An innovative approach to design SOFC air electrode materials: high entropy La1−xSrx(Co,Cr,Fe,Mn,Ni)O3−δ (x = 0, 0.1, 0.2, 0.3) perovskites synthesized by the sol–gel method

Abstract

Among the for the first time reported Cr-containing high entropy La_1−xSr_x(Co,Cr,Fe,Mn,Ni)O_3−δ (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5) perovskite-type oxides, the selected Sr-doped La_0.7Sr_0.3(Co,Cr,Fe,Mn,Ni)O_3−δ material is documented to possess attractive properties as a candidate air electrode material for Solid Oxide Fuel Cells (SOFCs). Nanosized powders of the considered oxides are obtained using a modified Pechini sol–gel method. In the formed solid solution with a simple perovskite structure the strontium solubility limit is found to be at least x = 0.3. Room temperature (RT) structural data indicate the presence of rhombohedral structural distortion (Rc symmetry) in the materials. High-temperature structural studies for the selected La_0.7Sr_0.3(Co,Cr,Fe,Mn,Ni)O_3−δ indicate the occurrence of a phase transition to an aristotype Pmm structure at ca. 800 °C. The linear thermal expansion coefficient in the RT-1000 °C range is found to be moderate, 16.0(3) × 10⁻⁶ K⁻¹. The results of impedance spectroscopy measurements support the semiconducting-type behavior of the electrical conductivity for all single-phase materials, in a temperature range of RT-1000 °C. The maximum recorded conductivity for the La_0.7Sr_0.3(Co,Cr,Fe,Mn,Ni)O_3−δ composition exceeds 16 S cm⁻¹ in the 900–1000 °C range, being suitable for application. Furthermore, chemical stability toward the La_0.8Sr_0.2Ga_0.8Mg_0.2O_3−δ (LSGM) electrolyte is proven. Considering the presence of chromium, typically deleterious to the performance, the measured value of the total cathodic polarization resistance for the La_0.7Sr_0.3(Co,Cr,Fe,Mn,Ni)O_3−δ-based electrode, being 0.126 Ω cm⁻² at 900 °C, seems to be very attractive. The results obtained for a button-type fuel cell indicate power densities at a level of 550 mW cm⁻² at 900 °C. Therefore, it can be considered that the high entropy-based approach enables to propose alternative SOFC air electrode materials, with otherwise inaccessible chemical compositions.