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Electrochemical behaviour of dysprosium in the eutectic LiCl–KCl at W and Al electrodes
Electrochimica Acta 50 (2005) 2047–2057
Electrochemical behaviour of dysprosium in the eutectic LiCl–KCl at W and Al electrodes Y. Castrillejoa,∗ , M.R. Bermejoa , A.I. Barradoa , R. Pardoa , E. Barradoa , A.M. Mart´ınezb a
Dpto de Qu´ımica Anal´ıtica, Facultad de Ciencias, Universidad de Valladolid, Prado de la Magdalena s/n, 47005 Valladolid, Spain b Department of Materials Technology, Sem Sælands vei 6, 7491 Trondheim, Norway Received 21 April 2004; received in revised form 10 July 2004; accepted 12 September 2004
Abstract The electrochemical behaviour of DyCl3 was studied in the eutectic LiCl–KCl at different temperatures. The cathodic reaction can be written: Dy(III) + 3e ↔ Dy(0) which can be divided in two very close cathodic steps: Dy(III) + 1e ↔ Dy(II) and Dy(II) + 2e ↔ Dy(0) Transient electrochemical techniques, such as cyclic voltammetry, chronopotentiometry, and chronoamperometry were used in order to study the reaction mechanism and the transport parameters of electroactive species at a tungsten electrode. The results showed that in the eutectic LiCl–KCl, electrocrystallization of dysprosium seems to be the controlling electrochemical step. Chronoamperometric studies indicated instantaneous nucleation of dysprosium with three dimensional growth of the nuclei whatever the applied overpotential. Mass transport towards the electrode is a simple diffusion process, and the diffusion coefficient of the electroactive species, i.e. Dy(III), has been calculated. The validity of the Arrhenius law was also verified by plotting the variation of the logarithm of the diffusion coefficient versus 1/T. In addition, the electrode reactions of the LiCl–KCl–DyCl3 solutions at an Al wire were also investigated by cyclic voltammetry and open circuit chronopotentiometry. The redox potential of the Dy(III)/Dy couple at the Al electrode was observed at more positive potentials values than those at the inert electrode. This potential shift was thermodynamically analyzed by a lowering of activity of Dy in the metal phase due to the formation of intermetallic compounds. © 2004 Elsevier Ltd. All rights reserved. Keywords: Molten chlorides; Dysprosium chloride; Electrodeposition; Nucleation and crystal growth; Diffusion coefficient; Dy–Al alloy formation
1. Introduction Electrochemical processes of metals (electrowinning, electrorefining, electroplating, and electroforming) in molten salts media, present some advantages in comparison with those carried out in aqueous solution. Higher efficiency of the ∗ Corresponding author. Tel.: +34 983423000x4245; fax: +34 983423013. E-mail address: [email protected] (Y. Castrillejo).
0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.09.013
electrolysis, lower energy consumption, often high electrodeposition rates and much better characteristics of the deposits [1] can be pointed out as some of the main advantages. Moreover, of all the elements in the periodic table only about one quarter may be electrodeposited from aqueous electrolytes in their elemental form or as alloys suitable for coatings, the rest are of engineering and scientific interest because their specific properties, and methods of depositing them are desirable. The electrodeposition in molten salts is a specific way to obtain compounds of reactive and refractory elements, such
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as the refractory metals, the actinides and rare earth metals [2]. The accumulated knowledge concerning molten salts high-temperature electrochemistry leads to openings for the deposition of metals in the solid state and in alloys. Their possibilities lie in the fact that, because of their variety, one can always find a solvent whose chemical and electrochemical characteristics and melting point are suitable to carry out a given process. In the last years a new field has been developed, namely the use of molten salts media for pyrochemical separation as a promising option in the nuclear fuel in the future [3–9]. The interest is due to the progress in the assessment of new concepts for transmutation and the corresponding fuel cycles [10]. In order to assess the feasibility of such pyrochemical separation, several processes have been considered for the recovery of minor actinides from spent metallic, nitride, oxide nuclear fuels, and high level radioactive liquid wastes [11,12]. One of the most important steps in the pyrometallurgical reprocessing is the electrorefining from molten chlorides. In this step, spent metal fuel is anodically dissolved into molten chlorides, and the minor actinides are selectively recovered at the cathodes due to the differences among the redox potentials of the elements, while fission products remain in the anode and/or in the electrolyte salt [9,13]. Alloy electrodeposition is involved in the separation of minor actinides from rare earths, the most difficult fission products to separate due to their similar properties. Each of these category of elements is well known to yield easily and with high rates alloys or compounds with more noble elements such as cadmium, nickel or aluminium [2,9,13–19]. The deposition in an alloyed form of these elements proceeds at considerably more anodic potentials, so that their separation from other reactive elements could become more easy. The determination of the electrochemical behaviour of the elements on different substrates, with and without alloy formation, is of crucial importance for the understanding of the process and the design of the separation cell. The work presented here is part of a series focused on the study of separation of actinides from rare earths. Hence, obtaining basic data of fission products in molten halide salts is a major concern. The work presents a study of the electrochemical properties of dysprosium trichloride and has been carried out in the eutectic LiCl–KCl. Few studies have been conducted relating to the chemical and electrochemical behaviour of dysprosium in molten chlorides. It is reported by Plambeck [20], that in the eutectic LiCl–KCl, Dy metal reacts slowly with the melt to give a dark solid and a pale-violet solution containing dysprosium in some form. The Dy(III) species has proven stable in this melt [21], so the reaction might give Dy(III), Dy(II), or possibly both [22]. The existence of Dy(II) has been noticed by some authors [23,24], in this way Franklin et al. [24] indicate that when a solid metal Dy is immersed in LiCl–KCl–DyCl3 , the metal reacts with DyCl3 to form DyCl2 . The DyCl2 is dispersed in the bulk of the molten slats and appear to dis-
proportionate into Dy and DyCl3 . According to Sheng et al. [25] the Dy formed in the bulk by this mechanism may be considered a “metal fog”. On the contrary, Chang et al. [26] propose that the electroreduction of Dy(III) solutions takes place in only one three electron reversible electrochemical step Dy(III)/Dy(0). Although the mechanism of electroreduction of Dy ions at one inert electrode is not clear yet, the electrochemical preparation of Dy–Fe and Dy–Ni alloys has been conducted by Konishi et al. [14–17] in the eutectic LiCl–KCl. The purpose of our investigation was to determine the electrochemical behaviour of a LiCl–KCl–DyCl3 melt at temperatures between 673 and 823 K at different substrates: (i) inert electrodes; and (ii) more noble metals than Dy with possibility of alloy film formation. In order to study the deposition of pure metal, an inert working electrode must be used. Preliminary experiments showed that refractory metals such as tungsten was a convenient electrode material and transient electrochemical techniques, such as cyclic voltammetry, chronopotentiometry, and chronoamperometry, were used to determine the reaction mechanism and the transport parameters of electroactive species. Moreover nucleation studies of Dy metal were carried out. The results showed that under the experimental conditions deposition of dysprosium at a tungsten electrode can be explained in terms of a model involving instantaneous and three-dimensional nucleation where the growth of the crystals is controlled by hemispherical or linear diffusion, over the whole cathodic potential range studied. The electrode reactions of the LiCl–KCl–DyCl3 solutions at an Al wire were also investigated by cyclic voltammetry and open circuit chronopotentiometry. The redox potential of the Dy(III)/Dy couple at the Al electrode was observed at more positive potentials than those at the inert electrode. This potential shift was thermodynamically analyzed by a lowering of the activity of Dy in the metal phase due to the formation of intermetallic compounds.
