Introduction

The electrochemical capacitors, also referred as supercapacitors, have been widely used in various fields with high power density and excellent life cycle [1,2,3,4]. There are two typical energy storage classifications of supercapacitors: electrical double layer capacitors (EDLC) and pseudocapacitors [5]. Recently, research on EDLCs has aroused great interest because of the rise in popularity of electric and hybrid electric vehicles. Carbon-based supercapacitors have attracted a lot of attention due to the diversified material structures of higher stability and conductivity [6,7,8]. The supercapacitors’ superior properties benefit from their unique energy storage mechanism. The energy arises from the electrical charges’ accumulation at the electrode/electrolyte interface to form the electrical double-layer through rapidly adsorption-desorption without faradic reactions [9,10,11,12]. However, the weakness for the supercapacitors is the low energy density. Many studies have been carried out to improve the energy density to meet the expanding demands. As is known, the energy density of supercapacitors can be described by the equation E = ½CV2, where C is the capacitance, and V is the operative voltage of supercapacitors. It is evident that increasing the supercapacitors’ operative voltage can have a great impact on the supplied energy. Generally, the operative voltage of a supercapacitor does not exceed 1.0 V in aqueous media due to the decomposition voltage of water, and for most of organic electrolytes, the safe operative voltages are generally lower than 2.7 V [13,14,15].

Ionic liquids (ILs) are room temperature molten salts with negligible vapor pressure, low flammability, wide electrochemical windows, and high electrochemical and thermal stabilities [16,17,18]. Among the alternative electrolytes proposed so far, ILs can offer a wide range of electrochemical stability, which are currently considered as one of the most promising materials as high-performance electrolytes. Many studies showed that the operative voltage of IL-based supercapacitors can realize 3.5 V or even higher [16, 19,20,21].

However, the typically high viscosity and low conductivity of the ILs limit the powers of IL-based supercapacitors, especially at room temperature. In order to improve the power of high-voltage supercapacitors, the low viscosity mixtures resulted from a combination of ILs, organic solvents, or other liquids have been a useful and easy method [16, 21, 22]. Krause et al. have shown the mixtures of 1-butyl-1-methylpyrrolidinium bis{(trifluoromethyl)sulfonyl}imide ([Pyrr14][TFSI]), and PC exhibits higher conductivity and lower viscosity at 298 K than those of the neat [Pyrr14][TFSI], while still keeping an operative voltage as high as 3.5 V [23]. Christoph Schütter et al. have reported to the blends of nitriles (butyronitrile and adiponitrile) and [Pyrr14][TFSI] in EDLCs revealed a good electrochemical stability allowing a maximum operative voltage up to 3.7 V [24]. Brandt et al. studied mixtures of the trimethyl-sulphoniumbis[(trifluoromethyl)sulphonyl]imide (Me3STFSI) and PC as electrolyte for carbon-based electrochemical double layer capacitors (EDLC). They demonstrated that the EDLC life cycle strongly depends on the concentration of Me3STFSI in the mixture [25].

Herein, we synthesized trialkyl substitution imidazolium-based IL [Emmim][BF4] and characterized it by 1H nuclear magnetic resonance spectroscopy (1HNMR), infrared spectrum (IR), thermogravimetry (TGA), and elemental analysis (EA), respectively. For the solid state of [Emmim][BF4], the ACN was used as solvent to prepare a series of IL binary mixture system (concentration of IL from 0.2 to 1.2 mol·L−1). First, the density, viscosity, and electrical conductivity of the [Emmim][BF4] + ACN binary system were measured at temperature range from 288.15 to 333.15 K. The temperature dependences on electrical conductivity of the binary system under different concentrations were described by the Vogel-Fulcher-Tamman (VFT) equation and Arrhenius equation, respectively. Then, the IL mixtures were employed as possible electrolytes in the active carbon-based supercapacitors. The cyclic voltammetry (CV), electrochemical impedance spectrum (EIS), and galvanostatic charge-discharge (GCD) of the supercapacitors were measured and discussed. In addition, the energy and the power of the investigated devices were calculated to evaluate the electrochemical performances of the fabricated supercapacitors. The results showed the [Emmim][BF4] + ACN binary mixture electrolytes could be a good candidate of electrolyte with great potential for supercapacitors.

Experimental section

Chemicals

Bromoethane, sodium tetrafluoroborate, and acetonitrile are purchased from Sinopharm Co (China) (purity > 0.990). The 1,2-dimethylimidazolium is purchased from Zhejiang doubleport Co. Ltd. (China) and distilled at reduced pressure before use. The materials’ purities and sources are summarized in Table 1.

