Since the goal of net zero emissions has been set, a great attention was directed towards the novel environmental-friendly energy technologies. Hydrogen appears as the promising option due to its carbon-free emission [1]. Solid Oxide Cells (SOCs), known as a clean energy technology, could be operated in either fuel cell or electrolyser mode, which is crucial to the development of energy technologies in modern society [2]. Meanwhile, it was shown that the oxygen electrode often limits SOC performance, derived from its large polarization resistance at intermediate temperature (600-800 °C). Thus, studying and developing novel oxygen electrode materials is indispensable, as it should maintain desirable performance output for the prolonged operation time [3].

Currently, high-entropy oxides (HEOs) have appeared as a novel option in materials science, due to their high configurational entropy, generated by introduction of diverse elements at either a varied or equimolar proportion. HEOs may exhibit enhanced physicochemical properties, compared to the typical perovskite-type oxides that are applied as electrode materials in SOCs [4]. Meanwhile, the cobalt-containing perovskites are still widely-used as oxygen electrodes in SOCs, ascribed to their remarkable electrocatalytic activity. Nevertheless, environmental toxicity and high capital costs of cobalt should be highlighted. Therefore, based on the adjustable characteristics of HEOs, and the similarity of other 3d metal elements (e.g. Mn, Fe, Ni and Cu), novel candidate oxides based on single perovskite skeleton can be proposed, curbing the negative effects of the cobalt element and maintaining the favorable catalytic activity at working temperatures.

In this work, multicomponent La_0.6Sr_0.4Ni_0.15Mn_0.15Fe_0.15Cu_xCo_0.55-xO_3-δ (LSNMFCC) perovskite-type oxides were studied as oxygen electrode materials for SOCs. A role of copper substitution for cobalt and the influence of the increased entropy were evaluated. Electrospinning was adopted for morphological modification. Characterization methods include X-ray diffraction measurements at different temperatures, iodometric titration, scanning electron microscopy, pseudo 4-probe electrical conductivity and Seebeck effect tests, to assess lattice structure during heating/cooling processes, oxygen nonstoichiometry, microstructural and charge transport characteristics, respectively. For the electrocatalytic activity and performance, symmetrical and full cells were prepared with selected electrodes. La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-δ (LSGM) and Ce_0.9Gd_0.1O₂ (GDC) were used as electrolyte in symmetrical cells, while LSGM was selected for electrolyte-supported full cells.

The as-synthesized LSNMFCC materials, by either sol-gel or electrospinning method, all crystalize in the R-3c symmetry, the structure is stable up to 900 °C. Specifically, no impurities or inhomogeneity could be detected among compositions with x=0.05, 0.1, 0.15 and 0.2. Selected four oxides exhibit similar oxygen nonstoichiometry at room temperature (δ ≈ 0.12-0.14). The total conductivity of the materials is very high in the range of 25-850 °C, above 200 S cm^-1. The negative Seebeck coefficient indicated the electrons is the charge carriers. Symmetrical cells with LSNMFCC electrodes were prepared and tested. The studies revealed the lowest polarization resistance at 850 °C is 0.015 Ω cm² for the x = 0.05 (LSNMFCC0550) electrode obtained by sol-gel method, while a little bit higher value of 0.017 Ω cm² obtained for x = 0.2 (LSNMFCC2035) electrode at 850 °C. Since LSNMFCC2035 has lower cobalt content and exhibits higher entropy, it was selected for further modifications. After electrospinning, the polarization resistance of LSNMFCC2035 electrode could be decreased to 0.014 Ω cm² at 850 °C. In further tests with full cell configuration, a maximum power density over 1.1 W cm^-2 at 850 °C could be reached, with promising performance obtained also in the electrolysis mode. Those results imply that the optimized LSNMFCC can be applied as the perspective oxygen electrode material in SOCs.