The Research Of Oxygen Vacancies Diffusing Mechanisms In Aliovalent Elements Doped Zirconia

Solid Oxide Fuel Cell(SOFC)is a new type of renewable energy system with high energy conversion rate and wide fuel adaptability.However,the high operating temperature(1000 ?)causes severe chemical reactions between various parts of the system,resulting in attenuation or even scrapping of fuel cell performance,which greatly limits the development and use of solid oxide fuel cells.Reducing the operating temperature of the fuel cell and preparing a new medium-low temperature fuel cell system are the current research directions.SOFCs mainly consist of cathodes,anodes and solid electrolytes.Solid electrolytes exhibit excellent ion conductivity at high temperatures and are key components affecting the operating temperature of fuel cells.In this thesis,oxygen vacancies were introduced into zirconia by doping with low-valence elements(Mg,Zn,Ga,Sc,In,Y,Bi,or lanthanoids,etc.).The oxygen vacancy conduction mechanism was systematically studied by density functional theory calculations and experiments.Firstly,the intrinsic mechanism of the stability of cubic phase structure of zirconia electrolyte was studied from the aspects of volume effect,coordination effect and oxygen ion shift.Then,the effects of mono-and co-doped elements on the formation of oxygen vacancies,overcoming the binding forces with doping cations,and kinetically diffusion processes in zirconia electrolytes were investigated.Based on the above calculation results,large radius elements were used as co-dopants to improve the conductivity of ScSZ electrolyte.The diffusion mechanism of oxygen vacancies at different temperatures was studied.Aiming at the problem of low conductivity in the grain boundary of ScSZ electrolyte,two new types of concentration gradient structure electrolyte and ScSZ/GDC dual-phase electrolyte were constructed,respectively.The concentration of oxygen vacancy in grain boundary was improved and lattice distortion was introduced.The conductivity of ScSZ is improved through the above studies.The main research results are as follows:1.The change of phase structure of zirconia at different doping concentrations of scandia was systematically studied based on a combination of density functional calculation and experimental verification.The stability mechanism of the cubic phase is explained from the aspect of volume effect,coordination effect and oxygen ion displacement.The results show that when the concentration of Sc is lower than 18.75 mol%,the tetragonal phase is a stable phase.When the doping concentration is higher than 18.75 mol%,the cubic phase becomes a stable phase.Doping leads to an increase in the volume of the unit cell,a decrease in the coordination number of atoms and exchange potential energy,so that the total energy of cubic phase can be reduced,and thus can be stable at room temperature.2.The oxygen vacancy conduction process is decomposed into oxygen vacancy formation,overcoming the binding of doped cation-oxygen vacancy and kinetically diffusion process.The theoretical calculations and experimental methods were used to study the mechanism of mono-and co-doped elements on the oxygen vacancy conduction behavior in zirconia,and the key factors influencing the conductivity of electrolytes were summarized.The results show that the degree of random formation of oxygen vacancies in ScSZ is greater than that of YSZ at different positions,and the binding energy of oxygen vacancies and doping cations in ScSZ is lower than YSZ.The random distribution of oxygen vacancies and low defect binding energy are beneficial to improve the conductivity of zirconia electrolytes.This is explained from the aspect of charge densities.The doping of scandia affects the charge density of the oxygen atoms in the nearest and next nearest neighbors.The doping of yttria only causes electron loss in the nearest neighbor oxygen atom.The electron loss state is conducive to the detachment of oxygen ions to participate in conduction.The effect of the valence and radius of the co-doped element on the conductivity of the ScSZ electrolyte was summarized.The oxygen vacancies of the divalent element Mg or Zn co-doped ScSZ tend to form at the nearest neighbor position of the doped cations under Coulomb interaction.When the co-doped element radius is in the range of 0.075 nm-0.103 nm,the elastic interaction between the oxygen vacancies and the doped cations can be partially cancelled out by the electrostatic interaction,and the binding energy required for the vacancies to participate in conduction is low.3.Based on the above mechanistic studies,the selection principles of 10ScSZ electrolyte co-doped elements were summarized.Based on the theoretical prediction,large-radius(lanthanide or Bi)element co-doped ScSZ electrolyte was designed and synthesized to stabilize the cubic structure,broaden the doping level,and improve conductivity performance.The experimental results show that the conductivity of Ce,Tb,or Bi co-doped 10ScSZ electrolytes(600 ?-700 ?)are 8%higher than that of undoped electrolytes,respectively.The occurrence of rhombohedral phase is suppressed by co-doping.The conductivity is more than 4 times higher than that of the undoped electrolyte at low temperature(550 ?-600 ?).The diffusion mechanism of oxygen vacancies in zirconia electrolytes at different temperature ranges are studied.Results show that the transport of oxygen vacancies in the electrolyte is affected by both the defect interaction and the kinetically diffusion barrier under the medium temperature range.In the high temperature(800 ?-950 ?)range,the transport of oxygen vacancies is mainly affected by the kinetically diffusion barrier.4.In order to improve the grain boundary conductivity of ScSZ electrolytes,a concentration gradient grain boundary structure electrolyte and a ScSZ/GDC dual-phase grain boundary structure electrolyte were constructed from the viewpoint of improving the concentration of oxygen vacancy at the grain boundary and introducing lattice distortion.The effects of grain boundary doping element segregation and grain boundary oxygen vacancy concentration on the conductivity of the electrolyte were studied by Scanning Transmission Electron Microscopy-Energy Dispersive X-ray(STEM-EDX)and Electrochemical Impedance Spectroscopy(EIS),The results show that the segregation of the doping elements can effectively improve the conductivity of the grain boundary of the electrolyte.On this basis,the concentration gradient grain boundary structure ScSZ electrolyte was designed and synthesized.The influence mechanism of the concentration gradient distribution of doping elements on the conductivity of the electrolyte was studied.The conductivity of the electrolyte was more than 30%higher than that of the traditional electrolyte at 550 ?.The ScSZ/GDC dual-phase grain boundary structure electrolyte is designed and synthesized.When the GDC addition ratio is 15%,the conductivity of the composite electrolyte reaches 195 mS/cm(850 ?),which is more than 20%higher than that of the traditional ScSZ electrolyte.