Wei
Xiao
a,
Xin
Wang
a,
Huayi
Yin
a,
Hua
Zhu
a,
Xuhui
Mao
a and
Dihua
Wang
*ab
aSchool of Resource and Environmental Sciences, Wuhan University, Wuhan, 430072, P. R. China. E-mail: wangdh@whu.edu.cn; Fax: +86 27 68775799; Tel: +86 27 68775799
bState Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, P. R. China.
First published on 13th June 2012
With the verification of the existence of the dissolution–electrodeposition mechanism during the electro-reduction of solid silica in molten CaCl2, the present study not only provides direct scientific support for the controllable electrolytic extraction of nanostructured silicon in molten salts but it also opens an avenue to a continuous silicon extraction process via the electro-deposition of dissolved silicates in molten CaCl2. In addition, the present study increases the general understanding of the versatile material extraction route via the electro-deoxidization process of solid oxides in molten salts, which also provokes reconsiderations on the electrochemistry of insulating compounds.
SiO2(s) + 4e = Si(s) + O2− | (1) |
In fact, the electrochemistry of insulating solid compounds possesses crucial roles in sustaining many electrochemical processes with important applications ranging from energy storage (i.e., secondary batteries) to experimental technologies (e.g. Ag/AgCl reference electrodes).7 The basic electrode process of solid PbSO4 (solubility product, Ksp is 1.6 × 10−8), which mediates the reversible charge–discharge process of lead-acid batteries,8 involves a dissolution–electrodeposition mechanism as depicted in the consecutive reactions of reaction (2 and 3). While a solid-state ion transport mechanism (reaction 4) is believed to regulate the redox reaction of the solid Ni(OH)2 (Ksp = 2.0 × 10−15) electrode which is used as the positive electrode material in nickel–cadmium or nickel–metal hydride batteries.8
PbSO4(s) = Pb2+ + SO42− | (2) |
Pb2+ + 2e = Pb(s) | (3) |
Ni(OH)2(s) + OH− = NiOOH(s) + e + H2O(l) | (4) |
Although the solubility of SiO2 in molten CaCl2 was too low to trigger a dissolution–electrodeposition mechanism, the situation might be changed with CaSiO3 as an intermediate product during the reduction process. Practically, the CaCl2 melt always contains some CaO due to the hydrolysis reaction of CaCl2·xH2O during the conventional CaCl2 preparation process, and inevitably, the in situ generated CaO during the reduction of SiO2 on the cathode. Therefore, reactions (5 and 6) might take place and generate soluble silicates in the cathodic region and/or inside the porous cathode since the Gibbs free energy change of reaction 5 at 850 °C is −139.76 kJ mol−1. This is understandable if one considers the basic nature of CaO and the acidic characteristic of silica. A recent reference reported that reaction 5 becomes significant in molten CaCl2 with the addition of 4.8 mol% CaO.9 The increased solubility of silica in molten CaCl2 with adscititious CaO also facilitates the electrorefining of silicon in CaCl2–CaO–SiO2 melts via the dissolution–electrodeposition mechanism.10
Furthermore, it was estimated in our previous work that the concentration of in situ generated oxygen ions at the electrochemical interfaces during the electro-reduction of solid silica in molten CaCl2 could be higher than 1 mol L−1.5 Such a high concentration is believed to be adequate to trigger the formation of silicates in the silica cathode. Therefore, the presence of CaSiO3 at the cathode is inevitable during the electro-reduction of solid silica in molten CaCl2, regardless of the addition of CaO in the melt. The involvement of CaSiO3 at the cathode during the reduction process was proposed and reported before.6,11 If the solubility of CaSiO3 in the melt is high enough, Si could also be produced through the electrodeposition of the dissolved silicate as described in reactions (5–8).
