Nonstationary Dependences during the Polarization of Liquid Metal Electrodes in Molten Salts with Circulation Cell Structures. 2. Influence of External Conditions and Sequentially Applied Current Pulses

Abstract

This work is a continuation of the studies of the nonstationary dependences (inclusion curves) recorded during the cathodic polarization of liquid electrodes made of lead, bismuth, cadmium, and aluminum in chloride melts, when the Marangoni-microeffect-induced structures in the form of vortices, i.e., circulation cells (CCs), dominate at the interface in the range from the current-free potential to the zero-charge potential. In this case, the time dependences of overvoltage η and current density i at steplike setting of the current under galvanostatic conditions or the potential under potentiostatic conditions (inclusion curves) have current or overvoltage extrema, which decrease until a steady state is formed. The values of η and i in the first extremum (η_ext, i_ext) always have the highest amplitude. The time τ_ext of reaching η_ext and i_ext and the values of η_ext and i_ext characterize the process of CC formation. τ_ext is the most convenient for analysis, since it does not depend on the concentration of electrochemically active particles. We assume that the higher the mass-transfer processes at the electrode–electrolyte interface, which is determined by the polarization conditions, the slower the CC formation and the higher the value of τ_ext. When the temperature increases, τ_ext is shown to increase, which means that the CC formation proceeds more slowly. When an external magnetic field is applied to decelerate the mass transfer, τ_ext decreases. When an electrode is immersed in a quartz holder, τ_ext also decreases. These results generally indicate the correctness of the assumption about the factors influencing the rate of CC formation. When current pulses are sequentially applied to a polarized electrode, the inclusion curves contain a well-pronounced overvoltage extremum, the magnitude of which decreases in the pulse sequence. The values of τ_ext also decrease. The existence of overvoltage extrema can be related to the energy difficulties of incorporating newly formed CCs into the structure of existing ones.