Closure to “Discussion of ‘Passivation of Zinc Anodes in  KOH  Electrolytes’ [M.‐B. Liu, G. M. Cook, and N. P. Yao (pp. 1663–1668, Vol. 128, No. 8)]”

Abstract

of the zincate ion. 82 Furthermore, this calculated diffusion rate is too high because in making the calculation it was assumed that the concentration of zincate ions at the electrode was determined by the total number of zincate ions produced during the entire discharge process. These calculations show that the zincate ions do not diffuse away from the electrode surface as fast as they are formed. As a result, l~he concentration of zincate ions at the electrode surface builds up. The well-known ability of aqueous KOH to become supersaturated with zincate ions allows these accumulated zincate ions to remain in solution rather than precipitate, A similar calculation can be made with respect to the experimental discharge cited above. In that discharge 0.064 tool of zincate ions was formed at the electrode surface. The rate of diffusion needed to clear the electrode surface of these zincate ions was 4.3 X 10 -7 tool cm-~ sec -i while the calculated maximum rate of diffusion was 4.8 X 10 .9 mol cm-2 sec-1. Thus the diffusion of zincate ions away from the electrode surface was about 0.01 of what was needed to clear the electrode surface so that OH-ions could have access to the zinc for continuing the discharge reaction.One conclusion of all this is that while passivation of the zinc electrode in many cases may be due to the formation of Type I or Type II ZnO on or at the electrode surface, there are also situations in which passivation occurs without the formation of such a film. 38 This probably corresponds to ta in the paper and is due to the fact that the reaction products accumulate at the electrode surface and prevent access of OHions to the electrode surface.No one of these mechanisms will describe the passivation of the zinc electrode. This passivation is determined by local conditions that vary from place to place on the electrode surface and that may be quite different from average values that are used in analyses such as these.M:-B. Liu, G. M. Cook, and N. P. Yao: 34 Much of what Dirkse discusses is in complete agreement with the thinking that went into the development of our proposed scheme for the processes leading to the formation of the passivation film on zinc anodes. His primary concerns seem to be (i) our apparent specification of the film composition as Type I or Type II ZnO, and (if) the possibility that the passivation process is related to a limited rate of OH-diffusion across a boundary layer.We had hoped to avoid the apparent specification of film composition through our statements, in the second paragraph of the paper, that the composition was indeed unknown. In our scheme, the term ZnO was used only for simplicity. The logic of the scheme depends upon (i) the formation of soluble zincate species from the combination of zinc atoms with excess OH-, (ii) the precipitation of a zinc oxide-hydroxide with the release of the excess OH-, and finally (iii) the direct formation of insoluble zinc oxide-hydroxide species from the combination of zinc atoms with stoichiometric amounts of OH-. In practice, t...

Utilization of Hyper-Dendritic Zinc during High Rate Discharge in Alkaline Electrolytes

“…density (mA/cm 2 ), which decreases the k parameter in 12. 16 Therefore, as expected, the HD Zn performs significantly better at higher absolute currents, given its order of magnitude higher surface area, and correspondingly lower surface-area-normalized current density.…”

“…Several relationships have been proposed to link passivation time to current. 16 For diffusion-dominated mechanisms, the supersaturation of zincate, the diffusion of OH − through the porous ZnO passivation layer, or a combination of both, limit the time to passivation. For a diffusion dominated process, the relationship between current and time to passivate is generally of the form:…”

“…In this passivation-controlled case, it is t pass that determines utilization, whereas in the corrosion case, the parasitic current acts concurrently with discharge to reduce utilization. In general terms, this implies that the greater the particle surface area, the longer the time will be until passivation, 16 as the porous type I ZnO passivation layer that forms over the zinc surface will have a reduced thickness for higher surface area zinc, reducing diffusion limitations. Figure 5 shows this passivation effect (red dot-dash line).…”

“…Therefore, passivation effects are expected to be felt more rapidly at higher current densities (i.e., high current per unit surface area). 14,16 It follows, then, that high surface area electrodes are beneficial for minimizing passivation and maximizing power output.…”

Galvanostatic Growth of Passivating Films Under Transient Conditions. I. Model and Quantitative Analysis for the Zn/ZnO System

“…From a galvanostatic point of view, there are studies in sulfuric acid [22], in alkaline solutions [23][24][25][26][27][28], sodium carbonate and bicarbonate solutions [29] and phosphate buffer solutions [30,31] and even in sodium borate solutions [32].…”