Electrochemistry Volume 4

A Review of the Literature Published up to December 1972

By H. R. Thirsk

The Royal Society of Chemistry

Copyright © 1974 The Chemical Society
All rights reserved.
ISBN: 978-0-85186-037-4

Contents

Chapter 1 The Electrochemical Behaviour of Zinc in Alkaline Solution By R. D. Armstrong and M. F. Bell,
Chapter 2 The Electrochemical Behaviour of Cadmium in Alkaline Solution By R. D. Armstrong, K. Edmondson, and G. D. West,
Chapter 3 The Nickel Hydroxide and Related Electrodes By G. W. D. Briggs,
Chapter 4 Electrode Reactions in Liquid Ammonia By O. R. Brown,
Chapter 5 Electrochemistry of Molten Salts By D. Inman, J. E. Bowling, D. G. Lovering, and S. H. White,
Chapter 6 Membrane Phenomena By N. Lakshminarayanaiah,
Chapter 7 Organic Electrochemistry–Synthetic Aspects By P. M. Robertson,
Author Index, 337,

CHAPTER 1

The Electrochemical Behaviour of Zinc in Alkaline Solution

BY R. D. ARMSTRONG AND M. F. BELL

1 Introduction

This Report discusses the electrochemical behaviour of zinc in alkaline solution with particular emphasis on the electrochemical kinetics of this system. A typical anodic current–voltage curve for Zn in alkaline solution is shown schematically in Figure 1. The curve can be considered as exhibiting the active region, AB, and the passive regions, BC and DE. These regions will be discussed in separate sections. Related topics, i.e. soluble zinc species and the important forms of zinc oxide, will also be covered.

2 Soluble Zinc Species in Alkaline Solution

Substantial experimental evidence points to the fact that the predominant soluble zinc species in alkaline solutions is the tetrahedral ion Zn(OH): 2/4-.

Potentiometric experiments, carried out by Dirkse, gave results which he evaluated as follows. The initial assumption was made that the dissolution of zinc could be described by the equations

Zn[right arrow]Zn2+ +2e- (1)

xZn2+ + yOH- + zH2O [right arrow] + Znc (2)

where Znc is the complex ZnII species in solution. The potential for this reaction would be given by

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

which, because the activity of solid zinc can be assumed to be unity, reduces to

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)

The equilibrium constant for reaction (2) is given by

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (5)

Ezn then becomes

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (6)

No attempt was made to evaluate z. From plots of Ezn against log Mzn* and considering several possibilities for the solution-soluble species [i.e. Zn(OH)2, Zn(OH)-3, Zn(OH)2-4] Dirkse obtained a value of 1 for x. He also obtained a value of 4 for y and gives evidence that water is a reactant rather than a product. The thermodynamic data, shown in Table 1, are derived from these results.

From i.r. and Raman studies, Fordyce and Baum found absorptions for zincate solutions at the wavenumbers shown in Table 2. They attribute these to the Zn(OH)2-4 species and conclude that the experimental results can be explained by a single tetrahedral species in which there is some repulsion between the ligands and in which bond stretching is easier than is usually the case. They also suggest possible hydrogen-bonding in the structure.

In n.m.r. studies of the proton resonance in alkaline zinc solutions, Newman and Blongen found an average value of the chemical shift (dependent on the KOH concentration) which is consistent with a Zn(OH)2-4 species. They also state that the evidence is in favour of the partial (35–45%) covalency of the Zn — O bond.

3 The Active Dissolution of Zinc

Amstrong and Bulman studied the active dissolution of zinc in solutions having compositions in the range 3 x 10-2 — 2M-NaOH [of constant ionic strength (3 mol l-1) with NaCl] using the rotating-disc technique. They found that the current was dependent on rotation speed at constant potential and assumed this effect to be due to the simultaneous occurrence of deposition and dissolution. By extrapolating out diffusion from i-1vs. ω-1/2 plots, they obtained dissolution currents which showed a Tafel slope of 42 [+ or -] 5 mV decade-1. This would give a corresponding cathodic Tafel slope for zinc deposition of 105 mV decade-1, as the anodic and cathodic Tafel slopes are linked by the equation

1/30 = 1/ba + 1/bc (7)

This work also showed that the reaction order with respect to zincate for zinc deposition is unity. These results are in substantial agreement with the work of Bockris et aL. who found, by potentiostatic and galvanostatic techniques, an anodic Tafel slope of 49 mV decade-1 and a cathodic Tafel slope of 113 mV decade-1. Bockris lists the reaction orders, calculated from the dependence of exchange current density on concentration. These are shown in Table 3 where they are compared with those found by Armstrong and Bulman. They agree with the orders measured from the concentration dependence of current density at constant potential. As Armstrong and Bulman studied only two solutions, not too much weight should be placed on their high reaction order with respect to hydroxide ion.

