The mechanism of the deposition of manganese dioxide

.i Electroanal. &hem., 201 (1986) 113-122 Elsevier Sequoia S.A., Lausanne - Pnnted

THE MECHANISM

113 m The Netherlands

OF THE DEPOSITION

PART II. ELECTRODE

IMPEDANCE

OF MANGANESE

DIOXIDE

STUDIES

R.L. PAUL and A. CARTWRIGHT Councti for M~nernl TechnoIo~, (Received

Pnuate Bag X3015, Randburg 2125 {South Afrtcaj

Zlst May 1985; in revised form 18th November

1985)

ABSTRACT The impedance of electrodes undergomg the deposition of manganese dioxtde from acidic manganous sulphate electrolytes has been measured at various steady-state dc potentials, temperature, and electrolyte compositions. The dominant component of the electrode impedance is a Warburg response. The results are shown to fit a model of the deposition process in which the magnitude of the current is determmed by the rate of diffusion of manganous ions through a porous layer. The layer 1s presumed to be an intermediate product in the overall mechanism of the oxidatton of manganous tons to manganese dioxide.

INTRODUCTION

In 1979 an intensive research programme aimed at the development of a process flowsheet for the production of electrolytic manganese dioxide (EMD) was initiated at the Council for Mineral Technology (Mintek). A novel feature of the process was the use of furnace sludge (which is produced during the pyrometallurgical reduction of manganese ores to ferromanganese) as a feed material to the process [1,2]. The high organic content of the furnace sludge led to certain difficulties in the final electrowinning stage, which prompted a detailed exa~nation of various aspects of the deposition of EMD from acidic manganous sulphate electrolytes [3,4]. Of particular relevance to the present study is the earlier paper by Cartwright and Paul [3] in which a mechanism for the electrodeposition of manganese dioxide was proposed (which is referred to here as part I). Briefly, the mechanism is thought to involve an initial oxidation of manganous ions at the growing MnO, surface to produce some solid intermediate (like MnOOH, Mn,O,, Mn(OH),, etc.). If the rate of conversion of this intermediate into MnOz is slow compared to its rate of formation, a layer of material will gradually grow on the outer surface of the MnO,. This layer will retard the transport of manganous ions from the bulk of the electrolyte to the interface between the MnO, deposit and the thickening intermediate where electron exchange occurs. The diffusion of protons, which are produced in the overall reaction, from the reaction interface to the bulk of the 0022-0728,‘86,‘$03.50

0 1986 Elsevier Sequoia

S.A.

114

electrolyte is assumed to be much faster than the diffusion of manganous ions. When the thickness of the layer has increased to the extent that the rate of diffusion of Mn2+ ions to the surface of the MnO, deposit is reduced to the rate at which the intermediate is transformed to MnO,, a steady-state condition is achieved. This mechanism was used as the basis for the development of a mathematical relationship between current and time, which was shown to be in good agreement with experimental current-time transients measured over extended periods [3]. Furthermore, various aspects associated with the operating parameters required for the production of EMD were interpreted successfully in terms of the proposed mechanistic model [4]. In the present investigation, additional support for the model was obtained by a study of the impedance characteristics of electrodes during the electrodeposition of MnO,.

EXPERIMENTAL

Materials All chemicals were of analytical-reagent grade and were used without any additional purification. Water of conductivity grade (a < lop5 S m-i), prepared by distillation and polished by the use of a Millipore-Q ion-exchange system, was used for the preparation of all solutions. The rotating electrode was fabricated from a vitreous-carbon rod (Le Carbone) of 3.0 mm diameter, which was fitted into a polytetrafluoroethylene sleeve, exposing a surface area of 0.071 cm2. Apparatus The rotating electrode assembly was constructed at Mintek. Potentials were measured with a saturated calomel reference electrode, and are quoted with respect to this electrode. Ohmic losses in the measured potentials were minimized by the use of a glass Luggin capillary positioned close to the centre of the rotating electrode. The electrolyte was maintained at the desired temperatures by means of a j-acketed cell fed with heated water from a thermostatted water-bath. The electrode potentials were controlled with a Solartron 1186 potentiostat. Current-time curves were recorded by means of an interface comprising a 13-bit analogue-to-digital converter and a real-time clock, which was connected to the current output of the potentiostat and an Apple IIe microcomputer. The real and imaginary components of the impedance spectra were measured by modulation of the electrode potential with a 10 mV rms sinusoidal signal and analysis of the resulting ac component of the electrode current with a Solartron 1170 frequency response analyser. Data were transferred to the microcomputer via a 1181 BCD interface.

