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Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Study of the BMIM-PF6 : Acetonitrile binary mixture as a solvent for
electrochemical studies involving CO2
Anthony M. Rizzuto, Rachel L. Pennington, Karl D. Sienerth ∗
Chemistry Department, Elon University, Elon, NC 27244, United States
a r t i c l e
i n f o
Article history:
Received 9 November 2010
Received in revised form 22 March 2011
Accepted 23 March 2011
Available online xxx
Keywords:
Ionic liquid
Binary mixture
BMIM-PF6
1-n-Butyl-3-methylimidazolium
Carbon dioxide
CO2
Electrochemistry
Electrochemical
a b s t r a c t
A series of binary mixtures ranging from 0 vol.% to 100 vol.% CH3 CN in BMIM-PF6 were investigated
to identify an optimum ratio for use in electrochemical studies involving CO2 . Density, viscosity and
conductivity were measured for the range of binary mixtures and compared to previously published
data. The electrochemistry of a model compound, azobenzene, was studied as well. The data indicated
that a binary mixture containing 15–20 vol.% (approximately 0.5 mol fraction) CH3 CN in BMIM-PF6 was
optimal for electrochemistry, and FTIR of CO2 saturated solutions demonstrated that the solubility of CO2
in the 15–20% CH3 CN mixtures was only about 10% lower than that seen in the neat BMIM-PF6 .
© 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Over the last 50 years, room temperature ionic liquids (RTIL)
have been used in a wide variety of chemical processes. For
instance, several researchers have taken advantage of the variable Lewis acidity of RTIL-AlCl3 melts to facilitate reactions that
are impossible, or at best very difficult, to achieve in aqueous
or common organic solvent systems [1–3]. These solvents have
also been the focus of interest for many years because of their
potential use as green solvents [4–6]. But the majority of workers
report taking advantage of the inherently conductive nature and
very wide electrochemical windows of RTILs in using them as solvents for electrochemical studies [2,7,8]. In general RTILs also are
excellent solvents for spectroscopic studies because they exhibit
low absorptivities over large wavelength ranges in typical spectroscopic instrumental methods [1–3,7,8].
The early RTILs typically required an inert environment because
of the high hygroscopicity of the counterion (formed from species
such as AlCl3 ) used in combination with an organic salt [1–8].
Research progressed in finding new RTILs that were far less
hygroscopic and so were not subject to the problems of dissolved
protic and oxide species. Wilkes and Zaworotko were among the
∗ Corresponding author. Tel.: +1 336 278 6217; fax: +1 336 278 6258.
E-mail address: [email protected] (K.D. Sienerth).
first to report the use of a counterion other than tetrachloroaluminate with a large, asymmetric organic cation in order to minimize
water-sensitivity [9]. Following up on those studies, researchers
synthesized 1-butyl-3-methylimidazolium chloride and then used
a variety of counter ions (BF4 − , sulfonamides, PF6 − ) to replace the
chloride [10,11]. In most cases, the resulting RTIL was substantially
less water-sensitive than the chloroaluminate melts. It was also
found that the single-salt RTILs dissolved CO2 to a much greater
extent than virtually any other standard solvent, leading to propositions to use it as a sequestering medium and a suitable medium
for studies of CO2 chemistry [12–14].
The primary disadvantage observed was that the single-salt,
large-anion RTILs were highly viscous and, although ionic in nature,
generally exhibited lower molar conductivity than previous RTILs.
Diffusion was severely compromised in such melts and so electrochemical studies were constrained as well (see Fig. 1). To overcome
those limitations, researchers began studying co-solvent systems to
achieve lower viscosity and special properties. Stoppa, et al., conducted a study of the conductivities of binary mixtures involving
several polar solvents with BMIM and EMIM (ethylmethylimidazolium) with various anions [15]. Yang et al., studied volumetric
properties of water and hydrophilic solvents with BMIM-Cl [16],
while Domanska and Królikowska considered the density and viscosity of BMIM-SCN with a homologous series of n-alcohols [17].
