Characteristics of copper-doped SrFe0.75Mo0.25O3 − δ ceramic as a cathode material for solid oxide fuel cells

In this study, the densification, microstructure and electrical properties of SrFe0.75Mo0.25 − xCuxO3 − δ ceramics were investigated. The densification of SrFe0.75Mo0.25O3 − δ ceramics were significantly enhanced by Cu substitution through transient liquid phase sintering at high temperatures. Rietveld refinement of the diffraction data confirmed the structure as an orthorhombic (Pnma) perovskite and no secondary phase was observed. 7.6% and 45.0% of Fe and Mo in the SrFe0.75Mo0.25O3 − δ ceramic were in the valence states of 3 + and 5 +, respectively, while these percentages in SrFe0.75Mo0.05Cu0.20O3 − δ ceramic increased to 13.8% and 50.2%, respectively. On the other hand, 92.4% and 55.0% of Fe and Mo for the former were in 2 + and 6 + valence states respectively. However for the later these percentages dropped to 86.2% and 49.3% respectively. The calculated δ value increased from 0.46 for the SrFe0.75Mo0.25O3 − δ ceramic to 0.87 for the SrFe0.75Mo0.05Cu0.2O3 − δ ceramic, indicating that the concentration of oxygen vacancies increased with Cu substitution. The average thermal expansion coefficient was approximately 21.6 × 10− 6 K− 1 for the SrFe0.75Mo0.05Cu0.2O3 − δ ceramic in the temperature range from 25 to 600 °C, and larger than that of the SrFe0.75Mo0.25O3 − δ ceramic (14.4 × 10− 6 K− 1). The electrical conductivities of SrFe0.75Mo0.25 − xCuxO3 − δ ceramics were consistent with the small polaron hopping conduction mechanism at low temperatures and with a metallic behavior above the transition temperature of 600 °C attributed to loss of lattice oxygen. The electrical conductivity increased by more than three times from 36.5 S·cm− 1 for x = 0 to 135.6 S·cm− 1 for x = 0.2 at 600 °C, due to an increase in oxygen vacancies, the change in the proportions of Fe2 +/Fe3 + and Mo6 +/Mo5 + redox couples, and the increase in the grain size in the microstructure.