115

Procedure The vitreous-carbon electrode was lightly abraded with 1000 grit Carborundum before use, and polished with 5 pm alumina. After all the electrodes had been positioned in the cell and the temperature of the electrolyte had been allowed to stabilize at the desired value, the required potential was applied to the carbon working electrode. The current-time transient was recorded for 6000 s, the electrode impedance being measured as a function of frequency after 1000 s. The frequency was scanned from 10 kHz to 10 mHz at an interval of 5 points per decade. RESULTS AND DISCUSSION

Analysis of i-t transients In part I of the present investigation [3], a mathematical relationship between current and time was developed on the basis of the mechanism described in the introduction above. If it is assumed that the intermediate layer is composed of some manganese(II1) compound (e.g. MnOOH, Mn,O,, etc.) that is slowly oxidized to MnO,, the mathematical relationship has the form -~-ln[i-~]=4~~~ct 2i- i where i is the current at any time t, i, is the current at t = 00, M is the molar mass of the layer, p is the density of the layer, c is the concentration of Mn*’ in the bulk electrolyte, A is the electrode area, F is the Faraday constant, and d is the diffusion coefficient of Mn2’ through the porous layer. If the layer is assumed to be some manganese(W) compound, which undergoes a slow chemical transformation to Mn02, an equation that is only slightly different to eqn. (1) is obtained, and the results and conclusions are essentiahy the same. A typical experimental curve of current versus time is shown in Fig. 1 for the deposition of MnO, on a rotating vitreous-carbon electrode (1000 rpm) from an electrolyte containing 1, 0.1, and 0.9 mol of MnSO,, H,SO,, and Na,SO, per litre, respectively. The very slow decay in the current is typical of this electrochemical process and, even after 6000 s, the current has not attained a steady-state value. The curve of i-t is unaffected by the rotation speed, even down to 0 ‘pm. Also shown in Fig. 1 is a curve calculated from eqn. (1) in which the only two unknown variables in the equation (i.e. i, and D) were calculated by an iterative-regression procedure to yield the best fit to the experimental results. (Values of 90 for M and 4.3 g cme3 for p were used in the calculation.) The mathematical form of eqn. (1) clearly represents the results very well. Similar results were obtained under a variety of experimental conditions, although poorer fits were obtained at high concentrations of sulphuric acid (i.e. above about 0.5 mol 1-l). This effect, which was more pronounced at low temperatures, may be attributed, at least in part, to the

116 5

=Experlmenfal values = Calculated values

-

I

0

lcho

3oilo

2&o

4oba

skim

Tfme/s

Fig. 1. Curves of i-t for the deposition of MnO, at 1.10 V on a rotating carbon electrode (1000 rpm) from an electrolyte (40%) containing MnSO,, H,SO.,, and Na,SO., at concentrations of 1.0,0.1, and 0.9 mol I-‘, respectively.

diffusion of soluble manganic ions away from the disc, resulting in reduced coulombic efficiencies. The stability of manganic ions, Mns’, is increased by high concentrations of sulphuric acid and low temperatures [5]. Significant amounts of manganic ion leaving the disc were detected when a rotating ring-disc electrode was used with the potential of the platinum ring held at -0.1 V to reduce Mn3’ to Mn2*. It is also of interest that the quality of the EMD produced from electrolytes containing sulphuric acid at above about 0.6 mol I-’ is definitely poorer that at lower concentrations [4]. Values of i, and D determined under a variety of experimental conditions, are shown in Table 1. Inspection of these figures reveals the following: (i) i, and D are independent of electrode potential (at constant temperature and electrolyte composition), (ii) i, and D increase sharply with increasing temperature, (iii) the values of I> are smaller by factors of 100 to 1000 than would be expected for the diffusion of Mn*’ in the bulk electrolyte (i.e. 5 x lop6 cm* s-l), (iv) i, appears to be directly proportional to the bulk concentration of Mn*‘, whereas the values of D, although somewhat scattered, do not exhibit any clear dependency upon the concentration of manganese, and (v) as the temperature is increased, the values of i, approach the m~mum current density that can be employed for the production of EMD (i.e. 10 mA cm-* at 95 to 98°C [4,6]). Values of i, measured at 95°C would be desirable, but bubbles of vapour tended to collect on the horizontal surface of the electrode at temperatures above about 85°C causing the current to fall rapidly to zero. This effect is barely noticable on a short time-scale, but was difficult to control over the extended period (i.e. 6000 s) required for measurement of the current-time curves.

