Surface reactivity and cation non-stoichiometry in BaZr_1−xY_xO_3−δ (x = 0–0.2) exposed to CO₂ at elevated temperature

The reactivity of BaZr_1−xY_xO_3−δ (x = 0–0.2) ceramics under 1 atm CO₂ at 650 °C for up to 1000 h was investigated in order to elucidate possible degradation processes occurring when the material is applied as a proton-conducting electrolyte in electrochemical devices. The annealed ceramics were characterized by a range of techniques (SEM, TEM, GIXRD, XPS and SIMS) with respect to changes in the phase composition and microstructure. Formation of BaCO₃ was observed on the surfaces of the annealed samples and the amount increased with time and was higher for the Y-doped compositions. The subsurface regions were found to be deficient in Ba and, in the case of the Y-doped compositions, enriched in Y in two distinct chemical states as identified by XPS. First-principles calculations showed that they were Y residing on the Zr and Ba-sites, respectively, and that local enrichment of Y both in bulk and on the surface attained a structure similar to Y₂O₃. Overall, it was substantiated that the reaction with CO₂ mainly proceeded according to a defect chemical reaction involving transfer of Y to the Ba-site and consumption of BaZrO₃ formula units. It was suggested that a similar degradation mechanism may occur in the case of Ba(OH)₂ formation under high steam pressure conditions.