Synthesis, Conducting Properties And Application Of Ba_x Ce_0.8Y_0.2O_3-α + YZnO

Potonic conductors with ABO₃-type perovskite structure, e.g. SrCeO₃ and BaCeO₃ based oxides, have a wide range of technological applications in solid oxide fuel cells (SOFCs), hydrogen sensors and ammonia synthesis at atmospheric pressure, etc.Among the proton-conducting oxides reported so far, barium cerate ceramics showed the high proton conductivity, particularly when doped with 15—25 mol% Y³⁺, reached high conductivities around 10-2 S cm-1 at 873 K. However, the sintering temperatures of such oxides were above 1923 K through a traditional solid state reaction method without sintering aid. Previous research demonstrated that the addition of ZnO could significantly lower the sintering temperatures of BaCeO₃-BaZrO₃ based solid solution. Ma, et.al. synthesized and investigated nonstoichiometric BaxCe0.9Y0.1O₃-α(x = 0.8—1.2) ceramics, discovered that the sample of x = 0.95 showed both the highest conductivity and better chemical stability among the investigated samples.To our best knowledge, there has been no report on 20 mol% Y³⁺ doped nonstoichiometric BaCeO₃, BaxCe0.8Y0.2O₃-α, to date. In this paper, the ceramic samples BaxCe0.8Y0.2O₃-α+ 0.04ZnO (x = 1, 0.98, 0.96, 0.94) were prepared by a solid-state reaction method, and the conducting properties at intermediate temperature (673—1073 K) of the samples were investigated. Main works and results are as follows:1. The stoichiometric ratio of Ba kept invariable and the synthetic condition of BaCe0.8Y0.2O₃-α+ yZnO (y = 0.02,0.04) was investigated. ZnO was added before or after calcining of reaction mixtures. The mixture was calcined and sintered at different temperatures. The results indicated that when the sample after the mixture calcined at 1523 K for 10 h was added with 4 mol% ZnO, and then sintered at 1623 K (300 K lower than the traditional solid-state reaction method without sintering aid) for 10 h, the ceramic sample exhibited a single phase of orthorhombic perovskite-type structure and a higher relative density as well as the higher conductivity.2. Based on the optimum synthetic conditions mentioned above, the ) nonstoichiometric ceramic samples BaxCe0.8Y0.2O₃-α+ 0.04ZnO (x = 1, 0.98, 0.96, 0.94) were prepared. The conducting properties of the ceramic samples were investigated by using electrochemical methods including alternating current (AC) impedance spectra, gas concentration cells and electrochemical hydrogen permeation (hydrogen pumping) etc. in the intermediate temperature range of 673—1073 K. It was found that the conductivities were affected by the nonstoichiometric amount of Ba2+, and increased in the order:σ(x = 0.94) <σ(x = 1) <σ(x = 0.96) <σ(x = 0.98). It was also found that the samples were almost pure ionic conductors and contributed mainly by proton and partially by oxide ion under wet hydrogen atmosphere at 773—1073 K; Under water-vapor-containing air atmosphere, the ceramic samples were mixed proton, oxide-ion and electron hole conductors.3. The ammonia synthesis at atmospheric pressure was successfully conducted using an electrolytic cell based on Ba_0.98Ce_0.8Y_0.2O_3-α + 0.04ZnO. The maximum ammonia formation rate reached 2.36×10^-9 mol s^-1 cm⁽-2 at 773 K with an applied current of 0.8 mA. The value in this study was comparable to 2.1×10^-9 mol s^-1 cm⁽-2 using BaCe0.85Y0.15O₃-αreported by us (Guo Y.X., Liu B.X., Yang Q., et al., Electrochem. Commun, 2008, 11, 153).It indicated that Ba0.98Ce0.8Y0.2O₃-α+ 0.04ZnO is a promising solid electrolyte material for ammonia synthesis at atmosphere pressure.