Synthesis And Characterization Of Carbon Dioxide And Boiling Water Stable Proton Conducting Double Perovskite-type Metal Oxides

摘要

In this paper, we report the synthesis, chemical stability and electrical properties of three new Ta-substituted double perovskite-type Ba_2Ca_(2/3)Nb_(4/3)O_6 (BCN). The powder X-ray diffraction (PXRD) confirms the formation of double perovskite-like structure Ba_2(Ca_(0.75)Nb_(0.59)Ta_(0.66))O_(6-δ), Ba_2(Ca_(0.75)Nb_(0.66)Ta_(0.59))O_(6-δ) and Ba_2(Ca_(0.79)Nb_(0.66)Ta_(0.55))O_(6-δ). The PXRD of CO_2 treated (800℃; 7 days) and water boiled (7 days) samples remain the same as the as-prepared samples, suggesting a long-term structural stability against the chemical reaction. The electrical conductivity of the investigated perovskites was found to vary in different atmospheres (air, dry N_2, wet N2, H_2 and D_2O + N_2). The AC impedance investigations show bulk, grain-boundary and electrode contributions in the frequency range of 0.01 Hz to 7 MHz. Below 600℃, the bulk conductivity in wet H_2 and wet N_2 was higher than in air, dry H_2 and dry N_2. However, an opposite trend was observed at high temperatures, which may be ascribed to p-type electronic conduction. The electrical conductivity of the investigated perovskites was decreased in D_2O + N_2 compared to that of H_2O + N_2 atmosphere. This clearly shows that the investigated Ta-doped BCN compounds exhibit proton conduction in wet atmosphere which was found to be consistent with water uptake. The water uptake was further confirmed by thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR) characterization. Among the samples investigated, Ba_2(Ca_(0.79)Nb_(0.66)Ja_(0.55))O_(6-δ) shows the highest proton conductivity of 4.8 × 10~(-4) S cm~(-1) (at 1 MHz) at 400℃ in wet (3% H_2O) N_2 or H_2, which is about an order of magnitude higher than the recently reported 1% Ca-doped LaNbO_4 at the same atmosphere and at 10kHz.