2. Experimental 2.1. Preparation and purification of the melt The experimental cell was carefully prepared. Therefore all handling of the salts was carried out in a glove box mBraun Labstar 50 under an argon atmosphere. Moreover, purification of the cell was very important in order to obtain consistent results. The chloride mixture (LiCl–KCl, analytical-grade) was melted in an alumina crucible placed in a quartz cell inside a Taner furnace. A West 3300 programmable device controlled the temperature of the furnace to ±2 ◦ C. The working temperature was measured with a thermocouple protected by an alumina tube inserted into the melt. The mixture was fused under vacuum, then raised to atmospheric pressure using dry argon; afterwards it was purified
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by bubbling HCl through the melt for at least 20 min, and then kept under an argon atmosphere which removed the residual HCl and maintained an inert atmosphere during experiments. This procedure has been used previously [27–35]. 2.2. General features Solutions of Dy(III) in the concentration range 0.04–0.09 mol/kg, were prepared by direct additions of solid DyCl3 (Sigma Aldrich 99.99%). The experimental problems related to the low solubility of Dy-O compounds prevented preparation of stable solutions of Dy(III) for longer periods, more than 1 day, and it was hard to know the exact amount of salt introduced into the melt, for which HCl was bubbled each day prior to determinations. The total concentration of dissolved rare earth was calculated by ICP-AES analysis of frequently taken melt samples 2.3. 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. The reference electrode consisted of a silver wire (1 mm diameter) dipped into a silver chloride solution (0.75 mol kg−1 ) in the LiCl-KCl molten mixture, contained in a Pyrex tube. Potentials were measured by reference to the Ag|AgCl couple and translated into potentials versus Cl2 /Cl− . The working electrodes consisted of 1 mm tungsten and aluminium wires (Sigma Aldrich, 99.99% and 99.999%). Another tungsten wire was used as the counter electrode. The lower end of the tungsten working electrode was polished thoroughly by using SiC paper, and then cleaned in ethanol using ultrasound. Potentiostatic electrolysis was conducted with a 1 mm thick aluminium foil cathode (Sigma Aldrich, 99.999%) and a graphite rod (Sofacel) as counter electrode. Auxiliary techniques such scanning electron microscopy (SEM), EDX analyses and ICP-AES analyses were also used. 3. Results and discussion 3.1. Electrochemical behaviour of LiCl–KCl–DyCl3 solution at a tungsten electrode 3.1.1. Electrochemical characterization of the systems The stable oxidation states of dysprosium were identified by different electrochemical techniques, i.e. cyclic voltammetry, chronopotentiometry, and square wave voltammetry. Fig. 1 shows representative examples of cyclic voltammograms of a LiCl–KCl–DyCl3 solution at a tungsten electrode. The electroreduction takes place in two very close electrochemical steps A and B in the potential range close to the cathodic limit of the melt (electrodeposition of liquid lithium). The shape of the cathodic wave A it is not clear enough, while the shape of the following wave B, is characteristic of the formation of a new phase, Dy(0), steep rise and slow decay. In the
Fig. 1. Typical cyclic voltammogram for the reduction of dysprosium trichloride (1.4191 × 10−4 mol cm−3 ) on a tungsten electrode in the eutectic LiCl–KCl melt at 723 K. Sweep rate 0.2 V s−1 .
positive potential scan direction, the reduced species Dy(0) are reoxidized in one overall peak B , with the expected characteristics for a stripping peak. The electrochemical signals could be explained by one of the following mechanisms: i.- The formation of dissolved Dy, which takes place at the cathode at potentials more anodic, wave A, than the reversible potential for metal deposition. ii.- The electrochemical reduction of Dy(III) takes place in two very close consecutive steps: Dy(III) + 1e → Dy(II) (wave A) and Dy(II) + 2e → Dy (wave B). More information could be obtained by chronopotentiometry. Chronopotentiometric transients show that the reduction mechanism takes part in two successive steps. From theoretical considerations, Berzins and Delahay [36] have shown that for two consecutive diffusion controlled reaction, the ratio of the two transition times is given by Eq. (1) n22 τ2 n2 (1) =2 + τ1 n1 n21 where n1 and n2 are the numbers of electrons involved in the two steps. For n1 = 1 and n2 = 2 the calculated value of the ratio of the transition times is 8. The experimental values τ 2 /τ 1 ∼ = 7 was in agreement with the theoretical value if we take into account that Eq. (1) assumes that Dy(III) and Dy(II) are reduced at sufficiently different potentials, which is not completely accomplished in this case. In addition, the measurement of τ 1 it is not accurate (see Fig. 2).
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Fig. 2. Chronopotentiometric curve obtained on a tungsten electrode (0.54 cm2 ) in the eutectic LiCl–KCl melt at 773 K containing DyCl3 (1.0664 × 10−4 mol cm−3 ).
Square wave voltammograms were also obtained. This technique was described in detail by Osteryoung and Osteryoung [37] and Ramaley and Krasue [38]. The potential–time function consists of the sum of a synchronised square wave and a staircase potential ramp. The current is sampled at the end of every half wave and then differentiated. This allows capacitive and residual currents to be eliminated and makes the method highly sensitive. For a simple reversible reaction, the net current-potential curve is bell-shaped and symmetrical about the half-wave potential, and the peak height is proportional to concentration of the electroactive species [38,39]. The width of the half-peak, W1/2 , depends on the number of electrons exchanged and on the temperature as follows: W1/2 = 3.52
RT nF
(2)
Fig. 3 presents a net-current square wave voltammogram for the reduction of a Dy(III) solution at a tungsten electrode, which is typical of a multicomponent system with interfering waves. The deconvolution model used to process the curve reveals two peaks (A and B). By measuring the width at might-height of peaks A and B it is possible to determined by means of Eq. (2) the number of electrons [40]. The n-values obtained are close to 1 for the first peak (A) and 2 for the second peak (B) if we consider that Eq. (2) is accomplished for a soluble–insoluble system. These results point out that the electrodeposition of Dy(0) at an inert electrode takes place via the intermediate formation of Dy(II), then the solubility of Dy metal in the molten chloride is expected to increase in the presence of Dy(III). 3.1.2. Electrochemical nucleation of Dy in the eutectic LiCl–KCl A typical cyclic voltammogram of LiCl–KCl–DyCl3 solution on a tungsten electrode with evidence of nucleation and
Fig. 3. Net-current square wave voltammogram for the reduction of Dy(III) at a tungsten electrode in the eutectic LiCl–KCl melt at 723 K. Pulse height: 25 mV; potential step: 1 mV; frequency: 30 Hz. S = 0.44 cm2 . (♦) Experimental and (—) theoretical curves.
crystal growth is shown in Fig. 4. The cathodic part of this voltammogram presents a “crossover” of the direct and the reverse curve. The deposition of dysprosium does not commence until a potential well past Erev is reached. The reason for this behaviour is that the formation of stable Dy nuclei on an inert surface requires a potential more negative than the reduction of Dy(III) ions on a Dy surface. Chronoamperometry is a technique particularly sensitive to nucleation and growth phenomena [41–45]. If the metal is deposited on a dissimilar substrate, the initial response of the current yields information about the nucleation behaviour of the metal on the foreign substrate in question. I–t transients of Dy(III) for various potential pulses at a tungsten electrode are shown in Fig. 5. The shape of these curves indicated that nucleation and growth of Dy covering a foreign substrate play a part in the overall deposition process. The initial regime of each transient is characterised by a decrease in current which corresponds, after charging of the double layer, to the formation of the first nuclei. This is followed by an increase in the current associated with crystal growth on the electrode. Finally the current decays in the usual way with time due to diffusion control. The rising part of the current culminates in a maximum, im , as the individual diffusion zones of the growing nuclei merge, and we can see that the higher the overpotential, the greater the value of im . The position of this maximum on the time axis, tm , depends on the magnitude of the potential step, and decreases as the applied potential is more negative.
