The intermittent nature of renewable energy resources makes imperative the development of efficient energy storage technologies. Solid oxide electrolysis cells (SOECs) are a promising alternative to energy conversion devices. SOECs can play an important role in the control of greenhouse gases by improving processes such as CO2 electrolysis. In order to enhance SOEC performance, exsolution of metal nanoparticles is emerging for the catalytic surface functionalization of electrodes, preventing sintering issues related to classical impregnation methods and enabling tailoring specific catalytic functions. In this work, a medium-entropy, double perovskite system Sr x FeCo0.2Ni0.2Mn0.1Mo0.5O6−δ (x = 2.0, 1.9, and 1.8) was studied. We provide evidence of Fe–Co–Ni ternary alloyed exsolved nanoparticles, revealing that the alloy composition can be tuned by adjusting the reducing conditions. Exsolution temperature is critical for Fe content in nanoparticles, increasing as temperature increases, but Ni and Co are not significantly affected. Temperature adjustments allowed control over nanoparticle size and population, shrinking and growing, respectively, as temperature decreases. In contrast to what is usually described, A-site deficiency resulted in a decrease in nanoparticle exsolution because of NiO phase formation in x = 1.9 and 1.8, so that the x = 2.0 compound outperformed both non-stoichiometric materials, showing significantly larger populations. The three compounds exhibit important conductivity under both oxidizing and reducing atmospheres, which makes them promising electrodes. The Sr2FeCo0.2Ni0.2Mn0.1Mo0.5O6−δ material was integrated as a cathode in an asymmetrical electrolyte-supported cell, and its electrochemical performance under CO2 electrolysis conditions was studied. Our results showed a boost in electrocatalytic activity upon exsolution at 600 °C when compared to the fuel electrode without exsolved nanoparticles or exsolved at 800 °C, where the appearance of the secondary Ruddlesden-Popper phase was observed. Overall, here, we proved the possibility of obtaining ternary alloy exsolved nanoparticles and tuning their composition to enhance the performance of SOEC devices, paving the path for optimized metal-alloyed exsolved nanoparticle design, which might extend its applicability to other electrocatalytic processes in energy conversion and storage.