Pulse current electrodeposition of Al from an AlCl3-EMIC ionic liquid

Article abstract of DOI:10.1016/j.electacta.2011.03.047

Electrodeposition of aluminum from an AlCl₃-EMIC ionic liquid with or without the addition of saturated LaCl₃ was carried out by both direct- and pulse-current plating methods. The effects of various parameters, including current density, pulse frequency, current on/off duration (t_on and t_off), and temperature, on deposit morphology and crystal size were investigated. Deposits prepared by pulse-current plating gave a brighter and flatter surface than those prepared by direct-current plating at appropriate pulse current parameters. Temperature and pulse-current frequency (t_off) were shown to significantly affect deposit morphology. Coalescence of grains during t_off periods in the pulse current plating was observed, especially at temperatures above 60 °C. Increasing the temperature from 25 to 90 °C caused an increase in deposit grain size and resulted in a change of grain shapes from a small sphere-like form to a feather-like form. As a result, the adhesion of the deposited aluminum to the substrate was lowered. Smaller grain sizes and well-adhered deposits were achieved at lower temperatures. For example, deposition at 25 °C resulted in the smallest crystal size of about 0.3 μm under the conditions of t _on = 80 ms, t_off = 20 ms, and i = 8 mA/cm². Furthermore, the addition of LaCl₃ to the melt at 60 °C effectively reduced the porosity and improved compactness of deposits.

Full text of DOI:10.1016/j.electacta.2011.03.047

Electrochimica Acta 56 (2011) 5478–5482
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Pulse current electrodeposition of Al from an AlCl3-EMIC ionic liquid
Bing Li∗, Chunhua Fan, Yan Chen, Jingwei Lou, Lingguang Yan
School of Resource and Environmental Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 May 2010
Received in revised form 12 March 2011
Accepted 13 March 2011
Available online 20 April 2011
Electrodeposition of aluminum from an AlCl3-EMIC ionic liquid with or without the addition of saturated
LaCl3 was carriedoutby both direct-andpulse-currentplatingmethods. The effectsofvarious parameters,
including current density, pulse frequency, current on/off duration (ton and toff), and temperature, on
deposit morphology and crystal size were investigated. Deposits prepared by pulse-current plating gave
a brighter and flatter surface than those prepared by direct-current plating at appropriate pulse current
parameters. Temperature and pulse–current frequency (toff) were shown to significantly affect deposit
morphology. Coalescence of grains during toff periods in the pulse current plating was observed, especially
at temperatures above 60 ◦C. Increasing the temperature from 25 to 90 ◦C caused an increase in deposit
grain size and resulted in a change of grain shapes from a small sphere-like form to a feather-like form. As
a result, the adhesion of the deposited aluminum to the substrate was lowered. Smaller grain sizes and
well-adhered deposits were achieved at lower temperatures. For example, deposition at 25 ◦C resulted in
the smallest crystal size of about 0.3 ␮m under the conditions of ton = 80 ms, toff = 20 ms, and i = 8 mA/cm2.
Furthermore, the addition of LaCl3 to the melt at 60 ◦C effectively reduced the porosity and improved
compactness of deposits.
Keywords:
Electrodeposition
Pulse current
Al coating
Ionic liquid
LaCl3
© 2011 Elsevier Ltd. All rights reserved.
1. Introduction
current density to reach extremely high values. Therefore, pulse
current can also be used to electrodeposit Al in order to reduce the
Room temperature ionic liquids have been employed as media
for the electrodeposition of aluminum and its alloys, e.g., Al–La,
Al–Mo, Al–Zr, AlInSb [1–19]. In particular, the AlCl3-ethyl-3-methyl
imidazolium chloride (EMIC) system has been considered the most
promising electrolyte due to its excellent physicochemical proper-
ties. As previously reported [16,20–22], direct-current plating can
be used to electrodeposit Al coatings from an AlCl3-[EMIC] melt.
