Electrodeposition of aluminium from 1-butyl-1-methylpyrrolidinium chloride/AlCl3 and mixtures with 1-ethyl-3-methylimidazolium chloride/AlCl3

Article abstract of DOI:10.1016/j.electacta.2012.03.056

In this paper we show that nanocrystalline aluminium can be electrodeposited from the ionic liquid 1-butyl-1-methylpyrrolidinium chloride ([Py_1,4]Cl/AlCl₃) (40/60 mol%). The study comprises electrochemical experiments such as cyclic

Full text of DOI:10.1016/j.electacta.2012.03.056

Electrochimica Acta 70 (2012) 210–214
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Electrochimica Acta
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Electrodeposition of aluminium from 1-butyl-1-methylpyrrolidinium
chloride/AlCl and mixtures with 1-ethyl-3-methylimidazolium chloride/AlCl
3
3
P. Giridhar , S. Zein El Abedina,b, F. Endresa,∗
a
a
Institute of Particle Technology, Clausthal University of Technology, Arnold-Sommerfeld-Strasse 6, 38678 Clausthal-Zellerfeld, Germany
Electrochemistry and Corrosion Laboratory, National Research Centre, Dokki, Cairo, Egypt
b
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper we show that nanocrystalline aluminium can be electrodeposited from the ionic liquid
-butyl-1-methylpyrrolidinium chloride ([Py1,4]Cl/AlCl3) (40/60 mol%). The study comprises electro-
Received 4 February 2012
Received in revised form 1 March 2012
Accepted 13 March 2012
Available online 23 March 2012
1
chemical experiments such as cyclic voltammetry, constant potential electrolysis, SEM and XRD
measurements. Thick and uniform layers of aluminium are obtained from [Py1,4]Cl/AlCl3 with an aver-
◦
age crystal size of 20 nm at 100 C. Aluminium deposits obtained from 90:10 vol% and 80:20 vol% of
◦
[
Py1,4]Cl/AlCl3 and [EMIm]Cl/AlCl3 are nanocrystalline at 100 C. The presence of imidazolium cations
Keywords:
seems to alter the electrode/electrolyte interface to some extent as nanocrystalline and microcrys-
talline aluminium deposits are obtained from the ionic liquid mixtures at compositions of 70:30 vol%
of [Py1,4]Cl/AlCl3 and [EMIm]Cl/AlCl3 and below.
Electrodeposition
Aluminium
Ionic liquids
Ionic liquid mixtures
XRD
© 2012 Elsevier Ltd. All rights reserved.
SEM
1
. Introduction
found that [Py1,4]TFSA/AlCl and [EMIm]TFSA/AlCl mixtures show
3
3
the same behaviour and the electrochemically active Al-species
–
The Electrodeposition of aluminium and its alloys from ionic
in both mixtures was found to be [AlCl (TFSA) ] . Therefore we
2
2
+
liquids has been extensively studied in ionic liquids composed of
organic halides and aluminium halides. Already in 1928, an attempt
to electrodeposit aluminium from a mixture of tetraethyl ammo-
nium bromide and aluminium bromide (as a source of aluminium)
supposed that the [Py1,4] cation acts as a grain refiner and it
might play its role by adsorption on the substrates and on growing
nuclei, thus hindering the further growth of crystallites. Together
with Atkin we further confirmed this interpretation by probing the
interfacial structure at the electrode/ionic liquids interface using
atomic force microscopy (AFM) and scanning tunneling microscopy
measurements (STM) [22]. The AFM results showed that several
solvation layers form at the electrode/ionic liquids interface, and
that the strength of the interaction between the innermost layer
and the substrate is dependent on the cation type. The force
required to rupture the solvation layers is greater for [Py1,4]TFSA
than for [EMIm] TFSA, signifying the increased strength of interac-
tion for the [Py1,4] cation compared to [EMIm] cation. Furthermore,
the STM results revealed that the interaction of [Py1,4]TFSA with
Au(1 1 1) leads to strongly structured surface and on the contrary
[EMIm]TFSA/Au(1 1 1) does not show surface restructuring [22].
