Design And Characterization Of Perovskite Oxide Electrodes For Solid Oxide Fuel Cells

Solid oxide fuel cell(SOFC)is an all-solid energy conversion device,which can convert the chemical energy stored in the fuel directly into electric energy.It has the advantages of high conversion efficiency,clean and pollution-free.However,the operating temperature of traditional SOFC is high,generally between 800?1000?.High operating temperature not only brings many problems,but also puts forward harsh requirements for cell materials.Therefore,reducing the operating temperature of SOFC is an important means to facilitate their commercialization.However,lowering the operating temperature will lead to a sharp decline in the electrochemical performance of the whole cell,which is mainly due to the sluggish catalytic activity of the air electrode at lower temperature.Therefore,it is necessary to develop and design air electrode materials with high oxygen reduction catalytic activity.On the other hand,hydrocarbon fuel has the advantages of high energy density and wide sources,thus being the ideal fuel for the commercial development of SOFC.However,when Ni based cermet is used as fuel electrode,the serious carbon deposition problem limits the application of hydrocarbon fuel in SOFC.Perovskite oxide fuel electrodes have attracted much attention due to their excellent resistance to carbon deposition and sulfur poisoning.However,compared with Ni-based fuel electrodes,the electrical conductivity and catalytic performance of these perovskite oxides need to be improved.Therefore,in view of the urgent requirements of electrode materials for medium and low temperature SOFC and fuel diversification,this dissertation aims to develop and design high-performance perovskite electrode materials to obtain efficient and stable SOFC.The work of dissertation can be mainly divided into two parts.In the first part,based on the problem of decreased catalytic activity of the air electrode at reduced operating temperature,taking PrBaCo2O5+?(PBC)perovskite oxide as the object,the oxygen reduction catalytic performance of air electrode material is improved by means of investigating the oxygen reduction reaction kinetics at the three-phase-boundary and fluoride anion doping;In the second part,aiming at the problem of carbon deposition of traditional Ni-based fuel electrode,the methods of A site doping of bismuth and in-situ exsolution of Ni nanoparticles are used on perovskite oxide substrate to improve the catalytic activity of the fuel oxidation.Chapter 1 briefly introduces working principle of SOFC and the main factors that affect the cell performance.The materials and performances of typical electrolyte,fuel electrode and air electrode for SOFC are summarized.The future development trend of SOFC and requirements of electrode materials are briefly discussed.Finally,the main research basis and content of this dissertation are presented.In chapter 2,the influence of the addition of Sm0.2Ce0.8O1.9(SDC)on the performance of PBC is studied.PBC has high surface exchange coefficient,kchem,and bulk diffusion coefficient,Dchem.However,the thermal expansion coefficient of PBC is high,which results in poor thermal matching with electrolyte.On the one hand,the addition of SDC can enhance the termal matching between PBC and the electrolyte.On the other hand,it also increases the three-phase boundary,thus enhancing the oxygen reduction reaction activity.In this chapter,electrical conductivity relaxation method is used to characterize the oxygen reduction reaction kinetics of PBC-SDC composites with different weight ratios.For the PBC-SDC composites,the oxygen reduction reaction can occur not only at the PBC-gas two-phase interface(2PB)but also at the PBC-SDC-gas three-phase boundary(3PB).But the reaction at 3PB is more facile than that at 2PB.When 60 wt.%SDC is introduced,the oxygen surface exchange coefficient and oxygen bulk diffusion coefficient are improved by a factor of 3.06 and 3.37 at 600?,respectively.Meanwhile,the reaction at 3PB contributes up to 80%,In addition,The polarization resistance of the symmetric cell is reduced from 0.281 ? cm2 for bare PBC to 0.107 ? cm2 for PBC-60 wt.%SDC at 700?.In chapter 3,based on the problem of high thermal expansion coefficient of PBC,another method is proposed to decrease the thermal expansion coefficient of PBC,i.e.by doping fluorine anion.The fluorine doped materials PrBaCo2O5+?Fx(x=0,0.1,0.2)are synthesized via combustion method.The fluorine doping effects on the material performances including the thermal expansion behavior,conductivity,oxygen transport properties,and electrochemical performances are investigated.Fluorine doping reduces the thermal expansion coefficient of PBC from 24.03×10-6 K-1 to 16.78×10-6 K-1.Therefore,the thermal cycling stability of symmetric cell between 200 and 800? improves by about three times.Fluorine doping also improves the oxygen transport properties and electrochemical performances of PBC.In chapter 4,based on the low conductivity and poor catalytic activity of La0.75Sr0.25Cr0.5Fe0.5O3-?(LSCrF)oxide fuel electrode,the enhancement effect of A-site bismuth doping on the performance of LSCrF is studied.Firstly,bismuth can maintain sub-oxidation state between zero and positive trivalence in LSCrF,which elevates the valence states of B-site elements Cr and Fe,thus increasing electronic conductivity.On the other hand,it also improves the oxygen vacancy concentration of the material in the atmospheres of air,wet hydrogen and CO2/CO mixture.Secondly,bismuth doping significantly improves the fuel oxidation,oxygen reduction and CO2 reduction catalytic properties of LSCrF.For example,the polarization resistance of the symmetrical cell at 800?,hydrogen atmosphere is reduced by 60%with 10%bismuth doped LSCrF.When bismuth doped LSCrF is used as the fuel electrode,the peak power density of the single cell using hydrogen fuel increases by about 40%.The polarization resistance of the symmetrical cell in air atmosphere is reduced by about 74%at 800?.The peak power density of the symmetrical structure single cell with bismuth doped LSCrF electrode using ethanol fuel is increased by more than 80%.The polarization resistance of the symmetrical cell in the CO2/CO mixing atmosphere is reduced by about 83%at 800?.The current density of the symmetrical structure single cell with bismuth doped LSCrF electrode at the 1.5 V electrolytic voltage increases from 0.54 A cm-2 to 0.79 A cm-2 at 800? in CO2 atmosphere.In addition,the cells with bismuth doped LSCrF electrodes show good stability when using hydrogen and hydrocarbon fuels and electrolyzing CO2.In chapter 5,the SrV0.5Mo0.5O3-?(SVM)perovskite oxide fuel electrode with in-situ exsolution Ni nanoparticles is designed.The conductivity of SVM in reducing atmosphere is about 1000 S cm-1 at 800?.But the catalytic activity of SVM is very poor.The Ni metal nanoparticles with high catalytic activity can be introduced onto the surface of SVM with high electrical conductivity by in-situ exsolution method.Ni exsolution greatly improves the performance of SVM fuel electrode.The peak power density of the single cell increases by 160%when using hydrogen as the fuel at 800?,and the polarization impedance is reduced by 56%from 0.81 ? cm2 to 0.36 ?cm2.In addition,the single cell also shows good performances in hydrocarbon and sulfur-containing fuels,providing peak power densities of 0.36,0.22 and 0.43 W cm-2 for the single cell when using syngas,ethanol and H2-50 ppm H2S at 800?,respectively.