Xu et al. studied the excess molar volumes of BMIM-PF6 with acetonitrile [18] and Ma and co-workers measured the densities of
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Fig. 1. Cyclic voltammograms of 10 mM azobenzene on a glassy carbon electrode
at a scan rate of 100 mV/s. (A) (. . .) was obtained in CH3 CN/0.1 M KPF6 ; (B) (—) was
obtained in pure BMIM-PF6 ; (C) (- - -) is the same as B but magnified 5× to more
clearly show features.
binary mixtures of BMIM-X (X = PF6 − and BF4 − ) with acetonitrile,
benzene and propanol [19]. Zafarani-Moattar and Roghayeh [20]
considered a variety of physical properties of BMIM-PF6 with some
standard organic electrochemical solvents. Because the research in
our lab focuses on the use of BMIM-PF6 and acetonitrile as individual solvents, we were interested in these later studies and in other
physical properties of the binary mixture of those two solvents. In
particular we are interested in the use of RTILs and binary mixtures involving RTILs in the study of the electrocatalytic reduction
of CO2 . Therefore, we undertook to measure the pertinent physical properties of binary mixtures of BMIM-PF6 with CH3 CN over
the entire range of molar ratios: viscosity, density, conductivity,
and CO2 solubility and the feasibility of electrochemical studies.
While CH3 CN is a common nonaqueous electrochemical solvent, it
is considered a hazardous organic substance, and so the addition of
it to BMIM-PF6 diminishes the “green-ness” of the latter. Thus, the
overarching goal was to find a minimum level of CH3 CN that would
provide a notable increase in conductivity and decrease in viscosity compared to pure BMIM-PF6 , but that would not represent a
significant sacrifice in terms of CO2 solubility.
2. Materials and methods
2.1. Synthesis of BMIM-PF6
Synthesis of the ionic liquid was accomplished following procedures similar to those found in the literature [21,22]. Prior to use,
all glassware was washed with organic soap, rinsed five times with
deionized water, rinsed with acetone, and then placed in 110 ◦ C
oven overnight.
A 1 L round bottom flask and stir bar were preheated in a heating mantle. To this flask was added approximately 100 g of molten
BMIM-Cl (Fluka, 95%) followed by slow addition of 100 mL of acetone (Sigma–Aldrich, 99.5% ACS Reagent Grade) with stirring. A 1.25
mole:mole ratio of KPF6 (Acros Organics, 99%) was added along
with another 200 mL of acetone to begin the reaction. The mixture
was allowed to reflux with stirring for 48–72 h to ensure a complete
reaction.
The product mixture was allowed to cool to a temperature at
which it could be handled and then filtered through Celite (Fisher,
545), followed by washing of the Celite twice with 50 mL of acetone. Acetone was removed by rotary evaporation (65 ◦ C), during
which a white precipitate (presumably KCl) formed. The impure
product, distinctly yellow–orange liquid with the white precipitate,
was placed into a large separatory funnel and 50 mL each of milli-Q
water and chloroform (Fisher, HPLC grade) were added with vigorous shaking in order to dissolve any ionic/inorganic and organic
impurities, respectively. The chloroform and aqueous layers were
then removed and discarded leaving only the melt, and the washing
process was repeated three times. A 100 mL aliquot of ethyl acetate
(Sigma–Aldrich, 99.8%) was added to dissolve the product, and then
50 mL of milli-Q water were added. After shaking and allowing the
layers to separate, the water was removed from the top of the separatory funnel by pipet. Several drops of saturated AgNO3 solution
were added to a small amount of the aqueous wash. The formation
of an AgCl precipitate was an indication that not all chloride had
been removed and the solution was washed further until no AgCl
precipitate was observed.
