| Global energy crisis and environmental pollution become more and more serious, whichthreat sustainable development and human survival. So the utilization of efficient and cleanenergy sources has caught people’s attention all over the world. Solid oxide fuel cell（SOFC）is considered as one of the most promising power generation devices because of its highenergy conversion efficiency and environmentally-friendly features, becoming the researchfocus of new energy sources home and abroad. The key to the application of SOFCtechnology is to reduce the operating temperature to intermediate temperature（600-800℃）.Cathode material, as an important component of IT-SOFC, the performance has to fulfill theneeds of IT-SOFC in order to realize the popularization and application.Based on performance requirements and theory analysis, we designed two novelcobaltate and fenrrate salts rare earth based IT-SOFC cathode material, which are Sr, Ca andFe multi-doped Ln1-x-ySrxCayCo1-zFezO3-δ（denoted as LnSCCF, Ln=La, Pr, Nd; x=0.1,0.2; y =0.1,0.2; z= 0.2,0.3,0.4, sample NO.LnSCCF-81182/72173/62264）and Sr, Cu dopedLn1-xSrxFe1-yCuyO3-δ（denoted as LnSFCu, Ln=La, Pr, Nd; x=0.1, 0.3; y=0.1, 0.2; sampleNO.LnSFCu-7382/7391/9191）.Thermogravimetry-differential thermal（TG-DTA）, Fourier Transform Infrared（FT/IR）and X-ray diffraction（XRD）techniques were applied to analyze phase forming process andbehaviors at different presintering temperatures of LnSCCF and LnSFCu materials for theimprovement and optimization of preparation process. Traditional solid state sintering andmicrowave solid state sintering were chosen to prepare LnSCCF and LnSFCu cathodematerials. Crystal structure, chemical composition, element valence, microstructure andchemical compatibility were characterized by using X-ray diffraction（XRD）, energyspectrum（EDS）, X-ray photoelectron spectroscopy（XPS）and scanning electronmicroscopy（SEM）. Thermal expansion behavior in 20-1000℃and mixed conductivity in50-950℃of sintered samples were carried out using thermal expansion device and DCfour-terminal method to analyze the changing laws of thermal expansion behavior andconductivity with temperature rising. The influence of doping elements, preparationprocessing parameters on structure and performances of sintered samples were furtherdiscussed.XRD test results show that LnSCCF（1200℃-6 h , 10 h traditional solid state sinteringå'Œ1200℃-3 h microwave solid state sintering）and LnSFC（u1200℃, 1280℃-4 traditionalsolid state sintering and 1200℃-2 h microwave solid state sintering）cathode materials formsingle perovskite structure and good chemical compatibility with YSZ electrolyte. EDS testresults confirm that impurities elements do not exist in sintered samples. SEM observationand analysis indicate that 1-2μm spherical shaped grains exist in porous LnSCCF and LnSFCu samples after 1200℃-6 h and 4 h traditional solid phase sintering. With sinteringtime extends（10 h）and temperature rising（1280℃）, grain grows with its shape changinginto polygon and porosity content decrease. While, after 1200℃-3 h and 2 h microwavesolid state sintering, samples show irregular shaped grain and less pores and partial excessivesintering trace show up. Suitable porosity and homogeneous grain size benefit to theconductivity of samples.The experimental results show that thermal expansions curves of LnSCCF and LnSFCusamples are approximately straight lines. For LnSCCF samples, after1200℃-6 h traditionalsolid state sintering, TEC values are in 1.8×10-5-2×10-5K-1.TECs decrease as sintering timeextends to 10 h. After 200℃-3 h microwave solid state sintering, TECs decrease to1.36×10-5-1.6×10-5K-1. Although the TECs of samples still have mismatch with extensivelyused electrolyte, TECs of Sr, Ca and Fe multi-doped samples are obviously reduce comparedwith only Sr doped samples. Ca-O bond is stronger than Sr-O band, meanwhile Fe-O bond isstronger than Co-O bond, which make the lattice more stable.Ca and Fe doping is helpful toimprove heat matching between cathode materials and electrolyte. For LnSFCu samples,TECs increase as Sr, Cu doping amount increase. In addition, preparation process also hasinfluence on TEC. After 1200℃-4 h traditional solid state sintering, TECs are in 1.2×10-5-1.47×10-5K-1. After 1200℃-2 h microwave solid state sintering, TECs reduced somewhat.Rising traditional solid state sintering temperature to 1280℃, TECs reduce to1.11×10-5-1.33×10-5K-1,which close to the TEC of commonly used electrolyte. Experimentalresults show that LnSFCu cathode materials have better heat matching with electrolyte.Electrical performance test results show that the mixed conductivity of LnSCCF andLnSFCu samples increases up to a maximum value, and then decreases as the temperature rise.At lower temperature, Ln（σT）versus 1000/T is nearly linear, which is consistent with a smallpolaron hopping mechanism and a lower activation energy. There is a turning point in thecurve, conductivity decrease when temperature is higher than this point. This may be due tothe creation of oxygen vacancies for lattice oxygen losing, which lower the carrierconcentration and mobility of small polaron. In 600-800℃temperature range, formulti-doped LnSCCF cathode materials, conductivity values are in 530-770 S/cm after 1200℃-6 h traditional solid state sintering. Extending the sintering time to 10 h, conductivityvalues increase to 540-870 S/cm. After 1200℃-3 h microwave solid state sintering,conductivity values are in 190-440 S/cm. Although the conductivity values have significantlydecrease compared with traditional solid state sintering, it still meets the need of IT-SOFCcathode material. Doping level also has influence on conductivity when using the samepreparation process. LnSCCF-72173 has the highest conductivity while LnSCCF-62264 hasthe lowest ones. This illustrate that doping properly can improve conductivity. For Sr and Cudoped LnSFCu cathode materials, conductivity increase as Sr, Cu doping amount increase.After 1200℃-4 h traditional solid state sintering, samples have relatively low conductivity, which far lower than LnSCCF samples. Improving the sintering temperature to 1280℃,conductivity increase to 100-220 S/cm. After 1200℃-2 h microwave solid state sintering, theconductivity of LSFCu-7382 is 140 S/cm, but to NSFCu-9191, is only 99 S/cm.XPS tests show that, in traditional and microwave solid state sintered samples, rare earthelements La, Pr and Nd are all + 3 valent, which refer to three chemical states. The chemicalstates of Co and Fe elements are all of + 3 and + 4 valent. Cu element exists as Cu2+. There isadsorbed oxygen in O element, which is corresponding to the existence of oxygen vacancies.But less Co4+, Fe4+and oxygen vacancies amounts in microwave solid state sintering samplescan explain the low conductivity of samples.Research results show that the developed LnSCCF and LnSCu are very promisingIT-SOFC cathode materials. |
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