| It is well known that because high-temperature proton conductors have special proton-conducting function, they have valuable and potential applications in some electrochemical devices and membrane reactors such as fuel cells, gas sensors, hydrogenation and dehydrogenation of some organic compounds, ammonia synthesis at atmospheric pressure, steam electrolyzer, and separation and purification of hydrogen, etc.Y2O3 doped BaCeO3 is one of the most favourite proton-conducting functional ceramic material. It is not only due to its highest proton conductivity among the proton-conducting ceramics discovered heretofore, but also due to relatively inexpensive cost of Y2O3 dopant compared with the other rare earth oxides. However, there are two deficiencies in the researches on Y2O3 doped BaCeO3 ceramics as follows: (1) BaCeO3-based ceramics were usually prepared through a conventional solid-state reaction at high sintering temperature up to 1650℃, resulting in problems such as stringent requirement for the equipment, high energy-consuming and high cost. (2) In the past, the reports on proton conduction in BaCeO3-based ceramics were mainly localized in high temperature range of 6001000℃, whereas there are only few reports on the proton conduction in the materials at intermediate temperatures of 300600℃. This limits their applications in the intermediate temperatures range. For this reason, in this paper, BaCe1-xYxO3-α (x = 0.05, 0.10, 0.15, 0.20) ceramic samples were synthesized, and the conducting properties at intermediate temperatures of 300600℃were investigated.Main works and results are as follows:1. The powders of BaCe1-xYxO3-α (x = 0.05, 0.10, 0.15, 0.20) are prepared via microemulsion method. For the comparison, the powders of sample with x = 0.05 are prepared via hydrothermal precipitation and sol-gel method, respectively. The dense ceramic samples were obtained through sintering of green compacts of the powers prepared by the wet chemical methods as above at lower temperatures. The sintering temperatures for the microemulsion method, hydrothermal precipitation method and sol-gel method were reduced 150℃, 100℃and 50℃compared with the temperature (1650℃) of traditional solid state reaction, respectively. The obtained products were characterized by Differential Scanning Calorimetry and Thermogravimetric Analysis (DSC/TGA), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD). The results indicate that the samples calcined at about 11501200℃for 10 h have almost formed a single phase of orthorhombic perovskite-type structure.2. The conducting properties of the ceramic samples were investigated by using electrochemical methods including AC impedance spectra, gas concentration cells and electrochemical hydrogen permeation (hydrogen pumping) etc. in the intermediate temperature range of 300600℃. It was found that the electrical conductivities of the samples varied with the atmosphere, temperature, doping level and preparation method. Under hydrogen atmosphere, the ceramics were pure proton conductors. Under water-vapor-containing air atmosphere, the ceramics were mixed proton, oxide-ion and hole conductors. While under dry oxygen atmosphere, they were mixed oxide-ion and hole conductors. Among the samples prepared by microemulsion method, the sample of x = 0.15 has the highest conductivities, and its conductivities in the wet hydrogen, wet air and dry air atmosphere at 600℃were 1.04×10-2 S·cm-1, 1.18×10-2 S·cm-1, 1.24×10-2 S·cm-1, respectively. For the same sample of BaCe0.85Y0.15O3-α in the same conditions, the conductivities vary in the order:σ(hydrothermal precipitation method) >σ( microemulsion method)>σ(sol-gel method).3. Ammonia was synthesized successfully in an electrolytic cell based on BaCe0.85Y0.15O3-α prepared via the three methods. For the samples prepared by the hydrothermal precipitation method, microemulsion method, and sol-gel method, the rate of NH3 formation was 2.11×10-9` mol·s-1·cm-2, 2.06×10-9` mol·s-1·cm-2 and 1.18×10-9` mol·s-1·cm-2 under 0.75 mA or 1mA at 500℃, respectively. These results are much higher than the rate of NH3 formation based on SrCe0.95Yb0.05O<sub>3-α (G. Marnellos, M. Stoukides, Science, 1998, 282, 98.). The order of the rate of NH3 formation is same as the conductivity order as shown in works and results 2. |
