A228
Journal of The Electrochemical Society, 161 (3) A228-A238 (2014)
0013-4651/2014/161(3)/A228/11/$31.00 © The Electrochemical Society
Comparative Study of Two Protic Ionic Liquids as Electrolyte
for Electrical Double-Layer Capacitors
Laure Timperman,a,z François Béguin,b Elzbieta Frackowiak,b and Mériem Anoutia,z
a Université François Rabelais, Laboratoire PCM2E, Parc de Grandmont,37200 Tours, France
b Institute of Chemistry and Technical Electrochemistry, Poznan University of Technology, 60-965
Poznan, Poland
This study describes preparation, characterization, application of two protic ionic liquids (PILs), Pyrrolidinium
bis(trifluoromethanesulfonyl)imide ([Pyrr][TFSI]) and Diisoproyl-ethyl-ammonium bis(trifluoromethanesulfonyl)imide ([DIPEA][TFSI]),as electrolyte for supercapacitors. Their modelisation by Density Functional Theory shows that their most evident
difference is the Cosmo Volume, and the structure dissymmetry of their cation. Physicochemical and electrochemical properties
are discussed according to the cation structure. Ionic conductivity increases with temperature up to 8.4 and 17.3 mS cm−1 at 80◦ C
while viscosity decreases to 20 and 12 mPa s, at 70◦ C for [DIPEA][TFSI] and [Pyrr][TFSI], respectively. Ionicity was then studied
using the Walden diagram, showing that they are good ILs. Their behavior was then evaluated as electrolytes for supercapacitor, on
activated carbon electrodes by cyclic voltammetry, galvanostatic cycling and Electrochemical Impedance Spectroscopy. Depending
on the nature of binder used in electrodes preparation, a difference of wettability was observed. [Pyrr][TFSI] displays a better
diffusivity on the electrode allowing good cycling capacitance (120 F g−1 ), whereas [DIPEA][TFSI] allows higher voltage (E =
2 V) and specific energy. According to the results, the cation nature is a decisive parameter on PILs electrochemical behavior for
supercapacitor systems.
© 2013 The Electrochemical Society. [DOI: 10.1149/2.016403jes] All rights reserved.
Manuscript submitted October 2, 2013; revised manuscript received December 2, 2013. Published December 14, 2013.
Supercapacitors have attracted increasing interest due to their high
power storage capability that is highly desirable for electric vehicles
(EVs), hybrid electric vehicles (HEVs), and domestic applications.1–6
Furthermore, supercapacitors can be coupled with fuel cells or batteries to deliver the high power required during acceleration of vehicles
and to recover then energy during braking.7–9 Carbon-based capacitors have also attracted great attention mainly driven by the diversified textures of these materials dealing also with their high stability
and conductivity.10–16 The electrical double-layer capacitors (EDLC)
based on high specific surface area carbon electrodes, that operate by
separation of charges at the electrode / electrolyte interface, are the
most advanced and are already on the market.17
Activated Carbons (ACs) are the most commonly used electrode
materials for EDLC applications due to their relatively low cost and
high surface area in comparison with other carbon materials. The attainable voltage of any supercapacitor device essentially depends on
the electrolyte potential window, while the equivalent series resistance
(ESR) depends on the electrode nature and electrolyte conductivity;
therefore the choice of the electrolyte is prominent. Although aqueous
solutions are environment friendly and a low cost solution, the low cell
voltage attainable, and consequently low specific energy, is an important disadvantage.18,19 Organic electrolytes display a higher potential
window, which is an advantage for high energy applications.20,21
Owing to their unique properties of high chemical, thermal
and electrochemical stability, good conductivity, low volatility, nonflammability, and recyclability, ionic liquids (ILs) are considered as
“key electrolytes” for the development of novel and safe electrochemical devices.22–26 “Protic Ionic Liquids” (PILs) are referred as
a protic subgroup in the class of ambient temperature fluid systems,
formed by the transfer of one proton between a Brönsted acid and a
Brönsted base.27,28 The labile proton creates donor and acceptor sites
and can lead to the formation of hydrogen bonds.29 These conducting
electrolytes are starting to emerge as useful materials owing to various potential applications including electrolytes in batteries, fuel cells,
double-layer capacitors and biosensors.30–40 In the literature, few studies report on the use of PILs for supercapacitors,41,42 while there is a
larger number of reports concerning aprotic ILs (AILs)15,16,18,22–24,43–46
electrolyte formulations.
We have recently reported the use of PILs pure and in mixtures as electrolytes for supercapacitor applications, pyrrolidinium
nitrate,47 triethylammonium bis(trifluoromethane-sulfonyl)imide,48,41
or pyrrolidinium methane sulfonate in water49 and phosphonium
z
E-mail: [email protected]; [email protected]
tetrafluoroborate in acetonitrile.42 The present work reports on
the preparation, characterization and application of two PILs
based on bis(trifluoromethanesulfonyl)imide anion and diisopropylethyl-ammonium cation [DIPEA][TFSI] or pyrrolidinium cation
[Pyrr][TFSI], as electrolytes for supercapacitor. The choice of these
two cations was led by their opposite characteristics, in fact, one is
cyclic and the other one aliphatic, they are also different with their pKa
and the size of ions. Thus, they represent two cations models allowing understanding the effect of cation on physico-chemical properties
for supercapacitor application. The aim of this work is to understand
and compare two protic cations models very different, with controlled
residual amount of water on activated carbon, taking into account
structural effects of cations and their physico-chemical properties.
The objective of this publication is not to evaluate performances of
those PILs for real EDLC applications. The purpose is not to put interest on the effect of residual water, which was already studied in a
previous work from our group.50–53
Experimental
Materials.— Diisopropyl-ethyl-amine, pyrrolidine, hydrochloric
acid (37%) and 1,2-dichloroethane (DCE, > 99%), commercially
available from Sigma Aldrich, were used without further purification.
Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) (≥ 99.0%) was
obtained from Solvionic. Water was purified using a Mili-Q 18.3 M
system.
Preparation of PILs.— The pyrrolidinium and diisopropyl-ethylammonium bis(trifluoromethanesulfonyl)imide ([Pyrr][TFSI], [DIPEA][TFSI]) PILs were synthesized by an equimolar metathesis method, as previously reported for triethylammonium
bis(trifluoromethanesulfonyl)imide.41 The obtained PILs areviscous
gels, with melting temperature Tm = 21◦ C and Tm = 35.9◦ C for [DIPEA][TFSI] and [Pyrr][TFSI], respectively. They were analyzed for
water content by coulometric Karl-Fisher titration and were found to
contain approximately 600 ppm, just after drying, but after leaving
at the equilibrium in air, the water content raised close to1000 ppm
(0.1 wt%).
