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Journal of Solution Chemistry (2019) 48:1119–1134https://doi.org/10.1007/s10953-019-00898-8

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Imidazolium Based Ionic Liquids as Electrolytes for Energy Efficient Electrical Double Layer Capacitor: Insights from Molecular Dynamics and Electrochemical Characterization

Upasana Mahanta, et al. [full author details at the end of the article]

Received: 19 December 2018 / Accepted: 24 April 2019 / Published online: 24 July 2019 © Springer Science+Business Media, LLC, part of Springer Nature 2019

AbstractIonic liquids (ILs) have attracted considerable interest as electrolytes for electrical double layer capacitors (EDLC) bringing in enhancement of energy efficiency. This work studied three imidazolium based ILs mixed with a co-solvent as the electrolytes for EDLC. A com-bined study involving molecular dynamics (MD) and electrochemical experiments was car-ried out to interpret the potential of the electrolyte solution. Initially, MD simulation was employed to compute ionic conductivity and viscosity of pure ILs, 1-ethyl-3-methylimi-dazolium bis(trifluoromethylsulfonyl)imide ([EMIM][Tf2N]), 1-propyl-3-methylimidazo-lium bis(trifluoromethylsulfonyl)imide ([PMIM][Tf2N]) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][Tf2N]) and the study was further extended to 1 mol·dm−3 solutions of these three ILs in acetonitrile (ACN). The MD results were sequentially validated by experiments. Based on the ionic conductivity and viscosity values obtained from MD and experiments, 0.5, 1.0 and 2.0 mol·dm−3 solutions of the ILs in ACN were further investigated as electrolytes for carbon based EDLC. Cyclic voltammetry, elec-trochemical impedance spectroscopy and galvanostatic charge discharge techniques were employed. From cyclic voltammetry, the observed highest value of the operating potential window was 3 V. The nearly rectangular and symmetric shape of cyclic voltammograms and vertical line of Nyquist plot at lower frequencies indicated good capacitive behavior of the system. The highest specific capacitance of 122 F·g−1 was achieved for the 1 mol·dm−3 solution of [PMIM][Tf2N] at 0.5 A·g−1. The highest energy density values were found to be 152 and 149 W·h·kg−1 for 1 mol·dm−3 solutions of [PMIM][Tf2N] and [BMIM][Tf2N], respectively. Overall, 1 mol·dm−3 solutions of the less explored [PMIM][Tf2N] and [BMIM][Tf2N] provided better electrochemical stability, energy and power density.

Keywords Ionic liquids · Ionic conductivity · Electrical double layer capacitor · Operating potential window · Specific capacitance

Electronic supplementary material The online version of this article (https ://doi.org/10.1007/s1095 3-019-00898 -8) contains supplementary material, which is available to authorized users.

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1120 Journal of Solution Chemistry (2019) 48:1119–1134

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1 Introduction

Electrical double layer capacitors (EDLCs), a form of hybrid capacitors or supercapaci-tors, have gained significant attention from the researchers in the field of energy storage because of their high power density and long cycle life. The energy storage in EDLC is due to adsorption of the electrolyte’s ions on the porous and electrically conductive electrodes. EDLCs are high power electrochemical devices where charge separation takes place at the electrode/electrolyte interface, which results in capacitance. Lower charge transfer resist-ance compared to conventional batteries makes EDLCs capable of delivering higher power. The capacitive nature of EDLC is in the order of 100 F·g−1 and superior to that of dielec-tric capacitors, which yield capacitances in the µF·g−1 range. The capacitance of an EDLC is mainly governed by the nature of the electrode. Reported literature describes the use of different forms of carbon electrode [1–6] for enhancing the capacitance of EDLCs. This work focuses on the choice of electrolyte for energy efficient EDLCs. In this regard, ionic liquids (ILs) are considered as potential electrolytes for EDLCs [6–8]. ILs generally consist of an organic cation and either an organic or inorganic anion. Due to their lower volatility, non-flammability, higher thermal and chemical stability and better ionic conductivity, ILs are suitably employed as electrolytes for EDLCs. In addition to that, ILs possess wider operating potential windows (OPW) [6–8], which makes them more favorable than aque-ous electrolytes. Aqueous solutions of KOH, H2SO4, Na2SO4, (NH4)2SO4 and LiCl have been investigated earlier [9, 10] and have reported OPW in the range of 0.8–1.2 V. The volatile, flammable and toxic nature of most of the organic electrolytes limits their usability in EDLCs [11, 12]. The OPW for common organic electrolytes is limited to 2.5–2.8 V [12], which can be improved by using ILs as electrolytes. However, the higher viscosity of ILs is a drawback for their use in EDLCs. It was also suggested in the literature [8, 11] that the addition of organic solvents such as acetonitrile (ACN), butyronitrile and benzonitrile sig-nificantly lowers the viscosities of ILs.

