J. Cent. South Univ. Technol. (2008) 15: 617−621
DOI: 10.1007/s11771−008−0115−7
Electrochemical behavior of CoCl2 in
ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate
ZHOU Zhou(周 舟), HE De-liang(何德良), CUI Zheng-dan(崔正丹),
ZHONG Jian-fang(钟建芳), LI Guo-xi(李国希)
(State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082, China)
Abstract: The electrochemical behavior of CoCl2 in 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]PF6) was
investigated by cyclic voltammetry. The cyclic voltammograms were obtained from electrochemical measurement under different
temperatures, and the reversible behavior for Co2+/Co3+ redox couple on glassy carbon electrode in [bmim]PF6 was confirmed by the
characteristic of the peak currents. The diffusion coefficients (about 10−11 m2/s) of Co2+ in [bmim]PF6 under different temperatures
were evaluated from the dependence of the peak current density on the potential scan rates in cyclic voltammograms. It is found that
the diffusion coefficient increases with increasing temperature. Diffusion activation energy of Co2+ in [bmim]PF6 is also calculated to
be 23.4 kJ/mol according to the relationship between diffusion coefficient and temperature.
Key words: CoCl2; electrochemical behavior; ionic liquid; cyclic voltammetry; diffusion coefficient; diffusion activation energy
1 Introduction
Ionic liquids(ILs) are organic melting salts at
ambient temperatures[1]. Typically they are comprised of
bulky N,N-dialkylimidazolium, quaternary ammoniums,
quaternary phosphonium or alkylpyridinium organic
cations and a variety of anions such as chloroaluminate
(AlCl4−), hexafluorophosphate (PF6−), tetrafluoroborate
(BF4−), bis(perfluoromethyl-sulfonyl)imide ((CF3SO2)2N−). ILs exhibit many excellent physical and chemical
properties, for example, low volatility, high electrical
conductivity, excellent thermal and electrochemical
stability, excellent solubility properties, and they are
widely studied in electrochemistry, organic synthesis and
other
fields[2−5].
In
comparison
with
early
chloroaluminate-based ILs, the non-chloroaluminatebased ILs that are stable to moisture can adapt to
common atmosphere environment. So the number of
publications related to new type ILs grows rapidly in
recent years[6−9].
ILs are applied widely for electrochemical
investigation as both supporting electrolytes and
solvents[10−11]. However, the investigations on basic
electrochemical parameters in ILs are limited, and the
former work mainly is focused on chloroaluminate-based
ILs, for example, Fe[12] and I[13]. Especially, the studies
on electrochemical properties and the measurements of
electrochemical parameters for metal ions in nonchloroaluminate-based ILs are few[14].
It is significant to investigate the redox of some
metal ions in ILs, a new electrolyte environment. ZEIN
EL ABEDIN[15] reported the electroreduction of Se, In
and Cu in ILs. Electrodeposition of cobalt and different
cobalt alloys from ILs was studied in Refs.[16−18], but
the major studies were correlative to the reduction of
cobalt from chloroaluminate-based ILs. In this work, the
electrochemical behavior of Co2+ on glassy carbon
electrode was investigated in non-chloroaluminate-based
IL i.e. 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]PF6). The effects of temperature on
diffusion coefficient of Co2+ in [bmim]PF6 were studied
by cyclic voltammetry. And diffusion activation energy
of Co2+ in [bmim]PF6 was derived from the relationship
between diffusion coefficients and temperature.
2 Experimental
[bmim]PF6 was prepared according to Ref.[19]. The
equal molar amounts of 1-methylimidazole and
chlorobutane were heated and stirred, then
hexafluorophosphoric ammonium was added to the
resulting viscous liquid. The mixture was allowed to
proceed with continuous stirring. The product was
washed with redistilled water, and then heated under
vacuum for 48 h to control the water content less than
Foundation item: Project(2005-383) supported by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, Ministry of Education,
China
Received date: 2008−03−06; Accepted date: 2008−05−12
Corresponding author: HE De-liang, Professor, PhD; Tel: +86−731−8821449; E-mail: [email protected]
618
3×10−6, as determined by Karl-Fischer titration. The
result of NMR of [bmim]PF6 was the same to that in
Ref.[20] without multi-resonance peaks of impurities.