Theoretical measurements.— The charge distribution, polarizability and COSMO volume of the two cations and the anion investigated
during this study were firstly generated using DFT within Gaussian
version 3.0, utilizing the B3LYP method and the DGTZVP basic set.
The resultant optimized structure of each ion was then used as an input
for the generation of the COSMO file within the Turbomole program,
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Journal of The Electrochemical Society, 161 (3) A228-A238 (2014)
in this case, using the BP-DFT method and the Ahlrichs-TZVP basic set for all species. The COSMO volume and the sigma profile
of each ion were then generated by using COSMO-RS (Conductorlike Screening Model for Real Solvent) methodology, within the
COSMOthermX program (version 2.1, release 01.08).
Physicochemical and electrochemical measurements.— The viscosity was determined using an AR 100 rheometer (TA Instrument)
with a conical geometry in a temperature range from 20 to 70◦ C. A
GLP 31 (Crison) digital multifrequency conductimeter was utilized
to measure the ionic conductivity of PILs in the temperature range
from –10 to 80◦ C, using a F25 thermostated bath (JULABO), with an
accuracy of ± 0.2◦ C, the uncertainty for conductivity did not exceed
± 2%. Differential scanning calorimetry (DSC) was carried out on a
Perkin-Elmer DSC 4000 under nitrogen atmosphere, coupled with an
Intracooler SP VLT 100. The contact angles were measured according
to the sessile drop method using a G-11 goniometer (Krüss, Germany)
at room temperature (25 ± 2◦ C). For each support, the contact angle was determined five times at different positions on the material,
when the difference was more than 10%, values were removed, and
the average of consistent values are reported.
Electrochemical measurements were carried out on a Versatile
Multichannel Potentiostat (Biologic S.A) piloted by an EC Lab
V10.20 interface. Galvanostatic charge-discharge experiments and
cyclic voltammetry were conducted using teflon Swagelok cells, either
in AC/AC two-electrode configuration or in three-electrode configuration, with 12 mm diameter AC as the working and counter electrodes,
and a silver wire as pseudo-reference electrode. A porous Whatman
membrane (thickness h = 675 μm and pore diameter Ø = 2.7 μm)
soaked with the electrolyte solution was utilized as separator. For this
study, two type of electrode were used, the electrodes, kindly supplied
by Batscap (France), made of aluminum foil coated with AC with
polyvinylidene fluoride (PVDF) binder (PVDF-AC) (80 wt% AC,
10 wt% PVDF, 10 wt% carbon black), or home-made with carboxymethyl cellulose (CMC) by coating using a doctor blade
(CMC/SBR-AC). It is well-known that CMC is soluble in water, and
PILs based electrolyte used here contain 0.1 wt% of water. However,
as it was previously shown,54 below 3 wt% of water in PILs, H2 O cannot be considered as free molecule, therefore the CMC could not be
solubilized by the electrolyte in this study. The ink for the home-made
electrodes was obtained in water by mixing AC (82 wt%) with a conductive additive (C65, TIMCAL, 10 wt%), carboxymethyl cellulose
(CMC, 4 wt%) and styrene-Butadiene Rubber (SBR, 4 wt%).
Results and Discussion
The Conductor-like Screening Model (COSMO) is a continuum
approach,55 while more complicated, which is computationally quite
efficient. The COSMO procedure generates a conducting polygonal
surface around the system (ion or molecule), at the van der Waals’ distance. The expression for the total screening energy is simple enough
to allow the first derivatives of the energy with respect to atomic coordinates to be easily evaluated. This method has been previously used
for RTIL properties analysis.56
To understand the differences on the structure, charge distribution and volume between both investigated PILs, computational
methods were harnessed. Literature data regarding research on
bis(trifluoromethanesulfonyl)imide anion confirm that, in liquid state,
two conformers of anion, presented in Table I, coexist. Reported energy for each conformer pointed that the difference between them is
only a few kilojoules, which explains the high flexibility of an anion
caused by facilitated conversion of discussed conformers.57 The common characteristic of [TFSI]− is that its negative charge is delocalized
among the nitrogen and the four oxygen, inducing lower coordinating
power of this anion. Concerning cations, the most evident difference
is their Cosmo Volume, which is two times higher for [DIPEA]+ than
for [Pyrr]+ , and the dissymmetry of their structure. This fact influences the lowering of melting point of investigated PILs. The charge
A229
densities presented in sigma profiles are more localized in the nitrogen
atom of Pyrrolidinium cation.
Physicochemical properties of PILs.— The measured density, viscosity and specific ionic conductivity as well as calculated molar mass,
molar volume and equivalent ionic conductivity of [DIPEA][TFSI]
and [Pyrr][TFSI], are presented in the Table II. Values are reported
in this table at 50◦ C, as PILs are viscous ([DIPEA][TFSI]), or solid
([Pyrr][TFSI]) at room temperature. Besides, electrochemical characterizations will be presented, below, at 50◦ C.
In general, the density of ionic liquids such as alkylimidazoliumbased is very strongly affected by the nature of the anion. ILs with
an inorganic counteranion exhibit a larger density than with an organic anion. Furthermore, the molecular packing in the liquid phase
is enhanced when the anion possesses an electron acceptor group
as the CF3 group in the [TFSI]− anion. Here, both PILs exhibit a
large density, e.g., 1.37 g cm−3 and 1.57 g cm−3 , for [DIPEA][TFSI]
and [Pyrr][TFSI], respectively, compared to their homologue based
on the acetate anion, e.g., 0.982 g cm−3 and 1.054 g cm−3 , for
[DIPEA][CH3 COO] and [Pyrr][CH3 COO], respectively. This feature
can be explained by the molar mass of the [TFSI]− anion even if its
interaction with the two cations is smaller. Otherwise, the two times
smaller COSMO volume for the [Pyrr]+ cation than for [DIPEA]+ , as
shown in Table I, can explain the difference in density between the
two PILs.
