On continuous polarization of the lead electrode within the Pb/PbO/PbSO4 potential region (from –0.40 V to +0.95 V vs. Hg/Hg2SO4), a layer of tet-PbO is formed between the metal surface and the PbSO4 layer. Oxidation of lead continues via transfer of oxygen vacancies through the PbO layer. It has been established that, due to the tet-PbO layer, the Pb/PbO/PbSO4 electrode attains photo-electrochemical properties. As a result of the photo-electrochemical processes that occur, PbO is oxidized to a non-stoichiometric oxide PbOn (1<n<2) with higher electro- and photo-conductivity. A semi-conducting mechanism has been proposed for the processes of photo-oxidation of PbO.
At the interface PbO/solution in the pores of the PbSO4 membrane layer a photo-electrochemical reaction of H2O oxidation proceeds, as a result of which oxygen is evolved. The latter penetrates into the PbO layer and oxidizes it to PbOn. The PbOn layer has higher electron conductivity and the rate of the reaction of oxygen evolution increases. Consequently, the degree of oxidation of the PbOn layer increases, too. The influence of alloying additives (Sb, Sn, Ag, etc.) on the above processes has been investigated at the LABD.
Researchers from USA, Canada, England, Russia and Australia have confirmed the above photo-electrochemical behaviour and the mechanism of the processes of oxidation of the Pb/PbO/PbSO4 electrode system.
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Keywords: electrochemical oxidation of lead, tet-PbO layer, photo-electrochemical properties of Pb/PbO/PbSO4 electrode, semi-conducting mechanism of photo-oxidation, PbOn, non-stoichiometric lead oxide