Preparation And Performance Of Ce_0.9Gd_0.1O_1.95 Composite Alkaline Earth Metal Oxides（MgO,SrO） Electrolyte Materials For Intermediate-temperature Solid Oxide Fuel Cells

As energy conversion devices,solid oxide fuel cells（SOFCs）have attracted much attention because of their higher electrical conversion efficiency,environmental friendliness and fuel flexibility.Electrolyte is the key component of SOFCs.The operating temperature of SOFCs is usually determined by electrolytes.Standard electrolyte materials based on stabilized zirconias（YSZ）are required that the fuel cells must be operated at 900-1000 °C in order to obtain sufficient ionic conductivity.But such high temperature will bring some negative effect to the cells,such as undesirable chemical reactions between cell components,thermal expansion mismatch,material aging,electrode sintering,then degrade the performance of the cells.In recent years,much work has been done on the development of intermediate-temperature SOFCs（IT-SOFCs）to accelerate the commercialization of SOFCs.Intermediate-temperature（500–800 °C）SOFC（IT-SOFC）has received much interest because a replacement of YSZ by a reduced-temperature oxygen-ion conductor in SOFCs would greatly reduce material and fabrication problems and improved cell reliability during prolonged operation.Hence,there is a strong motivation to develop electrolyte materials with high ionic conductivities in the intermediate-temperature range.Doped ceria is regarded as one of the most promising electrolytes for IT-SOFCs because of its excellent oxide ionic conductivity and chemical compatibility with electrodes.Gadolinia doped ceria（GDC）is confirmed to be attractive electrolytes because of its high grain conductivity and better stability at low oxygen partial pressure.In polycrystalline ceramic materials,grain boundaries often have a significant influence on overall conducting properties.In low and intermediate temperature regions,the grain-boundary resistivity contribution to the overall resistivity is significant.The specific grain-boundary conductivity in typical polycrystalline ceria-based electrolytes has been widely reported to be several orders of magnitude lower than the grain interior conductivity.The grain-boundary effects have been mainly attributed to space-charge layer and resistive siliceous films.High-purity materials are very costly.The SiO₂ impurity is ubiquitous in ceria precursor materials.Furthermore,SiO₂ contamination can be inadvertently introduced during the manufacturing process.Therefore,it is of great significance to develop new electrolyte materials with high conductivity and high tolerance to SiO₂.In addition,the output performance of SOFC is related to the performance of the electrolyte and the electrochemical characteristics of the electrode/electrolyte interface.Therefore,it is necessary to study the electrochemical properties of the interface between the electrode and the new electrolyte in the development of new electrolyte materials.In this paper,doped CeO₂ based electrolytes are the main research object.GDC electrolyte samples with different grain sizes are prepared.By using the design idea of composite materials,some novel electrolyte materials with good conductivity and high SiO₂ tolerance are prepared.The electrochemical characteristics of the electrode and the new electrolyte interface are studied.To lay the foundation for the design of IT-SOFC electrolyte materials with higher conductivity and higher tolerance to SiO₂.Finally,the purposes of improving the output performance of SOFC and prolonging the working life of SOFC are expected to be achieved.To improve the grain-boundary conductivity,the influence of grain size on the grain-boundary conductivity has been investigated.To understand the behavior of grain-boundary conductivity with grain size,it is important to understand the behavior of grain size with space-charge potential.However,there is no report on the effect of grain size on the space-charge potential of Ce_0.9Gd_0.1O_1.95（GDC）electrolytes.In this paper,a method is used to evaluate the influence of two factors,the space charge layer effect and the impurity blocking effect,on the GDC grain boundary blocking effect.The relationship between grain size and electrical properties of GDC is analyzed.GDC electrolyte samples with different grain sizes are prepared.The results of XRD test show that the GDC samples sintered at 1150,1250,1350 and 1450 °C for 10 h are cubic structure.The SEM test results show that the average grain size increases with the increase of sintering temperature.The results of AC impedance spectroscopy show that when the relative density is less than 95%,the grain conductivity and macroscopic grain boundary conductivity will decrease.When the relative density is greater than 95%,the variation of the grain size has little effect on the bulk conductivity;with the decrease of the grain size of samples,the macro grain boundary conductivity gradually increases,mainly due to the dilution of impurities,has nothing to do with the space charge layer effect.The change of grain size has a great influence on the electrode polarization resistance.Single cell test