Synthesis And Transport Properties Of Doped Lanthanum Gallate Solid Electrolytes By High Temperature And High Pressure

Solid oxide fuel cell（SOFC）,as a kind of fuel cell,has the characteristics of all solid-state structure,low emission,low noise,rapid electrode reaction,high energy conversion efficiency,strong fuel adaptability and building blocks.SOFC has a sandwich structure consisting of dense electrolyte,porous cathode and anode.As the core component of SOFC,electrolyte should have high ionic conductivity,minimum electronic conductivity,better stability,compactness and good electrode matching.Sr and Mg co-doped LaGaO₃ is a widely used SOFC electrolyte material.Its outstanding characteristics are high density and high ionic conductivity.However,there are still some problems to be solved:pure phase of LSGM is difficult to synthesize,which requires higher sintering temperature and longer sintering time.As a result,the gasification of gallium and the structural instability of perovskite are caused.Moreover,as a common oxygen ionic conductor,the proton conductivity of LSGM has been seldom studied.Finding a new preparation method of LSGM and exploring its proton conductivity is a subject worthy of further study.The main contents and conclusions of this work are as follows:La_0.9Sr_0.1Ga_0.8Mg_0.2O₃-δ（LSGM）electrolyte materials were prepared by solid-state method and high-pressure method respectively.The structure,morphology,thermal properties,mechanical properties and conductive transport properties of LSGM in different atmospheres were studied to further understanding and enriching the related characteristics of LSGM electrolyte provides a basis for the design of perovskite structural electrolyte.LSGM electrolyte materials were synthesized by solid-state method.XRD results show that the LSGM electrolyte samples sintered at 1450°C for 20 h form a single phase monoclinic perovskite structure,and no impurities are found.The cell parameters are a=7.8032?,b=5.5231?,c=5.5309?,respectively.And the cell volume is 238.37?³.SEM results show that the structure of LSGM electrolyte synthesized by solid-phase method is densificated and the grain size is about 6.403μm.The relative density of the sample was more than 95%by Archimedes gravity bottle method.The results of thermal expansion coefficient measurement show that the thermal expansion coefficient of LSGM electrolyte material synthesized by solid-phase method is 11.5?10^-66 K^-1 at 30-900°C in air and 11.9?10^-66 K^-1 in hydrogen.The coefficient of thermal expansion increases slightly in hydrogen atmosphere.The results of thermogravimetric analysis show that the overall trend of thermogravimetric curves of LSGM materials in air and hydrogen atmospheres is approximately the same at 30-1000°C.In reducing atmosphere,LSGM materials are more likely to lose oxygen and form oxygen vacancies.The Vickers hardness of LSGM electrolyte prepared by solid-state method is 7.39 GPa,which is higher than that synthesized by glycine nitrate method.Electrochemical impedance spectroscopy（EIS）results show that the grain resistance and grain boundary resistance of the samples decrease with the increase of temperature in air and wet hydrogen atmosphere.The Arrhenius curve of the sample bends around 600°C,and the activation energy changes from 0.68 eV to1.1 eV,which is mainly attributed to the change of activation energy caused by the agglomeration of void defects.In air atmosphere,the total conductivity of LSGM can reach 0.1 Scm^-1 at 850°C.In wet hydrogen atmosphere,the bulk,grain boundary and total conductivity of the sample are slightly lower than those measured in air atmosphere.In wet hydrogen atmosphere,the total activation energy in high temperature region is lower than that measured in air atmosphere.LSGM electrolyte materials were synthesized by high pressure method at1.7GPa,with the temperature is 1050°C,1180°C and 1.9 GPa and with 1050°C.The results of XRD show that the LSGM electrolyte samples synthesized by high pressure method at 1.7 GPa and 1.9 GPa at 1050°C form a single-phase perovskite structure.Single-phase perovskite structure can also be formed at 1.7 GPa at 1050°C and1180°C.The sintering time is 30 minutes.It can be seen that the high pressure method can effectively reduce the synthesis temperature and shorten the synthesis time.For the sample synthesized at 1.7 GPa and 1050°C,the cell parameters are a=7.7990?,b=5.5082?,c=5.5279?,respectively.And the cell volume is 237.47?³.SEM results show that the structure of LSGM electrolyte synthesized by solid-state method is densificated,the relative density is over 95%,and the grain size is about 0.728μm,which is obviously smaller than that of the sample synthesized by solid-state method.The results of thermal expansion coefficient measurements show that the thermal expansion coefficient of LSGM electrolyte materials synthesized by high pressure method is 15.9?10^-66 K^-1 at 100-900°C in air and 17.9?10^-66 K^-1 at hydrogen.The thermal expansion coefficient is higher than that of sample synthesized by the solid phase method,which can be attributed to a stress relaxation process when the sample is heated at atmospheric pressure,which shows that the thermal expa nsion coefficient increases.The thermal expansion coefficient in hydrogen atmosphere is slightly higher than that in air atmosphere,which is consistent with that in solid phase synthesis.The results of thermogravimetric analysis show that the overall trend of thermogravimetric curves of LSGM materials in air and hydrogen atmospheres is approximately the same at 30-1000°C.In reducing atmosphere,LSGM materials are more likely to lose oxygen and form oxygen vacancies.The Vickers hardness of LSGM electrolyte prepared under high pressure is 7.91 GPa.The Vickers hardness of LSGM electrolyte prepared under high pressure is higher than that prepared by solid state method.EIS results show that the bulk resistance and grain boundary resistance of the samples decrease with the increase of temperature in air and wet hydrogen atmosphere.The Arrhenius curve of the sample bulk process bends around 500°C,and the activation energy changes from 0.67eV to 0.23eV,which is mainly attributed to the activation caused by the agglomeration of void defects.Because of the small grain size and the increase of grain boundary,the grain boundary conductivity of high-pressure synthesized samples is obviously smaller.The activation energy of grain conduction process decreases due to stress in high pressure synthetic samples.In wet hydrogen atmosphere,the activation energy of grains in high temperature region is lower than that measured in air atmosphere.In summary,we provide a new method for preparing LSGM solid electrolytes under high pressure.The obtained LSGM materials have changed their thermal expansion properties,mechanical strength and electrical properties,which provides a basis for the modification and optimization of LSGM electrolyte materials.LSGM electrolyte material has good conductivity in wet hydrogen atmosphere,and is a promising mixed conductor of oxygen ion and proton.