The effect of silicate ion addition on the dissolution of zinc in alkaline solution

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COMMUNICATION

THE EFFECT OF SILICATE ION ADDITION ON THE DISSOLUTION OF ZINC IN ALKALINE SOLUTION R. D. ARMSTRONG Electrochemical

Research

and M. F. BELL

Laboratories, Department of Physical Chemistry, University Newcastle upon Tyne NE1 7RU, England (Received

6 March

Recently it has been reported[i, 21 that the charge necessary for passivation of a zinc/air cell containing approximately 3 M potassium hydroxide with the addition of 2% v/v potassium silicate solution is infinite, so that complete utilization of the zinc electrode in a power source application can be achieved[3]. Hampson[l] has suggested that this phenomenon is due to a silicate-rich layer, at the electrode surface, in which the zincate ion is more soluble. In this communication we report measurements designed to test this hypothesis using techniques described previously[4]. Both in 3 M and 1 M KOH, the effect of the addition of silicate ions (potassium silicate solution, BDH, s.g. 1.33) was to lower the steady-state current at any potential as shown in Fig. 1 (A and B). By analysis of the rotation speed dependence in a manner described previously[4], it was found that on the addition of silicate ions, the Tafel region moved to more anodic potentials (Fig. 2), which

of Newcastle upon Tyne,

1975)

reflects the decrease in current. However, no appreciable change in Tafel slope could be detected. This suggests that the mechanism of dissolution does not depend on the silicate ion and can therefore be written as[4] Zn(0) * Zn(I),,,

(1)

Zn(I),,,:

(2)

Zn(II).

The electrode impedance was measured in 1 M KOH as a function of potential and silicate ion concentration. An example of the silicate ion concentration dependence is shown in Fig. 3. Similar behaviour was also observed at all potentials. Calculations of the anodic Tafel slope Cram Ihe zero-frequency resistance. using equation (3). showed these results to be consistent with those obtamed from the steady state measurements Tafel slope = 2.303 i,,. Rde

4

(3)

iilicote free

._‘

E.

Fig.

1. Steady-state

current/potentials curves as function of silicate rotation speed of 53.3 Hz: A-3 M KOH; B-l 1.55

ion concentration M KOH.

V

(% v/v) at a

156

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COMMUNICATION

Table

r

1. Results

of the viscosity

Observed time (=I

Solution E i..

Water 3M KOH 3 M KOH/2%

a E \ .40

* Obtained

i

II-1.25

I

-1z.5

-I 30

E.

V

Fig. 2. The effect of silicate ion addition on the anodic Tafel slope in 1 M KOH at a rotation speed of 53.3Hz. 0 Silicate free; 0 1%; A 2%; A 4%.

experiments

v/v KZSiO,

379 435 489

in 3 M KOH

Density k/ml)

Viscosity (CP)

0.997 1.127 1.13

0.890* I.15 I .29

from tab&

These results show that the effect of silicate ion addition is to inhibit the rate of dissolution of zinc, by adsorption of the silicate ion on the surface. The anomalous continuous dissolution can therefore not be related to surface phenomenon as suggested by Hampson and Marshall[l]. Viscosity experiments using an Ostwald viscometer, showed that there is only a small change in viscosity on the addition of silicate ions to 3 M KOH (Table 1). This change is too small to be important in the operation of the battery. It must be postulated, therefore, that the addition of silicate to the electrolyte delays the precipitation ofzinc oxide (or hydroxide) from the bulk solution as suggested originally by Flerov[5,6] thus giving the battery system additional capacity.

REPERENCES

1. J. S. Drury, N. A. Hampson and troanal. C&m. 50, 292 (1974). 2. J. S. Drury, N. A. Hampson and Application No. 2904/73. 3. H. Espig, Private Communication. 4. R. D. Armstrong and M. F. Bell, 55, 201 (1974). 5. V. N. Flerov, Zh. prikl Khirn. 30. 6. V. N. Flerov, Zh. prikl Khim. 31,

Fig. 3. The effect of silicate ion on the electrode impedance, plotted in the complex plane, in I M KOH at a rotation speed of 53.3 Hz. 0 Silicate free; A 2%.

.

A. Marshall,

J. elec-

A. Marshall,

Patent

J. elecrroanal. 1326 (1957). 49 (1957).

Chem.