Electrochemistry of thulium on inert electrodes and electrochemical formation of a Tm–Al alloy from molten chlorides

Electrochimica Acta 54 (2009) 6212–6222

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Electrochemistry of thulium on inert electrodes and electrochemical formation of a Tm–Al alloy from molten chlorides Y. Castrillejo a,∗ , P. Fernández a , M.R. Bermejo a , E. Barrado a , A.M. Martínez b a b

Dpto de Química Analítica, Facultad de Ciencias, Universidad de Valladolid, Prado de la Magdalena s/n, 47005 Valladolid, Spain SINTEF Materials and Chemistry, Sem Sælands vei 12, 7465 Trondheim, Norway

a r t i c l e

i n f o

Article history: Received 11 February 2009 Received in revised form 25 May 2009 Accepted 30 May 2009 Available online 10 June 2009 Keywords: Thulium Molten chlorides Nucleation Al–Tm alloys TmOCl Tm2 O3

a b s t r a c t The electrochemical behaviour of TmCl3 solutions was studied in the eutectic LiCl–KCl in the temperature range 673–823 K using inert and reactive electrodes, i.e. W and Al, respectively. On an inert electrode, Tm(III) ions are reduced to metallic thulium through two consecutive steps: Tm(III) + 1e ↔ Tm(II)

and

Tm(II) + 2e ↔ Tm(0)

The electroreduction of Tm(III) to Tm(II) was found to be quasi-reversible. The intrinsic rate constant of charge transfer, k0 , as well as of the charge transfer coefficient, ˛, have been calculated by simulation of the cyclic voltammograms and logarithmic analysis of the convoluted curves. Electrocrystallization of thulium plays an important role in the electrodeposition process, being the nucleation mode affected by temperature. The diffusion coefficients of Tm(III) and Tm(II) ions have been found to be equal. The validity of the Arrhenius law was verified by plotting the variation of the logarithm of the diffusion coefficients vs. 1/T. The electrode reactions of Tm(III) solutions at an Al electrode were also investigated. The results showed that for the extraction of thulium from molten chlorides, the use of a reactive electrode made of aluminium leading to Al–Tm alloys seems to be a pertinent route. Potentiometric titrations of Tm(III) solutions with oxide donors, using a ytria stabilized zirconia electrode “YSZE” as a pO2− indicator electrode, have shown the formation of thulium oxychloride and thulium oxide and their corresponding solubility products have been determined at 723 K (pks (TmOCl) = 8.0 ± 0.3 pks (Tm2 O3 ) = 18.8 ± 0.7). © 2009 Elsevier Ltd. All rights reserved.

1. Introduction Rare earth metals and alloys based on a rare earth and a light or transition metal are increasing in importance, particularly in the fields of magnetism, energy and high technology [1,2]. Since rare earths elements are very active, it is not possible to deposit them in aqueous solution by electrochemical methods. The use of molten salts, as reaction media, provides a unique opportunity for the electrowinning and electrorefining of high purity rare earth metals, as well as for the electrochemical synthesis of their alloys, being possible to control the composition and thickness of the alloys by controlling the electrochemical parameters. Another important issue concerning rare earths and molten salts is pyrochemical reprocessing of nuclear fuel, which is considered a promising option for the proposed advanced fuel cycles [3–5]. In this sense, the recycling or destruction (transmutation) of the

∗ Corresponding author. Tel.: +34 983 423000x4245; fax: +34 983 423013. E-mail address: [email protected] (Y. Castrillejo). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.05.095

more hazardous radionuclides in the waste into less hazardous or shorter lived elements would significantly reduce the volume and the required storage time for the waste [5]. However, before this transmutation can be achieved, it is necessary to separate the minor actinides (MAs), from other fission products (FPs). This separation process can be carried out in a molten salt media, due to their properties such as their radiation resistance, which allows a high actinide content, shorter cooling times and inherent proliferation resistance. Pyrochemical reprocessing methods can be rationally conceived via a thermodynamic and kinetic analysis based on a deep understanding of the actinides (Ans) and FPs chemistry and electrochemistry in molten salt. Although fuels proposed for transmutation tolerates a small content of FPs, most of them have to be removed. Special attention must be paid to the lanthanides (Lns) not only for its chemical similarity with respect to the actinides, that makes difficult its mutual separation, but also because of their neutronic poison effect, that could reduce the efficiency of the transmutation process. Another important issue is to consider that during the electrochemical separation process the FPs build up in

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the solvent; this fact can modify the characteristics of the electrolyte and contaminate the final cathodic product. When the FPs concentration exceeds about 10 wt% in the melt it must generally be purified or regenerated to avoid affecting the actinide/FP separation efficiency [6]. Our team has been engaged in a research program in which a two-step process is foreseen, corresponding to (i) the selective extraction of Ans and (ii) the extraction of Lns for decontamination of the salt. Our studies are devoted to the acquisition of fundamental data of Lns to allow conceptual design and assessment of reprocessing process involving as separation steps electrolytic extraction via liquid (Cd or Bi) or solid Al cathodes, and oxide selective dissolution/precipitation [7–20]. Specifically, this paper is concerned with the electrochemistry of thulium, as a part of the program to look into the Ln series in chloride melts which has impacts for fuel reprocessing. Despite that Tm has a small fission yield and consequently it is present in a small amount in the nuclear waste, the study of Tm is interesting as it has exactly the same electrochemical behaviour as Am [21]. The study has been carried out with a solution of TmCl3 in the eutectic LiCl–KCl mixture at temperatures between 673 and 823 K using different substrates: (i) W as an inert electrode and (ii) Al, a more noble metal than Tm with possibility of alloy formation. In addition, the reaction of Tm(III) with oxide ions, has also been studied. 2. Experimental 2.1. General features

