOCs with nanofibrous electrodes decorated by in situ exsolved nanocatalysts. [1] J. Lach, M. Gogacz, P. Winiarz, Y. Ling, M. Zhou, K. Zheng, Materials 2025, 18(6), 1272 [2] J. Lach, Zheng, M. Gogacz, P. Czaja, J. Luo, A. Brzoza-Kos, ECS Transactions 2023, 111(6), 1025−1034 [3] K. Zheng, J. Lach, P. Czaja, M. Gogacz, P. Czach, A. Brzoza-Kos, P. Winiarz, J. Luo, J. Power Sources 2023, 587, 233707 The work is funded by the National Science Centre Poland (NCN) based on the decision number UMO-2021/43/D/ST5/00824. Jakub Lach acknowledge the financial support of research project supported by the program „Excellence Initiative – Research University” for the AGH University of Krakow. HRTEM, EDS and SAED results were a part of CERIC-ERIC grant no. 20232033 and were conducted in National Institute of Materials Physics, Măgurele in Romania. Figure 1