However, the current density is limited to a range of 5–70 mA/cm2
at a temperature range of 20–90 ◦C [21,22]. Higher current densities
result in a gradually decreasing concentration gradient of reactant
species caused by both a relatively low ion-diffusion rate and a
gradually increasing diffusion-layer thickness. Of note, the mor-
phology, grain size and compactness of the deposits are usually
dependent on electroplating parameters such as current density
and overpotential. Specifically, a larger current density or larger
overpotential will help to form a smaller-size deposit.
grain size of deposits. For example, when employing a square-wave
pulse current, which changes current with time, current density
can be maintained at a high value during the on-current period by
keeping a high instantaneous concentration gradient of reactant
species. During the off-current period, the concentration gradi-
ent for the reactant species immediately disappears, eliminating
the possibility of forming a smaller concentration gradient over
a long-duration electrodeposition. These processes are repeated
continuously during the entire electrodeposition to ensure a larger
current density or overpotential, which consequently increases the
nucleation rate of electrodeposition and reduces the grain-size of
deposits [24]. Literature reports have shown that pulsed currents
greatly improve surface morphology [25]. The objectives of this
study were to employ a square-wave pulse current-plating method
to electrodeposit an aluminum coating from an AlCl3-EMIC ionic
liquid with or without the addition of LaCl3 and to study the effects
of pulse current parameters and temperature on deposit morphol-
ogy and crystalline size.
According to the literature [23], in pulse plating with short pulse
durations, two distinct cathodic diffusion layers exists between the
cathode and the solution, including the pulsating diffusion layer
and the stationary diffusion layer. The concentration gradient in
the pulsating diffusion layer can be very high, allowing the pulse
2. Experimental
Anhydrous aluminum chloride was prepared as previously
reported by sublimation of a two-phase molten salt consist-
ing of 1 wt% NaCl and AlCl3 (>99.0 wt%) with a small amount
of high-purity Al flakes (99.999 wt%)[16,26]. Anhydrous lan-
∗
Corresponding author.
E-mail address: bingli@ecust.edu.cn (B. Li).
0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.03.047

B. Li et al. / Electrochimica Acta 56 (2011) 5478–5482
5479
40000
1
35000
30000
25000
20000
15000
10000
5000
0
1-Cu
2-Al
2
200
400
600
800
1000
1
2
2
2
1
Fig. 1. Schematic for square-pulse train for electrodeposition of Al from the ionic
liquid.
10
20
30
40
50
60
70
80
thanum chloride was prepared by gradually heating a solution
of LaCl3·7H2O and NH4Cl (1:8 molar ratio) to 120 ◦C, 200 ◦C and
340 ◦C and keeping for 3 h at each temperature. Upon completion of
the dehydration, the anhydrous AlCl3 and LaCl3 were immediately
transferred to a glove box until further use. Prior to the prepara-
tion of the AlCl3-EMIC ionic liquid, EMIC (>97 wt%) was dried in
phosphorus pentoxide (P2O5) in a glove box for 24 h. An acidic
ionic liquid of AlCl3-EMIC with a molar ratio of 2:1 was prepared
as previously reported [26]. Finally, a LaCl3-saturated AlCl3-EMIC
ionic liquid was obtained by adding anhydrous LaCl3 to the above-
prepared AlCl3-EMIC ionic liquid while stirring for 12 h.
A tungsten wire served as a working electrode and two high-
purity aluminum (99.999%) wires served as a counter electrode and
a reference electrode, respectively. Cyclic voltammograms were
carried out in a nitrogen-filled glove box in a three-electrode elec-
trochemical cell using a Princeton Applied Research Corporation
Model 2273 Potentiostat/Galvanostat. Direct- and pulse-current
depositions were performed using a two-electrode setup (i.e.,
cathode and anode) with a pulse power (SMD-30 Digital Pulse Elec-
troplating Power, China). A copper plate (>99%) was employed for
the cathode substrate and a high-purity aluminum (99.999%) wire
was used for the anode. Both electrodes were pretreated according
to the procedure described previously [27].
