Preparation And Properties Of Lanthanum Strontium Colbalt Ferrite Micro Or Nano-structure Cathode For SOFC

As a type of power generator with characteristics of low pollution and highefficiency, solid oxide fuel cell (SOFC) has great application potential in manyfields such as energy resource, chemical engineering, traffic, aerospace, and military.Recently, intermediate temperature SOFCs have become the hotspot and maintendency in the research and development of SOFC because of the reduced cost,extended operational life, and widened SOFC materials choice accompanied by thedecrease of the operational temperature which meet the commercial requirements.However, the resistance of SOFC cathode increases quickly as the operatingtemperature decreases, resulting in low cell performances. Therefore, thedevelopment and fabrication of SOFC cathodes with high electrochemicalperformances have been one of the most important issues. As we know, the cathodeelectrochemical performance is determined by its intrinsic properties andmicrostructure. Designing and optimizing the cathode microstructure has becomeone of the important approaches to obtain high performance cathodes. In thisresearch, according to the triple-phase boundary theory and relevant electrochemicalprinciples, one dimensional La0.8Sr0.2Co0.2Fe0.8O3-δ(LSCF) based cathodematerials were designed and prepared. Furthermore, LSCF/GDC composite cathodeswith high performance were successfully fabricated.The controllable fabrication of LSCF based cathode materials was achieved byadjusting working parameters using electrospinning technique. After determinationof electrospinning processing parameters (electrospinning voltage is20kV, and thedistance is15cm, and the size of nozzle is0.8mm, and the humidity is20-35%, andat ambient temperature), LSCF cathode materials including nanoparticles (170nm),nanofibers (100nm), nanorods (220nm), and nanotubes (280nm) were successfullyobtained via adjusting precursor solution concentration (30%,20%, and10%) andsintering temperature. Subsequently, the LSCF cathode materials with the structuresof nanoparticles (specific surface area15.7475m2g-1), nanofibers (specific surfacearea10.7810m2g-1), and nanorods (specific surface area5.3178m2g-1) weresintered to form LSCF cathodes, and their electrochemical performances wereinvestigated by means of AC impedance spectroscopy. At650°C, the polarizationresistances for nanoparticle, nanofiber, and nanorod structured LSCF cathodes are6.72,12.29, and17.85Ω cm2, respectively. The impedance results illustrate that thecathode performance is closely related to specific surface area of LSCF materials.The LSCF material with high specific surface area would provide a large contactinterface area between electrode and electrolyte, which contributes to a large TPB length, thus leading to a low polarization resistance. One dimensional LSCF/GDCcomposite materials (specific surface area9.2290m2g-1) were subsequentlyfabricated by electrospinning of mixed solution composed of LSCF precursorsolution and Ce0.8Gd0.2O1.9(GDC)(oxygen ion conductor), followed by sintering toform LSCF-GDC cathode. The corresponding polarization resistance is2.07Ω cm2at650℃, which is lower than that of the above-mentioned LSCF nanoparticlecathode, indicating that the addition of oxygen ion conductor,GDC, can enhance thecathode electrochemical performance due to the increase in ionic conductivity ofcathode and the extending of TPB length.A fast sintering technique was established for the fabrication of onedimensional LSCF cathode scaffolds with stable structure and relatively lowpolarization resistance, which were prepared using the as-electrospun onedimensional nanorods, nanotubes and nanofibers. Based on the above result that theO2-conductor of GDC favors the enlarging of the TPB length, the nanorod,nanotube, nanofiber and nanoparticle structured LSCF scaffolds were infiltratedwith GDC precursor solution followed by sintering to form the LSCF/GDCcomposite cathode. Additionally, the optimal LSCF/GDC mass ratios for thenanorod, nanotube, nanofiber, and nanoparticle structured LSCF/GDC compositecathode are1/1,1/0.52,1/0.56and1/0.23, respectively. At650°C, the minimumpolarization resistances for these LSCF/GDC cathodes are0.1,0.07,0.27, and0.51Ω cm2, respectively. Especially, the polarization resistances for the nanorod andnanotube structured LSCF/GDC cathodes are lower than those of LSCF-basedcomposite cathodes reported previously, which indicates that these cathodes haveexcellent electrochemical performance.The performance stability of the LSCF/GDC composite cathodes was furtherinvestigated. At650°C, after120h polarization under a current density of100mAcm-2, the polarization resistance changed from0.80to1.37Ω cm2for thenanoparticle structured LSCF/GDC composite cathode prepared by the mixingmethod, and the polarization resistance of the nanoparticle structured LSCF/GDCcomposite cathode prepared by the infiltration method increased from0.61Ω cm2to0.98Ω cm2. Results indicate that the performance degradation can be effectivelyslowed down by the infiltration of GDC. Subsequently, the performance stability ofone dimensional nanofiber and nanorod structured LSCF/GDC composite cathodeswas studied. Results indicate that the nanorod structured LSCF/GDC compositecathode with high GDC loading (LSCF/GDC mass ratio of1/1) exhibits excellentperformance stability. At650°C, after144h polarization under a high currentdensity of300mA cm-2, the polarization resistance of the nanrod structuredLSCF/GDC composite cathode decreased from0.097Ω cm2to0.077Ω cm2.Additionally, after20thermal cycles, the polarization resistance decreased from 0.117Ω cm2to0.111Ω cm2, revealing outstanding thermal shock resistance. Resultsindicate that the performance degradation of LSCF cathode has been suppressed byinfiltrating GDC phases onto the surface of LSCF scaffold surface. Furthermore, themore GDC phases are infiltrated, the better the cathode stability will be. X-raydiffraction testing verifies that the crystal face spacing of LSCF decreases with theaddition of GDC phases, which indicates that a compressive strain is applied onLSCF scaffold surface by GDC phases, suppressing Sr surface segregation. This willprovide a new approach to hinder the degradation of LSCF/GDC composite cathode.To evaluate practical application of the nanorod structured LSCF/GDCcomposite cathodes, a GDC electrolyte layer was deposited onto a NiO/GDCsubstrate by electrophoretic deposition, and the high performance nanorodstructured LSCF/GDC composite cathode was successfully sintered onto the GDClayer to form an anode-supported single cell. The practical application of theLSCF/GDC composite cathode was studied in a single cell. Results indicate that,after75h polarization under500mA cm-2, the cathode polarization resistancedecreased from0.103Ω cm2to0.095Ω cm2, exhibiting an excellent performancestability. This research lays the foundation for the practical application of onedimensional nano-structured LSCF/GDC composite cathodes.