Lead Dioxide Electrocrystallization from Nitrate and Methanesulfonate Electrolytes: The Influence of Various Dopants on Initial Stages

Abstract

Properties of electrodeposited lead dioxide depend on the composition of the electrolyte and deposition conditions (1). Various ions or dispersed particles added in the electrolyte affect regularities of lead dioxide electrodeposition (2, 3). It is also known (4), that additives in the deposition electrolyte can distort the shape of crystals due to changing of the surface energy of the growing crystal faces or their incorporation into the crystal, thereby disturbing the crystallization process. In the present work we examine early stages of electrocrystallization of PbO2 from methanesulfonate electrolytes that contain various ionic additives (Bi3+, Ce3+, Sn4+, [NiF6]2-, [SnF6]2-, sodium dodecyl sulfate (SDS), colloid of TiO2) in order to get integrated data about the influence of dopants with different nature on crystallization rate constants and crystal shape. Electrodeposition regularities of lead dioxide both in nitrate and methanesulfonate electrolytes were studied on Pt disk electrode (0.19 cm2) by steady-state voltammetry, chronoamperometry. Voltammetry measurements were carried out in a standard temperature-controlled three-electrode cell. All potentials were recorded and reported vs. Ag / AgCl / KCl (sat.). According to crystallization model, proposed by Abyaneh et al. (5), two distinct growth mechanisms are considered: instantaneous and progressive growth. Current densities for both types of nucleation were calculated from current-time transients of lead dioxide electrocrystallization (6). As one can see, there is an increase of current delay, corresponding to the induction period on chronoamperometric curves in the presence of cationic additives in the deposition electrolyte. This indicates difficulties in initial stages of lead dioxide phase formation. The presence of cations in the deposition electrolyte alters the ratio between α- and β-phase crystallization constants in different amount. Thus, in the presence of the complex ion [SnF6]2- the growth of β-phase dominates. For other cationic additives the prevalence of α-phase growth is observed. It should also be noted, that the presence of complex nickel and tin fluoride ions reduces the beginning of nucleation. Most clearly this effect is observed for [NiF6]2- ion. It has been also found that surfactants are incorporated into the growing coating through adsorption on PbO2 crystals. That in turn will lead to changes in initial stages of the crystallization. It is known (7), that surfactant additives selectively adsorbed on certain faces, usually parallel to faces, reducing the growth rate of these faces, and thereby altering the shape of growing crystals. The process of coating formation of lead dioxide begins with the formation of α-phase crystals. After a certain period of time, the formation of β-phase crystals takes place. Wherein, the α- and β-phases can be formed simultaneously. Predominance in the growth of one or the other phase is determined by the ratio between the kinetic constants of the crystal growth of α- and β-phase. In all cases involved the crystallization of lead dioxide from methanesulfonate electrolytes proceeds through the progressive mechanism. A preferred form of formed crystals in the case of electrolytes, based on nitric acid is a cone, and electrolytes, based on methanesulfonic acid is a cylinder. For nitrate electrolytes there is a change in the mechanism from progressive on instantaneous at high concentrations of added surfactants. The preferred form of crystals in the presence of surfactant additives in nitrate electrolytes is semi-spheroid, and in the case of methanesulfonate electrolytes cylinder becomes the preferred form of crystals. The geometric shape of crystals is differs in the case of suspension electrolytes and depends on the polarization of the electrode. Thus, at low polarization (E=1400 mV) it is a semi-spheroid, with an increase of polarization on 50 mV change in crystal form on tapered takes place, and at high polarizations (E=1550 mV), the formation of cylinder crystals may occur.