Electrochemical behaviour of magnesium ions in the equimolar CaCl2NaCl mixture at 550 °C

Pergamon PII:

Electrochimica Acta. Vol. 42, No. 12, pp. 1869-1876, 1997 0 1997 Elsevier Science Ltd. All rights reserved. Printed in Great Britain 00 134686/97 $17.00 + 0.00 SOO13-4686(96)00399-4

Electrochemical behaviour of magnesium ions in the equimolar CaClz-NaCl mixture at 550°C Y. Castrillejo,“* “Dpto de Quimica bDepartment

Analitica,

A. M. Martinez,a

R. Pardo” and G. M. Haarbergb

Facultad

of Electrochemistry,

de Ciencias, Universidad de Valladolid, Prado de la Magdalena s/n, 47005 Valladolid, Spain The Norwegian Institute of Technology, University of Trondheim, 7034 Trondheim, Norway

(Received

29 July 1996)

Abstract-The electrochemical behaviour of magnesium ions was studied in molten CaClrNaCl at 550°C by cyclic voltammetry, chronoamperometry and chronopotentiometry. We observed that the electrode process of the reduction of Mg(II) on a tungsten electrode is quasireversible. The values of the transfer coefficient, tl, and the charge-transfer rate constant, k”, were calculated by means of the logarithmic analysis of the convoluted curves. Those values were compared with the ones obtained from the steady-state current-potential curves. On the other hand, the diffusion coefficient of the Mg(II) ions, has also been determined by different electrochemical techniques, giving values ranging from 7.2 to 9.9 x 10e6 cm2 s-‘. 0 1997 Elsevier Science Ltd. All rights reserved Key words: Molten

chlorides,

magnesium

chlorides,

INTRODUCTION Despite the many improvements of the thermal methods, electrolysis is still the main technique for the industrial production of magnesium, because of: (i) the manufacturing technique has improved considerably; (ii) the magnesium chloride used as raw material is cheap; and (iii) the cost of electrolytic magnesium has kept decreasing. The high energy consumption has been a motivation to develop processes with higher current and energy efficiencies. The electrolytic methods of magnesium production use as reaction media mixtures containing mainly sodium chloride and calcium chloride: MgC12 (8-16%); CaC12 (30-40%); NaCl (25-45%) and KC1 (O-10%) (weight percent) [l]. These methods are divided according to cell feed into hydrous (developed by the Dow Chemical Company) and the anhydrous (I. G. Farbenindustrie, Norsk Hydro and Vami). The main difference between them is that the former accepts a MgC12 feed prepared from sea water and containing 25% H20, whereas the latter operates on dehydrated molten carnallite or anhydrous MgC12. A detailed description of the two processes has been given by Krenzke [2]. In both types of cell, *Author to whom correspondence

should be addressed.

kinetic parameters,

chemometrics.

the electrical consumption is about 18 kW h/kg Mg. Recent major technical advances include the development of a cell without a diaphragm, an increased current capacity and the introduction of the multipolar cell [3]. When it comes to the electrochemical studies of magnesium in chloride melts, Duan ef al. [4] have used voltammetric techniques to characterize the Mg(II) reduction process at different electrode substrates, such as platinum, tungsten and stainless steel, in the MgClrNaCl-KC1 ternary eutectic melt and in dilute solutions of MgClz in a NaCl-KC1 eutectic solvent. For all concentrations of MgC12 studied, the reduction process was found to be diffusion controlled. In concentrated solutions, the current transients, as well as the voltammograms, showed nucleation and growth effects. Yongjin and Liangming [5] as well as Mel&s [6] have also studied the nucleation of magnesium from NaCl-KC1 mixtures with 2% and 5% of MgC12, respectively. The former used steel electrodes whereas the latter studied the electrodeposition of magnesium on glassy carbon and iron substrates. They found that the nucleation is an instantaneous process, which is followed by a three-dimensional growth by hemispherical diffusion. They also calculated the number of nuclei. 1869

