Synthesis, Conducting Properties And Applications At Intermediate Temperature Of BaCe_1-xM_xO_3-α(M=Ca, Yb)

BaCeO₃-based ceramics as a kind of proton-conducting functional material have attracted considerable attention due to their potential applications in hydrogen fuel cell, hydrogen sensor, steam electrolyzer, separation and purification of hydrogen, extraction of hydrogen isotopes from fusion reactor core exhaust gas and ammonia synthesis at atmospheric pressure, etc.Typically, dopant cations for BaCeO₃ are trivalent rare earth metal ions and Ca²⁺ ion. In spite of usually lower proton conductivity, compared to rare earth metal oxide doped BaCeO₃, calcium oxide doped BaCeO₃ still is interesting due to rich source and cheapness of calcium oxide. However, there are two deficiencies in the researches on CaO doped BaCeO₃ ceramics as follows: (1) the calcining and sintering temperatures for synthesis of BaCe1?xCaxO₃?αby solid-state reaction were as high as 1400°C and 1665°C, resulting in problems such as stringent requirment for equipment, high energy-consuming and high cost. (2) Iwahara et al reported that BaCe1?xCaxO₃?αexhibited an almost pure ionic conduction in wet hydrogen at 600 1000°C. Slade et al reported that BaCe1?xCaxO₃?αpossessed the protonic conduction as confirmed by electromotive force (emf) measurements using the ceramics as the solid-electrolyte membranes separating moist and dry nitrogen at 400 800°C. However, the investigation on protonic conduction in BaCe1?xCaxO₃?αat intermediate temperatures of 300 600°C until now was still insufficient. In addition, there is no report ahout its application in ammonia synthesis at atmospheric pressure. Furthermore there were few reports about applications of BaCeO₃-based ceramics in hydrogenation of some organic compounds. Therefore, this thesis aims to investigate the above problems.Main works and results are as follows:1. The powders of BaCe1?xCaxO₃?α(x = 0.05, 0.10, 0.15, 0.20) ceramics were prepared by a microemulsion route. The dense ceramic samples were obtained through calcining at 1400°C and sintering at 1665°C of the powders. The calcining and sintering temperatures were reduced by 350°C and 165°C, compared with those (1400°C and 1665°C) of traditional solid-state reaction, respectively. For the comparison, the powders of sample with x = 0.10 were also prepared via sol-gel method. The obtained samples were characterized by Diffraction Scanning Calorimetry and Thermogravimetric Analysis (DSC / TGA), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD).2. The conducting properties of BaCe1-xCaxO₃-αin the intermediate temperature range of 300?600°C were investigated by employing the techniques of AC impedance, hydrogen concentration cell and hydrogen pumping. It was discovered that the ceramic samples were almost pure proton conductors in wet hydrogen, and the sample of x = 0.10 has the highest proton conductivities.3. It was found that proton conductivities of samples were markedly affected by preparation method. The proton conductivity of sample prepared by a microemulsion route was higher than that prepared by sol-gel route. For example: the proton conductivity of BaCe0.90Ca0.10O₃-αprepared by a microemulsion route and sol-gel route were 7.64×10?4 S cm?1 and 1.22×10-4 S·cm-1 at 600°C, respectively.4. Anmmonia was synthesized successfully from nitrogen and hydrogen gases at atmospheric pressure using BaCe0.9Ca0.1O₃-αwith the highest proton conductivity among BaCe1-xCaxO₃-αsamples as an electrolyte of ammonia synthesis reactor, and the maximum rate of NH3 formation was observed to be 2.69×10-9 mol s-1 cm-2 at 480°C with an applied current of 0.8 mA. The ammonia formation rate was higher than the report from Science (electrolyte: SrCe0.95Yb0.05O₃-α, formation rate: 7.5×10-11 mol?s-1?cm-2).5. The powders of BaCe1?xYbxO₃?α(x = 0.05, 0.10, 0.15, 0.20) were prepared by a microemulsion route. The dense ceramic samples were obtained at a lower sintering temperature. The highest proton conductivity of 1.17×10-2 S·cm-1 was observed for x = 0.10 at 600°C. Ammonia was synthesized successfully from nitrogen and hydrogen gases at atmospheric pressure using BaCe0.9Yb0.1O₃-αas an electrolyte of ammonia synthesis reactor, and the maximum rate of NH3 formation was 1.98×10-9 mol?s-1?cm-2 at 480°C with an applied current of 0.6 mA.6. Methanol was first synthesized from formaldehyde and hydrogen gases at atmospheric pressure using Yb3+doped BaCeO₃ as an electrolyte of methanol synthesis reactor. The maximum formation rate of CH3OH was observed to be 1.60×10-8 mol·s-1·cm-2 at 260°C with an applied current of 10 mA in this study.