An enormous variety of ILs is available due to the combination of different cations and anions. This unique feature leads to IL applications in diverse fields. However, the issues of higher viscosity and lower conductivity prevent some of the ILs from being used as electro-lytes for EDLCs. Among the common cations for ILs, viz imidazolium, pyrrolidinium, pyri-dinium, ammonium, etc., the most extensively used cation for EDLC is found to be imida-zoilium, more specifically 1-ethyl-3-methylimidazolium (EMIM) [6, 12]. Typical anions are tetrafluroborate (BF4), hexaflurophosphate (PF6), bis(trifluromethylsulphonyl) imide (Tf2N/TFSI) and bis(flurosulphonyl) imide and dicyanamide (DCA) [6, 12]. Generally imidazo-lium based ILs are recognized for their higher ionic conductivity and reasonable OPW [6, 12]. In the case of EDLC, the wider the OPW the greater is the energy and power density [5]. The [EMIM] cation with different anions have been tested for graphene based EDLC [13]. Among different ion pairs, [EMIM] [DCA] gave the best value of specific capacitance (95 F·g−1) due to having the lowest viscosity, molecular weight and size among all the other ILs, followed by [EMIM] [BF4] and [EMIM] [Tf2N]. However, the corresponding OPW was quite low (2.3 V) whereas [EMIM] [BF4] and [EMIM] [Tf2N] delivered OPW values of ~ 4 V and ~ 3.5 V, respectively. Again, [EMIM] [Tf2N] was found to be less viscous than [EMIM] [BF4]. Therefore, it can be concluded that [EMIM] [Tf2N] is expected to be the more favorable IL. The main criteria to fulfil for an optimized ion pair for ILs are lower viscosity, higher conductivity and wider OPW. Specific capacitance is affected more by the nature of the electrode than the size of the ions [2]. Apart from viscosity and ionic conduc-tivity, the Tf2N anion also imparts more thermal stability to the IL [14].

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[EMIM][Tf2N] was studied extensively as an electrolyte for EDLC [6]. An OPW of 3.5 V has been obtained for graphene-derived carbon EDLC with [EMIM][Tf2N] in ACN [15]. 1-Butyl-3-methylimidazolium (BMIM) with the Tf2N anion was investigated for an oxide-coated silicon nanowire array capacitor and the reported OPW was 1.6 V with a specific capacitance 0.7 F·g−1 [4]. A functionalized carbon nanotube (CNT) based EDLC was studied using 1-butyl-2,3-dimethylimidazolium (BDMIM) with Tf2N where a superior value of OPW (4.4 V) was measured [16]. Furthermore, a nanostructured thin film based EDLC was explored with [BMIM][Tf2N] as electrolyte [17]. The reported OPW is 3.5 V and the highest specific capacitance was found to be 75 F·g−1. Further [BMIM][Tf2N] was studied together with poly(methyl metacrylate) (PMMA) as a gel polymer electrolyte [18].

There is an absence of literature studies on a cation such as 1-propyl-3-methylimidazo-lium ([PMIM]) for carbon based EDLC. Having gone through the basic requirements of electrolytes, imidazolium based cations, EMIM and the less explored PMIM and BMIM with the Tf2N anion have been chosen for our study. The choice of the widely used EMIM cation was made to have a comparative study on the performance PMIM and BMIM. Ace-tonitrile (ACN) was used as the co-solvent to reduce the viscosity of ILs. MD simulation is recognized as a reliable tool to investigated physical properties such as viscosity and ionic conductivity for liquid systems [19, 20]. Initially molecular dynamics (MD) studies have been carried out to compute the viscosity and ionic conductivity of the electrolytes before purchasing expensive ILs, which is further validated with literature and experimental find-ings. Later, in the second part of the study, electrochemical characterization techniques were performed to determine specific capacitance, energy and power density for IL solu-tions and carbon electrode system.