Y. Castrillejo

1870

Tunold [7] has performed studies of the cathode process in the equimolar CaClz-NaCl melt containing MgC12 on magnesium, platinum, steel and glassy carbon electrodes by voltammetric and other transient techniques. Plots of peak current c’s the square root of the scan rate were found to be linear, which indicates that the process is mainly diffusion controlled. However, at scan rates higher than 2 V s-’ deviation from linearity was observed, implicating some irreversibility of the discharge process. The deposition of magnesium in CaCl2-KCI-NaCl (6:18:65) melt has also been studied by Yoon [8] using silver and platinum electrodes, finding that the process is quasi-reversible on both substrates. The cathodic process during electrochemical deposition of magnesium in pure molten magnesium chloride at temperatures ranging from 725,-C to 780°C has been extensively studied by Bsrresen [9] by electrochemical measurements and visual observations. He has established a model for the process on the basis of the experimental results obtained, where it is claimed that the deposition process and the formation of dissolved metal are parallel processes. Bsrresen et al. [IO] have recently studied the kinetics of deposition of Mg from chloride melts including pure molten MgC12. By combining ac impedance measurements with potential relaxation the charge transfer was found to follow a two step the second step being very fast mechanism; (i,, z 25-30 A cm-2 at 750°C). In this paper, we studied the cathodic reaction in dilute solutions of magnesium chloride in 1: 1 mixtures of sodium chloride and calcium chloride. We have the stability of the different oxidation states of magnesium as well as the standard potential of the Mg(II)/Mg(O) couple. The properties of this exchange were studied by using voltammetric and pulse techniques.

EXPERIMENTAL Preparation and purification of the melt The CaClrNaCl melt, was contained in a 100 cm3 alumina crucible placed in a quartz cell. The temperature was maintained constant within + 2°C by means of a Taner furnace and a West 3300 programmable device. All handling of the salt was carried out in a glove box with an argon atmosphere. The mixture was fused under vacuum, then raised to atmospheric pressure using dry argon, purified by bubbling HCl through the melt for at least 30 min, and then kept under argon atmosphere [l 11. Electrochemical

apparatus and electrodes

Cyclic voltammetry and pulse techniques were performed with a PAR EG&G Model 273A potentiostat/galvanostat controlled with the PAR EG&G M270 software package.

et al.

Two tungsten wires (1 mm diameter) were used as working and counter electrodes. The working electrode active surface area was determined by the depth of immersion. The results were corrected for the height of the meniscus. The reference electrode consisted of a silver wire (0.5 mm diameter) dipped into a solution of silver chloride in a CaCll-NaCl molten mixture placed in a quartz tube. Potentials were measured by reference to the Ag/AgCl couple. Chemicals Both anhydrous calcium chloride and sodium chloride were of analytical grade. Since only chloride 6-hydrate is commercially magnesium available, the addition of Mg(II) ions into the melt was made in three different ways: (i) anodic dissolution of the metal at constant current; (ii) addition of magnesium chloride partially dehydrated in the heater (subsequent HCl bubbling into the melt could dissolve any MgO formed); and (iii) by addition of dehydrated magnesium chloride, which was prepared from magnesium chloride-hexahydrate predried under HCI atmosphere up to 450°C and subsequently distilled under vacuum.

