THE CATHODIC REDUCTION OF OXYGEN ON URANIUM-DIOXIDE IN DILUTE ALKALINE AQUEOUS-SOLUTION

The cathodic reduction of oxygen on uranium dioxide (UO2) in dilute alkaline aqueous solutions has been investigated using rotating UO2 disc and Au ring-UO2 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. Tafel slopes of 130-280 mV decade(-1) were observed spanning 2-3 decades of current. At high overpotentials, the (transport-compensated) kinetic-limiting current invariably approached a constant value. The reaction order with respect to the O-2 concentration was determined to be unity. In perchlorate solutions, an overall four-electron reduction process was observed, with only a weak dependence on pH. In carbonate solutions, O-2 reduction currents were suppressed by up to an order of magnitude (compared with those in perchlorate) and copious amounts of peroxide were released to solution. The theory developed by Presnov and Trunov for O-2 reduction on transition metal oxides with p-type semiconductivity was adapted to interpret the results for UO2. Variations in the number density of surface donor-acceptor sites as a function of applied potential can then be invoked to explain a diverse range of experimental observations. These claims are consistent with the conclusions of previous voltammetric experiments, which showed that the degree of oxidation of the UO2 surface changes substantially over the region of Tafel behaviour for O-2 reduction.