Preparation And Properties Of Doping CeO₂ Based Electrolyte For Solid Oxide Fuel Cells

Solid Oxide Fuel Cell（SOFC）is a new type of energy device that can directly convert chemical energy into electric energy.It has advantages of high conversion efficiency and environmental friendliness.However,the high operating temperature（800℃-1000℃）of traditional yttria stabilized zirconia（YSZ）electrolyte limits the selection of packaging materials and accelerates the aging of the materials.Therefore,reducing the operating temperature of SOFC becomes the main research work,and the development of new electrolyte materials with high ionic conductivity in the mid-low temperature range has also become a research hotspot.Studies show that the conductivity of doped cerium-based electrolyte materials is higher than that of YSZ between 800℃and 1000℃.Therefore,doped cerium-based electrolyte is considered as the promising electrolyte materials for the mid-low temperature solid oxide fuel cell.The main work of this thesis focuses on two common cerium oxide electrolyte materials Ce_0.8Sm_0.2O_1.9and Ce_0.8Gd_0.2O_1.9.The effects of different elements and different doping amounts on the sintering performance,chemical stability and electrical conductivity of these electrolyte materials are studied.Co-doped ceria-based electrolyte materials of Ce_0.8Gd_0.2-xCa_xO_2-δ（x=0.00、0.02、0.04、0.06、0.08）、Ce_0.8-xSm_0.2Ti_xO_2-δ（x=0、0.10、0.20、0.30）and Ce_0.8Gd_0.2-xBa_xO_2-δ（x=0.00、0.02、0.04、0.06、0.08）are prepared by sol-gel method.The phase composition of powder is characterized by X-ray diffraction.Archimedes method is used to measure the relative density of sintered body.The microstructure of electrolyte materials are observed by scanning electron microscope.Electrochemical properties of electrolyte materials are measured by electrochemical impedance technique.The results show that:Ce_0.8Gd_0.2-xCa_xO_2-δ（x=0.00-0.08）can form cubic-fluorite solid solution at 500℃.The doping of Ca O makes the relative density of Ce_0.8Gd_0.2-xCa_xO_2-δ（x=0.02-0.08）much higher than that of Ce_0.8Gd_0.2O_1.9.The conductivity of electrolyte Ce_0.8Gd_0.2-xCa_xO_2-δis mainly affected by the testing temperature as well as the doping content of Ca O.When the testing temperature is in the range of 400-800℃,the conductivity of the electrolyte material increases with the rising temperature.When the Ca O doping content is 6 mol%,the conductivity of electrolyte at 800℃can reach0.082 S/cm,which is higher than Ce_0.8Gd_0.2O_1.9（σ=0.0305 S/cm）.Ce_0.8-xSm_0.2Ti_xO_2-δ（x=0.00-0.3）can form single-phase fluorite structure at lower calcination temperature,the grain sizes are all in the range of 50-100 nm.Doping a certain amount of Ti O₂can promote the growth of grains,which is beneficial to improve the density of electrolyte materials.However,doping Ti O₂will increase the grain boundary resistance,resulting in the decrease of conductivity.Therefore,Ti O₂can be used as an efficient sintering agent,but cannot be used to improve the electrochemical performance of cerium oxide electrolyte.BaO and Gd₂O₃co-doped CeO₂-based electrolyte,Ce_0.8Gd_0.2-xBa_xO_2-δ（x=0-0.08）shows the cubic fluorite structure in the doping range.The density and conductivity of electrolyte materials can be improved by doping BaO.At the same time,the sintering temperature of electrolyte is also the main factor affecting the conductivity.The conductivity of the electrolyte sintered at 1400°C is higher than that sintered at1300°C.The conductivity of electrolyte Ce_0.8Gd_0.16Ba_0.04O_1.9sintered at 1400°C for 4h reaches 0.051 S/cm at 800℃.In addition,electrolyte Ce_0.8Gd_0.2-xBa_xO_2-δ（x=0.02-0.06）has a good linear expansion coefficient,which can provide good thermal stability for battery package.