Electrochemical and physicochemical properties of redox ionic liquids using electroactive anions: Influence of alkylimidazolium chain length

Article abstract of DOI:10.1016/j.electacta.2016.03.075

Ferrocenylsulfonyl(trifluoromethylsulfonyl)imide [FcNTf] is reported as a redox anionic species used to obtain an electroactive ionic liquid with a typical alkylimidazolium cation. Here, several of these redox ionic liquids (RILs) were prepared by combining FcNTf with alkylimidazolium cations in order to understand the influence of symmetry and alkyl chain length on the RIL system. The physicochemical and electrochemical properties are characterized for [C_xC_yIm][FcNTf], where x and y were varied between 1 and 8, both in the pure, undiluted state and in solutions of acetonitrile. In pure form, the ionic conductivity of [C_xC_yIm][FcNTf] was found to range between 0.22 and 0.42 mS cm^-1 at 60 °C, and the alkyl chain length was found to have a similar effect on viscosity as in conventional imidazolium-NTf₂ ionic liquids. While an increase in alkyl chain length increases viscosity and decreases mass transport, it was found to have no effect on the redox potential of the ferrocene center. The cyclic voltammetry of [C_xC_yIm][FcNTf] solutions diluted in acetonitrile exhibited behavior which depends on concentration and on the presence of a supporting electrolyte. At concentrations above 50 wt.% and in the absence of any supporting electrolyte, the electroactive anion was found to deposit as a dense film on the electrode upon oxidation. This behavior is linked to formation of Fc⁺NTf^- zwitterions, which accumulates in the double-layer and precipitates on the electrode surface. This study also investigates the electrochemical properties of film deposition.

Full text of DOI:10.1016/j.electacta.2016.03.075

Electrochimica Acta 200 (2016) 283–289
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrochemical and physicochemical properties of redox ionic liquids
using electroactive anions: influence of alkylimidazolium chain length
1,
Han Jin Xie, Bruno Gélinas, Dominic Rochefort *
Département de chimie, Université de Montréal, CP6128 Succ. Centre-Ville, Montréal, Québec H3C 3J7, Canada
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 18 November 2015
Received in revised form 5 February 2016
Accepted 12 March 2016
Ferrocenylsulfonyl(trifluoromethylsulfonyl)imide [FcNTf] is reported as a redox anionic species used to
obtain an electroactive ionic liquid with a typical alkylimidazolium cation. Here, several of these redox
ionic liquids (RILs) were prepared by combining FcNTf with alkylimidazolium cations in order to
understand the influence of symmetry and alkyl chain length on the RIL system. The physicochemical and
Available online 22 March 2016
electrochemical properties are characterized for [C
1 and 8, both in the pure, undiluted state and in solutions of acetonitrile. In pure form, the ionic
x y
C Im][FcNTf], where x and y were varied between
Keywords:
Redox ionic liquids
ionic conductivity
ferrocenyl
imidazolium
cyclic voltammetry
ꢀ1
ꢁ
conductivity of [C
chain length was found to have a similar effect on viscosity as in conventional imidazolium-NTf
liquids. While an increase in alkyl chain length increases viscosity and decreases mass transport, it was
found to have no effect on the redox potential of the ferrocene center. The cyclic voltammetry of [C Im]
FcNTf] solutions diluted in acetonitrile exhibited behavior which depends on concentration and on the
x
y
C Im][FcNTf] was found to range between 0.22 and 0.42 mS cm at 60 C, and the alkyl
2
ionic
x y
C
[
presence of a supporting electrolyte. At concentrations above 50 wt.% and in the absence of any
supporting electrolyte, the electroactive anion was found to deposit as a dense film on the electrode upon
+
ꢀ
oxidation. This behavior is linked to formation of Fc NTf zwitterions, which accumulates in the double-
layer and precipitates on the electrode surface. This study also investigates the electrochemical
properties of film deposition.
ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction
counter-ions. Examples of RILs include iodide-based ([I]) [6–8],
metal complexes based on ([Ru],[Co],[Fe]) [9,10] and metallocenium
Ionic liquids (ILs) are composed entirely of ions and present a
melting point below 100 C [1,2]. Their potential application in the
4 7
[11] or viologen ([C VC ]) [12]—based counter ions. Additionally, ILs
ꢁ
may be made electroactive by grafting a redox functional group on a
common cation or anion structure. Functional groups such as
ferrocenyl (Fc) [13–21], 2,5-di-tert-butyl-1,4-dimethoxybenzene
field of electrochemistry is very attractive due to the potential –
depending on the selected ion – to obtain liquid phases with a low
vapour pressure, good thermal and chemical stability, good ionic
conductivity, and a large electrochemical window of stability. Ionic
liquids become increasingly appealing when considering the
numerous possibilities for structure modification and ion combi-
nations. The incorporation of functional groups on the cation or
anionstructurecanbeusedinorderto accomplishspecific tasksfora
specific application [3–5]. The use of redox-active ionic liquids (RILs)
is increasingly prevalent in the literature for fundamental studies on
electron transfer in concentrated phases as well as for application in
energy storage devices. Redox ionic liquids may be considered
intrinsically electroactive with the presence of redox-active
(DDB)
[22],2,2,6,6-Tetramethylpiperidin-1-yl)oxyl
(TEMPO)
[23,24], and metal complexes [25,26] have been reported in the
literature. Redox ionic liquids could replace traditional redox
systems which are used in several electrochemical applications to
transfer electrons in homogeneous or heterogeneous reactions.
RelatedtoenergystorageandconversionRILcanincreasethespecific
energy inpseudocapacitors [6,27], achieve internal electron transfer
in dye-sensitized solar cells [8,23] and act as redox shuttles in
lithium-ion batteries to prevent cathode over-oxidation [15,28].
