Synthesis and characterization of carbon dioxide and boiling water stable proton conducting double perovskite-type metal oxides

Abstract

In this paper, we report the synthesis, chemical stability and electrical properties of three new Ta-substituted double perovskite-type Ba 2Ca 2/3Nb 4/3O 6 (BCN). The powder X-ray diffraction (PXRD) confirms the formation of double perovskite-like structure Ba 2(Ca 0.75Nb 0.59Ta 0.66)O 6− δ , Ba 2(Ca 0.75Nb 0.66Ta 0.59)O 6− δ and Ba 2(Ca 0.79Nb 0.66Ta 0.55)O 6− δ . The PXRD of CO 2 treated (800 °C; 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 N 2, 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 °C, 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 0.79Nb 0.66Ta 0.55)O 6− δ shows the highest proton conductivity of 4.8 × 10 −4 S cm −1 (at 1 MHz) at 400 °C 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 10 kHz.