Thermal properties.—Figure 1 presents the DSC thermograms of the
PILs recorded at a scan rate of 5◦ C min−1 during heating from −60◦ C
to 100◦ C (step I) followed by cooling from 100◦ C to −60◦ C (step II),
all characteristics values are reported in Table III. Two melting peaks
were observed for [DIPEA][TFSI], at −4.9◦ C (268.1 K) and 21.9◦ C
(294.9 K) and three melting peaks for [Pyrr][TFSI] at −5◦ C (268 K),
15.5◦ C (288.5 K) and 35.9◦ C (308.9 K).The complicated phase behavior of PILs is likely explained by the fact that the salts have solid-solid
phase transitions forming disordered phases.58–60 In fact, the presence
of TFSI conformers and the free rotation of isopropyl group on [DIPEA] cation favor a large number of solid states and facilitate the
transitions phase. The melting point obtained here for [Pyrr][TFSI]
(Tm = 35.9◦ C) is consistent with the value previously reported by
Susan et al.36 or by Greaves et al.61 with a difference of less than
1◦ C. For each PIL one can see a particular shape of the peaks which
corresponds typically to the melting of a binary mixture of immiscible
solids leading to the formation of a eutectic. As both PILs contain a
relatively important amount of water (∼1000 ppm), the peaks at −6◦ C
(Figure 1a) and 2.6◦ C (Figure 1b) (Te ) correspond to the PIL-H2 O eutectic melting and the second peaks at Tm1 = −4.9◦ C and Tm2 = 15.5◦ C
to the liquidus line, for [DIPEA][TFSI] and [Pyrr][TFSI], respectively.
One advantage on the synthesized PILs is their supercooling character
which allows using them to be liquid at room temperature. In fact, one
can see on Figure 1 numerous peaks corresponding to crystallization
phases at Tc ranging from 17.1 to −20◦ C for [DIPEA][TFSI] and a
narrow and intense peak at −40◦ C for [Pyrr][TFSI]which denotes a
high structural organization. In Ionic liquids usually have supercooled
state higher than 40◦ C which is quite stable and this state is thermodynamically favored in confined environment like the microporosity
of activated carbon.
It has been established that increasing the operating temperature
for electric systems could significantly improve their effectiveness and
reduce then the level of cost, for similar capacitance, compared to the
current state of the art (increasing of mass or volumetric capacitance).
Besides, the higher temperature is obtained from engine heat and not
from external devices. For both PILs, the crystallization and melting
enthalpy values are very close; the small difference could be due to
the uncertainty on the melting enthalpy, because of the intersection
of the two peaks corresponding to the melting point of the eutectic
(Te ) and the pure PIL (Tm ). Those results allow considering the use
of those PILs from room temperature to 100◦ C; they have been reported to be stable up to 350◦ C for [DIPEA][TFSI],62 and 373◦ C for
[Pyrr][TFSI].36
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A230
Journal of The Electrochemical Society, 161 (3) A228-A238 (2014)
Table I. Structure, abbreviation, Cosmo volume and sigma profiles of the cations and anion.
Structure
and abbreviation
Cosmo Volume
(Å3)
[DIPEA]+
207.37
[Pyrr]+
106.01
Sigma profiles
219 69
219.69
[TFSI]- conformer 1
222.21
222 21
[TFSI]- conformer 2
Table II. Molar mass, density, molar volume, viscosity, specific and equivalent ionic conductivity of [DIPEA][TFSI] and [Pyrr][TFSI] at 50◦ C.
PILs
M g mol−1
ρ g cm−3 ± 0.1%
Vm cm3 mol−1 ± 0.1%
η mPa s ± 0.1%
σ mS cm−1 ± 2%
S cm2 mol−1 ± 2%
[DIPEA][TFSI]
[Pyrr][TFSI]
409.5
351.3
1.37
1.57
298.9
223.8
40.4
20.1
3.6
8.1
1.08
1.81
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Journal of The Electrochemical Society, 161 (3) A228-A238 (2014)
4
10
(a)
Tm1
Tm2
Te
5
Heat flow (Endo up) / mW
Heat Flow (Endo up) / mW
6
A231
2
0
Tc3
-2
-4
Tc2
-6
Tc1
-8
-10
-60
-40
-20
0
20
40
60
80
100
(b)
Te Tm2 Tm3
Tm1
0
-5
-10
-15
-20
-25
-30
Tc
-35
-60
-40
-20
0
20
40
60
80
100
Temperature / °C
Temperature / °C
Figure 1. DSC thermograms of [DIPEA][TFSI] (a) and [Pyrr][TFSI] (b) in the temperature range from −60◦ C to 100◦ C.
Transport properties.— Conductivity.—Among the most important
properties of ionic liquids in terms of potential application as electrolytes for electrochemical devices is their conductivity. There are
a number of studies on the specific conductivity of neat ILs, as well
as on their solutions in molecular solvents48,63 and composites with
polymers.64–66 In general, ionic liquids have good ionic conductivity,
and in the same range than standard electrolyte for energy storage like
mixture of alkyl carbonate with LiPF6 (for example: σ = 12 mS cm−1
for EC/DMC 1M LiPF6 ).67 However, in the case of Et4 N BF4 + ACN
electrolyte, the conductivity is higher (60 mS cm−1 ) than neat ILs, but
in the same range than ILs in mixture with molecular solvent, such
as acetonitrile.68 However, at room temperature, their conductivity is
usually lower than for concentrated aqueous electrolytes, based on the
fact that ionic liquids are composed exclusively of ions either associated or free. PILs conductivity is higher than for aprotic ILs.69–72
At 25◦ C, the conductivity of [DIPEA][TFSI] and [Pyrr][TFSI] was
found to be 1.2 and 3.2mS cm−1 , respectively. These values are in the
same range as for AILs based on pyrrolidinium and the TFSI anion
with a conductivity around 2mS cm−1 ,73 and as for other PILs based
on the TFSI anion and alkylimidazolium cations with a conductivity
between 0.08 and 4 mS cm−1 .61 In our previous work we reported on
[Et3 NH][TFSI] (600 ppm of water) with comparable conductivity of
5mS cm−1 .74 The difference of water content in these two different
studies is negligible and will not influence drastically the conductivity.
