Study For LSF Based Perovskites Materials As Solid Oxide Cell Cathodes

Coal,petroleum and natural gas are still the main energy resources now,and the transfer process limited by carnot cycle leads to low transfer efficiency and environmental pollution.Therefore,developing new energy technology and improving the fuel availability are impending.Solid oxide fuel cell?SOFC?is an energy-conversion convert technology,which can convert chemical energy from the fuel and oxygen molecules directly into electricity.As the device is not limited by carnot cycle,it has a high energy conversion efficiency.SOFC energy loss is primarily determined by the overpotential associated with oxygen reduction reaction?ORR?at the cathode,where oxygen molecular is electrochemically transferred to oxygen ions.Accordingly,it is necessary to reduce the cathodic overpotential by accelerating the ORR kinetics and enhancing the activity of cathode catalyst,thus improving the electrochemical properties of SOFC and lowing the work temperature to intermediate temperature range of 600-800?.The preparation and operating cost of SOFC will be reduced by the lower work temperature.The ferrite based perovskites materials possessed high ionic and electronic conductivity and good catalytic activity are often used as intermediate-temperature SOFC cathode.La0.8Sr0.2FeO₃-??LSF?at intermediate-temperature range has a maximal conductivity of about 100 Scm-1.In addition,the thermal expansion coefficient of 12.2×10-6 K-1 is similar to those of the traditional electrolyte materials,and then LSF has a good thermal match with electrolytes.Based on LSF material,we will improve the electrochemical properties of LSF by doping in A site and modifying in surface in this paper.Chapter 1 mainly introduces the research basics and the operating principle of SOFC,and factors affecting cell performance.Summarizing the basic characteristics and structure of SOFC composition,including cathode,anode and electrolyte materials.Presenting the research and analyze methods for electrochemical performance,and studying the cathode ORR mechanism.Extracting the main points in this paper.In Chapter 2,Bismuth is doped into La0.8Sr0.2FeO₃-? by solution method to produce ferrite-based perovskites with a composition of La0.8-xBixSr0.2FeO₃-??0?x?0.8?.The perovskite structures and properties including oxygen nonstoichiometry coefficient???,average valence of Fe,sinterability,thermal expansion coefficient?TEC?,electrical conductivity???,oxygen chemical surface exchange coefficient?Kchem?,and chemical diffusion coefficient?Dchem?are explored as a function of bismuth content.Meanwhile,the interfacial polarization resistance?Rp?and ORR activation energy are also explored as a function of bismuth content with Sm0.2Ce0.8O1.9?SDC?electrolyte.For La0.8-xBixSr0.2FeO₃-?,Rp and ? decreases with x,while conductive activity energy,Dchem and Kchem increase with Bi doping.The perovskite with x= _0.4 is suggested as the most promising composition as solid oxide fuel cell cathode material since it has demonstrated applicable electrical conductivity of 9.3 S cm-1 and low interfacial polarization resistance of 0.12 ?cm2 at 700 0C.In addition,this composite has Kchem of 9.5×10-4 cms-1 and Dchem of 1×10-4 cm2s-1 at 700?.The properties of La_0.4Bi_0.4Sr0.2FeO₃-??LBSF4?are studied particularly in Chapter 3.The properties of LBSF4 are further investigated in the present study using thermogravimetric?TG?analysis,oxygen temperature-programmed desorption?TPD?method,iodometric titration and high-temperature X-ray?HT-XRD?diffraction refinement methods to reveal its structural properties including oxygen non-stoichiometry coefficient???,valence state of Fe,and lattice parameters at the different temperatures.In addition,the rate determining steps of the oxygen reduction process on the single phase LBSF4 is explored using the distribution of relaxation time?DRT?method,based on the electrochemical impedance spectroscopy measurements conducted in oxygen partial pressure from 0.01 to 1.0 atm.The LBSF cathode electrochemical performance is effectively improved by cooperating Sm0.2Ce0.8O1.9?SDC?,an oxygen ion conductor,resulting in interfacial polarization resistance less than 0.1 ?cm2 at 700 ? when SDC is used as the electrolyte.Chapter 4 presents the catalytic improvement by decorating LSF surface with cobalt oxide,Co3O₄.Cobalt oxide is usually used as a dopant to improve the catalytic activity of Mn and Fe based perovskites cathode materials,however,the doping of Co will lead to the increase of TEC.This work reports that the performance can be improved by introducing the cobalt oxide onto the LSF surface using infiltration technique,and the surface modification will not affect TEC of LSF.XRD,TG,SEM?scanning electron microscopy?and HRTEM?high resolution electron microscopy?analysis indicate that Co3O₄ is thermal stability and chemically compatible with LSF at work conditions.Electrochemical impedance spectrum demonstrates substantial reduction in interfacial polarization resistance,from 0.22 to 0.083 ?cm2 at 700? when 5.84 wt.%Co3O₄ is infiltrated into the bare LSF electrodes.Further analysing the impedance spectrum with DRT calculation suggests that the performance improvement is associated with the charge-transfer processes of the surface reaction processes.Meanwhile,the electrochemical conductivity relaxation?ECR?measurement shows that Co3O₄ particles can improve the surface reaction kinetics,increasing the oxygen surface exchange coefficient by a factor of about 5 at 700?.In addition,Co3O₄ particles can increase the peak power density of the single cells from 0.865 Wcm-2 to 1.3 Wcm-2 at 800? with LSF based cathodes.In Chapter 5,SrCO₃ is used for surface modification of LSF.SrCO₃ particles are infiltrated into LSF to improve the electrochemical performance.XRD and HRTEM indicate good chemical compatibility between LSF and SrCO₃ particles.ECR measurement shows that SrCO₃ particles improve the value of Kchem by a factor of 2.Electrochemical impedance spectroscopy demonstrates substantial reduction in interfacial polarization resistance,from 0.22 to 0.15 ?cm2 at 700? when 5.2 wt.%SrCO₃ is infiltrated into the LSF electrode.DRT analysis shows the reduction is strongly related to the charge-transfer process.In addition,SrCO₃ particles can increase the peak power density from 596 mWcm-2 to 857 mWcm-2 at 750? for the single cells,achieving an enhancement of more than 40%.Chapter 6 aims to verify the effect and propose the mechanism by decorating SrCO₃ nanoparticles on La_0.6Sr_0.4Co0.2O₃-??LSCF?surface.In recent years,Sr surface segregation in long-term operation has been reported to have the contradicting effects that either degrade or improve the reaction.Thus,it is critical to understand the mechanism of surface Sr compounds on ORR kinetics.ECR measurement shows that SrCO₃ particles improve the value of Khem by up to a factor of 100.The electrochemical performance is significantly improved by the infiltration of SrCo3,which is comparable to those obtained by typical electrocatalysts including precious metals such as Pd and Rh.DRT analysis shows that the performance enhancement is strongly related to the improved kinetics of charge transfer and oxygen incorporation processes.Density functional theory calculations show that the surface SrCo3 reduces the O2 dissociation energy barrier from 1.34 eV to 0.33 eV,in addition,SrCO₃ affects the charge density distribution at LSCF-SrCO₃ interface,then the charge could transfer from SrCO₃ to the LSCF surface,thus enhancing the ORR kinetics.