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oxide and silver ions (3 × 10−2 and 0.75 mol kg−1 , respectively) into which a silver wire was immersed (inner reference Ag|AgCl). All the electrochemical studies were performed with a PAR EG&G Model 273A potentiostat/galvanostat controlled with the PAR EG&G M270 software package, and a multimeter Fluke 45. The Tm–Al alloys were analyzed by XRD using an X-ray diffractometer Philips PW1710, and SEM (JEOL JSM-820) was used to observe the morphology of the alloys. 3. Results and discussion 3.1. Electrochemistry of thulium at an inert electrode 3.1.1. Characterization of the electrochemical systems Fig. 1 shows typical cyclic voltammograms of the Tm(III) ions, presumably as TmCl6 3− as it has been reported in the case of other lanthanides [22], in LiCl–KCl at 673 K on a tungsten electrode. In these voltammograms two cathodic processes are clearly seen. The wave A and a well defined peak B in the potential range close to the cathodic limit of the melt (electrodeposition of lithium). During the reverse sweep, two signals A and B are observed. The cyclic voltammograms recorded with varying the cathodic limit, showed that the wave A is due to the oxidation of the product formed during the process A, and the sharp anodic peak B is related to B. The shape of waves A/A was the expected for a soluble–soluble electrochemical exchange, whereas the features of the peaks B/B were typical of the formation/dissolution of a new phase [23–25], presumably Tm metal.

The electrolytic bath was a mixture of LiCl–KCl (Sigma–Aldrich 99.0% and 99.5%) with the eutectic composition (58.8:41.2 mol%), and the container was an alumina crucible placed in a cylindrical quartz cell. All the reagents were handled in a glove box MBraun to avoid contamination of moisture. The electrolyte was initially fused under vacuum in a ramp mode and then raised to atmospheric pressure using dry argon. Purification of the melt was realized by bubbling HCl(g) through the melt for 30 min, and residual HCl was removed by bubbling argon gas. The experiments were performed under an inert argon atmosphere (99.999%, Air Liquide). The cell was heated using a tubular furnace connected to a West 8100 programmable device. The working temperature was varied from 673 to 823 K and was controlled with a nickel–chromium thermocouple and kept to ±2 K. Tm(III) ions, in the concentration range 0.06–0.09 mol/kg, were introduced into the bath in the form of TmCl3 (Sigma–Aldrich 99.9% or Alfa Aesar 99.9%) powder. In order to avoid thulium oxide or oxychloride formation, HCl(g) was bubbled prior to determinations. 2.2. Electrodes and instrumentation As reference electrode the Ag|AgCl system was used. It consisted of a Ag wire of 1 mm diameter dipped into a closed-end Pyrex glass tube, in which the LiCl–KCl eutectic salt containing 0.75 mol kg−1 AgCl was placed. Unless other is stated, all the potentials are referred to this reference. As working electrodes, tungsten and aluminium wires of 1.0 mm diameter were used. The counter electrode was a tungsten wire of 1.6 mm diameter, in order to have a higher surface and to ensure uniform current lines distribution. In the potentiostatic electrolysis experiments, 0.5 mm thick aluminium foil (Sigma–Aldrich, 99.99%) and a graphite rod of 6.0 mm diameter were used as working and counter electrodes, respectively. The pO2− indicator electrode consisted of a tube of yttriastabilised zirconia, supplied by Interbil Spain (inner diam. 4 mm, outer diam. 6 mm), filled with molten LiCl–KCl solution containing

Fig. 1. Cyclic voltammogram for the reduction of a TmCl3 solution in the eutectic LiCl–KCl melt using a tungsten electrode: T = 673 K. C0 = 1.16 × 10−4 mol cm−3 . Scan rate 0.2 V s−1 . Dotted line corresponds to a cyclic voltammogram of the eutectic LiCl–KCl mixture in absence of TmCl3 .

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sian signal, that accordingly with the literature [21,26,27] can be attributed to a nucleation overpotential due to the formation of a solid phase, which delays the rise of the current. A mathematical analysis of the Gaussian peak yields, in the case of a reversible soluble–soluble system, a simple equation associating the width of the half-peak, W1/2 , and the number of exchanged electrons [24,25]: W1/2 = 3.52

RT nF

(1)

It is possible to apply Eq. (1) to peak A, as long as the differential peak current is linear with the square root of the frequency, i.e. between 10 and 80 Hz. When Eq. (1) was applied to peak A, a value of n = 0.95 ± 0.4 was obtained, indicating that the electrochemical exchange corresponds to the Tm(III)/Tm(II) system. When the linear potential sweep data obtained with a solution of Tm(III) were transformed, according to the convolution principle [28,29], into a form resembling a steady-state voltammetric curve, one observes the occurrence of two plateaux proving the existence of two electrochemical exchanges Ox/Red1 , Red1 /Red2 (Fig. 3). It has been shown from theoretical considerations that when the diffusion coefficients of Ox and Red1 are equal, Eq. (2) is accomplished [30–32]: m∗1 + m∗2

=

m∗1

Fig. 2. Net-current square wave voltammogram for the reduction of a TmCl3 solution (1.16 × 10−4 mol cm−3 ) at 673 K. Pulse height: 25 mV; potential step: 3 mV; frequency: 20 Hz. (–) Experimental and (–) theoretical curve.

