Electrochemical Properties Of Perovskites Sm-90.5-x0 Gd- X Sr-0.5 Coo -93-δ0 With Mixed Conduction

The polarization resistance of conventional LSM cathode increases with the decrease of SOFC operation temperature. Therefore, it is more important to develop new cathode materials. Sm_0.5Sr_0.5CoO_3-δ shows good performances at 500-700℃, which is a promising cathode material. In addition, based on the fact that Sm, Gd elements coexisting in Baotou Rare Earth ore, the synthesis and electrochemical properties of Sm_0.5-xGd_xSr_0.5CoO_3-δ was studied.The powders of perovskite-type oxides Sm_0.5-xGd_xSr_0.5CoO_3-δ(x=0.0, 0.1, 0.2, 0.3, 0.4, 0.5)were prepared using the solid state reaction method. Sm_0.2Ce_0.8O_2-δ (SDC) powders were synthesized by the low-temperature combustion method. The phase and thermal expansion coefficient of Sm_0.5-xGd_xSr_0.5CoO_3-δ were characterized by XRD and dilatometer. The cathode polarization resistance and overpotential of the composite cathodes Sm_0.5-xGdxSr_0.5CoO_3-δ/Sm_0.2Ce_0.8O_2-δ were measured by AC impedance and three-electrode methods. The microstructure of the composite cathodes was observed by SEM.The results show that all Sm_0.5-xGd_xSr_0.5CoO_3-δ sintered at 1200℃for 6h show a orthogonal perovskite structure. The thermal expansion coefficient (TEC) of Sm_0.5-xGd_xSr_0.5CoO_3-δ is (11.43-14.35)×10^-6℃^-1 at 25-800℃. The TEC of Sm_0.5-xGd_xSr_0.5CoO_3-δ is higher than that of Sm_0.5Sr_0.5CoO_3-δ. The polarization resistance and overpotential of Sm_0.5-xGd_xSr_0.5CoO_3-δ/Sm_0.2Ce_0.8O_2-δ composite cathodes is closely relation to the content of Gd³⁺ and SDC. The polarization resistance decreases from 0.104Ωcm² for the composite cathodes with x=0.0 to 0.039Ωcm² for the composite cathodes with x=0.1, then increases to 0.149Ωcm² for the composite cathodes with x=0.5 at 800℃, and it increases from 0.039Ωcm² for the SDC with doping 10% to 0.149Ωcm² for the SDC with doping 30%. In addition, the polarization resistance decreases with the testing temperature from 500℃to 800℃.As Gd³⁺ content increases, the overpotential of Sm_0.5-xGd_xSr_0.5CoO_3-δ /Sm_0.2Ce_0.8O_2-δ composite cathodes decreases for the composite cathodes with x=0.1 and then increases for the composite cathodes with x=0.5 at 800℃. The overpotential of the composite cathodes increases for the SDC with doping 10% to 30%, and it also decreases with the testing temperature from 500℃to 800℃. The law is same to that of polarization resistance. The porous of composite cathode is uniform in microstructure, and the thickness is about 20μm. This composite cathode Sm_0.5-xGd_xSr_0.5CoO_3-δ/Sm_0.2Ce_0.8O_2-δ is a promising potential cathode material for intermediate temperature solid oxide fuel cells.