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Fig. 6. Plots of i/t1/2 constructed from the rising portion of the curves shown in Fig. 5.
Fig. 4. Cyclic voltammogram illustrating the “nucleation crossover effect” on the return sweep for deposition of dysprosium onto tungsten electrode.
After every run the deposited metal was removed from the surface by polarizing the working electrode anodically. The most interesting part of the cathodic i–t transients is the rising portion of the curve which corresponds to the current before overlapping of diffusion zones of the first monolayer of the growing nuclei and therefore can be used to determine the kinetics of nuclei growth. Then, the rising part of the chronoamperograms was analysed and compared to models
Fig. 5. Potentiostatic current–time transients of LiCl–KCl–DyCl3 at various overpotentials [(1) −3.325, (2) −3.330, (3) −3.335, (4) −3.340 an (5) −3.345 V vs. Cl2 /Cl− ] with 1.0941 × 10−4 mol cm−3 of DyCl3 on a tungsten electrode (S = 0.34 cm2 ) at 693 K.
developed to describe instantaneous nucleation in which all the Dy nuclei are created at the same moment at the beginning of the electrolysis, and progressive nucleation in which new crystals are continuously created throughout electrolysis. In order to identify the dysprosium nucleation mode, we have analysed: (i) the relationship between current, i and time, t; and (ii) the non-dimensional plots of the chronoamperometric curves according to Scharifker and Hills [41]. According to the literature [41,45], the relationship between current, i and time, t, at the initial response following double-layer charging is given by an equation of the following type: i = αt x
(3)
where the exponent x depends on the type of nucleation, the geometry of the nuclei, and the growth conditions. The various models, which corresponding values of α and x, were presented by Allongue and Souteyrand [45]. The proportionality between i and t1/2 is shown in Fig. 6 for various overvoltages demonstrating that initial stages of electrochemical deposition of dysprosium at a tungsten electrode can be explained in terms of a model involving instantaneous and three-dimensional nucleation and the growth of the crystals is controlled by hemispherical or linear diffusion [45]. Scharifker and Hills [41] developed non-dimensional model adapted for this situation. Since the entire transient curve is analysed, the non-dimensional model was found to be more accurate. The entire dimensionless experimental current-time transients obtained at different applied cathodic potentials were compared to the appropriate theoretical transients reported by the authors for instantaneous and progressive nucleation (Eqs. (4) and (5) respectively). 2 I [1 − exp(−1.2564(t/tmax ))]2 = 1.9542 (4) Imax (t/tmax )
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3.1.3. Determination of the Dy(III) diffusion coefficient: verification of the Arrhenius law The diffusion coefficient, DDy(III) has been calculated by chronopotentiometry measuring the global transition time τ, being τ = (τ 1 + τ 2 ), by means of Eq. (6): Iτ 1/2 =
Fig. 7. Comparison of the dimensionless experimental data derived from the current–time transients with the theoretical models for (1) instantaneous and (2) progressive nucleation at different overvoltages: () −3.340, ( ) −3.345, and (♦) −3.360 V vs. Cl2 /Cl− .
I Imax
2 = 1.2254
[1 − exp(−2.3367(t/tmax )2 )] (t/tmax )
(5)
The results (see Fig. 7) confirmed the above statement: initial stages of electrochemical deposition of dysprosium on a tungsten electrode in the eutectic LiCl–KCl mixture, can be explained in terms of a model involving instantaneous nucleation with three-dimensional growth of the nuclei, whatever the overvoltage. As shown in Fig. 8, the experimental data always fit the model in the 673–823 K temperature range. Thus, the nucleation mode of dysprosium is not influenced by temperature in this range.
Fig. 8. Comparison of the dimensionless experimental data derived from the current–time transients with the theoretical models for (1) instantaneous and (2) progressive nucleation at different temperatures: () 693 K, () 723 K, (♦) 773 K, and ( ) 823 K.
3FSCo D1/2 π1/2 2
(6)
which is easily deduced from the Sand’s equation [39] for the first step and the Berzins and Delahay’s relation for two consecutive reactions [36,39] The validity of Eq. (6) requires that the mass transport towards the working electrode is only diffusion controlled, excluding any convective movement. Moreover, although a tungsten wire was used as working electrode and Eq. (6) is relevant to plane semi-infinite diffusion, it has to be assumed that under the experimental conditions the corrections related to cylindrical geometry can be neglected [46–49]. In addition short measurements times should be avoided to minimize the charging effect. Transition times for several current densities were measured and the resultant I versus τ −1/2 plot yielded a straight line (Fig. 9), indicating that all the requirements for using Eq. (6) are fulfilled. In the determination of the diffusion coefficient of the electroactive species it is really difficult to define the exact electroactive area of the working electrode in the molten salt electrolyte, mainly due to the wetting between the electrode and the molten salt, i.e. meniscus effect. Nevertheless, it is possible to cancel this effect by comparing the obtained transition times at various immersion depths of the working electrode when the concentration of Dy(III) in solution, the current passed through the working electrode, and the temperature
Fig. 9. Verification of Sand’s law. Data obtained from the chronopotentiometric curves obtained at different cathodic current density. [DyCl3 ] = 1.2364 × 10−4 mol cm−3 . S = 0.32 cm2 . T = 723 K.
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Fig. 10. Chronopotentiograms for LiCl–KCl–DyCl3 solutions with various surface areas of working electrodes (W). Current: −28 mA. [DyCl3 ] = 1.2380 × 10−4 mol cm−3 . T = 723 K.
of the system are kept constant. Under these conditions, the transition time should only be a function of the surface area of the working electrode, in agreement with Eq. (7) τ 1/2 =
nFCo D1/2 π1/2 nFCo D1/2 π1/2 S= (S0 + S) 2I 2I
(7)
Fig. 10 shows typical chronopotentiograms obtained for LiCl–KCl–DyCl3 changing the immersion height of the tungsten electrode. The plateau corresponding to the reduction of Dy(III) and potential jump portions before and after the plateau can be clearly defined. The transition time became shorter with decreasing surface area of the working electrode. Such a relation is plotted in Fig. 11. The square root of the transition time changed linearly with the change in the surface area of the working electrode at all temperatures.
Fig. 12. Variation of diffusion coefficient of Dy(III) with temperature in LiCl–KCl. Verification of the Arrhenius law.
The diffusion coefficient of Dy(III) was calculated from the slopes in Fig. 11 by Eq. (7). Fig. 12 shows the variation of the logarithm of the diffusion coefficient versus 1/T. The straight lines obtained showed the validity of the Arrhenius law. Finally, the diffusion coefficient of Dy(III) in LiCl–KCl is formulated as follows: log DDy = −2.63 −
1702 T
(8)
From this equation the temperature dependence of the diffusion coefficient was established, and the activation energy value for the diffusion process could be extracted, being H = −32.54 kJ/mol and independent of the temperature, a value in agreement with those obtained for another rare earths with similar ionic radius, i.e. Ce3+ [35,50,51]. 3.2. Electrochemical reduction of Dy(III) ions at an Al wire electrode: Dy–Al alloy formation In the previous section the metal deposition was studied on an inert electrode. However, dysprosium can form alloys with more noble metals such as aluminium. As general information of the Dy–Al system, the reported phase diagram [52] is shown in Fig. 13 The diagram shows the presence of five intermetallic compounds (DyAl3, DyAl2 , DyAl, Dy3 Al2 , and Dy2 Al). It has been shown that the alloying process can conveniently be studied by electrochemical techniques. [14,16,19,53–56]. The analysis of voltammograms and open-circuit chronopotentiograms provides interesting information concerning the alloy formation.
