Research On The Preparation And Electrical Properties Improvement Of YSZ Electrolytes

Solid oxide fuel cells（SOFCs）have attracted much attention due to their environmental friendliness,simple operation,and high-power generation efficiency.The electrolyte is an indispensable part of SOFCs,and its electrical performance determines the output performance of cell.Yttrium oxide stabilized zirconia（YSZ）is a kind of oxygen ion conductor with excellent chemical stability and is regarded as one of the most promising electrolytes materials for industrial application.However,YSZ electrolytes still have some problems,such as difficult sintering,unclear grain boundary conductance mechanism and poor electrical performance at medium and low temperatures.To solve the above problems,the preparation and performance improvement of YSZ electrolytes were studied in this paper.The main research contents and results are as follows:（1）Increasing the sintering activity of raw material powders is the key to reducing the sintering temperature of YSZ electrolytes.8 mol.%Y₂O₃ stable Zr O₂（YSZ）nano-powders with high activity were synthesized by an improved solid-state reaction at room temperature.The synthesis process,sintering properties,electrical properties and single cell properties of electrolytes were studied.The results show that when the ratio of yttrium,zirconium nitrate and sodium carbonate is 1:2,80 wt.%Tween-80 dispersant is added,and the calcination temperature is 700℃,the YSZ nano-powders with favourable dispersion can be obtained（average particle size～20 nm）.The electrolytes prepared by the powders can be densified（relative density is 97.9%）and sintered at a lower sintering temperature（1400℃）,which is about 100℃lower than the sintering temperature reported in the literature.The optimal ionic conductivity of YSZ electrolytes is up to 0.034 S cm^-1 at 800℃.The peak power density of the anode-supported single cell is 827 mW cm^-2 at 800℃.（2）To elucidate the mechanism of grain boundaries conductance,micrometer（3.73μm）and submicron（0.48μm）YSZ electrolytes were prepared by microwave one-step sintering and two-step sintering,respectively,and the effect of grain size on electrical properties was investigated.The results show that the total conductivity,grain boundary conductivity and grain conductivity of micrometer YSZ electrolytes are slightly higher than that of submicron YSZ electrolytes.Based on the analysis of the brick layer and Schottky model,the space charge potential（0.16 V）at the grain boundaries of the former is lower than that of the latter（0.44 V）,which is conducive to improving the oxygen ion conduction.However,its impurity blocking term（0.29）is lower than that of the latter（0.89）,which is not conducive to oxygen ion conduction.Therefore,in the range of grain size studied in this paper,due to the combined effect of space charge potential and impurity blocking,increasing the grain size of YSZ electrolytes is beneficial to improve the ionic conductivity.The peak power density of the micron electrolyte-supported single cell is 418 mW cm^-2 at 800 ℃.（3）xESB-YSZ（x=0,5,10,15,20 mol.%）composite electrolytes were prepared by introducing Er_0.2Bi_0.8O_1.5（ESB）oxygen ion fast conductor to improve the electrical properties of YSZ electrolytes at medium and low temperatures.The electrical properties and grain boundary conductance mechanism were studied.The results show that the introduction of ESB can effectively improve the sintering and electrical properties of YSZ.When the amount of ESB was 20 mol.%,the sintering temperature was reduced by 500 ℃,and the ionic conductivity(0.093 S cm^-1,750 ℃)was 4 times that of YSZ electrolytes.The improvement of electrical performance is attributed to the lower space charge potential,higher impurity blocking term and higher oxygen vacancy content.The peak power density of the electrolyte-supported single cell is 572 mW cm^-2 at 750 ℃.