Thus, it can be stated without doubt that the difference in sur-
face interactions of the ionic liquid species can totally influence the
electrochemical behaviour of ionic liquids leading to interesting
effects. Even a slight change in the substitution of an imidazolium
cation, such as incorporation of a methoxy group into the side
chain, significantly influences the electrochemical behaviour as
nanocrystalline, shiny aluminium deposits can be obtained from
1-(2-methoxyethyl)-3-methylimidazolium chloride/AlCl3 [23].
◦
at 100 C was reported [1]. Later, Hurley and Wier electrodeposited
aluminium and some metals such as Cu, Zn and others from solu-
tions composed of ethylpyridinium bromide and the respective
metal chlorides [2,3]. These ionic liquids (sometimes called first
generation ionic liquids) are hygroscopic in nature, which might
limit their applications. Nevertheless, a lot of work was done on
the electrodeposition of aluminium from chloroaluminate ionic
liquids with imidazolium halides and in most of the cases a micro-
crystalline aluminium deposit was obtained [4–19]. In [20] we
showed that nano- and microcrystalline aluminium can be elec-
trodeposited from the ionic liquids [Py1,4]TFSA and [EMIm]TFSA,
respectively. At the first glance it was suggested that the specia-
tion and/or the cation of the ILs might be the reason of such effect.
Therefore, we systematically investigated the biphasic mixtures by
means of RAMAN/IR and 27Al- as well as F-NMR together with
DFT (density functional theory) calculations [21]. Interestingly, we
19
∗ Corresponding author. Tel.: +49 5323 722980; fax: +49 5323 722460.
E-mail address: frank.endres@tu-clausthal.de (F. Endres).
0
013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2012.03.056

P. Giridhar et al. / Electrochimica Acta 70 (2012) 210–214
211
Based on our earlier results described above we were interested
to investigate, if and to which extent imidazolium cations added
to [Py1,4]Cl/AlCl3 can alter the grain size of aluminium. For this
purpose, various compositions of ionic liquid mixtures have been
prepared. The ionic liquid mixtures were composed of 1-butyl-
1
-methylpyrrolidinium chloride, [Py1,4]Cl and AlCl3 (40:60 mol%)
and of 1-ethyl-3-methylimidazolium chloride [EMIm]Cl and AlCl3
40:60 mol%). Nanocrystalline aluminium deposits were obtained
(
both from [Py1,4]Cl/AlCl3 ionic liquid and also from mixtures
of ionic liquids up to a vol% of 80:20 of [Py1,4]Cl/AlCl3 vs.
[
EMIm]Cl/AlCl . In the case of [EMIm]Cl/AlCl , a microcrystalline
3
3
aluminium deposit was obtained. These results indicate that the
pyrrolidinium cation indeed seems to be responsible for the depo-
sition of nanocrystalline Al.
2
. Experimental
1
-ethyl-3-methylimidazolium chloride, [EMIm]Cl and 1-butyl-
1
-methylpyrrolidinium chloride, [Py1,4]Cl, were purchased from
Fig. 1. Cyclic voltammogram of [Py1,4]Cl/AlCl3 (40/60 mol%) at a scan rate of
IOLITEC, Germany. The quality of the substances, [EMIm]Cl and
Py1,4]Cl, given by the supplier are 99% and 99% respectively. The
water contents of as received [EMIm]Cl and [Py1,4]Cl, are 0.28% and
.27% respectively (by Karl–Fischer Titration). Aluminium chloride
−
1
◦
10 mV s on gold at 100 C.
[
0
scanned initially from the open circuit potential (OCP) to the
grains (99.99%) were procured from FLUKA.
