Mechanism of the Elementary Electrochemical Processes Taking Place during Oxygen Evolution on the Lead Dioxide Electrode

Abstract

When a Pb/ electrode immersed in solution is subjected to polarization in the lead dioxide potential region (φ > 1.0 V vs. Hg/ reference electrode), is decomposed releasing oxygen. The aim of this investigation is to elucidate the mechanism of the reactions taking place on oxygen evolution. Linear‐sweep‐voltametric cycling according to various cycling programs has been performed, and the structure of the anodic layer has been examined through scanning electron micrscopy and x‐ray diffraction. It has been established that at potentials in the region 1.0 < φ < 1.3 V the electrode has passive behavior (i.e., only a weak current passes through it), and in the range 1.3 > φ > 1.6 V extensive oxygen evolution is observed (active potential zone). This oxygen evolution is a result of two consecutive electrochemical reactions. While the first reaction proceeds at (φ > 1.0 V and leads to the formation of OH radicals, the second takes place at (φ > 1.3 V. It is assumed that these reactions proceed in the hydrated layer of the lead dioxide. Both reactions are localized in a certain number of active centers in the hydrated layer. At (φ < 1.3 V, the products of the first electrochemical reaction block these active centers and hence the current decreases significantly. At φ > 1.3 V, the second electrochemical reaction proceeds, as a result of which oxygen is evolved due to oxidation of the OH radicals and consequent unblocking of the active centers. The electrode is activated, and the reaction resistance is the dominant rate‐limiting factor. The present contribution proposes a mechanism of the elementary processes that occur on oxygen evolution in light of the gel‐crystal structure of the layer. This mechanism involves the hydrated polymer chains in the gel layer.