This journal is c the Owner Societies 2012 Phys. Chem. Chem. Phys., 2012, 14, 8199–8207 8199

Cite this: Phys. Chem. Chem. Phys., 2012, 14, 8199–8207

Triethylammonium bis(tetrafluoromethylsulfonyl)amide protic ionic liquid

as an electrolyte for electrical double-layer capacitorsw

Laure Timperman,*aPiotr Skowron,

abAurelien Boisset,

aHerve Galiano,

c

Daniel Lemordant,aElzbieta Frackowiak,

bFrancois Beguin

band Meriem Anouti*

a

Received 1st February 2012, Accepted 28th March 2012

DOI: 10.1039/c2cp40315c

This study describes the preparation, characterization and application of [Et3NH][TFSA], either

neat or mixed with acetonitrile, as an electrolyte for supercapacitors. Thermal and transport

properties were evaluated for the neat [Et3NH][TFSA], and the temperature dependence of

viscosity and conductivity can be described by the VTF equation. The evolution of conductivity

with the addition of acetonitrile rendered it possible to determine the optimal mixture at 25 1C,

with a weight fraction of acetonitrile of 0.5. This mixture was also evaluated for transport

properties, and showed a Newtonian behavior, as the neat PIL. An electrochemical study

demonstrated, at first, a passivation on Al after the second cyclic voltammogram. Subsequently,

the electrochemical window was estimated using a three-electrode cell to 4 V on a platinum

electrode, and to 2.5 V on activated carbon. Finally, the neat PIL was found to exhibit good

performances as promising electrolyte for supercapacitor applications.

1. Introduction

Ionic liquids (ILs) are organic molten salts with melting points

below 100 1C. They have been extensively investigated for

various applications,1–10 because of their unique physicochemical

properties, such as the favorable solubility of organic and inorganic

compounds, relatively high ionic conductivity, low vapor pressure,

high thermal stability and low flammability. By modifying the

cations and anions according to the nature of the desired

reactions, the properties of ILs can be customized.11 Thus, ILs

can be named ‘‘designer solvents’’. The ability to carefully and

predictably control physical properties has led to a vast number

of papers concerning the use of ILs as solvents.1,12–16

ILs can be classified into two groups: protic (PILs) and

aprotic ionic liquids (AILs).17,18 PILs are synthesized by mixing

equimolar amounts of a Brønsted acid with a Brønsted base.19–21

The proton transfer from the acid to the base creates proton

donor and acceptor sites and can lead to the formation of

hydrogen bonds.21 Due to the recent interest in proton-

conducting electrolytes, PILs appear as promising materials for

batteries, fuel cells, solar cells, actuators, or double-layer capacitors.

Supercapacitors have attracted increasing interest because

of their high power storage capability, which is highly desirable

for applications in electric vehicles (EVs) and hybrid electric

vehicles (HEVs).22–24 To deliver the high power needed during

the acceleration and to recover the energy during braking,

supercapacitors can be coupled with fuel cells or batteries.

Supercapacitors have two energy storage mechanisms: the

electrical double-layer (EDL) and pseudocapacitance.25–28

Carbon-based capacitors have been devoted great attention

since these materials possess diversified structures/nanotextures

with high stability and conductivity.29–35 Activated carbons

(ACs) are the most commonly used electrode materials for

EDLC applications, because of their relatively low cost and

high surface area in comparison with other carbon-based

materials.

Another factor that influences the properties of a super-

capacitor is the selected electrolyte. Currently, aqueous solutions

have been the most utilized.36–39 However, the main drawback of

the acidic and basic electrolyte-based supercapacitors is that

they display a narrow cell voltage and low energy density.40 By

contrast, higher voltage values are observed with neutral

aqueous electrolytes either in symmetric AC/AC capacitors37

or in asymmetric systems.38 The power depends on the equivalent

series resistance (ESR), which is affected by the electrode nature

and the electrolyte conductivity. A proper choice of the electro-

lyte is therefore essential.

