Electrical Double-Layer Capacitor Based on Low Aqueous Electrolyte Contents in EmimTFO Ionic Liquid

Kitenge, V. N.; Tarimo, D. J.; Oyedotun, K. O.; Rutavi, G.; Bakhoum, D. T.; Manyala, N.

doi:https://doi.org/10.1155/2023/8659009

International Journal of Energy Research

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Abstract Introduction Experimental Results and Discussion Conclusion Data Availability Conflicts of Interest Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2023 | Article ID 8659009 | https://doi.org/10.1155/2023/8659009

Electrical Double-Layer Capacitor Based on Low Aqueous Electrolyte Contents in EmimTFO Ionic Liquid

V. N. Kitenge,¹D. J. Tarimo,²K. O. Oyedotun,²G. Rutavi,²D. T. Bakhoum,²and N. Manyala²

Academic Editor: Pranav Kalidas Katkar

Received15 Feb 2023

Revised11 Apr 2023

Accepted13 Apr 2023

Published25 Apr 2023

Abstract

A study has been conducted on the electrochemical properties of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EmimTFO) protic ionic liquid enhanced by adding potassium nitrate (2.5 M) aqueous solution. The properties of EmimTFO as well as mixtures diluted by molar fractions of 0.6, 0.7, 0.8, and 0.9 of KNO₃ were also investigated through measurements of viscosity, density, and conductivity. In a three-electrode test run at 0.25 A g^-1, the addition of 2.5 M KNO₃ solution generated peak specific capacities of ~40.2 and ~85.8 mAh g^-1 on the positive and negative potentials, respectively. These performances surpassed the specific capacities obtained for EmimTFO in a three-electrode run at 0.25 A g^-1 using the same electrode material (activated carbon). The top-performing electrolyte mixture ([EmimTFO]_0.8[2.5 M KNO3]_0.2) was then used to assemble a symmetric supercapacitor, which could run at a voltage of ~2.1 V. The device was able to retain 71.35% of its capacitance after 10,000 cycles of charge and discharge. It also displayed higher specific energy and power of 22.21 Wh kg^-1 and 520 W kg^-1, respectively, at 0.5 A g^-1 as compared to specific energies of 4.73 Wh kg^-1 and 11.2 Wh kg^-1 for the devices assembled with single EmimTFO and 2.5 M KNO₃ as the electrolytes, respectively.

1. Introduction

Seeking effective, dependable, and economical electrochemical energy devices is still a global essential responsibility [1, 2]. Promising energy storage technologies, involving both physical and chemical energies, are being studied to alleviate the challenge of a continuous energy supply. Supercapacitors (SCs) make part of the leading technologies for energy storage [3]. SCs are known for their superior specific power output, reasonable production cost, safety, and long cycle life [4]. Electrodes and electrolytes are the key components of SCs that could impact their performances [5, 6]. Developing novel electrode materials or electrolytes can result in improving SCs’ specific energy [7].

Electric double-layer capacitors (EDLCs) are typical supercapacitors. EDLC electrode materials exhibit excellent electrical conductivity and have a high specific surface area which allows them to efficiently store charge [8]. These properties promote EDLCs’ charge storage mechanism which is accomplished through swift electrostatic adsorption of electrolyte ions on the surface of the electrode, resulting in the formation of an electric double layer (EDL) at the juncture between the electrode and electrolyte [9]. One such material that is commonly used in EDLCs is activated carbon (AC). EDLCs have a notable drawback which is their lower energy density compared to other types of capacitors. The energy density of EDLCs is closely tied to the operating voltage range of the device. Therefore, it is essential to design electrolytes that allow wide potential windows while also offering high ionic conductivity to achieve optimal performance [10]. Ionic liquids (ILs) are a cutting-edge category of electrolytes used in energy storage devices, particularly in supercapacitors. [11]. These electrolytes enable higher cell potential (U) which in turn enhances the specific energy (E) in energy storage devices as per the formula: ( – capacitance). The chemical structure of ILs categorizes them as aprotic (AILs) or protic (PILs) ionic liquids. PILs differ from AILs because they can carry one or more protons on their cation or anion, whereas AILs cannot [12]. ILs demonstrated great electrochemical stability when used as electrolytes in SCs [13–15]. They possess a wider potential window than traditional aqueous and organic electrolytes. However, their room-temperature ionic conductivity is typically lower than that of these other electrolytes [16]. This is due to their viscous nature which restricts their use in their pure form at room temperature [17]. Preparing mixtures of previously recognized ILs for energy storage applications is one technique for improving their transport properties [18–20]. Mixtures of ILs with other types of solvents have also been explored. A study by Palm et al. [21] revealed that incorporating 16% by weight of acetonitrile (AN) into 1-ethyl-3-methylimidazolium tetrafluoroborate (EmimBF₄) decreased the EmimBF₄’s viscosity by 20% while also significantly increasing the mixture’s conductivity by 2.5 times. Chaban [22] also found that adding AN to several imidazolium ILs improves their conductivity by up to 50 times their original values. Sethuraman and Montree [23] reported a dual ionic liquid (IL) and 1.0 M LiTFSI (aq.) “water in salt” mixture with enhanced specific capacitance and specific energy as compared to ionic liquid electrolyte alone.