2 Computational Details

2.1 Molecular Dynamics Simulation Details

Optimized geometry for the cations and anion of ILs and ACN, drawn by Gauss View 5 [21], were obtained using Gaussian 09 [22] using the hybrid B3LYP/6-31G* [23] functional and basis set. 100 molecules of both pure ILs and 1 mol·dm−3 solution of ILs in ACN were packed in low-density configurations using the Packmol package [24]. The required force field param-eters for MD simulation were generated according to the Generalized Amber Force Field (GAFF) [25] functional form with the help of the ANTECHAMBER [26] module of AMBER 12 [27]. The partial charges obtained from Gaussian were then fitted with the restricted elec-trostatic potential module of AMBER 12. Optimized geometry and partial charges are pro-vided in Fig. S1 and Table S1, respectively, in the Supporting Information. The reliability of the force field parameters were validated by comparing the simulated density and experimen-tal densities of all the systems (Table 1). To predict the transport properties of ILs realistically, force fields must take into account the dynamic electronic polarization effects. In this regard, reduction of partial charges has become an alternative approach to the use of polarizable force fields, which involves higher computational cost [20]. Literature suggests that reducing the charges by a factor of about 0.7 for ions in general can take care of the electronic polariza-tion effect [28]. For [BMIM][Cl] a scaling factor of 0.73 has been already used [29]. In our study, a series of MD simulations was performed with and without charge reduction. For pure ILs, the more accurate scaling factor was found to be 0.72 from comparing the self-diffusion coefficient and ionic conductivity with available literature. For mixtures of ILs with ACN, a


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scaling factor of 0.85 has been adopted from the literature [20] where similar charge reduction was done for IL + ethanol mixtures and this particular choice of scaling factor eventually came up as the effective one when comparison of transport properties with experimental values was made. For simulation of IL solutions the scaling factor was used only for ionic species and not for the organic solvent [20].

MD simulations were performed using NAMD version 2.9 [30]. The system was brought to its experimental density through a 2 ns equilibration run in the NPT ensemble. Thereafter the equilibrated configuration was taken for a production run of 50 ns in the NVT ensemble at 298.15 K. A Langevin thermostat [31] with a damping coefficient of 1 ps and Nose–Hoover Langevin barostat [32] with a period of 100 fs and decay of 50 fs were applied to monitor temperature and pressure throughout the simulation. Pressure imposed for the simulation was 1.01325 bar. The Verlet algorithm was implemented with a time step of 2 fs [33] to integrate the equations of motion. A 12 Å cut off distance was employed for short-range L–J potential. In a similar manner, long-range electrostatic interactions were taken care of using the Particle Mesh Ewald Approximation.

2.2 Determination of Ionic Conductivity and Viscosity

Ionic conductivity being a transport property is associated with self-diffusion coefficients of the respective ions. In this regard, first the self-diffusion coefficient or self-diffusivity (D) was estimated for the ion pairs using the Einstein relation [34]:

where the term inside the angular bracket stands for mean square displacement (MSD) of the molecules, the angular bracket represents the ensemble average and the factor 1/6 is attributed to the three-dimensionality of the system. MSD data were obtained from the last 10 ns production run of the MD simulation.

Further, ionic conductivity ( � ) was computed with the help of the Nernst–Einstein relation [20].

(1)D =1

6limt→∞

d

dt

⟨

N∑

i=1

[

r⃗i(t) − r⃗i(0)]2

⟩

(2)� =q2(n+D+ + n−D−)

VkBT

Table 1 Density of pure ILs and their 1 mol·dm−3 solutions in ACN at T = 298.15 K

a MD simulation from this workb Experiment from this workc Experiment from literature

�a (kg·m−3) �b (kg·m−3) �c (kg·m−3)

[EMIM][Tf2N] 1.55 1.55 1.52 [38][EMIM][Tf2N] + ACN 0.99 0.95[PMIM][Tf2N] 1.46 1.47 1.474 [39][PMIM][Tf2N] + ACN 0.97 0.97[BMIM][Tf2N] 1.43 1.44 1.434 [40][BMIM][Tf2N] + ACN 0.96 0.97


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where q is the electron charge, D+ and D− are the self-diffusion coefficients, n+ and n− are the number of cations and anions, V is the system volume, kB is the Boltzmann constant and T is the absolute temperature (K).

Viscosity ( � ) calculations for equilibrium MD was performed using the Green–Kubo relation [35].

where Pxy represents the xy component of the pressure tensor, V is the system volume, kB is the Boltzmann constant and T is the temperature. The angular bracket indicates the average over all the time origins and the summation considers all three off diagonal elements of the pressure tensor, i.e. Pxy, Pyz and Pzx.

