Sm0.5-xGdxSr0.5CoO3-δ (SGSC) compositions were synthesized by the solid state reaction method.Their crystal structure, thermal expansion coefficient and electrochemical properties were investigated.All SGSC samples are the perovskite structure with an orthorhombic symmetry.The substitution of Gd3+ for Sm3-results in an increase in thermal expansion coefficient.Compared with Sm0.sSr0.5CoO3-δ(SSC),Srn0.4Gd0.1Sr0.sCoO3-δ-SDC composite cathode shows an improved electrochemical performance.However,the cathodic properties are worsened with the further increase of Gd3+ substituting.At 800C, the area specific resistance of Sm0.4Gd0.1Sr0.sCoO3-δ-SDC composite cathode measured by impedance analysis is 0.10 Ω cm2,which is approximate to the value of 0.11 Ω cm2 under cathodic polarization.Moreover, the Sm0.4Gd0.1Sr0.sCoO3-δ-SDC composite cathode demonstrates the highest exchange current density and the lowest overpotential under the same conditions.