The assignment of the electrochemical system to the signals can be carried out by square wave voltammetry, convolution of the voltammetric curves, chronopotentiometry or a combination of them. In square wave voltammetry, two peaks (A and B) are obtained (Fig. 2), corresponding to the two steps of the reduction of Tm(III) observed in the voltammetric experiments. Peak A has the expected Gaussian shape, indicating an exchange between two soluble species, on the contrary peak B exhibits an asymmetrical Gaus-

n2 + n1 n1

(2)

where m1 *and m2 * are the maximum values of the semiintegral for the first and second step, respectively, and n1 and n2 the corresponding electron numbers. The experimental ratio (m2 * + m1 *)/m1 * was close to 3.0, whatever the studied temperatures, a value in good agreement with an exchange of one and two electrons for the first and second reduction steps, respectively. Two well defined transitions can also be seen on the Chronopotentiograms for Tm(III) reduction (Fig. 4). Berzins and Delahay [33] have shown that for two consecutive diffusion controlled reaction, the ratio of the two transition times is given by Eq. (3) [24,33]: 2 = 1

 n 2 2

n1

+2

n2 n1

(3)

in which,  1 and  2 are the transition times involved in the two steps. The experimental ratio  2 / 1 is close to 8 and confirms once again that the electrodeposition of Tm(0) at an inert electrode takes place in two consecutive steps with the intermediate formation of Tm(II), and that the diffusion coefficients of Tm(III) and Tm(II) are quite similar.

Fig. 3. Cyclic voltammogram for the reduction of Tm(III) and its corresponding convoluted curve. T = 673 K; C0 = 1.16 × 10−4 mol cm−3 .

Y. Castrillejo et al. / Electrochimica Acta 54 (2009) 6212–6222

Fig. 4. Chronopotentiogram of Tm(III) in LiCl–KCl. [Tm(III)] = 1.04 × 10−4 mol cm−3 ; temperature 723 K; I = −38 mA.

3.1.2. The Tm(III)/Tm(II) electrochemical system A series of voltammograms corresponding to the A/A exchange were recorded in a LiCl–KCl purified bath containing TmCl3 , at different temperatures for a variety of concentrations and scan rates. A reversibility test was performed by comparing their recorded convoluted voltammograms. The convoluted curves were independent of the scan rates, nevertheless they do not remain identical from the direct to the reverse scan in the cathodic potential domain. This behaviour pointed out that the electrochemical exchange is not fast [23,34]. A deeper analysis of the recorded voltammograms, as those in Fig. 5a, was carried out in the interval 693–823 K. The analysis is based on the measure of the peak currents and peak potentials, following the methodology indicated in the literature [23–25], and

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their variation with the scan rates. The main information extracted from the voltammograms is the following: (i) the ratio ipa /ipc holds a value close to 1, irrespective of the sweep rate; (ii) the plots of the cathodic and anodic peak currents vs. the square root of the scan rate were linear (Fig. 5b), being the ratio between the slopes of the plots close to unit, and (iii) the peak potentials do not depend on the sweep rate up to a value close to 0.4 V s−1 (Fig. 5c). According to the theory of cyclic voltammetry [23–25,34], it is possible to state that: (i) up to a sweep rate of 0.4 V s−1 the electron process is controlled by the rate of mass transfer and reversible voltammograms are recorded, whereas for higher scan rates, the electron transfer rate is significantly lower than that of the mass transport causing an increase in the peak separation, therefore, the electrochemical system can be considered as quasi-reversible, (ii) the diffusion coefficients of Tm(III) and Tm(II) are equal if the charge transfer coefficient, ˛, remains close to 0.5 [23,24]. According to these results, the electro-reduction of Tm(III) to Tm(II) takes place through a one-electron transfer step characterized by the intrinsic rate constant k0 and the charge transfer coefficient ˛, being the diffusion coefficients of both Tm(III) and Tm(II) also involved in the mechanism. The kinetic parameters k0 and ˛ can be obtained in practice: (i) by simulation of the cyclic voltammograms and (ii) from the analysis of the convoluted curves.

3.1.2.1. Determination of the charge transfer kinetic parameters by simulation of the cyclic voltammograms. The cyclic voltammograms obtained were very reproducible, and it was found that the information of interest could be easily extracted from the comparison of the experimental curves and the simulated voltammograms calculated using a simulation computer program (M271 COOL kinetic analysis software 1.10), for a quasi-reversible mechanism in which kappa, , which is related to the apparent standard rate constant,

Fig. 5. (a) A series of voltammograms related to the Tm(III)/Tm(II) electrochemical exchange at 673 K, [Tm(III)] = 1.16 × 10−4 mol cm−3 , S ≈ 0.35 cm2 , scan rates: (1) 0.2 V s−1 ; (2) 0.3 V s−1 ; (3) 0.4 V s−1 ; (4) 0.5 V s−1 ; (5) 0.6 V s−1 and (6) 0.8 V s−1 . (b) Variation of the cathodic () and anodic () peak current with the square root of the sweep rate. (c) Variation of the cathodic (♦) and anodic () peak potential with the logarithm of the sweep rate.

