Preparation And Performances Of Pr_2-xSr_xNiO₄Materials For Intermediate-temperature Solid Oxide Fuel Cells

Solid oxide fuel cell （SOFC） is one of the modern, environmentally friendlyand most efficient power generation devices with low greenhouse gas emission andis attracting a lot of attentions. At present, much work has been done on thedevelopment of intermediate temperature solid oxide fuel cell （IT-SOFC） toaccelerate the commercialization of SOFC. The lower operating temperature wouldsolve many problems associated with the high temperature operation includingelectrode sintering and interfacial diffusion between electrolyte and electrodes. Asthe operation temperature is lowered, however, the cathodic processes at theinterface between the cathode and the electrolyte become predominant, limitingSOFC performance. Thus much work has gone into finding suitable materials forlow temperature cathodes.Up to now, cathode materials for SOFC are normally based on mixed oxygenionic and electronic conducting oxides （MIEC）, such as perovskite-type oxidesABO3. Relatively, A₂BO₄oxides with K₂NiF₄type structure are less intensivelyinvestigated. Its thermodynamic stability is higher and the thermal expansioncoefficient （TEC） is generally lower than that of the perovskite-type oxides.Furthermore, the K₂NiF₄-type oxides exhibit high electrical conductivity as well ashigh values of oxygen diffusion coefficient and surface exchange coefficient,which is about one order of magnitude higher than those of the best perovskitematerials. But by now, most researches on A₂BO₄-type oxides have focused on Laand Nd at A-site, and there are few reports about other rare earths like Pr.Therefore, in this work, Pr_2-xSr_xNiO₄materials were prepared, and its physical andelectrochemical properties for used as cathodes of IT-SOFC were investigated. Theresults show that:1. The synthesized PSNO powders have pure K₂NiF₄-type structure;2. The PSNO materials are chemically stable with Sm_0.2Ce_0.8O_1.9（SCO）electrolyte;3. The sintered PSNO samples have porous and fine microstructure with poresize smaller than1μm; 4. The average thermal expansion coefficient of the PSNO materials is about12-13×10^-6K^-1at200-800°C;5. The electrical conductivity is in the range of70^-120Scm^-1at800°C;6. Area specific resistance （ASR） value for the Pr_2-xSr_xNiO₄materials on SCOelectrolyte is0.407cm²,0.126cm²and0.112cm²at800°C, respectively,for x=0.3,0.5and0.8;7. The open circuit voltages of the Pr_2-xSr_xNiO₄（x=0.3,0.5and0.8） singlecells are0.66V,0.72V and0.75V, the power densities of the cells are179mWcm-2,220mWcm-2and288mWcm-2, which indicates thatPr_2-xSr_xNiO₄may be a potential cathode material for IT-SOFC.The above research shows Pr_1.2Sr_0.8NiO₄having the best performances.Therefore, in this work, Pr_1.2Sr_0.8NiO₄materials were prepared, and its physicaland electrochemical properties for used as cathode of IT-SOFC were investigated.The results show that:1. Pr_1.2Sr_0.8NiO₄materials with K₂NiF₄-type structure can be obtained bycalcining the precursors at temperature above1050°C;2. The sintered Pr_1.2Sr_0.8NiO₄samples have an average thermal expansioncoefficient （TEC） about12×10-6K^-1in200-800°C;3. The sintered Pr_1.2Sr_0.8NiO₄samples have an electrical conductivity of155Scm^-1at450°C and have an average conduction activation energy of0.034eV at400-800°C;4. Electrochemical impedance spectroscopy （EIS） shows that area specificresistance （ASR） of the Pr_1.2Sr_0.8NiO₄cathode on Sm_0.2Ce_0.8O_1.9electrolyte is0.112cm²at800°C;5. The single Ni-SCO/SCO/PSNO cell produces a power density of288mWcm-2, and an open circuit voltages of0.75V at800°C, respectively.Composite cathodes which can expand the electrochemically activetriple-phase boundaries （TPB）, at the two-dimensional interface between theelectrolyte and the cathode have recently gained great interests. The performancesof Pr_2-xSr_xNiO₄（x=0.3,0.5,0.8） materials have been investigated and it was shownthat Pr_1.2Sr_0.8NiO₄has the best performances. Therefore, in this work,Pr_1.2Sr_0.8NiO₄-xSm_0.2Ce_0.8O_1.9（x=0,25,50） composite cathode materials wereprepared. The result shows that:1. PSNO cathode and SCO electrolyte have a good chemical compatibility; 2. Area specific resistance value for the Pr_1.2Sr_0.8NiO₄-xSm_0.2Ce_0.8O_1.9（x=25,50） composite cathode on SCO electrolyte is0.087and0.076cm²at750°C.The result indicates Pr_2-xSr_xNiO₄and the composite materials may be apromising cathode material for intermediate temperature solid oxide fuel cell（IT-SOFC）.