Ionic liquids containing ammonium cations have been

extensively studied. In particular, the PIL [Et3NH][TFSA]

has been investigated in fundamental research, such as the

preparation of ILs with a new, mild and expedient method,41

aUniversite Francois Rabelais, Laboratoire PCM2E,Parc de Grandmont 37200 Tours, France.E-mail: meriem.anouti@univ-tours.fr,laure.timperman@univ-tours.fr; Fax: +33 247367073;Tel: +33 247366951, +33 247367156

b Institute of Chemistry and Technical Electrochemistry,Poznan University of Technology, Piotrowo 3, 60-965 Poznan, Poland

c CEA, DAM, le Ripault, F-37260 Monts, Francew Electronic supplementary information (ESI) available. See DOI:10.1039/c2cp40315c

PCCP Dynamic Article Links

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8200 Phys. Chem. Chem. Phys., 2012, 14, 8199–8207 This journal is c the Owner Societies 2012

for the dynamic solvation in ILs,42 critical properties,43 and

dynamical and structural information.44 To the best of our

knowledge, this PIL has not been reported for supercapacitor

applications, but it has been the subject of electrochemical and

non-electrochemical studies, like fuel cells,45,46 batteries,47–49

irradiation and its consequence on PILs,50,51 the emissions of

Taylor cones from ILs52 and catalysis.53,54

We have recently reported on PILs, i.e., pyrrolidinium

nitrate55 and phosphonium tetrafluoroborate56,57 for super-

capacitor applications. The present work presents the preparation

and characterization of a triethylammonium bis(tetrafluoro-

methylsulfonyl)amide PIL. The physicochemical properties

were explored, as well as the thermal characteristics and

transport properties for the PIL, both neat and in a mixture

with acetonitrile. The temperature effect of those transport

properties is also reported. Finally, the electrochemical behavior

of the PIL, both pure or together with acetonitrile, has been

evaluated in three- or two-electrode cells. The obtained

results were compared with those of tetraethylammonium

tetrafluoroborate in acetonitrile, used as reference.

2. Experimental section

2.1. Materials

Triethylamine (Z 99%) and hydrochloric acid (37%) were

purchased from Sigma Aldrich and used without further

purification. The lithium bis(tetrafluoromethylsulfonyl)amide

(LiTFSA, 99.9%) was obtained from Solvionic. The anhydrous

acetonitrile (99.8%) and 1,2-dichloroethane (DCE, >99%) were

purchased from Sigma Aldrich. The water was purified using a

Milli-Q 18.3 MO water system.

2.2. Preparation of the PIL

Triethylammonium bis(tetrafluoromethylsulfonyl)amide [Et3NH]-

[TFSA] PIL was synthesized using a metathesis method. First,

triethylamine (16.38 g; 0.1602 mol) was introduced into a

three-necked round-bottom flask immersed in an ice bath

and topped by a reflux condenser. A dropping funnel was

utilized to add the acid, and a thermometer to control the

temperature. A hydrochloric acid solution 37% (15.80 g;

0.1602 mol) was added dropwise to the amine under vigorous

stirring (30 min). As the acid–base reaction was slightly

exothermic, the ice bath was used to maintain the temperature

below 25 1C. After addition of all the acid to the amine,

stirring was maintained for 2 h at room temperature, before

charging 165 g of DCE. Subsequently, in order to remove the

water, the mixture was distilled under normal pressure until

the water–DCE hetero-azeotropic boiling point was reached

(346 K). Residual DCE was finally evaporated under reduced

pressure enabling a white solid to be collected.

Subsequently, triethylammonium chloride (22.06 g; 0.16 mol)

was mixed with 22 mL of water, and maintained under stirring

until total dissolution. The LiTFSA (48.30 g; 0.16 mol) was

also mixed in about 45 mL of water. Those two solutions

were then blended together and the stirring was continued for

2 hours. This process enabled replacing the chloride anion by

the TFSI anion. Two phases were obtained, with the aqueous

phase at the top and the PIL below. For a better separation of

these two phases, approx. 10 mL of chloroform was added,

and the product was introduced into a separating funnel. The

PIL diluted in chloroform was separated from the aqueous

phase, and then washed with small amounts of water, in order

to eliminate the chloride, as LiCl, formed during the metathesis

process. After being washed with water several times, tests were

performed with AgNO3 on the aqueous phase, in order to

determine whether there remained any chloride in the PIL. In

fact, silver reacts with the chloride to form AgCl, which is a

well-known white precipitate, and which darkens upon exposure

to light. Thus, the absence of this solid leads to the supposition

that almost all chloride had been removed from the PIL. After

the purification process, chloroform was evaporated under

reduced pressure. Finally, the PIL was dried under a high

vacuum using a liquid nitrogen trap, during two or three days.

