Electrochemical Deposition and Nucleation of Aluminum on Tungsten in Aluminum Chloride-Sodium Chloride Melts

Transcription

1 J. Mater. Sci. Technol., Vol.24 No.6, Electrochemical Deposition and Nucleation of luminum on Tungsten in luminum Chloride-Sodium Chloride Melts Zhaowen WNG, Hongmin KN, Zhongning SHI, ingliang GO, Yungang N and Xianwei HU School of Materials and Metallurgy, Northeastern University, Shenyang , China [Manuscript received ugust 19, 2008, in revised form October 16, 2008] Electrochemical deposition and nucleation of aluminum on tungsten electrode from lcl 3 -NaCl melts were studied by cyclic voltammetry, chronopotentiometry and chronoamperometry. Cyclic voltammetry and chronopotentiometry analyses showed that l (III) was reduced at 200 C in two consecutive steps in an electrolyte of molten lcl 3 -NaCl system with a composition 52:48 molar ratio. The current-time characteristics of nucleation aluminum on tungsten showed a strong dependence on overpotentials. Chronoamperometry showed that the deposition process of aluminum on tungsten was controlled by an instantaneous nucleation with a hemispherical diffusion-controlled growth mechanism. The results could lead to a better understanding of the lcl 3 -NaCl melt system that has technological importance in electrodeposition of metals as well as in rechargeable batteries. KEY WORDS: Electrochemical deposition; luminum-sodium chloride melts; luminum; Tungsten; Nucleation 1. Introduction The lcl 3 -NaCl melts have been used as the electrolytes in electrodeposition of metals and rechargeable batteries [1]. Several investigators have examined the electrodeposition of pure aluminum from lcl 3 -alkali metal chloride systems [2 5]. The electrochemical behavior of aluminum in chloroaluminate melts has been studied using primarily aluminum electrodes [6,7]. In neutral melts with an equimolar mixture of lcl 3 and NaCl, the dominant ionic species are Na + and lcl 4. In basic melts, the relative concentrations deviate from equi-molar concentrations and additional ionic species are introduced; Cl is present in melts containing excess NaCl. In acidic melts, however, aluminum complexes, such as l 2 Cl 7, are present with excess lcl 3. The chemical equilibria operative in lcl 3 -NaCl melt under a wide range of lcl 3 concentrations above the equimolar point are well known [8]. This melt is often considered as an acid-base system, where the acid (l 2 Cl 7 ) is defined as a chloride ion acceptor and the base (lcl 4 ) is defined as a chloride ion donor: 2lCl 4 l 2Cl 7 + Cl (1) The lcl 3 -NaCl melts possess a number of interesting features, such as relatively high electrical conductivity, and a wide electrochemical potential window. These properties also render the lcl 3 -NaCl melt a potential electrolyte for the electrolytic extraction and recycling of aluminum. The electrocrystallization of metal deposit on various substrates have attracted much interest in modern electrochemistry due to its technological importance. Nucleation kinetics and the growth of the first metallic nuclei formed on a substrate are critical steps that determine the physicochemical properties of electrodeposits and are crucial in the understanding and control of electrochemical deposition processes. Prof., Ph.D., to whom correspondence should be addressed, zhaowenw@mail.neu.edu.cn. critical overpotential must be reached before the onset of nucleation can occur. oth the metal ions and the substrate material can affect this critical overpotential. In previous nucleation studies, it was observed that the nuclei are randomly distributed crystallites of nearly identical size and grown under mass transfer control [9]. Rising current transients were observed in a chronoamperometric run, which reflect the increase in current as each nucleus grows in size and are accompanied by an increase of the total area of the electroactive surface. The models describing the threedimensional (3D) nucleation growth process during the bulk metal deposition on foreign substrate have been summarized by llongue and Souteyrand [10]. mong the models, the one involving hemispherical diffusion-controlled growth of the nuclei has been found to be appropriate for representing metal