−
1
negative direction at a sweep rate of 10 mV s . The cyclic voltam-
The organic halides were further dried for two days at 363 K
and stored in closed bottles in an argon filled glove box. This
procedure usually reduces the water content to values of about
mogram exhibits a sharp rise in reduction current below −50 mV
(
c ) followed by a stripping peak (a ) upon scan reversal. The
4
4
observed crossing of forward and backward scan close to 0 V
can be attributed to nucleation and growth of aluminium on the
working electrode. Fig. 1 reveals that the aluminium bulk deposi-
tion is reversible. In addition, three cathodic processes have been
2
2
0 ppm. The water and oxygen contents in the glove box are below
ppm (OMNI-LAB from vacuum atmospheres). Acidic chloroalu-
minate ionic liquids were prepared by careful mixing of 40 mol%
of either [Py1,4]Cl or [EMIm]Cl and 60 mol% of AlCl3 in the glove
box. A viscous and light yellow colour solution was obtained in the
observed (c , c , and c ) prior to the bulk deposition of aluminium.
1
2
3
The processes at c and c might be attributed to the under potential
1
2
case of [Py1,4]Cl/AlCl . The ionic liquids, i.e., pure [Py1,4]Cl/AlCl3
3
deposition of Al and the corresponding counter parts are observed
at a and a . The cathodicwaveat c mightbe attributedthe alloying
(
40:60 mol%), pure [EMIm]Cl/AlCl (40:60 mol%), and various com-
3
1
2
3
positions of ionic liquid mixtures (mixtures of [Py1,4]Cl/AlCl3
40:60 mol%) and [EMIm]Cl/AlCl3 (40:60 mol%) are clear at the
studied temperature. A clear and light brown colour solution was
of Al with gold before the onset of the nucleation and growth pro-
cess of three dimensional particles [25–27]. The anodic wave at a3
(
is associated with the cathodic process at c , which is attributed to
3
obtained in the case of [EMIm]Cl/AlCl . According to results of
Abbott’s group the light brown colour of [BMIm]Cl/AlCl3 has no
considerable effect on the electrodeposition characteristics [24].
3
the dissolution of this aluminium-gold alloy. It is worth mentioning
that in the case of the aluminium deposition from [Py1,4]TFSA/AlCl3,
where TFSA is bis(trifluoromethylsulfonyl)amide, no underpoten-
tial processes were observed [25]. Furthermore, the deposition was
not reversible.
In order to get information on the deposit, a constant potential
electrolysis was carried out at −0.3 V for 1 h. The obtained deposit
was washed and then analyzed by high resolution scanning elec-
tron microscopy and X-ray diffraction. A bright and uniform deposit
was obtained and the high resolution SEM image shown in Fig. 2a
reveals fine crystallites with some aggregations.
For XRD analysis, Al was deposited on mild steel under the same
conditions reported for gold. The XRD patterns of Al made at −0.3 V
and −0.5 V on mild steel for 1 h are shown in Fig. 2b. As seen, the
deposited Al is crystalline and a strong diffraction peak (2 0 0) is
obtained along with other characteristic diffraction peaks (1 1 1),
Since the viscosity of [Py1,4]Cl/AlCl3 is relatively high compared to
◦
[
EMIm]Cl/AlCl , experiments were carried out at 100 C in order
3
to have comparable viscosities. Various compositions of ionic liq-
uid mixtures, i.e., from 90 (vol%) of [Py1,4]Cl/AlCl3 and 10 (vol%) of
[
EMIm]Cl/AlCl to 50/50 vol% of [Py ]Cl/AlCl and [EMIm]Cl/AlCl3
3 1,4 3
mixtures were prepared. All the prepared acidic chloroalumi-
nate ionic liquids and their mixtures were purified by electrolysis
between an Al anode and a stainless steel cathode for up to 2 days
◦
at 100 C and used for the reported studies. The electrochemical
measurements were carried out using a PARSTAT 2263 potentio-
stat/galvanostat controlled by PowerCV and PowerStep software.
A 25 mL glass beaker was used as an electrochemical cell. Gold
substrates (gold on glass) from Arrandee Inc. and mild steel sheets
were used as working electrodes, Al sheets and Al wires were used
as counter and reference electrodes respectively. Prior to use the
working electrodes were annealed in a hydrogen flame to red glow
for a few minutes. High resolution SEM (Carl Zeiss DSM 982 Gem-
ini) was employed to investigate the surface morphology of the
deposited films. X-ray diffraction patterns were recorded using a
Siemens D-5000 diffractometer with Co K␣ radiation.
