Hydration and oxidation kinetics of a proton conductor oxide, SrCe_0.95Yb_0.05O_2.975

Abstract

The oxidation and hydration kinetics of a proton conductor oxide, SrCe(0.95)Yb(0.05)O(2.975), were examined via conductivity relaxation upon a sudden change of oxygen activity in a fixed water-activity atmosphere, and vice versa, in the ranges of -4.0 < log a(O(2)) < or = 0.01 and -5.0 < log a(H(2)O) < -2.0 at 800 degrees C. It was found that under an oxygen-activity gradient in a fixed water-vapor-activity atmosphere, the conductivity relaxation with time is monotonic with a single relaxation time (as usual), yielding a chemical diffusivity that is unequivocally that of the component oxygen. In a water-activity gradient in a fixed oxygen activity atmosphere, on the other hand, the conductivity relaxation appears quite unusual, exhibiting an extremum after an initial transient. The conductivity relaxation upon hydration or oxidation, in general, is quantitatively analyzed in terms of two apparent chemical diffusivities for component oxygen and hydrogen, respectively. The inner workings of hydration is discussed, and the as-evaluated chemical diffusivities are reported and compared with the conventional chemical diffusivity of water.

Crystal Structure, Hydration, and Two-Fold/Single-Fold Diffusion Kinetics in Proton-Conducting Ba0.9La0.1Zr0.25Sn0.25In0.5O3−a Oxide

“…The activation energy of DH2O in this range was evaluated as 0.77(1) eV. The reported values are comparable in magnitude to the published data for doped BaCeO3 and similar materials ( [30,35] and references therein).…”

“…Consequently, in e.g., the electrical relaxation experiments conducted during change of the water vapor partial pressure in the sample's surrounding atmosphere, the respective relaxation curves should be of a single fold nature. This is in contrast to numerous literature reports showing a two-fold character of the relaxation curves recorded in specific conditions [30][31][32][33][34][35][36].…”

“…Both processes can be described, respectively, as ambipolar diffusion of protons and electronic holes, as well as ambipolar diffusion of oxygen anions and electronic holes. Furthermore, as shown in work [30], the observed change in the total conductivity (∆σ) as a function of time (i.e., in relaxation experiments) is a sum of the H-related and O-related conductivities, which proceed in opposite directions: ∆σ(H) is decreasing with time, while ∆σ(O) is increasing.…”

On the Origin of Non-monotonic, Twofold Conductivity Relaxation of BaCe0.95Yb0.05O2.975 upon Hydration/Dehydration

“…• h ) in the thermodynamic conditions examined (1,6), it is suggested that the increasing proton conductivity overrides the decreasing hole conductivity with increasing 2 H O a in the reducing atmospheres, but as the atmosphere goes more and more oxidizing, the situation is gradually reversed. examined, as shown in Fig.…”

“…1) that the conductivity relaxes always nonmonotonically to a new equilibrium value leaving an extremum (minimum for hydration and maximum for dehydration) in between. The hydration kinetics of a proton conductor oxide used to be understood as being due to the chemical diffusion of H 2 O as a whole or ambipolar diffusion of oxide ions (or vacancies) and protons (2,3).…”