It is obvious that the important molar mass and charge delocalization
of the [TFSI]− anion reduce its mobility by comparison with other
smaller and more polarizable anions like [HCOO]− and [NO3 ]− .75
In the same time, the ammonium cation [DIPEA]+ seems to be less
favorable to ionic mobility due to its mass and size (M = 130 g mol−1 ,
Vm = 119.188 cm3 mol−1 ) compared with the pyrrolidinium cation
(M = 72 g mol−1 , Vm = 53.12 cm3 mol−1 ).49
The temperature dependence of the ionic conductivity for both
synthesized PILs is presented in Figure 2. As expected, the conductivity increases with the temperature to reach 8.4 and 17.3 mS
cm−1 at 80◦ C for [DIPEA][TFSI] and [Pyrr][TFSI], respectively.
Moreover [Pyrr][TFSI] presents a residual conductivity of 0.4 mS
cm−1 at −10◦ C. This last is the conductivity remaining at very low
temperature, probably due to the labile proton mobility by Grotthus
mechanism, and not to the ionic mobility. The same aspect has been
observed previously for [Et3 NH][TFSI] with a residual conductivity
of 0.7 mS cm−1 at −10◦ C, and also for morpholonium-based PILs
with 1 or 1.5 mS cm−1 for N-ethylmorpholonium formate and Nmethylmorpholonium formate, respectively.76
Since the pure PILs exhibit a non-Arrhenius (equation 1), the
Vogel-Tamman-Fulcher (equation 2) was used to determine the temperature dependence of conductivity (Figure 3).
−Bσ
[1]
σ = σ0 ex p
T
−Bσ
σ = σ0 ex p
T − T0
[2]
Here, σ0 (mS cm−1 ), Bσ (K), Bσ ’ (K) and T0 (K) are fitting parameters. The product Bσ R or Bσ ’R (where R is the molar gas constant) has
the dimension of the pseudo-activation energy (kJ mol−1 ). From VTF
fitting analysis important information on rotational pseudo-energy
linked to the electrolyte fragility especially with T0 linked to the ideal
Tg can be determined.27,77 These data are related to the PILs behavior in small porosity in a vitreous state in activated carbon. Figure 3
represents the VTF plot. Following equation 2, the fitting parameters
could be determined for both PILs, and are presented in Table IV.
According to the previous argumentation about ionic mobility, we
can justify that the activation energy of ionic transport has a ratio of
1.32. Otherwise, one can see that the ideal glassy transition temperatures obtained here from the fitting parameters, are very close for
both PILs.
Viscosity.—The viscosity has a strong effect on the rate of mass transport within the solution, and is therefore an important parameter for
electrochemical studies. As mentioned previously, the viscosity can be
influenced by several parameters such as, a higher alkalinity, size, relative capacity to form hydrogen bonds, van der Waals interactions70
of anionic species and size of the cation.49,78 In the present work,
Table III. Thermal properties of the PILs: Melting Point (Tm ), Melting Enthalpy (Hm ), Crystallization temp. (Tc ), Crystallization Enthalpy
(Hc ), Eutectic temp.(Te ).
PILs
Tm(i) (◦ C)
Hm(i) (kJ mol−1 )
Tc(i) (◦ C)
Hc(i) (kJ mol−1 )
Te (◦ C)
[DIPEA][TFSI]
−4.9
21.9
5.7
4.4
2.7
2.2
5.4
−6.0
−5.0
15.5
35.9
0.0
17.1
−20.5
6.7
4.8
4.9
−40.0
17.8
2.6
[Pyrr][TFSI]
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A232
Journal of The Electrochemical Society, 161 (3) A228-A238 (2014)
9
18
(a)
-1
7
6
5
4
3
2
14
12
10
8
6
4
2
1
0
0
-2
20
(b)
16
Conductivity / mS cm
Conductivity / mS cm
-1
8
30
40
50
60
70
-10
80
0
10
20
30
40
50
60
70
80
Temperature / °C
Temperature / °C
Figure 2. Effect of temperature on the conductivity of [DIPEA][TFSI] (a) and [Pyrr][TFSI] (b). The solid line serves as a guide to the eye.
the viscosity of pure PILs at 25◦ C is 110 mPa s and 47 mPa s for
[DIPEA][TFSI] and [Pyrr][TFSI], respectively. For [Pyrr][TFSI], the
viscosity is in the same range than AILs based on pyrrolidinium and
the TFSI anion, with a value around 55–53 mPa s73 and as for other
PILs based on the TFSI anion and alkylimidazolium cations with a
viscosity between 54 and 100 mPa s.61
Figure 4 shows the evolution of viscosity with temperature for
[DIPEA][TFSI](a) and [Pyrr][TFSI] (b).The first scan was recorded
from 30 or 35◦ C to 70◦ C, and the second one from 70 to 20◦ C.
Thanks to supercooling, PILs are still liquid at 20◦ C. As expected,
the viscosity decreases when temperature increases to 70◦ C, due to
the higher mobility of ions. We can also remark, from the Figure 4, a
different effect of temperature on viscosity depending on the nature
of the cation. The inset in Figure 4 displays the relation between the
shear stress and the shear rate. In the case of [Pyrr][TFSI] (Figure 4b)
at 35◦ C, the shear stress increases monotonically with the shear rate
up to a shear rate of 5000 s−1 , revealing a Newtonian behavior with
a constant shear stress/shear rate ratio. However, a negative deviation
from the Newtonian behavior was observed at 20◦ C, with the onset of a
shear thinning at a shear rate value of 3000–3500 s−1 . This deviation
was also observed for [DIPEA][TFSI] (Figure 4a) at 20 and 30◦ C.
Furthermore, it appears that the flow behavior of the investigated PILs
changes from Newtonian to non-Newtonian by changing temperature.
Herein, all viscosity data reported were measured at a shear rate value
close to 500 s−1 , since each PIL has a Newtonian behavior at this
value. Since both PILs exhibit a non-Arrhenius (equation 3) behavior,
the Vogel-Tamman-Fulcher (VTF) equation 4 was used to represent
the temperature dependence of viscosity:
Bη
η = η0 ex p
T
η = η0 exp
Bη
[3]
[4]
T − T0
Here, η0 (mPa s), Bη (K), Bη ’ (K) and T0 (K) are fittings parameters. The product Bη R or Bη ’R (where R is the molar gas constant)
has the dimension of the pseudo-activation energy (kJ mol−1 ). The
best parameters obtained from the VTF plots for [DIPEA][TFSI] and
[Pyrr][TFSI]are listed in Table IV. As observed for conductivity, the
ideal glassy transition temperatures are very close for both PILs and
the pseudo-activation energies have a ratio of 1.29, close to the value
1.32 calculated from conductivity for the pseudo-activation energies,
which can be also explained by the ionic mobility.
