Isotopic exchange between hydrogen from the gas phase and proton-conducting oxides: Theory and experiment

Abstract

The interaction of hydrogen isotopes from the gas phase with proton-conducting oxides with the perovskite structure La1−x SrxScO3−α (x = 0; 0.04) was studied by means of the hydrogen isotope exchange with gas phase equilibration in the temperature range T = 300–800 °C and in hydrogen pressure range pH2 = 2–20 mbar.A novel kinetic model for the hydrogen isotope exchange experimental data treatment taking into account the isotopic effects was developed. This model was implemented for the obtained experimental results. The heterogeneous exchange rates of hydrogen isotopes with investigated oxides La1−xSrxScO3−α (x = 0; 0.04) were calculated. The mole fractions of hydrogen isotopes were determined for the investigated materials. It was found that deuterium saturation level is higher in comparison with protium, whereas the deuterium surface exchange coefficient for the proton-conducting oxide La0.96Sr0.04ScO3–α is smaller in comparison with the protium surface exchange coefficient. The thermodynamic isotope effect can be caused by the difference of energy of zero-point oscillations between OH- and OD-defects and molecular H2 and D2. The kinetic isotope effect can be explained by the different strength of OH and OD bonds. It is shown that the rate determining stage of hydrogen surface exchange is the process of the exchange between the forms of hydrogen in the gas phase and in the adsorption layer of the proton-conducting oxides (the stage of dissociative adsorption of hydrogen). A new statistical criterion is proposed for the first time allowing dividing the observed surface inhomogeneities caused by not only the natural surface roughness but also the presence of different isotopes of hydrogen (protium and deuterium) with different binding energies on a solid surface.The activity of the investigated proton-conducting oxides with respect to the hydrogen heterogeneous exchange is comparable to the hydrogen heterogeneous exchange activity for the oxides based on cerates and zirconates of alkaline earth metals. High catalytic activity with respect to the process of hydrogen exchange from the gas phase in reducing atmospheres allows us to consider proton-conducting oxides based on the lanthanum scandates as very promising electrolytes for numerous electrochemical applications.