2-Theta(º)
Fig. 2. X-ray diffraction pattern and EDS analysis (the inset) of deposit prepared
under im = 20 mA/cm2; ton = 9 ms; toff = 1 ms; T = 60 ◦C and t = 30 min.
using either pulse-current plating (Fig. 2) or by direct-plating (data
not shown). The deposits prepared under 20 mA/cm2 by direct-
current plating at 60 ◦C were composed of ∼10 ␮m, strip-like grains
(Fig. 3a) and showed a more compact surface than those prepared
under 5 mA/cm2 with ∼20 ␮m, sheet-like grains (Fig. 3b). These
results indicated that a higher current density (or overpotential)
could improve deposit compactness and reduce grain size. How-
ever, upon further increasing the current density to 30 mA/cm2,
the deposits become darker and were composed of dispersed Al2O3
granules (as determined by EDS). As a result, the deposits exhibited
a lowered coherence to the copper substrate. This phenomenon was
also observed in the AlCl3-TMPAC melt [21] and may be due to the
ability to deposit small-size aluminum grains (e.g., nano-aluminum
grains) at a higher current density; these grains exhibited a higher
activity and could react with oxygen to form Al2O3.
Pulse current plating was applied to electrodeposit aluminum
at conditions of average current density of 5 mA/cm2, ton = 9 ms,
and toff = 1 ms (Fig. 3c). The deposits consisted of uniform and reg-
ular sphere grains of about ∼15 ␮m and gave a brighter and flatter
surface than those deposited by direct-current plating at the same
average current density (Fig. 3b).
The pulse-current parameters employed in this work, average
current density (im), pulse current density (ip), pulse frequency
(f), on-current period (ton) and off-current period (toff), satisfy the
relations in Eqs. (1) and (2),
After electrodeposition, all the samples were thoroughly
cleaned with deionized water and dried in air. The morphology
of the deposits was visualized using scanning electron microscopy
(SEM, JSM-6360LV) and the chemical compositions were mon-
itored using an auxiliary X-ray energy dispersive spectroscopy
(EDS). Phase compositions were analyzed using inductively cou-
pled plasma atomic emission spectroscopy (ICP-AES) and X-ray
diffraction (XRD).
ipton
3. Results and discussion
im
=
(1)
(2)
ton + toff
Electrodeposition of aluminum was conducted on a copper sub-
strate from the 2:1 molar ratio AlCl3-EMIC melt by direct- and
pulse-current plating. The square-pulse current train that was used
is shown in Fig. 1, where ton and toff are current on and off dura-
tions in ms and ip is the pulse current density in A/cm2. XRD and
EDS analyses showed that the deposits were composed of Al when
1
f =
ton + toff
where im is average current density in A/cm2, ip is pulse current
density in A/cm2, ton is the on-current period in ms, toff is the off-
current period in ms, and f is frequency in Hz.
Fig. 3. SEM micrographs of deposits on Cu substrate of an AlCl3-EMIC (2:1 molar ratio) at 60 ◦C for 30 min; (a) im = 20 mA/cm2; (b) im = 5 mA/cm2; (c) im = 5 mA/cm2; ton = 9 ms;
toff = 1 ms.

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B. Li et al. / Electrochimica Acta 56 (2011) 5478–5482
Fig. 4. Effects of pulse frequency on deposits of AlCl3-EMIC (2:1 molar ratio) at 60 ◦C for 30 min at ton/toff = 9:1; im = 20 mA/cm2 at (a) 10 Hz; (b) 100 Hz; (c) 500 Hz.
Fig. 5. Effects of ton and toff on deposits in AlCl3-EMIC (2:1 molar ratio) melt im = 20 mA/cm2, f = 100 Hz at 60 ◦C for 30 min at (a) ton = 8 ms; toff = 2 ms; (b) ton = 5 ms; toff = 5 ms.
such as 40 mA/cm2, these were not observed. The deposits shown
in Figs.4b and 5a were prepared under current densities of 25 and
22 mA/cm2, respectively, and were characterized by more com-
pact and flat surfaces. The different toff values of 5 ms, 2 ms and
1 ms may be responsible for these results, as the deposited grains
would coalescence during the toff period to form irregular and larger
grains. From the above results, a toff value of 1 ms was found to be
most optimal in the AlCl3-EMIC (2:1 molar ratio) melt with the
conditions of f = 100 Hz and im = 20 mA/cm2 at 60 ◦C.