(
(
2 2 0), (3 1 1), and (2 2 2). At a more negative potential (−0.5 V), the
1 1 1) peak vanishes, indicating a preferential orientation of the
deposit. Furthermore, the diffraction peaks are quite broad, and
the average crystallite size could be determined by the Scherrer
equation [28]. The average size was found to be about 55 nm. Fig. 2a
shows smallest grain size of about 20 nm, which is more or less in
agreement with the XRD result.
Fig. 3 shows how the cyclic voltammogram is altered if both
ionic liquids are mixed. All cyclic voltammograms exhibit a similar
electrochemical behaviour, i.e., a nucleation loop is observed when
the scan is reversed and a corresponding anodic peak associated
with the complete stripping of deposited aluminium is observed
3
. Results and discussion
The cyclic voltammogram of [Py1,4]Cl/AlCl3 (40/60 mol%) on
◦
gold at 100 C is shown in Fig. 1. The electrode potential was

2
12
P. Giridhar et al. / Electrochimica Acta 70 (2012) 210–214
Fig. 4. XRD patterns of electrodeposited aluminium layers on mild steel from vari-
◦
ous compositions of ionic liquid mixtures at 100 C.
carried out to deposit Al from the ionic liquid mixtures at −0.3 V
◦
for one hour at 100 C either on gold or on mild steel.
The obtained deposits were washed in a stream of isopropanol
and water. The washed samples are analyzed by high resolution
scanning electron microscopy and X-ray diffraction. The XRD pat-
terns of the electrodeposited aluminium at an applied potential
of −0.3 V on mild steel for 1 h are shown in Fig. 4. As shown,
the diffraction peaks of the deposits obtained in the ionic liq-
uid mixtures are broad indicating the small particle size of the
deposit. However, the diffraction peaks for the deposit obtained
from [EMIm]Cl/AlCl3 (40/60 mol%) are comparably narrow. SEM
images of the deposits obtained from various compositions and
from [EMIm]Cl/AlCl3 are shown in Fig. 5a–f. The deposits are uni-
form with very fine crystals (∼25 nm) obtained from 90:10 vol%
and 80:20 vol% mixtures (Fig. 5a and b). In the case of Al deposits
obtained from 70:30, 60:40, and 50:50 vol% mixtures and from pure
Fig. 2. (a) SEM image of electrodeposited Al obtained on gold in [Py1,4]Cl/AlCl3
◦
(
40/60 mol%) at −0.3 V for 1 h at 100 C. (b)XRD patterns of electrodeposited Al layer
on mild steel from [Py1,4]Cl/AlCl3 (40/60 mol%) at two different potentials −0.3 and
◦
−
0.5 V for 1 h at 100 C.
in all cases. In the case of [EMIm]Cl/AlCl , the oxidation waves
3
slightly differ from those ones obtained in ionic liquid mixtures.
Furthermore, an increase in the current is observed from pure
[
EMIm]Cl/AlCl3 (40/60 mol%), coarse and cubic particles are seen
in the SEM images (see Fig. 5c, d and f). However, with a higher
magnification nanostructures are seen on the microcrystals at com-
positions below 80:20 vol%.
