The mechanism of the deposition of manganese dioxide

J Electroanal. Chem., 201 (1986) 123-131 Elsevier Sequota S.A.. Lausanne - Printed

THE MECHANISM

OF THE DEPOSITION

PART III. ROTATING

R.L. PAUL

123 in The Netherlands

RING-DISC

OF MANGANESE

DIOXIDE

STUDIES

and A. CARTWRIGHT

Councrlfor Mineral Technology, Prrvate Bag X3015, Randburg 2125 (South Afrrca) (Received

21s.t May 1985; m revised form 18th November

1985)

ABSTRACT

The effect of ferrous ions on the current-time behaviour of an electrode during the deposition of manganese dioxide from acidic manganese sulphate electrolytes has been measured by the use of rotating ring-disc electrodes. Analysis of the ring and disc currents revealed that the deposrt can be reductively corroded by ferrous ions at electrode potentials that appear to be sufficiently positive to afford anodic protection to the manganese depostt. The results provide further support for a model described earlier for the deposition of manganese dioxide.

INTRODUCTION

In a study of the deposition of manganese dioxide onto vitreous-carbon electrodes, Cartwright and Paul [1,2] proposed a mechanism involving the oxidation of manganous ions at the growing MnO, deposit to produce a relatively stable intermediate. The rate of conversion of that intermediate into MnO, is assumed initially to be slow in relation to its rate of formation so that, as a result, a porous layer of the intermediate material would be formed on the outer surface of the deposit, retarding the diffusion of manganese ions to the site of electron exchange, and hence the rate of formation of the layer. Although experimental current-time and impedance spectra, which were measured under various conditions, were interpreted successfully in terms of that model, no direct evidence to support the existence of such a porous layer was provided. In this third, and final, part of the investigation, use was made of rotating ring-disc electrodes in an attempt to demonstrate the presence of a poorly conducting layer on the outer surface of the electrode. Measurement of the rate of diffusion of manganous ions through a massive sample of electrolytic manganese dioxide (EMD) negated the possibility that the layer is the entire deposit. 0022-0728/86/$03.50

0 1986 Elsevier Sequoia

S.A.

124 EXPERIMENTAL

Materials All the solutions were prepared from analytical-grade reagents and water of conductivity grade [2]. The massive samples of electrolytic manganese dioxide used had been prepared at the Council for Mineral Technology (Mintek) during the operation of a pilot-plant [3] using full-scale titanium anodes. The anodes (supplied by Imperial Metal Industries, Birmingham) were coated with ruthenium dioxide before use, and were operated at a current density of 9 mA cmp2. Fine-grained graphite rods were used as cathodes. The electrolyte contained MnSO, and H,S04 at concentrations of 1.0 and 0.4 mol l-i, respectively, and was maintained at 90-92°C. Samples of the EMD were tested by 3 international manufacturers of dry-cell batteries, and were shown to be of high purity and activity. A number of homogeneous, massive samples of EMD were carefully removed from the anodes and ground to discs of the desired thickness, and the flat surfaces were polished with 5 pm alumina. The orientation of the flat surfaces in relation to the anode and the direction of current flow in the electrolyte during deposition were maintained as closely as possible. Apparatus

and procedure

The rotating ring-disc electrode consisted of an outer platinum ring (with an internal diameter of 4.5 mm and an external diameter of 6.0 mm) and an inner vitreous-carbon disc (diameter 4.0 mm; area 0.126 cm’). The ring and disc were isolated electrically by a layer of Araldite M Epoxy resin and were fitted into a polytetrafluoroethylene sleeve. The electrode was polished with 5 pm alumina and washed with purified water before use. The rotating electrode assembly, potentiostats, and thermostatted cell have been described previously [2]. Current-potential curves were recorded on a Hewlett-Packard 7046A 2-pen X-Y recorder. Potentials were measured with respect to a saturated calomel reference electrode, and are quoted as measured. All electrochemical experiments were carried out in an electrolyte containing 1.0 mol of MnSO,, 0.1 mol of H,SO, and 0.9 mol of Na,SO, per litre (referred to here as “stock” electrolyte), unless stated otherwise. The electrolyte was maintained at 40°C and the electrode rotated at 500 rpm. The collection efficiency of the ring-disc electrode was measured periodically by use of the ferrous-ferric couple, and values of 0.39 to 0.41 were found to be typical of the electrode system. For ready comparison of the magnitudes of the ring and disc currents, all measured ring currents were divided by the collection efficiency and the exposed surface area of the disc (i.e. ring currents are expresssed as an equivalent current density originating at the disc). The diffusion rates of manganous ions were measured across a massive polished sample of EMD clamped between two glass reservoirs. Silicone rubber gaskets

125

ensured a leak-free seal. Pure sulphuric acid (0.5 mol I-‘) was placed in one reservoir, and the test solution in the other. The surface area of manganese dioxide exposed to the solutions was 2.01 cm2. The entire apparatus was immersed in a water bath which permitted the solutions in the reservoirs to attain a steady temperature of 89°C. Samples of the pure sulphuric acid were removed at regular intervals and analysed for their concentration of manganese by atomic-absorption spectroscopy.