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Taken into account that, with a 95% of confidence level, there are no significant differences between the diffusion coefficients of the electroactive species (that will be demonstrated below), we can ap r . assume that E0 ≈ E1/2 3.1.2.2. Determination of the charge transfer kinetic parameters from the analysis of the convoluted curves. As an alternative approach to the estimation of the kinetic parameters an analysis of the convoluted curves was carried out according to a quasi-reversible reaction, by applying the following Eq. (7) [22,29,34]: r + E = E1/2

2.3RT log  ˛nF



Fig. 6. Cyclic voltammograms of the electrochemical system Tm(III)/Tm(II) at 723 K. [Tm(III)] = 1.14 × 10−4 mol cm−3 , sweep rate 0.6 V s−1 . (—) Experimental curve, (—) simulated curve corresponding to a quasi-reversible process, (- - -) simulated curve corresponding to a reversible model.

k0 , by means of Eq. (5), the charge-transfer coefficient, ˛, and the r , were adjusted to give the best reversible half-wave potential, E1/2 fit between the experimental and calculated results: =

k0

(4)

1/2 1−˛ ˛ DTm(II) ) (DTm(III)

The simulation method is based upon a non-linear simplex optimization of the parameters in a normalized space derived from linear regression of the measured current on a calculated dimensionless current function [35,36]. We also tried to fit the system to a reversible process, however the best results were found for a quasi-reversible process. Representative example of this simulation is shown in Fig. 6 and the average values obtained are gathered in Table 1. r , On the other hand, the reversible half wave potential, E1/2 obtained by this methodology, can be used for calculating the ap apparent standard potential in solution, E0 , of the electrochemical system Tm(III)/Tm(II) by using the following equation [23–25,34]: 1/2

r E1/2 = E00 +

DTm(II) Tm(III) RT RT + ln ln 1/2 F Tm(II) F DTm(III) 1/2

ap

= E0 +

DTm(II) RT ap ∼ ln = E0 1/2 F DTm(III)

(5)

k0 (1−˛)

˛ DTm(II) DTm(III) ap

m∗ − m − m exp{(nF/RT )(E − E0 )} 2.3RT + log ˛nF i

 (6)

Which can be transformed in Eq. (7) assuming that DTm(III) ≈ DTm(II) : ap

E = E0 +

k0 2.3RT log ˛nF D1/2



2.3RT + log ˛nF



ap



m∗ − m − m exp (nF/RT )(E − E0 ) i

(7)

The average values of k0 and ˛ obtained from the plot E vs. the logarithmic function of convoluted current, using the diffusion coefficients values obtained later on, are gathered in Table 1, where it is possible to see the very good agreement between the two methods employed. According to Matsuda and Ayabe criteria [23,24,37], the exchange Tm(III)/Tm(II) can be qualified as quasi-reversible. 3.1.3. Electrochemical nucleation of Tm at a tungsten electrode Chronoamperometry was used to study the nucleation/growth process of thulium on W electrodes. The experiments were conducted with a solution of Tm(III) by stepping the potential from a value where no reaction of thulium deposition takes place, to those potentials sufficiently negative to induce the onset of nucleation. After every run the deposited metal was removed from the surface by polarizing the working electrode anodically. The current–time transients resulting from one of these experiments are shown in Fig. 7. These transients initially exhibited a sharp current spike due to charging of the electrode double

Table 1 Values of the kinetic parameters corresponding to the Tm(III)/Tm(II) exchange calculated by means of (a) simulation of the voltammograms; kinetic analysis software; (b) logarithmic analysis of the convoluted curves. T/K

rev E1/2 /V vs. Cl2 /Cl−

˛

Log k0

673

−2.904 ± 0.019(a) –

0.49 ± 0.05 (a) 0.43 ± 0.03 (b)

−2.55 ± 0.15 (a) −2.61 ± 0.01 (b)

698

−2.875 ± 0.015 (a) –

0.47 ± 0.04 (a) 0.49 ± 0.15 (b)

−2.52 ± 0.08 (a) −2.55 ± 0.21 (b)

723

−2.860 ± 0.008 (a) –

0.47 ± 0.03 (a) 0.49 ± 0.1 (b)

−2.21 ± 0.07 (a) −2.28 ± 0.30 (b)

748

−2.853 ± 0.012 (a) –

0.50 ± 0.08 (a) 0.49 ± 0.12 (b)

−2.37 ± 0.09 (a) −2.17 ± 0.03 (b)

773

−2.850 ± 0.008 (a) –

0.48 ± 0.05 (a) –

−2.29 ± 0.06 (a) –

823

−2.823 ± 0.022 (a) –

0.47 ± 0.06 (a) 0.48 ± 0.07 (b)

−2.21 ± 0.11 (a) −1.99 ± 0.05 (b)

Fig. 7. Potentiostatic current–time transients of a TmCl3 solution (1.164 × 10−4 mol cm−3 ) at various applied overpotentials [(1) −2.470 V, (2) −2.480 V, (3) −2.490 V, (4) −2.500 V, (5) −2.505 V, (6) −2.515 V and (7) −2.525 V vs. Ag|AgCl]. Temperature: 723 K.

