In this study, the feasibility of a new group of alkali-free high-entropy ABO_3-δ perovskite cathodes, in which the B-site is occupied by a mixture of 3d and 4d/5d elements, is examined. Six different materials with a general formula of La(Cu_0.2Ni_0.2X1_0.2X2_0.2Y1_0.2)O_3-δ (where X1, X2 = mixture of two Co, Ga, Fe, and Y1 = one of Nb/Ta): La(Cu_0.2Ni_0.2Co_0.2Ga_0.2Nb_0.2)O_3-δ, La(Cu_0.2Ni_0.2Co_0.2Ga_0.2Ta_0.2)O_3-δ, La(Cu_0.2Ni_0.2Fe_0.2Ga_0.2-Nb_0.2)O_3-δ, La(Cu_0.2Ni_0.2Fe_0.2Ga_0.2-Ta_0.2)O_3-δ, La(Cu_0.2Ni_0.2Co_0.2Fe_0.2Nb_0.2)O_3-δ, La(Cu_0.2Ni_0.2Co_0.2Fe_0.2Ta_0.2)O_3-δ were synthesized, with five of them possessing a single-phase, Pnma perovskite structure. While in the case of the basic properties, such as electrical conductivity or thermomechanical behavior, the studied oxides show a number of similarities, the differences between them become more apparent when low-temperature Oxygen Evolution Reaction (OER) and high-temperature Oxygen Reduction Reaction performance is evaluated. The overall best performance is achieved by La(Cu_0.2Ni_0.2Co_0.2Fe_0.2Nb_0.2)O_3-δ and La(Cu_0.2Ni_0.2Co_0.2Fe_0.2Ta_0.2)O_3-δ compositions, with the former offering slightly faster OER kinetics, and the latter exhibiting superior polarization resistance as an SOFC cathode. Overall, the materials exhibit a strong correlation between composition and properties, with potential for further development into non-equimolar compositions of superior performance.

Currently, multicomponent modification of cobalt-based SrCoO_3-δ (SCO) perovskites is one of the most popular new paths of development of solid oxide fuel cells (SOFCs) cathode materials. SCO-type oxides are interesting because of their excellent mixed ionic-electronic conductivity and high oxygen reduction reaction activity. However, due to a lack of stability below 900 °C and the too-high value of the thermal expansion coefficient, caused by the high Co content, it cannot be applied under such work conditions. Therefore, a new group of perovskites with the general formula SrCo_0.7-xTa_0.1Mo_0.1Mn_0.1X_xO_3-δ (where X: Cu, Fe, Ni) was synthesized and studied. The strong impact of additional elements on the characteristics of the materials was confirmed by structural and functional measurements, including methods such as XRD, SEM, iodometric titration, DIL and 4-probe DC. Based on XRD measurement and Rietveld analyses, it can be assumed that 3 materials are single phase with Pm-3m structure, while for one Ni-doped material additional phase can be observed. Due to the limited amount of Co in the material, it was possible to reduce the value of the thermal expansion coefficient to a moderate level - Cu-doped material exhibits 15.7 ·10^-6 K^-1 in the temperature range 25 - 450 °C and 22.0 ·10^-6 K^-1 in the temperature range 450 - 900 °C. The obtained materials exhibit high total conductivity (68.9 S·cm^-1 at ~380 °C and 53.2 S·cm^-1 at ~425 °C for base materials and Cu-doped materials, respectively). An important aspect in the study of the behavior of materials is the charge and spin state of Co. Therefore, X-ray absorption spectroscopy measurements were carried out. It allowed to determine the oxidation state of cobalt, which indicates that for all materials, the element occurs in the oxidation state range of 2.68-2.91. For all materials predominant the high spin state of cobalt is predominant, but materials Fe- and Ni-dopped have the highest tendency toward the formation of the low spin configuration. Also, for materials with Fe and Ni, lower covalency behavior of the Co-O bond was observed to directly affect the local structure distortion of CoO₆ octahedra. The possibility of using the oxides considered as cathode materials in SOFC cells was examined. The Cu-doped material turned out to be particularly promising: using symmetric cells with GDC as an electrolyte support, a polarization resistance value of about 0.538 Ω·cm² was achieved at 700 °C for a phase-pure cathode, and as low as 0.081 Ω·cm² at 700 °C for a composite 60:40 wt% layer cathode, making the material an interesting alternative to state-of-the-art analogues. The next stage of research is the fabrication and testing the performance of a complete anode-supported button cell.