Ionicity.—One way of assessing the ionicity of ionic liquids is to use
the classification diagram based on the classical Walden rule.27,77 The
Walden rule correlates the ionic mobility, represented by the molar
conductivity,with the fluidity η−1 of the medium through which
the ions move. The molar conductivity is calculated by using the
relation = Vm σ where Vm is the molar volume.
Figure 5 shows ln() versus ln(1/η) plots at various temperatures
for [DIPEA][TFSI], [Pyrr][TFSI]and selected PILs for comparison.75
The ideal line is obtained on the basis that ions have a mobility
which is determined only by the viscosity of the medium, and that
-4.5
3.0
(a)
2.0
-1
-1
Ln (σ / mS cm )
-5.0
Ln (σ / mS cm )
(b)
2.5
-5.5
-6.0
-6.5
1.5
1.0
0.5
0.0
-0.5
-7.0
-1.0
5.0
5.5
6.0
6.5
7.0
-1
1000 / T-T0 (K )
7.5
8.0
-1.5
5
6
7
8
9
10
11
12
-1
1000 / T -T0 (K )
Figure 3. VTF plots of ionic conductivity for [DIPEA][TFSI] (a) and [Pyrr][TFSI] (b). The solid lines represent the VTF fitting.
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Journal of The Electrochemical Society, 161 (3) A228-A238 (2014)
A233
Table IV. VTF equation parameters of conductivity and viscosity (T0 , σ 0 (mS cm−1 ), η0 (mPa s), Bi (K), R2 ) for [DIPEA][TFSI] and [Pyrr][TFSI].
Conductivity
Viscosity
Conductivity
Viscosity
[DIPEA][TFSI]
[Pyrr][TFSI]
σ0 or η0
B’σ or B’η
R2a
164
179
175
177
936
0.185
764
0.3289
891.2
775.2
673.9
600.0
0.9993
0.99967
0.9996
0.99946
coefficient
the number of ions present in the equivalent volume is the one deduced from the salt composition, i.e. all ions contribute equally.79
The position of the ideal line was established by using aqueous one
molar KCl solution. The two PILs studied in this work present a
good iconicity according to the Walden rule. Furthermore, they lie
significantly closer to the ideal line with unit slope than the aprotic ILs such as quaternary ammonium tetrafluoroborate.80 Besides,
[DIPEA][TFSI] and [Pyrr][TFSI] are closer to the “ideal line” (KCl)
than other PILs previously studied, like [Pyrr][HCOO], with Walden
product (η) equal to 0.5, 0.55 and 0.21 S cm2 mol−1 poise at 50◦ C,
respectively. This indicates that studied PILs have a higher ionic nature. From the almost identical Walden plots, it can be concluded that,
in this temperature range, the two PILs of the study have a similar
ionicity.
Electrochemical study.— Electrochemical Impedance spectroscopy
(EIS) on two-electrode cells.—Electrochemical Impedance Spectroscopy (EIS) measurements were conducted at open circuit voltage
(OCV) with a sinusoidal signal of 5 mV over the frequency range
from 1 mHz to 100 kHz. In general, the Z” = f(Z’) Nyquist plot of an
EDLC comprises three domains: i) the semicircle at high frequencies
is representative of the contact resistance between the active material
and the current collector; the equivalent resistance (sum of resistances
of active material, current collectors, electrolyte in the separator and
contact resistance) is the value obtained at ω→ ∞; ii) the middlefrequency region or Warburg region, represented by a 45◦ line, is
rather due to the frequency dependent resistance R(ω) associated with
the ionic resistance in the porosity;81 iii) the low-frequency region
corresponds to purely capacitive phenomena and is represented by a
quasi-vertical line;82 the associated resistance, named equivalent diffusion resistance (EDR), is obtained from the intercept of the line with
the Z’ axis.
Figure 6 compares the Nyquist plots of supercapacitor made
with CMC/SBR-AC electrodes in [Pyrr][TFSI] and [DIPEA][TFSI]
electrolytes with the [Et4 N][BF4 ] based electrolyte. In the case of
PILs electrolytes, the almost non-existent semi-circle at high frequency could be linked to the pores size (micro/mesoporous) and
the cations size83 (similar results - not shown here - were observed
with PVDF-AC electrodes on both PILs). The ESR values are re-
ported in Table V for each electrolyte and each activated carbon
electrode.
One can see that the values are lower in [Pyrr][TFSI] than in
[DIPEA][TFSI]; this could be explained by the higher conductivity
and lower viscosity of [Pyrr][TFSI],which lead to a more compact
(Helmholtz) layer of [Pyrr]+ and [TFSI]− ions next to the electrode
surface. The nature of cations is directly linked to their mobility, Van
Der Waals interactions and their polarizability (dipole moment)84–86
which control the viscosity. These two parameters are themselves
linked to the charge density and to the volume occupied by ions,
as calculated and presented in the Table I. Those two parameters
influence drastically the internal Helmholtz layer. Indeed, the ions
form a layer on the surface of material at a distance of about 1 nm. This
distance is considered as the layer thickness. It depends on the ion size
and on the voltage. This effect is more pronounced with CMC/SBRAC, which can be related by different wettability of the CMC/SBR-AC
based electrodes with the PILs. This aspect will be further discussed
with cycling tests. However, the ESR values obtained here in the pure
PILs are lower than the ones obtained in pure AILs based on TFSI
anion, such as [MPPip][TFSI], [MPPyr][TFSI] and [EMIm][TFSI],
which display ESR values of 58 , 30.2 and 15.4 , respectively,
but in the same range as in AILs mixed with propylene carbonate (PC)
or acetonitrile (ACN).87
Three-electrode cell configuration.—Figure 7 presents the threeelectrode cell cyclic voltammograms of PVDF-AC in the two PILs.
According to the Figure 7, and regarding the CVs shape, the working potential window can be estimated to 2 V and 1.7 V for [DIPEA][TFSI] and [Pyrr][TFSI], respectively. When the CVs deviate
from the rectangular shape, with oxidation and reduction peak, the
maximum operative voltage is exceeded.We have previously reported
that the cathodic limit observed in an ionic liquid is due to the reduction of the ammonium cation via its deprotonation and the reduction
of the obtained proton, to give nascent hydrogen that might further
bind either with the carbon surface or with another atom to form a
di-hydrogen molecule.