Designing redox-active ILs with improved properties is required
to achieve an application in this area, but insufficient fundamental
understanding of the relationship between various structure
properties is currently impeding progress. While most examples
of RILs were obtained through modification of the cation, our
group reported on the first example of anion modification with
ferrocene in order to generate an electroactive ionic liquid.
*
Corresponding author. Tel: +1-514-343-6733; fax: +1-514-343-7586.
E-mail address: dominic.rochefort@umontreal.ca (D. Rochefort).
ISE Member.
1
http://dx.doi.org/10.1016/j.electacta.2016.03.075
013-4686/ã 2016 Elsevier Ltd. All rights reserved.
0

2
84
H.J. Xie et al. / Electrochimica Acta 200 (2016) 283–289
A ferrocenylsulfonyl(trifluoromethylsulfonyl)imide anion (FcNTf)
14] was combined with 1-butyl-3-methylimidazolium, and the
impurities from the final product. In this case, sodium imidazolate
was added to a 10 ml of THF solution. Two molar equivalents of
alkyl halide (1-iodomethane, bromooctane) were then added to
the mixture. The mixture was maintained under reflux for four
hours. The resulting substance was washed with THF, followed by
dichloromethane and diethyl ether (yields: 67% and 99%). The final
steps in RIL synthesis consisted of a recombination reaction of
cation and redox anion from the metathesis reaction with NaFcNTf
dissolved in water, carried out in an ultrasonic bath for two hours
(Fig. S1D). This step was followed by intensive washing with water
to eliminate the sodium halide salt. The RILs were dried overnight
under vacuum and stored in the glove box for further analysis.
Following this drying step, the resulting water concentration in
[
resulting RIL’s physicochemical and electrochemical properties
were discussed in a previous paper. Since only one cation was used
in the aforementioned study, the effect of its structure on the ionic
liquid’s properties was not established. Systematic studies of the
effect of structural modifications of the cation in conventional
ionic liquids have been previously reported, for example by
Watanabe [29,30]. In this study, we wish to determine the role of
the imidazolium alkyl’s chain length on the transport and
electrochemical properties of electroactive ionic liquids employing
a redox anion.
To do so, seven different RILs were prepared based on the
ferrocenylsulfonyl(trifluoromethylsulfonyl)imide anion (FcNTf)
undiluted RIL was found at 144 ppm for [C
1 2
C Im][FcNTf] and
and imidazolium cation (C
x
y
C Im). Their structures are presented
170 ppm for the [C Im][FcNTf]. These values are similar to those
1 4
C
in Fig. 1. The effect of alkyl chain length and symmetry on the
imidazolium was investigated for all cation structures, and
viscosity, conductivity and redox properties were measured for
the RIL, in pure form as well as in acetonitrile solutions. These
properties were compared with conventional, unmodified IL
systems found in the literature. Results of cyclic voltammetry
experiments on the RIL at high concentrations are discussed to
provide additional data on the electrochemistry of these redox
liquid phases.
reported for vacuum-dried unmodified ionic liquids [32].
2.3. Characterization
All synthesized compounds were characterized by H, 13C and
F NMR on a Bruker ADVANCE 300 MHz spectrometer at room
1
19
temperature and by mass spectrometry (MS) on an Agilent ESI-LC-
MSD TOF. Elemental analysis was performed on an EAS 1108 appa-
ratus from Fisons Instruments SPA. Detailed results of this
characterization are provided in the supporting information. The
presence of water in IL was determined through coulometric Karl
Fisher titration (Mettler Toledo Coulometric KI Titrator). The
determination of wH2O was performed three times for each sample.
The density measurement was made through a very simple
method with an electronic balance (Mettler Toledo XS105 Dual
Range) and a 1 ml syringe. The viscosity values were obtained by a
2. Experimental section
2.1. Materials
All reagents and precursor chemicals were obtained from
Sigma-Aldrich unless otherwise stated, and all solvents were
obtained from Fisher. The trifuoromethanesulfonamide was
ordered from Synquest Laboratories. All products were used as
received.
ꢁ
viscometer from Cambridge Applied Systems between 50 C and
ꢁ
ꢁ
80 C, at temperature intervals of 4 C. Thermal properties were
obtained by DSC analysis on a TA Q2000. Two heating cycles were
ꢁ
ꢀ1
done on each RIL with a scan rate of 1 C min at temperatures
ꢁ
ꢁ
2.2. Synthesis of RILs
ranging ꢀ70 C to 100 C. The ionic conductivity was measured by
AC impedance using a Parstat 2273 potentiostat from Princeton
Applied Research, with a flow conductivity Orion (018012) cell
composed of two platinized Pt electrodes that were calibrated
Synthesis of the redox anion ferrocenylsulfonyl(trifluorome-
thylsulfonyl)imide (FcNTf) was performed in accordance with a
method published earlier [14]. All alkylimidazolium cations were
prepared following established procedures, and detailed synthetic
ꢀ
1
ꢁ
daily with a standard solution of 0.117 M KCl (0.015Scm at 25 C).
The measurement was carried out from 1 MHz to 1 Hz at
ꢁ
ꢁ
ꢁ
steps for the imidazolium cations [C
C
1 2
Im], [C
4
4
C Im], [C
2
2
C Im] and
25 C- 80 C, at temperature intervals of 4 C. A BioLogic SP-
50 potentiostat was used for cyclic voltammetry (CV) and AC
impedance measurements, using a three-electrode setup com-
posed of a silver wire quasi-reference electrode, a Pt wire counter
electrode, and a platinum disk working electrode with a surface
1 8
[C C Im] are provided in Fig. S1B [31]. In short, a solution of the
corresponding alkylimidazole and alkylhalides was mixed in an
acetonitrile solution. The resulting reaction product was washed
ꢁ
with ethyl ether and the final product dried under vacuum at 75 C
2
before characterization (yield: 98%). For the synthesis of [C
and [C Im], a different procedure (Fig. S1C) was used due to a low
reaction yield of pathway B and some difficulty in eliminating
1
1
C Im]
area of 0.0194 cm . All RILs were diluted in acetonitrile solution
C
8 8
with 1 M TBAP and the total volume of solution in the cell for each
experiment was about 1.5 mL. All potentials are reported to the
x x
Fig. 1. General structure of [C C Im][FcNTf] with cation structure variations and abbreviations.