117

TABLE

1

Calculated 1000 rpm

values of I,

Temp./V Electrolyte.

and D for the deposition

r,/mA

E/V I.0 mol of MnSO,

40 40 40 40 80 Electrolyte:

0.1 mol of MnSO,

cm-*

0.47 0.47 0.46 0.48 1.85

+ 0.1 mol of H,SO,

1.160 1.140 1.160

70 70

on a vitreous-carbon

+ 0.1 mol of H2SOd + 0.9 mol of Na,SO,

1.100 1.120 1.140 1.160 1.100

40

of MnO,

108D/cmz

rotating

at

s-l

per lrtre

0.77 0.76 0.79 0.76 8.85 + 1.8 mol of Na,SO,

0.046 0.23 0.19

per Ltre

3.17 9.3 11.9

These observations provide strong support for the proposed anomaly being the apparent independence of i, on potential. Analysis

electrode

mechanism,

the only

of impedance spectra

In the present investigation, all impedance data are considered as lying in the complex plane, and to be composed of a real, Z’, and an imaginary, Z”, term as follows: Z = Z’ - iZ”, (2) where i = (- 1)lj2. Preliminary results [3] indicated that the impedance characteristics of the process can be expressed satisfactorily in terms of the Warburg equation [7] -1/2(I _ i) Zw=ao (3) However, for a complete analysis of all the results it was found necessary for the more complete Randles-Ershler circuit shown in Fig. 2 to be utilized. Because of the interaction between the ac and dc diffusion layers at low frequencies, the more exact equation for the Warburg impedance [2] was used, i.e. Z,

= ~o-‘/~(

P - iQ)

where u is the Warburg potential; p=

coefficient;

w is the angular

velocity

(4) of the modulating

sinh(u)+sin(u) (5)

cosh( U) + cos( u) sinh( U) - sin(~)

Q= cosh( U) + sin(~) u = s(20/D)1’2 and S is the dc diffusion

layer thickness.

118

Fig. 2. Randles-Ershler between the electrode capacitance, and Z,

resistance circuit used for the analysis of results, where R u IS the uncompensated and the Luggin capillary, 0 is the charge-transfer resistance, C, IS the double-layer is the Warburg impedance.

The hyperbolic terms P and Q have an effect upon the Warburg impedance only when the magnitude of u is less than 2, and cause the Nyquist plot of the impedance data to bend over at low frequencies to the extent where the imaginary component of the impedance falls to zero. The total impedance of the circuit in Fig. 2 is as follows: Z=R,+{(e+Zw)-l+iWc~}~l. The substitution impedance, i.e. Z=R

of eqn.

+ ew+2P-l(e2 U

1 + 2aw”‘QCd

(8) (5) into

eqn.

WC, + 2a&“PC, +

(8), and

evaluation,

+ 02P2C, + u~-“~Q

02uQ2C,’ + w2d2C: + 2a03/28PC:

yields

the total

cell

+ a2Q2C, + a2~P2C:

(9)

Values for R,, 8, Cd and (I were calculated by the use of an iterative procedure so that a good fit could be obtained between the experimental impedance results obtained under various conditions and the values calculated by the use of eqn. (9). The hyperbolic terms were evaluated by the use of values of lo-’ and 10 -’ cm’ s- ’ for the diffusion coefficient of manganese at 40 and 70 to 80°C respectively; the diffusion-layer thickness was determined from the Nernst diffusion equation 6 = FADc/i,,,

(10)

where i,,, is the dc current measured after 1000 s. The applicability of eqn. (10) in this instance is valid provided that i,,, z+ i,. Good correlations between experimental and calculated data were obtained in all experiments (Figs. 3 and 4), which confirmed the applicability of the Randles-Ershler circuit in Fig. 2. Of particular importance is that those curves (Figs. 4a and 4d) that exhibit extensive deviations from the simple Warburg expression, i.e. eqn. (3), due to

Fig. 3. Comparison of expe~ment~ (0) and calculated (0) impedance data for the deposition of MnO, from an electrolyte containing MnSO,, H,SO, and Na,SO, at concentrations of 1.0, 0.1. and 0.9 mol I-‘, respectively.

interaction between the ac and dc diffusion layers, were successfully expressed in terms of eqn. (9) and the values of D and 6 that had been determined from the modelling of the results for the r-t transients. In this regard it is of interest that the values of 6 in Table 2 (2 to 40 pm) are of the same magnitude as the value of the diffusion layer (i.e. 12.4 pm) which was calculated from the Levich equation [8] for 1000 r-pm, values of 0.01 and 5 X lop6 cm2 s-l being used for the kinematic viscosity of the electrolyte and the diffusion coefficient of bulk manganese, respectively. However, if this value of 12.4 pm for 6, and the value of 5 x 10e6 cm2 s- ’ for D, are substituted into eqn. (9), it is readily

.

300

0

2’4

0

‘UK

50

cm:

Fig 4. Comparison of experimental (0) and calculated for various electrolytes. potentials. and temperatures.