The PIL was analyzed for water content using coulometric

Karl-Fisher titration, and was found to contain approximately

100 or 200 ppm, just after drying. However, at equilibrium,

i.e., in normal use outside of a glove box, the water content

would remain constant at around 400–600 ppm.

2.3. Measurements

A Crison (GLP 31) digital multifrequency conductimeter was

utilized to measure ionic conductivities. The temperature control,

from 25 to 80 1C, was ensured by a JULABO F25 thermostated

bath, with an accuracy of �0.2 1C. The conductimeter was

calibrated using standard solutions of known conductivity

(0.1 and 0.01 mol L�1 KCl); the uncertainty for the conduc-

tivities did not exceed �2%. Each conductivity was recorded

when the stability was superior to 1% within 2 min. Differential

scanning calorimetry (DSC) was carried out on a Perkin-Elmer

DSC 4000 under a nitrogen atmosphere, coupled with an

Intracooler SP VLT 100. The samples for the DSC measure-

ments were sealed in Al pans. They were first heated from 20 to

100 1C at a scan rate of 5 1C min�1, after which thermograms

were recorded during cooling, from 100 to �60 1C with a scan

rate of 2 1C min�1, and during a second heating run, from �60to 100 1C at a scan rate of 10 1C min�1. Vapor pressure

measurements were performed using an isobaric Vapor–Liquid

Equilibrium (VLE). Details of the design of the apparatus were

previously described by Husson et al.58

Electrochemical measurements were carried out on a Versatile

Multichannel Potentiostat (Biologic S.A) piloted by an EC

Lab V9.97 interface. The potential window of the PIL was

measured by cyclic voltammetry using a three-electrode cell,

with platinum as the working electrode, a stainless steel grid as

the counter electrode and a silver wire as the reference electrode.

Galvanostatic charge–discharge experiments and cyclic voltam-

metry were conducted using a symmetric AC/AC Teflon

Swageloks-type two-electrode cell. A glass microfiberWhatmans

filter paper (thickness h=675 mmand pore diameterØ=2.7 mm)

was utilized as separator. The activated carbon electrode

material (12 mm diameter, 7.2 mg, with an active mass of

5.76 mg) was kindly supplied by Batscap. This carbon material

exhibits a specific surface around 1500 m2 g�1 (BET), with

high microporosity (do 1 nm). It was coated on aluminium as

current collector, using PVDF as binder. The temperature was

controlled at 50 and 80 1C by a thermostatic oven.

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3. Results and discussion

3.1. Physical properties of the PIL

3.1.1. Thermal properties. The thermal behavior of the

studied PIL [Et3NH][TFSA] was investigated by DSC from

�60 to 100 1C. Fig. 1 presents a thermogram showing the

different phase transitions from�60 to 300 1C. The characteristic

temperatures determined from the maximum of the respective

peaks are listed in Table 1.

First, the sample was heated from 20 to 100 1C at 10 1C min�1,

and then cooled from 100 to �60 1C at a lower heating scan,

2 1C min�1 to be more precise for the crystallization peak.

Thus, a crystallization peak occurred at �29 1C (244 K).

Subsequently, the sample was maintained at �60 1C for

3 minutes, and heated to 300 1C.