The ionic liquid–ethyl acetate solution was placed into a 500 mL
Erlenmeyer flask with a stir bar. To this were added several scoops
of anhydrous MgSO4 (Fisher, Cert.) and the resulting slurry was
allowed to stir for approximately 1 h followed by Celite filtration
and washing with two 10 mL aliquots of ethyl acetate. The filtrate was placed in a 500 mL round bottom flask containing several
scoops of activated carbon (Aldrich, Norit RO 0.8) and was refluxed
for over an hour followed by filtration through celite and washing
with ethyl acetate. Most of the ethyl acetate was removed by means
of rotary evaporation (70–75 ◦ C) with the final product placed (still
hot from the Rotovap) into a high vacuum chamber overnight in
order to remove any excess ethyl acetate.
The BMIM-PF6 product was colorless to very slightly yellow.
Infrared and NMR spectra of the melt indicated a high degree of
purity with respect to organic side-products and water.
2.2. Binary mixtures of acetonitrile and BMIM-PF6
For most measurements, mixtures were prepared volumetrically using calibrated 50 mL burets. Mixtures containing 5, 10, 15,
20, 25, 30, 40, 50, 60, and 70% CH3 CN (Fisher, HPLC grade) were
prepared and placed in clean, dry vials with Teflon-sealed caps for
storage.
All CH3 CN used in this study was treated prior to use in the following manner. Molecular sieves were baked over 48 h at 125 ◦ C
in a glass 500 mL bottle, then the bottle was transferred while still
hot to a high vacuum chamber and evacuated for over 48 h. Acetonitrile was poured from the reagent bottle directly into the 500 mL
bottle containing the cold sieves and the bottle was sealed with a
Teflon-lined cap. The bottle was shaken periodically over the next
24 h after which the 1× dried CH3 CN was transferred to a second
baked/evacuated set of sieves in a different bottle. This procedure
was repeated one more time and from that point the 3× dried
CH3 CN was stored over molecular sieves until use.
2.3. Density measurements
Mixtures for each density measurement were prepared by first
weighing three 5.00 mL volumetric flasks (class-A) with ground
glass stoppers. The appropriate amount of CH3 CN was then pipetted (class-A volumetric pipets) into each flask, which were then
capped and massed. Finally, the flasks were made to volume with
BMIM-PF6 , capped and massed once more.
2.4. Viscosity measurements
Viscosity of the binary mixtures was measured as a function
of time-of-flow from a 10.00 mL buret [23]. In all measurements,
a buret was filled to 0.00 mL with the solution being measured,
then the stopcock was opened fully. A stopwatch was started as the
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meniscus passed the 1.00 mL mark and stopped when it reached the
6.00 mL mark (5.00 mL total flow). The process was repeated for a
total of 5 measurements for each solution, and if the relative standard deviation among measurements was not below 2%, additional
trials were conducted until the %RSD was below 2%.
Because of the large number of solutions and trials needed, multiple burets were required. Three burets were inter-calibrated using
flow-time of pure methanol (Fisher, HPLC grade) and a correction
factor from this inter-calibration was applied as appropriate.
The method described above does not provide a direct measure
of viscosity, so standard solutions were used to correlate the timeof-flow measurements to literature standards [24]. In addition to
water, dry acetonitrile and dry ethylene glycol, aqueous solutions
of 30, 40, 50 and 65% sucrose were prepared. The temperature at the
end of each trial was recorded. Throughout all studies the temperature averaged 294.0 K with a %RSD of only 0.3% but in each case the
viscosity was corrected to the literature 293 K value through linear
interpolation. The linear regression of flow time vs. literature viscosity (R2 = 0.995) prepared from the standard solutions was used
to determine the viscosity of each binary mixture solution.
2.5. Conductivity measurements
Conductivity was determined indirectly through resistivity
measurements using an AC voltage to avoid charging effects on the
large-area electrodes. A 1.00 cm conductivity cell with platinized
platinum electrodes was filled with the solution to be measured
and placed in readiness. A Fisher EMD1512 wave generator was
used to supply 1.5 V AC at 60 Hz, and the voltage was recorded in
open circuit using a Keithley 2700 multimeter with no filter applied
but with the meter set to report a mean of 5 consecutive measurements that all fell within 1% of each other. The open circuit
measurement was taken as the reference, and then the voltage was
measured again with the conductivity cell in circuit. The voltage
drop between the short and in-circuit measurement was used as a
direct measure of resisitivity. Fifteen of those measurements were
taken for each solution.
