Electrodeposition of aluminum from aluminum chloride-trihexyl(tetradecyl) phosphonium chloride

Article abstract of DOI:10.1149/1.2804422

Ionic liquids (ILs) are solvents of interest for applications such as electroplating, winning, and refining of metals. In this study, the conductivity of the Al Cl3 -trihexyl(tetradecyl) phosphonium chloride ([P14,6,6,6] Cl) system was characterized over a wide range of Al Cl3 concentration and temperature. Cyclic voltammetry was used to determine the electrochemical window of the neat IL using a Pt substrate. The anodic and cathodic potentiodynamic polarization behavior of Al was measured in Al Cl3 - [P14,6,6,6] Cl at an Al Cl3 concentration of 0.67 mol fraction (XAl Cl3). Aluminum was electrodeposited at a constant potential over a range of potentials (0.3-0.75 V); the deposit morphology, current efficiency, and power consumption are provided. The presence and effects of impurities such as H2 O and HCl in the ionic liquid are also addressed.

Full text of DOI:10.1149/1.2804422

D68
Journal of The Electrochemical Society, 155 ͑1͒ D68-D72 ͑2008͒
0013-4651/2007/155͑1͒/D68/5/$23.00 © The Electrochemical Society
Electrodeposition of Aluminum from Aluminum
Chloride–Trihexyl(tetradecyl) Phosphonium Chloride
z
*
James Vaughan and David Dreisinger
Department of Materials Engineering, The University of British Columbia, Vancouver, British Columbia,
Canada V6T 1Z4
Ionic liquids ͑ILs͒ are solvents of interest for applications such as electroplating, winning, and refining of metals. In this study, the
conductivity of the AlCl3–trihexyl͑tetradecyl͒ phosphonium chloride ͓͑P14,6,6,6͔Cl͒ system was characterized over a wide range of
AlCl3 concentration and temperature. Cyclic voltammetry was used to determine the electrochemical window of the neat IL using
a Pt substrate. The anodic and cathodic potentiodynamic polarization behavior of Al was measured in AlCl3–͓P14,6,6,6͔Cl at an
AlCl3 concentration of 0.67 mol fraction ͑XAlCl ͒. Aluminum was electrodeposited at a constant potential over a range of poten-
3
tials ͑0.3–0.75 V͒; the deposit morphology, current efficiency, and power consumption are provided. The presence and effects of
impurities such as H2O and HCl in the ionic liquid are also addressed.
© 2007 The Electrochemical Society. ͓DOI: 10.1149/1.2804422͔ All rights reserved.
Manuscript submitted August 14, 2007; revised manuscript received October 2, 2007.
Available electronically November 13, 2007.
Hurley and Weir were some of the early investigators into the use
pure. The Al plate working and counter electrodes were 99.99%
pure. The Al wire reference and working electrodes and foil were
99.999% pure. The Pt working and counter electrodes were 99.99%
pure. The Cu working electrode was 99.9% pure. The IL
͓͑P14,6,6,6͔Cl͒ used in this work was provided by Cytec Inc. The
moisture content of the IL was determined to be 0.15 wt % by Karl
Fischer titration ͑ASTM D4377͒. The purity of the IL was
ϳ96 to 97%, with tetradecene isomers, HCl and trialkyl phospho-
nium salts ͑R3PHCl͒ being the other main impurities.15
of ionic liquids ͑ILs͒. They electrodeposited aluminum at room tem-
perature from solutions of ethyl pyridinium bromide ͑EPB͒, alumi-
num chloride, and toluene or benzene.1 In these systems, the pyri-
dinium cation was easily reduced, limiting the plating bath lifetime.