3 Experimental Details

3.1 Chemicals

The ILs, [EMIM][Tf2N], [PMIM][Tf2N], [BMIM][Tf2N], of purity ≥ 98% and anhydrous ACN with 99.8% purity were supplied by Sigma-Aldrich. Extra Pure silver nitrate (99.5%) was obtained from Sisco Research Laboratories Pvt. Ltd., India. Exfoliated carbon elec-trodes (BET surface area 635 m2·g−1) with butyl rubber binder were supplied by Vikram Sarabhai Space Center, Thiruvananthapuram. Prior to any experiment, the ILs were kept overnight for vacuum drying at 120 °C to diminish the effect of volatile impurities.

3.2 Density, Ionic Conductivity and Viscosity Measurement of Electrolytes

Densities of the pure ILs as well as their mixtures with organic solvent were measured experimentally using an Anton-Paar 4500 (Switzerland) density meter to confirm the consistency of the simulation parameters. Subsequent to MD simulation, ionic conduc-tivity was also measured using a Microprocessor based water-soil analysis kit (VSI 302, VSI Electronic Private Ltd.) at 25 °C. Further, viscosities obtained from MD simulation were validated by measuring the dynamic viscosity using an Anton Paar Phsica MCR301 Rheometer using the cone plate (CP25-2/S) method at 25 °C. Experimentally, both ionic conductivity and viscosity were measured for pure ILs and 0.5, 1.0 and 2.0 mol·dm−3 solu-tions of all the ILs (Fig. S2).

3.3 Electrochemical Characterization of EDLC

Electrochemical experimentation was carried out with a three-electrode cell configuration at 25 °C using a PARSTAT-3000A potentiostat. A spiral Pt wire was used as counter elec-trode (CE) for all the systems. Ag/Ag+, 0.1 mol·dm−3 AgNO3 in ACN + IL solution [36] was employed as reference electrode (RE) for the [PMIM][Tf2N] and [BMIM][Tf2N] sys-tems. Due to the observed tendency of [EMIM][Tf2N] to react with AgNO3 solution in ACN, Ag wire was used as pseudo-reference electrode for this IL. Exfoliated carbon sheet was used as working electrode (WE). Double layer capacitive behavior of 0.5, 1.0 and

(3)� =V

kBT

�

∫0

⟨

∑

Pxy(t).Pxy(0)⟩

dt


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2.0 mol·dm−3 solutions of ILs in ACN was studied against the carbon electrode. Initially cyclic voltammetry (CV) was performed followed by the galvanostatic charge–discharge (GCD) characterization technique, to examine the performance of the EDLC. In addition to evaluating the specific capacitance and OPW from CV and GCD, electrochemical imped-ance spectroscopy (EIS) was carried out for the best IL solutions. All the experiments were repeated three times to confirm the reproducibility of our findings.

CV is an effective electrochemical method to evaluate the double layer capacitive behav-ior. Multiple CV scans were performed with 5, 10 and 20 mV·s−1 scan rates. Capacitance from CV was estimated using the following equation [37]:

where C is the capacitance, v is the scan rate, Vc − Va represents OPW and I(V) is the current.

Followed by CV, a GCD test was carried out to study the reproducibility of the OPW obtained from CV and the cycle stability of the exfoliated carbon based EDLC. In addition, calculations of capacitance were also done using data obtained from GCD, using the fol-lowing relation [11, 37]:

where C is the capacitance, I is the current applied and dV∕dt is the slope of the charge–discharge curve excluding the IR drop.

Eventually, potentiostatic EIS measurements were conducted in the frequency range from 100 kHz to 10 mHz by applying a sinusoidal signal of 10 mV RMS. The EIS study was carried out at potentials of 0 V, 50 mV and 100 mV versus the OCP. Frequency dependent capacitance was computed using the relation given below [37]:

where C(f ) is the frequency dependent capacitance, f denotes frequency and Z∕∕(f ) stands for imaginary part of the impedance.

4 Results and Discussion

4.1 Physical Properties

4.1.1 Density

In order to validate the accuracy of the force field parameters used in the MD simulation, density ( � ) of the simulated IL systems was computed after equilibration for 2 ns. Further, density obtained from simulation was compared with experimental density (Table 1). It can be noted that density from simulation matches the experimental one, which indicates the

(4)C =1

2v(Vc − Va)

Vc

∫Va

I(V)dV

(5)C =I

dV∕dT

(6)C(f ) =−1

2�fZ∕∕(f )


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reliability of the force field parameters. One important observation made is that there was no significant variation of density with the scaling factor.