On the thermogram two peaks were observed at�9 1C (264 K)

and �0.8 1C (272.2 K). The shape of those peaks corresponds

typically to the melting of a binary mixture of immiscible solids

leading to the formation of an eutectic. As the [Et3NH][TFSA]

PIL contains a relatively important amount of water (o600 ppm,

3.72 mol L�1), the first peak at �9 1C (Te) corresponds to the

PIL–H2O eutectic fusion and the second to the liquidus line,

meaning that at Tm all PIL is melted. The melting temperature

was close to the one found by Matsumoto et al.49 for

[Et3NH][TFSA], with a difference of only 3 1C. Moreover, this

value was lower than the temperature values observed for other

PILs based on a triethylammonium cation, like [Et3NH][BF4],

with 103.4 1C, [Et3NH][NO3], with 113–114 1C, [Et3NH][HSO4],

with 84.2 1C, or [Et3NH][CH3SO3], with 21.6 1C.18 An aprotic

IL, with a TFSI anion [Et4N][TFSA], also exhibits a higher

melting point than the synthesized PIL, with 105–114 1C.42,49,59

Thus, it can be seen that the heat capacity of the PIL

[Et3NH][TFSA], 0.56 J K�1 g�1 (214 J K�1 mol�1), was lower

than the corresponding values of aprotic ILs, such as

[BMIm][BF4] with 1.66 J K�1 g�1,60 and protic ILs, based

on pyrrolidinium cations, such as [Pyrr][NO3], [Pyrr][HSO4],

[Pyrr][HCOO], [Pyrr][CH3COO], [Pyrr][CF3COO], with 1.70,

1.54, 1.55, 1.50 and 2.25 J K�1 g�1, respectively.61 Besides, the

heat capacity of the studied PIL is in the same range than

[Pyrr][C7H15COO], with 0.45 J K�1 g�1.61 The crystallization

and melting enthalpy values were close; this difference is due to

the incertitude for the melting enthalpy, because of the inter-

section of the two peaks corresponding to the melting point of

the eutectic (Te) and the pure PIL (Tm). Those results allowed

considering the use of this PIL from 0 to 300 1C. The DSC of

PIL–acetonitrile mixture was presented in (ESIw, Fig. S2).Vapor pressure determines the volatility of a solvent. It

governs the exchange rate of ILs across the vap–liq interface.

The greatest difficulty and uncertainty arise in the determination

of the vapor pressure of low volatile compounds like ILs. In this

study vapor pressure measurements were performed using an

isobaric Vapor–Liquid Equilibrium (VLE). Protic ionic liquids

are volatile by their nature because the acidic proton can be

abstracted by the basic anion at room temperature. The

acid–base equilibrium for the abstraction reaction allows the

formation of neutral molecular species that readily evaporate. In

fact, the first published vapor pressures on ionic liquid mixtures,

by Wilkes et al., brought into the same type of chemical

equilibrium.62 In this study for [Et3NH][TFSA], Pv (22 1C) =

7.5 mbar (Table 1). This value is lower than the Pv(H2O) =

26 mbar, Pv(CH3CN) = 102 mbar or Pv(EtOH) = 79 mbar,

but higher than propylene carbonate Pv(PC) = 0.04 mbar,

or than an extremely low vapor pressure of 0.1 � 10�12 mbar

(100 pPa) for AILs like [C2mim][TFSA] at 25 1C.63

3.1.2. Transport properties of the PIL

3.1.2.1. Conductivity. At 25 1C, the conductivity of [Et3NH]-

[TFSA] was found to be 5 mS cm�1, which was not far from

the value of 4.4 mS cm�1, at 25 1C reported by Matsumoto

et al.49 This conductivity was somewhat higher than for other

PILs based on pyrrolidinium, for example, with the TFSI anion,

which exhibited a conductivity around 2.2 or 1 mS cm�1.64

The temperature dependence of the ionic conductivity for the

synthesized PIL [Et3NH][TFSA] is presented in Fig. 2.

Fig. 1 DSC thermogram showing the different phase transitions of

[Et3NH][TFSA].

Table 1 Thermal properties of the PIL: crystallization temp. (Tc), eutectictemp. (Te), melting point (Tm), isobaric heat capacity (Cp), crystallizationenthalpy (DHc), melting enthalpy (DHm), and vapor pressure (Pv)

Tc/1C

Te/1C

Tm/1C

Cp (25 1C)(J K�1 mol�1)

DHc/kJmol�1

DHm/kJmol�1

Pv (22 1C)(mbar)

�29 �9 �0.8 214 5 7.7 7.5Fig. 2 Influence of temperature on the conductivity of the

[Et3NH][TFSA]. The solid line serves as a guide to the eye.