Fig. 11. Relation between square root of transition time and surface area working electrode at 723 K.
3.2.1. Results obtained by cyclic voltammetry Cyclic voltammograms of a LiCl–KCl–DyCl3 solution with W and Al electrodes are shown in Fig. 14. The shape of
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Fig. 13. Phase diagrams of Dy–Al system.
the curve obtained at the W electrode is very different from the curve obtained at the Al wire. The tungsten electrode was used as a reference to compare with the Al substrate because no alloys exist for the W–Dy binary systems. As we showed in the previous section, if the electrode substrate is a W wire the reaction scheme comprises two
very close electrochemical steps as follows: Dy(III) + 1e ↔ Dy(II)) Dy(II) + 2e ↔ Dy(0)
(9) (10)
For the Al electrode, one cathodic peak A is observed from −2.69 V (versus Cl2 /Cl− ). Since this potential value is more positive than the potential of Dy metal deposition, the cathodic peak is thought to be caused by the formation of a Dy–Al alloy, which according to the phase diagram is probably DyAl3 according to the reaction: Dy(III) + 3Al + 3e ↔ DyAl3
(11)
The potential shift was thermodynamically interpreted in terms of a lowering of activity of the deposited metal in the Al phase. When the scanning direction is reversed, one anodic peak A is observed, which corresponds to Dy dissolution from the Dy–Al alloy. The final rises of the voltammograms correspond to the anodic dissolution of the electrode material, limiting the potential window when Al is used as working material. The Dy–Al chemical interaction: Dy(0) + 3Al ↔ DyAl3
Fig. 14. Cyclic voltammograms for the reduction of a LiCl–KCl–DyCl3 solution at an Al electrode (curves 1–3) and at a W electrode (curve 4) at 773 K. Scan rate: 0.1 V/s.
(12)
may be considered as one chemical reaction following the charge transfer, then the reaction scheme at the Al electrode may be viewed as an EC process. The chemical step (12) is considered an equilibrium, and the product of the charge transfer Dy(0) is stabilized by this step, then the potential corresponding to reaction (11) is shifted in the positive direction compared with the reduction potential of Dy(III) to Dy(0).
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of DyAl3 + Al, according to the phase diagrams of Dy–Al compounds. Due to the reaction: 2Dy(III) + Dy(0) ↔ 3Dy(II)
(13)
it is not possible to use the emf of the cell Dy|DyCl3 in LiCl–KCl|Dy–Al
(14)
for determining the standard Gibbs energy of formation G0f of the intermetallic compound DyAl3 . The standard potential for reactions (9) and (10) could be estimated from logarithmic analysis of voltammograms and deconvolution of square wave voltammograms. It is obtained at 723 K: 0 EDy(III)/Dy(II) = −3.243 V (versus Cl2 /Cl− )
Fig. 15. Open circuit transient curve for an Al electrode after electrodepositing Dy metal at −2.829 V vs. Cl2 /Cl− for 60 s at 673 K.
3.2.2. Results obtained by open circuit chronopotentiometry Open-circuit potentiometry was carried out to investigate the formation potential of Dy–Al alloys. The measurements were conducted as follows. Firstly, Dy metal is electrodeposited at an Al electrode by potentiostatic electrolysis for a little period. Then, a transient curve of the open circuit potential was measured. Since the deposited Dy metal reacts with the Al and diffuses into the bulk of the electrode, the electrode potential gradually shifts to more positive values. During this process, a potential plateau is observed when a composition of the electrode surface is within a range of two phase coexisting state. Fig. 15 shows one example of the open circuit potential transient curves for an Al wire after depositing Dy metal by potentiostatic electrolysis at 723 K. During this process, one potential plateau is observed before the abandon potential of the Al wire, which corresponds to two phase coexisting state
0 EDy(II)/Dy(0) = −3.254 V (versus Cl2 /Cl− )
In the vicinity of a pure dysprosium electrode, the concentration of Dy(III) and Dy(II) obey the equation K=
[Dy(II)]3 [Dy(III)]
2
0 0 (EDy(III)/Dy(II) −EDy(II)/Dy(0) )2F/RT
=e
= e(−3.243+3.254)2F/RT = e0.35 = 1.4 mol/kg This result show that, in the concentration range of the experiments (∼0.076 mol kg−1 ), the concentrations of dysprosium species are: [Dy(II)] = 0.048 and [Dy(III)] = 0.028 mol kg−1 . Thus, in the vicinity of the dysprosium electrode, most of the dysprosium ions are in the oxidation state (II), whereas around the aluminium-dysprosium electrode they are in the oxidation state (III). This situation has not been taken into consideration in some previous papers [16,57]. However, the problem has been considered by Sheng et al. [25] who indicate that “the valency of Dy at the salt/Dy metal interface is not very clear”. They observed a
Fig. 16. (a) Surface SEM image of a DyAl3 electrodeposit obtained by potentiostatic electrolysis and (b) EDX analysis of the sample.
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fluctuation of emf which they considered to be due to the presence of a metal fog at the electrode surface. 3.2.3. Potentiostatic electrolysis Based on the results obtained by cyclic voltammetry and chronopotentiometry and in order to confirm the formation of Dy–Al alloys, potentiostatic electrolysis were conducted at −2.98 V (versus Cl2 /Cl− ), a potential value more positive than the Dy metal deposition potential, for 2 h at 723 K, using an aluminium foil as working electrode. After the electrolysis, the samples were washed by anhydrous ethylene glycol (Aldrich 99.8%) and stored inside the glove box until their analysis. The formation of DyAl3 alloy was detected by SEM-EDX. The surface morphology of the deposits was observed by scanning electron microscopy (SEM) (Fig. 16a), and the samples were analysed by EDX (Fig. 16b) showing the existence of both Dy and Al. In addition, cross section of the sample analysis showed that the thickness of the deposit was comprised between 66 and 166 m.
4. Conclusions Electrochemical properties of dysprosium were studied in the eutectic LiCl–KCl using different substrates: (i) W as an inert working electrode and (ii) Al a more noble metal than Dy with possibility of alloy formation. Different behaviours were found for the two electrodes. By combining different electrochemical techniques (i.e. voltammetry, chronopotentiometry, and square wave voltammetry), it was possible to determine the mechanism for the Dy(III) electroreduction in the molten chloride mixture. The electrodeposition of Dy(0) at an inert electrode seems to take place via the intermediate formation of Dy(II) in two very close electrochemical steps. Dy(III) + 1e ↔ Dy(II) and Dy(II) + 2e ↔ Dy(0) Then, the solubility of Dy metal in the molten chloride is expected to increase in the presence of Dy(III). Voltammetric and chronoamperometric techniques showed that nucleation and growth of the metallic dysprosium deposit show an important role in the overall electrodeposition process. Analysis of the transient curves (both of the rising part and the complete curve according to a non-dimensional model) showed that the initial stages of electrochemical deposition of dysprosium on a tungsten electrode can be explained in terms of a model involving instantaneous nucleation (i.e. all the nuclei are formed “immediately” after applying the potential step) with three-dimensional growth of the nuclei. Identical results were found when varying the working temperature from 673 up to 823 K. Moreover, the diffusion coefficient of the Dy(III) ions was calculated by chronopotentiometry and showed a temperature dependence according to the Arrhenius law.
Cyclic voltammetry and open circuit chronopotentiometry measurements have proven that dysprosium alloys with solid Al electrodes. The electroreduction of Dy(III) on Al proceeds via a one-step mechanism leading to the formation of a stable Dy–Al alloy. SEM-EDX analysis of the obtained deposits confirmed the DyAl3 alloy formation. Cross section analysis indicates a good transport of Dy metal into the Al cathode.
