Electrical properties of proton-conducting BaCe0.8Y0.2O3-δ and the effects of bromine addition

Abstract

A detailed analysis of the electrical properties of the proton-conducting BaCe0.8Y0.2O3-δ phase (BCY20) and the effects of Br doping has been undertaken. Members of the system of nominal stoichiometry BaCe0.8Y0.2O2.9-(x/2)±δBrx (x = 0, 0.05, 0.1 and 0.2) were synthesised by a citrate-nitrate process and high-temperature annealing up to 1500 °C. X-ray diffraction revealed the formation of single-phase material with a monoclinically distorted perovskite structure (space group I2/m) with greater distortion for Br-synthesised phase. Significant contrasts in electrical behaviour and stability between BCY20 and Br-synthesised series members were also found. However, chemical analysis by total X-ray fluorescence spectroscopy indicated that only trace amounts of Br remain after synthesis, indicating that Br addition influences stoichiometry but not directly the physicochemical properties. Higher conductivity is observed for the Br-synthesised compositions in wet and dry oxidising conditions in the temperature range 300–900 °C, reaching a value of 5.8 S m−1 at 800 °C for the x = 0.2 member in wet O2 (pH2O ∼ 0.022 atm). Determination of partial-conductivity components indicated that, in humid conditions and low temperature, the conductivity of 20% Br-synthesised material is principally protonic (tH = 0.91 at 600 °C and pO2 = 0.2 atm) and superior to that of BCY20. Mixed oxide-ionic-electronic conductivity dominates at high pO2 (1 atm) and oxide-ionic conductivity at low pO2 (∼10−4 atm) in the temperature range 600–900 °C for both BCY and Br-doped samples. Stability was found to be poorer in CO2 for the Br-synthesised phases as determined by thermogravimetry and prolonged conductivity measurements.