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layer after the potential step was applied (Zone I). This capacitive transient was followed by a rise in the current due to an increase in effective electrode area, either as each independent nucleus forms and grows in size and/or the number of nuclei increase (Zone II). The rising current culminates in a broad maximum, im , as the individual diffusion zones of the Tm nuclei begin to overlap, 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. Finally the current decays in the usual way with time due to diffusion control (Zone III). In order to identify the thulium nucleation mode, the dimensionless experimental current–time transients obtained at different applied cathodic potentials, with the dimensionless theoretical transient models derived by Scharifker and Hills [38]. The models utilize the coordinates of the chronoamperometric peaks in order to distinguish between the two limiting nucleation mechanisms: instantaneous and progressive, and are represented by Eqs. (8) and (9), respectively:

 i 2 im

 i 2 im

2

= 1.9542

[1 − exp(−1.2564(t/tm ))] t/tm

= 1.2254

[1 − exp(−2.3367(t/tm ) )] t/tm

2

(8) 2

(9)

Instantaneous nucleation corresponds to immediate activation of all nucleation sites, where the rate of further nuclei formation is negligible in the time frame of the experiments, and then all the Tm nuclei are created at the same time at the beginning of the electrolysis. On the contrary, during progressive nucleation, the rate of new nuclei formation in the time frame of the experiment is not negligible, therefore new crystals are continuously created throughout electrolysis. In Fig. 8 the dimensionless experimental data, extracted from the chronoamperometric curves, are represented together with the theoretical ones for instantaneous (curve 1) and progressive nucleation (curve 2) at different temperatures. According to Fig. 8, in the 723–823 K temperature range, the experimental curves are in good agreement with theoretical models based on instantaneous nucleation with three dimensional growth of the nuclei whatever the applied overpotential, whereas at lower temperatures such as 673 and 698 K, a transition between progressive and instantaneous models is observed.

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3.1.4. Determination of the Tm(III)and Tm(II) diffusion coefficients The stepwise reduction of Tm(III) according to the equations: Tm(III) + 1e− ↔ Tm(II)

(10)

−

Tm(II) + 2e ↔ Tm(0)

(11)

are produced at sufficiently different potentials to yield a potential–time curve exhibiting two steps. Therefore, the diffusion coefficients of the oxidized, Tm(III), and reduced, Tm(II), species can be calculated by chronopotentiometry by using the appropriate equations. By measuring the transition times  1 of the first electrochemical step, Tm(III)/Tm(II), the diffusion coefficient of Tm(III) can be calculated on the basis of the treatment developed by Sand [24,25]: 1/2

1/2

I1

=

n1 FSC0 DTm(III) 1/2

(12)

2

After the transition time  1 , the concentration of Tm(III) at the electrode surface is equal to zero, but this substance continues to diffuse towards the electrode where it is reduced directly to Tm(0) in a process involving (n1 + n2 ) electrons. Furthermore, the Tm(II) species, which were produced in the first step of the electrolysis, diffuse towards the electrode at which they are reduced according to reaction (11). According to Refs. [24,33] the diffusion coefficient of Tm(III) and Tm(II) are related with the global transition time , being  = ( 1 +  2 ), by means of Eq. (13), allowing us to calculate the diffusion coefficient of Tm(II), once that the diffusion coefficient of Tm(III) has been calculated by means of Eq. (12): 1/2

I

1/2

=

1/2

(n1 DTm(III) + n2 DTm(II) )CTm(III) FS1/2

(13)

2

For the specific case where the diffusion coefficients DTm(II) and DTm(II) are equal, Eq. (13) can be transformed into Eq. (14): 1/2

I 1/2 =

(n1 + n2 )FSC0 DTm(III) 1/2

(14)

2

The validity of Eqs. (12), (13) and (14) requires the absence of any convective movement. Moreover, although a tungsten wire was used as working electrode and Eqs. (12), (13) and (14) are relevant to plane semi-infinite diffusion, it was assumed that under the experimental conditions the corrections related to cylindrical geometry can be neglected [25,39]. In the determination of the diffusion coefficient of the electroactive species it is difficult to define the exact electroactive area of the working electrode in the molten salt electrolyte, mainly because to the wetting effects between the electrode and the molten salt, i.e. meniscus effect. Nevertheless it is possible to take into account this effect by comparing the obtained transition times at various immersion depths of the working electrode when the concentration of Tm(III) in solution, the current passed through the working electrode, and the temperature of the system are kept constant. Under these conditions, the transition times should only be a function of the surface area (S = S0 + S) of the working electrode, and Eqs. (12), (13) and (14) can be rewritten as (15), (16) and (17): 1/2

1/2

1

=

1FC0 DTm(III) 1/2 2I 1/2

 1/2 =

1/2

S=

1FC0 DTm(III) 1/2 2I

(S0 + S)

(15)

1/2

(1DTm(III) + 2DTm(II) )C0 F1/2 2I

(S0 + S)

(16)

1/2

Fig. 8. Comparison of the dimensionless experimental data derived from the current–time transients with the theoretical model for (1) instantaneous and (2) progressive nucleation at different working temperatures: (+) 673, (♦) 698, (×) 723, 748, () 748, () 773 and () 823 K.

 1/2 =

3DTm(III) C0 F1/2 2I

(S0 + S)

(17)

Fig. 9 shows typical chronopotentiometric curves obtained with a solution of Tm(III) changing the immersion depth of the tungsten

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one factor (ANOVA) [40], was carried out. The null hypothesis was the non-existence of significant differences between the calculated units. In all the cases, the calculated F statistics was lower than a critical F value, then the null hypothesis must be maintained, and it can be concluded that: (i) with a 95% confidence level, there are no significant differences between the diffusion coefficients of Tm(III) and Tm(II), and (ii) with a 95% confidence level, Eqs. (15) and (17) lead to the same DTm(III) value. Moreover, it was found that the results follow the Arrhenius law: log D = −3.00(±0.086) −

1556(±63) T

(18)

and the activation energy for the diffusion was found to be 29.8 ± 1.2 kJ/mol. Fig. 9. Chronopotentiograms obtained at 723 K with a TmCl3 solution, C0 = 1.164 × 10−4 mol cm−3 , at various immersion deep of the working electrode. Applied current −46 mA.