This limitation was overcome when Wilkes et al. developed a class
of IL cations based on dialkyl-imidazolium salts in an effort to cre-
ate a battery with an Al metal anode and a chlorine cathode.2 Imi-
dazolium chloride salts have been shown to be technically suitable
solvents for the production and refining of Al. A recent patent details
the production and refining and recycling of reactive metals. Ex-
amples were given where Al was electrodeposited from solutions of
AlCl3 in 1-butyl-3-methylimidazolium chloride ͑BMIC͒.3 Unfortu-
nately, the high cost of the imidazolium ILs may limit their com-
mercial application. Phosphonium ILs are known to be thermally
stable over a wide range of temperatures and are less expensive to
produce than imidazolium ILs.4 They are also understudied com-
pared to both pyridinium and imidazolium. It is believed that phos-
phonium types are good alternatives to other ILs in many situations
and thus deserve further research.5 This is a reported study of Al
electrodeposition from phosphonium ILs. For additional background
information, a thorough review of the electrodeposition of Al was
written by Zhao and VanderNoot in 1997.6 More recently, the elec-
trodeposition of metals from ionic liquids was reviewed by Endres7
in 2002 and Abbott and McKenzie in 2006.8 Buzzeno et al. also
reviewed the electrochemistry of nonhalo aluminate ionic liquids in
2004.9
Solution purification
.— Because the presence of H2O and HCl in
the melt was found to detract from the current efficiency of Al
electrodeposition,16 a purified solution was prepared. The as-
received ͓P14,6,6,6͔Cl was first exposed to pure Al foil to remove any
excess HCl by way of corrosion
3H2͑g͒
Al + 3HCl → AlCl3 +
͓2͔
2
The AlCl3 was then sublimated prior to mixing with the ͓P14,6,6,6͔Cl.
After combining the purified AlCl3 and ͓P14,6,6,6͔Cl, the
AlCl3–͓P14,6,6,6͔Cl mixture was again exposed to the Al foil to re-
move HCl.
By comparing Al foil corrosion rates for the melts prepared using
The primary equilibrium determining the anion species in chloro-
aluminate systems is described by
Table I. The equilibrium constant for the self dissociation of
AlCl−4 in NaCl, BPC, and MEIC.
2AlCl−4 = Al2Cl−7 + Cl−
͓1͔
Temperature
Equilibrium constant
of Eq. 1
The equilibrium constants in NaCl, butyl pyridinium chloride ͑BPC͒
and 1-methyl-3-ethyl-imidazolium chloride ͑MEIC͒ are summarized
System
͑K͒
Ref.
11
AlCl3–NaCl
448
473
523
573
628
313
322
328
313
323
333
473
1.1 ϫ 10−7
2.2 ϫ 10−7
8.6 ϫ 10−7
2.4 ϫ 10−6
5.8 ϫ 10−6
1.2 ϫ 10−13
4.5 ϫ 10−13
7.5 ϫ 10−13
5 ϫ 10−17
8 ϫ 10−17
3 ϫ 10−16
in Table I. The low value of the equilibrium constant ͑10−7–10−17
͒
indicates that AlCl−4 is very stable, especially in the IL systems ͑BPC
and MEIC͒. There is no equilibrium data currently available for the
AlCl3–͓P14,6,6,6͔Cl system.Because of the high stability of AlCl−4, Al
͑XAlCl = 0.5–0.7͒
3
can only be electrodeposited from acidic melts ͑Ͼ0.5 XAlCl ͒ due to
AlCl3–BPC
12
13
14
3
the presence of the coordinately unsaturated Al2Cl−7 ion.10
͑XAlCl = up to 0.67͒
3
Experimental
AlCl3–MEIC
͑XAlCl = 0.6͒
3
Materials used.— All of the samples were prepared in a Vacuum
Atmosphere Co. glove box ͑HE Series͒ under a dry nitrogen atmo-
sphere ͑Ͻ10 ppm O2,H2O͒. The anhydrous AlCl3 used was 99.99%
a
AlCl3–MEIC
7.4 ϫ 10−15
͑XAlCl = 0.7–0.82͒
3
a Calculated from thermodynamic equilibrium parameters presented in
Øye’s model14 including data from Hussey13 and vapor pressure mea-
surements at higher temperature and AlCl3 concentration.