The reduction stability limit of both PILs does not follow the hyR
drogen potential reflected by their pKa. In fact, the lower is the Elim
the higher is the acidic character of the cation, as it can be seen in
60
160
700
600
Shear stress / Pa
140
η / mPa s
120
100
50
500
400
40
300
200
100
80
0
0
60
1000
300
(b)
30 °C
20 °C
2000
3000
4000
5000
-1
Shear rate / s
η / mPa s
(a)
35 °C
20 °C
250
Shear stress / Pa
a correlation
T0 / K
200
150
100
50
0
30
0
1000
2000
3000
4000
5000
-1
shear rate / s
20
40
10
20
20
30
40
50
Temperature / °C
60
70
20
30
40
50
60
70
Temperature / °C
Figure 4. Evolution of viscosity with temperature for [DIPEA][TFSI] (a) and [Pyrr][TFSI] (b). Inset: Shear stress versus shear rate at 30 (a) and 35◦ C (b) and
20◦ C (a and b).
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Journal of The Electrochemical Society, 161 (3) A228-A238 (2014)
50
2
Good Ionic
Liquid
[Pyrr][NO3]
3
40
-Im (Z) / Ohm cm
2
-1
Ln (Λ / S cm mol )
[DIPEA][TFSI]
[Pyrr][TFSI]
[Pyrr][HCOO]
[Pyrr][CH3COO]
[Pyrr][HSO4]
4
Super Ionic
Liquid
2
1
30
1.4
20
1.2
2
5
-Im (Z) / Ohm cm
A234
10
0
-1
0
1
2
3
-1
-1
Ln (η / poise )
4
0.4
0.6
0.8
1.0
1.2
1.4
Re (Z) / Ohm cm
10
20
30
40
50
2
Re (Z) / Ohm cm
Figure 6. Nyquist plot of a supercapacitor with CMC/SBR-AC electrodes in
[Pyrr][TFSI] () and [DIPEA][TFSI] (O) at 30◦ C. Inset: Nyquist plot of a
supercapacitor with CMC/SBR-AC electrodes in 1 mol.L−1 [Et4 N][BF4 ] in
ACN.
Table V. ESR obtained from the Nyquist plot at ω→ ∞ for each
selected electrolyte and electrode.
Table VI. This is in contradiction with the evolution which shows a
better stability in reduction of the [DIPEA]+ cation in presence of an
activated carbon electrode, which is presumably linked to the kinetic
of electronic transfer. Reactions in which electron and proton transfer
are performed were described either by two distinct steps Electron
Proton Transfer or Proton Electron Transfer (EPT or PET), or in the
same concerted step: Concerted Proton Electron Transfer (CPET).88
Contrary to simple proton or electron transfer, CPET is more complicated and the coupling at the activated carbon /molecule (cation)
interface influences thermodynamically and kinetically the process.
In a concerted mechanism, the electrons and protons are transferred
in the same supplementary step; this mechanism is defined by only
one transition state and consequently it allows high energy reaction
intermediates to be avoided. At the microscopic scale, the kinetic
description is not the same for both processes. The electron transfer dynamic is governed by the molecular dynamic, which includes
intramolecular reorganization, and also the solvent reorganization.
Consequently, potential energies of reactants (here cations) and products (here hydrogen and amine) can be described by the deformations
of the systems, both internal (variation of bond lengths and angles) and
external (solvent reorganization). In the case of the studied cations, the
dynamic transfer (electron and proton) which determines the reduction potential at the electrode is more favorable for the pyrrolidinium
cation because of its planar geometry, which is more accessible at
Electrode
Electrolyte
PVDF-AC
Rs ( cm2 )
CMC/SBR-AC
Rs ( cm2 )
[DIPEA][TFSI]
[Pyrr][TFSI]
1 mol L−1 [Et4 N][BF4 ] in ACN
4.75
4.4
1.07
6.2
4.6
1.0
the interface with activated carbon, than for the [DIPEA]+ cation.
This can be explained by an easy rearrangement of the pyrrolidinium
cation.
Two other parameters influence the comportment of electrolyte in
the EDLC system: the van der Waals volume of ions (Table I), which
has to adapt itself to the material porosity, and their polarizability
(Table VI). These two parameters are correlated, e.g., the volume of the
[DIPEA]+ cation (207.37 Å3 ) is twice higher than the one of the pyrrolidinium cation(106.01 Å3 ), but at the same time [DIPEA]+ displays
a higher polarizability (Table VI), which allows a better distortion in
order to adapt its size to the carbon pore size.89,90 Thus, the [DIPEA]+
cation counterbalances its higher size by its higher polarizability. This
believable hypothesis linking reduction stability limit to the geometry
and the volume of cations should be verified on a larger number of
2
2
(a)
(b)
1
-1
1
0
j/Ag
-1
45 °
0.2
2
5
Figure 5. Molar conductivity () versus fluidity (η−1 ) for [DIPEA][TFSI]
(red), [Pyrr][TFSI] (blue) and some other PILs.75 The solid line for Good
ionic liquids is the “ideal” Walden line obtained with 1 mol L−1 aqueous KCl.
j/Ag
0.4
0.0
0
-1
0.6
0.0
0
-2
-2
0.8
0.2
45°
Poor Ionic
Liquid
1.0
-1
-1
-2
-3
-1.5
0
-2
-1.0
-0.5
0.0
E / V vs. Ag
0.5
1.0
1.5
-3
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
E / V vs. Ag
Figure 7. Three-electrode cyclic voltammograms of a PVDF-AC electrode in [DIPEA][TFSI] (a) and [Pyrr][TFSI] (b), with Ag wire as pseudo-reference, at
5 mV s−1 , at 30◦ C.