H.J. Xie et al. / Electrochimica Acta 200 (2016) 283–289
285
mid-point potential of the ferrocene/ferrocenium (Fc/Fc + ) redox
couple which was determined separately in the given electrolyte.
3. Results and discussion
3.1. Physicochemical properties of RILs based on FcNTf
The thermal properties of the RILs obtained by DSC analysis (see
curves in Fig. S2) are summarized in Table 1. An endothermic peak
ascribed to the melting point (T ) is observed for all RILs, but only
upon the first heating cycle. No exothermic peaks corresponding to
m
ꢁ
ꢀ1
crystallization temperature (Tcr) are seen at a 1 C min rate due to
slow crystallization kinetics, which is a common behavior of ionic
liquids. Such slow kinetics are caused by the asymmetric structure
of the FcNTf anion moiety and the high viscosity of RILs. All FcNTf-
based RILs are solids at room temperature; however, they are
frequently found in a super-cooled state for several days at ambient
conditions prior to crystallization[14], thus permitting cyclic
voltammetry experiments on the pure phase (see Section 3.2).
Fig. 2. Temperature dependence tendencies for ionic conductivity in pure [C
FcNTf].
X Y
C Im]
g
For all cooling cycles, a glassy transition temperature (T ) of similar
[
value is observed, despite different cooling cycles. Common ILs
generally tend to form a glass state rather than a crystalline state
upon cooling. This is due to low energy gain in crystal formation
rather than a loss of entropy [33–35], which often leads to
3.2. Ionic conductivity of undiluted RILs
g m
observation of a T instead of a Tcr. In Table 1, a decreasing T is
observed along with an increasing alkyl side chain length on
imidazolium, due to larger cation size and increased interactions
between the chains. Furthermore, linear alkyl substituents on the
cation may also generate multiple rotation degrees of freedom,
Next, the ionic conductivity and electrochemical response of
the pure redox ionic liquid phases were studied to provide a better
understanding of the cation’s impact on RIL properties. The ionic
conductivity of pure RILs, as measured by impedance spectroscopy,
is shown in Fig. 2 as a function of temperature. As a comparison
between conventional and modified ionic liquids, the ionic
conductivity of RIL [C C Im][FcNTf] and [C C Im][NTf ] is 0.136 mS
cm and 13.78 mS cm , respectively, at 40 C [29]. The decrease
in ionic conductivity of the ferrocenyl-modified IL is due to the
presence of a larger anion, which significantly lowers ion mobility.
With different cation sizes, higher ionic conductivity is observed
with shorter cation alkyl chains due a decreased number of van der
Waals interactions between side chains. This tendency is similar to
that of conventional ILs reported in the literature [29,42,43]. The
ionic conductivity’s dependency on temperature does not follow
the typical linear Arrhenius law seen with diluted solutions, and a
slight curvature is observed at lower temperatures. Most ionic
liquids do exhibit such fragile behavior, which is not attributable
specifically to the structure of the FcNTf anion [44]. Therefore, the
ionic conductivity curve in Fig. 2 has been fitted with an empirical
Vogel-Fulcher-Tamman model (VFT) shown in Eq. (1):
m m
which also contribute to a lower T value [36]. The variance in T
along with alkyl side chain for RILs follows a tendency similar to
the one observed with common redox ILs [29]. No clear trend may
be established as a function of chain length symmetry on the cation
structure. The presence of a symmetrical structure generally
provides more efficient ion–ion packing ꢀ and thus a higher
1
2
ꢀ1
1
2
ꢁ
2
ꢀ
1
crystalline order ꢀ and increases T
m
. For the systems studied,
symmetry has a limited influence over columbic attraction, which
appears to be the dominant force. In comparison with conventional
unmodified ionic liquids, the physicochemical properties of RILs
showed an increase of T
intermolecular interactions between ferrocene units between the
FcNTf anions. The DSC analysis of [C Im][FcNTf] and [C Im]
FcNTf] showed no T and T due to their super cooled state within
the temperature range measured. Density values are also listed in
Table 1 for all RILs except [C Im][FcNTf] which is in a solid form
m g
and T , likely due to additional p-p
C
1 8
8 8
C
[
m
g
1 1
C
at room temperature. A decrease in density with an increased
cation size and decreased symmetry are observed [37,38]. This
tendency is typical for common imidazolium-based ILs and results
from less densely-packed ions [39–41].
ꢀ
ꢁ
ꢀ
B
s
¼
s0exp
ð1Þ
T ꢀ T
0
where
s
is ionic conductivity,
s
0
is the infinite ionic conductivity, T
is the absolute temperature, and B and T
0
are fitting parameters,
with the B parameter in this model related to the activation energy
Table 1
x Y
Physicochemical properties of [C C Im][FcNTf] and its corresponding common IL. The values in italic are from other sources (see footnote).