Standard solutions (0.1000, 0.0100 and 0.0010 M) of KCl
(Aldrich, 99.8%) were prepared as conductivity standards. A plot
of voltage drop vs. literature [24] conductivity was used to determine the conductivity of binary mixture solutions. Throughout
these measurements, the temperature was monitored and averaged 293.1 K ± 0.5%RSD.
2.6. FTIR measurements of soluble CO2
The relative amount of CO2 dissolved in the 0, 15, 20 and
100% (v/v) CH3 CN was demonstrated with FTIR spectroscopy using
a constant-pathlength sandwich cell. The neat and binary mixture solutions were prepared, and then each was spiked with
the same concentration of benzil (1,2-diphenyl-1,2-ethanedione,
Aldrich, 98%) as an internal standard. Benzil exhibits a peak in the
mid-IR that is distinct from all absorbance attributed to the other
components of the mixtures. Each solution was placed in a 50 mL
Buchner flask containing a Teflon-coated stir bar, and a 3-way valve
was attached to the side-arm of the flask with Tygon tubing. One of
the remaining valve ports was connected to a tank of CO2 (Roberts
Oxygen, Inc., bone dry grade) while to the other was attached an
inflatable bladder (essentially a balloon). The bladder was filled
with CO2 , and then the valve was rotated to expose the solution
to the CO2 at 1 atm pressure. This exposure was maintained under
constant stirring for several hours until immediately before the
spectroscopic measurement was made for a given solution.
A 0.1 mm constant-pathlength cell (KBr windows, Teflon spacer)
was assembled and was not disassembled until all solutions had
been measured. The background single-beam spectrum was taken
3
(Nicolet Magna 560 FTIR Spectrometer) with the empty cell. A 1 mL
volume of solution (>20-fold excess of the cell volume) was flushed
through the cell with a syringe and the ports were sealed with
Teflon caps prior to each measurement. Between solutions, the cell
was rinsed thoroughly with CH3 CN followed by syringe-removal of
excess CH3 CN.
The baselines of the spectra were normalized using the range
from 1960 to 1940 cm−1 (a region completely free of absorbance)
and the absorbance values throughout the spectra were normalized using the 1680 cm−1 peak of the internal standard (benzil).
Throughout all of these procedures the temperature was recorded
and averaged to be 293.1 K ± 0.5%RSD.
2.7. Electrochemistry
Azobenzene (Alfa Aesar, 98%) was selected as the test compound due to its well-known electrochemical behavior in CH3 CN
and because it is substantially soluble in pure BMIM-PF6 [25–27].
Solutions containing 10.0 mM azobenzene were prepared in pure
BMIM-PF6 and in 0.1 M KPF6 /CH3 CN. These solutions were used to
prepare a series of binary mixtures containing 0, 15, 20, 40, 60, 80
and 100 vol.% CH3 CN. Prior to use the KPF6 was baked at 125 ◦ C
followed by high vacuum treatment.
Cyclic voltammetry was conducted using a PAR Versastat on a
3 mm diameter glassy carbon working electrode (BAS, Inc.) with a
Pt coil counter electrode and an AgNO3 /Ag reference electrode prepared in acetonitrile solvent. The counter and reference electrodes
were separated from the bulk solution by Vycor frits (BAS, Inc.).
The working electrode was polished on felt with slurry of 0.5 ␮m
alumina in distilled H2 O prior to each CV. The electrode was then
dried thoroughly with a lint-free paper cloth and then rinsed with
acetonitrile to ensure no water entered the system. The potential
was cycled from −0.35 V to −1.35 V and back at 100 mV/s, with
an acquisition rate of 50 s−1 . Three voltammograms were obtained
with each solution to ensure reproducibility.
