Kinetics of the anodic reaction occurring during the electrolysis of molten nitrates on platinum electrodes at different temperatures

Abstract

The kinetics of the anodic reaction occurring during the electrolysis of molten nitrates on bright platinum electrodes has been studied. The kinetic parameters were obtained from conventional current/potential curves, build-up of electrode potential at constant current density and decay of anodic overpotential, working at temperatures between 220 and 470°C. The current density was varied between 1 and 500 mA/cm². In the region 220–290°C the Tafel slope approaches 2RT/F as given by the current/potential curves, but for decay curves, when overpotential is plotted against log t, two straight line portions, one with slope 2RT/F and the other with slope RT/F were observed. The transition from one straight line to the other occurs abruptly. The exchange current densities for both portions were calculated. These results are explained considering a consecutive scheme of reaction where at least two steps differently dependent on electrode potential can be rate-controlling in the mechanism of reaction. In the range between 350°C and above, the Tafel slope is clearly RT/F, independent of the method of measurement. The exchange current density was also determined and the mechanism of the reaction interpreted in terms of the increase of oxide-ion concentration with temperature and the discharge of these ions on platinum electrodes.