Design of multi-doped strontium ferrate perovskite-based electrocatalysts with medium-entropy and reduced content of critical elements

The issue of critical raw materials and the energy crisis forces the scientific community to find alternative chemical compositions for clean energy production without compromising too much on the final materials functionality. A possible strategy is reducing the concentration of critical elements and, at the same time, increasing the entropy of the material, that has a considerable effect on most of the relevant properties of electrocatalytic materials. In this work, non-equimolar medium-entropy perovskite oxides with reduced content of critical elements and medium entropy were here proposed as sustainable oxygen electrode materials for intermediate temperature solid oxide fuel cells and solid oxide electrolysis cells devices. Five powders with nominal composition Sr1−(a+a′)BaaCaa′Fe1−bMobO3−δ (a = 0.25, 0.05, 0.075, a′ = 0.85, 0.25, 0.10, 0.075; b = 0.2, 0.5) were prepared by solution combustion synthesis and studied by powder X-ray diffraction with Rietveld refinement and by Raman spectroscopy, temperature programmed reduction, thermal gravimetric analysis, N2 adsorption, scanning electron microscopy with energy dispersive spectroscopy and electrochemical impedance spectroscopy. Results indicate strong competition for Mo between a doped SrFeO3-type perovskite structure, a Sr2FeMoO6-type double perovskite structure and a doped BaMoO4-type tetragonal structure, whereas a predominantly single-phase SrFeO3-type medium-entropy perovskite oxide with reduced content of critical elements was thermodynamically stabilized by high Sr-to-Mo ratio. The best electrocatalyst, Sr0.85Ba0.05Ca0.10Fe0.8Mo0.2O3−δ, had a stable electrochemical performance, both in oxygen reduction and evolution mechanisms, according to the harmonized stress test protocols. Although calcium in the A-site generally still induces the formation of more than one phase, the oxygen reduction/evolution activity of calcium-rich sample approaches the best Sr-rich sample at 800 °C. Therefore, calcium-rich compositions may still be valid alternatives with a reasonable compromise between critical element content and performance.