The results reported by Kabanov are in fair agreement with the work discussed above. He gives a value of 3 for the pH dependence of the anodic current density, ([partial derivative]logia/[partial derivative]pH). However, his reported anodic Tafel slope of 30 mV decade-1 is rather low. This would imply that the cathodic Tafel slope was infinite.

The Tafel slopes of approximately 40 mV decade-1 are consistent with an overall mechanism

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (8)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (9)

where Zn1 could be either an adsorbed or a solution-soluble intermediate. The reaction order with respect to hydroxide ion would suggest that two hydroxide ions are involved in the rate-determining step. However, it is difficult to be certain about the state of complexation of the zinc intermediates. Bockris favours a solution-soluble intermediate of the type Zn(OH)-2.

Hampson and co-workers have studied the zinc system by a number of techniques. Using the faradaic impedance method they fitted their results to a Randles plot, with the intercept as ω [right arrow] ∞ independent of both hydroxide and zincate ion concentrations. It was stated that this behaviour could be attributed to slow adatom diffusion on the electrode surface. This cannot be true since Rangarajan has shown that, when adatom diffusion is considered, a ‘Randles’ plot can be found if (i) [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and io<Da/x2o, in which case the intercept is RT/nFio, or (ii) [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], when the intercept is [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

Thus in either case the exchange current density is an important component of the intercept and it is generally accepted that the exchange current depends on cOH- and czn(OH)2-4- , as was found in the previously mentioned investigations.

Hampson reported from galvanostatic i-η transients that io did not depend on zincate concentration, which is at variance with the results of other workers, as are his reported Tafel slopes and the value of the double-layer capacitance. Bockris has suggested that some of these discrepancies are due to the lack of correction for ohmic resistance in the solution.

The Dissolution of Zinc from a Zinc Amalgam in Alkaline Solution. — Gerischer reported that, whilst the overall reaction for the dissolution of zinc amalgam in alkaline solution is

Zn+4OH- [right arrow]Zn(OH)42-+2e- (10)

it can be broken down into two mechanistic steps, the rate-determining step being

Zn(Hg)+20H-[right arrow]Zn(OH)2+2e- (11)

followed by

Zn(OH)2+20H-[right arrow]Zn(OH4) 2-4 (12)

The arguments for this reaction depend on the following experimental results:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (13)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (14)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (15)

According to the above mechanism, assuming that the difference between concentration and activity and also double-layer effects may be ignored, io can be expressed by

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (16)

where Ki is the stepwise equilibrium constant, given by [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. On the basis of these results, a number of authors have reported values for the parameters k°, K° = [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], and α, of equation (16). These are summarized and compared in Table 4.

4 Properties and Crystal Structures of Zinc Oxides, Hydroxides, and Peroxides

The zinc oxide–hydroxide system is complicated by the number of forms these compounds can take. The method of preparation of zinc oxide affects the catalytic activity of the product. For instance, the oxide prepared from the carbonate is more active than that from the nitrate. These both possess the same crystal structure but the active form is more disordered. (The amount of catalytic activity is associated with the degree of disorder.)

Inactive zinc oxide crystals usually exist as either short or long needles of hexagonal structure. In these crystals, the zinc and oxygen atoms form a hexagonal structure of the wurtzite type which from atomic radii must be rather open. Under certain conditions, zinc atoms may be lodged in interstitial sites giving rise to non-stoicheiometry. It is believed that this is the reason for zinc oxide’s semiconductor properties (of the n type).

Each atom is surrounded by four oxygen atoms situated at the apices of a tetrahedron. Analysis by electron diffraction shows that the zinc atom is displaced from the centre of the tetrahedron by 11 pm in a direction parallel to the c-axis. Moreover, zinc oxide is partially covalent and the crystals are not purely ionic.

The ratio of the crystal axes, measured by X-ray diffraction, is a:c = 1.60200; a = 324.265 pm, c = 519.48 pm. The space group is 6 mm and there are two molecules per unit cell.

The zinc hydroxide system is much more complex. The solid exists in five crystalline forms, designated α, β, γ, δ, and ε, and also in an amorphous form. At ordinary temperatures the only stable form is the &-hydroxide and all the other forms are converted into this form, although the transformations are not simple. Specific conditions are necessary for the preparation of any of the metastable forms.

A gelatinous form (amorphous zinc hydroxide) is obtained by adding a weakly alkaline solution to a solution of zinc nitrate. If the precipitation is not complete, this form is converted into the α-form.