Fig. 10. Relation between the square root of the transition times ( 1 () and  t ()) and the electroactive area at 723 K. Applied current = −46 mA; [TmCl3 ] = 1.164 × 10−4 mol cm−3 .

electrode. Both transition times,  1 and , became shorter decreasing the surface area of the working electrode. Such a relation is 1/2 plotted in Fig. 10. The square root of the transition times, 1 and 1/2  , changed linearly with the change in the surface area, S, of the working electrode at all temperatures. Table 2 reports the values of the diffusion coefficients of Tm(III) and Tm(II) obtained at different temperatures by using the above Eqs. (15), (16) and (17). In order to determine if there are significance differences between: (1) the diffusion coefficients of Tm(III) and Tm(II) and (2) the diffusion coefficient values of Tm(III) using Eqs. (15) or (17), a statistical treatment, one-way analysis of variance of

3.2. Electrochemical reduction of thulium at an Al electrode. Tm–Al alloy formation According to the Al–Tm phase diagram [41], thulium can form five solid intermetallic compounds with aluminium at the temperature of the experiments. Three of them determined experimentally (Al3 Tm, Al2 Tm, AlTm) and two extrapolated (Al2 Tm3 and AlTm2 ) from other aluminium–lanthanide systems. In order to investigate Tm–Al alloy formation, cyclic voltammetry, anodic striping voltammetry and potentiostatic electrolysis were conducted. 3.2.1. Qualitative information 3.2.1.1. Results obtained by cyclic voltammetry (CV). Fig. 11 compares the cyclic voltammograms for a Tm(III) solution on tungsten and aluminium electrodes, the former was used because no alloys exist for the W–Tm binary systems. The shape of the curves on both electrodes is different. As it was shown in the previous section, if the electrode substrate is a W wire the reaction scheme proceed in the two consecutives and well separated electrochemical steps: Tm(III) + 1e ↔ Tm(II) and Tm(II) + 2e ↔ Tm(0). When Al is used as working electrode material, the electrochemical window of the melt is limited cathodically by the reduction of Li(I) forming a Li–Al alloy, and anodically by the oxidation of the Al electrode [16,18,19]. In the cathodic scan, a well defined peak A is observed, and an important current background is also noticeable in the region between −2.1 V and the cathodic limit of the melt. When the scanning direction is reversed, 3 peaks are also observed, a sharp oxidation peak D and two shoulders labelled C and E. These indicate the possibility of formation of more than one kind of Tm–Al. 3.2.1.2. Results obtained by anodic stripping voltammetry. Identical information was obtained by anodic stripping voltammetry. Samples of thin layers of Alx Tm alloys were prepared by cathodic deposition of a solution of Tm(III) at the Al electrode for a short period at different cathodic potentials. Following the deposition

Table 2 Variation of the diffusion coefficient of Tm(III) and Tm(II) ions with the temperature in the eutectic LiCl–KCl. 673

698

723

748

773

823

5.0 ± 1.0

6.7 ± 1.7

7.2 ± 1.0

8.2 ± 1.5

10.2 ± 1.5

12.2 ± 1.6

DTm(II) × 10 Eq (16) Fcalculated Fcritical

4.6 ± 1.2 0.6645 4.2597

6.2 ± 1.3 0.0946 5.3176

7.4 ± 1.2 0.1784 4.1596

7.8 ± 0.9 0.0425 4.6001

10.0 ± 1.7 0.1861 4.6672

12.7 ± 1.2 0.0537 4.8443

DTm(III) × 106 Eq (15)

5.0 ± 1.0

6.7 ± 1.7

7.2 ± 1.0

8.2 ± 1.5

10.2 ± 1.5

12.2 ± 1.6

DTm(III) × 106 Eq (17) Fcalculated Fcritical

4.7 ± 1.1 0.4912 4,2597

6.4 ± 1.0 0.0473 5.3176

7.5 ± 0.9 0.3021 4.1596

8.0 ± 0.9 0.0538 4.6001

10.2 ± 1.4 0.0833 4.6672

13.4 ± 1.1 1.2485 4.9646

DTm(III) × 106 Eq (15) 6

Y. Castrillejo et al. / Electrochimica Acta 54 (2009) 6212–6222

Fig. 11. Cyclic voltammograms at 723 K for the reduction of a TmCl3 solution 1.132 × 10−4 mol cm−3 in the eutectic LiCl–KCl mixture at an Al electrode (black line) scan rate 50 mV s−1 , and at a W electrode (grey line) at 50 mV s−1 .

time, the anodic stripping voltammograms were recorded, and stripping curves like the ones shown in Fig. 12 were obtained. A sharp peak and two shoulders were observed between the reoxidation of the deposited Li–Al alloy and the Al dissolution, which corresponds to peaks D, C and E in cyclic voltammetry. 3.2.2. Potentiostatic electrolysis Based on the above results, potentiostatic electrolysis was carried out at −1.9, −2.1; −2.3,−2.4, and −2.45 V for 2 h, using an aluminium foil as working electrode. After the electrolysis, the samples were washed with ethylene glycol (Aldrich 99.8%) and stored inside the glove box until their analysis. The samples were analysed by XRD and the main results are summarized in Table 3. XRD pattern of samples (2) and (5) are shown in Figs. 13 and 14. The surface morphology of the obtained deposits was also observed by scanning electron microscopy (SEM) (Fig. 15). The stabilization of Tm metal in the Al phase by alloy formation, prevents the corrosion of Tm(0) in presence of Tm(III). Therefore, for the extraction of thulium from molten chlorides, the use of a reactive electrode made of aluminium leading to different Tm–Al alloys seems to be a pertinent route. In addition, reducing Tm(III) at

