The technology of solid oxide fuel cells (SOFCs) is widely considered to be one of the essential components of the next-generation power grid, offering unmatched efficiency, low pollution level, and the possibility of both producing (fuel cell mode) and storing energy (electrolysis mode). Its main deficiencies are high operating temperatures and still limited longevity, both factors being affected primarily by insufficient performance and degradation of the cathode materials. Unfortunately, in most state-of-the-art solutions, the excellent performance of materials walks hand in hand with their susceptibility to deterioration under cell’s operating conditions. A common reason for this behavior is the presence of alkali ions, mainly Sr and Ba, which on the one hand are considered indispensable for achieving sufficient catalytic activity, but on the other tend to form secondary phases, such as carbonates, which over time limit the amount of active cathode sites. One of the possible solutions proposed to counter these effects is the application of the high-entropy approach, which enables obtaining properties beyond the rule-of-mixture, and therefore overcoming some of the limitations of more conventional analogs. However, strict adherence to the high-entropy guidelines might become a limitation by itself, as the more elements we combine, the more difficult it becomes to preserve the single-phase structure and avoid the unwanted interactions between them.

In this study, we present the possibility of obtaining highly functional, alkali-free perovskites of superior performance, obtained through the progressing simplification of the starting La-Co-Cu-Fe-Mn-Ni and La-Cu-Fe-Mn-Ni-Ti high entropy systems. The properties of all materials are systematically studied, including their structure, oxygen nonstoichiometry, thermal expansion behavior, electrical properties, and stability against both typical electrolytes and under the operating conditions of the fuel cell. For the selected, most promising materials, the level of electrochemical performance is assessed using both symmetrical cells (cathodic polarization value R_p) and full cells (power output). The results show that most of the beneficial properties characterizing the base, high entropy systems, can be preserved in simpler, 3- and 4-component compositions while at the same bringing several new features, thanks to eliminating some of the suboptimal elements from the mixture. The electrochemical performance of the best materials, such as La(Co,Cu,Fe)O₃ and La(Cu,Fe,Ni)O₃, despite the lack of alkali ions is superior to the state-of-the-art LSCF (La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ) by a considerable margin, reaching the cathodic polarization value of 0.15 Ω·cm² in the vicinity of 675 °C, making these materials potential candidates even for the low-temperature SOFCs (LT-SOFCs). This is especially impressive for the La(Cu,Fe,Ni)O₃ composition, which not only does not contain alkali ions, but cobalt as well. Overall, it can be stated that the proposed approach can lead to materials characterized by a superior combination of functionality and electrochemical performance with respect to the currently utilized alternatives, while simultaneously addressing some of the most pressing issues of the SOFC cathode materials.