4.1.2 Ionic Conductivity and Viscosity

Following density, ionic conductivity (σ) was calculated as explained in Sects. 2.2 and 3.2. A 10 ns long trajectory from MD simulations was considered in order to compute the MSD and hence σ. The corresponding σ and D values are reported in Table 2. The increasing MSD (Fig. 1) for 1 mol·dm−3 solutions of ILs in ACN clearly indicates the enhancement of D and σ over pure ILs. ACN breaks the large ion pair clusters by weakening the ionic inter-action, which helps raise σ. The smaller size of the cation EMIM resulting from shorter alkyl chain facilitates a higher diffusivity D than PMIM and BMIM (Table 2). Overall, MD results provide better insights on the effect of alkyl chain length on diffusivity and agree with the trend reported earlier [41]. The use of a particular scaling factor takes care of the effect of charge transfer and polarizability [20, 28]. Ionic conductivity values pre-dicted from MD simulation show a close match with experimental findings. Effects of co-solvent on the increase of the ionic conductivity of [EMIM][Tf2N] was studied extensively by Bozym et al. [42]. The trend obtained clearly matches our findings for all the three ILs (Fig. S2). It may be expected that at 1 mol·dm−3 the [EMIM][Tf2N] solution will have the highest ionic conductivity from the trend in the results for the pure ILs. However, the IL content in this case (6 mol%) is less than the optimum IL content (10 mol%) reported for this IL [42]. This leads to slightly lower ionic conductivity than for 1 mol·dm−3 [PMIM][Tf2N].

Since the viscosities of the IL solutions could not be found in the literature, we per-formed a dynamic viscosity study of the IL solutions in ACN, as mentioned in Sect. 3.2, to interpret the extent of reduction upon addition of ACN. Viscosity of pure ILs was also measured, to have a comparative idea about the reliability of the findings from MD. As expected, significant viscosity reduction of ILs was obtained with the addition of co-sol-vent (Table 3; Fig. S2). Addition of the less viscous ACN mobilizes the ions of the IL by lowering intermolecular friction. Table 3 reports the viscosity computed from our study for pure ILs and their 1 mol·dm−3 solutions along with available literature data for comparison.

Table 2 Self-diffusion coefficients and ionic conductivity of pure ILs and their 1 mol·dm−3 solutions in ACN at T = 298.15 K


D (10−7 cm2·s−1) σa (mS·cm−1) σb (mS·cm−1) σc (mS·cm−1)

D+ D−

[EMIM][Tf2N] 2.17 1.13 4.91 4.33 6.67 [38][EMIM][Tf2N] + ACN 50.63 40.55 30.31 28.50 32 [42][PMIM][Tf2N] 1.80 0.78 3.48 3.51[PMIM][Tf2N] + ACN 46.14 34.47 30.99 30.40[BMIM][Tf2N] 1.24 0.92 2.44 2.76 3.80 [40][BMIM][Tf2N] + ACN 31.89 34.79 24.67 27.30


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Viscosity of pure [PMIM][Tf2N] obtained from simulation data agrees reasonably well with the literature [39]. However, for [BMIM][Tf2N] it is slightly lower than the literature values (deviation 3.3% and 9.23%) [40, 43] and [EMIMTf2N] is higher than the available

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Fig. 1 Mean square displacement (MSD) plots a [EMIM][Tf2N], b 1 mol·dm−3 [EMIM][Tf2N], c [PMIM][Tf2N], d 1 mol·dm−3 [PMIM][Tf2N], e [BMIM][Tf2N] and f 1 mol·dm−3 [BMIM][Tf2N]

Table 3 Viscosity of pure ILs and their 1 mol·dm−3 solutions in ACN at T = 298.15 K


�a (mPa·s) �b (mPa·s) �c (mPa·s)

[EMIM][Tf2N] 37.76 33.2 33.9 [38][EMIM][Tf2N] + ACN 2.04 1.70[PMIM][Tf2N] 43.04 39.70 43.70 [39][PMIM][Tf2N] + ACN 2.98 1.00[BMIM][Tf2N] 43.57 42.90 48 [40], 45 [43][BMIM][Tf2N] + ACN 3.12 1.50


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data [38]. Additionally, experimental viscosities for pure ILs obtained in our study show a fair match with those obtained from simulation (deviations ~ 1.5–7.7%) except for [EMIM][Tf2N], where the deviation is ~ 11%. On the contrary, experimental viscosities observed for mixtures of ILs and organic solvent show significant deviations from the MD results. However, results from both the studies show a decreasing trend with the addition of co-solvent. Based on insights from MD and experimentally derived viscosity and ionic con-ductivity, the three imidazolium ILs with co-solvent ACN were adopted for electrochemi-cal experimentation.