Temperature is also expected to have significant effects on
the morphology and grains size of deposits. Electrodeposited alu-
minum from the same melt (2:1 molar ratio AlCl3-EMIC) at the
conditions of ton/toff = 9:1, f = 100 Hz, and im = 20 mA/cm2 for 30 min
at 80 and 90 ◦C are shown in Fig. 6a and b. Both deposits were
less compact and composed of grains larger than ∼50 ␮m. The
deposit prepared at 80 ◦C was composed of stone-like grains, while
those deposited at 90 ◦C contained feather-like grains. These results
indicate that increasing the temperature during pulsed-current
electrodeposition not only results in an increase in the deposit grain
size but also in a change in grain shape. Both deposits showed a
lowered adhesion to the substrate, especially those prepared at
90 ◦C. Electrodepositions of aluminum by direct and pulse cur-
rent plating were also carried out at 25 ◦C (Fig. 7a–c). As shown
in Fig. 7a and b, the deposits by pulse-current plating under the
conditions of 8 mA/cm2, ton = 80 ms, and toff = 20 ms showed a more
In order to examine the effects of pulse frequency on the
deposits, different pulse frequencies were applied to electrode-
posit aluminum at a fixed ratio of ton/toff = 9:1 with an average
current density of im = 20 mA/cm2 at 60 ◦C, the results are shown
in Fig. 4a–c. The corresponding pulse current density (ip) was
22.2 mA/cm2 as calculated from Eqs. (1) and (2). By comparison,
the deposits in Fig. 4b showed a more compact and smooth surface
with a grain-size of ∼5 ␮m and lowered porosity. In contrast, the
deposits prepared under 10 Hz (toff = 10 ms and ton = 90 ms) gave a
flat surface with ∼15 ␮m grain size and appeared more porous than
others, which is likely due to H2 evolution. In fact, even when pre-
pared by normal methods, the EMIC ionic liquid invariably contains
small quantities of water, providing a source of H2 evolution dur-
ing Al electrodeposition [28]. Pulse-current plating at a frequency
of 500 Hz (toff = 0.2 ms and ton = 1.8 ms) gave deposits with the high-
est surface roughness with some island-like nodules and contained
the biggest grains of about ∼25 ␮m because they were nearly con-
tinuously plating (Fig. 4c).
The effects of toff values on deposits were investigated in
the same melt as above at the conditions of f = 100 Hz and
im = 20 mA/cm2 at 60 ◦C. The morphologies of the deposits as visu-
alized with SEM are shown in Fig. 4b, 5a and b. When toff = 5 ms
(ton = 5 ms), the deposits appeared black with thin, porous surfaces
that were less adhered to the substrate (Fig. 5b). Although qual-
ity deposits were expected with a higher pulse current density,
Fig. 6. Temperature effects on electrodeposits from AlCl3-EMIC (2:1 molar ratio) melts; ton/toff = 9:1; f = 100 Hz; im = 20 mA/cm2; t = 30 min at (a) 80 ◦C and (b) 90 ◦C.

B. Li et al. / Electrochimica Acta 56 (2011) 5478–5482
5481
Fig. 7. Direct-current plating and pulse-current plating from an AlCl3-EMIC melt at 25 ◦C for 30 min at (a) 10 mA/cm2; (b) 8 mA/cm2; ton = 80 ms; toff = 20 ms; (c)10 mA/cm2;
ton = 80 ms; toff = 30 ms.
overpotential at 25 ◦C was much greater than that for 60 ◦C at the
same current density. Overpotential ꢀ values influence the nucle-
ation rate of deposited aluminum according to the relationship
shown in Eq. (3). The greater overpotential ꢀ at 25 ◦C would result
in a larger nucleus formation rate of deposited aluminum than that
at 60 ◦C, resulting in a smaller grain size at 25 ◦C.
