Surface exchange kinetics and chemical diffusivities of BaZr_0.2Ce_0.65Y_0.15O_3−δ by electrical conductivity relaxation

Highlights: • Electrical conductivity relaxation in BaCe_0.65Zr_0.2Y_0.15O_3−δ was monitored. • Monotonic relaxation behavior was observed during oxidation/reduction. • Nonmonotonic twofold relaxation behavior was observed during hydration/dehydration. • Surface exchange coefficients and diffusivities of O and H were calculated. - Abstract: Perovskite-type oxide BaCe_0.65Zr_0.2Y_0.15O_3−δ (BCZY2015) was synthesized by a solid state reaction method. BCZY2015 samples were characterized by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). The time dependent variation in electrical conductivity of BCZY2015 was monitored during the oxidation/reduction in oxygen partial pressure (pO₂) range of −2.28 ⩽ log (pO₂/atm) ⩽ −0.68 at a fixed water vapor pressure (pH₂O), and during the hydration/dehydration in −3.15 ⩽ log (pH₂O/atm) ⩽ −2.35 range in air. The electrical conductivity showed a monotonic relaxation behavior by the ambipolar diffusion of V_o^·· and OH_o^· during the oxidation/reduction and the relaxation process was governed by the diffusivity of oxygen (D-tilde_vO). On the other hand, during the hydration/dehydration process, a non-monotonic twofold relaxation behavior was observed due to the decoupled diffusion of H and O components with the mediation of holes, and the conductivity relaxation process was governed by the diffusivities of both H (D-tilde_iH) and O (D-tlde_vH). The values of surface exchange coefficients and diffusivities of oxygen and hydrogen were calculated from Fick's second law by the nonlinear least squares fitting of the conductivity data, as proposed by Yoo et al. (2008)