RESULTS

AND

DISCUSSION

Rotating ring-disc studies Manganese dioxide was deposited onto the vitreous-carbon disc of the rotating ring-disc electrode for 400 s at 1.14 V from stock eIectrolyte. The slow decay of the curve of the i-t transient in Fig. 1 is typical for this process 121. A small amount of manganic ions produced at the disc was detected at the ring (E, = -0.1 V). Actins et al. [4] also detected solution-free manganic ions by the ring-disc technique, and suggested that the chemical reaction of Mn’” with Mn4’ were the source of the Mn3’ ions. After the plating period of 400 s, the disc and the ring were both open-circuited (point A in Fig. 1). The thermostatted cell containing the stock electrolyte was rapidly removed, the electrodes were rinsed with purified water, and a second thermostatted cell containing a preheated solution of sulphuric acid and ferrous ammonium sulphate (at respective concentrations of 0.1 and 0.02 mol 1-r) was

&-I 14v E,=-0,lV

3

0 0

200

400

6M)

800

Ical

Tlmds Fig. 1. Current-time curves for the disc and ring durmg the depositton of MnO, from stock electrolyte point A and from H2S04 (0.01 mol I-‘) and Fe’+ (0.02 mol I-‘) from point B

to

126

attached to the apparatus. The potentials previously applied to the electrodes were reapplied (point B). It was found that the ring responded immediately to the presence of ferric ions generated at the disc (Fig. 1). The constant cathodic current at the ring, due to the reduction of ferric to ferrous ions, was equivalent to an anodic current density of 9.5 mA cm-’ at the disc. This is in good agreement with the value of 8.7 mA cm-’ calculated by the use of the Levieh equation [5] for a rotation speed of 500 rpm and viscosity and diffusion values of 0.01 and 1 X 10W5 cm2 s- ’ for the kinematic coefficient of ferrous ions, respectively. The ring responded immediately to the generation of ferric ions at the disc, but the measured disc current required approximately 300 s to attain its maximum value * (Fig. 1). This observation is consistent with the existence of a of 9.5 mA cmpoorly conducting layer on the outer surface of the electrode, which is capable of oxidizing ferrous ions to ferric ions. Although the exact chemical composition of this layer has yet to be characterized, a compound like MnOOH (groutite) has been suggested [1,2]. Sugimori and Sekine [6] also suggested that groutite may be an intermediate in the formation of MnO,, but made no attempt to construct a detailed model of the electrode-electrolyte interface. Certainly, the presence of manganese with a valency of less than 4 in electrolytic manganese dioxide has been well established [7]. The rapid generation of ferric ions at the disc, therefore, could occur via the reaction Fe’++

3 H++

MnOOH

-+ Fe3*+

MnZCf

2 H,O

(1)

As the thickness of the layer is reduced by reaction with ferrous ions, the ohmic drop across the poorly conducting layer will decrease, and the potential at the outer surface of the electrode will rise towards the apparent value of 1.14 V. Hence, an increasing fraction of the ferric ions will therefore be produced by “electrochemical” oxidation, Fe’+ -+ Fe”++

e-

(2)

which accounts for the gradual increase in the measured disc current. If the effective potential at the surface of the disc does indeed increase to the point where reaction (1) ceases and is replaced entirely by reaction (2), then a certain fraction of the deposit produced during the initial deposition period of 400 s will always remain on the surface of the disc. This remaining deposit should, in effect, be anodically protected. The above implication is readily tested by the deposition of material on the disc from stock electrolyte for 400 s while the disc current is integrated to yield the total anodic charge passed. The disc was open-circuited, the manganese electrolyte was replaced by the ferrous solution (0.02 mol l-i), and the disc and ring were held at respective potentials of 1.14 and -0.1 V for varying lengths of time. The disc and ring currents up to this point are given in Fig. 1. The disc was then open-circuited, which caused the disc current to fall to zero. The ring current was unaffected by this