3. Results and discussion
Fig. 1 demonstrates the limitations researchers encounter when
trying to use BMIM-PF6 as an electrochemical solvent. The extreme
viscosity of the ionic liquid coupled with its low inherent conductivity lead to poor electrochemical outcomes. Not only is the
measured peak current for azobenzene dramatically diminished
(93% lower than that seen in CH3 CN/0.1 M TBAP), but the chemistry is clearly different in the two solvents: what appears as a
well-behaved reversible redox couple in CH3 CN reduces to a broad,
poorly defined irreversible wave in pure BMIM-PF6 . For use as a
solvent for the study of CO2 electroreduction catalysts, the high
solubility of CO2 in BMIM-PF6 is definitively outweighed by the substantial degradation of electrochemical response. One approach to
resolving this issue is to diminish viscosity and increase conductivity through the use of a co-solvent, attempting to achieve a balance
between gains in those areas and possible loss of CO2 solubility.
3.1. Density measurements
The data acquired from density measurements is presented in
Table 1 and depicted in Fig. 2. The linear regression (Eq. (1)) shows
excellent fit with a coefficient of
Density = (−5.94 × 10−3 )(vol.%CH3 CN) + 1.382
(1)
(R2 )
Determination
of 0.999, a standard error in the slope of
1.1% and a standard error in the intercept (the calculated density of
BMIM-PF6 , 1.382 g/cm3 ) of 0.24%. These measurements agree with
those reported previously [19].
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Table 1
Density of binary mixtures of CH3 CN and BMIM-PF6 .
CH3 CN (vol.%)
0.00
5.00
10.0
15.0
20.0
25.0
30.0
40.0
50.0
60.0
70.0
100.
a
b
Mole fraction BMIM-PF6
Density (g/cm3 )a
b
1.00
0.828
0.696
0.590
0.504
0.433
0.372
0.276
0.203
0.145
0.098
0.00
1.382
1.343
1.330
1.291
1.262
1.235
1.201
1.138
1.090
1.032
0.972
0.778
RSD%a
–
0.85
0.67
0.11
0.96
0.72
0.48
0.51
0.23
0.69
0.21
0.14
Each point represents the mean and %RSD of three measurements.
Determined from the intercept of the linear regression.
Fig. 3. Calibration plot of flow time vs. literature [24] value of viscosity for some
pure solvents and a series of standard aqueous sucrose solutions.
Fig. 2. Plot of measured density vs. volume percent of CH3 CN for a series of binary
mixtures with BMIM-PF6 .
3.2. Viscosity of the binary mixtures
Table 2 provides data for time-of-flow measurements for the
viscosity standards and Fig. 3 shows a plot of the flow-time for
the viscosity standard solutions vs. the reference viscosity value in
mN s/m2 . The correlation is excellent, yielding a linear relationship
Flow time = (2.61)(viscosity) + 8.83
(2)
With R2 = 0.999 and a standard error in the slope of 0.54%. Fig. 4
shows the viscosity calculated using Eq. (2) for a range of binary
mixtures of BMIM-PF6 and CH3 CN. The observed trend is as
expected, with a dramatic decrease in viscosity as the mole fraction of BMIM-PF6 decreases initially, reaching a relatively steady
state after the amount of CH3 CN predominates.
Fig. 4. Plot of calculated viscosity vs. mole fraction of BMIM-PF6 for a series of binary
mixtures of BMIM-PF6 and CH3 CN. Black circles (䊉) represent data from the current
work; each point represents the mean of at least 5 trials. The gray squares () and
line are from data reported in Ref. [20].
The time-of-flow method described here is not a standard technique for measuring viscosity, and it is important to verify its
validity for providing a reasonable estimate of that physical parameter.
Included in Fig. 4 are results from a previous report by ZafaraniMoattar and Roghayeh [20] who obtained viscosity data using an
industrial viscometer. Similarly, the results of the current study are
in close agreement with Han and co-workers [28], especially at the
lower concentrations of the RTIL. The correlation deviates slightly at
the higher concentrations due to the difficulty of measuring liquids
Table 2
Time-of-flow measurements for viscosity standards.
