The Lead Anode in Alkaline Solutions. II. The Mechanism of PbO Film Formation

The mechanism of the initial stages of PbO formation has been investigated at a Pb anode in 1M NaOH solutions, with the use of cyclic voltammetry, transient and steady-state potential step methods, and modulated surface reflectance spectroscopy (MSRS). It has been found that, following the deposition and dissolution of Pb(OH) sub 2 at less positive potentials, PbO deposition initiates according to an instantaneous three-dimensional nucleation and growth mechanism under diffusion control. This is supported by the good match of the experimentally obtained i--t response with the theoretically predicted one, by the observed changes in surface reflectivity with time and the marked effects of electrode rotation on the observed currents. Some dissolution of PbO also occurs in these early stages of oxidation, forming the HPbO sub 2 md species, consistent with the observed Tafel slope and the excess of the anodic to cathodic charge in cyclic voltammetry. At still higher potentials or longer times, very thick porous PbO films can be formed, consistent with film initiation by a nucleation and growth mechanism. 16 ref.--AA