Electrochemical behaviour of galena (PbS) in aqueous nitric acid and perchloric acid solutions

The electrochemical reactions of natural n-type PbS in nitric and perchloric acid solutions have been studied using cyclic voltammetry, SEM and XPS. Several oxidation stages, which differ in their electrode kinetics, surface topography and sulfide phase composition, particularly Pb/S ratios and the state of sulfur, were demonstrated. The quantities of surface sulfate and thiosulfate were rather low at PbS oxidized in HNO₃ solutions, even at high potentials. The magnitude of the reoxidation peaks of cathodically formed products, primarily Pb⁰, was found to increase as the electrodissolution of PbS on the preliminary positive-going sweep was hindered. It is proposed that the predominant donor-like defects in the disordered reaction layers limit the concentration and mobility of holes, thus inhibiting PbS oxidation and promoting metallic lead deposition in the course of potential cycling. The model developed as an alternative to the traditional explanations uses the concept of negative correlation energy centres resembling those in non-crystalline chalcogenide semiconductors.