Synthesis And Electrical Conductivity Of LaGaO₃-based Ceramics

High-temperature proton conductors have potential applications in fuel cells, gassensors, membrane separators and membrane reactors, etc. Since proton conduction inSrCeO3 perovskite was discovered by Iwahara, many another high temperature protonconductors have been found one after another.Ishihara reported in 1994 that LaGaO₃ doped with Sr and Mg is a pure oxide-ionicconductor over a wide range of oxygen partial pressures (1～10²⁰atm), and it isconsidered as a promising candidate electrolyte for intermediate temperature fuel cell.And there are many papers On the LaGaO₃ based electrolyte so far. However, all theresearchers' eyes were focus on its oxide-ionic conduction in the past decade, and thereis no report on its proton conduction. In addition, LaGaO₃ based electrolytes weresynthesized by solid state reaction, but there are second-phases by this method, and thewet-chemical methods were developed to solve this problem.The La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-α ceramic was synthesized successfully via solid statereaction, sol-gel and microemulsion methods in this study, and its electrochemicalproperties were studied by some kinds of electrochemical methods. The results of XRDshowed that there are some second phases in the ceramic prepared via solid state reaction,but it is a single-phase perovskite prepared via sol-gel and micrlemulsion methods. Gasconcentration cell, AC-impedance and gas electrochemical permeation experiment wereused to study the ionic conduction behavior, conductivity and ionic transference numberat 600～1000℃by using the La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-α ceramic as solid electrolyte, theproperties of H₂-Air fuel cell were also studied. We also studied the synthesis ofammonia at atmospheric pressure successfully by using La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-α ceramicas a solid electrolyte. And we attained some important results as follows: (1)La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-α ceramic was synthesized successfully via three methods, and the microemulsion method was used for synthesis of LaGaO₃ for the first time, and thesingle-phase perovskite was attained via this method. (2) The proton conduction inLa_0.9Sr_0.1Ga_0.8Mg_0.2O_3-α ceramic under hydrogen atmosphere was discovered for the firsttime, it is a pure proton conductor under hydrogen atmosphere with the proton transportnumber t_H⁺=1 and proton conductivityσ_H⁺= 0.15 S·cm^-1 at 1000℃. (3) It was foundthat it is a mixed ionic conductor (proton and oxide) under wet air atmosphere for thefirst time. (4) It is a pure oxide ionic conductor under dry oxygen.atmosphere. (5) Theproperties of H₂-air fuel cell using the sample as solid electrolyte prepared via abovethree methods were studied, and the sample prepared via microemulsion method has thehighest fuel cell output performance. (6) Ammonia was synthesized at atmospherepressure successfully by using La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-α ceramic as a solid electrolyte forthe first time, and the efficiency of electrochemical prton was more than 70%, and therate of ammonia formation is 2.37×10^-9 mol·s^-1·cm^-2.