Transport Properties of Fluorite-Type Ce_0.8Pr_0.2O_2−δ: Optimization via the Use of Cobalt Oxide Sintering Aid

Dense Ce 0.8 Pr 0.2 O 2−δ ceramic membranes with submicron grain size can be formed at 1000 °C by minor additions of cobalt oxide. X-ray energy-dispersive spectroscopy shows the additive to be located in the grain boundary. Although a very fine grain interface is obtained for the composition containing 2 mol % cobalt oxide, pronounced grain boundary layers and Co-rich fringes are noted at 5 mol %. The cobalt oxide additions enhance electronic conductivity by around 2−3 times. For 2 mol % additions, no change to the level or nature of ionic conductivity is observed, whereas at 5 mol %, a depleted ionic conductivity is noted at lower temperatures. Coulombic titration studies show the bulk Pr oxidation state to be unaffected by the additions. Jointly, these results prove that the Co additions are not accommodated in the bulk material but instead form additive-rich grain boundary networks that are electronically conductive. Materials essentially free from oxygen surface exchange limitations are produced on addition of 2 mol % cobalt oxide, in contrast to that noted for 5 mol %. The combination of enhanced ambipolar conductivity and enhanced oxygen surface exchange kinetics boosts oxygen permeability in 2 mol % cobalt oxide doped Ce 0.8 Pr 0.2 O 2−δ to offer one of the highest levels of oxygen permeability reported to date for a single component mixed conducting fluorite material.