deposition in most systems, including the electrodeposition of aluminum, copper and silver from lcl 3 based ionic liquids [11 14]. The deposition process of aluminum on tungsten substrates was controlled by instantaneous nucleation with diffusion-controlled growth, while the deposition processes of aluminum on aluminum electrodes were found to be associated with kinetic limitations from 2:1 molar ratio aluminum chloride (lcl 3 )-1-ethyl-3-methylimidazolium chloride ([EMIm]Cl) ionic liquids [15]. The deposition processes of aluminum on both tungsten and aluminum substrates from 2:1 molar ratio aluminum chloride (lcl 3 )-trimethylphenylammonium chloride (TMPC) ionic liquids were controlled by an instantaneous nucleation with diffusion-controlled growth [16]. In the early stage of electrodeposition of aluminum onto graphite from a molten electrolyte containing lcl 3 -NaCl-KCl and at low cathodic potentials, two-dimensional (2D) nucleation and growth proceed through instantaneous and progressive steps following the initial double layer charging [17]. Despite the previous studies, there has been a lack of understanding of the nucleation at initial stage of aluminum electrodeposition on tungsten electrode from lcl 3 -NaCl melts, except for the study that the

2 916 J. Mater. Sci. Technol., Vol.24 No.6, 2008 electrochemical deposition of aluminum onto glassy carbon electrode from NalCl 4 saturated with NaCl at 175 C was found to be progressive [5]. lcl 3 -NaCl melts has been studied before, but little work has been carried out in about the last 20 years. With better technology and methods developed over this period, it is necessary to study electrochemical deposition and nucleation of aluminum in the lcl 3 -NaCl melts in much detail with precise electrochemical equipment (a potentiostat PGSTT30 and a OOSTER 20) and improved methods. The purpose of the present work was to determine the kinetic and mechanism of the electrodeposition of aluminum from lcl 3 - NaCl melts onto tungsten substrate, where the methods of cyclic voltammetry, chronopotentiometry and chronoamperometry have been used. The lcl 3 -NaCl melt is a potential electrolyte for the electrolytic extraction and recycling of aluminum, as well as aluminium plating using a very small device. Our research would provide fundamental basis for such practice. For example, according to the information obtained for the electrochemical reactions, one may control the current or potential in electrolytic extraction and recycling as well as electroplating of aluminum. One may also determine the experimental parameters, such as pulsed currents, to obtain better results [1 7]. 2. Experimental ll chemicals were handled in a dry argon-filled glove box. lcl 3 and NaCl were both analytically pure and crystal. The electrochemical experiments, including cyclic voltammetry, chronopotentiometry and chronoamperometry, were carried out using a three-electrode electrochemical cell and a potentiostat PGSTT30 and a OOSTER 20 (utolab Co., Netherlands). The electrolyte was 52:48 molar ratio lcl 3 -NaCl melts system using continuous magnetic bar stirring. The electrolyte temperature was controlled at 200 C with silicone oil circulation. The working electrode was a tungsten (99.95%) wire of 1 mm in diameter. The counter electrode was a tungsten (99.95%) rod of 5 mm in diameter. The reference electrode was a high purity aluminum wire (99.999%) of 1 mm in diameter. To lower the ohmic drop, both the reference electrode and the counter electrode were directly immersed into the electrolyte. The distance between the aluminum tip of the reference electrode and the surface of working electrode was controlled at about 5 mm. Prior to the electrochemical measurements, the tungsten working electrode and the counter electrode were polished with increasingly finer grades of emery papers. The aluminum reference electrode was polished further in a mixture of 25% (by volume) H 2 SO 4 (98%), 70% H 3 PO 4 (85%) and 5% HNO 3 (52.5%) for min at room temperature to remove residual oxides [18]. The experiment of chronopotentiometry was performed as follows: the negative value of the current was given at s and the positive value of the current is given at s. 3. Results and