ꢁ
ꢁ
ꢀ3
ꢁ
ꢁ
ꢁ
ꢀ3
ꢁ
RILs [FcNTf]
Tm/ C
Tg/ C
Density/g cm (20 C)
ILs [NTf
2
]
Tm/ C
Tg/ C
Density/g cm (20 C)
C
C
C
C
C
C
C
1
1
2
1
4
1
8
C
1
C
2
C
2
C
4
C
4
C
8
C
8
Im
Im
Im
Im
Im
Im
Im
89
65
58
55
51
–
ꢀ23
ꢀ32
ꢀ35
ꢀ36
ꢀ39
–
–
C
1
C
1
C
2
C
1
C
4
C
1
C
8
C
1
C
2
C
2
C
4
C
4
C
8
C
8
Ima
26
-18
-11
-3
–
–
1.57
1.53
1.47
1.44
–
a
b,c
a
1.53
1.42
1.51
1.35
1.39
1.28
Im
Im
Im
Im
Im
Im
-87
–
-87
–
a
–
-80
–
1.33
–
–
–
–
a
b
c
Watanabe et al. [29].
Andrzej et al. [48].
Gardas et al. [49].

2
86
H.J. Xie et al. / Electrochimica Acta 200 (2016) 283–289
for ionic conductivity. The values obtained are 684 K, 1199 K, 915 K,
and 1159 K for [C Im][FcNTf], [C Im][FcNTf], [C Im][FcNTf],
Im][FcNTf], respectively. An increase in the value of the B
C
1 2
C
2 2
1 4
C
4 4
[C C
parameter with increased alkyl chain length is observed for our
RILs (Table 2). Watanabe et al. published a systematic study of the
physicochemical properties of several imidazolium-NTf
2
ionic
liquids with different alkyl chain lengths [29]. They found B
parameter values from the VFT fitting ranging between 604 and
1 1 2 1 4 2
625 K for ILs with short chains ([C C Im][NTf ] to [C C Im][NTf ])
as well as increasing values for chains with more than four carbon
atoms. They also demonstrated that conductivity, diffusivity and
+
viscosity had the same structural dependency. The [C
1
C
1
Im] and
+
1 2
[C C Im] cations provided ILs with the same properties, and a
decrease in conductivity and diffusivity (along with increased
viscosity) was observed with alkyl chains having more than four
carbon atoms. This tendency appears to apply also to ionic liquids
modified with ferrocene. Fig. 3 shows a comparison of the
conductivity of three families of ionic liquids with different alkyl
chains: the unmodified [C
1
C
Y
Im][NTf
] where the cation bears the ferrocene, and
Im][FcNTf] with the redox anion. All three ionic liquids show
2
], the redox ionic liquids
Fig. 3. Ionic conductivity tendencies based on variations in alkyl side chain length
Y 2
[FcEC Im][NTf
for pure [C
X
C
Y
Im][FcNTf] and [FcEIMC
x
][NTf
2
]
[17] RILs and corresponding
ꢁ
[C
1
C
Y
uncommon ILs [47] at 60 C.
a very similar decrease in conductivity with a single alkyl chain
length increase.
the 1 mM solution, where all diffusion coefficient values are
located within the same range. This is likely caused by maximum
solvation of both species as the concentration of ionic liquid is
lower in solution. Therefore, in diluted solutions the cation size has
little impact on the transport properties of the anionic electro-
active species. In comparison with ferrocene, the [FcNTf] anions
diffuse less rapidly because of a significantly larger radius. Our
previous work showed that the diffusion coefficient of [FcNTf]
3.3. Diffusion in diluted solutions
The diffusion of RILs diluted in acetonitrile has been character-
ized at 0.3 M and 1 mM in the presence of 1 M TBAP as supporting
electrolyte, in order to assess the role of cation size in transport.
This would suggest that the strong interaction between RIL ions
would be maintained even at low concentrations. The CVs obtained
ꢀ5
2 ꢀ1
(DFcNTf = 1.03 ꢃ10 cm s ) was only half the value for ferrocene
ꢀ
5
2 ꢀ1
at various scan rates for [C
C
1 2
Im][FcNTf] at both concentrations are
(DFc = 1.99 ꢃ10 cm s ) in diluted solutions (1 mM in CH CN
3
presented in Fig. 4. The curves show a reversible electrochemical
system with a mid-point potential shifted to values higher than
that of ferrocene, due to the electron-withdrawing effect of the
with 0.1 M TBAP) [14]. The electrochemical parameters of RIL are
listed in Table 4 for 0.3 M and 1 mM solutions. As the redox active-
group is linked to anion structure, no obvious cation influence can
be reported for the 1 mM solutions. All mid-point potential (E’)
ꢀ
2 2 3
(SO )N(SO )CF group in FcNTf. A large peak splitting (DEp,a-p,c) is
observed for all scan rates at a concentration of 0.3 M (Fig. 4(A)),
which is caused by solution resistance. The diffusion coefficients
were calculated using the Randles-Sevcik equation [45]:
values, as well as
within the same range of values. The
D
E
pa-pc and oxidation and reduction limits, fall
pa-pc values obtained at
D
E
0.3 M are significantly larger than those in the 1 mM solutions due
to higher solution viscosity. The mid-point potentials (E’) for the
0.3 M solutions are also found to be slightly higher than those
obtained in the 1 mM solutions and some effect of the cation on the
E’ is noted. This is a result of a difference between the diffusion
coefficients of the oxidized (neutral) and reduced (anionic) species
which distorts the CV at high concentrations of RIL. This effect was
explained in a previous publication [14].