The crystals of the a-hydroxide are usually only small and never well formed. Certain anions, such as carbonate, stabilize the α-hydroxide under certain conditions and the stabilized form possesses a hexagonal structure with alternate ordered and disordered layers. Anion incorporation into the lattice is the cause of the disordered structure. The length of the edge, a, is well defined and equal to 311 pm. The length of the other edge, c, depends on the anion in-corporated into the lattice.

The β-hydroxide can exist in two forms (β1 and β2. These both possess layer lattices in which the distance between the planes is 567 pm. These forms are converted slowly into the ε-form. The γ-hydroxide is found as protracted prismatic crystals but has been reported to crystallize in other forms, especially needles. The unstable δ-form, which is obtained by slow crystallization from supersaturated zincate solution, exists as rhombohedral flakes. It is converted quite rapidly into the ε-form.

Large monocrystals of ε-hydroxide are obtained if the solution from which it is obtained is only slightly supersaturated. This form belongs to the ortho-rhombic system and the lattice dimensions are a = 516 pm, b = 853 pm, and c = 492 pm. Each zinc atom is tetrahedrally surrounded by hydroxide ions and the unit cell contains four molecules. The tetrahedra form zig-zag chains along the c-axis. Each hydroxide ion belongs to two tetrahedra and the lattice extends in three dimensions. The Zn — OH length is 195 pm and the OH — OH distance is 283 pm. The space group of this form is P212121 and z=4.

The peroxide is formed from the hydroxide at low temperatures but loses oxygen rapidly at higher temperatures. It has been shown recently that this is a true peroxide, and a product corresponding to 4ZnO2,ZnO, H20 has been isolated.

The stabilities of the hydroxide forms are in the order amorphous

Table 5 shows the free energy and enthalpy changes for the conversion of the different forms into the ε-form. Solubility products ([FORMULA NOT REPRODUCIBLE IN ASCII]) and the enthalpy and free-energy changes for the conversion of the hydroxides into the oxide are shown in Table 6.

5 The Passive Regions of Zinc in Alkaline Solution

Despite intensive investigations, these regions remain the most complex and unresolved subjects in the electrochemistry of zinc in alkaline solution. Many authors have studied this system using a wide range of concentrations of both zincate and alkali and also numerous techniques. Because of this, correlation of their results is very difficult. Experimentally it is generally found that there are two main regions, BC and DE, in the i-E curve (Figure 1). In this Report, the active–passive transition will be taken to occur at the point where the first deviation from the active region occurs. This corresponds to the point B in Figure 1.

Most of the literature of the mechanism of passivation suggests an adsorption model or a model involving the nucleation and growth of a two-dimensional layer, though a dissolution–precipitation mechanism has been suggested. Thus, Kabanov et al. found that a quantity of charge equivalent to 1 mC cm-2 was sufficient to confer passivity on the electrode. This would seem to support the adsorption model. Hull et a1. are also in favour of this model because the current–voltage curves which they obtained on rotating-wire and rotating-disc electrodes would be difficult to explain in terms of a dissolution–precipitation model. These authors also carefully observed the colour changes seen on sweeping anodically (Figure 2).

The two-dimensional model of passivation was further substantiated by Armstrong and Bulman, who from results obtained by potentiostatic cathodic reduction of the film on wire electrodes with cOH-<0.3 mol 1-1 suggest that passivity over at least a range of 500 mV is due to formation of no more than a monolayer of an anodic film. This monolayer film was stated to be analogous to the type II film of Powers and Breiter even though the latter authors used 7M-KOH. One anomaly of this film, pointed out by Armstrong and Bulman, was that the calculated reversible potential of the Zn|ZnO electrode is 100 mV more negative than the potential at which the active–passive transition occurs. Powers and Breiter suggest that this film starts to form earlier but that the active–passive transition is not seen until almost monolayer coverage has been achieved. It should be noted that the distinction between the two-dimensional nucleation model and the adsorption model is difficult to make because in order to do so it would be necessary to demonstrate the absence/presence of two-dimensiocal nucleation on a polycrystalline solid metal surface.

Powers and Breiter, by in situ photomicroscopy, suggested that under quiescent conditions the type II film, which they reported, was formed by direct reaction beneath a loose, flocculent, precipitated film, designated type I. Numerous authors have attempted to describe the nature and composition of the ‘passivating’ film but it seems unlikely that they were studying the mono-layer film because it is so thin. It was postulated by Powers and Breiter that this type I film could consist of either Zn(OH)2 or ZnO although Powers later explained his X-ray diffraction results obtained using horizontal electrodes in terms of zinc oxide.

(Continues…)Excerpted from Electrochemistry Volume 4 by H. R. Thirsk. Copyright © 1974 The Chemical Society. Excerpted by permission of The Royal Society of Chemistry.
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