RESULTS Characterization

AND DISCUSSION

of the reduction mechanism

The purity of the solvent, after its purification, was checked by voltammetry (Fig. la) using tungsten electrodes. The electrochemical stability domain was limited on the cathodic side by the reduction of sodium ions, and on the anodic side by the oxidation of chloride. The electrochemical window was then found to be -2.5 V to 1.25V vs the Ag/Ag+ reference. Figure l(b) shows a voltammogram for the reduction of a solution of magnesium chloride on a tungsten electrode. The electroreduction takes place in a single step characterized by the cathodic wave A at about - 1.85 V, associated with the reoxidation peak A’. Peaks B, C and D, can be related to the formation of different alloys between magnesium and sodium and/or calcium [12]. The shape of the A/A’ system was characteristic of a soluble-insoluble exchange, the Mg(II)/Mg(O) couple. This has been confirmed by an examination of voltammograms, since the ratio of the anodic to the cathodic current was greater than unity and the ratio of the total anodic to the total cathodic charge, was close to unity and independent of scan rate. Moreover, analysis of the deposit obtained under potentiostatic conditions, by means of X-ray diffraction analysis, confirmed the presence of solid magnesium. Thus, we can conclude that II and 0 are the stable oxidation states of magnesium in the equimolar CaClz-NaCl mixture at 550°C.

Electrochemical

behaviour of magnesium ions

We wish to emphasize that under our experimental conditions and throughout the study, we have not observed nucleation and growth phenomena, either by voltammetry or I-t current transients. Kinetics of the Mg(II)/Mg(O)

The reduction of magnesium three magnesium concentrations.

1871

the cathodic peak current vs the square root of the sweep rate were found to be linear (Fig. 2), which proves that the mass-transport is diffusion controlled. From the slope of such plots, the diffusion coefficient has been calculated. Furthermore, this value has been compared with the ones found by means of chronopotentiometry (see Fig. 3). This study confirmed the verification of Sand’s law. The values

exchange

ions was studied at In all cases, plots of

0.3-

(a)

I

0.2-

I

O-l-

-0.2(a) -0.3 -3 !

-2.5

-2

-1.5

-0.5

-1

0

0.5

1

. 5

E/V

0.15 1 0.1 0.05 0

i

4 1

-0.05

C

-0.1 -0.15 -0.2 I 4.25 : -3

lb) -I

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

l.! 5

EN Fig. 1. (a) Cyclic voltammogram obtained at a tungsten electrode in a pure CaClrNaCl melt (sweep rate 0.2 V s-r; electrode area 0.25 cm2); (b) typical cyclic voltammogram for the reduction of magnesium chloride (0.1137 mol kg-‘) on a tungsten electrode (sweep rate 0.2 V s-l).

Y. Castrillejo

1872 0.05

0.04

et al.

Table 1. Magnesium techniques

I

(II) diffusion

coefficients

obtained

lo6 D/cm2 s-l

Technique

/

Voltammetry Semi-integral Chronopotentiometry

0.03 a +

by different

7.2 f 1.0 9.9 f 0.9 8.0 + 1.0

0.02

0.01

;/ T/J ,+ ./

o

+

0

OS

1

1.5

““z/(v.s’

2:5

2

1’”

Fig. 2. A plot of the cathodic peak current as a function of the square root of the sweep rate for the reduction of Mg(II) ions at a tungsten electrode (S = 0.25 cm*). (0) [MgClJ = 0.07850 mol kg-’ (m) [MgC12] = 0.04575 mol kg-’ (A) [MgClz] = 0.02957 mol kg-‘.

of the diffusion coefficient obtained are summarized in Table 1. The differences between these are mainly related to the difficulty in determining the exact active electrode areas. On the other hand, the potential difference between the peak potentials for the cathodic and reoxidation process, as well as the difference between the peak potential and the half-peak potential for the cathodic process are greater than would be expected for a simple two-electron reversible process [I 31, probably due to partial control by the electrode reaction itself.

Thus, the data suggest some irreversibility of the discharge reaction. A depth analysis of the voltammograms (Fig. 4a) gives more information about the mechanism of the reduction process of the magnesium ions. The fact that the cathodic peak potentials are shifted towards negative potentials for scan rates higher than 1 V s-’ (Fig. 4b) indicated that, in these conditions and after correction of the ohmic drop, the electron transfer rate is significantly lower than that corresponding to the mass transport [14], which is in agreement with the results obtained above. When the electrochemical reaction is diffusion controlled, we can apply the following equation to calculate the standard potential, I?, of the Mg(II)/ Mg(0) system:

EP = I?’ + 2.3 5

log co - 0.849 s

(1)

where EP is the cathodic peak potential, CO the bulk concentration and n the number of electrons exchanged. R and T have the usual meanings. The value obtained is - 1.500 V (us Ag/Ag+ reference, mol cm-’ scale).