*
Electrochemical Society Student Member.
z E-mail: drei@interchange.ubc.ca
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Journal of The Electrochemical Society, 155 ͑1͒ D68-D72 ͑2008͒
D69
Figure 2. The conductivity of AlCl3–͓P14,6,6,6͔Cl as a function of tempera-
ture and concentration of AlCl3.
Figure 1. Electrochemical experiment setup. Heating was provided by an
external heating band ͑not shown in diagram͒.
Heating was provided by an external heating band. The solution was
agitated by a Teflon-coated magnetic stir bar during the cyclic vol-
tammogram and one of the electrodeposition experiments.
sublimated to nonsublimated AlCl3, it was determined that the sub-
limation of the AlCl3 had little to no effect on the concentration
proton containing impurities.
It was difficult to remove HCl or H2O from ͓P14,6,6,6͔Cl by cor-
roding Al as the corrosion rate low ͑Ͻ0.1 mmpy͒. However, in the
Sample characterization.— A Hitachi S-3000N SEM with an en-
ergy dispersive spectroscopy ͑EDS͒ system and Quartz analysis
software was used. The working distance for the EDS measurements
was 15 mm. The mass of the electrodeposit was measured using a
precision scale sensitive to four decimal places ͑0.0001 g͒.
chloroaluminate mixture ͓AlCl3–͓P14,6,6,6͔Cl͑XAlCl = 0.67͔͒ at
3
Results and Discussion
373 K, the Al corrosion rate was much higher ͑1.6 mmpy͒. After
one week of exposure to the Al foil the corrosion rate in the
Conductivity
.— The conductivity of the AlCl3–͓P14,6,6,6͔Cl sys-
XAlCl = 0.67 solution dropped below 0.4 mmpy indicating that a
tem increased with increasing temperature and AlCl3 concentration
͑Fig. 2 and Table II͒. The increased conductivity at higher concen-
tration of AlCl3 is attributed to a decrease in the cation-anion inter-
action as larger anions are formed ͑AlCl−4 and Al2Cl−͒. The conduc-
3
significant amount of the HCl had been consumed. The total mass of
Al corroded into the 75 g of AlCl3–͓P14,6,6,6͔Cl ͑XAlCl = 0.67͒ was
0.169 g corresponding to 6.26 mmoles of Al. Based o3n the stoichi-
tivity of the purified XAlCl = 0.67 solution was sligh7tly less than the
ometry of 3 HCl for 1 Al, the HCl removed from the 0.67 XAlCl
3
3
solution was ϳ0.25 mol/kg solution. The concentration of HCl in
the ͓P14,6,6,6͔Cl was calculated to be 0.12 mol/kg based on the
R3PHCl and HCl impurity values provided by Cytec Inc. An addi-
tional 0.11 mol/kg of HCl may have been produced when the AlCl3
was added to ͓P14,6,6,6͔Cl if all the H2O present was converted to
nonpurified, indicating that the presence of HCl as an impurity in the
ionic liquid increases the conductivity. In this system, the cation is
considerably larger than the anions; the estimate ion volumes are
presented in Table III.
The imidazolium chloroaluminate systems are more conductive
than AlCl3–͓P14,6,6,6͔Cl due to the smaller cations whose charge is
relatively delocalized. The imidazolium systems also exhibit a peak
conductivity near XAlCl3 = 0.5, which is not the case with
AlCl3–͓P14,6,6,6͔Cl. A more thorough comparison of the effects of
the cations and anion on the conductivity in these systems was pre-
sented previously.19
HCl. Thus, the AlCl3–͓P14,6,6,6͔Cl ͑XAlCl = 0.67͒ should only have
3
contained ϳ0.23 mol/kg of HCl. Because the corrosion rate was not
completely diminished after removing 0.25 mol/kg HCl, it is sus-
pected that some HCl was being introduced to the solution, probably
from small amounts of moisture contamination from the glove box
or from moisture adsorbed onto the corrosion coupons during clean-
17
ing ͑H2O reacts with AlCl3 to produce HCl and AlOCl2−
AlOHCl2 ͒.
