The Influence Of Ca Doping On The Physicochemical And Electrochemical Properties Of Double Perovskite As Cathode Material

Double perovskite oxides LnBaCo₂O_5+??Ln=Lanthanide?are considered to be potential cathode materials for solid oxide fuel cells?SOFCs?due to their extremely high oxygen surface exchange and bulk diffusion rates.In particular,the cobalt and copper-based double perovskite structure material LnBaCoCuO_5+?has received special attention due to its good thermal compatibility with other component materials of SOFCs.At present,improving the electrochemical performance of such materials is the key to their practical application.To address this problem,the effects of calcium doping on the physicochemical and electrochemical properties of NdBa_1-xCa_xCoCuO_5+?and PrBa_1-xCa_xCoCuO_5+?were studied systematically.The main conclusions are as follows:?1?NdBa_1-xCa_xCoCuO_5+?:A decrease in the cell volume and thermal expansion coefficient,and an increase in oxygen vacancy concentration and electron conductance acquired for the partial substitution of Ba with Ca in NdBa_1-xCa_xCoCuO_5+?.The electrochemical performance of NdBa_1-xCa_xCoCuO_5+?cathodes significantly improved with higher Ca content.For example,the area specific resistances?ASRs?of the NdBa_1-xCa_xCoCuO_5+?-based symmetric cell at 700°C decreased from 0.062?cm² for x=0.0 to 0.038?cm² for x=0.3 and the highest power densities for the corresponding single cells at the temperatures of 800°C increased from 1.42Wcm^-2?x=0.0?to 1.84 W cm^-2?x=0.3?.?2?PrBa_1-xCa_xCoCuO_5+?:With the Ca doping in PrBa_1-xCa_xCoCuO_5+?,the crystallographic symmetry of the double perovskite changes from orthorhombic Pmmm?x=0.0?to tetragonal P4/mmm?x=0.3?.Meanwhile,the distortion of the cobalt and copper octahedron was alleviated and the space of the PrO_?layer was increased.Ca doping in PrBa_1-xCa_xCoCuO_5+?cathode enhanced the oxygen vacancy concentration,lower the coefficient of thermal expansion and improved the electronic conductivity.The electrochemical performance and stability of surface microstructure for PrBa_1-x-x Ca_xCoCuO_5+?had greatly enhancement with Ca doping.For example,the ASRs of the PrBa_1-xCa_xCoCuO_5+?-based symmetric cell at 700°C decreased from0.037?cm² for x=0.0 to 0.028?cm² for x=0.3.The highest power densities for corresponding single cells at the temperatures of 800°C increased from 1.47 W cm^-22 for x=0.0to 2.04 W cm^-22 for x=0.3 and the output current density retention after 100 h?at 700°C and0.7V?increases from 82.2%?x=0.0?to 94.7%?x=0.3?.The greatly enhancement in stability for surface microstructure and electrochemical performance in PrBa_1-xCa_xCoCuO_5+?was closely related to the Ca doping which could lower the A site cation mismatch,optimize the crystal structure and increase the oxygen vacancy concentration and electronic conductivity.Through a systematically study of both systems mentioned above,we found that Ca doping is an effective way to enhance the electrochemical performance of the cobalt and copper-based double perovskites NdBa_1-x-x Ca_xCoCuO_5+?and PrBa_1-x-x Ca_xCoCuO_5+?.The boost of electrochemical performance for this kind of material relate to the lowered A site cation mismatch,optimized bulk/surface microstructure and chemical defects.The results are expected to lay a certain experimental and theoretical foundation for the design and development of high-performance SOFCs cathode materials.