Discussion 3.1 Cyclic voltammetry In order to determine the potentials, at which the electrocrystallization nucleation of aluminum occurs, ' ' v=0.05 V s -1 2 v=0.08 V s -1 3 v=0.1 V s -1 4 v=0.2 V s Potential / V (vs l) Fig.1 Typical voltammograms recorded on W electrode in an electrolyte composition 52:48 molar ratio lcl 3 -NaCl at 200 C at various scan rates, v, (active area of working electrode, s=0.636 cm 2 ). The cathodic and anodic limits were set to 0.6 V and +2.8 V, respectively a cyclic voltammetry technique was first applied. Figure 1 shows typical voltammograms obtained at various scan rates. The lcl 4, l 2Cl 7 and Cl concentrations in the electrolyte were reported to be 7.68 mol L 1, 0.66 mol L 1 and 0.01 mmol L 1, respectively. The two pronounced cathodic peaks in Fig.1 show that l (III) is reduced in two consecutive steps: 4l 2 Cl 7 + 3e l + 7lCl 4 (2) lcl 4 + 3e l + 4Cl (3) The reduction of the relatively weak l 2 Cl 7 complex to l (peak ) is seen at a potential of V with respect to the aluminum reference electrode in the melts. luminum deposition results in a localized acidity decrease near the electrode surface as l 2 Cl 7 was consumed while lcl 4 produced. The reduction of the more stable lcl 4 tetrahedral complex (peak ) occurs at approximately 0.5 V. When the potential sweep is reversed, two anodic peaks ( and ), both associated with the stripping of aluminum, were observed in the cyclic voltammograms. The first at 0.45 V was the anodic dissolution of metallic aluminum to form lcl 4 with the presence of Cl. The stripping wave was terminated with the consumption of Cl in the vicinity of the electrode. The second stripping wave occurred at 0.15 V, also associated with the anodic dissolution of metallic aluminum, forming l 2 Cl 7 with the presence of lcl 4. The process at V was a result of the oxidation of the lcl 4 anions[11] : C 4lCl 4 2l 2Cl 7 + Cl 2 + 2e (4) ccording to the above analysis, aluminum can be electrodeposited from the aluminum complexes of either lcl 4 or l 2Cl 7. The reduction of lcl 4 occurred at potentials more negative than that required for l 2 Cl 7, and became prominent when the acidity (lcl 3 content) of the melt was decreased. Figure 2 shows the cyclic voltammograms performed at different sweep rates. The magnitude of the l 2 Cl 7 and lcl 4 reduction wave (I p) is seen to increase linearly with the square root of the sweep rate (v 1/2 ) (see Fig.3). This is a clear indication that the reductions followed a diffusion-controlled process. The diffusion coefficients of l 2 Cl 7 and lcl 4 are

3 J. Mater. Sci. Technol., Vol.24 No.6, v=0.05 V s -1 2 v=0.06 V s -1 3 v=0.08 V s -1 4 v=0.1 V s -1 5 v=0.2 V s Potential / V (vs l) Fig.2 Typical voltammograms recorded on W electrode in an electrolyte composition 52:48 molar ratio lcl 3 -NaCl at 200 C at various scan rates, (s=0.636 cm 2 ). The cathodic and anodic limits were set to 0.65 V and V, respectively I p / ( cm -2 ) v 1/2 / V 1/2 s -1/ Peak Peak Fig.3 Peak current (I p ) as a function of the square root of the scan rate (v 1/2 ) in 52:48 molar ratio lcl 3- NaCl melt at 200 C Potential / V (vs l) v=0.04 V s -1 2 v=0.05 V s -1 3 v=0.06 V s -1 4 v=0.08 V s -1 5 v=0.1 V s -1 Fig.4 Typical voltammograms recorded on W electrode in an electrolyte composition 52:48 molar ratio lcl 3 -NaCl at 200 C at various scan rates (s=0.636 cm 2 ). The cathodic and anodic limits were set to 0.55 V and V, respectively different in this system. So the slopes of the two processes are different. To obtain more details on the aluminum electrodeposition, the measurements were focused on the potential range between 0.55 V and V. The cathodic peaks and anodic peaks in Fig.4 were attributed to the deposition and dissolution of aluminum, respectively. When the cathodic limit was set to 0.55 V, the voltammogram exhibited a current loop hysteresis, typical of a deposition process requiring a nucleation overpotential. The hys- Potential / V , , 0.06 ' Fig.5 Chronopotentiometry recorded on W electrode in 52:48 molar ratio lcl 3 -NaCl melt at 200 C teresis loop is indicative of that the deposition of aluminum proceeded by a nucleation and growth mechanism. These current loops occurred because the deposition of aluminum on tungsten during the negative scan requires a considerable overpotential in order to initiate the nucleation and subsequent growth of an aluminum deposit. 