6219

Fig. 12. Anodic stripping voltammograms for thulium, [Tm(III)] = 1.123 × 10−4 mol cm−3 , on an Al electrode at 773 K. Deposition time 30 s, sweep rate 10 mV s−1 . Deposition potentials: (a) −2.00, −2.20, −2.35 and −2.40 V vs. Ag|AgCl.

a reactive Al electrode at less cathodic potentials than the solvent reduction Li(I)/Li(Al), should allow the electrolytic recovery of Tm into Aluminium without co-reduction of the solvent. 3.3. Stability of thulium–oxychloride. Determination of the solubility product Experimentally, thulium (III) combinations with oxide ions can be characterized by composition as well as by their solubility products. These data can be determined by analysis of the curves for potentiometric titrations of the oxoacid Tm(III) with an oxide ion. The variation of pO2− was measured experimentally by means of an oxide ion selective electrode, the yttria-stabilized zirconia

Table 3 Electrolysis experiments and identification of the different phases in the obtained samples. Sample no.

Electrolysis potential/V

Time

Identified phase

1 2 3 4 5

−1.9 −2.1 −2.3 −2.4 −2.45

2h 2h 2h 2h 2h

Al3 Tm, Al2 Tm, Al, KCl Al3 Tm, Al2 Tm, Al, KCl Al3 Tm, Al2 Tm, AlTm, Al, KCl Al3 Tm, Al2 Tm, AlTm, Al, KCl Al3 Tm, Al2 Tm, AlTm

Fig. 13. X-ray diffraction analysis of the deposits obtained under potentiostatic electrolysis. Eapplied = −2.1 V vs. Ag|AgCl t = 2 h. () Al3 Tm, () Al2 Tm, (♦) Al and () KCl.

6220

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Fig. 14. X-ray diffraction analysis of the deposits obtained under potentiostatic electrolysis. Eapplied = −2.45 V vs. Ag|AgCl t = 2 h. () Al3 Tm, () Al2 Tm and (×) AlTm.

membrane electrode (YSZME), which has been proved to work satisfactorily in molten chlorides [7,8,42–44]. Several potentiometric titrations of Tm(III) solutions were carried out, using solid Li2 CO3 or BaO as source of free O2− ions, according to the reactions: Li2 CO3 → 2Li+ + CO3 2−

(19)

CO3 2− ↔ CO2 + O2−

(20)

BaO → Ba

2+

+O

2−

Tm

+O

2−

−

+ Cl ↔ TmOCl (s)

2TmOCl + O

2−

↔ Tm2 O3 (s) + 2Cl

These results were confirmed by XRD analysis of the solids recovered at the bottom of the crucible when the experiments were stopped for x < 1 (Fig. 17a) and x > 1.5 (Fig. 17b). The theoretical equation corresponding to the titration curve can be elucidated from the mass balance equation: [O2− ]total = [O2− ]free + [TmOCl]precipitated + 3[Tm2 O3 ]precipitated (24)

(21)

It has been proven [42–44] that the dissociation constant of the carbonate ions (20) is sufficiently large that a lowering of the partial pressure of CO2 leads to dissociation (the solubility of CO2 in the molten salt is very low). A very low partial carbon dioxide pressure (about 10−5 atm) can be obtained by bubbling pure Ar at 1 atm, then the concentration of O2− in solution is equal to that of the carbonate dissolved initially [42–44]. All the experimental curves obtained exhibited similar habits to the one shown in Fig. 16. One can see that the YSZME potential values (converted into pO2− values after calibration of the electrode), showed two equivalence points: x (defined as the ratio of added oxide ion to the initial Tm(III) concentration, C0 ) at 1.0 and 1.5 indicating the formation of two insoluble products TmOCl and Tm2 O3 , respectively. Then the reactions follow the path: 3+

Fig. 16. Experimental values obtained with an YSZME during the potentiometric titration of a Tm(III) solution in LiCl–KCl (0.117 mol kg−1 ) with O2− ions, introduced as Li2 CO3 , at 723 K.

−

[Tm(III)]total = [Tm(III)]free + [TmOCl]precipitated + 2[Tm2 O3 ]precipitated

(25)

the solubility product of TmOCl and Tm2 O3 : ks (TmOCl) = [Tm(III)] [O2− ]

(26)

ks (Tm2 O3 ) = [Tm(III)]2 [O2− ]3

(27)

and the equilibrium constant of reaction (23): K=

1 2−

[O

]

=

ks2 (TmOCl) ks (Tm2 O3 )

(28)

The following expressions are obtained for:0 < x < 1 2−

pO

= pks (TmOCl) + log C0 (1 − x)

(22)

1 < x < 1.5

(23)

pO2− = pks (Tm2 O3 ) − 2pks (TmOCl)

(29)

(30)

Fig. 15. (a) SEM micrographs of an Al–Tm film formed by potentiostatic electrolysis at −2.1 V vs. Ag|AgCl during 2 h. (b) SEM micrograph of the cross-section of an Aluminium electrode after electroreduction of TmCl3 at −1.9 V vs. Ag|AgCl.

