The Influence of Anode Composition on the Electrochemical Ferrate (VI) Production in Molten KOH

摘要

Cyclic voltammetric and electrochemical impedance spectroscopic analysis were utilized to study aprocess of electrochemical ferrate(VI) synthesis in the molten potassium hydroxide at three anodematerials: pure iron (Fe), white cast iron (FeC) and silicon-rich steel (FeSi) electrodes. Several anodicand corresponding cathodic voltammetric peaks are observed during electrode treatment. The mostnegative anodic peaks are assigned to Fe(0)瓼e(II) and Fe(II)瓼e(III) reactions and subsequent layerreconstruction and/or transformation. Immediately after passivity region the transpassive irondissolution, including ferrate(VI) formation together with an oxygen evolution occurs and the currentpeaks are visible for all electrodes used. This peak is the best developed for Fe electrode. In the case ofthe FeC electrode, only current shoulder is visible. With increasing temperature the visibility of thispeak is getting better for all three electrodes. For all systems linear dependences of the current densityon square root of the polarization rate at low polarization rates were found indicating that the electrodereaction kinetic is controlled by the mass transport under the semi-infinitive linear diffusionconditions. At higher scan rates current densities decrease indicating that a chemical step becomes ratedetermining at high polarization rates. Impedance data confirm the physical model of the polarizedsurface based on the concept of two, inner and outer surface sublayers. For all three electrodesresistances of inner and outer layers decrease with increasing applied potential. With increasingtemperature the resistance of inner and outer layer decreases. The capacities of both layers increasewith increasing potential, i.e. layers are getting thinner and/or more disintegrated.