ꢀ
ꢂ
−K2
−1 cm−2
(3)
ꢁ ꢁ
V = K1 exp
s
ꢁ ꢁ
ꢀ
At 25 ◦C, a distinct column crystal growth was observed along
the cross-section of the pulse-plated deposits (Fig. 7c). In sum-
mary, we found that lower temperatures significantly reduce grain
coalescence and decrease deposit porosity.
As previously reported [16,29], the addition of LaCl3 to the
AlCl3-EMIC melt can improve the morphology of deposits. Indeed,
upon the addition of LaCl3, the deposits obtained by direct-current
plating showed a regular face-centered cubic arrangement of Al
as determined by XRD. The addition of LaCl3 did not, however,
change the way nucleus formation occurred in the deposits. The
effects of direct- and pulsed-current parameters on deposits of
AlCl3-EMIC melts that were saturated with LaCl3 were investi-
gated. The resulting changes in surface morphology of deposits
were shown in Figs. 9 and 10. As compared with the deposits with-
out LaCl3 (Fig. 3a), those with added LaCl3 showed very compact
and flat surface without any distinct porosity Obviously, LaCl3 effec-
tively improved the morphology of the deposit. But pulse-current
plating did not exhibit significant effects (Figs. 9 and 10). The
deposits were prepared under the conditions of im = 20 mA/cm2,
t = 20 min, ton = 8 ms, toff = 2 ms, and f = 100 Hz and were composed
of Al with 0.1 wt% La content (as determined by ICP-AES analysis).
These deposits gave nearly the same-size grain as those formed by
direct-current plating. However, the pulse-current plating deposits
exhibited a distinct coalescence between grains indicating that
pulse-current plating does not result in deposits of better quality
than the direct-current plating method when LaCl3 is added to the
ionic melt.
Fig. 8. Cyclic voltammograms recorded on a tungsten electrode in a 2:1 molar ratio
AlCl3-EMIC melt. WE, RE and CE: high-purity aluminum wires. WE area: 0.2512 cm2.
Scan rate: 100 mV/s. (a) 60 ◦C; (b) 25 ◦C.
compact and flatter surface than those deposited by direct-current
plating at 10 mA/cm2. Although the pulse current density was
also 10 mA/cm2, the same value as the direct current plating in
Fig. 7a, the deposits by pulse current plating in Fig. 7b were com-
posed of smaller, ∼0.3 ␮m spherical grains. Furthermore, although
toff = 20 ms, it did not cause an increase in deposit grain size. These
results imply that temperature has a significant effect on grain size
and the morphology of the deposit in an AlCl3-EMIC melt. Lower
temperatures are favorable for obtaining deposits with smaller
grain sizes. This finding can be explained by the cyclic voltam-
mograms recorded on a tungsten electrode in the 2:1 molar ratio
AlCl3-EMIC melt at 25 ◦C and 60 ◦C (Fig. 8). The onset potential for
aluminum ion reduction did not change with temperature, but the
Fig. 9. Direct-current plating of an AlCl3-EMIC-LaCl3 melt at 60 ◦C at im = 20 mA/cm2 and t = 30 min.

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B. Li et al. / Electrochimica Acta 56 (2011) 5478–5482
Fig. 10. Addition of LaCl3 to a 2:1 molar ratio AlCl3-EMIC melt at 60 ◦C at conditions of im = 20 mA/cm2; t = 20 min; ton = 8 ms; toff = 2 ms; f = 100 Hz.
4. Conclusions
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Aluminum deposits were obtained from a 2:1 molar ratio AlCl3-
EMIC melt by both direct-current and pulse-current plating. At the
appropriate conditions, the pulse-current plated deposits exhibited
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same average current density by direct-current plating. In addi-
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pulse frequency, ton and toff values, and temperature, were also
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molar ratio AlCl3-EMIC ionic liquid during pulse current electrode-
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Acknowledgement
This study was financed by the National Natural Science Foun-
dation of China (Research Project 50474027).
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