-2.15

‘(6)

(5)

(4)

(3)

0.5

1

('1

(2) I

I

1.5

2

3

2.5

3.5

4

4.5

5

! 5

t/s Fig. 3. Chronopotentiometric study of magnesium chloride reduction. MgCl2 concentration, current density: (1) 107.4; (2) 115.4; (3) 127.3; (4) 131.3; (5) 139.3; (6) 199 mAcmm2.

0.1137 mol kg-‘. Cathodic

Electrochemical behaviour of magnesium ions In order to confirm these results, we have performed the convolutional analysis of the voltammograms [15-171. Figure 5 shows some voltammograms of Mg(I1) reduction and their corresponding semi-integral, which is independent of the scan rate. By analysing the convoluted curves, it

1873

can be observed that the direct and reverse scans are not identical, as well as a hysteresis behaviour between the up and down sweeps. All these characteristics confirm the fact that the reduction of Mg(I1) on a tungsten electrode is not a reversible reaction.

0e5/

(a) -0.2 ! -2.2

-2.1

, -2

-1.9

-1.8

-1.7

-1.6

I -1.5

-1.4

-’ .3

EN

-1.6

-1.7

P 4

-1.8

-1.9

-2 r

Fig. 4. (a) Voltammetric reduction of magnesium chloride. [M&h] = 0.1137 mol kg-‘; electrode area 0.25 cm’; sweep rates: (I) 0.1; (2) 0.5; (3) 1; (4) 2; (5) 3 V s-l; (b) variation of the cathodic and anodic peak potentials with the logarithm of the sweep rate.

Y. Castrillejo

1874

et al -0.64

-0.72

-0.1 I -2

1

1

-1.95

-1.9

I -1

-1.8

-1.85

EN Fig. 5. Cyclic voltammograms of MgClz reduction. their corresponding the convoluted data following a quasireversible model. From the limiting value, m*, of the semi-integral. and by applying the equation: m* = nFScoD”2

(2)

S being the active electrode area, we can calculate the diffusion coefficient of Mg(I1) species, D, yielding a value of the same order of magnitude as the ones obtained above (see Table I). Moreover, we can use the convolution in order to know the mechanism, as well as to estimate the kinetic parameters of the electrode reaction. We have carried out the logarithmic analysis of the convoluted curves according to the model of a quasireversible

convoluted

curves and the logarithmic

exchange with formation of an insoluble by applying the equation: E = .I?’ + 2.3 gF

analysis

of

product

[ 181,

log B

(3)

log k” + 2.3 gF

B being:

BEtrn* -

m)D-“2 + nFS exp[(nF/RT)(E I

- P)]

The plot of E vs log B, was a straight line, from which slope and intercept we can obtain the value of the transfer coefficient, a, and the charge-transfer rate constant, kO, respectively. The average values so obtained were: a = 0.7 + 0.1 and log (k”/ cm s-l) = -3.9 k 0.8. Taking into account the Matsuda and Ayabe criteria [19], in which the charge-transfer rate constant and the sweep rate are related, we can confirm that the Mg(II)/Mg(O) exchange is quasireversible on a tungsten electrode. of the charge Determination parameters from the steady-state curves

transfer kinetic current-potential

According to these results, the reduction of Mg(II) involves a diffusion step (diffusion of Mg(II) ions 0

0:2

0:4

0:6

0:s

i

alpha

Fig. 6. Pit-mapping graph depicting the variation of the sum S(k*, a) as a function of the values of the charge-transfer rate constant, /co, and the transfer coefficient, r. .S(kO, a) values: (1) 2.2 x 10e5; (2) I.7 X 10e5; (3) 1.3 X IO-‘; (4) I x 10-5; (5) 0.8 x 1O-5 ; (6) 0.7 x 10-j and (7) 0.6 x 10e5.

