Materials and Corrosion 2009, 60, No. 12
DOI: 10.1002/maco.200905238
Corrosion and electrochemical behaviors of pure aluminum
in novel KOH-ionic liquid-water solutions
J. M. Wang*, J. B. Wang, H. B. Shao, X. X. Zeng, J. Q. Zhang and
C. N. Cao
The corrosion and electrochemical behaviors of pure aluminum in KOH-ionic
liquid-water solutions with variable volume ratios of water and the ionic liquid
1-butyl-3-methyl imidazolium tetrafluoroborate (BMIMBF4) were for the first
time investigated by means of hydrogen collection, polarization curve,
galvanostatic discharge, and electrochemical impedance spectroscopy (EIS). The
results of hydrogen collection experiments showed that aluminum has a low
corrosion rate in KOH-BMIMBF4-H2O solutions, and the corrosion rate decreases
with increase in BMIMBF4 content in the electrolytes. The results of
electrochemical experiments revealed that aluminum is electrochemically
active over a very wide potential window in the KOH-BMIMBF4-H2O solutions,
and its electrochemically kinetic mechanism is similar to that in the
corresponding aqueous solution; the increase in KOH and water contents in
the electrolytes may improve the anodic dissolution performance of
aluminum. It was found that aluminum presents excellent galvanostatic
discharge performance in the 2.0 M KOH BMIMBF4-H2O mixed solution with
60% water.
1 Introduction
Aluminum is considered to be a very attractive anode material
for electrochemical power sources because of its high specific
capacity, very negative standard electrode potential, environmentally benign characteristics, and almost unlimited reserves
[1, 2]. Among several investigated aluminum batteries systems
[2–5], alkaline aluminum batteries display excellent discharge
performance especially at high discharge rates. However, in
alkaline solutions aluminum anode undergoes self-discharge
with the production of large amount of hydrogen gas. This
destructive self-corrosion results in unacceptably high-energy
loss during standby and gives rise to the safety problem in the
use of batteries. There are two ways to reduce the self-corrosion
of the aluminum anode. The first is to dope aluminum with
J. M. Wang, J. B. Wang, H. B. Shao, X. X. Zeng, J. Q. Zhang, C. N. Cao
Department of Chemistry, Zhejiang University, Hangzhou 310027, (P.R.
China)
E-mail: [email protected]
J. Q. Zhang, C. N. Cao
State Key Laboratory for Corrosion and Protection of Metal, Institute of
Metal Research, Chinese Academy of Sciences, 62 Wencui Road,
Shenyang 110016, (P.R. China)
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other elements. Aluminum alloys based on high purity grade
metal doped with elements such as Ga, In, Sn, Zn, Mg, Ca, Pb,
and Mn have been investigated [6–9]. The second is to modify
the electrolyte by adding inhibitors, additives, or complexing
agents in order to make the electrolyte less corrosive [10–13].
Although significant progresses in the above two aspects have
been obtained, the corrosion behavior of aluminum in alkaline
solutions needs to be further improved.
Many researches [2, 14, 15] showed that the electrochemical
behaviors of aluminum largely depend on the applied electrolyte.
A successful electrolyte system should keep aluminum anode
electrochemically active whilst reducing its corrosion rate to a low
level. The solvent water with relatively high protic activity could be
mainly responsible for the large corrosion rate of aluminum
anode in alkaline aqueous solutions [14]. Hence, it can be
concluded that the application of an aprotic solvent in alkaline
solutions may effectively inhibit the corrosion of aluminum.
Based on the above idea, in this work the corrosion and
electrochemical behaviors of aluminum anode have been
investigated in a novel electrolyte system based on KOH-ionic
liquid-water solutions for the first time. In this electrolyte
medium, aluminum anode is electrochemically active due to the
use of KOH, and hydrogen evolution may be inhibited because of
the partial substitution of aprotic ionic liquid [1-butyl-3-methyl
imidazolium tetrafluoroborate (BMIMBF4)] for solvent water
with relatively high protic activity.
