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Synthesis and Electrical Conductivity of LaYO3-based Ceramics

Author: LiuBaoXin
Tutor: MaGuiLin
School: Suzhou University
Course: Inorganic Chemistry
Keywords: LaYO3 La1-xSrxYO3-α proton conduction impedance spectroscopy gas concentration cell
CLC: TQ174.12
Type: Master's thesis
Year: 2009
Downloads: 43
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Abstract


Since Iwahara reported that proton conductor based on perovskite-type SrCeO3 exhibited high proton ion conductivity in 1981, many high-temperature proton conductors have been found one after another. They are very important functional materials for their significant and extensive applications in some electrochemical devices and membrane reactors such as fuel cell, hydrogen sensor, steam electrolyzer, separation and purification of hydrogen, hydrogenation and dehydrogenation of some organic compounds and ammonia synthesis at atmospheric pressure, etc.Until now, there were only a few reports about the conduction in doped LaYO3. In 1980, Iwahara reported that in hydrogen or steam atmosphere, LaYO3 doped with Ca or Ba at La sites had proton conduction. Alcock reported that LaYO3 doped with Sr at La sites was pure oxide-ionic conductor at low oxygen partial pressure, and a p-type semiconductor at high oxygen partial pressure, but lack of proton conduction. Ruiz-Trejo reported that La1-xSrxYO3-α had proton conduction under 550℃. But until now, the proton conduction in Sr doped LaYO3 hasn’t been reported at 600-1000℃and its proton conduction under 600℃also hasn’t been proved fully.The La1-xSrxYO3-α （x = 0.00, 0.05, 0.10, 0.15） ceramics were synthesized successfully via solid state reaction. La0.95Sr0.05YO3-α which has the highest conductivity was also prepared via sol-gel method in this study, and its electrochemical properties were studied by some electrochemical methods. The results of X-ray powder diffraction pattern showed that the ceramics via these two methods were all single monoclinic perovskite structure. Using the ceramics as electrolytes and porous platinum as electrodes, the conductive properties in various atmospheres from 400 to 1000℃were investigated by means of ac impedance spectroscopy, gas concentration cells and electrochemical hydrogen permeation and the performances of H2-air fuel cells were also studied. The synthesis of ammonia at atmospheric pressure was successfully researched by using La0.95Sr0.05YO3-α ceramic which synthesized via sol-gel method. The main research contents and results are as follows: （1） With the increase of doping content, the conductivity of La1-xSrxYO3-α ceramics synthesized via solid state reaction increased firstly and then decreased, the overall conductivities increase in the order:σx = 0.05 >σx = 0.10 >σx = 0.15 >σx = 0.00. At 1000℃, the conductivity of La0.95Sr0.05YO3-α was 0.018 S·cm-1 （in air）,0.0077 S·cm-1（in wet air）,0.00455 S·cm-1（in hydrogen） and 0.00464 S·cm-1（in wet hydrogen）. （2） The dependence of conductivity on oxygen partial pressure revealed that the doped samples were mixed conductors of ions (H+ + O2-) and hole at high oxygen partial pressures, but pure ionic (H+ + O2-) conductors at low oxygen partial pressures. （3） The observed values of hydrogen concentration cell coincided with theoretical ones perfectly, indicating that the ceramics were pure ionic conductors under hydrogen atmosphere. The results of hydrogen pumping confirmed that the ceramics mainly were proton conductors under hydrogen atmosphere. The results of oxygen concentration cell exhibited that the samples were mixed conductors of oxide-ion and hole in dry oxidizing atmosphere. The results of water vapour concentration cell and oxygen concentration cell exhibited that the samples were mixed conductors of oxide-ion, proton and hole in wet oxidizing atmosphere. （4） The properties of H2-air fuel cell were studied using the samples as solid electrolyte which synthesized via solid state reaction. Among the samples, La0.95Sr0.05YO3-α sample which used Ag/Pd as electrode material had the highest fuel cell output performance, current density was 366 mA·cm-2 and power density was 86 mW·cm-2 at 1000℃. （5） Ammonia was synthesized successfully from nitrogen and hydrogen at atmospheric pressure by using Pt as electrode material, and La0.95Sr0.05YO3-α as electrolyte which prepared via sol-gel method. The maximum rate of NH3 formation was found to be 9.83×10-10 mol·s-1·cm-2.

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