Acknowledgements The authors are grateful to Ministerio de Educaci´on y Ciencia MEC-FEDER (Spain) Project ENE200400317/CON for financial support.
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Electrochemical behaviour of dysprosium in the eutectic LiCl–KCl at W and Al electrodes Y. Castrillejoa,∗ , M.R. Bermejoa , A.I. Barradoa , R. Pardoa , E. Barradoa , A.M. Mart´ınezb a
Dpto de Qu´ımica Anal´ıtica, Facultad de Ciencias, Universidad de Valladolid, Prado de la Magdalena s/n, 47005 Valladolid, Spain b Department of Materials Technology, Sem Sælands vei 6, 7491 Trondheim, Norway Received 21 April 2004; received in revised form 10 July 2004; accepted 12 September 2004
Abstract The electrochemical behaviour of DyCl3 was studied in the eutectic LiCl–KCl at different temperatures. The cathodic reaction can be written: Dy(III) + 3e ↔ Dy(0) which can be divided in two very close cathodic steps: Dy(III) + 1e ↔ Dy(II) and Dy(II) + 2e ↔ Dy(0) Transient electrochemical techniques, such as cyclic voltammetry, chronopotentiometry, and chronoamperometry were used in order to study the reaction mechanism and the transport parameters of electroactive species at a tungsten electrode. The results showed that in the eutectic LiCl–KCl, electrocrystallization of dysprosium seems to be the controlling electrochemical step. Chronoamperometric studies indicated instantaneous nucleation of dysprosium with three dimensional growth of the nuclei whatever the applied overpotential. Mass transport towards the electrode is a simple diffusion process, and the diffusion coefficient of the electroactive species, i.e. Dy(III), has been calculated. The validity of the Arrhenius law was also verified by plotting the variation of the logarithm of the diffusion coefficient versus 1/T. In addition, the electrode reactions of the LiCl–KCl–DyCl3 solutions at an Al wire were also investigated by cyclic voltammetry and open circuit chronopotentiometry. The redox potential of the Dy(III)/Dy couple at the Al electrode was observed at more positive potentials values than those at the inert electrode. This potential shift was thermodynamically analyzed by a lowering of activity of Dy in the metal phase due to the formation of intermetallic compounds. © 2004 Elsevier Ltd. All rights reserved. Keywords: Molten chlorides; Dysprosium chloride; Electrodeposition; Nucleation and crystal growth; Diffusion coefficient; Dy–Al alloy formation
1. Introduction Electrochemical processes of metals (electrowinning, electrorefining, electroplating, and electroforming) in molten salts media, present some advantages in comparison with those carried out in aqueous solution. Higher efficiency of the ∗ Corresponding author. Tel.: +34 983423000x4245; fax: +34 983423013. E-mail address: [email protected] (Y. Castrillejo).
0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.09.013
electrolysis, lower energy consumption, often high electrodeposition rates and much better characteristics of the deposits [1] can be pointed out as some of the main advantages. Moreover, of all the elements in the periodic table only about one quarter may be electrodeposited from aqueous electrolytes in their elemental form or as alloys suitable for coatings, the rest are of engineering and scientific interest because their specific properties, and methods of depositing them are desirable. The electrodeposition in molten salts is a specific way to obtain compounds of reactive and refractory elements, such
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as the refractory metals, the actinides and rare earth metals [2]. The accumulated knowledge concerning molten salts high-temperature electrochemistry leads to openings for the deposition of metals in the solid state and in alloys. Their possibilities lie in the fact that, because of their variety, one can always find a solvent whose chemical and electrochemical characteristics and melting point are suitable to carry out a given process. In the last years a new field has been developed, namely the use of molten salts media for pyrochemical separation as a promising option in the nuclear fuel in the future [3–9]. The interest is due to the progress in the assessment of new concepts for transmutation and the corresponding fuel cycles [10]. In order to assess the feasibility of such pyrochemical separation, several processes have been considered for the recovery of minor actinides from spent metallic, nitride, oxide nuclear fuels, and high level radioactive liquid wastes [11,12]. One of the most important steps in the pyrometallurgical reprocessing is the electrorefining from molten chlorides. In this step, spent metal fuel is anodically dissolved into molten chlorides, and the minor actinides are selectively recovered at the cathodes due to the differences among the redox potentials of the elements, while fission products remain in the anode and/or in the electrolyte salt [9,13]. Alloy electrodeposition is involved in the separation of minor actinides from rare earths, the most difficult fission products to separate due to their similar properties. Each of these category of elements is well known to yield easily and with high rates alloys or compounds with more noble elements such as cadmium, nickel or aluminium [2,9,13–19]. The deposition in an alloyed form of these elements proceeds at considerably more anodic potentials, so that their separation from other reactive elements could become more easy. The determination of the electrochemical behaviour of the elements on different substrates, with and without alloy formation, is of crucial importance for the understanding of the process and the design of the separation cell. The work presented here is part of a series focused on the study of separation of actinides from rare earths. Hence, obtaining basic data of fission products in molten halide salts is a major concern. The work presents a study of the electrochemical properties of dysprosium trichloride and has been carried out in the eutectic LiCl–KCl. Few studies have been conducted relating to the chemical and electrochemical behaviour of dysprosium in molten chlorides. It is reported by Plambeck [20], that in the eutectic LiCl–KCl, Dy metal reacts slowly with the melt to give a dark solid and a pale-violet solution containing dysprosium in some form. The Dy(III) species has proven stable in this melt [21], so the reaction might give Dy(III), Dy(II), or possibly both [22]. The existence of Dy(II) has been noticed by some authors [23,24], in this way Franklin et al. [24] indicate that when a solid metal Dy is immersed in LiCl–KCl–DyCl3 , the metal reacts with DyCl3 to form DyCl2 . The DyCl2 is dispersed in the bulk of the molten slats and appear to dis-
proportionate into Dy and DyCl3 . According to Sheng et al. [25] the Dy formed in the bulk by this mechanism may be considered a “metal fog”. On the contrary, Chang et al. [26] propose that the electroreduction of Dy(III) solutions takes place in only one three electron reversible electrochemical step Dy(III)/Dy(0). Although the mechanism of electroreduction of Dy ions at one inert electrode is not clear yet, the electrochemical preparation of Dy–Fe and Dy–Ni alloys has been conducted by Konishi et al. [14–17] in the eutectic LiCl–KCl. The purpose of our investigation was to determine the electrochemical behaviour of a LiCl–KCl–DyCl3 melt at temperatures between 673 and 823 K at different substrates: (i) inert electrodes; and (ii) more noble metals than Dy with possibility of alloy film formation. In order to study the deposition of pure metal, an inert working electrode must be used. Preliminary experiments showed that refractory metals such as tungsten was a convenient electrode material and transient electrochemical techniques, such as cyclic voltammetry, chronopotentiometry, and chronoamperometry, were used to determine the reaction mechanism and the transport parameters of electroactive species. Moreover nucleation studies of Dy metal were carried out. The results showed that under the experimental conditions deposition of dysprosium at a tungsten electrode can be explained in terms of a model involving instantaneous and three-dimensional nucleation where the growth of the crystals is controlled by hemispherical or linear diffusion, over the whole cathodic potential range studied. The electrode reactions of the LiCl–KCl–DyCl3 solutions at an Al wire were also investigated by cyclic voltammetry and open circuit chronopotentiometry. The redox potential of the Dy(III)/Dy couple at the Al electrode was observed at more positive potentials than those at the inert electrode. This potential shift was thermodynamically analyzed by a lowering of the activity of Dy in the metal phase due to the formation of intermetallic compounds.