Table 2 Kinetic parameters

of Mg(II)/Mg

Technique Semi-integral (E us log B) Z-E curve (Gauss-Newton)

exchange log /co/cm s - ’

a

- 3.9 5 0.8 - 5.0 f 05

0.7 * 0.1 0.7 * 0.1

Electrochemical

behaviour

a

of magnesium

ions

1875

calculated by using the following faradaic current Ir: Zr = 2FS(k,

expressions

for the

- ~,[Mg(II),i]).

(4)

The concentration of Mg(I1) near the electrode as a function of its concentration in the bulk can be calculated from: [Mg(II)],,

In these constants, reduction -0.016+ -2.3

-2.2

-2.1

-1.9

-1.6

=

expressions, respectively, ways:

kO+ D/~b’fg(II)lbulk ’ k, + D/S k, and k, denote for the oxidation

k, = k” exp[2F( I - a)(E - Eb)/RT]

-1.7

(5)

the rate and the

(6)

ii/

k, = k” exp[ -2Fa(E

Fig. 7. Reconstruction of the steady-state I-E curve. (0) experimental data (from chronoamperograms, Fig. 4). The traced curve corresponds to the theoretical one calculated using the values of the kinetic parameters obtained.

by

from the bulk to the electrode, characterized by the diffusion coefficient D), and a two-electron transfer step (characterized by the intrinsic rate constant k” and the charge transfer coefficient a). These kinetic parameters can be obtained from the analysis of steady state current-potential curves [20].

solution

electrode

Mg(I1) + 2e- t* Mg(s). The theoretical

steady-state

I-E curve can then be

- .!?‘)/RT].

(7)

We have to seek the values of k” and a that minimize the sum S(k”, a) of the squares of the differences between the measured faradaic current (I;“‘““‘)and the calculated values (F”) obtained from equations (4) and (5) at each potential: S(k”, a) = ,f (v

-

rlF”“)2.

(8)

E,

Figure 6 gives the variation of S(k”, a) as a function of the values of the intrinsic rate constant k” (ranging from 1Om7to 1Oe2 cm SC’) and the transfer coefficient a (ranging from 0 to 1). The contour lines show a pit for which the corresponding values of k” and a lead to the best fit of the I-E curve. We have calculated these parameters by using the Gauss-Newton non-linear least-squares method. The values are

0 -0.005 -0.01 -0.015 -0.02 a 1 -0.025 -0.03 -0.035 -0.04 -0.045 0

1

2

3

4

5

t/s Fig. 8. Chronoamperometric study of magnesium (4) -1900; (5) -1975; (6) -2OOOmV. EA 42/12-E

chloride

reduction

(0.0918 mol kg-‘):

(I) - 1200; (2) - 1800; (3) - 1850;

Y. Castrillejo

1876

gathered in Table 2 and are in the same order of magnitude to those obtained by other authors [lo] in other molten chlorides. Figure 7 shows a very good agreement between the theoretical steady-state I-E curve obtained by using the values of the kinetic parameters calculated before and the experimental values extracted from the steady-state potential-dependent current values of the chronoamperometric curves (see Fig. 8). That confirms once more that the electrochemical system Mg(lI)/Mg(O) must be considered as quasireversible on a tungsten electrode.