A certain mass of CoCl2·6H2O was dehydrated to
turn blue by vacuum drying, and then added into
[bmim]PF6 quickly. The concentration of Co2+ was
controlled to be 0.03 mol/L. All chemicals were
analytical grade. The glass apparatus was washed by
dilute nitric acid and redistilled water, and then dried in
vacuum.
The electrochemical characteristics of CoCl2 in
[bmim]PF6 were investigated in a one-compartment cell
by using CHI 660B electrochemical working station (CH
Instrument Inc). The working electrode was a 3 mmdiameter glassy carbon disc (CH Instrument Inc). Before
the electrochemical measurements, the surface of the
working electrode was polished with 0.5 µm alumina
powders and then washed with redistilled water, ethanol
and acetone. Pt coil was used as the counter electrode.
The reference electrode was a self-designed Ag/AgCl
electrode. Cyclic voltammograms were scanned at a
series of potential scan rates of 0.050, 0.100, 0.200,
0.300, 0.400 and 0.500 V/s. All electrochemical
experiments were performed in a nitrogen-filled constant
temperature box.
3 Results and discussion
3.1 Cyclic voltammogram of Co2+ in [bmim]PF6
Cyclic voltammgram of pure [bmim]PF6 on glassy
carbon electrode is shown in Fig.1(a). It can be found
that the electrochemical window of [bmim]PF6 is 3.7 V
at 308.15 K, which is in agreement with the result in
Ref.[21]. The limits of redox correspond to the
decomposition of PF6− and the reduction of [bmim]+.
Fig.1(b) shows the cyclic voltammogram of Co 2+ in
J. Cent. South Univ. Technol. (2008) 15: 617−621
[bmim]PF6 on glassy carbon electrode at 308.15 K. The
oxidation peak (0.750 V) and reduction peak (−0.245 V)
can be observed obviously when potential scan rate is
0.500 V/s.
3.2 Electrochemical oxidation of Co2+ in [bmim]PF6
It is well known that some metal ions, such as
2+
Fe /Fe3+ and Co2+/Co3+, are reversible redox couples in
aqueous solutions. However, the reversibility is greatly
affected by the medium[12]. Here, it is necessary to study
the reversibility of Co2+/Co3+ redox couple in
[bmim]PF6. Cyclic voltammgrams of 0.03 mol/L Co2+ on
glassy carbon electrode in [bmim]PF6 were measured at
318.15 K. The results are shown in Fig.2. The potential
scan rates are 0.050, 0.100, 0.200, 0.300, 0.400 and
0.500 V/s, respectively. The detailed data at various
potential scan rates are listed in Table 1. It can be found
that the distance between the anodic and cathodic peak
potential is above 59 mV. The anodic peak moves more
positively, and the cathodic peak moves negatively when
the potential scan rate increases. The potential shift may
be attributed to the depressing reversibility of Co2+/Co3+
redox couple and/or ohmic resistance of the ionic liquid
[bmim]PF6. Nevertheless, the anodic and cathodic peak
currents (Ip, a, Ip,c) are almost symmetrical. The ratios of
Ip,a to Ip,c are approximately equal to 1, and Ip,a and Ip,c
can fit well with the square root of scan rate (v1/2) to
linear relationships (Ra=0.997, Rc=0.991). Therefore, it
can be presumed that the Co2+/Co3+ redox couple in
[bmim]PF6 is a reversible electrochemical reaction
according to Ref.[22].
Fig.2 Cyclic voltammograms of 0.03 mol/L Co2+ in [bmim]PF6
at different potential scan rates and 318.15 K: (a) 0.050 V/s;
(b) 0.100 V/s; (c) 0.200 V/s; (d) 0.300 V/s; (e) 0.400 V/s;
(f) 0.500 V/s
Fig.1 Cyclic voltammograms of glassy carbon electrode in
[bmim]PF6 at 308.15 K: (a) Pure [bmim]PF6, 0.005 V/s;
(b) 0.03 mol/L Co2+ in [bmim]PF6, 0.500 V/s
When the plot of ln[(Ip−I)/I] vs potential(E) is a line,
the corresponding oxidation or reduction process is very
reversible[23−24], where Ip is the peak current, I is the
instantaneous current relevant to potentials. The data of
J. Cent. South Univ. Technol. (2008) 15: 617−621
619
Table 1 Parameters of cyclic voltammograms of 0.03 mol/L Co2+ in [bmim]PF6 at various potential scan rates and 318.15 K
v/(V·s−1)
v1/2/(V·s−1)1/2
Ep,c/V
Ip,c/µA
Ep,a/V
Ip,a/µA
∆E/V
0.050
0.224
−0.070
49.8
0.570
56.6
0.640
0.100
0.316
−0.110
70.5
0.620
74.8
0.730
0.200
0.447
−0.165
97.0
0.680
103.0
0.845
0.300
0.548
−0.185
113.0
0.730
130.0
0.915
0.400
0.632
−0.195
130.0
0.760
151.0
0.955
0.500
0.707
−0.215
156.0
0.780
170.0
0.995
2+
Note: Current values are obtained by subtracting the background current (no Co in ILs) from current responses.