Not much studies in the literature have explored the addition of water-based (aqueous) electrolytes (potassium nitrate, potassium hydroxide, sodium hydroxide, and so on) to improve IL properties. This present work analyzes, for the first time, the electrochemical properties of an innovative electrolyte mixture made up of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EmimTFO) and 2.5 M KNO₃, at different molar ratios of 0.6, 0.7, 0.8, and 0.9. EmimTFO (PILs) was selected due to its hydrophilic and protic nature, which is advantageous for dilution with the aqueous electrolyte. To investigate the electrochemical properties of these mixtures, techniques such as impedance spectroscopy (EIS), galvanostatic charge-discharge (GCD), and cyclic voltammetry (CV) were used to evaluate the performance of the IL/aqueous mixtures as electrolytes in a three- and two-electrode configuration, where AC was utilized as the electrode material. As PILs mainly carry available protons, these electrolytes allow mixtures to combine them with an aqueous medium through hydrogen bonding to form the Grotthuss like protons [24]. The mixture may also prevent the use of highly flammable, volatile organic solvents while obtaining an economical electrolyte with high potential windows. The electrolyte mixture has illustrated an interesting electrochemical feature that unlocks the limits of aqueous electrolyte and ionic electrolyte systems, providing an important direction for the development of high-energy supercapacitors.

2. Experimental

EmimTFO IL ( ppm, 99.9%) was obtained from Solvionic (Toulouse, France). Deionized (DI) water was utilized to prepare the 2.5 M KNO₃ solution. KNO₃ was purchased from Sigma Aldrich (Missouri, United States). The four electrolyte mixtures were prepared via magnetic stirring at 120 rpm for 1 hour (h) within a glovebox (provided by Inert Technologies, USA) which ensured that the oxygen and water levels were maintained at 0.0%. The in the formula represents the ionic liquid’s molar fraction, and 1- the 2.5 M KNO₃ molar fraction in the solutions. The prepared mixtures are: (1) [EmimTFO]_0.9[2.5 M KNO₃]_0.1, (2) [EmimTFO]_0.8[2.5 M KNO₃]_0.2, (3) [EmimTFO]_0.7[2.5 M KNO₃]_0.3, and (4) [EmimTFO]_0.6[2.5 M KNO₃]_0.4, respectively.

The conductivities of EmimTFO, 2.5 M KNO₃, and the mixtures were determined using a Jenco model 3020M conductivity meter (Jenco, USA) with conductivity accuracy of ~0.5 F. S at a temperature of 18.2°C. The viscosities were evaluated using an RV model NDJ-8S viscometer (W & J Instrument Co., PRC). The densities (ρ) were measured using an Attension model Sigma 700 densimeter (KSV Instrument, Finland).