2. Experimental 2.1. Preparation and purification of the melt The experimental cell was carefully prepared. Therefore all handling of the salts was carried out in a glove box mBraun Labstar 50 under an argon atmosphere. Moreover, purification of the cell was very important in order to obtain consistent results. The chloride mixture (LiCl–KCl, analytical-grade) was melted in an alumina crucible placed in a quartz cell inside a Taner furnace. A West 3300 programmable device controlled the temperature of the furnace to ±2 ◦ C. The working temperature was measured with a thermocouple protected by an alumina tube inserted into the melt. The mixture was fused under vacuum, then raised to atmospheric pressure using dry argon; afterwards it was purified
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by bubbling HCl through the melt for at least 20 min, and then kept under an argon atmosphere which removed the residual HCl and maintained an inert atmosphere during experiments. This procedure has been used previously [27–35]. 2.2. General features Solutions of Dy(III) in the concentration range 0.04–0.09 mol/kg, were prepared by direct additions of solid DyCl3 (Sigma Aldrich 99.99%). The experimental problems related to the low solubility of Dy-O compounds prevented preparation of stable solutions of Dy(III) for longer periods, more than 1 day, and it was hard to know the exact amount of salt introduced into the melt, for which HCl was bubbled each day prior to determinations. The total concentration of dissolved rare earth was calculated by ICP-AES analysis of frequently taken melt samples 2.3. 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. The reference electrode consisted of a silver wire (1 mm diameter) dipped into a silver chloride solution (0.75 mol kg−1 ) in the LiCl-KCl molten mixture, contained in a Pyrex tube. Potentials were measured by reference to the Ag|AgCl couple and translated into potentials versus Cl2 /Cl− . The working electrodes consisted of 1 mm tungsten and aluminium wires (Sigma Aldrich, 99.99% and 99.999%). Another tungsten wire was used as the counter electrode. The lower end of the tungsten working electrode was polished thoroughly by using SiC paper, and then cleaned in ethanol using ultrasound. Potentiostatic electrolysis was conducted with a 1 mm thick aluminium foil cathode (Sigma Aldrich, 99.999%) and a graphite rod (Sofacel) as counter electrode. Auxiliary techniques such scanning electron microscopy (SEM), EDX analyses and ICP-AES analyses were also used. 3. Results and discussion 3.1. Electrochemical behaviour of LiCl–KCl–DyCl3 solution at a tungsten electrode 3.1.1. Electrochemical characterization of the systems The stable oxidation states of dysprosium were identified by different electrochemical techniques, i.e. cyclic voltammetry, chronopotentiometry, and square wave voltammetry. Fig. 1 shows representative examples of cyclic voltammograms of a LiCl–KCl–DyCl3 solution at a tungsten electrode. The electroreduction takes place in two very close electrochemical steps A and B in the potential range close to the cathodic limit of the melt (electrodeposition of liquid lithium). The shape of the cathodic wave A it is not clear enough, while the shape of the following wave B, is characteristic of the formation of a new phase, Dy(0), steep rise and slow decay. In the
Fig. 1. Typical cyclic voltammogram for the reduction of dysprosium trichloride (1.4191 × 10−4 mol cm−3 ) on a tungsten electrode in the eutectic LiCl–KCl melt at 723 K. Sweep rate 0.2 V s−1 .
positive potential scan direction, the reduced species Dy(0) are reoxidized in one overall peak B , with the expected characteristics for a stripping peak. The electrochemical signals could be explained by one of the following mechanisms: i.- The formation of dissolved Dy, which takes place at the cathode at potentials more anodic, wave A, than the reversible potential for metal deposition. ii.- The electrochemical reduction of Dy(III) takes place in two very close consecutive steps: Dy(III) + 1e → Dy(II) (wave A) and Dy(II) + 2e → Dy (wave B). More information could be obtained by chronopotentiometry. Chronopotentiometric transients show that the reduction mechanism takes part in two successive steps. From theoretical considerations, Berzins and Delahay [36] have shown that for two consecutive diffusion controlled reaction, the ratio of the two transition times is given by Eq. (1) n22 τ2 n2 (1) =2 + τ1 n1 n21 where n1 and n2 are the numbers of electrons involved in the two steps. For n1 = 1 and n2 = 2 the calculated value of the ratio of the transition times is 8. The experimental values τ 2 /τ 1 ∼ = 7 was in agreement with the theoretical value if we take into account that Eq. (1) assumes that Dy(III) and Dy(II) are reduced at sufficiently different potentials, which is not completely accomplished in this case. In addition, the measurement of τ 1 it is not accurate (see Fig. 2).
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Fig. 2. Chronopotentiometric curve obtained on a tungsten electrode (0.54 cm2 ) in the eutectic LiCl–KCl melt at 773 K containing DyCl3 (1.0664 × 10−4 mol cm−3 ).
Square wave voltammograms were also obtained. This technique was described in detail by Osteryoung and Osteryoung [37] and Ramaley and Krasue [38]. The potential–time function consists of the sum of a synchronised square wave and a staircase potential ramp. The current is sampled at the end of every half wave and then differentiated. This allows capacitive and residual currents to be eliminated and makes the method highly sensitive. For a simple reversible reaction, the net current-potential curve is bell-shaped and symmetrical about the half-wave potential, and the peak height is proportional to concentration of the electroactive species [38,39]. The width of the half-peak, W1/2 , depends on the number of electrons exchanged and on the temperature as follows: W1/2 = 3.52
RT nF
(2)
Fig. 3 presents a net-current square wave voltammogram for the reduction of a Dy(III) solution at a tungsten electrode, which is typical of a multicomponent system with interfering waves. The deconvolution model used to process the curve reveals two peaks (A and B). By measuring the width at might-height of peaks A and B it is possible to determined by means of Eq. (2) the number of electrons [40]. The n-values obtained are close to 1 for the first peak (A) and 2 for the second peak (B) if we consider that Eq. (2) is accomplished for a soluble–insoluble system. These results point out that the electrodeposition of Dy(0) at an inert electrode takes place via the intermediate formation of Dy(II), then the solubility of Dy metal in the molten chloride is expected to increase in the presence of Dy(III). 3.1.2. Electrochemical nucleation of Dy in the eutectic LiCl–KCl A typical cyclic voltammogram of LiCl–KCl–DyCl3 solution on a tungsten electrode with evidence of nucleation and
Fig. 3. Net-current square wave voltammogram for the reduction of Dy(III) at a tungsten electrode in the eutectic LiCl–KCl melt at 723 K. Pulse height: 25 mV; potential step: 1 mV; frequency: 30 Hz. S = 0.44 cm2 . (♦) Experimental and (—) theoretical curves.
crystal growth is shown in Fig. 4. The cathodic part of this voltammogram presents a “crossover” of the direct and the reverse curve. The deposition of dysprosium does not commence until a potential well past Erev is reached. The reason for this behaviour is that the formation of stable Dy nuclei on an inert surface requires a potential more negative than the reduction of Dy(III) ions on a Dy surface. Chronoamperometry is a technique particularly sensitive to nucleation and growth phenomena [41–45]. If the metal is deposited on a dissimilar substrate, the initial response of the current yields information about the nucleation behaviour of the metal on the foreign substrate in question. I–t transients of Dy(III) for various potential pulses at a tungsten electrode are shown in Fig. 5. The shape of these curves indicated that nucleation and growth of Dy covering a foreign substrate play a part in the overall deposition process. The initial regime of each transient is characterised by a decrease in current which corresponds, after charging of the double layer, to the formation of the first nuclei. This is followed by an increase in the current associated with crystal growth on the electrode. Finally the current decays in the usual way with time due to diffusion control. The rising part of the current culminates in a maximum, im , as the individual diffusion zones of the growing nuclei merge, and we can see that the higher the overpotential, the greater the value of im . The position of this maximum on the time axis, tm , depends on the magnitude of the potential step, and decreases as the applied potential is more negative.