3.2 Chronopotentiometry luminum deposition has also been studied in order to examine the time-dependent electrode potential under galvanostatic conditions. typical result given in Fig.5 shows an initial sharp drop in potential associated with the nucleation of aluminum from reduction of l 2 Cl 7. This was followed by relaxation to a plateau ( 0.1 V) associated with the growth of aluminum deposit. fter a time interval, which is a function of the applied current (plateau from to 0.06 ), the potential began to decrease with the nucleation of aluminum from reduction of lcl 4. This was followed by relaxation to a plateau (about 0.52 V) for the growth of the aluminum. When the current was reversed, plateau and plateau were observed due to the stripping of aluminum. Therefore, the chronopotentiometry results also indicated that the deposition of aluminum proceeded by a nucleation and growth mechanism. This is in good agreement with the cyclic voltammograms described above. ' 3.3 Chronoamperometry The nucleation process of l (III) in the lcl 3 - NaCl melt was further investigated by means of chronoamperometry. The obtained current-time transients are shown in Fig.6. The shape of the curve is apparently dependent on the overpotential. When the potential is beyond 0.58 V, these transients show the typical nucleation characteristics. When the potential abruptly reaches a constant value (beyond 0.58 V), the current also follows an abrupt change, due to the charge of the double layer. With the decrease of the charge current, the current begins to decrease. Subsequently, the current begins to increase due to an increase in effective electrode area, either because each independent nucleus forms and grows in size, or because the number of nuclei increases [19]. When the diffusion zones of the growing nuclei begin to overlap, the current reaches a maximum value (I m ), after that it resumes the usual I t 1/2 behavior. The time

4 918 J. Mater. Sci. Technol., Vol.24 No.6, mv (a) (b) 610 mv 650 mv (c) 660 mv (d) 670 mv (e) 680 mv (f) Fig.6 Chronoamperograms recorded on the W electrode in 52:48 molar ratio lcl 3 -NaCl melt at various overpotentials at 200 C (s=0.636 cm 2 ) reaching the current maximum (t m ) depends on the overpotential and decreases as the applied potential is made more negative. Several models have been proposed to describe the cathodic deposition of metals [20]. One of them is the instantaneous nucleation model, in which nucleation is completed instantaneously and there are no new nuclei formed in the subsequent growth process. nother is the progressive nucleation model, in which new nuclei increasingly form during the growth process. We know, for instantaneous nucleation: I(t) = zf N 0π(2DC) 3/2 M 1/2 ρ 1/2 t 1/2 (5) for progressive nucleation: I(t) = 2zF K nn 0 π(2dc) 3/2 M 1/2 3ρ 1/2 t 3/2 (6) where N 0 is the initial nucleation number; z the valence number; M the atomic weight for deposits, g mol 1 ; D the diffusion coefficient for ions deposited, cm 2 s 1 ; C the bulk concentration of ion deposited, mol cm 3, ρ the density for deposits, g cm 3 ; I(t) the polarization current when the time is t, ; t the polarization time, s; and K n the nucleation constant. ased on these models, we can see that the instantaneous nucleation model on a fixed number of active sites is applicable if the current increases linearly with the increase in t 1/2, whereas a progressive model on an infinite number of active sites implies that the current increases linearly with the increase in t 3/2. Figure 7 shows the results obtained from the rising portion of the current transients shown in Fig.6. The good linearity in I vs t 1/2 for the various overpotentials suggests an instantaneous 3D nucleation of l (III) on the tungsten electrode. more definitive analysis can be made by comparing the experimental data in the entire current-time transient range to the appropriate dimensionless theoretical equations derived on the basis of the models. The expressions given in Eqs.(7) and (8) relate the dimensionless current to the dimensionless time for