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Wang, Wang, Shao, Zeng, Zhang and Cao
Materials and Corrosion 2009, 60, No. 12
2 Experimental
2.1 Electrochemical measurements
Electrochemical measurements were performed in a typical
three-electrode glass cell. The working electrode was made of
pure aluminum (supplied by Johnson Matthey, purity no less than
99.9995%) in the form of 6 mm in diameter rods. The electrode
was insulated with epoxy resin except for the surface to be tested.
Before each test the electrode surface was polished by 2000 grit
waterproof abrasive paper, degreased in acetone, and rinsed in
deionized water. The counter electrode was a platinum foil and
the reference electrode a Hg/HgO electrode. The ionic liquid
BMIMBF4 was prepared as prescribed in the literature [16]. Other
reagents are of AR grade. All the solutions were prepared by
deionized water, and they were purged with nitrogen to remove
oxygen before measurements were made.
The measurement system used a potentiostat/galvanostat
(EG&G model 273A) and a lock-in amplifier (model 5210),
controlled by a microcomputer. Electrochemical impedance
spectroscopy (EIS) were obtained in the frequency range of
120 kHz–5 MHz and at a.c. single amplitude of 5 mV. Zview
software was used for the fitting of EIS. The potentiodynamic
polarization curves were measured at a scanning rate of 1 mV/s,
and galvanostatic discharge was performed at different current
densities. The electrochemical measurements were taken until
the electrode reached a steady open-circuit potential (OCP). All
the above electrochemical experiments were conducted at a
constant temperature of 25 8C.
2.2 Determination of corrosion rates
Corrosion rates were determined by a hydrogen collection
method. The apparatus used was as described elsewhere [6]. The
electrode was made of the same aluminum rod as that used in the
electrochemical experiments. Before each test the electrode
surface was degreased in acetone and rinsed in deionized water;
the solutions used were purged with nitrogen to remove oxygen.
After the specimens had been immersed in the test electrolytes at
25 1 8C for certain time (1–52 h), the gas volumes were
determined. The corrosion current (icorr) of the aluminum
electrode was calculated as follows [15]:
icorr ¼
2pVF
tSRT
(1)
where p is the atmospheric pressure, V the gas volume, F the
Faraday constant, t the collecting time, S the electrode area, R the
gas constant, and T is the experimental temperature.
3 Results and discussion
The corrosion data of aluminum in various KOH-BMIMBF4water solutions, obtained from the hydrogen collection method,
are shown in Fig. 1, where water content is defined as the volume
ratio of water versus the sum of water and BMIMBF4. If KOH
concentration is larger than 3.0 M, the KOH-BMIMBF4H2O solution becomes a biphasic mixture (its chemistry is
ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 1. Corrosion current densities of aluminum in variable KOHBMIMBF4-H2O solutions
unclear), similar to the behavior of several liquids based on
the bis(trifluoromethylsulfonyl)imide systems described by
Wasserscheid and coworkers [17]; if water content is lower than
0.1 (this critical value is 0.7 for the 3.0 M KOH containing
electrolyte), KOH has very low solubility. Because of the above
points, the corrosion data of aluminum in the electrolytes with
KOH concentration larger than 3.0 M or water content lower than
0.1 are absent in Fig. 1. It can be seen from Fig. 1 that the corrosion
rate of aluminum increases with progressive increase in water and
KOH contents in the electrolytes. It is noted that aluminum
displays relatively lower corrosion rate in the electrolytes with water
contents lower than 60%. Moreover, the corrosion rates of
aluminum determined by the weight loss method (data not shown)
are almost consistent with those obtained from the hydrogen
collection method. Therefore, it can be concluded that the selfcorrosion reactions of aluminum in alkaline medium are as follows
[18, 19]:
Al þ 4OH ! AlðOHÞ
4 þ3e
(2)
2H2 O þ 2e ! H2 þ 2OH
(3)
Larger KOH concentration facilitates the removal of the
dense oxide film of aluminum surface and may enhance the
electrochemical dissolution of aluminum in terms of reaction (2),
resulting in the higher corrosion rate of aluminum in the
electrolytes with larger KOH content. The addition of BMIMBF4
in the electrolytes decreases water content (activity), and this leads
to the decrease in the evolution rate of hydrogen according to
reaction (3), hence the corrosion rate of aluminum decreases with
increase in BMIMBF4 concentration (decrease in water content).