or
Electrochemical window.— The electrochemical window of
͓P14,6,6,6͔Cl was determined to be 3.6 V at 373 K from a cyclic
18
Conductance.— The solution conductivity was measured using a
Jenway conductivity meter with an “enterprise” black platinum cell
probe ͑cell constant = 1 cm−1͒. An inert atmosphere was maintained
during the conductivity measurements by purging with dry nitrogen
͑Ͻ10 ppm O2, H2O͒. The temperature was regulated using an agi-
tated oil bath. The steady-state conductivity measurements were
taken following one hour of stabilization at each temperature. The
experimental error in the conductance measurements can be consid-
ered to be less than 4% with 95% confidence at all conditions
except at temperatures below 340 K when no AlCl3 is added. At
lower temperatures with the neat IL, the experimental error is con-
sidered to be less than 10% with 95% confidence as the solution
conductivity is considerably lower and the sensitivity limit of the
probe is being approached.19
Table II. The conductivity (S/m) of AlCl3–†P14,6,6,6‡Cl as a func-
tion of temperature (K).
Mole fraction
AlCl3, ͑XAlCl
͒
Conductivity, S/m ͑T is temperature in K͒
3
0.67
0.67 ͑purified͒
0.5
0
=−3 ϫ 10−8T3 + 7 ϫ 10−5T2 − 0.0338T + 4.546
=−4 ϫ 10−8T3 − 2 ϫ 10−5T2 − 0.0006T + 0.4769
=5 ϫ 10−8T3 − 3 ϫ 10−5T2 + 0.0030T + 0.0990
=3 ϫ 10−8T3 − 2 ϫ 10−5T2 + 0.0056T − 0.4888
Table III. Ion volumes calculated by Jenkins method.20
Ion volume
Electrochemical experiments.— The general setup for the elec-
trochemical experiments is described in Fig. 1. A Solartron 1286
potentiostat was employed to control the potential during these ex-
periments. The reference electrode used was an Al wire positioned
1 mm from the working electrode. For the potentiodynamic scan-
ning experiments, the solution resistance was partially compensated
for by current interrupt feedback ͑a CorrWare v2.9 software feature͒.
Ion
͑nm3͒
Al2Cl−7
AlCl−4
Cl
0.265
0.156
0.047
0.819
0.118
+
͓P14,6,6,6
͔
C6H11N+2 ͑EMI+͒
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D70
Journal of The Electrochemical Society, 155 ͑1͒ D68-D72 ͑2008͒
Figure 5. Potentiostatic electrodeposition of Al from purified
AlCl3–͓P14,6,6,6͔Cl ͑XAlCl = 0.67͒ at 395 K. Cu plate cathode ͑1 cm2͒ and
3
two Al plate anodes ͑11 cm2 total͒. The cathode-anode spacing was 2 mm.
effectively stopping the electrodeposition from these areas. The ca-
thodic passivation is lessened with increasing temperature, and a
diffusion limited current is observed. Two anodic dissolution reac-
tions are seen, the first peak occurs at 0.1–0.2 V and the second
occurs at 0.7–0.8 V. The first anodic peak is likely due to the strip-
ping of Al ͑reverse of Eq. 4͒. The second anodic peak may also be
the stripping of Al to the form of Al3Cl−10 as in Eq. 5. This was not
confirmed experimentally, but the existence of Al3Cl−10 in ionic liq-
uids was observed by Abdallah et al. using negative-ion fast-atom-
bombardment mass spectrometry23
Figure 3. Cyclic polarization of Pt in ͓P14,6,6,6͔Cl at 5 mV/s and 373 K. Pt
wire working electrode ͑0.096 cm2͒, Pt foil counter electrode ͑2 cm2͒, Al
wire reference electrode. The Pt wire-foil spacing was 2 mm, and the Pt
wire-Al wire spacing was 1 mm. The scan proceeded from the open-circuit
potential ͑ϳ0.7 V vs Al wire͒ to +2 to − 3 to + 2 V. The solution was
stirred using a magnetic stir bar.
voltammogram on a Pt substrate with reference to an Al wire ͑Fig.
