Engineering Analysis of Shape Change in Zinc Secondary Electrodes: I . Theoretical

Abstract

Shape change, the redistribution of active material over the zinc electrode surface as a result of cell cycling, is hypothesized to be caused by convective flows driven primarily by membrane pumping. A mathematical model is formulated based on the convective flow hypothesis for the zinc‐silver oxide secondary cell. The numerical solutions predict redistribution of zinc material over the zinc electrode, fluid flow rates, and variations of current distribution and cell potential with the number of cycles. These calculated results can be compared to experimental results. The results suggest that shape change can be eliminated if the convective flow in the zinc electrode compartment parallel to the electrode surface is stopped.

Mathematical Modeling of a Primary Zinc/Air Battery

“…Therefore, an alkaline zincate solution can be reasonably treated as a ternary electrolyte consisting of KOH, K2Zn(OH)4(1), and H20. When ionic zinc species (Zn 2 § are introduced into the solution by dissolution of Zn, they may remain in the electrolyte as zincate ions or deposit as ZnO (s) Zn(OH)~-~ ZnO(s) + 2 OH-+ H20 [1] In addition, Zn(OH)~-ions can also precipitate in the form of solid K2Zn(OH)4(1) if the solution is oversaturated with K2Zn(OH)4 (9). Although the kinetics of these processes are not clearly understood, a correlation between the alkaline concentration and the solubility of zincate is well established, which is an approximate linear function of hydroxide concentration over a certain range (8).…”

“…where K is the equilibrium constant for reaction [1], a v is the initial number of moles of ZnO per unit volume of the anode, ko is a proportional constant, and (e~ -e) represents the volume of the precipitated ZnO if the amount of K2Zn(OHh precipitate is negligible compared to that of ZnO. Similarly, the precipitation rate of solid potassium zincate may be expressed as…”

“…It is assumed here that C1"~ also has a linear relation with the concentration of KOH, as shown in Table I. According to reaction [1] and the direct precipitation of potassium zincate, R~ and R~ can be written in terms of Rz~o and Rk…”

The cycling behaviour of zinc electrodes in a nickel oxide-zinc accumulator

“…The mechanism based on convective downward flows by (electro)-osmotic pumping of the membrane separator [8] does explain shape change in the y-direction (see Fig. 17), but fails to explain shape change in the z-direction.…”

“…Two different mechanisms have been proposed to explain shape change: one based on electro-osmosis (Choi et al [8]) and the other on current distribution (McBreen [9]). Neither of these models can adequately and consistently describe the observed phenomena.…”