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As expected, the conductivity increases with the temperature,

to reach 20 mS cm�1 at 80 1C. Moreover this PIL presents a

residual conductivity of 0.7 mS cm�1 at �10 1C. The same

aspect has been observed previously for morpholonium-based

PILs with a residual conductivity of 1 or 1.5 mS cm�1 at�10 1Cfor N-ethylmorpholonium formate and N-methylmorpholonium

formate, respectively.65

As shown in Fig. 3a, the pure PIL exhibits a non-Arrhenius

(eqn (1)) behavior, wherefore the Vogel–Tamman–Fulcher

(eqn (2)) was used to determine the temperature dependence

of conductivity.

s ¼ s0 exp�B1

T

� �ð1Þ

s ¼ s0 exp�B01

T� T0

� �ð2Þ

Here, s0 (mS cm�1), B1 (K), B01 (K) and T0 (K) are fitting

parameters. The product B1R or B01R (where R is the molar gas

constant) has the dimension of the activation energy (kJ mol�1).

Fig. 3b represents the VTF plot. According to eqn (2), fitting

parameters could be determined for the [Et3NH][TFSA], and are

presented in Table 2.

3.1.2.2. Viscosity. The viscosity has a strong effect on the

rate of mass transport within the solution, which is why it is an

important parameter for electrochemical studies. As mentioned

previously, the viscosity can be influenced by several parameters

such as the anionic species, a higher alkalinity, size, relative

capacity to form hydrogen bonds, van der Waals interactions

and size of the cation.66 In the present work, the viscosity of the

pure PIL was 39 mPa s at 25 1C. The same PIL has previously

been reported to present a viscosity of 48 mPa s, at 25 1C,49 which

is a comparable value. This viscosity is in the same range as that

of for example pyrrolidinium-based PILs with TFSA anions, with

55–53 mPa s.64 Other pyrrolidinium-based PILs with TFSA

anions have been reported to have higher viscosities, at 20 1C,

i.e., 72.5 and 95.1 mPa s,67 as also exhibited by imidazolium-based

PILs with TFSA anions, with 90 and 117 mPa s.42

As expected, Fig. 4 shows that viscosity decreases as the

temperature increases from 20 to 70 1C. This effect is due to

the higher mobility of ions. The inset in the figure depicts a

linear relation between the shear stress and the shear rate.

This observation leads to the conclusion that the PIL has a

Newtonian behavior in this temperature range.

The VTF equation (eqn (3)) was used to represent the

temperature dependence of viscosity for the synthesized PIL.

Z ¼ Z0 exp�B02

T� T0

� �ð3Þ

Here, Z0 (mPa s), B02 (K) and T0 (K) are fitting parameters.

The product B02R has the dimension of the activation energy

(kJ mol�1). According to eqn (3), fitting parameters could be

determined for [Et3NH][TFSA], and are presented in Table 2,

compared with those obtained from the VTF fitting of the

ionic conductivity. First, one can observe that, for this pure

PIL, the fitted temperature T0 is similar for these two different

properties, conductivity and viscosity, with 185 K and 175 K,

respectively. Secondly, the parameter B0i, which is related to

Fig. 3 Arrhenius (a) and VTF (b) plots of ionic conductivities for the PIL

[Et3NH][TFSA]. The solid lines represent the Arrhenius or VTF fitting.

Table 2 VTF equation parameters for the conductivity and theviscosity (T0, s0, Z0, B0 i) of the pure PIL [Et3NH][TFSA]

VTF equation parameters T0/K s0/mS cm�1, Z0/mPa s B0/K R2 a

Conductivity 185 345.7 465 0.9994Viscosity 175 2.53 � 10�3 898 0.9987

a Correlation coefficient.

Fig. 4 Influence of temperature on the viscosity of [Et3NH][TFSA].