The dependences of anodic dissolution behavior of aluminum on KOH concentration in KOH-BMIMBF4-H2O solutions
with a 60% constant water content are shown in Fig. 2.
Aluminum may provide more negative potential and larger
anodic current density in the electrolytes with higher KOH
concentration, consistent with the variation trend of corrosion
rate with KOH content. In our experimental range aluminum
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Materials and Corrosion 2009, 60, No. 12
Electrochemical behavior of pure aluminum
Figure 2. Effects of KOH concentration on the anodic dissolution
behavior of aluminum in KOH-BMIMBF4-H2O solutions with a 60%
constant water content
presents better anodic dissolution performance in the electrolyte
with 2.0 M KOH. Although the corrosion rate of aluminum
increases with increase in KOH concentration, in the electrolytes
with 10–60% water contents the effects of KOH content on the
corrosion rate are relatively minor, as shown in Fig. 1. Therefore,
KOH concentration in the electrolytes is fixed at 2.0 M in the
following investigations.
The effects of solvent composition in the electrolytes with
2.0 M KOH on the electrochemical performance of aluminum are
shown in Fig. 3. In the anodic polarization curve of aluminum in
the electrolyte with 10% water (90% BMIMBF4) an almost
constant potential-independent current density is achieved at
relatively positive potentials, as shown in Fig. 3A. This behavior
can be interpreted in terms of the dissolution of aluminum
through a porous product layer that permits charge transfer and
ionic conduction, suggesting that aluminum may be in a pseudopassivation state [20]. In the electrolytes with 40–100% water
content the anodic current density of aluminum generally
becomes larger with increase in the electrode potential, indicating
the good dissolution performance of aluminum anode. As water
content in the electrolytes increases, the hydration of the surface
product layer enhances [14]. This improves the active dissolution
of aluminum, and the OCP of the electrode system shifts in the
negative direction. Figure 3B shows the cathodic polarization
curve of aluminum in the electrolytes with 2.0 M KOH and
various solvent compositions. In the tested systems, the
dominant cathodic reaction is the reduction of water [reaction
(3)]. The increase in water content (activity) accelerates the
evolution of hydrogen; thus larger cathodic current densities are
obtained in the electrolytes with higher water content. The above
results of polarization curves show that the increase in water
content in the electrolytes enhances both anodic and cathodic
reactions, hence increases the corrosion rate of aluminum,
consistent with the results of hydrogen collection experiments.
In order to investigate the electrochemical kinetic behavior of
aluminum in novel KOH-BMIMBF4-H2O solutions, EIS at OCP
were tested, and the results are shown in Fig. 4. The EIS of
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Figure 3. Anodic (A) and cathodic (B) polarization curves of aluminum
in the 2.0 M KOH-containing electrolytes with variable water contents
aluminum in the electrolytes with various water contents (40–
80%) are similar to that obtained in the corresponding aqueous
solution (water content 100%) [13, 14]. This indicates that the
partial substitution of BMIMBF4 for solvent water in 2.0 M KOH
solutions does not change the electrochemical kinetic mechanism
of aluminum. It can be seen from Fig. 4A that the EIS consists of
three parts: a high-frequency capacitive loop caused by the charge
transfer resistance (Rt) in parallel with the double-layer
capacitance (Cdl), a small middle-frequency inductive loop
resulting from the adsorbed intermediate, and a low-frequency
capacitive loop originating from the growth and dissolution of the
surface film [14, 21]. The model for the essential features of
aluminum electrode system may be represented by the electrical
equivalent circuit shown in Fig. 4B [13], where Rs is the total
Ohmic resistance of the electrode system, RL and L reflect the
information of the adsorbed intermediate, and Rc and Cf relate to
the growth and dissolution of the surface film. The fitted lines are
included in Fig. 4A, and the corresponding fitted parameters are
shown in Table 1. It can be seen that all the fitted parameter values
increase with the decrease in water content in the electrolytes with
2.0 M KOH. The increase in Rs with decrease in water content
implies that the addition of BMIMBF4 decreases the ionic
conductivity of the electrolytes to some extent. In the electrolytes
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Wang, Wang, Shao, Zeng, Zhang and Cao
Materials and Corrosion 2009, 60, No. 12
Figure 5. Galvanostatic discharge curves of aluminum in the KOHBMIMBF4-H2O solution with 2.0 M KOH concentration and 60% water
content
Figure 4. EIS of aluminum in the 2.0 M KOH-containing electrolytes
with various water contents at OCP (A) and their equivalent circuit
model (B)
with lower water content both anodic and cathodic reactions are
inhibited as shown by the results of polarization curves (Fig. 3),
hence the Rt value increases with decrease in water content. The
increase in Rc value with decrease in water content results from
decrease in the hydration extent of the surface product layer,
consistent with the results of anodic polarization curves (Fig. 3A).
