Stability of BaCeO3-based proton conductors in water-containing atmospheres

The stability of Gd-doped and La-doped BaCeO3 in water-containing atmospher es was examined. Samples containing up to 20 mol % Gd and 30 mol % La were synthesized by calcining the requisite mixtures of precursors. Three types of tests were performed: (i) thermal treatment in an atmosphere containing water vapor (p(H2O) = 0.0964 atm) between 150 and 400 degrees C, (ii) boili ng in water, and (iii) simultaneous exposure to boiling water and water vap or at essentially the same temperature. Samples were characterized by X-ray diffraction. All powders boiled in water decomposed within a few hours, wh ereas none of the powders decomposed when exposed to similar to 1 atm water vapor pressure at the same temperature. This difference was rationalized o n the premise that the mechanisms of decomposition depend upon the form of water, liquid or vapor. In liquid water, the decomposition kinetics appear to be interface-controlled. By contrast, in water vapor, the kinetics appea r to be dictated by bulk diffusion of water into the lattice. (C) 1999 The Electrochemical Society. S0013-4651(98)08-072-0. All rights reserved.