Inset: shear stress versus shear rate at 20 1C.

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the activation energy, is lower for conductivity than for

viscosity, which means that the energy is also lower.

3.1.3. Transport properties of the PIL in the mixture with

acetonitrile. Fig. 5 shows the evolution of conductivity with the

addition of acetonitrile to the PIL, at 25 1C. At first, one can see

that the conductivity increases quickly to reach a maximum at a

weight fraction of acetonitrile of wCH3CN= 0.5 (1.36 mol L�1),

with s=50mS cm�1. Subsequently, the conductivity drops very

fast to reach, by extrapolation, the value of pure acetonitrile.

The optimal PIL mixture, obtained at 25 1C, was then

evaluated for the temperature dependence of conductivity

(Fig. 6a) and viscosity (Fig. 6b).

As expected, the conductivity increases with temperature,

from 50 mS cm�1 to 83.2 mS cm�1, at 80 1C. Fig. 6b shows

that the viscosity decreases when the temperature is raised

from 20 to 70 1C. The inset displays a linear relation between

the shear stress and the shear rate. Thus, one can conclude that

this PIL had a Newtonian behavior in the mixture with

acetonitrile (wCH3CN= 0.5).

3.2. Electrochemical study

3.2.1. Behavior of aluminium in the PIL electrolyte. The

electrodes supplied by Batscap are made of aluminium coated

by activated carbon. The native passive Al2O3 layer existing on

the metal surface provides protection against corrosion. It is

well known that, depending on the nature of the electrolyte

anions, this passive layer can be broken leading to the corro-

sion of Al at high potential.

The corrosion behavior of Al was evaluated in the PIL

[Et3NH][TFSA] electrolyte. Fig. 7 shows cyclic voltammograms

of the Al current collector in [Et3NH][TFSA], from the first to the

fourth cycle. The first cycle illustrates a hysteresis loop initiated

at about 0.25 V vs. Ag, with a large irreversible current peak

(1 mA cm�2) at about 1.1 V vs. Ag. For the second cycle, and

the following, the initiation potential of the hysteresis loop was

significantly shifted to the more anodic side (1.25 V vs.Ag), while

the current sharply decreased (0.1 mA cm�2). This observation

shows that aluminium is easily oxidized in [Et3NH][TFSA] since

0.5 V vs. Ag and that the result of this oxidation leads to the

formation of a passive layer on the electrode surface. This layer is

able to prevent further oxidation, when its thickness is sufficient.

The mechanism involved is analogous to the one performed in

aqueous media demonstrating anodic passivation on aluminium.

Aluminium ions as Al3+ are precipitated on the electrode surface

as Al(OH)2, Al2O3, nH2O or AlOOH, all hydroxyl groups have

been provided by traces of water present in the PIL.

3.2.2. Three-electrode cell configuration. Fig. 8 presents

cyclic voltammograms recorded in a three-electrode cell configu-

ration using a platinum wire as the working electrode. For the

PIL, the working potential window was near 4 V, considering

Fig. 5 Evolution of conductivity, s, as a function of the weight

fraction of CH3CN added to the PIL [Et3NH][TFSA] at 25 1C.

Fig. 6 Influence of temperature on the conductivity (a) and viscosity

(b) of [Et3NH][TFSA] with CH3CN (wCH3CN= 0.5). Inset: shear stress

versus shear rate at 20 1C.

Fig. 7 Cyclic voltammograms of the Al current collector in

[Et3NH][TFSA] at 20 1C. Scan rate: 5 mV s�1.

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current density less than 1 mA cm�2. The reversible peaks

observed (a and b) in Fig. 8, at 0.6 V vs. Ag (a) and �0.5 V vs.

Ag (b), could be attributed to the presence of a small amount

of water (oxidation/reduction of H+/H2) in the PIL after

purification (around 400–600 ppm).

It has been previously reported that the cathodic limit observed

in an ionic liquid is due to the reduction of the cations.68,69 Such a

reduction of ammonium cations proceeds by the CE (Chemical–

Electrochemical) mechanism. First, the deprotonation of the

triethylammonium cation occurs (eqn (4)), followed by the proton

reduction (eqn (5)). For the anodic limit, the ammonium could

in this case be oxidized in the presence of oxygen.