Activated carbon derived from mangosteen shell (MS-AC), which was synthesized in our previous work, was used as the electrode material. A one-step carbonization/activation method was used to synthesize the material from the mangosteen shells (MS) [25]. The synthesis process involved pyrolyzing a mixture of 5 grams (g) of mangosteen shells and 5 g of K₂CO₃ (activating agent) in a tubular furnace. The furnace was set to a temperature of 700°C and heated at a ramp rate of 5°C per minute for 2 h while argon gas was flown at 200 cm³ min^-1. Postsynthesis, the material was filtered, thoroughly rinsed until a neutral pH was attained, and dried at 60°C for a period of 12 h. Figure 1 displays the textural properties obtained with MS-AC. These properties were analyzed using a NOVA touch LX² Quanta chrome pore size distribution and surface area analyzer (NOVA touch NT 2 LX-1, 220 V, USA). The Brunauer-Emmett-Teller (BET) method was used to evaluate the material’s specific surface area through isotherms of adsorption and desorption, utilizing a relative pressure (P/Po) range of 0.05 to 0.95 (as depicted in Figure 1(a)). The density functional theory (DFT) method enabled determining the material’s pore-size distribution (as seen in Figure 1(b)). The results illustrated the type-I isotherm with an H4 hysteresis loop, representing a high percentage of micropores and mesopores in the MS-AC. The morphological image of the sample, as seen in the inset, was acquired with a Zeiss Ultra Plus 55 field emission scanning electron microscope (FE-SEM, Akishimashi, Japan) set at 2.0 kV. The image displays a flower-like morphology having a mesoporous arrangement with open cavities. Table 1 indicates the different textural features obtained from the material.

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In order to prepare the electrodes, a blend of 80% active material, 10% polyvinylidene difluoride (PVDF) as a binder, and 10% conductive carbon acetylene black (CAB) was made. A total of 3.2 mg of electrode material was used per electrode, consisting of 2.56 mg of active material (activated carbon), 0.32 mg of CAB, and 0.32 mg of PVDF. CAB was added to boost the conductivity lost due to the presence of the nonconductive PVDF. The mixture was then made homogenous by producing a slurry with the addition of two 0.5 ml drops of N-methyl-2-pyrrolidone (NMP). This slurry was then coated on the surface of a 1 cm² nickel foam current collector. The produced electrodes were finally dried in an electric furnace for 12 h at 60°C.

The measurements were conducted at room temperature using a Biologic VMP300 potentiostat (made in the USA) ran with EC-Lab® V11.33 software. The measurements were completed in both a three-electrode and a two-electrode configuration. The three-electrode comprised a T-cell consisting of the fabricated electrode as the working electrode, a silver (Ag) disc used as the reference electrode, and a glassy carbon disc used as the counter electrode. Except from the aqueous-assembled T-cell, all the T-cells made for three-electrode measurements were assembled in the glovebox with all three components separated by filter paper. Before assembly, the electrodes were vacuum-dried at 60°C for 10 hours. The mass of the active material used in the electrodes varied between 2.5 and 2.8 mg cm^-2 for all the materials under study. CV measurement was carried out at various scan rates within a potential window ranging from -1.6 to 0.0 V for the negative electrode and 0.0 to 1 V for the positive electrode, with both potentials relative to the reference electrode. The GCD test was performed at different specific currents for both the positive and negative tests. EIS was evaluated at frequencies ranging from 10 mHz to 100 kHz at open-circuit potentials.

2.1. Calculations

The specific capacitances () in the three electrodes for EDLC material were obtained from GCD curves using the following equation: where is the mass of the electrodes (mg); is the applied current (mA), is the discharge time (s); and is the potential window (volt: V).

For nonlinear GCD curves, the specific capacities () calculated in the three electrodes were determined as per the following formula: where specific is the discharge capacity (mAh g^-1); is the discharge time (s); is the applied current (mA); and is the active material mass (g).

Equation (3) was used to estimate how much charge is stored on each electrode as follows:

The specific capacitance of single electrode () for different symmetric devices GCD curves which showed EDLC behaviour was obtained using the following equation: where is the overall mass of both electrodes; is the applied current, is the discharge time, and is the voltage of the device.

The specific energy () and specific power () of symmetric devices were obtained using the following equations: where is the device capacitance, is the device voltage, and is the device’s discharge time.

