Preparation And Performance Of Fluorite Structure And Scheelite-type Electrolyte Materials

Solid oxide fuel cell（SOFC）has been recognized as a novel green energy in the 21 st due to its high efficiency on energy conversion,low pollution and environmentally friendly.Among all key materials of SOFC,the electrolyte plays a most important role.The high operating temperature for the traditional yttria stabilized zirconia（YSZ）electrolyte is too high（above 800℃）,which can cause aging of electrolytes,interface reaction between electrolytes and electrodes,electrode sintering,choice of interconnect materials and so on.These limited the development and commercial use of SOFC.Among new electrolyte materials,doped ceria and scheelite type composite oxides have been recognized the alternative of YSZ because of their high ionic conductivities.Gd³⁺ and Sm³⁺ doped Ce O2（GDC and SDC）are extensively investigated due to their high conductivity in the intermediate temperature ranges（500-700℃）,and scheelite type composite oxides seem to be particular interesting as a result of high conductivity and the negligible electric conduction.Therefore,this study focuses on the preparation and electrical properties of both mentioned-above electrolytes,and estimated the possibility for them used as electrolytes in SOFC.The crystal structure,sintering temperature and electrical properties of Gd³⁺,Mg²⁺ co-doped CeO₂ and Sm³⁺,Fe³⁺ co-doped Ce O2 were studied in detail.For MgO doped GDC,in low dopant content,the conductivity doesn’t increase,even has a slightly decrease.But as the increase of MgO dopant content,the conductivity increased,and reached 0.0203S·cm-1 at 800 ℃.The grain conductivity hardly changes due to the low solubility of Mg²⁺ in GDC lattice.However,it is found that the additions of MgO largely enhance the grain boundary conductivity.Sm³⁺,Fe³⁺ co-doped CeO₂ can form a sosoloid with cubic fluorite structure after calcined at 700 ℃.The prepared powder has good sinterable ability,and dense ceramic can formed after sintering at 1400 ℃ for 4h.The conductivity increased as the increasing of Fe³⁺ dopant content and reached the maximum 0.0263 S·cm^-1.The effects of Sm₂O₃ doping on the phase structure,sintering properties,mechanical properties and electrical properties of Y-TZP were investigated.The results show that Sm₂O₃ can be dissolved in Y-TZP to form a sosoloid with fluorite structure.The doping of Sm₂O₃ can also improve the mechanical strength and electrical conductivity of Y-TZP materials to meet the requirements of solid electrolyte materials for SOFC.The phase structure,sinteringability and electrical properties of rare earth element doped composite oxides with scheelite structure were investigated.The results show that the doping of trivalent ionic with larger radius,such as lanthanum ions or samarium ion,will increase the scheelite cell parameters and formed oxygen vacancy.Thus,the doping leads to the increase of the ionic conductivity of the substrate material.The composite oxide with scheelite structure has good sintering activity,and the dense electrolyte ceramics with more than 95% relative density can be obtained after sintering at 1250 ℃ for 4h.This dense electrolyte ceramics satisfied the use requirement of SOFC electrolyte materials.The oxygen concentration cell test found that the conductive type of scheelite type oxide is mainly ionic conductivity,thus this kind of material has great potential application in SOFC.In order to study the performance fuel cell of the doped ceria electrolyte,a single cell was built used GDC as the electrolyte,LSCF-GDC(LSCF = La_1–xSr_xCo_1–yFe_yO₃)as the composite cathode,and the Ni O/GDC as the anode materials.Electrochemical analysis shows that the single cell has good electrochemical performance,and the power density is 88 m W·cm^-2at 800 ℃.The improvement of battery performance can be attributed to the TPB（three phase interface）expansion caused by the composite cathode,which increases the ionic conductivity.Therefore,GDC can be used as electrolyte material,and LSCF-GDC can be used as composite cathode to obtain excellent battery performance.