4.2 Performance of EDLC

Figures 2, 3 and 4 present the cyclic voltammogram at scan rates 5, 10 and 20 mV·s−1 for [EMIM][Tf2N], [PMIM][Tf2N] and [BMIM][Tf2N], respectively, at different solvent con-centrations. Since different REs were used for different ILs, the potential of all the systems is expressed with respect to Ag/Ag+ (0.1 mol·dm−3 AgNO3 in [PMIM][Tf2N] + ACN). The nearly rectangular shape of the CVs at lower scan rates and their symmetry against the zero line of the y-axis indicate that the studied systems show nearly ideal double layer capaci-tive behavior for an EDLC [44]. Moreover, the absence of any peak in CV curves excludes the possibility of any redox reaction, which proves the pure double layer behavior of all the systems. The electrolyte comprising [PMIM][Tf2N] gives higher current than the others at any scan rate, which corresponds to higher ionic conductivity of this IL solution (Fig. S2).

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l vs A

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g+ (V

)

Time (s)

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(b)

(d)(c)

Fig. 2 CV for a 2 mol·dm−3, b 1 mol·dm−3 and c 0.5 mol·dm−3 [EMIM][Tf2N] solutions at different scan rates and d GCD for different [EMIM][Tf2N] concentrations at 0.5 A·g−1


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Following CV, GCD tests were performed at constant current values of 0.5 and 1 A·g−1. The OPW obtained from CV was successfully reproduced by the GCD (Figs. 2, 3, 4 and S3). A decrease in the electrochemical window by 0.5 V, as a result of reduction in the anodic voltage limit, is observed when the concentration decreases from 1 to 0.5 mol·dm−3. A similar pattern was observed by Scalia et al. [45] with increase of the co-solvent pro-pylene carbonate wt% for N-butyl-N-methylpyrrolidinium 4,5-dicyano-2-(trifluoromethyl) imidazole. Although the OPW remained the same for 2 and 1 mol·dm−3 IL solutions, higher resistance is observed from both CV and GCD for 2 mol·dm−3 solutions due to higher viscosity of the electrolyte solutions (Fig. S2).

The measured electrochemical stability is wider for [PMIM][Tf2N] and [BMIM][Tf2N] (3 V for 1 and 2 mol·dm−3 and 2.5 V for 0.5 mol·dm−3 solutions) than [EMIM][Tf2N] (2.5 V for 1 and 2 mol·dm−3 and 2 V for 0.5 mol·dm−3 solutions). One reason may be the use of Ag wire as pseudo RE for [EMIM][Tf2N] based systems. For REs like Li and Ag wires, the redox couple that determines the potential is undefined, which lowers the degree of accuracy in measurement of electrochemical stability [46]. Impurities present in ILs also lower the OPW [46]. The OPW using pure [BMIM][Tf2N] against a Si nanowire electrode was limited to only 1.6 V [4]. The same for pure [EMIM][Tf2N] was observed to be 3–3.5 V [6] against different kinds of carbon electrode. Further, Bettini et al. found an OPW of 3.5 V with pure [BMIM][Tf2N] for a cluster assembled nanostructure carbon EDLC [17]. Previous work suggested that the electrochemical stability window is also dependant on the type of working electrode [2, 7, 13, 46]. The OPW achieved for the elec-trolytes used in our work is comparable with literature data [6, 17] and still higher than for

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-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0-2.5-2.0-1.5-1.0-0.50.00.51.01.52.02.5

-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0-2.5-2.0-1.5-1.0-0.50.00.51.01.52.02.5

0 250 500 750 1000 1250 1500 1750 2000 2250-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

Cur

rent

(A g

-1)



Cur

rent

(A g

-1)



Cur

rent

(A g

-1)



Pote

ntia

l vs A

g/A

g+ (V

)

Time (s)

2 mol dm-3

1 mol dm-3

0.5 mol dm-3

(a) (b)

(c) (d)