The Coulombic efficiencies were calculated through the following method:

3. Results and Discussion

Figure 2 shows the changes in the density, viscosity, and conductivity properties as the molar fraction of the 2.5 M KNO₃ aqueous electrolyte is increased within the mixtures of EmimTFO. The trends illustrated that the density, viscosity, and conductivity of the electrolytes highly depend on the molar fraction of 2.5 M KNO₃ within the mixtures. A slight change in room temperature is observed due to minor variations in weather conditions during the measurement. Figures 2(a) and 2(b) illustrate that the addition of 2.5 M KNO₃ leads to a lower IL density and viscosity. This is expected due to the low density and viscosity of the aqueous solution contributing to lower values of the neat EmimTFO after dilution. In Figure 2(b), the change in viscosity is seen to be prominent when the molar fraction is 0.9, and small changes are observed from the molar fraction of 0.8 downward. This shows that a low amount of 2.5 M KNO₃ was sufficient to obtain a viscosity close to that of an aqueous electrolyte. The conductivity (Figure 2(c)) is shown to increase with the addition of 2.5 M KNO₃ which decreases the EmimTFO molar fractions in the mixture. This can be advantageous as a combination of high conductivity and low viscosity normally demonstrates high power in EDLCs [5]. Most of the ILs used for supercapacitors are reputed to have trivial conductivities [26]. The hydrophilic nature of EmimTFO enables good mixing with the 2.5 M KNO₃ and leads to better transport properties. Figure 2(d) illustrates a photograph of the single electrolyte and the prepared mixture. All the mixtures illustrated spontaneous miscibility with EmimTFO, and no biphasic states were noticed. However, little particle accumulations at the bottom of the container can be seen with [EmimTFO]_0.7[2.5 M KNO₃]_0.3 and [EmimTFO]_0.6[2.5 M KNO₃]_0.4. This phenomenon is suggested to determine the hydrophilic and miscibility limit of EmimTFO with regard to the 2.5 M KNO₃ solution.

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Figure 3 illustrates the electrochemical properties of the neat EmimTFO and 2.5 M KNO₃. All electrochemical measurements were performed using MS-AC described in the experimental section as the working electrodes. Figures 3(a) and 3(b) display the CV and GCD patterns of the ionic liquid, illustrating partially-rectangular CV curves and moderately linear GCD triangles in a three-electrode measurement. These shapes represent normal pseudocapacitor behaviour, and it is noticed that EmimTFO ILs have an impact on this behaviour. EmimTFO is also responsible for the IR drop occurring in Figure 3(b) due to poor transport properties leading to nonideal galvanostatic charge-discharge. Despite a low current response and discharge time, the electrolyte displayed an interesting potential window (0.0 to 1.1 V vs. Ag) on the positive potential and negative potential (-1.6 to 0.0 V vs. Ag). The potential window obtained on the positive and negative potentials could compare well with the previously reported three-electrode measurement utilizing hydrophobic ILs [13, 27]. Figures 3(c) and 3(d) present the results of CV and GCD patterns for the 2.5 M KNO₃ electrolyte. As displayed in Figure 3(c), the CV pattern of the 2.5 M KNO₃ electrolyte exhibits a greater current response and a more pronounced rectangular profile when compared to other electrolytes. Potassium (≈280 pm) and nitrate (≈280 μm) ion sizes are suitable for the MS-AC porous structure in enhancing the ion diffusion within the porous material’s framework when using 2.5 as the molarity. This led to an ideal GCD as seen in Figure 3(d) which confirms the observations in Figure 3(c). However, the potential window on the aqueous electrolyte was limited to 0.7 and -0.9 V vs. Ag on the positive and negative potentials, respectively. The difference in equivalent series resistance () between EmimTFO and 2.5 M KNO₃ can be observed in Figure 3(e). The EmimTFO electrolyte’s Nyquist plots showed a semicircular shape in the high-frequency range, showing substantial charge transfer resistance at the active material/electrolyte boundary, which supports the electrolyte’s pseudocapacitive behaviour mentioned earlier. Inversely, the absence of a semicircular shape in the 2.5 M KNO₃ electrolyte highlighted the outstanding ionic conductivity and revealed a low charge transfer resistance. This is supported by the aqueous medium’s short, vertically directed diffusion route.