Ip, I and potential are gained from the cyclic
voltammograms in Fig.2 when the scan rate is 0.050 V/s,
then the plot of ln[(Ip−I)/I] vs E is made and shown in
Fig.3. It is obvious that they can be fitted to a line as
Eqn.(1). This also proves that the oxidation of Co2+ in
[bmim]PF6 is reversible.
ln[(Ip−I)/I]=16.4−36.0E (R1=0.998)
(1)
where R1 is correlation coefficient of the linear fitting.
These plots exhibit excellent linear relationship. From
the slopes (k) of the fitting line observed between Ip and
v1/2, the values of D (the diffusion coefficient of Co2+ in
[bmim]PF6) can be calculated and are listed in Table 2.
The results are similar to those of Fe3+ in IL[25], but are
less than those of Co2+ in dimethylsolfoxide (about
10−9 m2/s)[26]. This may be attributed to the viscosity of
the medium and/or the inhibition of electron transfer
between the glassy carbon electrode and the redox center
due to bulkiness of PF6−. So it is surmised that Co2+ and
Co3+ are surrounded by PF6− to form [Co(PF6)n]2−n and
[Co(PF6)n]3−n that will transform reciprocally as follows:
[Co(PF6)n]2−n−e=[Co(PF6)n]3−n
(3)
Table 2 Diffusion coefficient of Co2+ in [bmim]PF6 at c(Co2+)=
0.03 mol/L and different temperature
T/K
k/106
R1
D/(10−11 m2·s−1)
308.15
206
0.998
1.35
318.15
238
0.997
1.86
328.15
268
0.997
2.43
338.15
299
0.999
3.12
Fig.3 Linear plot of ln[(Ip−I)/I ] vs potential at 318.15 K and
scan rate of 0.050 V/s
3.3 Diffusion coefficient of Co2+ in [bmim]PF6
The relationship between peak current and potential
scan rate for reversible electrochemical process accords
with the Randles-Sevcik equation:
Ip=0.446 3(nF)3/2(RT)−1/2Ac0D1/2v1/2
(2)
where n is the Faraday constant per mole of substrate
electrolyzed; A is the surface area of the electrode; c0 is
the bulk concentration of reactant and D is the diffusion
coefficient of reactant. The cyclic voltammograms of
0.03 mol/L Co2+ in [bmim]PF6 were measured under
different temperatures, and the plots of the anodic peak
current vs square root of scan rate are shown in Fig.4.
Fig.4 Plots of Ip,a vs v1/2 at different temperatures and c(Co2+)=
0.03 mol/L
620
J. Cent. South Univ. Technol. (2008) 15: 617−621
It can also be found that the diffusion coefficient
increases with the increase of temperature. This may be
attributed to a decrease in viscosity of solvent
([bmim]PF6) and an increase in kinetic energy of reactant
because of temperature rising.
3.4 Diffusion activation energy of Co2+ in [bmim]PF6
The relationship between diffusion coefficient and
temperature follows Arrenius law:
D=D0exp[−ED/(RT)]
(4)
approximate to those of other metal ions in ILs.
3) The results of the research lay the foundation for
metal ions’ electrochemical redox behavior in ILs, and
supply basic electrochemical parameters for Co2+ in
[bmim]PF6. Based on the results, it can be concluded that
IL [bmim]PF6 can be used extensively as solvent
material for electrochemical investigations.
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where D is the diffusion coefficient relevant to T, and
D0 is preexponential factor. Eqn.(4) is given below in
logarithmic form:
[2]
ln D=ln D0−ED/(RT)
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(5)
A proportional relationship between ln D and 1/T
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ln D=−2.81/T−15.9, R1=0.995
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2) The diffusion coefficients and diffusion
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