ꢀ
ꢁ
1
=2
nF
RT
v1=2D1=2
i
p
¼ 0:4463 n FAC
ð2Þ
where i
p
is the peak current (A), n the equivalent number of
electrons exchanged during the reaction, A the surface area of the
2
3
electrode (cm ), C the concentration (mol/cm ), D the diffusion
2
coefficient (cm /s), T the temperature (K),
V/s), and F and R the Faraday (C/mol) and ideal gas (J/mol
constants, respectively.
n
the potential scan rate
(
ꢂK)
3
.4. Electrochemistry
The CV curves and Randles-Sevcik plots obtained for all
solutions of RILs are shown in Figs. S3 to S6 and the diffusion
coefficient values are listed in Table 3. At high concentration
1 2
Due to its low viscosity in the pure state, the [C C Im][FcNTf]
RIL was selected for all remaining studies on the electrochemical
behavior of the redox anion. While the [C
provide slightly lower viscosity, its 90 C melting point complicates
the measurement process. Fig. 5(A) shows the cyclic voltammo-
1
C
1
Im][FcNTf] RIL may
(
0.3 M), an effect on chain length is noted in the [C
8 8
C Im][FcNTf]
ꢁ
and [C Im][FcNTf] ionic liquids. The impact of cation structure
1 8
C
on anion transport properties indicates a partial association
between cation and anion in the 0.3 M solution. However, the
ion association and impact of viscosity become less significant for
grams for pure [C
1
C
2
Im][FcNTf] at different temperatures between
0 C and 80 C. As discussed above, once melted, these RILs remain
in a super cooled liquid state for a substantial period of time,
allowing CV experiments to be made below T . The CV curves in
ꢁ
ꢁ
5
m
Table 2
Fig. 5(A) indicate that the transport properties are improved at
higher temperatures, as the liquid becomes less viscous. A
noticeable feature of these curves is the different shapes of the
oxidation and reduction peaks. The maximum peak current is
significantly higher during the oxidation process than during
reduction. This difference may be an effect of a migrational
contribution to mass transport which increases the flux of FcNTf
VFT fitting parameters of [C
X Y
C Im][FcNTf] in their undiluted state. The parameters
for the RILs with C
8
chains are not listed because of unreliable fitting.
ꢀ
1
RILs
B/K
T
0
/K
0
s /S m
C
1
C
2
C
1
C
4
C
C
C
C
2
Im FcNTf
2
Im FcNTf
4
Im FcNTf
4
Im FcNTf
684
1199
915
224
185
208
189
0,28
1,46
0,49
0,88
1159

H.J. Xie et al. / Electrochimica Acta 200 (2016) 283–289
287
1 2 1 2
Fig. 4. Cyclic voltammograms of (A) 0.3 M [C C IM][FcNTf] and (B) 1 mM [C C
IM][FcNTf] in acetonitrile with 1 M TBAP at different scan rates (25 to 500 mV s 1 range).
ꢀ
+
above 2 V vs. Ag/Ag (Supporting Information, Figure S-7) shows a
Table 3
peak at 640 nm which is indicative of ferrocenium [46] and
Diffusion coefficients of 0.3 M and 1 mM [C
TBAP.
X
C
Y
Im][FcNTf] in acetonitrile with 1 M
+
ꢀ
supports the hypothesis of the presence of Fc NTf . This process is
reversible in the sense that a reduction peak is observed during the
backwards potential sweep and no residual film can be noticed
RILs
0.3 M in acetonitrile 1 mM in acetonitrile
ꢀ7
2
ꢀ1
ꢀ5
2 ꢀ1
D
red (x10 )/cm s
Dred (x10 )/cm s
visually after the reduction occurred. However, there is
a
C
C
C
C
C
C
C
1
1
2
1
4
1
8
C
1
C
2
C
2
C
4
C
4
C
8
C
8
Im FcNTf
Im FcNTf
Im FcNTf
Im FcNTf
Im FcNTf
Im FcNTf
Im FcNTf
5.11
5.28
5.85
5.65
5.18
4.46
2.77
1.70
1.87
1.78
1.52
1.57
1.51
1.73
considerable difference in voltammetric charge between the
oxidation and the reduction processes which could be ascribe to
dissolution of the film or to incomplete reduction. The whole
deposition process is still under investigation.
Further details on the deposition process were obtained from
the CV curves recorded at different concentrations of the [C
[
(
1
C
2
Im]
FcNTf] RIL in acetonitrile with no supporting electrolyte
Fig. 6(A)). Although they were severely distorted because of the
high resistance of the solution (no TBAP), the curves for the 10 and
50 wt.%, show processes that may be ascribed to mixed diffusion
and migration transport. The oxidation current is maintained upon
inversion of the potential sweep in the range of 1.5 to 0.4 V,
showing that the electrode is not passivated upon oxidation of the
FcNTf anion. At 60 and 70 wt.%, the curve shows the same pattern
as for pure RIL, in which the oxidation current drops rapidly to
0 after the peak value, thus indicating passivation of the electrode
due to film formation. This film is dense and thick enough to
prevent any further oxidation, even of the solvent, as no other
oxidation processes are observed up to 5 V (Fig. 6(B)). In all cases,
anion to the positively-poised electrode, since it is the sole anionic
species found in the ionic liquid. This is further evidenced by
looking at the effect of scan rate on current (Fig. 5(B)). The current
increases with scan rate; however, this increase does not show the
1/2
linear dependency with
n
expected for a purely diffusional
regime (see inset of Fig. 5(B)). The oxidation current also decays
rapidly in a manner that cannot be explained by diffusion of the
electroactive species. We recently reported on this behavior, which
was also observed in a [C C Im][FcNTf] RIL solution at high
1 4
concentration and in the absence of a supporting electrolyte [14].
+
ꢀ
We demonstrated that a blue film was deposited on the electrode
this process is reversible and reduction of the Fc NTf species back
ꢀ
ꢁ
ꢀ
ꢁ
surface as a result of the oxidation of Fc NTf to the zwitterion
to soluble Fc NTf is observed during the reverse sweep. While the
exact process leading to film formation under such specific
conditions is not fully solved at the moment, it may be
hypothesized that at high concentrations, the combination of
high viscosity and strong interactions between the FcNTf anions
+
ꢀ
Fc NTf . The shape of the curves in Fig. 5 show that formation of
this film, which passivates the electrode, is therefore also obtained
in the pure RIL and for different cations, confirming that it is related
to the electroactive anion’s structure. The absorbance spectrum of
the film deposited on ITO obtained after sweeping the potential
(possible
p-p interactions between Fc) allows high enough
Table 4
X Y
Electrochemical parameters for 0.3 M and 1 mM [C C
Im][FcNTf] in acetonitrile with 1 M TBAP at a scan rate of 100 mV sꢀ1. Oxidation and reduction limit at 0.3 M.
