The influence of LiCl coexistence with Al electrodeposition was investigated in a dimethyl sulfone, DMSO
2, bath containing AlCl
3 at 403 K. The electrochemical behaviors of Li and Al ions were examined using Pt electrodes in the bath and the deposition mechanism was analyzed by cyclic voltammetry, CV, with an Al reference electrode in the bath. The coexistence of LiCl in the AlCl
3-DMSO
2 bath inhibited the cathodic current corresponding to Al deposition in the CV experiment. The amount of ca. 500 μmol Al deposits was obtained in constant potential electrolysis for 1 h at –2 V in the bath with 10 mol% AlCl
3. However, it decreased to 140 μmol Al in the bath with 10 mol% AlCl
3 and 5 mol% LiCl. It was clarified that LiCl addition led to the formation of Li(DMSO
2)
+ more than the formation of
Plating technology in surface treatment using aqueous solutions is one of the useful techniques for prolonging the life of metal products. Although aluminum, Al, is a less noble metal, Al reacts with O2 in the air to form a dense oxide film, a passive state, so it is excellent in corrosion resistance. Because Al is also lightweight and has good features such as a glossy and beautiful surface, it is appropriate for use as a plating material. Indeed, galvanized Al plating, i.e., hot dipping, has been applied to a variety of pipes, heat exchange tubes, boilers, bolts, and nuts. Although galvanized and electroplated Zn films have been widely used as anticorrosive plating, Al is attracting attention as an alternative material to Zn because of concerns about exhaustion of Zn resources. It is, therefore, considered that progress in the Al plating processing technique is very important.
Several surface treatment methods, such as ionic liquids [
In the cases of vacuum plating using chemical or physical vapor depositions, the difficulty of mass production leads to a higher unit production cost, and a thin coating film may result in a stain on the basic material upon postprocessing. Moreover, hot dipping cannot provide a fair-quality coating film. It is also known that unlike other commercialized coating methods, Al metal cannot be obtained from aqueous solution because hydrogen evolution is dominant [
These ionic liquids have been recently proposed as alternative electroplating solutions [
None of the less noble metals, such as aluminum and lithium, can be electrodeposited from a system using water as a solvent. Therefore, by dissolving chlorides such as aluminum chloride, AlCl3, or lithium chloride, LiCl, using DMSO2 instead of water and performing electrolysis, it is expected that the reduction reaction of metal ions occurs on the cathode electrode and electroplating can be performed.
27Al NMR [
4AlCl 3 + 3DMSO 2 = Al ( DMSO 2 ) 3 3 + + 3AlCl 4 − (1)
The electrodeposition of Al can occur from the solvated cation Al ( DMSO 2 ) 3 3 + , whereas the reduction of AlCl 4 − is not observed within the electrochemical window of the electrolytes. It has been demonstrated that dense, uniform Al coatings with a high corrosion resistance can be electrodeposited from AlCl3-DMSO2 baths at 383 K [
In this study, we investigated the electrochemical behavior in an AlCl3-DMSO2 bath with LiCl addition to obtain the effect of Li ions on the Al ( DMSO 2 ) 3 3 + formation and Al deposition from NMR analysis for the bath and constant potential electrolysis, respectively. The possibility of Li electrodeposition was also examined in a LiCl-DMSO2 bath.
Platinum plate and copper plate (>99% purity, Nilaco) were used as working electrodes for cyclic voltammetry, CV, measurement and electrolysis at a constant potential. They were masked with insulating tape, Nitoflon, leaving an exposed surface area of 1.0 cm2. The Cu plate was wet-polished with 400 emery paper, and the Pt plate was also wet-polished, immersed in hydrochloric acid, washed with distilled water, and then washed with acetone for 20 min before tape masking. An Al rod (>99.99% purity, Nilaco) was immersed in the bath as a reference electrode, Al/Al3+. Pt plate was also used as a counter electrode.
To remove moisture of ca. 300 ppm contained in DMSO2 (99 mass%, Tokyo Chemical Industry, Japan), the DMSO2 powder was maintained at 353 K for 72 h in a constant temperature drier (DO-450 A, AS ONE Corporation) and melted at 403 K. The amount of DMSO2 used in each experiment corresponded to 0.2 mol.