Y. Castrillejo et al. / Electrochimica Acta 54 (2009) 6212–6222

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Fig. 17. X-ray diffractograms of the solids obtained after titration of a Tm(III) solution by O2− ions in the eutectic LiCl–KCl mixture at 723 K: () TmOCl, (♦) (Tm2 O3 ) and () KCl. (a) The experiment was stopped at x = 0.72 and (b) the experiment was stopped at x = 1.8.

x > 1.5 2−

pO

= −log C0 (x − 1.5)

(31)

The corresponding solubility products pks (TmOCl) = 8.0 ± 0.3 and pks (Tm2 O3 ) = 18.8 ± 0.7 were determined by mathematical modelling of the experimental curves. Dry HCl gas is a rapid and effective agent to remove oxide from the eutectic LiCl–KCl to a minimum concentration of 10−12.6 [8,43]. The acid–base constant of the mixture HCl(g) + H2 O, previously obtained [8,42,43] as well as the solubility products of TmOCl and Tm2 O3 have been compared, showing that TmOCl and Tm2 O3 can be chlorinated by bubbling commercial HCl according to the reactions: 2HCl(g) + TmOCl(s) ↔ TmCl3 (dissolved) + H2 O(g)

(32)

6HCl(g) + Tm2 O3 (s) ↔ 2TmCl3 (dissolved) + 3H2 O(g)

(33)

Experimental solubilization tests of solid TmOCl, previously synthesised, and Tm2 O3 by bubbling gaseous HCl through the cell were carried out and the progress of the reaction was followed by the YSZME showing efficiencies of around 100%. 4. Conclusions The electrochemical behaviour of thulium was studied in the eutectic LiCl–KCl mixture using inert (W) and reactive (Al) electrodes in a range of temperatures from 673 to 823 K. Different behaviour was found for the two electrodes. On an inert electrode (W), Tm(III) ions are reduced to metallic thulium through two consecutive steps: Tm(III) + 1e ↔ Tm(II) and Tm(II) + 2e ↔ Tm(0) Consequently, corrosion of Tm metal in the molten chloride media is expected in the presence of Tm(III) according to the reaction: 2Tm(III) + Tm(0) ↔ 3Tm(II) This corrosion reaction hampers the emf measurements of the Tm(III)/Tm(II) system, and it could be responsible for a low current yield in the electrolysis and a low stability of the deposits. The electroreduction of Tm(III) to Tm(II) was found to be quasireversible. The values of the intrinsic rate constant of charge transfer, k0 , are in a similar order at the studied temperatures, varying from 10−2.6 at 673 K to 10−2.1 at 823 K, and the transfer coefficient, ˛, remains close to 0.5 whatever the studied temperature. Electrocrystallization of thulium plays an important role in the electrodeposition process, being the nucleation mode

affected by temperature. In the 723–823 K temperature range, the experimental curves are in good agreement with theoretical models based on instantaneous nucleation with three dimensional growth of the nuclei whatever the applied overpotential, whereas at lower temperatures such as 673 and 698 K, a transition between progressive and instantaneous models is observed. The diffusion coefficient of the Tm(III) and electrogenerated Tm(II) ions were also calculated by chronopotentiometry. The statistical treatment of the data, a one-way analysis of variance of one factor (ANOVA), showed that with a 95% confidence level, the diffusion coefficients of Tm(III) and Tm(II) are equal. The diffusion coefficient showed a temperature dependence according to the Arrhenius law. The activation energy for diffusion was found to be 29.8 ± 1.2 kJ mol−1 . Cyclic voltammetry and anodic stripping voltammetry, showed that Tm(III) can be reduced in an aluminium electrode forming different thulium–aluminium alloys. Potentiostatic electrolysis was used to form the alloys at different deposition potentials, and X-ray diffraction analysis of the obtained deposits proved the formation of Al3 Tm, Al2 Tm and AlTm. The stabilization of Tm metal in the Al phase by alloy formation avoids its corrosion in the presence of Tm(III). Therefore, for the extraction of thulium from molten chlorides, the use of a reactive electrode made of aluminium leading to an Al–Tm alloy seems to be a pertinent route. In addition, it is possible to obtain Al–Tm alloys at a more cathodic potential than the Li(I) reduction. This should allow the electrolytic recovery of Tm into Aluminium without coreduction of the solvent. On the other hand, using an YSZME, it was possible to follow the pO2− during the potentiometric titration of Tm(III) solutions by O2− ions. The experimental curves pointed out the formation of solid oxychloride TmOCl and oxide Tm2 O3 , which were identified by XRD analysis of the obtained solids. The corresponding solubility products, pks (TmOCl) = 8.0 ± 0.3 and pks (Tm2 O3 ) = 18.8 ± 0.7, were determined by mathematical modelling of the experimental curves. Experimental solubilization tests of solid Tm2 O3 and TmOCl carried out by bubbling gaseous HCl through the melt were carried out, showing efficiencies close to 100%. Acknowledgements This work was supported by the Ministerio de Educación y Ciencia MEC-FEDER (Spain) Project ENE2004-00317 and the Consejería de Educación Junta de Castilla y León–Programa de Ayudas a Grupos de Excelencia GR170 (SIENMAT). The authors thank R. Gómez for her technical assistance.

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