The increase in the solution resistance (Rs), charge transfer
resistance (Rt), and surface layer one (Rc) of the electrode system
could be responsible for the lower corrosion rates of aluminum in
the electrolytes with higher BMIMBF4 content. The larger RL and
L values in the electrolytes with lower water content imply the
higher covering densities of intermediates, suggesting that
aluminum has lower anodic dissolution rate [7, 21]. The above
EIS results are basically in accord with the preceding results of
corrosion rates and polarization experiments.
Figure 5 shows the galvanostatic discharge curves of
aluminum in the 2.0 M KOH BMIMBF4-water mixed solutions
with a BMIMBF4/water v/v of 2:3 (water content 60%) at various
current densities. The aluminum anode presents a very flat
discharge plateau at relatively low electrode potentials at the
current density of 10 mA/cm2, and the discharge plateau almost
remains unchanged after a discharge period of 5 h, suggesting
that aluminum has excellent discharge performance in the
applied electrolyte. With increase in discharge current density the
discharge plateau of aluminum moves in the positive direction.
This may mainly result from the increase in electrode
polarization, as shown by the results of EIS in Fig. 4.
4 Conclusions
(i) The corrosion of aluminum in novel KOH-BMIMBF4H2O solutions is notably inhibited. Aluminum shows low
corrosion rate in the electrolytes with water content lower
than 60%, although the corrosion rate increases with
increase in water content and KOH concentration.
(ii) Aluminum is electrochemically active over a very wide
potential window in the KOH-BMIMBF4-H2O electrolytes,
Table 1. The parameters obtained by EIS fitting
Electrolyte
2.0 M KOH with 80% water
2.0 M KOH with 60% water
2.0 M KOH with 40% water
Rs (V cm2)
Rt (V cm2)
Cdl (mF/cm2)
RL (V cm2)
L (H cm2)
RC (V cm2)
Cf (mF/cm2)
3.90
16.21
5.48
29.34
0.09
8.20
9.48
7.81
19.57
8.16
41.37
0.21
12.64
15.44
8.76
38.23
21.70
99.30
0.79
32.87
28.27
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Materials and Corrosion 2009, 60, No. 12
and its electrochemically kinetic mechanism is similar to
that in the corresponding aqueous solution. The increase in
KOH and water contents in the electrolytes may improve the
anodic dissolution performance of aluminum.
(iii) The aluminum anode shows excellent galvanostatic discharge performance in the 2.0 M KOH BMIMBF4-water
mixed solution with a BMIMBF4/water v/v of 2:3 (water
content 60%).
Acknowledgements: This work was supported by National
Natural Science Foundation of China (approved no. 50571091).
The authors also gratefully acknowledge the financial support of
National R&D Infrastructure and Facility Development Program
of China (no. 2005DKA10400-Z20).
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(Received: January 8, 2009)
(Accepted: January 23, 2009)
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