Mechanism of the Electrochemical Oxidation of Lead to Lead Dioxide Electrode in H₂SO₄ Solution

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The electrode system Pb/PbO layer/PbSO₄ membrane/H₂SO₄ solutionforms when a lead electrode immersed in an H₂SO₄ solution is in the potentialrange between –400 and +960 mV (vs. the Hg/HgSO₄ electrode). When theelectrode potential is raised above +960 mV the PbO and PbSO₄ layers areoxidized to PbO₂. The mechanism of this oxidation is determined by the electricproperties and the crystal structure of PbO and PbSO₄. The paper dealswith the electrochemical behavior of the electrode during potential sweepsfrom the PbO into the PbO₂ potential ranges. It was found that first PbO isoxidized via solid-phase reactions to nonstoichiometric PbO_n without a changeof its crystal structure. This takes place via the electrochemical reaction

which proceeds at the PbO/solution interface. Since PbO_n is a semiconductor,the oxidation of the Pb²⁺ ions in its crystal lattice probably takes placethrough a mechanism in which surface states and mobile acceptors take part.Such are O^– and O, evolved during the reaction of oxygen evolution. When nreaches a critical value n = d, -PbO₂ nucleation commences in the pores ofthe PbSO₄ membrane. When this phase reaches the H₂SO₄ solution the oxidationof PbSO₄ crystals to -PbO₂ begins. This takes place according to themechanism

The dissolution of PbSO₄ crystals begins at the apexes of the crystals and continuesat their edges and faces.

©1980 The Electrochemical Society, Inc.