A rotating disc electrode study of the electrodeposition of zinc from alkaline zincate solutions

Abstract

Separate zinc deposition and hydrogen evolution current density versus potential curves have been measured for electrodeposition from alkaline electrolytes similar to those used in zinc/alkali batteries. Rates of mass transport of zincate were controlled using a rotating disc electrode and by varying zincate concentrations in 3 M KOH. The results indicate that zinc deposition is always mass transport controlled and consequently that deposits are always dendritic. In contrast hydrogen evolution can be very strongly potential dependent. The results suggest that at low overall current densities two surfaces develop on the depositing zinc. The tips of a low surface density of long dendrites form an outer low area surface of points of very high current density. Hydrogen bubbles form on these points but are too few and too far away from the much larger, smoother inner surface significantly to increase mass transport to it by stirring. Instead the long dendrites act as protrusions that reduce, mass transport to the inner surface. Long dendrites grow preferentially perpetuating the two surface structure and amplifying the differences in deposition conditions on the two surfaces: conditions that cause “dendrite growth” battery failures. High overall current densities favour the nucleation of dendrites rather than their growth. This leads to a single surface deposit formed by the tips of a high surface density of short dendrites. The hydrogen bubbles formed on these tips not only stir the solution and enhance mass transport but also tend to smooth the concentration profiles in the plane of the tip surface. Hence at high current densities, gas evolution stirring favours the maintenance of a single planar surface of short dendrite tips. Furthermore, conditions have been found in which this surface appears to grow in area: the opposite of densification. The present results suggest that some hydrogen evolution may be beneficial in a zinc / alkali battery and that conditions can be found where gas stirring encourages the formation of stable planar high surface area dendritic zinc deposits that would not density nor grow long dendrites in a battery.