Preparation And Properties Research Of H~+-SOFC Perovskite Based Cathode Materials By Electrospinning

Solid oxide fuel cells（SOFCs）are energy conversion devices with high efficiency,high energy density and low pollution.Conventional SOFCs operate at an excessively high temperature of about 800-1000^oC,which limits the choice of cell materials and increases the cost of the cell,which in turn limits the practical application of SOFCs.For this reason,related scholars have conducted a lot of research work to try to reduce their operating temperature to the range of medium temperature（600-800^oC）.However,the operation of SOFCs at mid-temperature significantly increases the ohmic loss of electrolyte and polarization loss of cathode,which leads to a decrease in the overall performance and energy conversion efficiency of the cell.In order to improve the performance of cathode and single cell at medium temperature,this paper starts from two aspects.One is to prepare nanofiber cathode material with porous structure by improving the experimental method,thus reducing the polarization loss of cathode.The second is to apply the nanofiber cathode materials to proton conductor fuel cells（H⁺-SOFCs）operating at medium temperature,thus enhancing their electrochemical performance.To this end,the following works are specifically done in this paper:Iron-based cathode La_0.7Sr_0.3Fe O_3-δ（LSF）thin films with uniform morphology were prepared by a simple electrospinning process.The LSF films were sintered at different temperatures and analyzed by X-ray diffraction（XRD）.The results showed that pure perovskite phase LSF nanofiber cathode materials were obtained above 700^oC.Scanning electron microscopy（SEM）results showed that the sintered LSF nanofiber exhibited a porous network structure formed by stacking hollow fibers.The average diameters of the LSF nanofibers sintered at 700 and 800^oC were calculated to be about 80 nm and 57 nm.The Rietveld refinement of the XRD pattern of the LSF fibers showed that the LSF nanofibers had an orthogonal crystal structure with lattice parameters of a=5.5151（?）,b=5.5444（?）and c=7.8005（?）.The microstructure of a single LSF fiber was analyzed by transmission electron microscopy（TEM）and the lattice fringe spacing was given.The XPS full spectrum of LSF nanofibers was given by XPS energy spectrum analysis,the elemental composition was determined,and the Fe 2p nuclear energy spectrum was analyzed.Through BET analysis,the specific surface area of LSF nanofibers is 24 m²g^-1.The above test analysis shows that LSF nanofibers sintered at 700 and 800^oC are suitable for cathode materials of H⁺-SOFC.The nanofiber composite cathode La_0.7Sr_0.3Fe O_3-δ-Ba Zr_0.1Ce_0.7Y_0.2O_3-δ（LSF-BZCY）with proton,oxygen ion and electron three conductive phases was prepared by combining LSF nanofibers with proton conductor Ba Zr_0.1Ce_0.7Y_0.2O_3-δ（BZCY）.The chemical compatibility experiment of the two components of the composite cathode was carried out.The XRD analysis results showed that the LSF and BZCY mixed powder had no other derivative phase after 1000^oC sintering for 3 h,indicating that the two were chemically compatible.Anodic supported H⁺-SOFC（Ni-BZCY/BZCY/LSF-BZCY）was prepared by using BZCY as electrolyte material,Ni O-BZCY as anode material and LSF-BZCY as composite cathode material.The electrochemical performance of the single cell at500-700^oC was tested,and the volt-ampere characteristic curve and power density curve of the single cell were given.At 700^oC,H⁺-SOFC exhibits excellent power output.The corresponding peak power output is 849 m W cm^-2.The electrochemical impedance spectroscopy fitted by Zview software showed that the single cell obtained a lower polarization resistance,and the lowest polarization resistance corresponding to 700^oC was0.12Ωcm².LSF nanofibers were combined with Sm_0.2Ce_0.8O_2-δ（SDC）,an oxygen ionic conductor with high ionic conductivity,to prepare a nanofibrous composite cathode La_0.7Sr_0.3Fe O_3-δ-Sm_0.2Ce_0.8O_2-δ（LSF-SDC）with mixed electron and ionic conductivity.The chemical compatibility experiments of the two components of the composite cathode were carried out.The XRD analysis results showed that the LSF and SDC mixed powders had no other derivative phases after sintering at 1000^oC for 3 h,indicating that the two were chemically compatible.Ni-BZCY/BZCY/LSF-SDC single cell was obtained by applying LSF-SDC cathode to anode-supported H⁺-SOFC with BZCY as electrolyte and Ni O-BZCY as anode.The electrochemical performance of the single cell at 500-700^oC was tested,and good electrochemical output performance was obtained.At 700^oC,the minimum R_pof the cathode was 0.066Ωcm²,the maximum power density（MPD）of the single cell was 1180m W cm^-2,and the corresponding open circuit voltage（OCV）was 1.04 V.Nanofiber cathode material Ba_0.5Sr_0.5Fe O_3-δ（BSF）with high conductivity,uniform and continuous pores was prepared by electrospinning.XRD analysis of BSF cathodes after heat treatment at 700-1000°C showed that the sintering process above 800°C could result in pure chalcogenide phase of BSF.SEM test results show that the BSF fibers sintered at800^oC show a porous and continuous network structure formed by cross-stacking of nanofibers.The average diameters of BSF fibers before and after sintering are about 155nm and 60 nm,respectively.In order to balance the high thermal expansion coefficient of BSF,Ba_0.5Sr_0.5Fe O_3-δ-Nd Mn O₃（BSF-NM）was obtained by combining BSF nanofiber cathode with negative thermal expansion material Nd Mn O₃.The H⁺-SOFC was prepared with BSF-NM nanocomposite fiber as cathode,and good electrochemical performance was obtained.At 700°C,the peak power density（PPD）of the cell was 1170 m W cm^-2,the open circuit voltage（OCV）was 1.04 V,and the corresponding polarization impedance（R_p）of cathode was 0.037Ωcm².