results show that the grain size change of GDC electrolyte will affect the output characteristics of single cell.Single cell with GDC electrolyte sintered at 1250 °C has the maximum power density,up to 0.67 W cm-2 at 800 °C.Although some progress has been made in the study of GDC-MgO electrolytes,the influence of MgO addition on the bulk conductivity,grain-boundary conductivity,total conductivity and fuel cell performance of GDC has not yet been studied systematically.Because the grain-boundary conductivity is significantly influenced by the grain size,it is important to understand the behavior of grain-boundary conductivity with grain size in Ce_0.9Gd_0.1O_1.95-x mol% MgO（GDC-x MgO）.Furthermore,it is meaningful to know whether the grain-boundary behavior of GDC-x MgO with grain size follows the same trend of GDC.In this work,GDC-x MgO（x=0–15）electrolytes are synthesized and sintered in air at 1200,1300 and 1400 °C for 10 h.XRD results indicate that all samples show a cubic structure and no MgO incorporation into the GDC lattice.A small MgO peak is found in the 5 mol% MgO-doped specimen and the intensity of peaks corresponding to MgO phase increases with the doping content of MgO.SEM results indicate that addition of 1 mol% MgO to GDC could promote grain growth during the sintering process.Analysis of the impedance spectra shows that for each composition,the grain-boundary resistivity decreases with decreasing grain size for the samples with grain size of >0.4 μ m.Much too small grain sizes（0.2<dg<0.3 μ m）produce an increase in grain-boundary resistivity.The addition of MgO weakens the influence of grain sizes on the grain-boundary resistivity.There is no change in the space charge potential.Addition of 1 mol% MgO to GDC could significantly lower the electrode interfacial polarization resistance and enhance the fuel cell performance.The maximum power density of the cell based on the GDC-1MgO electrolyte reaches 0.73 W cm-2 at 800 °C.It is anticipated that the incorporation of the relatively large Sr²⁺ will have a different effect on the grain-boundary conduction compared to the effect resulting from the addition of the smaller Mg²⁺.So far,the influence of SrO addition on the bulk conductivity,grain-boundary conductivity,total conductivity and fuel cell performance of GDC has not yet been studied systematically.Recently it is reported that Ba_0.8Sr_0.2Co_0.7Fe_0.2Nb_0.1O_3-δ（BSCFN）is a promising novel cathode material.In order to understand the chemical compatibility with GDC and GDC-1SrO,the BSCFN-GDC composite oxides are synthesized.In this work,Ce_0.9Gd_0.1O_1.95-x mol% SrO（GDC-x SrO,x=0,1,5,10 and 15）electrolytes are synthesized and sintered in air at 1350 and 1450 °C for 10 h.Symmetric cells are fabricated.XRD results indicate that all the samples show a cubic structure.A small Sr Ce O3 peak is found in the 15 mol% SrO-doped specimen.SEM results indicate that addition of 1 mol% SrO to GDC could promote grain growth during the sintering process.Analysis of the impedance spectra shows that the bulk conductivity decreases with increasing the doping content of SrO.The grain-boundary conductivity increases with increasing the SrO doping content,and reaches the maximum value at the SrO content of 1 mol%,and then decreases.Compared with the GDC-x SrO samples sintered at 1450 °C,the samples sintered at 1350 °C have higher grain-boundary conductivity and total conductivity,as well as smaller interfacial polarization resistance.Addition of 1 mol% SrO to GDC could significantly lower the electrode interfacial polarization resistance and enhance the fuel cell performance.The maximum power density of the cell based on the GDC-1SrO electrolyte reaches 0.84 W cm-2 at 800 °C.Recently it is reported that addition of Ca O+Zn O to SDC could significantly improve the grain-boundary conductivity.It is anticipated that addition of MgO+SrO to GDC could produce a similar result.So far,the influence of MgO+SrO addition on the bulk conductivity,grain-boundary conductivity,total conductivity and fuel cell performance of GDC has not yet been studied systematically.Furthermore,there is no report on the chemical compatibility between Ce_0.9Gd_0.1O_1.95-1 mol% MgO-1 mol% SrO（GDC-1MgO-1SrO）and Ba_0.8Sr_0.2Co_0.7Fe_0.2Nb_0.1O_3-δ-GDC（BSCFN-GDC）.In this work,GDC and Ce_0.9Gd_0.1O_1.95-1 mol% MgO-x mol% SrO（GDC-1MgO-x SrO,x=0,1,5 and 10）novel electrolytes are synthesized.Symmetric cells are fabricated.XRD results indicate that all the samples show a cubic structure.SEM results indicate that addition of 1 mol% MgO+1 mol% SrO to GDC could promote grain growth during the sintering process.Analysis of the impedance spectra shows that the with the increase of SrO content,the conductivity of the samples decreases gradually.The grain-boundary conductivity increases with increasing the SrO doping content,and reaches the maximum value at the SrO content of 1 mol%,and then decreases.Addition of 1 mol% MgO+1 mol% SrO to GDC could significantly lower the electrode interfacial polarization resistance and enhance the fuel cell performance.The maximum power density of the cell based on the GDC-1MgO-1SrO electrolyte reaches 0.77 W cm-2 at 800 °C.