Solution or solvent
Temperature (K)
Viscosity (N s m−2 )a
Time of flow (s)b
%RSD in time of flowb
CH3 CN
Water
30% sucrose(aq)
40% sucrose(aq)
50% sucrose(aq)
Ethylene glycol
65% sucrose(aq)
294.8
294.2
294.8
294.8
294.6
294.2
294.5
0.3576
0.9797
3.042
5.846
14.58
20.99
136.16
10.28
11.73
20.78
22.65
40.28
66.66
346.7
0.54
0.71
1.31
1.01
2.00
2.15
1.36
a
b
From Ref. [24], corrected by linear interpolation to the measurement temperature (column 2).
Mean and %RSD of 5 measurements.
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5
Fig. 5. Plot of literature [24] value of conductivity vs. measured voltage drop from
applied 1.5 V AC for a series of standard aqueous KCl solutions.
Fig. 6. Conductivity calculated from voltage drop from applied 1.5 V AC for a series
of binary mixtures of CH3 CN and BMIM-PF6 .
of a highly viscous nature. The correlation among the three sets
of results indicates that the time-of-flow method can be used to
estimate viscosity with reasonable accuracy when a viscometer is
unavailable.
It is particularly noteworthy, in consideration of the primary
goal of this work, that in the region of 15–20 vol.% CH3 CN the viscosity is only about one-tenth that of the neat BMIM-PF6 and further
addition of CH3 CN does not provide dramatic gains.
standard electrolytes (e.g., KPF6 ), which are normally soluble only
at in CH3 CN at a concentration of 0.1 M or lower.
With respect to the overarching goal of this study, it is most
important to note that where the CH3 CN volume percent is 15–20%
the conductivity observed is higher than that seen for aqueous
0.1 M KCl (a typical concentration for a supporting electrolyte in
electrochemical studies). That, added to the fact that viscosity is
decreased by 90% at that level of CH3 CN, suggests the 15–20%
CH3 CN region will be a reasonable target for further electrochemical studies in this binary solvent system.
3.3. Conductivity
Fig. 5 shows the correlation between the measured voltage drop
from the applied 1.5 V AC and the literature values of conductivity.
A quadratic regression (Eq. (3)) was deemed suitable
2
= 171.0(V ) + 19.45(V )
(3)
mS cm−1 ;
is conductivity in
V is voltage drop from 1.5 V AC
applied for estimation of conductivity from measured voltage drop
for values within the limits imposed by the standard KCl solutions
(which was true for all solutions measured except pure CH3 CN). The
general trend observed in Fig. 6 is as expected and is very similar
to those seen by Stoppa et al. [15], for binary mixtures of BMIMPF6 with other organic solvents (e.g., CH3 OH, CH2 Cl2 , DMSO). The
shape of the curve indicates that upon dissolution into a polar
solvent such as CH3 CN, the BMIM-PF6 dissociates and essentially
acts as a supporting electrolyte in electrochemical terms. However,
BMIM-PF6 is miscible with CH3 CN in all proportions, unlike most
3.4. Electrochemistry
As noted previously the chemistry observed in electrochemical
experiments in neat CH3 CN is significantly different from that seen
in neat BMIM-PF6 . Fig. 7 and Table 3 demonstrate that the chemistry undergoes a progressive transition as the relative amounts
of co-solvents are altered. In the pure solvents, a single reduction wave is observed (see Fig. 1), but in all binary mixtures of
the solvents, a pre-wave is observed and the main peak undergoes a progressive transition from the more cathodic, well-defined
wave seen in CH3 CN/0.1 M KPF6 to the broad, poorly defined wave
observed in pure BMIM-PF6 . The value of (Ep − Ep/2 ) for the main
peak remains relatively constant at ∼0.09 V until the volume percent CH3 CN drops below 20%, at which point it increases steeply
to a value of almost 0.2 V in the pure BMIM-PF6 . Similarly, the
(Ep − Ep/2 ) for the pre-wave is about 0.09 V for low BMIM-PF6 levels,
Table 3
Reduction peak data from cyclic voltammetrya of azobenzene in CH3 CN/BMIM-PF6 .
