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

The cathodic reduction of oxygen on uranium dioxide (UO2) in dilute al kaline aqueous solutions has been investigated using rotating UO2 disc and Au ring-UO2 disc electrodes. Steady state polarization measuremen ts were made using the current-interrupt method to compensate for the potential drop caused by ohmic resistance, mainly in the electrode. Ta fel slopes of 130-280 mV decade(-1) were observed spanning 2-3 decades of current. At high overpotentials, the (transport-compensated) kinet ic-limiting current invariably approached a constant value. The reacti on order with respect to the O-2 concentration was determined to be un ity. In perchlorate solutions, an overall four-electron reduction proc ess was observed, with only a weak dependence on pH. In carbonate solu tions, O-2 reduction currents were suppressed by up to an order of mag nitude (compared with those in perchlorate) and copious amounts of per oxide were released to solution. The theory developed by Presnov and T runov for O-2 reduction on transition metal oxides with p-type semicon ductivity was adapted to interpret the results for UO2. Variations in the number density of surface donor-acceptor sites as a function of ap plied potential can then be invoked to explain a diverse range of expe rimental observations. These claims are consistent with the conclusion s of previous voltammetric experiments, which showed that the degree o f oxidation of the UO2 surface changes substantially over the region o f Tafel behaviour for O-2 reduction.