Currently, one of the most promising directions in the development of air electrode materials for Solid Oxide Fuel Cells (SOFC) is stabilization by chemical doping of SrCoO_3-δ (SCO) perovskite. Typically, such oxides exhibit high oxygen reduction reaction (ORR) activity and excellent mixed ionic-electronic conductivity (MIEC). When applied in SOFCs, previously inaccessible levels of performance might be reached, creating a possibility of the significant lowering of the operating temperature. This may not only improve the longevity and portability of the cells, but also enables mitigating typical drawbacks associated with presence of alkali ions, such as formation of carbonates and excessive thermal expansion. However, if not properly doped, those materials are unstable at lower temperatures, making their further modification a required next step. The most effective way to effectively stabilize the SCO perovskite structure is to partially substitute cobalt at the B-site with higher-valence cations, such as Nb, Mo, Ru, Ta, or W. Dopants from the 4d/5d group are characterized by a relatively small ionic radius (thus stabilizing the perovskite structure by adjusting the Goldschmidt tolerance factor t), and high oxidation state (tailoring the oxygen content, and therefore MIEC properties). Also, in this approach modification of the compound’s electronegativity is enabled (correlated to the catalytic activity), as well as stabilization of Co³⁺ in the high-spin state, which promotes maintaining the thermal expansion coefficient at moderate levels. The most established materials in this field are SrCo_0.8Ta_0.2O_3-δ (SCT20), SrCo_0.8Nb_0.2O_3-δ (SCN20), SrCo_0.8Nb_0.1Ta_0.1O_3-δ (SCNT10) [1], which were found allowing the effective SOFC operation as low as in the 500-600 °C temperature range. However, the main disadvantage of the materials is still too high thermal expansion coefficient (TEC) of about ~25·10^-6 K^-1.

To address the above-mentioned issues and further enhance the performance and functionality, we propose introduction of additional 3d/4d/5d elements, combined with the composite-like approach to the electrode preparation, by mixing the respective perovskite and gadolinium doped ceria (GDC) powders. A series of SCO-based materials, in which cobalt is partially substituted by Nb, Ta, Mo, and W is successfully synthesized under different atmospheres, including argon, air, and pure oxygen. The XRD analyses show the possibility of obtaining phase-pure materials for a number of compositions, including the previously unreported W- and Mo-doped ones. For example, a single-phase oxide with the SrCo_0.7Ta_0.1Mo_0.1Mn_0.1O_3-δ composition was synthesized for the first time. The functional properties of those materials are tested, including the thermal expansion behaviour, transport properties, and chemical reactivity, including the stability toward typical electrolyte materials. Importantly, the addition of molybdenum and manganese reduced the value of the thermal expansion coefficient to 21·10^-6 K^-1 (favourably compared to 25.6·10^-6 K^-1 for SCNT10) in the temperature range 300-800 °C. Assessment of the electrochemical performance is also carried out, including the detailed optimization and analysis of the cathodic polarization resistance of both, single-phase and composite electrodes (mixture of powder and GDC electrolyte). It is possible to reach a limit value of 0.15 Ω·cm² below 750 °C for the classic and composite-form electrodes. Additionally, the use of a composite cathode may have a positive effect on the thermal expansion coefficient and slow down the degradation of the cell. The proposed novel compositions demonstrate new opportunities arising for by usage of 4d/5d-doped SCO, addressing a number of the most pressing issues known for this very promising group of air electrode materials [2].