3͒. The anodic current beginning at 1.7 V is the evolution of chlo-
rine gas. The cathodic current that begins at 0.35 V is attributed to
the evolution of hydrogen from the R3PHCl and HCl impurities. The
cathodic current beginning at about −1.9 V ͑determined by back
10Al2Cl−7 + Al → 7Al3Cl1−0 + 3e
͓5͔
¯
The anodic passivation diminishes with increased temperature as the
kinetics of the second anodic peak increase.
extrapolation of the Tafel slope͒ is due to the reduction of the cation.
+
The reduction of the cation ͓͑P14,6,6,6
͔
or R R P+͒ likely involves
Ј
3
electron transfer to an alkyl radical21
Potentiostatic electrodeposition of Al.— The potentiostatic po-
larization technique was employed to study the electrodeposition of
+
¯
R R P + e → PR + ·R
͓3͔
Ј
Ј
Al at various potentials from purified AlCl3–͓P14,6,6,6͔Cl ͑XAlCl
3
3
3
+
= 0.67͒. At this concentration the melt contains approximately
7 wt % Al. An electrochemical pretreatment was carried out prior to
the experiments, the cathode was cycled four times at 0.1 V/s over
a range of 2 V. The deposition time was 20 h; however, two of the
experiments were interrupted due to short-circuiting when the de-
posit contacted the anode. A temperature of 395 K was used. This
temperature was chosen to maximize the reaction kinetics while
minimizing possible losses of AlCl3 to the gas phase.
The electrochemical window of 3.6 V shows that the ͓P14,6,6,6
͔
cation is quite stable. Its stability is on the order of 0.8–1 V higher
than EMI+ as the electrochemical window of basic 1-ethyl-3-methyl
imidazolium chloride ͑EMIC͒ is 2.6–2.8 V.22
Potentiodynamic polarization.— Cathodic and anodic potentio-
dynamic scans were carried out in order to characterize the electrode
reactions and kinetics ͑Fig. 4͒. Evidence of a small amount of hy-
drogen evolution can be seen near at low cathodic overpotentials
and temperatures ͑347 and 366 K͒. Upon further cathodic polariza-
tion at the lower temperatures, Al is electrodeposited
The same solution was used for all five experiments. The reus-
ability of the bath is critical to the feasibility of industrial electro-
winning or refining applications of this technology. Kamavaram24
investigated the reusability of an AlCl3–BMIC bath used to recycle
Al metal matrix composites; he found that the current density slowly
decreased with each additional experiment, and the decrease was
ϳ5% after four 2 h experiments. The decrease was attributed to a
declining electrolyte concentration, this was not explained further
but presumably this meant that there was some contamination of O2
or H2O and the Al chloride species were slowly being converted to
oxides effectively lowering the concentration of electroactive Al.
Figure 5 describes the current density as a function of time dur-
ing the electrodeposition experiments. An increase in current density
is an indication of the formation of a dendritic deposit as the surface
area is increasing and the current density calculation is based on the
original surface area only. The experiments at higher potentials
͑−0.75 and −0.5 V͒ exhibited the highest current density and also
the fastest growing dendrites that ended the experiments prema-
turely by short-circuiting after 5.2 and 12.4 h, respectively. The ex-
periments at −0.3 and −0.4 V also exhibited some dendritic growth;
however, it was to a much lesser extent than that at −0.75 and
−0.5 V. The dendrites grew preferentially at the corners and edges
in all cases. The stirring of the solution slightly increased the current
density at −0.4 V.
−
−
¯
4Al2Cl7 + 3e → Al + 7AlCl4
͓4͔
A peak cathodic current is evident at lower temperatures; this is
likely due to the nature of the deposited Al. It was observed that
deposition on an Al substrate tends to occlude the IL and exhibits
passive behavior. The occluded IL would become deplete of Al2Cl7−
Figure 4. Potentiodynamic polarization of Al in the purified
AlCl3–͓P14,6,6,6͔Cl, XAlCl = 0.67 at 1 mV/s. Al plate working ͑0.72 cm2͒
3
and counter ͑6 cm2͒ electrodes, Al wire reference electrode. Current inter-
The electrodeposit current efficiency, power consumption and
deposit purity are presented in Table IV. The current efficiency
rupt resistance compensation was employed. No stirring.