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Fig. 6. Plots of i/t1/2 constructed from the rising portion of the curves shown in Fig. 5.
Fig. 4. Cyclic voltammogram illustrating the “nucleation crossover effect” on the return sweep for deposition of dysprosium onto tungsten electrode.
After every run the deposited metal was removed from the surface by polarizing the working electrode anodically. The most interesting part of the cathodic i–t transients is the rising portion of the curve which corresponds to the current before overlapping of diffusion zones of the first monolayer of the growing nuclei and therefore can be used to determine the kinetics of nuclei growth. Then, the rising part of the chronoamperograms was analysed and compared to models
Fig. 5. Potentiostatic current–time transients of LiCl–KCl–DyCl3 at various overpotentials [(1) −3.325, (2) −3.330, (3) −3.335, (4) −3.340 an (5) −3.345 V vs. Cl2 /Cl− ] with 1.0941 × 10−4 mol cm−3 of DyCl3 on a tungsten electrode (S = 0.34 cm2 ) at 693 K.
developed to describe instantaneous nucleation in which all the Dy nuclei are created at the same moment at the beginning of the electrolysis, and progressive nucleation in which new crystals are continuously created throughout electrolysis. In order to identify the dysprosium nucleation mode, we have analysed: (i) the relationship between current, i and time, t; and (ii) the non-dimensional plots of the chronoamperometric curves according to Scharifker and Hills [41]. According to the literature [41,45], the relationship between current, i and time, t, at the initial response following double-layer charging is given by an equation of the following type: i = αt x
(3)
where the exponent x depends on the type of nucleation, the geometry of the nuclei, and the growth conditions. The various models, which corresponding values of α and x, were presented by Allongue and Souteyrand [45]. The proportionality between i and t1/2 is shown in Fig. 6 for various overvoltages demonstrating that initial stages of electrochemical deposition of dysprosium at a tungsten electrode can be explained in terms of a model involving instantaneous and three-dimensional nucleation and the growth of the crystals is controlled by hemispherical or linear diffusion [45]. Scharifker and Hills [41] developed non-dimensional model adapted for this situation. Since the entire transient curve is analysed, the non-dimensional model was found to be more accurate. The entire dimensionless experimental current-time transients obtained at different applied cathodic potentials were compared to the appropriate theoretical transients reported by the authors for instantaneous and progressive nucleation (Eqs. (4) and (5) respectively). 2 I [1 − exp(−1.2564(t/tmax ))]2 = 1.9542 (4) Imax (t/tmax )
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3.1.3. Determination of the Dy(III) diffusion coefficient: verification of the Arrhenius law The diffusion coefficient, DDy(III) has been calculated by chronopotentiometry measuring the global transition time τ, being τ = (τ 1 + τ 2 ), by means of Eq. (6): Iτ 1/2 =
Fig. 7. Comparison of the dimensionless experimental data derived from the current–time transients with the theoretical models for (1) instantaneous and (2) progressive nucleation at different overvoltages: () −3.340, ( ) −3.345, and (♦) −3.360 V vs. Cl2 /Cl− .
I Imax
2 = 1.2254
[1 − exp(−2.3367(t/tmax )2 )] (t/tmax )
(5)
The results (see Fig. 7) confirmed the above statement: initial stages of electrochemical deposition of dysprosium on a tungsten electrode in the eutectic LiCl–KCl mixture, can be explained in terms of a model involving instantaneous nucleation with three-dimensional growth of the nuclei, whatever the overvoltage. As shown in Fig. 8, the experimental data always fit the model in the 673–823 K temperature range. Thus, the nucleation mode of dysprosium is not influenced by temperature in this range.
Fig. 8. Comparison of the dimensionless experimental data derived from the current–time transients with the theoretical models for (1) instantaneous and (2) progressive nucleation at different temperatures: () 693 K, () 723 K, (♦) 773 K, and ( ) 823 K.
3FSCo D1/2 π1/2 2
(6)
which is easily deduced from the Sand’s equation [39] for the first step and the Berzins and Delahay’s relation for two consecutive reactions [36,39] The validity of Eq. (6) requires that the mass transport towards the working electrode is only diffusion controlled, excluding any convective movement. Moreover, although a tungsten wire was used as working electrode and Eq. (6) is relevant to plane semi-infinite diffusion, it has to be assumed that under the experimental conditions the corrections related to cylindrical geometry can be neglected [46–49]. In addition short measurements times should be avoided to minimize the charging effect. Transition times for several current densities were measured and the resultant I versus τ −1/2 plot yielded a straight line (Fig. 9), indicating that all the requirements for using Eq. (6) are fulfilled. In the determination of the diffusion coefficient of the electroactive species it is really difficult to define the exact electroactive area of the working electrode in the molten salt electrolyte, mainly due to the wetting between the electrode and the molten salt, i.e. meniscus effect. Nevertheless, it is possible to cancel this effect by comparing the obtained transition times at various immersion depths of the working electrode when the concentration of Dy(III) in solution, the current passed through the working electrode, and the temperature
Fig. 9. Verification of Sand’s law. Data obtained from the chronopotentiometric curves obtained at different cathodic current density. [DyCl3 ] = 1.2364 × 10−4 mol cm−3 . S = 0.32 cm2 . T = 723 K.
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Fig. 10. Chronopotentiograms for LiCl–KCl–DyCl3 solutions with various surface areas of working electrodes (W). Current: −28 mA. [DyCl3 ] = 1.2380 × 10−4 mol cm−3 . T = 723 K.
of the system are kept constant. Under these conditions, the transition time should only be a function of the surface area of the working electrode, in agreement with Eq. (7) τ 1/2 =
nFCo D1/2 π1/2 nFCo D1/2 π1/2 S= (S0 + S) 2I 2I
(7)
Fig. 10 shows typical chronopotentiograms obtained for LiCl–KCl–DyCl3 changing the immersion height of the tungsten electrode. The plateau corresponding to the reduction of Dy(III) and potential jump portions before and after the plateau can be clearly defined. The transition time became shorter with decreasing surface area of the working electrode. Such a relation is plotted in Fig. 11. The square root of the transition time changed linearly with the change in the surface area of the working electrode at all temperatures.
Fig. 12. Variation of diffusion coefficient of Dy(III) with temperature in LiCl–KCl. Verification of the Arrhenius law.
The diffusion coefficient of Dy(III) was calculated from the slopes in Fig. 11 by Eq. (7). Fig. 12 shows the variation of the logarithm of the diffusion coefficient versus 1/T. The straight lines obtained showed the validity of the Arrhenius law. Finally, the diffusion coefficient of Dy(III) in LiCl–KCl is formulated as follows: log DDy = −2.63 −
1702 T
(8)
From this equation the temperature dependence of the diffusion coefficient was established, and the activation energy value for the diffusion process could be extracted, being H = −32.54 kJ/mol and independent of the temperature, a value in agreement with those obtained for another rare earths with similar ionic radius, i.e. Ce3+ [35,50,51]. 3.2. Electrochemical reduction of Dy(III) ions at an Al wire electrode: Dy–Al alloy formation In the previous section the metal deposition was studied on an inert electrode. However, dysprosium can form alloys with more noble metals such as aluminium. As general information of the Dy–Al system, the reported phase diagram [52] is shown in Fig. 13 The diagram shows the presence of five intermetallic compounds (DyAl3, DyAl2 , DyAl, Dy3 Al2 , and Dy2 Al). It has been shown that the alloying process can conveniently be studied by electrochemical techniques. [14,16,19,53–56]. The analysis of voltammograms and open-circuit chronopotentiograms provides interesting information concerning the alloy formation.
Fig. 11. Relation between square root of transition time and surface area working electrode at 723 K.