127

action at the disc, since ferric ions continued to be produced at the disc by reaction (1). When the remaining deposit was removed quantitatively by reaction with ferrous ions, the ring current fell to zero. The charge passed at the ring from the time that the disc is open-circuited can be converted to an equivalent charge at the disc if it is divided by the collection efficiency of the electrode assembly. A number of experiments in which the electrode was maintained in the ferrous solution for 500 to 2000 s before open-circuiting yielded residual amounts of material on the disc, which accounted for 24 to 27% of the total deposit on the disc. These results demonstrate the anodic protection afforded to the deposit when the layer has been sufficiently reduced in thickness by ferrous ions. Effect of Fe’ + during the deposition of MnO, When ferrous ions were added to the stock electrolyte during the deposition of MnO, onto the disc (point A in Fig. 2) a response similar to that described above was obtained, viz. a rapid rise in the ring current, indicating an immediate production of ferric ions at the disc, accompanied by a more gradual increase in the disc current. There are, however, two noticeable differences between Figs. 1 and 2. Firstly, it can be seen that the mass-transport-limited current for the oxidation of

Fig. 2. Current-time Fe’+ (0.02 mol 1-l)

curves for the disc and ring durmg bemg added at point A.

the depositlon

of Mn02

from stock electrolyte,

ferrous to ferric ions shown in Fig. 2 is approximately half the magnitude of the current shown in Fig. 1. This reduction in current occurs because the viscosity of the stock electrolyte, which reduces the diffusion coefficient of ferrous ions, is greater than that of the dilute ferrous -t sulphuric acid electrolyte (Fig. 1). Secondly, the rise in disc current is markedly faster after the addition of ferrous ions to the stock electrolyte during deposition of MnO,. A more rapid rise in disc current is to be expected since, in this experiment, any reduction in the thickness of the intermediate layer by reaction with ferrous ions will be accompanied by an increase in the rate of oxidation of m~ganous ions. The electrolyte was maintained at a potential of 1.14 V in the presence of ferrous ions at 0.02 mol I-’ (after an initial deposition period of 400 s) for respective periods of 100, 500, 1000, 2000, and 4000 s, and the residual charge remaining on the carbon electrode was equivalent to 91, 89, 95, 96, and 101% of the charge passed before the addition of ferrous ions. These figures should be compared with the charge recovery of 26% obtained when the stock solution was replaced by a ferrous electrolyte free of manganous ions. It may be concluded, therefore, that the production of ferric ions proceeds mainly according to eqn. (1) under the conditions of Fig. 2, even though the electrode is apparently maintained at a potential of 1.14 V. The increase in disc current is attributed to an accelerated rate of oxidation of rn~g~ous ions. The residual charge on the disc after continued polarization in the presence of ferrous ions (i.e. 90 to 100%) indicates that no net deposition of any material occurs after the ferrous ions have been added, even though the electrode is at 1.14 V. These results and conclusions are consistent with the presence of a poorly conducting layer of some manganese intermediate on the outer surface of the electrode.

* ----.

Ed= 1.14V &,--0lV

= DIFE = Ring

3

o-_ 1

0

800

400

lo@9

Time/s

Fig. 3. Current-time curves for the disc and ring during the depositlon of MnOz from stock electrolyte containing Fe2+ at 0.02 (curve A) or 0.04 (curve B) mol I- ‘.

129

Finally, the addition of ferrous ions to the stock manganese electrolyte at the commencement of the current-time transient would be expected to retard severely the overall deposition of manganese dioxide. Figure 3 shows the transients recorded in the presence of 0.02 and 0.04 mol of Fe’+ per litre over a period of 850 s. At the end of the deposition period, respective charges of only 430 and 40 mC cme2 were measured on the disc electrode. In the absence of ferrous ions, the charge measured after the same deposition period was approximately 3200 mC cmW2. measurement of rates of diff~ion Although the current-time behaviour [I], impedance characteristics [2], and ring-disc results are adequately interpreted in terms of the intermediate-layer model described above, it is conceivable that the layer could be interpreted as the MnO, deposit itself. This possibility, which requires the diffusion of manganous ions through the entire growing deposit to the surface of the vitreous-carbon electrode, is certainly feasible over the time scales (i.e. 1 to 2 h) employed in the studies described above. However, it is considered more difficult for such a mechanism to be envisaged over the durations employed for the production of electrolytic manganese dioxide (i.e. 20 to 30 d). Nevertheless, it was decided that the rates of diffusion of manganous ions across a sample of EMD, which had been produced during a pilot-plant campaign at Mintek [3], should be measured. Figure 4 shows plots of the mass of manganous ions apparently diffusing across the sample (exposed surface area 2.01 cm’) against time. The following comments pertain to these results.

A B C

6

H~SO~(O5molI~‘~+MnSO~~lmoll~‘~ HISO (0 5 mol l-‘)+MnSO4 (I moi 1.‘) H2SOI (OS mol I-‘)+ZnSOr (I mol I-‘)

ov 0

I

20

Fig. 4. Mass of Mn2+ apparently against trme.