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Journal of The Electrochemical Society, 155 ͑1͒ D68-D72 ͑2008͒
D71
Table IV. Potentiostatic electrodeposition of Al from purified †P14,6,6,6‡Cl–AlCl3 „XAlCl = 0.67… at 395 K. Cu plate cathode „1 cm2… and two Al
3
plate anodes (11 cm2 total). Electrode spacing = 2 mm.
Power
Experiment
͑in testing order͒
Deposition time
Current density
Current efficiency
consumption
͑kWh/kg-Al͒
Deposit purity
͑Al/Cl ratio͒
͑h͒
͑A/m2͒
Potential ͑V͒
͑%͒
Comments
1
2
3
4
5
5.2
12.4
20
20
20
67.5
44.0
5.8
14.3
17.0
−0.75
−0.5
−0.3
−0.4
−0.4
–
–
84
97
90
–
–
1.06
1.23
1.32
1.3
12.4
35.9
6.1
Short circuit
Short circuit
–
–
13.2
Stirred
ranged from 84 to 97%. There was some experimental error in the
measurement of the final cathode mass due to loss of sample when
rinsing with acetone following the experiment. Also, some IL is
occluded in the deposit and was not removed during the rinsing.
This became evident as Cl and P sometimes appear in the EDS area
scan of the electrodeposit. The current inefficiency is attributed to
hydrogen evolution; this was minimized by the purification proce-
dure but was not eliminated completely. The power consumption
values of 1.06–1.32 kWh/kg Al are low compared to the
2.5 kWh/kg Al reported by Zhang et al. from the AlCl3–BMIC
system.25 However, it should be noted that a proper comparison is
not possible because current densities and anode-cathode spacing
were also both lower in the present experiments.
dendrites in the unstirred solutions, deposits 2, 3, and 4 ͑Fig. 7, 8,
and 9b͒ were more pronounced than when the solution was stirred,
deposit 5 ͑Fig. 10b͒.
Deposits 2–4 were cross sectioned and polished with 1 ␮m
Al2O3 powder ͑Fig. 11 and 12͒. The Al deposit ͑the darker color͒
was generally found to be dense close to the Cu substrate.
Figure 12a is deposit number 3; the current passed during this
experiment was quite low, resulting in a thin deposit. In Fig. 12b,
there appears to be some cracking of the deposit near the bottom of
the micrograph. The cracking may have been introduced during the
The deposit purity was characterized by the Al/Cl ratio measured
using EDS, the impurity being the IL occluded into the deposit. A
higher Al/Cl ratio indicates a higher purity deposit. An Al/Cl value
of 0.22 would indicate no deposit ͑IL solution only͒. The deposit at
0.75 V was of the lowest purity because the fast dendritic growth
trapped a considerable amount of the solution. Although there was
no obvious degradation of the electroplating bath over the course of
the campaign, it was observed that the solution became a slightly
darker red color and more viscous. This is attributed to an increase
in the concentration of Al oxides and oxychlorides due to slow con-
tamination of moisture in the glove box and adsorbed on the elec-
trode surfaces.
The first Al electrodeposit ͑0.75 V͒ grew dendritically and short-
circuited after 5.2 h. The deposit occluded a significant amount of
the IL based on the Al/Cl ratio. The low Al/Cl ratio is also due to
the fact that the IL was not totally removed during rinsing because
the structure was fragile and efforts were made to minimize sample
loss. In Fig. 6a, the bulk deposit is seen, it appears as if there are
some Al crystals protruding from a layer of IL residue. The corner of
this electrodeposit is where a large dendrite formed and extended
out to the counter electrode ͑Fig. 6b͒.