SiO2(s) + CaO(l) = CaSiO3(s) | (5) |
CaSiO3(s) = Ca2+ + SiO32− | (6) |
SiO2(s) + O2− = SiO32− | (7) |
SiO32− + 4e = Si(s) + 3O2− | (8) |
However, the above mechanism has remained far from being well-addressed in the past literature concerning the electrochemical reduction of solid silica in molten CaCl2,1–6,12 although the liquid process was recently speculated to be present during the course of the reaction.11,13,14 Herein, we provide verification of the dissolution–electrodeposition mechanism during the electro-reduction of solid silica in molten chlorides through systematical investigations on the solubility and electrochemical properties of silicates, which not only form the scientific base for a controllable electrolytic extraction of nanostructured silicon in molten salts, but also opens an avenue to a continuous silicon extraction process via electro-deposition of dissolved silicates in molten CaCl2. In addition, the present study increases the general understanding of the versatile material extraction route via the electro-deoxidization process of solid oxides in molten salts, which also provokes reconsiderations on the electrochemistry of insulating compounds.
Electrodeposition experiments were performed in molten CaCl2 at 900 °C with 10 wt% CaSiO3 (CaSiO3 was supersaturated). A nickel foil was used as the deposition substrate.
The electrolytic samples were thoroughly rinsed in water and then collected after centrifugation and vacuum dried at 60 °C.
Fig. 1 (a) Images of quartz rods after being immersed in molten CaCl2 (1 #) and CaCl2–CaO (2–4 #); (b) the XRD pattern for the brown powder collected from the outer surface of the nickel foam. The inset shows an image of the Ni-sandwiched porous silica pellet cathode after electrolysis at 0.4 V (vs. Ca/Ca2+) at 850 °C in CaCl2 with 2 mol% CaO. |
The dissolution of SiO2 with the assistance of CaO was also confirmed by the cyclic voltammetry (CV) measurements. As shown in Fig. 2a, the cyclic voltammogram of the Mo electrode in molten CaCl2 (dotted curve) only exhibited cathodic peak C1 and its anodic counterpart A1 near the negative scan limit, ascribed to the formation and re-dissolution of Ca. With the addition of 0.1 mol silica, the cyclic voltammogram (dashed curve) remains unchanged at the potential range positive than that of Ca formation (onset of C1), indicating a low SiO2 concentration in the melt. With the consecutive addition of 0.1 mol CaO, another two pairs of redox peaks (C2/A2 and C3/A3) in the corresponding cyclic voltammogram (solid curve) were observed, which are ascribed to the redox reactions of the dissolved silicates.
Fig. 2 (a) Cyclic voltammograms of the Mo electrode in molten CaCl2 (434 g) (dotted), CaCl2 + 0.1 mol SiO2 (dashed) and CaCl2 + 0.1 mol SiO2 + 0.1 mol CaO (solid) at 850 °C; (b) Cyclic voltammograms of the Mo electrode in molten CaCl2 at 850 °C with the addition of different amounts of CaSiO3; (c) Cyclic voltammograms at different scan rates of the Mo electrode in molten CaCl2 at 850 °C with the presence of 0.051 wt% CaSiO3; (d) XRD pattern and SEM image of the powder electrodeposited on the Ni substrate via potentiostatic electrolysis at 0.6 V for 10 h in the CaSiO3-saturated molten CaCl2. |
The dissolution–electrodeposition of SiO2 was further verified by a potentiostatic electrolysis experiment. Using a silica pellet (wrapped in nickel foam) cathode, potentiostatic electrolysis was carried out in molten CaCl2 containing ∼2 mol% CaO. The inset of Fig. 1b shows the picture of the cathode after 12 h of electrolysis. The observed brown powder on the outer surface of the nickel foam which had no direct contact with the silica feedstock tells a different story from the solid-to-solid electro-reduction mechanism. This phenomenon was also found to exist in the same experiment performed in pure CaCl2 melt (without the addition of CaO). As per the solid-state electro-reduction mechanism, the cathode would retain its original integrity within the nickel foam. The XRD pattern shown in Fig. 1b suggests that the obtained brown powder contains crystalline silicon. Such anomalous deposited silicon provides evidence for the existence of the dissolution–electrodeposition mechanism. Upon cathodic polarizations, the dissolved silicates tend to be electro-reduced to solid silicon on the current collector, incurring the formation of silicon on the outer surface of the nickel foams, as depicted in the inset of Fig. 1b (reaction 8).
