Barium zirconate-based perovskites (BZY) are intensely studied proton conductors for high-temperature electrochemical applications; however, their refractory nature means excessive temperatures are required for sintering (~1700 °C). Although, different strategies have...
A detailed analysis of the electrical properties of the proton-conducting BaCe0.8Y0.2O3- phase (BCY20) and the effects of Br doping has been undertaken. Members of the system of nominal stoichiometry BaCe0.8Y0.2O2.9-(x/2)±Brx (x = 0, 0.05, 0.1 and 0.2) were synthesised by a citrate-nitrate process and high-temperature annealing up to 1500 °C. Xray diffraction revealed the formation of single-phase material with a monoclinically distorted perovskite structure (space group I2/m) with greater distortion for Brsynthesised 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 ºC 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, 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 ºC for both BCY and Br-doped samples. Stability was found to be poorer in CO2 for the Brsynthesised phases as determined by thermogravimetry and prolonged conductivity measurements.
Synthesis of the proton-conducting electrolyte SrZr 0.9 Y 0.1 O 3-δ (SZY) was undertaken by the sol-gel method employing an all-alkoxide route from reaction of strontium alkoxide produced in-situ and commercial zirconium and yttrium alkoxides. The solution was homogenized by a previous ligand exchange in 2-methoxyethanol to control the polycondensation rate and achieve SZY at the low final firing temperature of 700-800 °C. SZY thin films (~ 270 nm) were prepared by dip-coating on different substrates and characterised by scanning and transmission electron microscopy, grazing X-ray diffraction and confocal micro-Raman spectroscopy, revealing well crystallized SZY phase with orthorhombic symmetry (space group, Pnma). Impedance spectroscopy of a thin film deposited on a quartz substrate revealed that protons contribute to transport in wet conditions as confirmed by a lower conductivity in D 2 O-humidified air (1.02 eV) compared to H 2 O-wetted air (0.99 eV), with the difference in activation energy consistent with a conductive isotope effect.
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