Mechanism of formation of anodic excursion peaks on lead electrode in sulfuric acid

Potential step and linear sweep voltammetry are proposed in combination for the anodic excursion peaks (AEPs) measurement of lead in sulfuric acid during cathodic scanning in the positive potential range. The overall procedure of measurement is composed of three steps. The first one is to polarize the lead electrode with negative potential, which is necessary for passivated PbSO4(O) formation. Then, the polarization potential is stepped to positive to make sure that pure Pb could be oxidized to produce PbOn intermediate. Finally, a low-rate cathodic linear sweep voltammetry measurement is conducted in a small potential range. Four AEPs have been observed without reduction of PbO2 to PbSO4, which is in confliction with the conventional understanding that AEPs would appear accompanying with a large reduction peak for PbO2 discharge. A new mechanism is therefore proposed that AEPs corresponds to the oxidation of PbOn intermediate in the cathodic scanning, so AEPs can be used to indicate the difficulty for the oxidation of Pb to PbO2, and could be applied to characterize the corrosion resistance of lead alloy grid in the positive plate of a lead-acid battery.

► Four anodic excursion peaks (AEPs) appeared without the cathodic peaks for PbO2 discharge during the cathodic scanning, which can not be explained reasonably by existing mechanisms.
► AEPs’ formation depends on the reduction of PbSO4 to Pb and oxidation of Pb(II) to high-valent compounds, and the reduction of PbO to Pb has no effect on AEPs.
► A new mechanism is proposed that four AEPs correspond to the oxidation of PbOn intermediate compounds with different “n” values during the cathodic scanning.
► The AEPs reflects the incomplete oxidation of lead in sulfuric acid solution, so the AEPs measurements could be applied in determining the corrosion resistance of lead alloy in the positive plate of lead-acid battery.