Scheme 1 A schematic representation of the electro-reduction process of solid silica in molten CaCl2 governed by both the solid-state reaction and dissolution–electrodeposition mechanisms. |
When it comes to electrolytic silicon, Si is merely a semi-metal, if not a non-metal. Without sufficient delocalized electrons to prompt thermal-induced strain, the aggregation of generated Si seeds is not accelerated, facilitating the formation of ultrafine products. Such an ultra-refining mechanism is verified in the present study. As exhibited in Fig. 3, the precursory amorphous micro-spherical silica with diameters ranging from 2 to 5 μm was transformed into pure and crystalline silicon nanowires or silicon nanoparticles, upon constant-voltage electrolysis at 850 °C in molten CaCl2 for 10 h. The sample prepared via electrolysis at 2.2 V consists of high-aspect-ratio Si nanowires with diverse diameters ranging from 50 nm to 100 nm. While the sample formed at 2.4 V is composed of interconnected Si nanoparticles with diameters less than 80 nm. The above argument on retarded thermal-induced strain in less-metallic silicon is rational to address the formation of Si nanoparticles, but yet not adequate to interpret the generation of high-aspect-ratio Si nanowires. The formation of silicon nanowires through molten-salt electrolysis of solid silica was recently reported and the utilization of nano-sized silica precursors was claimed to be a prerequisite on the generation of silicon nanowires.13 Our present study demonstrates that molten-salt electrolysis is a controllable electrolytic extraction method of nanostructured silicon, which is powerful enough to ultrarefine micro-sized silica.
Fig. 3 SEM images (a–c) and XRD patterns (d) of micro-sized silica spheres before (a) and after constant-voltage electrolysis at 850 °C in molten CaCl2 at 2.2 V (b) and 2.4 V (c) for 10 h. |
Based on the solid-to-solid electro-reduction mechanism, the electro-deoxidation process leaves voids in the electrolytic samples due to the decrease in the molar volume from SiO2 (22.6 cm3 mol−1) to Si (12.1 cm3 mol−1). The observed ultrarefining phenomenon (Fig. 3) is two orders of magnitude higher than that of the inherent volume shrinkage from silica to silicon. Such an anomalous shrinkage therefore cannot be addressed by the solid-to-solid electroreduction mechanism, which however can be well-explained by the dissolution–electrodeposition mechanism.
During molten-salt electrolysis of solid silica, the sluggish kinetics of O2− diffusion and its high concentration at electrochemical interfaces allow a fast combination reaction between solid silica and in situ ionized O2−, causing the formation of silicates at the cathode (reaction 5). Both dissolved silicates and solid precursors (including solid silicates and solid silica) can be electro-reduced to silicon upon cathodic electrolysis. It should be noted that the proportion of the influence from the two coincident mechanisms should depend on the concentration of O2− ions in the vicinity of the reaction zone (reaction 7). The influence of dissolution–electrodeposition will be enhanced with the increased concentration of O2−. Many parameters can influence the O2− concentration. For example, the electrolysis potential can influence the concentration of O2− at the electrochemical interface. Upon electrolysis at higher overpotentials, the concentration of O2− at the electrochemical interface is higher.5 Therefore, a more significant influence of the dissolution–deposition mechanism occurs during the electrolysis at more negative potentials. The reported correlation between the electrode potential and the activity of O2− in the melts is a good reference for assessing contributions from the two mechanisms.11 The electrodeposition of dissolved silicates results in the formation of solid silicon, which can function as nucleation centers for the growth of silicon generated from solid feedstock and might lead to the growth of nano-silicon.13,21
Grain growth follows the nucleation process, during which competition between nucleation and growth also exists. The homogeneous nucleation process is relevant to overpotentials, indicating the possibilities of engineering the morphologies of products by tuning the overpotential. With a relatively low cathodic overpotential (for example, constant-voltage electrolysis at 2.2 V), the generation rate of nuclei is reasonably low, allowing the anisotropic growth and formation of Si nanowires (Fig. 3b). Upon electrolysis at higher cathodic overpotentials (more negative potentials), i.e. 2.4 V, the generation rate of Si nuclei is accelerated, facilitating the formation of Si nanoparticles (Fig. 3c). In addition, the growth process is speculated to be governed by both Ostwald-ripening and oriented-attachment mechanisms.22 The presence of high-aspect-ratio nanowires (Fig. 3b) indicates the existence of the classical Ostwald-ripening process, in which bigger crystals grow at the expense of smaller ones. The existence of the coincident oriented attachment process is evidenced from the generation of interconnected nanoparticles (Fig. 3c), in which smaller particles with matched orientations directly coalesce into larger ones. Despite its high-temperature nature, the molten-salt featuring strong polarity and high surface tension is capable of retarding the agglomeration of the tiny particles.23 The above justifications suggest that molten-salt electrolysis is a generic and self-templated preparation of nanostructured materials.