RILs
0.3 M
1 mM
Oxidation limit/V
Reduction limit/V
E’/V
D
E
pa-pc/V
E’/V
DEpa-pc/V
ꢀ
2
C
C
C
C
C
C
C
1
1
2
1
4
1
8
C
1
C
2
C
2
C
4
C
4
C
8
C
8
Im FcNTf
Im FcNTf
Im FcNTf
Im FcNTf
Im FcNTf
Im FcNTf
Im FcNTf
0.23
0.24
0.27
0.23
0.25
0.29
0.23
0.40
0.42
0.45
0.40
0.43
0.44
0.41
0.21
0.21
0.21
0.21
0.21
0.21
0.20
6.8 ꢃ 10
2.1
2.1
2.1
2.1
2.0
2.1
2.1
ꢀ1.2
ꢀ1.3
ꢀ1.2
ꢀ1.2
ꢀ1.3
ꢀ1.2
ꢀ1.3
ꢀ2
7.1 ꢃ10
7.4 ꢃ10
6.9 ꢃ10
ꢀ
2
ꢀ
2
2
2
2
ꢀ
ꢀ
7.3 ꢃ10
7.2 ꢃ10
6.9 ꢃ10ꢀ

2
88
H.J. Xie et al. / Electrochimica Acta 200 (2016) 283–289
ꢁ
ꢀ1
Fig. 5. Cyclic voltammograms of (A) pure [C
rates (5–200 mV s range) at 80 C.
1
C
2
Im][FcNTf] at different temperatures (50–80 C range) at scan rates of 100 mV s (B) pure [C
1
C
2
Im][FcNTf] at different scan
ꢀ
1
ꢁ
ꢀ
1
X Y
Fig. 6. Cyclic voltammograms of (A) different concentrations of [C C Im] [FcNTf] in acetonitrile (10–70 wt.% range)without supporting electrolytes at scan rates of 100 mV s
(
B) oxidation scan of pure [C Im][FcNTf] to 5 V.
1 2
C
+
ꢀ
amounts of the oxidized Fc NTf zwitterion to exist near the
existing interactions between the FcNTf anions. The potential
applications of this type of electroactive ionic liquids in redox-
active electrolyte supercapacitors are currently being investigated,
as film formation prevents diffusion of the oxidized species to the
negative electrode, thus suppressing device self-discharge [50].
electrode to enable deposition of this species into a film.
4. Conclusion
The properties of ionic liquids using alkylimidazolium cations
and the redox anion ferrocenylsulfonyl(trifluoromethylsulfonyl)
imide (FcNTf) are reported herein, in an effort to understand their
structure-property relationships. In particular, this study focuses
on the effect of alkylimidazolium chain length and symmetry on
the ionic liquid’s physicochemical and electrochemical properties.
Variations in cation structure have negligible impact when the RIL
is diluted in acetonitrile, due to the dissociation of cation and
anions in solution. At high concentrations of RILs and in pure
phases, shorter alkyl chains provide lower viscosity and higher
ionic conductivity and diffusion coefficients. However, reducing
the chain length increased the melting point due to higher lattice
free energy, as is the case in unmodified ionic liquids. In pure
phases but also in highly concentrated solutions (>50 wt.%) and in
the absence of supporting electrolytes, a film is deposed at the
electrode surface upon oxidation of the FcNTf anion to the neutral
Acknowledgements
The authors acknowledge the financial support of the Natural
Sciences and Engineering Research Council of Canada (NSERC),
under their Discovery Grant Program.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the
online
version,
at
http://dx.doi.org/10.1016/j.
electacta.2016.03.075.
References
+
ꢀ
species [Fc NTf ]. This deposition is believed to result from the
accumulation of the zwitterion at the electrode surface; such
accumulation is made possible by high viscosity and strong
[
1] K.E. Johnson, What’s an ionic liquid, Interface, Spring, 2007, pp. 38–41.
[2] K.R. Seddon, Ionic liquids for clean technology, J. Appl. Chem. Biotechnol. 68
1997) 351–356.
(

H.J. Xie et al. / Electrochimica Acta 200 (2016) 283–289
289
[
[
3] J.J.H. Davis, Task-specific ionic liquids, Chem. Lett. 33 (2004) 1072–1077.
4] R. Giernoth, Task-specific ionic liquids, Angew. Chem. Int. Ed. 49 (2010)
[27] G. Sun, K. Li, C. Sun, Electrochemical performance of electrochemical
capacitors using cu(ii)-containing ionic liquid as the electrolyte, Microporous
Mesoporous Mater. 128 (2010) 56–61.
2
834–2839.
[
5] A.C. Cole, J.L. Jensen, I. Ntai, K.L.T. Tran, K.J. Weaver, D.C. Forbes, J.H. Davis,
Novel brønsted acidic ionic liquids and their use as dual solvent-catalysts, J.
Am. Chem. Soc. 124 (2002) 5962–5963.
6] T. Tooming, T. Thomberg, L. Siinor, K. Tõnurist, A. Jänes, E. Lust, A type high
capacitance supercapacitor based on mixed room temperature ionic liquids
containing specifically adsorbed iodide anions, J. Electrochem. Soc. 161 (2014)
A222–A227.
[28] S.E. Khakani, J.C. Forgie, D.D. MacNeil, D. Rochefort, Redox shuttles for lithium-
ion batteries at concentrations up to 1 m using an electroactive ionic liquid
based on 2,5-di-tert-butyl-1,4-dimethoxybenzene, J. Electrochem. Soc. 162
(2015) A1432–A1438.
