Structural and Electrochemical Properties of Physisorbed and Chemisorbed Water Layers on the Ceramic Oxides Y2O3, YSZ, and ZrO2.

Abstract

A combination of operando Fourier transform infrared spectroscopy, operando electrochemical-impedance spectroscopy, and moisture-sorption measurements has been exploited to study the adsorption and conduction behavior of H2O and D2O on the technologically important ceramic oxides YSZ (8 mol % Y2O3), ZrO2, and Y2O3. Because the characterization of the chemisorbed and physisorbed water layers is imperative to a full understanding of (electro-)catalytically active doped oxide surfaces and their application in technology, the presented data provide the specific reactivity of these oxides toward water over a pressure-and-temperature parameter range extending up to, e.g., solid-oxide fuel cell (SOFC)-relevant conditions. The characteristic changes of the related infrared bands could directly be linked to the associated conductivity and moisture-sorption data. For YSZ, a sequential dissociative water ("ice-like" layer) and polymeric chained water ("liquid-like") water-adsorption model for isothermal and isobaric conditions over a pressure range of 10(-5) to 24 mbar and a temperature range from room temperature up to 1173 K could be experimentally verified. On pure monoclinic ZrO2, in contrast to highly hydroxylated YSZ and Y2O3, a high surface concentration of OH groups from water chemisorption is absent at any temperature and pressure. Thus, the ice-like and following molecular water layers exhibit no measurable protonic conduction. We show that the water layers, even under these rather extreme experimental conditions, play a key role in understanding the function of these materials. Furthermore, the reported data are supposed to provide an extended basis for the further investigation of close-to-real gas adsorption or catalyzed heterogeneous reactions.