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Journal of The Electrochemical Society, 161 (3) A228-A238 (2014)
0.8
0.6
0.8
(a)
PVDF-AC
(b)
0.4
-1
-1
0.2
j/Ag
j/Ag
PVDF-AC
0.6
0.4
0.0
0.2
0.0
-0.2
-0.2
-0.4
-0.4
-0.6
-0.6
0.0
0.5
1.0
1.5
2.0
2.5
0.0
0.5
1.0
E/V
2.0
2.5
0.8
(a)
CMC/SBR-AC
0.6
0.4
(b)
CMC/SBR-AC
0.4
-1
-1
0.2
j/Ag
I/Ag
1.5
E/V
0.8
0.6
A235
0.0
0.2
0.0
-0.2
-0.2
-0.4
-0.4
-0.6
-0.6
0.0
0.5
1.0
1.5
2.0
2.5
0.0
0.5
E/V
1.0
1.5
2.0
2.5
E/V
Figure 8. Cyclic voltammograms of two-electrode cells with active carbon (PVDF-AC or CMC/SBR-AC) on [DIPEA][TFSI] (a) and [Pyrr][TFSI] (b) at 5 mV
s−1 and at 30◦ C.
PILs and more generally ILs, as it was done in a previous work, an
exhaustive study of LiX salts in water.91
Cycling tests.—The symmetric AC/AC two-electrode cells were applied to evaluate the performance of the neat PIL electrolytes within
the voltage range from 0 to 2.5 V. Figure 8 shows the cyclic voltammograms (CVs) recorded at a scan rate of 5 mV s−1 , for the PVDF-AC
and CMC/SBR-AC electrodes; the CVs were performed at 25–30◦ C
in order to maintain the PILs in the liquid state.
The nearly rectangular shape of the CVs in the voltage range from 0
to 2 V characterizes typical capacitive properties. The less rectangular
shape in the case of [DIPEA][TFSI] reveals a worse charge propagation, which reflects the higher viscosity (110 mPa s), lower conductivity (1.2 mS cm−1 ) and higher volume of the cation (119.188 cm3
mol−1 ) for this PIL compared to 47 mPa s, 3.2 mS cm−1 and 53.12 cm3
mol−1 , respectively for [Pyrr][TFSI].49 Whatever the type of electrode,
either PVDF-AC or CMC/SBR-AC, in presence of [Pyrr][TFSI], the
shape of the cyclic voltammograms and the amplitude of the capacitive current are identical. By contrast, for [DIPEA][TFSI], a dramatic
decrease of capacitive current was observed when using CMC/SBRAC. This could be explained by the difference of binder between the
industrial electrodes (PVDF binder) and in the home-made electrodes
(CMC/SBR).
The humps observed in both electrolytes for a voltage scan up to 2.5
V, whatever binder, are the signature of pseudocapacitive phenomena
related with hydrogen storage in the negative carbon electrode. It
is worth to note that the faradaic response is more pronounced in
the case of [Pyrr][TFSI]. This property must be correlated with the
R
potential for
previous observation in Figure 7 showing a lower Elim
[Pyrr][TFSI], and resultant better ability for [Pyrr]+ oxidation and
hydrogen storage during the positive scan, and hydrogen reduction
during the negative one. In a recent work92 we clearly showed that
this pseudo-capacitive storage reaction of hydrogen is linked to the
residual water content. When using more dry PIL ([Et3 NH][TFSI]),
in the same conditions, no effect was observed on activated carbon.
The difference of electrochemical performance with the nature of
the binder in case of [DIPEA][TFSI] can be explained by the difference of wettability of the electrodes by the electrolyte, as shown
on Figure 9. It is obvious that the wettability of [DIPEA][TFSI] is
Activated Carbon electrode with
PVDF binder
Activated Carbon electrode with
CMC/SBR binder
t = 0 ms
t = 0 ms
t = 700 ms
t = 700 ms
Figure 9. Wettability and contact angle snapshots of [DIPEA][TFSI] on
PVDF-AC (left) and on CMC/SBR-AC electrodes (right), at t = 0 ms (top)
and after 700 ms (down).
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A236
Journal of The Electrochemical Society, 161 (3) A228-A238 (2014)
Table VI. Reduction potential, Ered , hydrogen potential, pKa, and polarizability of [DIPEA]+ and [Pyrr]+ cations.
Cation
Reduction potential Ered (V vs. NHE)
−1.1
−0.8
pKa*
10.73
11.40
Polarizability
16.96
8.83
*pKa characteristic of RN/RNH+
the specific capacitance, Csp , in farad per gram of active material
(Fg−1 ) is related to the capacitance of the cell Csp = 2 C/m, where m
is the mass of active material (g) per electrode.
The energy density (E) and power density (P) of an EDLC electrode were calculated with the equations, E max = 12 Csp (Vmax )2
E
and Pmax = t
, respectively. The values of specific capacitance, energy and power derived from AC (PVDF-AC and CMC/SBR-AC)
electrodes in both PILs are presented in Table VII. In all cases, the
capacitance increase with voltage, especially from 2 V to 2.5 V, can be
attributed to a pseudocapacitive contribution, e.g., hydrogen storage,
in particular for the pyrrolidinium based PIL. With the PVDF-AC
electrodes, at E = 1.6 V and 2 V, the values of capacitance are comparable in [Pyrr][TFSI] and [DIPEA][TFSI]electrolytes. By contrast,
a higher difference is observed on the CMC/SBR-based electrodes,
with 82 F g−1 and 55 F g−1 , respectively, at 2 V. The almost 30% lower
capacitance on the CMC/SBR-AC electrodes for the [DIPEA][TFSI]
electrolyte compared with [Pyrr][TFSI] could be explained by the
wettability and almost 30% higher contact angle of [DIPEA][TFSI]
compared with [Pyrr][TFSI] on the CMC/SBR-AC electrodes. However the wettability is not the only parameter to take into account, the
size of cations could also influence the capacitance.
The effect of temperature on specific capacitance is shown in
Table VIII for both synthesized PILs on PVDF-AC electrodes. The
capacitance increases from 30 to 50◦ C, for the three different electrochemical domains. This improvement of electrochemical performance
can be explained by the increase of conductivity and fluidity toward
the porosity of AC with temperature.
higher on PVDF-AC than on CMC/SBR-AC electrodes. When the
drop falls on the electrode surface, the contact angle (θ) is 74◦ ±
4◦ on PVDF-AC electrode and 98◦ ± 5◦ on CMC/SBR-AC electrode. By contrast, the contact angles of [Pyrr][TFSI] on PVDF-AC
and CMC/SBR-AC electrodes are in the same range, with 80◦ ± 5◦
and 74◦ ± 3◦ , respectively. Moreover, after a time of drop diffusion
(700 ms), the wettability of [DIPEA][TFSI] is completely different
on both electrodes, as the drop almost disappears on the PVDF-AC
electrode, with a total wettability, while the drop is still visible on the
CMC/SBR-AC electrode with a non-negligible contact angle. After
700 ms, for [Pyrr][TFSI], the wettability is good on both electrodes.
