Reversibility of electrosurface transfer through eutectic interfaces of MEWO4|WO3 (Me-Ca, Sr, Ba)

Abstract

It is found that electrosurface transition (EST) through eutectic interfaces of (-/+)WO3|MeWO4(+/-) induced by electric field is a reversible process. In the case of the (-/+) polarity, nominally, in the "direct experiment", macro amounts of WO3 from a WO3 (-) brick are drawn in the (+) direction onto the inner surface of MeWO4 forming a two-phase {WO 3MeWO4} composite. Simultaneously, nonequivalent countertransport of Me2+ within the WO3 (-) brick occurs, which changes the color of WO3 (-) from the natural hue to dark-green. Intercalation of Me2+ into WO 3 (-) is proved by several spectroscopic methods. The key role in the EST phe-nomenon belongs to a nonautonomous electrolytic phase of MeW-s formed on the contact interface with WO3|MeWO4. The composition of MeW-s is close to W/Me ≈ 2. As a result of EST, the cell acquires a more complicated structure: (Equation presented) Ultimately, WO 3 is transported in the (+) direction (into the composite) and Me2+ penetrates under the effect of the gradient in chemical potential into WO3 (-) forming a dark-green MexWO 3 phase with its front reaching the (-) Pt electrode. After the end of the "direct experiment", the cell polarity was changed to (+/-) and the "reverse experiment" was carried out. Now, on the cathodic boundary |4 of subcell (+){WO 3MeWO4}MeWO 4 3|4WO3|Pt(-) anions (WO4)2-are generated that are discharged on boundary 3| to oxide WO3 that is intercalated into the right boundary of MeWO4 - (3), where the rightmost composite region {WO 3MeWO4} is formed. Thus, the mass of W decreases; it becomes dark-green (see above) and the mass of the MeWO 4 disk continues growing and now its structure is as follows ( MeWO4 ⋮MeWO4 ⋮MeWO4 ). It is important that the left WO3 (+) disk that was dark-green after the "direct experiment" gradually becomes lighter in the "reverse experiment" up to its natural pale green color, i.e., Me 2+ is deintercalated from it: Me2+: MexWO3 + 1/2O2 → xMe2+ + 2e + WO3. It is found that dependences of variations of disk masses Δm(Q) practically coincide for the "direct" and "reverse" experiments. © Pleiades Publishing, Ltd., 2012.