CONCLUSIONS Vohammetric, chronopotentiometric, chronoamperometric and convolution measurements of magnesium ions on a tungsten electrode indicate that the oxidation states II and 0 are stable in the equimolar CaCIlNaCl mixture at 550°C. The electrochemical reduction of Mg(I1) occurs in a single step right down to solid magnesium. Diffusion is the only mass transport mechanism of the Mg(I1) species towards the electrode and the electrochemical exchange can be considered as quasireversible. Moreover, it was possible to determine accurately the kinetic parameters characterizing the mass and the charge transfer occurring in the reduction process. The charge-transfer rate constant value was extracted from the logarithmic analysis of convoluted curves by applying the equation corresponding to a soluble-insoluble quasireversible process, as well as from the steady-state I-E curves, by using different suitable mathematical treatments. Following the Matsuda and Ayabe criteria, the values so obtained confirmed the quasireversibility of the cathodic process. ACKNOWLEDGEMENTS The authors are grateful to DGICYT CE93-0017 (Spain) for financial support which enabled this study. A. M. Martinez wishes also to thank to DGICYT (Spain) for a doctoral grant.

et al. REFERENCES

I. N. Hyg-Petersen, T. Aune, T. Vralstad, K. Andreassen, D. Oymo, T. Haugerad and 0. Skane; Ullman’s Encyclopedia af Industrial Chemistry, 5th Ed. Vol. Al 5, p, 559. VCH Verlagsgesellschaft mbH, Weinheim, Germany (1990). 2. F. J. Krenzke, in The Encyclopedia of Electrochemistry (Edited by C. A. Hampel). Reinhold, New York (1964). 3. T. Ikeshima, 5th International Conference on Titanium, Munich, Germany, IO-14 September, 1984. 4. S. Duan, P. G. Dudley and D. Inman, Proc. Ftfth Int. Symp. Molten Salts, Vol. 86-1, p. 248. The Electrochem. Sot. (1986). 5. Z. Yongjin and L. Liangming, J. Cent.-South Inst. Min. Metal 22(5). 529 (1992). 6. J. Melas, Katodisk Magnesiumutfelling fra Kloridsmelte, Thesis, Dept. of Electrochemistry, NTH, Trondheim, Norway (1988). I. R. Tunold. Light Metals 1980. Proceedings of the 109th AIME Meeting, Ed. (J. McMinn), 949 (1980). A Spectroelectrochemical Study of 8. S. Y. Yoon, Aiuminium and Magnesium Electrolysis in Molten Chlorides, Ph.D. Thesis, MIT (1987). Electrodeposition of Magnesium from 9. B. Borresen, Halide Melts. Electrochemical Kinetics and Phase Formation, Thesis, Dept. of Electrochemistry, NTH, Trondheim, Norway (I 995). R. Tunold, A. Kisza 10. B. Borresen, G. M. Haarberg, and J. Kazmierczak, Proceedings of the Tenth International Symposium on Molten Salts, The Electrochemical Society 189th meeting, Los Angeles, California (1996). A. M. Martinez, G. M. Haarberg, B. 1 I. Y. Castrillejo, Borresen, K. S. Osen and R. Tunold, Electrochim. Acta 42, 1489 (1997). 12. P. Bocage, D. Ferry and G. Picard, Electrochim. Acta 36(l), 155 (1991). D. L. Maning and J. M. Dale, J. 13. G. Mamantov, Electroanal. Chem. 9. 253 (1965). 14. A. J. Bard and L. R. Faulkner, Electrochemical Methods Fundamentals and Applications, John Wiley, New York (1980). 15. J. C. Imbeaux and J. M. Saveant. J. Electroanal. Chem. 44, 169 (1973). 16. J. M. Saveant and D. Tessier, J. Electroanal. Chem. 61, 251 (1975) and 65. 57 (1975). 17. K. B‘. Oldham. Anal. Chem; 44 196 (1972). A. M. Martinez, M. Vega, E. Barrado 18. Y. Castrillejo, and G. Picard, J. Electroanal. Chem. 397, 139 (1995). and Y. Ayabe, Z. Electrochem. 59, 494 19. H. Matsuda (1955). 20. P. Pasquier and G. S. Picard, Electrochim. Acta 37, 163 (1963).