A bath with 10 mol% or 20 mol% AlCl3 was prepared by adding aluminum chloride, AlCl3 (98 mass%, Nacalai Tesque, crystallized), to the melted DMSO2. Further, a Li-Al bath was prepared by adding 0 - 20 mol% of lithium chloride, LiCl (98 mass%, Nacalai Tesque, crystallized) to each Al bath. Bath preparation was conducted in an Ar-filled glove box.
Electrochemical properties were measured in the Ar-filled glovebox with a potentiostat/galvanostat (BioLogic, SP 150). Cyclic voltammograms were obtained at the potential region between ?3.5 V and 3.5 V vs. Al/Al3+ at a scan rate of 100 mV∙s?1. Electrodeposition was attempted on the Cu plate by constant potential electrolysis under ?2 V for 1 h. The temperature was maintained at 403 K using a hotplate. The electrolyte was agitated at 150 rpm during the experiment using a magnetic stirring device at the bottom of the beaker.
7Li and 27Al NMR spectra were obtained at 130.3 MHz using a 500 MHz NMR spectrometer (Agilent Technology). All chemical shifts were referenced to D2O containing 1.5 M Al(NO3)3, which was used as an external reference. Samples were placed in 10 mm NMR tubes with a 5-mm coaxial tube filled with DMSO-d6 as a lock solvent. Spectra were gathered at 403 K, as in the electrochemical measurements.
After electrolysis, deposits were washed with an AlCl3?DMSO2 solution and vacuum-dried prior to characterization. Crystal orientation and surface morphology of the deposits were characterized by X-ray diffraction, XRD (Rigaku Ultima IV; 40 kV, ?30 mA, 0.4 deg∙min?1), and the nitric acid solution was collected by suction and analyzed for Al and Li by inductively coupled plasma-atomic emission spectrometry, ICP-AES (Seiko Instruments, SPS 7800).
The possible cathodic reactions on Pt electrode were examined, initially, by CV measurements in a DMSO2 electrolyte containing LiCl at concentrations between 5 and 20 mol% at 403 K. The electrode potential was scanned from the open-circuit potential near 1 V in the negative direction to −3.5 V followed by scanning in the positive direction to 3.5 V and then cycled.
7Li NMR spectroscopy was next employed as this technique allows the environment of Li atoms to be unambiguously determined in most cases. For consideration of the reaction mechanism of Li electrodeposition, NMR analysis was conducted as a state investigation of ions in the bath. The NMR analysis results for 7Li and 7Li chemical shift are shown in
vicinity of −2 ppm in the bath containing 15 mol% LiCl. Because this peak coincides with the chemical shift [
Li + + DMSO 2 = Li ( DMSO 2 ) + (2)
It is considered that Li was electrodeposited from the complex ion at the cathode by the reaction in Equation (3).
Li ( DMSO 2 ) + + e – = Li + DMSO 2 (3)
To confirm Li deposition, electrolysis was conducted at potentials of −2.0 V, −2.5 V, −3.0 V, and −3.5 V for 1 h using a Cu plate as a working electrode. The appearance of the electrode surface is shown in
As a confirmation that the precipitate must be Li metal, the deposits were dissolved in 1 M HCl and diluted 100-fold, then qualitative and quantitative analyses were carried out using ICP-AES analysis. The results by ICP-AES analysis of the amount of Li contained in the film electrodeposited on the Cu substrate are summarized in
It is known that Al is electrodeposited from a bath of AlCl3-DMSO2 [
Constant potential electrolysis was carried out for 1 h at −2 V in all baths to clarify the Al and Li deposition. The surface state of Cu electrodes after constant potential electrolysis in each bath is shown in
Potential | Li (mmol) |
---|---|
−2.0 V | 0 |
−2.5 V | 11 |
−3.0 V | 44 |
−3.5 V | 43 |
without LiCl, a black deposit and a glossy deposit were obtained, respectively. These deposits must be Al containing many impurities. With the increase in the amount of LiCl in the bath, the surface of the deposits after washing became whitened (Figures 5(b)-(e)). These white precipitates seem to be lithium hydroxide reacted with moisture in the air after electrolysis. Glossy deposits (
To identify the electrodeposits, XRD analysis was carried out as qualitative analysis of precipitates, and analysis by ICP-AES was decided as quantitative analysis. In
LiOH∙H2O peaks were detected in the results of 20 mol% LiCl. It is considered that Li metal electrodeposited on the Cu electrode reacts with moisture and appears on the surface as hydroxide.