Fig. 4 CV for a 2 mol·dm−3, b 1 mol·dm−3 and c 0.5 mol·dm−3 [BMIM][Tf2N] solutions at different scan rates and d GCD for different [BMIM][Tf2N] concentrations at 0.5 A·g−1

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.240

45

50

55

60

65

70

75

80

85

90

95

100

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.240

45

50

55

60

65

70

75

80

85

90

95

100[EMIM][Tf2N][PMIM][Tf2N][BMIM][Tf2N]

Sp. C

apac

itanc

e (F

g-1)

Molar concentartion of IL (mol dm-3)

[EMIM][Tf2N][PMIM][Tf2N][BMIM][Tf2N]

Sp. C

apac

itanc

e (F

g-1)

Molar concentartion of IL (mol dm-3)

(a) (b)

Fig. 5 a Specific capacitance from CV at 5 mV·s−1, b specific capacitance from GCD at 1 A·g−1


1 3

aqueous electrolytes reported in the literature [9–12]. Figure 5 represents specific capaci-tance from CV at 5 mV·s−1 scan rate and GCD at 1 A·g−1 current density. This agrees well with a concentration dependant capacitance study [42] where the authors studied a wide range of solvent concentrations to have a comprehensive study regarding the effect on specific capacitance upon dilution of an ionic liquid (IL) electrolyte with three differ-ent organic solvents. Both techniques provided greater values of specific capacitance for 1 mol·dm−3 solution of ILs. At the same concentration, the measured OPW is the largest, which increases the energy density. A high IL concentration, such as 2 mol·dm−3 (~ 25 wt% of ACN), still exhibits higher viscosity and lower ionic conductivity (Fig. S2) than at the other two concentrations, due to greater interionic interactions, which limit diffusion of the ions. This results in lower conductivity and capacitance in spite of having a high concentra-tion of charged species. The increase in solvent (1 mol·dm−3 IL, ~ 57 wt% ACN) improves both transport properties, which eventually provides the highest specific capacitance. Fur-ther, lowering of the IL concentration (0.5 mol·dm−3 IL, ~ 75 wt% ACN) decreases the vis-cosity; however, due to insufficient ion concentrations both ionic conductivity and specific capacitance decrease. Considering ionic conductivity, CV and GCD analysis together, the 1 mol·dm−3 solution of IL was taken to be the optimum electrolyte composition for subse-quent EIS, power and energy measurements.

Potentiostatic EIS measurements were performed over a frequency range of 100 kHz to 10 mHz at different potentials as described in Sect. 3.3. Nyquist plots for 1 mol·dm−3 solutions of three imidazolium ILs can be seen in Fig. 6. The Nyquist plot consists of three parts. The first part represents the insertion of the curve on the x-axis which depicts the solution resistance (Rs) and the semicircle whose diameter gives the charge transfer resistance (Rct). The second part involves a straight line with slope of 45° representing the

(a)(b)

(c) (d)

Fig. 6 a Nyquist plot for 1 mol·dm−3 solutions of ILs at 0 V versus OCP, frequency dependant capacitance for 1 mol·dm−3 solution of b [EMIM][Tf2N], c [PMIM][Tf2N] and d [BMIM][Tf2N]


1 3

Warburg impedance. The concluding part gives a nearly vertical tail at lower frequencies representing purely capacitive behavior. In our work (Fig. 6), the occurrence of the semi-circle indicates lower ionic conductivity than aqueous electrolytes [44]. The lowest solu-tion resistance is found for the [PMIM][Tf2N] solution due to its having the highest ionic conductivity at that particular concentration. The diameter of the semicircle appears to be nearly equal for all the electrolytes. The shorter 45° phase shift indicates lower Warburg impedance, which is associated with diffusion of ions into the porous carbon electrode. Reasonable capacitive behavior for both systems can be observed from the nearly vertical line at lower frequencies. Further, the capacitance from EIS data was calculated (Eq. 6) and plotted against frequency (Fig. 6) for OCP and other two overpotential measurements. A reduction in capacitance of ≤ 10 F·g−1 can be observed while disturbing the system from equilibrium (OCP) to a potential of 100 mV. The frequency dependant capacitance agrees well with already evaluated specific capacitance (Fig. 5) and the highest value of the same is obtained for [PMIM][Tf2N].