Figure 7. Deposit 2: ͑a͒ bulk deposit 200X and ͑b͒ edge 200X, 12.4 h,
395 K, 44.0 A/m2.
Dendritic growth occurred preferentially at the edges, particu-
larly, the bottom edge of the deposits. The edge effects are mainly
due to the concentration of the electric field. Another factor in this
setup is the solution flow due to convection. The less dense solution
from which Al was deposited flowed upward, resulting in a higher
concentration of Al in solution near the bottom of the cathode. The
Figure 8. Deposit 3: ͑a͒ bulk deposit 200X and ͑b͒ corner 100X, 20 h,
395 K, 5.8 A/m2.
Figure 6. Deposit 1: ͑a͒ bulk deposit 200X and ͑b͒ corner 60X, 5.2 h,
Figure 9. Deposit 4: ͑a͒ bulk deposit 200X and ͑b͒ edge 200X, 20 h, 395 K,
395 K, 67.5 A/m2.
14.3 A/m2.
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D72
Journal of The Electrochemical Society, 155 ͑1͒ D68-D72 ͑2008͒
Figure 10. Deposit 5: ͑a͒ bulk deposit 200X and ͑b͒ edge 200X, 20 h,
Figure 12. Cross sections: ͑a͒ deposit 3 corner 800X and ͑b͒ deposit 4
corner 1000X. The lighter shaded metal is the copper plate.
395 K, 17.0 A/m2.
thanked for donating the ͓P14,6,6,6͔Cl. Thanks are also given to Bé
Wassink, Jianming Lu, Al Robertson, and Charles Hussey who pro-
vided many helpful suggestions.
University of British Columbia assisted in meeting the publication costs
of this article.
References
1. F. Hurley and T. Weir, J. Electrochem. Soc., 98, 207 ͑1951͒.
2. J. Wilkes, J. Levisky, R. Wilson, and C. Hussey, Inorg. Chem., 21, 1263 ͑1982͒.
3. B. Wu, R. G. Reddy, and R. D. Rogers, U.S. Pat. 6,881,321 ͑2005͒.
4. C. Hilgers, in Ionic Liquids in Synthesis, P. Wasserscheid and T. Welton, Editors, p.
21, Wiley-VCH, New York ͑2003͒.
5. C. Bradaric, A. Downard, C. Kennedy, A. J. Robertson, and Y. Zhou, in Ionic
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6. Y. Zhao and T. J. VanderNoot, Electrochim. Acta, 42, p. 3 ͑1997͒.
7. F. Endres, ChemPhysChem, 3, 144 ͑2002͒.
Figure 11. Cross sections: ͑a͒ deposit 2 edge 1500X and ͑b͒ deposit 4 edge
4000X. The lighter color is the copper plate.
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grinding and polishing of the sample. The cross-sectioned mor-
phologies were definitely affected by the sample preparation be-
cause both Al and Cu are relatively soft metals.
Conclusion
Al was electrodeposited from a solution of AlCl3–͓P14,6,6,6͔Cl
͑XAlCl = 0.67͒. High current efficiencies were obtained, and it is
3
believed that with careful purification and control of the environ-
ment current efficiencies approaching 100% should be possible. The
power consumption required to electrodeposit Al from this system
was low; however, the power requirements will increase signifi-
cantly with increased anode-cathode spacing as the resistance due to
the solution is high due to its low conductivity. Although the lower
cost of the phosphonium ILs is an advantage compared to imidazo-
lium ILs, the effects of its lower conductivity must be carefully
considered.
Acknowledgments
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24. V. Kamavaram, Ph.D. Thesis, The University of Alabama, Tuscaloosa, AL ͑2004͒.
25. M. Zhang, V. Kamavaram, and R. G. Reddy, JOM, 55͑11͒, 54 ͑2003͒.
The authors acknowledge the financial support of the Electromet-
allurgy Consortium and the Canadian National Science and Engi-
neering Research Council of Canada ͑NSERC͒. Cytec Inc. is
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