Table 1 Information of sample
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Synthesis

The IL 1-ethyl-2,3-dimethylimidazolium tetrafluoroborate ([Emmim][BF4]) is synthesized in accordance with a two-step method (Scheme 1) described with the previous works [26, 27]. The final product is white solid salt. The IL is dried for 48 h under vacuum. The neat IL is then verified by 1H NMR, IR, TGA, and EA. (see supporting information).

Scheme 1
scheme 1

Synthesis route of IL [Emmim][BF4]

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Preparation of IL and IL + ACN binary system

A series of [Emmim][BF4] + ACN binary mixtures are prepared by weighing on a Shanghai JKFA2004N analytical balance with a precision of 0.0001 g. The concentration of [Emmim][BF4] in the mixtures (cIL) are 0.2, 0.4, 0.6, 0.8, 1.0, and 1.2 mol·L−1. These binary mixtures are called c0.2, c0.4, c0.6, c0.8, c1.0, and c1.2, respectively.

Preparation of the supercapacitors

A symmetrical supercapacitor device is designed by two identical electrodes. The electrodes are constituted of the viscous slurry of the active material, carbon black, and 60 wt% polytetrafluoroethylene in a small amount of ethanol mixed in 8:1:1 wt% [28]. The mass of each working electrode is about 80 mg. Each of the working electrodes is 13 mm in diameter and 2 mm in thickness [29,30,31]. Afterwards, the electrodes were dried at 353.15 K for 36 h.

Measurements

Density

A Mettler Toledo DM45 Delta Range Density Meter was used to measure the densities of the [Emmim][BF4] + ACN binary mixtures from 288.15 to 333.15 K with a step size of 5 K under atmospheric pressure. Resolution of the apparatus was 0.00001 g cm−3. The uncertainty was ± 0.0002 g cm−3 for the density measurement and ± 0.01 K for the temperature. The values are listed in Table 2.

Table 2 Density, viscosity, and electrical conductivity of binary system [Emmim][BF4] + ACN at T = (288.15–333.15) K and 101 kPa
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Viscosity

The viscosity of the binary mixtures was measured by an Anton Paar Lovis 2000 M viscometer, using rolling-ball principle. The viscosity range of the standard solution was 10 to 300 mPa s over the temperature range from 288.15 to 333.15 K at 5 K intervals with an accuracy of 0.0001 mPa s under atmospheric pressure. The uncertainty of the viscosity measurement was estimated to be ± 0.003 mPa s. The values are listed in Table 2.

Electrical conductivity

The electrical conductivities of binary mixtures were measured with a DDSJ-308A conductivity meter operating in a DJS-10C conductivity electrode under dry argon atmosphere. Temperature accuracy from 288.15 to 333.15 K was ensured within ± 0.01 K by means of a thermostatic water bath under atmospheric pressure. The uncertainty in the electrical conductivity measurements was estimated to be ± 0.1 mS cm−1. The values are listed in Table 2.

Electrochemical testing

The electrochemical property was measured on coin cells by electrochemical working station. All electrochemical behaviors of the fabricated supercapacitor were tested by cyclic voltammetry (CV), electrical impedance spectroscopy (EIS), and galvanostatic charge-discharge (GCD). The CV and the EIS were measured by an electrochemical working station (CHI 660E), and the GCD was measured by a battery testing system (CT 3008W). The CV curves for supercapacitors were measured at various scan rates (10, 20, 30, 50, 100 mV s−1). The CV curves in different voltage range were measured at the scan rate of 10 mV s−1, and the current densities of GCD were 20 mA g−1. The specific capacitance was calculated from the GCD according to equation the following equations [32]:

$$ C=I\Delta t/\left(m\Delta V\right) $$
(1)

The energy density was figured up from the GCD on the basis of equation:

$$ E=\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.{CV}^2 $$
(2)

The power density was computed from the GCD based on equation:

$$ P=E/\Delta t $$
(3)

where C (F g−1) is capacitance, m (g) is the mass of porous carbon on the working electrode, I (A) is the constant discharging current, Δt (s) is the discharging time, ΔV (V) is the potential window, E (Wh Kg−1) is energy density, and P (KW Kg−1) is power density. The whole electrochemical evaluation was performed at room temperature.

Results and discussion

Density, viscosity, and electrical conductivity

In this study, the density of binary system was measured at temperature from 288.15 to 333.15 K under atmospheric pressure, and the results are listed in Table 2. Figure 1 displays the experimental values of ρ against T. A liner correlation can be found between the density of the binary systems and the temperature, at the range of 288.15 to 333.15 K [33]. The density undulation can be embodied in thermal properties of the electrolyte [34]. From Fig. 1, the density values decreased with the rise of temperature. And at the same temperature, with the increase dosage of ILs, density values increase. The consequence of these binary systems showed a similar trend to the [Bmmim][BF4] + ACN binary systems in literature [35].