10”

I50

200

Z>R Ld

(0)

Impedance

data for the deposition

of MnO,

verified that all the curves should have exhibited extensive deviations from simple Warburg behaviour at frequencies as high as 1 Hz. Furthermore, the value of the mass-transport-limited current that was calculated from the Levich equation (i.e. 780 mA cme2 at an Mn” concentration of 1 mol 1-l) is approximately 500 times greater than that of the measured currents. The parameters used for the generation of the calculated values in Figs. 3 and 4 are shown in Table 2. Values of 6 increase with increasing potential, decrease with increasing temperature, and increase with reduced concentration of Mn2+. as expected. The values of the charge-transfer resistance, 8, are generally low, although

121 TABLE 2 Values of the parameters obtained by the fitting of eqn. (9) to the experimental results. Na,SO, added to all electrolytes to raise the total sulphate content to 2.0 mol 1-l

[Mn%I/

fHzS041/

E/v

Temp./

6/

a/

mol 1-l

mol 1-i

vs. SCE

“C

pm

s2 cm2 S-I/~

1.0

0.1 0.1 0.1 0.1 1.0 1.0 0.1 0.1

1.12 1.14 1.16 1.10 1.30 1.35 1.16 1.14

40 40 40 80 40 40 40 70

12 12 12 34 4.6 1.3 2.1 12

65 90 145 25 75 250 230 100

1.0 1.0 1.0 1.0 0.1 0.1 0.1

fv 0 cm2 0

0 0 5 25 80 20 100

was

cd/

pF cm-2 500 500 500 1500 400 200 100 400

higher values are observed for the electrolytes that are low in manganese or high in sulphuric acid, or both. This effect is attributed to positive shifts in the equilibrium potentials caused by these conditions, with a resulting drop in the overpotential. The values obtained for the double-layer capacitances are somewhat higher than expected. However, these values can be reduced substantially without any major effect upon the correlation between the calculated and experimental values, since the effect of C, is noticeable only at high frequencies. When (I and C, are both large, a pronounced curvature in the impedance spectrum is observed (particularly noticeable in Fig. 4d), which, in extreme cases [9], may assume the form of a semicircle.

CONCLUSIONS

The results of the present investigation demonstrated that the impedance characteristics of an electrode during the deposition of manganese dioxide can be interpreted in terms of a charge-transfer resistance and Warburg component connected in series, and shunted with the double-layer capacitance. Under most conditions, the effect of the Warburg diffusional impedance is dominant. Because the dc values of the electrode currents are at least 2 orders of magnitude smaller than the values calculated by use of the Levich equation and are independent of the rotation of the electrode, the influence of the mass transport of manganese ions through any aqueous diffusion layer is negligible. A mathematical relationship between current and time was developed on the basis that Mn*’ must diffuse through a porous, solid layer which grows on the outer surface of the MnO, deposit, the layer being a relatively stable intermediate before the formation of MnO,. The form of the relationship is shown to fit the experimentally measured current-time curves very well. Furthermore, the magnitudes of the diffusion coefficients calculated from the fitting of the model to the results appear to be consistent with the overall mechanism.

122

Finally, values of the thickness of the porous layer, calculated from the mathematical modeiling of the values for the i-t transients were used successfully for the calculation of certain aspects of the impedance spectra. ACKNOWLEDGEMENT

This paper is published

by permission

of the Council

for Mineral

Technology.

REFERENCES C.F.B. Coetzee, The Production of Electrolyhc Manganese Dioxide from Furnace Sludge. Report M60D, Council for Mineral TechnoIogy, Randburg, 1984. C.F.B. Coetzee and W.A.M. Te Rrete, The Production of Electrotytic Manganese Dioxide from Ferromanganese Furnace Sludge, Preprint, MINTEK 50. Conference on Recent Advances m Mineral Science and Technology, Sandton, 1984, Counctl for Mineral Technology, Randburg, 1985. pp. 715-721. A. Cartwright and R.L. Paul in B. Schumm, H.M. Joseph and A. Kosawa (Eds.), Second International Symposium on Manganese Dioxide, Tokyo, 1980, I.C. MnO, Sample Office, Cleveland, OH, 1981, pp. 290-304. R.L. Paul and A. Cartwright, The Electrodeposition of Manganese Dioxide: Theory and Practice, Preprmt. MINTEK 50: Conference on Recent Advances in Mineral Science and Technology, Sandton, 1984, Council for Mineral Technology. Randburg, 1985, pp. 453-461. J.W. Welsh, Electrochem. Technol., 5 (1967) 504. A. Kozawa in K.V. Kordesch (Ed.), Batteries, Vol. 1, Marcel Dekker. New York, 1974, p. 439. M. Sluyters-Rehbach and J.H. Sluyters in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 4, MarceI Dekker, New York, 1970, p. 16. V.G. Levich, Physic~he~~l Hydr~yna~~, Prentice Hall, Englewood Cliffs, NJ, 1962, p. 69. J.P. Diard and C. Hecker, J. EIectroanal. Chem., 121 (1981) 125.