The presence of alkyl substituents on [DIPEA] cation, with high van
der Waals interactions, can explain the bad wettability on CMC/SBR
based electrodes for [DIPEA][TFSI] electrolyte.
To evaluate the capacitance values in presence of the two studied PILs, galvanostatic (200 mA g−1 ) charge-discharge cycles were
recorded on two-electrode cell at various values of maximum voltage,
i.e., 1.6, 2.0 and 2.5 V (Figure 10). The difference of ohmic drop
between the two kinds of PILs, being larger with [DIPEA][TFSI] than
with [Pyrr][TFSI] is easily interpreted by their difference of conductivity.
The cell capacitance was calculated from the slope of the discharge
curve V/t, in volt per second (V s−1 ) (excluding the IR drop),
according to.C = I t/V, where C is the capacitance of the cell in
farad (F), I the discharge current in ampere (A). The cell capacitance
was also calculated from integration method and uncertainty between
the two ways of calculation is less than 5%. In a symmetrical system,
where the mass of active material is the same for the two electrodes,
(a)
2.0
1.5
E/V
1.5
E/V
PVDF-AC
CMC/SBR-AC
(b)
2.0
PVDF-AC
CMC/SBR-AC
1.0
1.0
0.5
0.5
0.0
0.0
0
100 200 300 400 500 600 700 800 900 1000
Time / s
0
100 200 300 400 500 600 700 800 900 1000
Time / s
Figure 10. Galvanostatic (200 mA g−1 ) charge-discharge at 30◦ C, for the synthesized PILs: [DIPEA][TFSI] (a) and [Pyrr][TFSI] (b) on the two kinds of electrodes.
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Journal of The Electrochemical Society, 161 (3) A228-A238 (2014)
A237
Table VII. Specific capacitance (Csp / F g−1 ), Specific energy (Emax / Wh kg−1 ), and specific power (Pmax / kW kg−1 ) for the AC-based electrodes
in neat PILs [DIPEA][TFSI] and [Pyrr][TFSI], at 30◦ C (j = 200 mA g−1 ).
V = 1.6 V
Maximum voltage
V = 2 V
V = 2.5 V
Electrolytes
Electrodes
Csp
Emax
Pmax
Csp
Emax
Pmax
Csp
Emax
Pmax
[DIPEA][TFSI]
PVDF-AC
CMC/SBR-AC
93
47
33.2
16.8
0.39
0.19
100
55
55.6
30.4
0.52
0.28
105
64
91.2
55.6
0.68
0.41
[Pyrr][TFSI]
PVDF-AC
CMC/SBR-AC
98
73
34.8
26
0.40
0.30
107
82
59.6
45.6
0.55
0.42
127
nd
110.4
nd
0.81
nd
Table VIII. Specific capacitance (Csp / F g−1 ) for neat PILs [DIPEA][TFSI] and [Pyrr][TFSI] on PVDF-AC electrodes, at three different
temperatures (j = 200 mA g−1 ).
Maximum voltage
T /◦ C
V = 1.6 V
Csp
V = 2 V
Csp
V = 2.5 V
Csp
[DIPEA][TFSI]
30
50
80
93
116
98
100
132
102
105
150
nd
[Pyrr][TFSI]
30
50
80
98
162
153
107
165
162
127
176
175
Electrolytes
By contrast, at 80◦ C, the capacitance decreases dramatically for
the [DIPEA][TFSI] electrolyte, which can be attributed to the thermal decomposition of [DIPEA] cation93 enhanced by the polarization
effect. However, this effect was not observed for [Pyrr][TFSI] electrolyte, which exhibits almost similar capacitance at 80◦ C and 50◦ C.
According to this observation, one can conclude that the latter is more
stable at high temperature under polarization than [DIPEA][TFSI].
Capacitances were also evaluated at different current densities, 500,
870 and 1200 mA g−1 , for [DIPEA][TFSI], the capacitance decreases
from 50 to 25 F g−1 . For [Pyrr][TFSI], capacitances evolved from
75 to 28 F g−1 , for current densities increasing from 1.5 to 3 A g−1 .
For those electrolytes there is a poor maintain of capacitance when
increasing current densities, which could be attributed to the cation
size.
electrode, in particular on the binder. The comparison between linear
and cyclic ammonium allows noticing how it affects the performances
and the physico-chemical properties.
Acknowledgments
This research was supported by the Conseil de la Région Centre
through the Sup’Caplipe project and by the “Polonium” project PHC
27731 VK. The Foundation for Polish Science is acknowledged for
supporting the ECOLCAP project realized within the WELCOME
program, co-financed from European Union Regional Development
Fund. Many thanks to Batscap (France) for providing the electrode
material.
References
Conclusions
In summary, this work reports on the preparation, comparative
characterization and application of neat PILs based on [TFSI]− anion,
[DIPEA][TFSI] and [Pyrr][TFSI], as electrolyte for supercapacitor.
The theoretical study with Density Functional Theory (DFT) allows a
comparison of the two studied PILs in terms of cation structure. The
thermal study shows eutectic with residual water in PILs, and crystallization and melting points for [DIPEA][TFSI] and [Pyrr][TFSI]. The
conductivity and viscosity dependences with temperature were then
evaluated for both PILs. The temperature dependences of those transport properties could be described by the VTF equation. Ionicity was
then evaluated thanks to the Walden diagram, showing that both synthesized PILs are conforming to the Walden rule and lie significantly
close to the ideal line.
Subsequently, the electrochemical behavior of both PILs was assessed on activated carbon electrodes (PVDF-AC and CMC/SBRAC). The EIS measurements show lower ESR for [Pyrr][TFSI] than
for [DIPEA][TFSI], in particular for CMC/SBR-AC electrodes. The
electrochemical window was measured on activated carbon with 1.6
and 2 V, respectively for [DIPEA][TFSI] and [Pyrr][TFSI]. Galvanostatic and voltammetry tests were performed to compare the activity
of both PILs, regarding capacitance, energy and power. The effect of
binder was compared on both PILs electrolytes showing a difference
of wettability. To conclude, the nature of the cation in PILs influences
the pH, the electrochemical stability, but also the wettability on the
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