Fig. 1
figure 1

Density ρ for the [Emmim][BF4] + ACN binary system as a function of temperature T at different concentrations of the IL

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The viscosity and electrical conductivity are usually considered the most important transport properties to evaluate IL for applications involving the use of IL as electrolytes [27]. We examined the viscosity of the binary system with the IL addition from 288.15 to 333.15 K. The relationship between viscosity and temperature is shown in Fig. 2. Like general trend [33], the viscosities of the binary mixtures decrease with the increased temperature. And with the addition of [Emmim][BF4], both the viscosity and conductivity increase.

Fig. 2
figure 2

Viscosity η for the [Emmim][BF4] + ACN binary system as a function of temperature T at different concentrations of the IL

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The electrical conductivities against concentrations of the IL for the binary mixtures are shown in Fig. 3. As seen from the Fig. 3, a raise of temperature leads to an increase of the electrical conductivity. The electrical conductivity values become larger with the enhanced concentration until the σmax is reached. When c ≈ 1.0 mol L−1, the values reached the maximum, where conductivity is 55.0 mS cm−1 at room temperature. Compared to the similar mixture systems of N-butyl-4-methylpyridinium bis(trifluoromethylsulfonyl)imide ([Bupy][Tf2N]) + ACN and N-hexylpyridinium bis(trifluoromethylsulfonyl)imide ([Hepy][Tf2N]) + ACN, whose electrical conductivities are 40.12 and 33.99 mS cm−1 [36], the electrical conductivities are smaller than the binary system in this work.

Fig. 3
figure 3

Electrical conductivities vs concentrations of the IL for the [Emmim][BF4] + ACN binary system at different temperatures T

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Then, the relation of temperature and electrical conductivity of IL [Emmim][BF4] + ACN can be fitted by the Vogel-Fulcher-Tamman (VFT) equation [26] as the following, respectively:

$$ D={D}_0\exp\ \left[\hbox{--} B/\left(T\hbox{--} {T}_0\right)\right] $$
(4)

where D is conductivity, D0 is the conductivity at the temperature limit, B is the quantity related to the activation barrier, and T0 is the absolute or “ideal glass transition” temperature. The fitted VFT parameters are listed in Table 3.

Table 3 Fitted parameter values of VFT equation and Arrhenius equation of conductivity for the binary system [Emmim][BF4] + ACN
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For electrical conductivity, the Arrhenius equation is:

$$ D={D}_{\infty}\exp \left[\hbox{--} {E}_{\mathrm{a}}/(RT)\right] $$
(5)

where Ea is the activation energy for conductivity, which showed the energy needed for an ion to hop to a free hole, D∞ is the maximum conductivity, and R is the gas constant. According to the Arrhenius equation, the 1/T relied on lnσ is plotted. (see Fig. 4b). The correlation coefficients of the VFT equation were all > 0.99993, and correlation coefficients of Arrhenius equation were nearly 0.999, which shows that the VFT equation should be proper for describing the effect of temperature on the conductivity of IL.

Fig. 4
figure 4

(a) σ vs T and (b) lnσ vs T−1 for the [Emmim][BF4] + ACN binary system at different concentrations of the IL.

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Electrochemical testing

The electrochemical stability of the binary mixtures is investigated by cyclic voltammetry (CV). Figure 5a presents the CV curves of the fabricated supercapacitors using the mixture electrolytes with various [Emmim][BF4] concentrations at scan rate of 10 mV s−1 under the room temperature. All the CV curves maintain well rectangular shape, which indicate a typical double layer behavior with the electrolyte ions unimpeded diffusion within the practicable electrode surface [37]. As the concentration of [Emmim][BF4] in binary electrolytes increase from 0.2 to 0.8 mol·L−1, the potential window enhanced to the maximum value(5.9 V). This result may be related to its high conductivity and medium viscosity. The CV curves of the c0.8 at different scan rates from 10 to 100 mV s−1 are presented in Fig. 5b. The rectangular shape of CV curves at low scan rates indicates that the mobile charge carriers diffuse at an almost constant rate and form the ion accumulation with low ohmic resistance [38]. The CV curves show slightly deviations at high scan rate because of electrode polarization effects and reduced electrolyte mobility in small pore regions [39].