The energy and power density (Fig. 7) of the carbon electrode was determined using the following relations [44].

where V is the cell voltage, C is the specific capacitance and t is the discharge time.Capacitance from GCD at 1 A·g−1 specific current is comparable to that from CV and

EIS for both systems. Application of a lower specific current of 0.5 A·g−1 to GCD lowers the IR drop and hence significant enhancement of capacitance is achieved (Fig. 7). Due to higher specific capacitance, the energy density at 0.5 A·g−1 is greater than that at 1 A·g−1 (Fig. 7). On the contrary, the calculated power density appears to be higher at 1 A·g−1 because of lower discharge time (Fig. S3). The three electrolytes exhibit reasonable per-formance in terms of energy and power density. Particularly, energy density is significantly

(7)E =1

2CV2

(8)P =E

t

(a) (b)

Fig. 7 a Specific capacitance from GCD test for 1 mol·dm−3 IL solutions at different current density, b power versus energy plot for 1 mol·dm−3 IL solutions


1 3

higher than for aqueous electrolytes [6, 9]. Energy density of EDLC consisting of LiPF6 as electrolyte at 1 A·g−1 is limited to only 18 W·h·kg−1 [6]. Specific energy and power obtained for pure [BMIM][Tf2N] with Si nanowire electrode are 0.23 W·h·kg−1 and 0.4 kW·kg−1, respectively, and the corresponding specific capacitance was only 0.7 F·g−1 [4]. The energy and power density obtained are similar for 1 mol·dm−3 of [PMIM][Tf2N] and [BMIM][Tf2N] at either of the current densities and are significantly greater than for [EMIM][Tf2N]. Therefore, a life cyclic study was conducted for only these two systems. The retention of original capacitance observed after 1000 cycles of GCD at 1 A·g−1 was 75% and 70% for 1 mol·dm−3 solutions of [PMIM][Tf2N] and [BMIM][Tf2N], respectively. The significant loss in capacitance can be understood from application of a high current (1 A·g−1). This invariably reduces the capacitance and cycle number as a consequence of the high IR drop.

5 Conclusions

In the initial part of the study, the MD tool has helped us in having a preliminary idea on the favorable ionic conductivity and viscosity resulting from the addition of a co-solvent, prior to any electrochemical study. Experimental findings then led to the proposal that the 1 mol·dm−3 solution of [PMIM][Tf2N] is a better electrolyte in terms of specific capaci-tance. However, 1 mol·dm−3 solutions of both [PMIM][Tf2N] and [BMIM][Tf2N] had the same OPW and nearly equal energy and power densities. Decrease in current with lowering the CV scan rate indicates moderate rate performance of the EDLC. Studies with [PMIM][Tf2N] can now bridge the gap between [EMIM][Tf2N] and [BMIM][Tf2N] in the field of EDLC. Future work is underway to interpret the performance at different temperatures.

Acknowledgements This work was funded by the Vikram Sarabhai Space Centre (ISRO), Thiruvanan-thapuram vide ISRO/RES/3/745/17-18. Authors acknowledge Prof. Mihir K. Purkait for providing the Microprocessor based water-soil analysis kit (VSI 302, VSI Electronic Private Ltd). Computational time from the Param Ishan supercomputer is duly acknowledged.

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Affiliations

Upasana Mahanta1 · R. Prasanna Venkatesh1 · S. Sujatha2 · S. A. Ilangovan3 · Tamal Banerjee1

* Tamal Banerjee [email protected]

1 Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati, India2 Advanced Power Systems, Vikram Sarabhai Space Centre (ISRO), Thiruvananthapuram, India3 Chemical Systems Division, Vikram Sarabhai Space Centre (ISRO), Thiruvananthapuram, India

http://orcid.org/0000-0001-8624-6586

Imidazolium Based Ionic Liquids as Electrolytes for Energy Efficient Electrical Double Layer Capacitor: Insights from Molecular Dynamics and Electrochemical CharacterizationAbstract1 Introduction2 Computational Details2.1 Molecular Dynamics Simulation Details2.2 Determination of Ionic Conductivity and Viscosity

3 Experimental Details3.1 Chemicals3.2 Density, Ionic Conductivity and Viscosity Measurement of Electrolytes3.3 Electrochemical Characterization of EDLC

4 Results and Discussion4.1 Physical Properties4.1.1 Density4.1.2 Ionic Conductivity and Viscosity

4.2 Performance of EDLC

5 ConclusionsAcknowledgements References

imidazoliumbasedionicliquidsaselectrolytes ......2019/07/24 · ionic liquids (ils) have attracted...

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