Preparation,Microstructure And Electrical Conductivity Of Doped ZrO₂-based Electrolyte Materials For IT-SOFC

Doped CeO₂（DCO）is currently considered to be the best electrolyte material for intermediate temperature solid oxide fuel cell（IT-SOFC）.However,CeO₂-based oxides are easy to be reduced in a reducing atmosphere,resulting in a certain degree of electron conduction,which leads to a decrease in the open circuit voltage of the cell.To solve this problem,the researchers proposed a two-layer electrolyte structure,in which a pure oxygen ionic conduction membrane was prepared as an electron blocking layer on the anode side surface of the DCO electrolyte.With its stable properties in reducing atmosphere,good mechanical properties,free conduction of electron,near thermal expansion coefficient with DCO,and low cost,8 mol%yttria stabilized zirconia（8YSZ）hasbeen used as electron blocking layer material of DCO electrolyte.However,poor ionic conductivity of pure 8YSZ at the temperature of below 800℃limits its applicability for the electrolyte materials of IT-SOFC.In view of the properties of Zr O₂-based electrolyte can be improved by doping and using ultrafine powder as raw material,we propose to prepare single-element or multi-elements doped 8YSZ electrolyte materials in this study.In this paper,pure 8YSZ,single element（iron,cerium,or copper）and multiple elements（iron-cerium,copper-cerium,and iron-cerium-copper）doped 8YSZ powders were prepared by sol-gel method,and doped and undoped 8YSZ electrolyte flakes were prepared by dry pressing-sintering method.The influence of calcination temperature on the phase structure and grain size of the powders was discussed.The surface morphology,cross section structure,sintering performance and electrical conductivity of pure 8YSZ electrolyte flake were characterized.The influence of single element doping,two elements three elements co-doping on surface morphology,cross section structure,sintering performance and electrical conductivity of 8YSZ electrolyte were studied respectively.The optimal doping amount of iron,cerium and copper were determined through analysis and comparison,so as to provide experimental basis for design and preparation of required DCO electron blocking layer materials.The main research results are listed as follows.In the process of synthesizing the powders by sol-gel method,the optimum calcination temperature of the precursor gel is 600℃.After calcined at 600℃for 2 h,the resultant powder is white,with a loose and porous network structure,a loose state and a large number of cavity micropores in the powder.The crystal phase of the powder is cubic zirconia,with an average grain size of 7.0 nm.Increasing calcination temperature enables the yttrium-doped zirconia to obtain larger microcrystalline size and higher crystallinity.The relative density of the pure 8YSZ electrolyte flake is 66.0%,the phase structure is cubic zirconia,and the average grain size is 0.79μm.In addition,there are many pores on the surface and cross section of the pure 8YSZ electrolyte flake.For single-element doped 8YSZ,the optimal doping amount of iron and copper are both about 2 mol%.All doped samples have simple cubic phase structure,and the relative density and ionic conductivity are higher than that of pure 8YSZ.Both iron and copper can be used as effective sintering aids for 8YSZ,which can improve the relative density of samples.The addition of cerium has little influence on the sintering performance of 8YSZ.Among the single-element doped samples,the 5%Fe-8YSZ has the maximum relative density of 90.5%and the maximum average grain size of 4.60μm.The 2%Fe-8YSZ has the highest ionic conductivity,which is 3.16 and 3.19 times that of pure 8YSZ at 500℃ and 750℃,respectively.For multiple elements co-doped 8YSZ,all doped samples are also cubic phase structure and their relative density and ionic conductivity are higher than that of pure 8YSZ.Among the co-doped samples,the 1%Fe-3%Ce-1%Cu-8YSZ has the maximum relative density of 84.0%,the maximum average grain size of 1.86μm and the highest ionic conductivity,which is 3.25and 3.56 times that of pure 8YSZ at 500℃ and 750℃,respectively.