Among the leading candidates for efficient energy conversion, Solid Oxide Fuel Cells (SOFCs) and Solid Oxide Electrolyzer Cells (SOECs) enable, respectively, the utilization of (green) hydrogen for electricity and heat generation, and production of hydrogen and other e-fuels from surplus renewable energy [1]. However, their large-scale application is still limited by unresolved challenges, particularly long-term stability issues and sluggish kinetics of the oxygen reduction (ORR) and oxygen evolution (OER) reactions at lowered operating temperatures [2]. Current research therefore focuses on tailoring both chemical composition and oxygen electrode morphology, with the aim of simultaneously enhancing electrochemical performance and stability, especially for reversible cell operation [3]. The family of double perovskites with the general formula AA’B₂O_5+δ (A: lanthanides; A’: alkaline earth metals, typically Ba; B: 3d metal elements, usually, with high amount of Co) represents attractive properties for oxygen electrodes in SOCs [4].

In this study, a series of Gd_1-xSm_xBa_0.5Sr_0.5CoCuO_5+δ (0 ≤ x ≤ 1) (GSBSCCO) double perovskites were synthesized via sol-gel method and evaluated with respect to their structural and transport properties. Among them, the Gd_0.75Sm_0.25Ba_0.5Sr_0.5CoCuO_5+δcomposition exhibited the lowest polarization resistance (R_p = 0.087 Ω cm^-2 at 800 °C), as determined by electrochemical impedance spectroscopy (EIS). Owing to this promising performance, this composition was further synthesized in the form of nanofibers via electrospinning, since such architectures are known to promote mass and charge transport [5]. Surprisingly, enhancement of the electrochemical performance of the GSBSCCO|LSGM|GSBSCCO symmetrical cell, where La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-δ(LSGM) was used as the solid electrolyte, was observed only when Ce_0.9Gd_0.1O_2-δ (GDC) was applied as a functional buffer layer. To better understand this phenomenon, a distribution of relaxation times (DRT) analysis of the EIS data was carried out, providing deeper insight into the performance of the electrodes and allowing for the identification of rate-limiting steps of electrochemical processes. The final step was to evaluate electrodes in the anode-supported button-type single cell with a Ni-Zr_0.92Y_0.08O_2-δ (YSZ) anode, YSZ electrolyte and GDC buffer layer. The considered cell achieved power densities of 0.46 W cm^-2 and 0.42 W cm^-2 at 700 °C with electrospun and sol-gel electrodes, respectively.