Effects Of MgO Addition On The Electrical Conductivity Of Ce_0.9Gd_0.1O_1.95-based Electrolytes

Doped CeO₂-based solid electrolytes are widely used in medium-temperature solid oxide fuel cells and oxygen sensors due to their high ionic conductivity and good electrode matching.For doped CeO₂-based solid electrolytes,the impedance of the grain boundary is the most important factor limiting the increase in total conductivity.In recent years,efforts have been made to introduce heterogeneous phases into the doped CeO₂ matrix to improve the grain boundary electrical conductivity of the doped CeO₂ thereby improving the electrical conductivity of the doped CeO₂-based solid electrolytes.At present,MgO is one of the commonly used heterogeneous phases,but the research of adding MgO is mostly concentrated in the case of its low content,in which,the added MgO only plays the role of scavenging the impurities in the grain boundary and optimizing the space charge layer structure.However,when the content of MgO is high,the research of the influence of heterogeneous interface on the electrical properties of doped CeO₂ has not been systematically studied.The composite powders of Ce_0.9Gd_0.1O_1.95(GDC)-x MgO(x=0,10%,20%,30%,40%,50mol%)were synthesized by sol-gel method in one step,and then sintered at1500^°C for 5h in an air atmosphere to obtain dense composite electrolyte samples.XRD results show that all the doped Gd-element are dissolved into the CeO₂ lattice,and MgO are micro dissolved into the CeO₂ lattice,making the lattice constant of GDC slightly reduced.SEM results show that the average grain size of pure GDC is 2.15?m and MgO are evenly dispersed in GDC matrix.The introduction of MgO can reduce the average grain size of GDC,when the content of MgO is 20mol%,the sample has the minimum grain size,is 0.41?m.The results of AC impedance show that the introduction of MgO can improve the specific grain boundary electrical conductivity of GDC-x MgO composite electrolytes dramatically.The GDC-20mol%MgO shows the highest specific grain boundary conductivity,which is 1.726×10^-5 S/cm at 400^°C,9.16times higher than that of pure GDC,which indirectly confirms that the formation of GDC/MgO heterogeneous interfaces are beneficial to the conductivity of GDC-x MgO composite electrolytes.In order to study the effect of sintering temperature on the properties of GDC-68mol%MgO composite electrolytes,GDC-68mol%MgO composite electrolytes were prepared at 1350^°C,1400^°C,1450^°C and 1500^°C,respectively.The(200)and(220)diffraction peaks of MgO can be clearly seen in the XRD pattern.SEM results show that the average grain size of GDC and MgO will increase with the increase of sintering temperature.When the sintering temperature was 1350^°C,the GDC and MgO have the smallest average grain size,239 nm and 680 nm,respectively.The HR-TEM results show that no other impurities precipitate at the grain boundary of GDC-MgO.The results of AC impedance spectroscopy show that with the increase of sintering temperature,the grain conductivity of the samples increase,while the apparent grain boundary conductivity and specific grain boundary conductivity decrease.The composite electrolyte sintered at 1400^°C has the largest total conductivity,which is4.4×10^-3 S/cm at 650^°C.In order to further reduce the sintering temperature of GDC-MgO composite electrolytes and obtain more heterogeneous interfaces,low melting point Bi₂O₃(825^°C)was introduced into GDC-10mol%MgO as sintering aid.The diffraction peak of Bi₂O₃was not found in XRD pattern.At the sintering temperature of 1300^°C,the grains of the composite electrolytes more uniform,but the density of the samples decreases slightly at this temperature.The results of EIS show that the addition of Bi₂O₃ is beneficial to the ionic conductivity of GDC-10mol%MgO.Although the evaporation of Bi₂O₃ will introduce pores,the ionic conductivity of GDC-10mol%MgO can still be improved when the doping amount of Bi₂O₃ is 3mol%.