Synthesis, Conducting Properties And Applications Of BaCe (1-x-y)Zr_xSm_y O_3-α

BaCeO3-based ceramics because of special proton-conducting function 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, separation and purification of hydrogen, etc.BaCeO3-based ceramics are one of the most favourite proton-conducting functional ceramic materials. However, these ceramics easily decompose into proton insulating barium carbonate (BaCO3) or barium hydroxide Ba(OH)2 and cerium oxide (CeO2) in CO2 and/or H2O containing atmosphere, which limits their practical applications. BaZrO3-based ceramics, in contrast, have excellent chemical stability and mechanical strength in spite of relatively low conductivity. In recent years, in order to obtain the novel materials with both good chemical stability and high conductivity, considerable efforts have been focused on the investigation of BaCeO3-BaZrO3 composites.It was reported that Sm-doped BaCeO3 ceramics had high conductivity. But there are very few reports of Zr4+ and Sm3+ doubled-doped BaCeO3 ceramics. Moreover, sintering temperatures above 1650℃are needed by traditional solid-state reaction. According to these reasons, a series of BaCe1-x-yZrxSmyO3-α(x = 0.10, y = 0.10, 0.15, 0.20; x = 0.10, 0.15, 0.20, 0.30, y = 0.10) ceramic samples were synthesized via modified hydrothermal precipitation method. The conducting properties of these samples were investigated by means of AC impedance spectroscopy and gas concentration cells at 300⁸00℃in different atmosphere. Additionally, ammonia synthesis at atmospheric pressure was also conducted in this paper.Main works and results are as follows:1. The precursors of BaCe1-x-yZrxSmyO3-α(x = 0.10, y = 0.10, 0.15, 0.20; x = 0.10, 0.15, 0.20, 0.30, y = 0.10) are prepared via modified hydrothermal precipitation method and the dense ceramic samples were obtained after calcined at 1150¹200℃and sintered at 1550℃. The calcined and sintered temperatures were reduced by 50¹00℃and 100℃compared with those (1250℃and 1650℃) of traditional solid-state reaction, respectively.2. The investigation of chemical stability in mixed gas (3% CO2 + 3% H2O + 94% N2) confirmed that the chemical stability of ceramics decreased with the content of Zr4+ increasing. Among BaCe_0.9-xZr_xSm_0.10O_3-α (x = 0.10, 0.15, 0.20, 0.30) ceramics, the samples of x = 0.20, 0.30 displayed a high chemical stability in the testing atmosphere.3. The conducting properties of BaCe1-x-yZrxSmyO₃-α(x = 0.10, y = 0.10, 0.15, 0.20; x = 0.10, 0.15, 0.20, 0.30, y = 0.10) was investigated by AC impedance spectroscopy. The results indicated that the sample of y = 0.10 in BaCe0.90-yZr0.10SmyO₃-α(y = 0.10, 0.15, 0.20) had the highest conductivity in wet air atmosphere at 300⁸00℃; the conductivities of BaCe_0.90-xZr_xSm_0.10O_3-α (x = 0.10, 0.15, 0.20, 0.30) in various atmosphere decreased with the content of Zr4+ increasing. BaCe_0.70Zr_0.20Sm_0.10O_3-α exhibited both adequate conductivity and better chemical stability among all the samples.4. The ionic conducting properties of BaCe_0.70Zr_0.20Sm_0.10O_3-α in hydrogen and wet air atmosphere at 500⁸00℃were investigated by using electrochemical methods including AC impedance spectroscopy, hydrogen concentration cells and electrochemical hydrogen permeation (hydrogen pumping), etc. The results were shown as follows: BaCe_0.70Zr_0.20Sm_0.10O_3-α was almost pure ionic conductor, and the ionic conduction was contributed mainly by proton and partially by oxide ion in wet hydrogen atmosphere at 500⁸00℃; the ionic conduction of BaCe_0.70Zr_0.20Sm_0.10O_3-α was a mixed conductor of proton, oxide ion and electron in wet air atmosphere.5. We successfully applied BaCe_0.90-xZr_xSm_0.10O_3-α (x = 0.10, 0.15, 0.20, 0.30) to the ammonia synthesis at atmospheric pressure. The peak ammonia formation rate of BaCe_0.70Zr_0.20Sm_0.10O_3-α achieved 2.67×10^–9 mol·s^–1·cm^–2 under a direct current of 0.80 mA at 500℃.