Et3NH+ ! Et3N + H+ (4)

H+ + 1e� ! 1/2H2 (5)

Both oxidation and reduction reactions are catalyzed on an

activated carbon electrode, as demonstrated in Fig. 9. This

figure represents cyclic voltammograms recorded on a symmetric

cell (Swageloks), with activated carbon as the working and

counter electrodes, and Ag wire as the reference. Compared to

a platinum electrode, one can easily see the capacitive current on

the activated carbon, and a smaller potential window reaching

almost 3 V. This domain was used as operating voltage, and

2 V was also taken for comparison.

3.2.3. Cycling tests. Test cycling performances were

obtained for two-electrode cells, stored at several temperatures:

20, 50 and 80 1C, both for the neat PIL [Et3NH][TFSA] and in

the mixture with acetonitrile. Fig. 10 exhibits the voltammo-

grams recorded for neat [Et3NH][TFSA], compared with those

of the mixture in acetonitrile, with the optimal mixture obtained

at 25 1C, ca. wCH3CN= 0.5. These curves were recorded in a

voltage domain between 0–2 V and 0–3 V.

First, one can see that for the curves of [Et3NH][TFSA] in

CH3CN, the shape is more rectangular than for neat PIL. This

effect is mainly due to the higher fluidity (lower viscosity,

higher mobility) of the PIL in acetonitrile than for the neat

PIL. However, for a domain of 2 V, the capacitance is not

really affected by the presence of acetonitrile, with 125 F g�1

for neat PIL and 128 F g�1 for the PIL in CH3CN, at 20 1C

(Table 3).

However, for a domain of 3 V, the capacitance was higher

when the PIL is mixed with acetonitrile. In fact, the capa-

citance increases from 144 F g�1 for neat PIL to 164 F g�1 for

the PIL in CH3CN, at 20 1C. Nevertheless, as shown in

Fig. 10, this increase in capacitance is mainly due to the

oxidation of acetonitrile which occurs above 2.7 V.70 For the

neat PIL, no oxidation peak is clearly detected in this range,

leading to the conclusion that a higher electrochemical window

could be reached. For the latter, a reversible wave appeared

centered at around 2.2 V during anodic scan, and around 1.2 V

during the cathodic one. Such a phenomenon is attributed to

hydrogen storage through proton reduction, as described in

eqn (5).

Self-discharge tests were realized (ESIw, Fig. S3) on the neat

PIL and in the mixture with acetonitrile, at two different

charge maximum voltages (2 and 3 V). For the neat PIL, the

self-discharge is 40 and 50% for DV= 2 and 3 V, respectively,

and 80% for the PIL in the mixture with CH3CN. These

values are higher than those expected for supercapacitor

systems, 15–20%.

Fig. 11 shows cyclic voltammograms recorded on the neat

PIL between 0 and 2 V, at 20 1C, for scan rates ranging from

2 to 50 mV s�1. The rectangular shape was almost maintained

Fig. 8 Cyclic voltammograms at a Pt electrode for [Et3NH][TFSA] at

20 1C. Scan rate: 10 mV s�1.

Fig. 9 Cyclic voltammograms for a symmetric AC/AC in [Et3NH]-

[TFSA], with Ag wire as reference, at 5 mV s�1.

Fig. 10 Cyclic voltammograms for two-electrode cells with activated

carbon in neat [Et3NH][TFSA], and in a mixture of [Et3NH][TFSA]

with CH3CN (w = 0.5) and in [Et4N][BF4] in a mixture with CH3CN

(molar fraction of CH3CN: x = 0.966, 1.5 mol L�1), at 20 mV s�1,

at 20 1C.

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from 2 to 20 mV s�1, which indicates a good capacitive

behavior of the AC electrode at the scan rates in question.

