The electrochemical reduction of hydrogen peroxide on uranium dioxide under intermediate pH to acidic conditions

The cathodic reduction of H2O2 has been studied at rotating SIMFUEL disc electrodes in sodium chloride solutions over the pH range 1–9. The surfaces were analyzed by X-ray photoelectron spectroscopy. The mechanism and rate of reduction did not change over the pH range 4–9. Within this pH range, H2O2 chemically oxidizes the UO2 surface to UV which is subsequently electrochemically reduced. The UV state is incorporated into a surface layer of UO2+x. For pH values below 3, reduction is catalyzed by the chemical creation of an adsorbed UV state. For solutions from pH 3 to 9, currents rise to the diffusion-limited value at sufficiently negative potentials since the rate of formation of UV (in UO2+x) is sufficiently rapid, and the UV state formed is sufficiently stable, that reduction is never limited by the availability of this state. However, the UV state formed in acidic solutions (pH 1, 2) is less stable and can be further oxidized to the insulating UVI state prior to its dissolution as UO22+, or cathodically reduced at less negative potentials than required in neutral solutions. This instability limits the availability of the catalytic state and the currents are always well below the diffusion-limited value.