Influence of the exchange current density and overpotential for hydrogen evolution reaction on the shape of electrolytically produced disperse forms - Review

Abstract

In this study, comprehensive survey of formation of disperse forms by the electrolysis from aqueous electrolytes and molten salt electrolysis has been presented. The shape of electrolitically formed disperse forms primarily depends on the nature of metals, determined by the exchange current density (j₀) and overpotential for hydrogen evolution reaction as a parallel reaction to metal electrolysis. The decrease of the j₀ value leads to a change of shape of dendrites from the needle-like and the 2D fern-like dendrites (metals characterized by high j₀ values) to the 3D pine-like dendrites (metals characterized by medium j₀ values). The appearing of a strong hydrogen evolution leads to formation of cauliflower-like and spongy-like forms (metals characterized by medium and low j₀ values). The other disperse forms, such as regular and irregular crystals, granules, cobweb-like, filaments, mossy and boulders, usually feature metals characterized by the high j₀ values. The globules and the carrot-like forms are a characteristic of metals with the medium j₀ values. The very long needles were a product of molten salt electrolysis of magnesium nitrate hexahydrate. Depending on the shape of the disperse forms, i.e. whether they are formed without and with vigorous hydrogen evolution, formation of all disperse forms can be explained by either application of the general theory of disperse deposits formation or the concept of "effective overpotential". With the decrease of j₀ value, the preferred orientation of the disperse forms changed from the strong (111) in the needle-like and the fern-like dendrites to randomly oriented crystallites in the 3D pine-like dendrites and the cauliflower-like and the spongy-like forms.