5 J. Mater. Sci. Technol., Vol.24 No.6, mv 650 mv t 1/2 / s 1/2 Fig.7 Current denisty as a function of t 1/2 obtained on the W electrode in 52:48 molar ratio lcl 3-NaCl melt system at various overpotentials at 200 C (s=0.636 cm 2 ). For 680 mv: I= , , , , , , , , and For 650 mv: I= , , , , , , , , , , , , and chosen to have a non-equimolar composition that is more stable than the 2:1 (lcl 3 :NaCl) system used in literature [23]. Our system has a lower vapor pressure. It appeared to be that we discussed lcl 4 rather than l 2 Cl 7 that was reduced on the tungsten electrode, as can be seen from the current-time plots shown in Fig.6. The compositions are 50.80:49.20 (lcl 3 : NaCl) of literature [23] and the reduction of l 2 Cl 7 was discussed in literature [3]. 4. Conclusion The electrochemical deposition process of aluminum on tungsten electrode in 52:48 molar ratio lcl 3 -NaCl melt system at 200 C has been investigated. The voltammetry studies show l (III) is reduced in two consecutive steps, 4l 2 Cl 7 +3e l+7lcl 4, lcl 4 +3e l+4cl. Certain nucleation overpotential was required during the deposition of aluminum on tungsten electrode. Chronopotentiometry analysis also shows that l (III) is reduced in two consecutive steps under certain current density. This is in reasonable agreement with cyclic voltammograms. The electrochemical deposition process of aluminum on tungsten electrode was found to proceed by a nucleation and growth mechanism. The current-time characteristics of nucleation on tungsten showed a strong dependency on overpotentials. Chronoamperometry analysis showed that the deposition of aluminum exhibited an instantaneous 3D nucleation with hemispherical diffusioncontrolled growth of nuclei. Fig.8 Dimensionless plots for overpotential of 650, 660, 670 and 680 mv and the theoretical curves for instantaneous nucleation and progressive nucleation the diffusion-controlled 3D progressive and instantaneous nucleation and growth, respectively: (I/I m ) 2 = (t/t m ) 1 (1 exp[ (t/t m ) 2 ]) 2 (7) (I/I m ) 2 = (t/t m ) 1 (1 exp[ (t/t m ) 2 ]) 2 (8) In order to make an appropriate comparison between the theory and experiment, it is necessary to apply a correction to the experimental data to account for the potential-dependent induction time, t 0, preceding the onset of nucleation. The method to obtain the value of t 0 has been described in literature [14]. The value of t 0 was used to redefine the time axis t =t t 0 and the time to reach the maximum current t m=t m t 0. The dimensionless plots (see Fig.8) suggest that the deposition of aluminum on tungsten electrode agrees well with the model of instantaneous nucleation. Interestingly, the results we have obtained are similar to some previous studies that have used either room temperature solutions [21,22] or high temperature (lcl 3 /NaCl) molten salts [3,23], where a good instantaneous behavior in the nucleation of aluminum on tungsten electrode was observed. Our system was cknowledgements The authors acknowledge the financial supported by the National asic Research Program of China (No. 2007C210305) and the National Natural Science Foundation of China (Grant No ). REFERENCES [1 ] H..Hjuler, R.W.erg and N.J.jerrum: Power Sources, 1985, 10, 1. [2 ] J.Vaughan and D.Dreisinger: J. Electrochem. Soc., 2008, 155(1), 68. [3 ] P.Rolland and G.Mamantov: J. Electrochem. Soc., 1976, 123(9), [4 ] Q.F.Li, H..Hjuler, R.W.erg and N.J.jerrum: J. Electrochem. Soc., 1991, 138(3), 763. [5 ] Q.F.Li, H..Hjuler, R.W.erg and N.J.jerrum: J. Electrochem. Soc., 1990, 137(2), 593. [6 ] M.Jafarian, M.G.Mahjani, F.Gobal and I.Danaee: J. ppl. Electrochem., 2006, 36(10), [7 ] K.Schulze and H.Hoff: Electrochim. cta, 1992, 17(1), 119. [8 ].J.Welch and R..Osteryoung: J. Electroanal. Chem., 1981, 118, 455. [9 ] G.J.Hills, D.J.Schiffrin and J.Thompson: Electrochim. cta, 1974, 19, 657. [10] P.llongue and E.Souteyrand: J. Electroanal. Chem., 1990, 286(1-2), 217. [11] R.T.Carlin, W.Crawford and M.ersch: J. Electrochem. Soc., 1992, 139(10), [12] X.H.Xu and C.L.Hussey: J. Electrochem. Soc., 1992, 139(5), [13] J.J.Lee,.Miller, X.Shi, R.Kalish and K..Wheeler: J. Electrochem. Soc., 2001, 148(3), C183.

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