To calculate the amounts of Al and Li contained in the deposits on the Cu substrate, ICP-AES quantitative analysis was performed as shown in
In addition, to estimate the effect of ions present in the bath on the deposit component, the NMR analysis on all electrolytic baths was carried out for 10 mol% AlCl3 bath (
Al 2 Cl 7 − was never observed. DMSO2 is therefore found to act as a Lewis base weaker than Cl? but stronger than AlCl 4 − leading to the following equilibria in DMSO2-based electrolytes [
4AlCl 3 + 3DMSO 2 = Al ( DMSO 2 ) 3 3 + + 3AlCl 4 − (1)
AlCl 3 + Cl – = AlCl 4 − (4)
A peak corresponding to AlCl 4 − was detected in the vicinity of 104 ppm in all baths [
Electrolytic bath | Relative abundance/% | |||
---|---|---|---|---|
AlCl3 (mol%) | LiCl (mol%) | Al ( DMSO 2 ) 3 3 + | AlCl 4 − | Li(DMSO2)+ |
10 | 0 | 37.77 | 62.23 | - |
5 | 14.42 | 64.66 | 20.92 | |
10 | 0 | 52.02 | 47.98 | |
15 | 0 | 51.54 | 48.46 | |
20 | 0 | 44.44 | 55.56 | |
20 | 0 | 37.58 | 62.42 | - |
5 | 20.25 | 70.10 | 9.65 | |
10 | 13.39 | 60.61 | 26.00 | |
15 | 7.81 | 54.95 | 37.24 | |
20 | 0 | 51.56 | 48.44 |
In this study, in addition to how Li ion behaves in a LiCl-containing DMSO2 bath, we also investigated how to change the behavior of Al ions and Li ions by adding LiCl in an AlCl3-DMSO2 bath capable of Al electrodeposition. Furthermore, after conducting constant potential electrolysis in each bath, surface analysis of the working electrode and analysis of ions in the electrolytic bath were carried out to obtain the following findings.
1) In the CV measurement in a LiCl-containing DMSO2 bath, the rise of current from around −1.4 V, which is considered to be the redox of Li ions, was observed. Li can also be precipitated from DMSO2 as in Al from the analysis result of electrodeposition obtained by constant potential electrolysis in DMSO2-15 mol% LiCl bath. From the NMR analysis in a DMSO2-15 mol% LiCl bath, both Li form a complex ion with DMSO2.
2) In CV measurement in a bath containing LiCl added to an AlCl3-DMSO2 bath, current responses considered as oxidation and reduction of Al ions and Li ions are observed simultaneously. In the constant potential electrolysis in a bath in which LiCl was added to an AlCl3-DMSO2 bath, the state of the electrodeposition changed as the amount of LiCl added increased. Because LiCl inhibits the formation of Al ( DMSO 2 ) 3 3 + , which is necessary for Al electrodeposition from DMSO2, it inhibits the electrodeposition of Al.
We thank Mie Torii for her experimental help in NMR measurements. And we also gratefully acknowledge that Otsuka Toshimi Scholarship Foundation for supporting our study in Japan.
The authors declare no conflicts of interest regarding the publication of this paper.
Kim, S., Kumeno, S., Kamebuchi, K., Kuroda, K. and Okido, M. (2018) Effect of Li Ions on Al Electrodeposition from Dimethylsulfone. Journal of Surface Engineered Materials and Advanced Technology, 8, 110-125. https://doi.org/10.4236/jsemat.2018.84010