The cyclic voltammograms of the Mo electrode in molten CaCl2 at 850 °C with different amounts of dissolved CaSiO3 are shown in Fig. 2b. Peak C3 indicating the formation of electrodeposited Si appears in the cyclic voltammograms. The onset potential of C3 is 0.75 V, in agreement with Fig. 2a. Peaks C2 and A2 related to Si–Ca are shown in the cyclic voltammograms with small silicate concentrations (dashed and dotted curves) and overlap other peaks with a high silicate concentration (solid curve). Peak C3 intensifies in the cyclic voltammograms recorded at faster scan rates, indicating the electrodeposition mechanism. The cyclic voltammograms collected at different scan rates (Fig. 2c) exhibit increased currents with scan rates, which is a typical characteristic of electrochemistry for dissolved species.7 While the cyclic voltammograms of the direct electro-reduction of solid silica display decreased cathodic currents with increased scan rate, which is an intrinsic part of the nature of the solid-state electro-reduction mechanism (second-type electrode process).6
Potentiostatic electro-deposition on the Ni substrate at 0.6 or 0.4 V (vs. Ca/Ca2+) for 10 h was then performed in the CaSiO3-saturated molten CaCl2 at 850 °C. The corresponding deposits show the morphology of the agglomerated granules (inset of Fig. 2d). The XRD pattern of the sample can be well indexed to cubic silicon (JSPDS. 27-1402), indicating the formation of high-purity crystalline silicon. It was found that deposits with thicknesses of ca. 3 mm can be produced upon electrolysis at 0.4 V for 10 h (Fig. 4, right), which was coherently connected with the Ni substrate. Upon electrolysis at 0.6 V for 10 h, only a thin-layer deposit can be generated (Fig. 4, left), which promises the fabrication of silicon film through the proposed method. This preliminary study promises the electro-deposition of dissolved silicates as a continuous silicon production method, however, the detailed information on the energy efficiency, productivity and property of the deposits deserves further investigation.
Fig. 4 Typical photos of the deposits on the Ni substrate prepared via potentiostatic electrolysis at 0.4 V (vs. Ca/Ca2+) (left) or 0.6 V (right) for 10 h in the CaSiO3-saturated molten CaCl2 at 850 °C. The dashed line in the right-hand image indicates the liquid level of the melt. |
On the one hand, the dissolved silicates can be electro-reduced into silicon which is capable of functioning as a nucleation centre for the growth of nanostructured silicon. Fewer nuclei are generated upon the electrolysis of porous silica at low overpotentials, facilitating the formation of high-aspect-ratio silicon nanowires. While silicon nanoparticles are produced upon the electrolysis of porous silica at high overpotentials, due to the presence of more nuclei.
On the other hand, electrochemical tests showed that the electrodeposition of silicon from dissolved silicates started at a potential of 0.75 V (vs. Ca/Ca2+), which is far more positive than the formation potential of Si–Ca intermetallics. This result provokes a continuous silicon production process via the electrodeposition of CaSiO3 in molten CaCl2, which was verified in the present study.
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