Charge transfer at oxygen/zirconia interface at elevated temperatures: Part 9: Room temperature

Abstract

The present paper reports the reactivity between the surface of zirconia and oxygen at room temperature. The experimental studies, performed with polycrystalline yttria-stabilised zirconia (YSZ), include in situ monitoring of charge transfer at the oxygen/zirconia interface using work function measurements and determination of mass transport at the oxygen/zirconia interface using the incorporation of oxygen isotope 18 O and its analysis using secondary ion mass spectrometry. Oxidation of YSZ at room temperature is found to result in oxygen chemisorption, leading to the formation of singly ionised molecular oxygen species, and subsequent oxygen incorporation into the lattice. Oxygen incorporation takes place by lattice diffusion from the entire surface exposed to the gas phase, resulting in oxygen penetration of depths of the order of 2 nm over 60 days, and diffusion of oxygen species along the surface from the adsorption sites towards grain boundaries with subsequent rapid diffusion of oxygen into the bulk along grain boundaries. These processes take place in the absence of an electronic conductor. The present findings contradict the widely held perceptions that YSZ at room temperature is quenched and not reactive, and that oxygen incorporation into YSZ requires the presence of a triphase boundary (TPB) formed by the gas phase (oxygen), electronic conductor and oxygen ionic conductor (zirconia).

Effect of Segregation on Surface and Near-Surface Chemistry of Yttria-Stabilized Zirconia

“…The studies of Nowotny et al − have shown that these two assumptions are in question. First, it has been documented that oxidation of YSZ does not require the presence of an electronic conductor .…”

“…It has also been commonly assumed that the presence of an electronic conductor attached to the surface of YSZ, such as platinum, is essential for oxygen incorporation from the gas phase into the YSZ lattice. However, the studies of surface semiconducting properties of YSZ indicate that (i) oxygen may react with the surface of YSZ without the presence of an electronic conductor and (ii) the charge transport within the YSZ-based electrochemical devices is determined by the electric field induced by gradients in chemical composition in the outermost surface layer rather than bulk diffusion. − In other words, it has been documented that, while YSZ is a poor electronic conductor, the local concentration of electronic charge carriers at the surface is high enough to allow the charge transfer during oxidation of YSZ in the absence of an electronic conductor.…”

Charge transfer at oxygen/zirconia interface at elevated temperatures: Part 2: Oxidation of zirconia

“…54,69 In addition, the present study shows that oxidation of electrodeless YSZ results in oxygen chemisorption and oxygen incorporation. [6][7][8][9][10] The preceding evidence leads to a general model as depicted in Fig. 5 (this is an analogue of Fig.…”

Charge transfer at oxygen/zirconia interface at elevated temperatures: Part 12: Summary

“…The present study has therefore determined the effect of temperature on the kinetics of the reactivity of YSZ with oxygen in terms of oxygen chemisorption and oxygen incorporation. 6,8,9 It has been shown that oxygen incorporates into the lattice in the entire temperature range studied (300-1173 K). The results obtained allow the effect of temperature on oxygen chemisorption to be established in terms of the stability of specific oxygen species and their surface coverage, and the determination of oxygen chemisorption isobars.…”

“…5,6 It has also established the effect of surface properties and temperature on reactivity between YSZ and oxygen in terms of the electrical effects accompanying both processes, namely oxygen chemisorption and oxygen incorporation. [7][8][9][10][11] Effect of temperature on reactivity It is well known that the performance of YSZ depends critically on temperature. The present study has therefore determined the effect of temperature on the kinetics of the reactivity of YSZ with oxygen in terms of oxygen chemisorption and oxygen incorporation.…”