Research On Cobalt-free Perovskite Cathode For Proton Conducting Solid Oxide Fuel Cells

Solid oxide fuel cell（SOFC）is a power generation device with high-efficiency and pollution-free.SOFC can be used for backup power,fixed power stations and distributed power stations,the industrialization of SOFC is of great significance for solving the energy crisis and environmental pollution.Currently SOFC is focused on the research of high temperature oxygen ion-conducting solid oxide fuel cell（O-SOFC）.High temperature（800～1000°C）operation has high electrode activity,promotes the oxygen reduction reaction,and it has the wider selectivity of hydrocarbon fuel;however high temperature operation causes many issues,such as components mismatch,serious solid-phase reaction,unstable operation,and seal connection is difficult at high temperature,which are not conducive to the industrial development of SOFC.Therefore intermediate-low temperature SOFC（550～750°C）has gradually become the main research direction.Compared with O-SOFC in the intermediate-low temperature,proton-conducting solid oxide fuel cell（H-SOFC）has higher nernst potential and lower conduction activation;at the same time the product water generates at the cathode,which avoids dilution of the anode hydrocarbon fuel and improves the fuel utilization rate.Therefore,H-SOFC has a broader application prospect in the intermediate-low temperature range,The decrease of working temperature adversely affects the working electrode,especially the cathode,as a result,the development of high performance cathode materials for intermediate-low temperature has become the research emphasis of current H-SOFC.This thesis is based on the existing high-performance lanthanum iron perovskite cathode materials of H-SOFC,after doping a new type of high performance cobalt-free perovskite cathode material La_0.6Sr_0.3Ca_0.1Fe_0.8Ni_0.2O_3-δ（LSCFN）was developed,at the same time the physical properties of LSCFN were characterized,and analyzed its possibility as H-SOFC cathode materials;Subsequently,LSCFN was combined with the electrolyte material Ba Zr_0.1Ce_0.7Y_0.1Yb_0.1O_3-δ（BZCYYb）to prepare a triple conductive composite cathode;an anode-electrolyte sheet substrate was prepared by co-compressed method,that was Ni O-Ba Zr_0.1Ce_0.7Y_0.1Yb_0.1O_3-δ（Ni O-BZCYYb）│BZCYYb;and finally full cell was prepared and tested for corresponding electrochemical performance.The following are main contents and results of the thesis:（1）The doped cobalt-free perovskite LSCFN material was prepared by the sol-gel method,and the physical and chemical properties were characterized.XRD phase analysis and refinement showed the space group of LSCFN was R-3c,and it had a single-phase rhombohedral structure;and the iodometric method experiment showed the room temperature oxygen vacancy of LSCFN wasδ₀=0.04,which combined with the data of TGA could be obtained the oxygen non-stoichiometric valueδincreased with the temperature,and reachedδ=0.45 at 800°C,LSCFN generated oxygen vacancies in high temperature;through XPS quantitative analysis of Fe and Ni elements,the oxygen vacancy at room temperature can be converted toδ=0.058,which is similar to the result of iodometry.（2）Investigated the chemical compatibility and physical compatibility of cathode LSCFN and electrolyte BZCYYb.High temperature co-firing and thermal expansion tests were performed,LSCFN and BZCYYb exhibited good chemical compatibility at 1000°C-2 h;the average thermal expansion coefficients of LSCFN and BZCYYb in the range of40～800°C were 13.680×10^-6 K^-1 and 10.788×10^-6 K^-1 respectively,and two materials achieved excellent thermal compatibility.（3）The LSCFN and BZCYYb were composited to form a composite cathode and researched its electrochemical performance.The composite cathode was applied to Ni O-BZCYYb│BZCYYb co-pressed tablet by screen printing to obtain full cell,It was characterized by I-V-P test and electrochemical impedance spectroscopy（EIS）,the highest power density at 750°C and 700°C reached 720.62 and 535.36 m W cm^-2 respectively,and the polarization resistances Rp were 0.045 and 0.114Ωcm²;The full cell carried out the long-term performance test at 650°C,the test maintained a voltage of about 0.75V without attenuation at a current density of 400 m A cm^-2,and it maintained for at least 140 h,after long-term operation the microstructure of the cell was still intact,the result showed that the full cell prepared by composite cathode LSCFN-BZCYYb had great long-term performnce.