Fig. 5
figure 5

(a) Cyclic voltammograms for the [Emmim][BF4] + ACN binary system at different concentrations of the IL. The voltage scan rate is 10 mV s−1 (b) CV curves for the c0.8 at scan rates range from 1 m to 100 mV s−1

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Figure 6a displays the Nyquist plots of the mixture electrolytes at room temperature. As seen from the figure, the Nyquist plots exhibit typical double-layer capacitor characteristics. In the high-frequency region, the series resistance (Rs) of the supercapacitor is the intercept on the X-axis. It is caused by electrolyte solution resistance, separator paper, and external circuit resistance. The semi-circle represents the contact resistance (Rct) between the electrode and the electrolyte. In the low frequency range, the straight line represents capacitance characteristics [40,41,42]. The 45° sloped curve is the Warburg resistance, and it is a result of the frequency dependence on ion diffusion/transport in the electrolyte to the electrode surface. As is shown in Fig. 6, the electrolyte solution resistance decreases with increasing concentration of [Emmim][BF4] ionic liquid. When the ionic liquid concentration reaches c0.8, the internal resistance of the electrolyte solution is the minimum. The high transferability and solubility can help ions insert into the hole of activated carbon. These reduce the interfacial resistance between electrode and electrolyte availably [41].

Fig. 6
figure 6

(a) Nyquist plots for the [Emmim][BF4] + ACN binary system at different concentrations of the IL. (b) Dependencies of log Z′ vs. log f plots for binary system in c0.8

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Figure 6b shows that the high frequency series resistance is independent of potential applied, and therefore, the electrolyte series resistance Rs can be calculated at f ≥ 10 kHz. At low frequencies, the values of log Z′ increased, which is adsorption and diffusion limited stages in the interior of the electrolyte [43]. The result is caused mainly by the surface roughness of the electrode [44,45,46].

The galvanostatic charge-discharge profiles of mixture electrolytes with different IL concentrations (Fig. 7a) were recorded with two-electrode supercapacitor at the current density of 20 mA g−1 in the voltage from 0.0 to 2.7 V. All the curves are linear and symmetrical triangle, indicating the excellent electrochemical reversibility and typical charge-discharge features of double layer capacitor [47]. The galvanostatic charging-discharging duration time for supercapacitor varied with the IL concentrations of the mixture electrolytes, which is in good agreement with the CVs and the impedance plots (Figs. 5 and 6). The charge/discharge performance of supercapacitor using c0.8 electrolyte with the longest duration times and without visible sudden voltage drop (IR drop) is obviously superior to those of other five systems electrolytes. After 3000 cycles at the working voltage of 2.7 V, capacitance retentions of EDLCs using c0.8 electrolyte (Fig. 7b) is reduced to 98%.

Fig. 7
figure 7

(a) Galvanostatic charge-discharge curve for the [Emmim][BF4] + ACN binary system at different concentrations of the IL under a constant current density of 20 mA g−1. (b) Cycle performance for the supercapacitor after 3000 cycles of charging and discharging at a current density of 20 mA g−1

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The electrochemical performance is also analyzed. The capacitance (C, F g−1), energy density (E, Wh Kg−1), and power density (P, kW kg−1) were calculated according to the Eqs. (1)–(3), and the results are listed in Table 4. After a short period of stabilization, the specific capacitance can achieve 142.6 F g−1, and the maximum storage energy is calculated to be 36.1 Wh Kg−1 at a current density of 20 m Ag−1. Compare with [Pyr14][TFSI] + ACN, [Pyrr][NO3] + gamma butyrolactone (γ-BL), the specific capacitance values of [PYR14][TFSI] + ACN, [Pyrr][NO3] + γ-BL are about 31,125 F g−1,which are smaller than our binary system [48, 49].

Table 4 Capacitance, energy, density and power density of binary system [Emmim][BF4] + ACN
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Conclusion

In this paper, an IL-based binary mixture system of [Emmim][BF4] + ACN was prepared, and the physicochemical properties and the electrochemical performances in supercapacitors of the system were studied. The different concentrations of [Emmim][BF4] in the mixture electrolyte can provide different voltage windows and capacitances. The electrical conductivity of the mixture of 1.0 mol L−1 can reach 55.0 mS cm−1 at the room temperature. The voltage window of the symmetrical AC supercapacitor with the IL mixture electrolyte could be expanded up on 5.9 V (cIL ≈ 0.8). The high solubility, low viscosity, and good conductivity of the ionic liquid can explain the capacity of the electrolyte in electrochemical applications. Due to introducing the ionic liquids into the ACN, the density, viscosity, and electrical conductivity of the mixture electrolyte were increased. And the specific capacitance, power density, and energy density of the fabricated supercapacitors using the mixture electrolyte are also improved.