CH3 CN (vol.%)
100
80
60
40
20
15
0
Mole fraction BMIM-PF6
0.00
0.060
0.145
0.276
0.504
0.590
1.00
Pre-waveb
Main peak
c
Ep (V)
Ep/2 (V)
Ip (␮A)
Ep (V)
Ep/2 (V)
Ip (␮A)
−1.107
−0.972
−0.910
−0.896
−0.876
−0.862
−0.754
−1.019
−0.883
−0.822
−0.812
−0.793
−0.766
−0.569
73.75
56.53
33.27
16.41
12.70
7.92
5.74
−0.658
−0.692
−0.722
−0.681
−0.666
−0.575
−0.606
−0.634
−0.560
−0.529
12.41
18.33
24.05
19.20
7.44
a
All data obtained from 10 mM solutions of azobenzene in the indicated solvent or binary mixture. Each point represents three CV’s obtained on a 3 mm diameter glassy
carbon electrode at 100 mV/s. Voltages are reported vs. Ag+ /Ag in CH3 CN/0.1 M KPF6 . Ep is the peak potential, Ep/2 is the potential at half-peak height and Ip is the peak current
(baseline corrected).
b
Only one peak (no pre-wave) seen in the neat solvents.
c
In the presence of a distinct pre-wave, the peak current of the main wave was measured from the baseline determined as an estimated extension of the current decay of
the pre-wave.
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Fig. 7. Cyclic voltammograms of 10 mM azobenzene obtained in solutions containing decreasing amounts of CH3 CN. (A) 100 vol.% CH3 CN (. . .); (B) 80% CH3 CN (- - -);
and (C) 40% CH3 CN (—) CH3 CN.
but rises rapidly after the amount of CH3 CN drops below 30 vol.%.
The reversibility (as indicated by the relative intensity of the anodic
wave associated with the main peak) diminishes rapidly with the
amount of BMIM-PF6 in the solvent, becoming totally irreversible
when the vol.% of CH3 CN drops below 40%. The shift in the oxidation peak observed in mixtures containing higher CH3 CN levels,
when using that seen in CH3 CN/0.1 M KPF6 as a basis, is as much
as 15% greater than that of the reduction peak. It should be noted
that the actual concentration of KPF6 varies with the percentage of
CH3 CN, and that, indeed, the conductivity itself varies as well (see
previous section of this paper). The goal of this work was to compare results obtained from solutions that would be prepared under
normal conditions in a working laboratory, rather than to attempt
to ensure constant ionic strength among all test solutions.
Several scenarios can be put forth to explain the electrochemical
results. Azobenzene has been reported to exhibit two 1-electron
reduction waves in aprotic media, the first attributed to the formation of a radical monovalent anion, the second being further
reduction to the stable dianion [25–27]. However, the second
reduction is reported to occur at a potential far more negative
than the first one, and would be expected to appear well beyond
the electrochemical window used in the current study. In protic
media, depending on pH, either a single 2-electron wave or two 1electron waves are observed [29], but the behavior seen in the series
of binary mixtures does not support attribution to trace water in
either solvent. The separation into two waves followed by collapse
into a single broad peak as the BMIM-PF6 content increases is most
likely a function of ion pairing between the azobenzene anion radical and the increasing amount of supporting electrolyte (BMIM-PF6
in this case). A full investigation of this curious behavior is beyond
the scope of this work; it will be the subject of further investigation
in the future.
The optimum binary mixture (15–20 vol.% CH3 CN) suggested by
the viscosity and conductivity studies should be considered here.
The data in Table 3 indicate that the 15% CH3 CN solution does not
provide a dramatic improvement over the neat BMIM-PF6 (per the
peak current value) but that using a mixture that 20 vol.% CH3 CN
represents an improvement of 200–300% depending on which peak
is used for comparison with the single wave observed in pure
BMIM-PF6 . Fig. 8 shows this comparison graphically. Note also that,
as described above, the chemistry in the 20% CH3 CN solution is still
distinctly different from that in the neat solvents, but that both
observed reduction peaks are well-defined and at least somewhat
Fig. 8. Cyclic voltammograms of 10 mM azobenzene obtained in solutions containing (A) 0 vol.% CH3 CN (- - -) and (B) 20 vol.% CH3 CN (—).