[29] H. Tokuda, K. Hayamizu, K. Ishii, M.A.B.H. Susan, M. Watanabe,
Physicochemical properties and structures of room temperature ionic liquids.
2. Variation of alkyl chain length in imidazolium cation, J. Phys. Chem. B 109
(2005) 6103–6110.
[30] H. Tokuda, K. Ishii, M.A.B.H. Susan, S. Tsuzuki, K. Hayamizu, M. Watanabe,
Physicochemical properties and structures of room-temperature ionic liquids.
3. Variation of cationic structures, J. Phys. Chem. B 110 (2006) 2833–2839.
[31] M.S. Gruzdev, L.M. Ramenskaya, U.V. Chervonova, R.S. Kumeev, Preparation of
1-butyl-3-methylimidazolium salts and study of their phase behavior and
intramolecular intractions, Russ. J. Gen. Chem. 79 (2009) 1720–1727.
[32] A.M. O’Mahony, D.S. Silvester, L. Aldous, C. Hardacre, R.G. Compton, Effect of
water on the electrochemical window and potential limits of room-
temperature ionic liquids, J. Chem. Eng. Data 53 (2008) 2884–2891.
[33] L.C. Branco, J.N. Rosa, J.J. Moura Ramos, C.A.M. Afonso, Preparation and
characterization of new room temperature ionic liquids, Chem. Eur. J. 8 (2002)
3671–3677.
[
[7] V.K. Thorsmølle, G. Rothenberger, D. Topgaard, J.C. Brauer, D.-B. Kuang, S.M.
Zakeeruddin, B. Lindman, M. Grätzel, J.-E. Moser, Extraordinarily efficient
conduction in a redox-active ionic liquid, ChemPhysChem 12 (2011) 145–149.
[8] M. Bidikoudi, L.F. Zubeir, P. Falaras, Low viscosity highly conductive ionic liquid
blends for redox active electrolytes in efficient dye-sensitized solar cells, J.
Mater. Chem. A 2 (2014) 15326–15336.
[
9] A. Komurasaki, Y. Funasako, T. Mochida, Colorless organometallic ionic liquids
from cationic ruthenium sandwich complexes: Thermal properties, liquid
5
6
properties, and crystal structures of [Ru(
5 5 6 5
h -C H )(h -C H R)][X] (X = N
(
SO CF , N(SO F) , PF ), Dalton Trans. 44 (2015) 7595–7605.
2
3
)
2
2
2
6
[
10] A.M. Leone, S.C. Weatherly, M.E. Williams, H.H. Thorp, R.W. Murray, An ionic
liquid form of DNA: Redox-active molten salts of nucleic acids, J. Am. Chem.
Soc. 123 (2001) 218–222.
[
11] T. Inagaki, T. Mochida, Metallocenium ionic liquids, Chem. Lett. 39 (2010)
[34] Z. Hu, C.J. Margulis, Room-temperature ionic liquids: Slow dynamics, viscosity,
and the red edge effect, Acc. Chem. Res. 40 (2007) 1097–1105.
[35] A.S. Larsen, J.D. Holbrey, F.S. Tham, C.A. Reed, Designing ionic
liquids: Imidazolium melts with inert carborane anions, J. Am. Chem. Soc. 122
(2000) 7264–7272.
[36] H.L. Ngo, K. LeCompte, L. Hargens, A.B. McEwen, Thermal properties of
imidazolium ionic liquids, Thermochim. Acta 357–358 (2000) 97–102.
[37] Y. Hu, X. Peng, Effect of the structures of ionic liquids on their physical
chemical properties, in: S. Zhang, J. Wang, X. Lu, Q. Zhou (Eds.), Structures and
interactions of ionic liquids, vol. 151, Springer, Berlin Heidelberg, 2014, pp.
141–174.
572–573.
[
12] N. Bodappa, P. Broekmann, Y.-C. Fu, J. Furrer, Y. Furue, T. Sagara, H. Siegenthaler,
H. Tahara, S. Vesztergom, K. Zick, T. Wandlowski, Temperature-dependent
transport properties of a redox-active ionic liquid with a viologen group, J.
Phys. Chem. C 119 (2015) 1067–1077.
[
13] R. Balasubramanian, W. Wang, R.W. Murray, Redox ionic liquid
phases: Ferrocenated imidazoliums, J. Am. Chem. Soc. 128 (2006) 9994–9995.
14] B. Gélinas, D. Rochefort, Synthesis and characterization of an electroactive
ionic liquid based on the ferrocenylsulfonyl(trifluoromethylsulfonyl)imide
anion, Electrochim. Acta 162 (2015) 36–44.
[
[
15] J.C. Forgie, S. El Khakani, D.D. MacNeil, D. Rochefort, Electrochemical
characterisation of a lithium-ion battery electrolyte based on mixtures of
carbonates with a ferrocene-functionalised imidazolium electroactive ionic
liquid, Phys. Chem. Chem. Phys. 15 (2013) 7713–7721.
[38] K.N. Marsh, J.A. Boxall, R. Lichtenthaler, Room temperature ionic liquids and
their mixtures—a review, Fluid Phase Equilib. 219 (2004) 93–98.
[39] H. Seemann, R. Winter, C.A. Royer, Volume, expansivity and isothermal
compressibility changes associated with temperature and pressure unfolding
of staphylococcal nuclease1, J. Mol. Biol. 307 (2001) 1091–1102.
[40] R. Gomes de Azevedo, J.M.S.S. Esperança, V. Najdanovic-Visak, Z.P. Visak, H.J.R.
Guedes, M. Nunes da Ponte, L.P.N. Rebelo, Thermophysical and
thermodynamic properties of 1-butyl-3-methylimidazolium
tetrafluoroborate and 1-butyl-3-methylimidazolium hexafluorophosphate
over an extended pressure range, J. Chem. Eng. Data 50 (2005) 997–1008.
