Performances Of Cathode Materials With Two Different Structures For Intermediate-Temperature Solid Oxide Fuel Cells

Solid oxide fuel cells（SOFCs）are a type of power generation devices with low pollution and high efficiency,which have a very good development prospect.Compared to traditional SOFCs,intermediate-temperature solid-oxide fuel cells（IT-SOFCs）have many advantages such wide choice of materials and low costs in cell system.At present,the development of SOFCs has been focused on IT-SOFC technology.Accordingly,it is important to develop a novel cathode material for application in IT-SOFCs.B-site ordered double perovskites Sr₂Co_1-xTi_xFeO₅+δ（SCTF;x=0.2,0.4,0.6 and 0.8）are prepared via solid-state reaction.The SCTF samples have a cubic structure,the space group is Pm-3m.The values of δ in SCTF samples are 0.37,0.43 and 0.48 for x = 0.2,x = 0.4 and x = 0.6,respectively.The SCTF materials have good chemical compatibility with the most common electrolytes such as SDC,GDC and LSGM at temperatures below 950 oC.The oxygen desorption peaks corresponding to the α-peaks for the reduction of Co and Fe as well as the β-peak for the reduction of Co are observed in the x = 0.2 and x = 0.4 samples.For x = 0.6 and x = 0.8 samples,only the α-peak for Fe and the β-peak for Co is observed.The results of oxygen temperature programmed desorption,thermogravimetric analysis and thermal expansion measurement showed that the introduction of Ti⁴⁺ without spin-state transition changes the valence state distribution of transition metal ions in the samples,improves the chemical stabilityalso cause a decreased TEC.The maximum conductivities of SCTF（x=0.2,0.4,0.6 and 0.8）are 95.5S cm^-1（350oC）,44.7 S cm^-1（375oC）,24.1 S cm^-1（400oC）,and 7.5 S cm^-1（475oC）,respectively.At 800oC,the polarization resistance values with SDC electrolyte for SCTF（x=0.2,0.4,0.6 and 0.8）cathodes are 0.040,0.043,0.052,and 0.069 Ω cm²,respectively;the maximum power densities of SDC electrolyte-supported single cells are 402,332,293,and 225 mW cm^-2 for SCTF（x=0.2,0.4,0.6 and 0.8）cathodes,respectively,showing that Ti substitution for Co decreases the electrochemical performances of SCTF cathodes.These results suggested the the Sr₂Co_1-xTi_xFeO₅+δ（x=0.2,0.4,0.6）double perovskites are potential cathode materials for applications in IT-SOFCs.K₂NiF₄-type oxides（LaSr）_xCoO_4+δ（x=1,0.95）are prepared via solid-state reaction.The（LaSr）_xCoO_4+δ（x=1,0.95）materials crystallize in a tetragonal structure（the space group,I4/mmm）.The（LaSr）_xCoO_4+δ（x=1,0.95）materials have good chemical compatibility with the most common electrolytes at temperatures below 950 oC.The weight losses of LaSr CoO4+δ and（LaSr）0.95CoO4+δ samples are 0.42% and 0.22% between 30o C and 1000 oC in air,respectively.The average TECs of LaSrCoO4+δ and（La Sr）0.95CoO4+δ samples are 15.2×10^-6 K^-1 and 16.2×10^-6 K^-1 between 30oC and 1000oC in air,respectively.the（LaSr）_xCoO_4+δ（x=1,0.95）materials show good chemical stability,and their average TECs are close to that of the frequently-used electrolytes.The introduction of A-site cationic deficiency improves the stability of（LaSr）_xCoO_4+δ material,but results in a slightly increased TEC.The electrical conductivities of（LaSr）_xCoO_4+δ（x=1,0.95）samples are 120-202 S cm^-1 and 110-182 S cm^-1 at 600-850oC,which are meet the requirements of the IT-SOFC cathode materials with regard to the conductivity.At 800oC,the polarization resistances values for（LaSr）_xCoO_4+δ（x=1,0.95）cathodes on the LSGM electrolyte are 0.143 and 0.097 Ω cm²,respectively;the maximum power densities of LSGM electrolyte-supported single cells are 386 and 463 mW cm^-2 for（LaSr）_xCoO_4+δ（x=1,0.95）cathodes,respectively.The introduction of A-site cationic deficiency is beneficial to improve the electrochemical performances of LaSrCoO_4+δ material.