40

60

diffusing

through

t

80

a piece of electroIytic

Ice

manganese

droxlde

plotted

130

(a) All the curves show that there is a continual decrease in the rate at which manganese is appearing in the reservoir containing the pure sulphuric acid. If manganous ions were diffusing from one reservoir, through the EMD, and into the other reservoir, the rate would be expected to be slow initially, and to increase to a constant value. Because the concentration of manganese in the reservoir initially filled with pure sulphuric acid never increased above 0.002 mol 1-l. a decrease in the diffusion gradient cannot account for the slopes of the curves. (b) The rate of diffusion across an EMD sample of 7 mm thickness from an electrolyte containing MnSO, at 1 mol 1-l (curve A) is approximately 8 mg h-’ at t = 0, decreasing to 1 mg h-i after 100 min. This rate of 1 mg h-i is equivalent to a lower than the value of 10 current density of 0.48 mA cm-‘, which is considerably mA cm-* usually employed [7] for the production of EMD. (c) A decrease in the thickness of the EMD sample from 7 to 2 mm (curves A and B respectively) should result in an increase in the rate of diffusion by a factor of 3.5. No such increase was observed. (d) When zinc ions were used as the diffusing species instead of manganous ions (curve C) no zinc could be detected in the sulphuric acid on the other side of the EMD sample. However, manganous ions appeared (curve C) at a rate only slightly lower than the values measured for manganous sulphate (curve A). These results can be interpreted in terms of the high percentage of entrained sulphuric acid and manganese sulphate commonly present in EMD. Even after milling of the material to a fine powder, typically smaller than a particle diameter of 75 pm, up to 24 h of washing is required to reduce the sulphate content to 2%. It is suggested, therefore, that metal cations (zinc or manganese) in the electrolyte are capable of penetrating only a short distance into the EMD. In order to maintain electroneutrality across the sample, occluded manganous ions will diffuse out of the other side of the EMD sample into the pure sulphuric acid. This interpretation is consistent with the four points discussed above. CONCLUSION

Previous studies involving the analysis of current-time [l] and impedance data [2] for the deposition of manganese dioxide from acidic manganese sulphate electrolytes have been interpreted successfully in terms of a model of the electrode-electrolyte interface. The electrochemical mechanism is thought to involve the oxidation of manganous ions to a relatively stable intermediate, which is subsequently transformed into manganese dioxide. Initially, the rate of formation of the layer is faster than its rate of transformation, and a porous layer of material develops on the outer surface of the electrode. If the layer is a poor electronic conductor, manganous ions must diffuse through the intermediate layer to the site of electron exchange. The thickening layer therefore retards its own formation until a steady state is achieved. The use of rotating ring-disc electrodes and the ferrous-ferric redox couple have enabled the existence of such a poorly conducting layer to be positively identified. Measurement of the diffusion rates of manganous ions through massive samples of

131

electrolytic manganese dioxide have shown that the diffusion barrier the intermediate layer cannot consist of the entire deposit. Although such as groutite (MnOOH) appears to be consistent with the properties the detailed chemical composition of the intermediate layer remains lished. The complexity of the crystal structure of bulk EMD proposed Ruetschi [S] served to illustrate that further advances in this area intensive study.

presented by a compound of the layer, to be estabrecently by will require

ACKNOWLEDGEMENT

This paper is published

by permission

of the Council

for Mineral

Technology.

REFERENCES 1 A. Cartwright and R.L. Paul in B. Schumm, H.M. Joseph and A. Kozawa (Eds.), Second InternatIonal Symposium on Manganese Dioxide, Tokyo. 1980, I.C. MnO, Samphng Office. Cleveland, OH, pp. 290-304. 2 R.L. Paul and A. Cartwright, J. Electroanal. Chem., 201 (1986) 113. 3 C.F.B. Coetzee, The Production of Electrolytic Manganese Dioxide from Furnace Sludge. Report M60D, Council for Mmeral Technology, Randburg, 1984. 4 A. Actins, G. Slaidins and V.S. Bagotskii, Latv. PSR Zmat. Akad. Vestls, Kim. Ser., 2 (1978) 380. 5 V.G. Levich. Physicochemical Hydrodynamics, Prentice Hall, Englewood Cliffs, NJ. 1962. p. 69. 6 M. Sugimori and T. Sekine. Denki Kagaku Oyobi Kagyo. Butsuri Kagaku, 37 (1969) 63. 7 A. Kozawa in H.M. Joseph (Ed.), Electrochermstry of Manganese Dioxide, U.S. Office of Electrochem. Sot. Jpn., Cleveland, OH, 1973, p. 72. 8 P. Ruetschl, J. Electrochem. Sot., 131 (1984) 2737.