The influence of temperature is shown by cyclic voltam-

metry in Fig. 12a, and by galvanostatic charge–discharge in

Fig. 12b. First, it is clear that the curves for 20 and 50 1C are

similar, which implies that the capacitance values are not

really affected by this temperature difference (approximately

120 F g�1).

By contrast at 80 1C, the shape of the curves changes

dramatically and the capacitance decreases to 80 F g�1 and

110 F g�1, respectively, for DV= 2 V and DV = 3 V, which is

attributed to electrolyte decomposition under the effect of

polarization.

Fig. 12c presents the efficiency at various maximum voltages

calculated for the neat PIL, at 20, 50 and 80 1C. The efficiency

is calculated according to eqn (5):

Z ¼ tdtc

ð6Þ

where td is the discharge time and tc the charge time. This

parameter is of great interest as a high efficiency indicates a

good reversibility for the system. First, one can see that, at 20

and 50 1C, the efficiency is quite stable up to 2.5 V, with values

higher than 95%. Then, it decreases to 93 and 83%, for

DV = 3 V, at 20 and 50 1C, respectively. However, at 80 1C,

it is clear that the efficiency is very low and decreases quickly

from 85 to about 40%. Consequently, the neat PIL could be

used with a maximum voltage of 2.5 V, at 20 and 50 1C, but it

is really not promising at 80 1C.

4. Conclusions

In summary, this work reports on the preparation, characterization

and application of [Et3NH][TFSA], neat or mixed with acetonitrile,

as an electrolyte for supercapacitor applications. A thermal study

first showed an eutectic, a melting point at �0.8 1C and a

crystallization point at�29 1C. Vapor pressure measurements were

performed using a Isobaric vapour–liquid equilibrium (VLE).

Table 3 Specific capacitance (calculated from the slope of galvanostatic discharge curves), in neat PIL [Et3NH][TFSA], [Et3NH][TFSA] + CH3CN(1.36 mol L�1)) and [Et4N][BF4] + CH3CN (1.5 mol L�1) as reference, at 20 1C

[Et3NH][TFSA], DV = 2 V [Et3NH][TFSA], DV = 3 V [Et3NH][TFSA] + CH3CN, DV = 2 V [Et4N][BF4] + CH3CN, DV = 3 V

C/F g�1 125 144 128 100

Fig. 11 Cyclic voltammograms for a two-electrode cell with activated

carbon in [Et3NH][TFSA] at different scan rates, at 20 1C.

Fig. 12 Influence of temperature as observed by (a) cyclic voltammetry

at 5 mV s�1, (b) galvanostatic charge–discharge at 2 mA (347 mA g�1)

and (c) efficiency at various maximum voltages for a two-electrode cell

with activated carbon in [Et3NH][TFSA] at 20, 50 and 80 1C.

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8206 Phys. Chem. Chem. Phys., 2012, 14, 8199–8207 This journal is c the Owner Societies 2012

The evolution of conductivity with temperature was then

evaluated, and was found to increase up to 20 mS cm�1

when the temperature was raised from �10 to 80 1C. This

temperature dependence could be described by the VTF

equation. The evolution of conductivity was evaluated with

the addition of acetonitrile to the PIL at 25 1C. The optimal

mixture was found at a weight fraction of 0.5 with a

conductivity of 50 mS cm�1. The viscosity of the neat PIL,

which could also be described by the VTF equation, exhibited

a Newtonian behavior. For this mixture, the evolution of

conductivity and viscosity was measured as a function of

temperature.

Subsequently, the electrochemical behavior of the neat PIL

was assessed on an Al electrode, which pointed out passivation

from the second cyclic voltammogram. The electrochemical

window was measured on platinum with 4 V, and on activated

carbon with less than 3 V. Galvanostatic and voltammetry

tests were performed to compare the activity of the neat PIL

with PIL in acetonitrile. The effects of the scan rate and

temperature were analyzed.

Acknowledgements

This research was supported by the Conseil de la Region

Centre through the Sup’Caplipe project. The Foundation for

Polish Science is acknowledged for supporting the ECOLCAP

project realized within the WELCOME program, co-financed

fromEuropeanUnionRegional Development Fund.Many thanks

to Batscap (France) for providing the electrode material.

Notes and references

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