Isotope exchange studies of electrochemical systems with solid oxide electrolytes

Abstract

It has been shown that electronic conductivity of oxides is one of the main factors affecting the rate of interphase exchange of oxygen: these compounds, except ZrO₂-based electrolytes with neglibible electronic conductivity, exchange with gas fairly actively (10¹⁶–10¹⁸ molecule/(cm²s) at 1000 K). It is possible to increase the exchange rate for ZrO₂-based electrolyte by a factor of 10²–10³ by modifying its surface with electrode materials - metals (Pt, Ag) and oxides (PrO_2−y, La_0.7Sr_0.3CoO_3−y). It has been shown that electrochemical exchange current I₀ and the rate of isotopic exchange of oxygen R are equal (in the same units) in electrode system O₂, Pt/ZrO₂-based electrolyte. It is due to the same reaction path connected with the Pt surface in both cases. In the electrode systems with electrolytes based on Bi₂O₃ and CeO₂, the rate R > I₀ because both the exchange on the Pt surface and the exchange on the electrolyte-gas interphase make contributions to the interphase exchange. A theoretical model have been proposed to describe the isotope exchange kinetics between gas and oxides. It makes it possible to determine not only the rate of interphase exchange and diffusivity of oxygen in oxides but also to analyze the contribution to the kinetics from various type of surface exchange taking diffusion into account. This method has been used for description of the oxygen exchange in a number of oxides (YBa₂Cu₃O_7−y, La_0.7Sr_0.3CoO_3−y, high-temperature protonic conductors CaTi_1−xFe_xO_3−y, YLa_1−xCa_xO_3−y).