I.3. Physico-chemical behaviour of the Pb/tet-PbO/PbSO₄/H₂SO₄ electrode

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 PbO_n. The PbO_n layer has higher electron conductivity and the rate of the reaction of oxygen evolution increases. Consequently, the degree of oxidation of the PbO_n 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.

References

1. D. Pavlov, S. Zanova, G. Papazov, Photoelectrochemical pro­perties of the lead electrode during anodic oxidation in sulphuric acid solution, J. Electrochem. Soc., 124 (1977) 1522.
2. D. Pavlov, Processes in solid state at anodic oxidation of a lead electrode in H₂SO₄ solution and their dependence on the oxide structure and properties, Electrochim. Acta, 23 (1978) 845.
3. D. Pavlov, Semiconductor mechanism of the processes during electrochemical oxidation of PbO to PbO₂, J. Electroanal. Chem. 118 (1981) 167.
4. D. Pavlov, Z. Dinev, Mechanism of the electrochemical oxidation of lead to lead dioxide electrode in H₂SO₄  solution, J.Electrochem.Soc., 127 (1980) 855
5. M. Dimitrov, K. Kochev, D. Pavlov, The effect of the thickness and stoichiometry of the PbO layer upon the photoelectric properties of the Pb/PbO/PbSO₄/H₂SO₄ electrode, J. Electroanal. Chem., 183 (1985) 145
6. B. Monahov, D. Pavlov, Hydrated structures in the anodic layer formed on lead electrode in H₂SO₄ solution, J. Appl. Electrochem., 23 (1993)1244
7. D. Pavlov, S. Zanova, M. Maja, E. Angelini, Influence of Ag, Ca and Sn ions on the photoelectrochemical properties of the lead electrode, 31 Meeting of ISE, Extended Abstracts, Vol.2, p. 609, Venice, Italy, 1980.
8. D. Pavlov, S. Zanova, B. Monachov, M. Maja, E. Angelini, Electrochemical oxidation of Pb alloys with Ag, Sn and Ca to PbO₂, 32 Meeting of ISE, Extended Abstracts, vol. 1, p. 142, Dubrovnik, Jugoslavia,1981.
9. D. Pavlov, Z. Dinev, Dependence of the lead electrode photo-electrochemical properties on the oxidation potential in H₂SO₄ solution, 29 Meeting of ISE, Part I, Extended abstracts, p. 455, Budapest, Hungary, 1978.
10.  B. Monahov, D. Pavlov, Influence of Antimony on the Amorphyzation of the Corrosion Layer, Proceedings of the International Conference LABAT '93, 7-11 June 1993, Varna, Bulgaria p. 61
11. M. Bojinov, D. Pavlov, Catalytic Effect of Bi on the Formation of Lead (IV) Oxides on Pb-Bi Alloys in H₂SO₄ Solutions, Proceedings of the International Conference LABAT '93, 7-11 June 1993, Varna, Bulgaria p. 72

Keywords: electrochemical oxidation of lead, tet-PbO layer, photo-electrochemical properties of Pb/PbO/PbSO₄ electrode, semi-conducting mechanism of photo-oxidation, PbO_n, non-stoichiometric lead oxide