| Solid oxide fuel cell?SOFC?is a high efficiency,environmentally friendly electrochemical energy conversion device,which has highly commercial value and application value for clean energy technology.The development of the traditional high-temperature SOFC is hampered by the various problems caused by high operating temperature.Therefore,developing new intermediate-temperature solid oxide fuel cell?IT-SOFC?materials has important practical significance and the more important key is to develop cathode material with high performance.In this paper,B-site-ordered Co-based double perovskite Sr2Co1-xNbxFeO5+??x = 0.0–0.2?are fabricated via a solid–state reaction and investigated as potential cathodes for IT-SOFC.The single phase SCNx F performing a cubic perovskite structure?space group Pm-3m?was obtained by sintering the samples at 1200 °C for 10 h in air.SCNxF materials exhibited good chemical compatibility with the La0.9Sr0.1Ga0.8Mg0.2O3–??LSGM?electrolyte at temperatures up to 1000 °C.The results of in situ high-temperature XRD and O2–TPD show that the SCNxF?x = 0.1,0.2?materials possess better structural stability compared to Sr2CoFeO5+??SCF?.The SCNx F series samples presented p-type small polaron conduction mechanism.The highest conductivities of the three samples were 321,163,and 115 S cm–1,respectively.Compared to the undoped Sr2CoFeO5+?,the thermal stability of the SCNx F were improved bythe introduction of Nb at Co-site,where the average TEC of the SCNx F?x = 0.0–0.2?series samples at 30–1000 °C were 24.4×10–6?22.8×10–6 K–1and 21.7×10–6 K–1,respectively.The differential curve of thermal expansion confirmed that no structural phase transition occurred in SCNxF this temperature range.The area specific resistances of the SCNx F with x = 0.00?0.20 cathodes on LSGM electrolyte were 0.014,0.026,and 0.058 ? cm2 at 800 °C,respectively.The maximum power densities of a single cell with x = 0.00?0.20 cathodes on a 300-?m thick LSGM electrolyte achieved 604.7,591.8,and 483.2 mW cm?2 at 800 °C,respectively.After long term stability test at 750 °C for 20 h,the performance of single cells for SCN0.1F cathode declines less.These findings indicated that the SCN0.1F double perovskite is a promising candidate cathode material of IT?SOFCs.The Nb doping improveed the thermal and structural stability of the SCNx F materials,however,the average TECs in these materials are still larger compared to the common electrolytes.Therefore,for the sake of balancing the electrochemical performance and the average TEC,the SCN0.1F–xCe0.8Sm0.2O1.9(SCN0.1F–xSDC)composite cathodes with different SDC contents were prepared and evaluated as potential cathodes for IT?SOFCs.SCNxF exhibited good chemical compatibility with the SDC electrolyte at temperatures up to 1000 °C.The average TECs at 30–1000 ° C for the optimum composition of SCN0.1F–40SDC,SCN0.1F–50SDC,and SCN0.1F–60SDC composite cathodes were 16.3 × 10–6,15.13 × 10–6 and 12.9 × 10–6 K–1,respectively,indicating that the appropriate addition of SDC into SCN0.1F further reduce the average TEC value of SCN0.1F.The polarization resistances of the SCN0.1F–30SDC and SCN0.1F–40SDC composite cathodes were 0.103 ? cm2 and 0.1021? cm2 at 700 °C,respectively,which are lower than the target value of 0.15 ? cm2.The fitting parameters by the Z-view software using the equivalent circuit show that the appropriate addition of SDC improved oxygen surface exchange,oxygen diffusion and dissociation in low temperature region.The maximum power density of the single cell with SCN0.1F–40SDC cathode on the LSGM electrolyte reached 325 mW cm–2 at 800 °C.The introduction of SDC also affects the electrochemical performance of the composite cathode materials. |
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