[
Py1,4]Cl/AlCl3 to pure [EMIm]Cl/AlCl , which is mainly attributed
3
to the decrease of viscosity. Constant potential electrolysis was
From the above results and discussion the following features can
be summarized. The electrochemical behaviour of Al species in all
ionic liquids or mixtures seems to be quite similar and in all cases
◦
the deposition is reversible at 100 C. Nanocrystalline deposits were
obtained from [Py1,4]Cl/AlCl3 (40/60 mol%) and from the ionic liq-
uid mixtures with a vol% of ≥80:20. Below this composition rather
nanocrystalline and microcrystalline Al deposits were obtained
together. The electrode/electrolyte interface does not seem to be
significantly affected upon the addition of [EMIm]Cl/AlCl3 to the
[
Py1,4]Cl/AlCl3 till 80:20 vol%. This result supports an earlier inter-
pretation that the pyrrolidinium ion interacts with the substrate in
a different manner than the imidazolium ion does. As the deposi-
◦
tion experiments were performed at 100 C it is unlikely that the
viscosity contributes to the obtained effect. At such temperature
the viscosity of both [EMIm]Cl/AlCl3 and [Py1,4]Cl/AlCl3 are almost
comparable. Furthermore, it was shown that the pyrrolidinium
based ILs are about 4 times more strongly adsorbed on the electrode
surface than imidazolium based ILs [22]. Therefore, the increased
strength of adsorption of [Py1,4]+ compared to [EMIm] can lead to
+
the difference in the microstructure of the obtained deposits. The
+
strong adsorption of [Py1,4
]
to the surface of the growing nuclei
hinders their further growth, leading to very fine particles with
sizes in the nanometer regime. However, the exact nature of the
Fig. 3. Cyclic voltammograms recorded on gold at various compositions of ionic
liquid mixtures at 100 C.
◦

P. Giridhar et al. / Electrochimica Acta 70 (2012) 210–214
213
Fig. 5. (a and b) SEM micrographs of Al deposits obtained from electrolysis of 90:10 vol% and 80:20 vol% of (40/60 mol%) [Py1,4]Cl/AlCl3: [EMIm]Cl/AlCl3 on gold at −0.3 V for
◦
1
h at 100 C. (c and d) SEM images of Al deposits (and their magnified images) obtained from 70:30 vol% and 60:40 vol% of (40/60 mol%) [Py1,4]Cl/AlCl3: [EMIm]Cl/AlCl3 on
◦
gold at −0.3 V for 1 h at 100 C. (e and f) SEM images of electrodeposited aluminium on gold after constant potential electrolysis of 50:50 vol% of (40/60 mol%) [Py1,4]Cl/AlCl3:
[
EMIm]Cl/AlCl3 (its magnified image) and pure [EMIm]Cl/AlCl3 (40/60 mol%) at an applied potential of −0.30 V for 1 h at 100 ◦C.

2
14
P. Giridhar et al. / Electrochimica Acta 70 (2012) 210–214
electrode/electrolyte interface cannot be commented based on the
present results alone. Quite recently, we showed that interfacial
structures of electrode/ultrapure ionic liquids are strongly influ-
enced by addition of a simple salt like LiCl even with very low
concentrations [29]. Therefore, we cannot account for the nature of
the electrode/employed electrolytes interface without in situ STM
and AFM experiments. However, such experiments with these liq-
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◦
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employed electrolytes. One can also think in nucleation studies
which can give useful information on the nucleation and growth
at the early stages; however it is not straightforward to get repro-
ducible results in such systems. Au-Al alloying and underpotential
processes can influence the results leading to a lack of reproducibil-
ity. Therefore, substrates such as graphene or even glassy carbon
should be employed to avoid the problems associated with alloy-
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[
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4
. Conclusions
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23] S. Zein El Abedin, P. Giridhar, P. Schwab, F. Endres, Electrochemistry Commu-
nications 12 (2010) 1084.
In this paper we have shown that nanocrystalline aluminium can
be electrodeposited from a 40/60 mol% mixture of [Py1,4]Cl/AlCl3
◦
at 100 C. Nanocrystalline aluminium is also obtained from var-
ious ionic liquid mixtures comprising both pyrrolidinium and
imidazolium cations. This study supports earlier results that pyrro-
lidinium cations interfere differently with the substrate than
imidazolium cations do. At mixtures of 70:30 vol% of both ionic
liquids and below ([Py1,4] vs [EMIm]) microcrystalline deposits are
obtained. However, even in these mixtures the pyrrolidinium ion
plays a role in the grain size of the Al deposit as nano on microcrys-
talline deposits were obtained from these mixtures.
[
[24] F. Endres, S. Zein El Abedin, Q. Liu, D.R. MacFarlane, K.S. Ryder, A.P. Abbott,
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