Oxygen Isotope Exchange in Proton-Conducting Oxides Based on Lanthanum Scandates

Abstract

The method of oxygen isotope exchange with the gas phase equilibration have been used to obtain the temperature dependences of the oxygen surface exchange and diffusion coefficients with proton-conducting oxides La1–xSrxScO3–δ (x = 0; 0.04; 0.09) in the temperature range of 600−900°C at oxygen pressure 1.01 kPa. The paper determines that the diffusion and oxygen surface exchange coefficients increase with the increasing of the strontium content in the oxides. We have found out the rates of the individual stages of the oxygen exchange process on the surface of the oxides. It is shown that oxygen incorporation is rate-determining stage of the oxygen exchange on the surface of the undoped oxide, whereas for the strontium-doped oxides La1–xSrxScO3–δ (x = 0; 0.04; 0.09) with increasing of strontium concentration, the difference between the rates of dissociative adsorption and oxygen incorporation decreases so that for the oxide La0,91Sr0,09ScO3–δ the stage of dissociative adsorption of oxygen becomes rate-determining stage. The paper analyzes the possible reasons of these differences in oxygen surface exchange kinetics. Moreover, the paper using the obtained oxygen diffusion coefficients that have been recalculated in the oxygen-ionic conductivities according to the Nernst-Einstein equation performs the contributions of the oxygen-ion and proton components of the total conductivity of oxides La1–xSrxScO3–δ (x = 0; 0.04; 0.09) in the wet reducing atmosphere (pH2O = 2.35 kPa, pO2 = 10−15 Pa). Proton transference numbers are shown to be close to unit in the temperature range of 500–600 °С at the wet hydrogen-containing reducing atmosphere.