Fig. 9. FTIR spectra of neat solvents and binary mixtures saturated with CO2 (g). Solid
line (—) is neat BMIM-PF6 ; dotted line (. . .) is 20 vol.% CH3 CN; dashed line (- - -) is
15 vol.% CH3 CN; combination line (– ··) is neat CH3 CN.
resolved. In terms of the goals of this project, the most important
fact is that in the 20% CH3 CN solution, the study of electrochemical reactions is considerably enhanced with respect to the pure
BMIM-PF6 ; the mechanisms involved in the electrochemistry of
azobenzene is not of specific import.
3.5. FTIR/CO2 solubility
As noted above, one of the conceivable advantages to using
BMIM-PF6 based solvents in studying CO2 chemistry, including its
electrocatalytic reduction, is that CO2 exhibits an extraordinarily high solubility in BMIM-PF6 . It is important, then, to ensure
that using a co-solvent such as CH3 CN, while providing advantages for electrochemistry, does not significantly diminish the CO2
solubility advantage. Fig. 9 shows the FTIR spectra obtained in a
constant-pathlength cell from solutions saturated in CO2 gas. The
peaks around 2250 and 2290 cm−1 is due to CH3 CN and the peak
at 2340 cm−1 is due to dissolved CO2 . The substantial difference in
CO2 solubility between neat CH3 CN and neat BMIM-PF6 is unmistakable. Most important, however, is the fact that the solubility of
CO2 diminishes only by about 10 vol.% even at 20 vol.% CH3 CN.
Please cite this article in press as: A.M. Rizzuto, et al., Study of the BMIM-PF6 : Acetonitrile binary mixture as a solvent for electrochemical studies
involving CO2 , Electrochim. Acta (2011), doi:10.1016/j.electacta.2011.03.106
G Model
EA-16967;
No. of Pages 7
ARTICLE IN PRESS
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7
4. Conclusions
References
A study of several properties of the binary mixtures of the room
temperature ionic liquid BMIM-PF6 and the typical electrochemical
aprotic solvent CH3 CN was undertaken. The goal of the work was
to assess the utility of a binary mixture of the two solvents to best
take advantage of the “green-ness” and CO2 solubility exhibited
by BMIM-PF6 while using a minimum amount of the organic solvent to enhance properties of the melt (e.g., viscosity, conductivity)
that are important to electrochemical studies. Viscosity measurements suggested that a binary mixture of 15–20 vol% CH3 CN would
be optimal and conductivity measurements supported that finding.
Electrochemical studies using azobenzene as a model analyte indicated that at 20 vol.% CH3 CN the signal is increased by as much
as 300% over that seen in the neat BMIM-PF6 . FTIR spectra confirmed that the solubility of CO2 diminishes only by about 10% in the
20 vol.% CH3 CN binary mixture. For the electrochemical and electrocatalytic studies involving CO2 , then, all results indicate that a
20 vol.% CH3 CN mixture with BMIM-PF6 will be an effective solvent
system. Furthermore, it is worth noting that these studies were
conducted at a primarily undergraduate institution, and indeed
was conducted by undergraduates. While standard instrumentation for viscosity and conductivity were not available, the methods
employed yielded reasonable results as indicated by correlation
with previously reported work.
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Acknowledgements
The authors wish to express appreciation for funding and other
support to the Elon University Chemistry Department and the Elon
University Undergraduate Research Program.
Please cite this article in press as: A.M. Rizzuto, et al., Study of the BMIM-PF6 : Acetonitrile binary mixture as a solvent for electrochemical studies
involving CO2 , Electrochim. Acta (2011), doi:10.1016/j.electacta.2011.03.106