[41] R. Gomes de Azevedo, J.M.S.S. Esperança, J. Szydlowski, Z.P. Visak, P.F. Pires, H.J.
R. Guedes, L.P.N. Rebelo, Thermophysical and thermodynamic properties of
[
16] P. Kubler, J. Sundermeyer, Ferrocenyl-phosphonium ionic liquids—synthesis,
characterisation and electrochemistry, Dalton Trans. 43 (2014) 3750–3766.
[
17] B. Gélinas, J.C. Forgie, D. Rochefort, Conductivity and electrochemistry of
ferrocenyl-imidazolium redox ionic liquids with different alkyl chain lengths,
J. Electrochem. Soc. 161 (2014) H161–H165.
18] J.E.F. Weaver, D. Breadner, F. Deng, B. Ramjee, P.J. Ragogna, R.W. Murray,
Electrochemistry of ferrocene-functionalized phosphonium ionic liquids, J.
Phys. Chem. C 115 (2011) 19379–19385.
[
[
19] T. Mochida, Y. Miura, F. Shimizu, Assembled structures and cation-anion
interactions in crystals of alkylimidazolium and alkyltriazolium iodides with
ferrocenyl substituents, Cryst. Growth Des. 11 (2011) 262–268.
ionic liquids over an extended pressure range: [BMIm][NTf
2
] and [HMIm]
[NTf ], J. Chem. Thermodyn. 37 (2005) 888–899.
2
[42] J.A. Widegren, E.M. Saurer, K.N. Marsh, J.W. Magee, Electrolytic conductivity of
four imidazolium-based room-temperature ionic liquids and the effect of a
water impurity, J. Chem. Thermodyn. 37 (2005) 569–575.
[
20] O. Fontaine, C. Lagrost, J. Ghilane, P. Martin, G. Trippé, C. Fave, J.C. Lacroix, P.
Hapiot, H.N. Randriamahazaka, Mass transport and heterogeneous electron
transfer of a ferrocene derivative in a room-temperature ionic liquid, J.
Electroanal. Chem. 632 (2009) 88–96.
[43] A. Stoppa, O. Zech, W. Kunz, R. Buchner, The conductivity of imidazolium-
ꢁ
based ionic liquids from (ꢀ35 to 195) C. A. Variation of cation’s alkyl chain, J.
[
21] V.O. Nyamori, M. Gumede, M.D. Bala, Synthesis, characterisation and
Chem. Eng. Data 55 (2010) 1768–1773.
properties of ferrocenylalkylimidazolium salts, J. Organomet. Chem. 695
[44] W. Xu, E.I. Cooper, C.A. Angell, Ionic liquids: Ion mobilities, glass temperatures,
and fragilities, J. Phys. Chem. B 107 (2003) 6170–6178.
(
2010) 1126–1132.
[
[
[
[
[
22] J.C. Forgie, D. Rochefort, Electroactive imidazolium salts based on 1,4-
dimethoxybenzene redox groups: Synthesis and electrochemical
characterisation, RSC Adv. 3 (2013) 12035–12038.
23] X. Chen, D. Xu, L. Qiu, S. Li, W. Zhang, F. Yan, Imidazolium functionalized
tempo/iodide hybrid redox couple for highly efficient dye-sensitized solar
cells, J. Mater. Chem. A 1 (2013) 8759–8765.
24] C.-T. Li, C.-P. Lee, C.-T. Lee, S.-R. Li, S.-S. Sun, K.-C. Ho, Iodide-free ionic liquid
with dual redox couples for dye-sensitized solar cells with high open-circuit
voltage, ChemSusChem 8 (2015) 1244–1253.
25] N. Audic, H. Clavier, M. Mauduit, J.-C. Guillemin, An ionic liquid-supported
ruthenium carbene complex: A robust and recyclable catalyst for ring-closing
olefin metathesis in ionic liquids, J. Am. Chem. Soc. 125 (2003) 9248–9249.
26] Q. Yao, M. Sheets, An ionic liquid-tagged second generation hoveyda–grubbs
ruthenium carbene complex as highly reactive and recyclable catalyst for ring-
closing metathesis of di-, tri- and tetrasubstituted dienes, J. Organomet. Chem.
[45] A.J. Bard, L.R. Faulkner, Electrochemical methods fundamentals and
applications, Second ed., John Wiley & Sons, Inc., 2001.
[46] A.W. Taylor, P. Licence, X-ray photoelectron spectroscopy of ferrocenyl- and
ferrocenium-based ionic liquids, ChemPhysChem 13 (2012) 1917–1926.
[47] H. Tokuda, S. Tsuzuki, M.A.B.H. Susan, K. Hayamizu, M. Watanabe, How ionic
are room-temperature ionic liquids? An indicator of the physicochemical
properties, J. Phys. Chem. B 110 (2006) 19593–19600.
[48] U. Doma nꢀ ska, A. R e˛ kawek, A. Marciniak, Solubility of 1-alkyl-3-
ethylimidazolium-based ionic liquids in water and 1-octanol, J. Chem. Eng.
Data 53 (2008) 1126–1132.
[49] L.I.N. Tomé, P.J. Carvalho, M.G. Freire, I.M. Marrucho, I.M.A. Fonseca, A.G.M.
Ferreira, J.A.P. Coutinho, R.L. Gardas, Measurements and correlation of high-
pressure densities of imidazolium-based ionic liquids, J. Chem. Eng. Data 53
(2008) 1914–1921.
[50] H.J. Xie, B. Gélinas, D. Rochefort, Redox-active electrolyte supercapacitors
using electroactive ionic liquids, Electrochem. Comm. 66 (2016) 42–45.
6
90 (2005) 3577–3584.