Research On The Preparation And Performance Of CeO₂/BaCeO₃-based Electrolytes Of Solid Oxide Fuel Cells

Solid oxide fuel cells(SOFCs)are one kind of clean and efficient electrochemical power generation devices,which can be widely used in fixed power stations,CHP(combined heat and power)and other fields.The electrolyte is one of the core components of SOFCs,and its electrical performance determines the performance of the cell.Among many electrolyte materials,CeO₂/Ba CeO₃ based composite electrolyte,which combines the advantages of CeO₂ and Ba CeO₃ electrolyte,has attracted much attention.However,the high sintering temperature of the electrolyte is likely to cause several problems such as interface diffusion/reaction and Ba element volatilization,leading to unsatisfactory electrical properties.Moreover,the grain boundary conductance mechanism of the electrolyte is still unclear.In order to solve the above problems,this work has carried out a series of studies on the low-temperature sintering and grain boundary conductance mechanism of CeO₂/Ba CeO₃-based composite electrolyte.The main research contents are as follows:(1)Adding sintering aids is an effective way to reduce the sintering temperature of the electrolyte.Sm2O₃ doped CeO₂(SDC)and Y2O₃,ZrO₂ co-doped Ba CeO₃(BZCY)synthesized by solid phase method were used as raw materials to prepare SDC-BZCY composite electrolyte.The effects of Fe2O₃ sintering aid on sintering performance and electrical properties of SDC-BZCY were studied.The results show that Fe2O₃ is an excellent sintering aid,which can reduce the sintering temperature of SDC-BZCY composite electrolyte from 1500?to 1400?.With the increase of Fe₂O₃ addition,the conductivity of the electrolyte increases firstly and then decreases.When the Fe2O₃ addition amount is1.5 wt.%,the conductivity reaches the maximum value of 8.06×10^-3 S cm^-1 at 700?,which is significantly better than similar electrolyte without sintering aids(1.78×10^-3 S cm^-1,700?).The prepared electrolyte-supported SOFCs has a power density of 36 m W cm^-2 at700?.(2)Improving the powder sintering activity is another effective way to reduce the sintering temperature of the electrolyte.The high-activity Bi,Sm co-doped CeO₂/Ba CeO₃nanopowders(Ce0.8Sm0.1Bi0.1O₂-?-Ba Ce0.8Sm0.1Bi0.1O₃-?,named as Bi SDC-BCSBi)were synthesized by the combustion method.Bi SDC-BCSBi composite electrolyte pellets were prepared.The sintering performance and electrical properties of the electrolyte were studied.The results show that the sintering temperature of the electrolyte can be reduced to 1200?by using the low melting point characteristics of Bi element and the high activity of nano powders,showing excellent sintering performance.When the mass ratio of Bi SDC and BCSBi is 80:20,the conductivity reaches the maximum value of 2.80×10^-2 S cm^-1 measured at 600?,the conductivity is 6 times higher than that of similar electrolytes without Bi element.This is because Bi element doping can increase theO-H groups and oxygen vacancy concentration of the electrolyte.In addition,the power density of electrolyte-supported SOFCs reaches 397 m W cm^-2 at 600?.(3)In order to clarify the mechanism of the grain boundary conduction,the micron-sized(grain size 1-2?m)and ultra-fine grained(grain size 100-300 nm)Bi SDC-BCSBi composite electrolyte were prepared by conventional sintering and two-step sintering process,respectively.The influence of grain size on electrical performance was studied by AC impedance test,blocking electrode method,concentration cell design and other methods.The results show that the apparent grain boundary conductivity of the ultra-fine grained electrolyte(2.83×10^-4 S cm^-1,350?)is higher than that of the micron-sized electrolyte(6.39×10^-6 S cm^-1,350?).The ultra-fine grained electrolyte exhibits excellent"grain boundary effect".The"grain boundary effect"is mainly due to the increase in the concentration ofO-H groups and oxygen vacancies at the grain boundary and the dilution of impurities.