The cathodic reduction of oxygen on n-type UO 2 in dilute alkaline aqueous solution

Abstract

The cathodic reduction of O 2 on ceramic UO 2 that exhibits an n-type photocurrent response has been studied in dilute alkaline aqueous solution using rotating disc and ring-disc electrodes. Steady state polarization measurements were made using the current-interrupt method to compensate for the potential drop caused by ohmic resistance, mainly in the electrode. Overall, four-electron reduction to OH − has been shown to predominate in perchlorate solution, with only a slight pH dependence. Although the reaction order with respect to the O 2 concentration in solution has been determined to be near square-root under these conditions, dissociative adsorption of O 2 appears very unlikely. Instead, the same basic reaction mechanism as proposed for p-type UO 2, involving consecutive single electron and proton transfer steps in a classic series route, is thought to apply. Reaction is presumed to occur exclusively at active sites formed by uranium ions in different valence states, after the theory originally developed by Presnov and Trunov for transition metal oxides with p-type semiconductivity. The near square-root reaction order, as well as unusually large Tafel slopes, can then be interpreted on the basis that the first electron-transfer step is tightly coupled with O 2 adsorption under Temkin conditions. This treatment follows ideas previously developed by Calvo and Schiffrin to explain similarly anomalous kinetics for O 2 reduction on passive iron in alkaline solution. In the present case, the observed behaviour is thought to reflect microscopic heterogeneity in the degree of hyperstoichiometry of the UO 2 + x electrode. Pronounced effects caused by gentle surface oxidation and the presence of carbonate in solution are consistent with the reaction scheme outlined.