3.2.1. Results obtained by cyclic voltammetry Cyclic voltammograms of a LiCl–KCl–DyCl3 solution with W and Al electrodes are shown in Fig. 14. The shape of
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Fig. 13. Phase diagrams of Dy–Al system.
the curve obtained at the W electrode is very different from the curve obtained at the Al wire. The tungsten electrode was used as a reference to compare with the Al substrate because no alloys exist for the W–Dy binary systems. As we showed in the previous section, if the electrode substrate is a W wire the reaction scheme comprises two
very close electrochemical steps as follows: Dy(III) + 1e ↔ Dy(II)) Dy(II) + 2e ↔ Dy(0)
(9) (10)
For the Al electrode, one cathodic peak A is observed from −2.69 V (versus Cl2 /Cl− ). Since this potential value is more positive than the potential of Dy metal deposition, the cathodic peak is thought to be caused by the formation of a Dy–Al alloy, which according to the phase diagram is probably DyAl3 according to the reaction: Dy(III) + 3Al + 3e ↔ DyAl3
(11)
The potential shift was thermodynamically interpreted in terms of a lowering of activity of the deposited metal in the Al phase. When the scanning direction is reversed, one anodic peak A is observed, which corresponds to Dy dissolution from the Dy–Al alloy. The final rises of the voltammograms correspond to the anodic dissolution of the electrode material, limiting the potential window when Al is used as working material. The Dy–Al chemical interaction: Dy(0) + 3Al ↔ DyAl3
Fig. 14. Cyclic voltammograms for the reduction of a LiCl–KCl–DyCl3 solution at an Al electrode (curves 1–3) and at a W electrode (curve 4) at 773 K. Scan rate: 0.1 V/s.
(12)
may be considered as one chemical reaction following the charge transfer, then the reaction scheme at the Al electrode may be viewed as an EC process. The chemical step (12) is considered an equilibrium, and the product of the charge transfer Dy(0) is stabilized by this step, then the potential corresponding to reaction (11) is shifted in the positive direction compared with the reduction potential of Dy(III) to Dy(0).
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of DyAl3 + Al, according to the phase diagrams of Dy–Al compounds. Due to the reaction: 2Dy(III) + Dy(0) ↔ 3Dy(II)
(13)
it is not possible to use the emf of the cell Dy|DyCl3 in LiCl–KCl|Dy–Al
(14)
for determining the standard Gibbs energy of formation G0f of the intermetallic compound DyAl3 . The standard potential for reactions (9) and (10) could be estimated from logarithmic analysis of voltammograms and deconvolution of square wave voltammograms. It is obtained at 723 K: 0 EDy(III)/Dy(II) = −3.243 V (versus Cl2 /Cl− )
Fig. 15. Open circuit transient curve for an Al electrode after electrodepositing Dy metal at −2.829 V vs. Cl2 /Cl− for 60 s at 673 K.
3.2.2. Results obtained by open circuit chronopotentiometry Open-circuit potentiometry was carried out to investigate the formation potential of Dy–Al alloys. The measurements were conducted as follows. Firstly, Dy metal is electrodeposited at an Al electrode by potentiostatic electrolysis for a little period. Then, a transient curve of the open circuit potential was measured. Since the deposited Dy metal reacts with the Al and diffuses into the bulk of the electrode, the electrode potential gradually shifts to more positive values. During this process, a potential plateau is observed when a composition of the electrode surface is within a range of two phase coexisting state. Fig. 15 shows one example of the open circuit potential transient curves for an Al wire after depositing Dy metal by potentiostatic electrolysis at 723 K. During this process, one potential plateau is observed before the abandon potential of the Al wire, which corresponds to two phase coexisting state
0 EDy(II)/Dy(0) = −3.254 V (versus Cl2 /Cl− )
In the vicinity of a pure dysprosium electrode, the concentration of Dy(III) and Dy(II) obey the equation K=
[Dy(II)]3 [Dy(III)]
2
0 0 (EDy(III)/Dy(II) −EDy(II)/Dy(0) )2F/RT
=e
= e(−3.243+3.254)2F/RT = e0.35 = 1.4 mol/kg This result show that, in the concentration range of the experiments (∼0.076 mol kg−1 ), the concentrations of dysprosium species are: [Dy(II)] = 0.048 and [Dy(III)] = 0.028 mol kg−1 . Thus, in the vicinity of the dysprosium electrode, most of the dysprosium ions are in the oxidation state (II), whereas around the aluminium-dysprosium electrode they are in the oxidation state (III). This situation has not been taken into consideration in some previous papers [16,57]. However, the problem has been considered by Sheng et al. [25] who indicate that “the valency of Dy at the salt/Dy metal interface is not very clear”. They observed a
Fig. 16. (a) Surface SEM image of a DyAl3 electrodeposit obtained by potentiostatic electrolysis and (b) EDX analysis of the sample.
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fluctuation of emf which they considered to be due to the presence of a metal fog at the electrode surface. 3.2.3. Potentiostatic electrolysis Based on the results obtained by cyclic voltammetry and chronopotentiometry and in order to confirm the formation of Dy–Al alloys, potentiostatic electrolysis were conducted at −2.98 V (versus Cl2 /Cl− ), a potential value more positive than the Dy metal deposition potential, for 2 h at 723 K, using an aluminium foil as working electrode. After the electrolysis, the samples were washed by anhydrous ethylene glycol (Aldrich 99.8%) and stored inside the glove box until their analysis. The formation of DyAl3 alloy was detected by SEM-EDX. The surface morphology of the deposits was observed by scanning electron microscopy (SEM) (Fig. 16a), and the samples were analysed by EDX (Fig. 16b) showing the existence of both Dy and Al. In addition, cross section of the sample analysis showed that the thickness of the deposit was comprised between 66 and 166 m.
4. Conclusions Electrochemical properties of dysprosium were studied in the eutectic LiCl–KCl using different substrates: (i) W as an inert working electrode and (ii) Al a more noble metal than Dy with possibility of alloy formation. Different behaviours were found for the two electrodes. By combining different electrochemical techniques (i.e. voltammetry, chronopotentiometry, and square wave voltammetry), it was possible to determine the mechanism for the Dy(III) electroreduction in the molten chloride mixture. The electrodeposition of Dy(0) at an inert electrode seems to take place via the intermediate formation of Dy(II) in two very close electrochemical steps. Dy(III) + 1e ↔ Dy(II) and Dy(II) + 2e ↔ Dy(0) Then, the solubility of Dy metal in the molten chloride is expected to increase in the presence of Dy(III). Voltammetric and chronoamperometric techniques showed that nucleation and growth of the metallic dysprosium deposit show an important role in the overall electrodeposition process. Analysis of the transient curves (both of the rising part and the complete curve according to a non-dimensional model) showed that the initial stages of electrochemical deposition of dysprosium on a tungsten electrode can be explained in terms of a model involving instantaneous nucleation (i.e. all the nuclei are formed “immediately” after applying the potential step) with three-dimensional growth of the nuclei. Identical results were found when varying the working temperature from 673 up to 823 K. Moreover, the diffusion coefficient of the Dy(III) ions was calculated by chronopotentiometry and showed a temperature dependence according to the Arrhenius law.
Cyclic voltammetry and open circuit chronopotentiometry measurements have proven that dysprosium alloys with solid Al electrodes. The electroreduction of Dy(III) on Al proceeds via a one-step mechanism leading to the formation of a stable Dy–Al alloy. SEM-EDX analysis of the obtained deposits confirmed the DyAl3 alloy formation. Cross section analysis indicates a good transport of Dy metal into the Al cathode.
Acknowledgements The authors are grateful to Ministerio de Educaci´on y Ciencia MEC-FEDER (Spain) Project ENE200400317/CON for financial support.
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