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In this section, we present an illustration of the electrochemical performance after the dilution of EmimTFO. Figure 4 illustrates the CV, GCD, and EIS results obtained with the four different mixtures: (1) [EmimTFO]_0.9[2.5 M KNO₃]_0.1, (2) [EmimTFO]_0.8[2.5 M KNO₃]_0.2, (3) [EmimTFO]_0.7[2.5 M KNO₃]_0.3, and (4) [EmimTFO]_0.6[2.5 M KNO₃]_0.4, respectively. All the electrolyte mixtures were tested within a positive potential range of 0.0 to 0.8 V vs. Ag and a negative potential range of -1.2 to 0.0 V vs. Ag. These limitations are caused by the existence of water in the mixture, which triggers oxygen and hydrogen evolutions. Many other activated carbon studies could not obtain such potential windows, especially on the negative potential, when using 2.5 M KNO₃ alone as the electrolyte [28–31]. The different potential limits are similar to the ones obtained with 2.5 M KNO₃ but with an extended potential window favoured by the presence of the EmimTFO. Figures 4(a) and 4(b) illustrate the CV curves obtained with the four distinct mixtures in the positive and negative potentials. [EmimTFO]_0.8[2.5 M KNO₃]_0.2 appears to have the highest current response as compared to the other mixtures. These results demonstrate the good interaction between EmimTFO and 2.5 M KNO₃ in the 0.8 to 0.2 molar fraction which triggered a high migration of ions across the carbon material interface. Additionally, this mixture allowed a significant amount of inaccessible micropores to be polarized because of the decent electrode surface wettability occurring when it is used as an electrolyte, thus yielding high ionic conductivity and great electrochemical performances. A clear decline in the current response is observed for [EmimTFO]_0.7[2.5 M KNO₃]_0.3 and [EmimTFO]_0.6[2.5 M KNO₃]_0.4. Figures 4(c) and 4(d) illustrate the distinct four mixtures’ GCD curves on both potentials. In this case, [EmimTFO]_0.8[2.5 M KNO₃]_0.2 also demonstrated longer discharge times on both the positive and negative potentials, confirming the high current response demonstrated in the CV curves. This is due to the improved transport properties acquired by combining the two electrolyte mediums, resulting in a larger charge build-up on the electrodes. The IR drop observed in Figure 3(c) with the [EmimTFO]_0.9[2.5 M KNO₃]_0.1 electrolyte is less significant for the [EmimTFO]_0.8[2.5 M KNO₃]_0.2 and the other electrolytes, as displayed in Figure 4(c). This confirms the ideal interaction at the electrode/electrolyte interface when using [EmimTFO]_0.8[2.5 M KNO₃]_0.2.

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Figures 5(a) and 5(b) illustrate a plot of several specific capacities since the materials show semilinear GCD curves against different specific currents determined for the four electrolyte mixtures in the positive and negative potentials calculated using Equation (2). The mixture [EmimTFO]_0.8[2.5 M KNO3]_0.2 demonstrates high specific capacity on both potentials. The electrolyte generated specific capacities of 40.2 and 85.8 mAh g^-1 on the positive and negative potentials, respectively, at 0.25 A g^-1. [EmimTFO]_0.8[2.5 M KNO₃]_0.2 also exhibited a sustaining capacity retention equivalent to 40% and 48% of its original capacity when raising the specific current to 2 A g^-1 on the negative and positive potentials, respectively. Figure 5(d) illustrates the EIS measurement obtained with the four mixtures. The figure displays a distinct semicircle in the region of high frequency for all the mixtures. However, [EmimTFO]_0.8[2.5 M KNO3]_0.2 displays a less pronounced semicircle indicating minimal charge transfer resistance at the electrolyte/electrode interface [32, 33]. The mixtures demonstrated values of 3.8, 2.5, 5.5, and 3.3 Ω for [EmimTFO]_0.9[2.5 M KNO3]_0.1, [EmimTFO]_0.8[2.5 M KNO3]_0.2, [EmimTFO]_0.7[2.5 M KNO3]_0.3, and [EmimTFO]_0.6[2.5 M KNO3]_0.4, respectively. The low value obtained with [EmimTFO]_0.8[2.5 M KNO3]_0.2 illustrates its enhanced conductivity and faster mass transport on the electrode’s interface as compared to the other mixtures. [EmimTFO]_0.8[2.5 M KNO3]_0.2 is therefore considered to be an optimal mixture to achieve good ion diffusion and adsorptions in this study.

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Figure 6 illustrates a comparison of the electrochemical properties of MS-AC//MS-AC symmetric devices assembled with EmimTFO, 2.5 M KNO₃, and the mixture [EmimTFO]_0.8[2.5 M KNO₃]_0.2 as electrolytes. The mixture [EmimTFO]_0.8[2.5 M KNO₃]_0.2 was selected for the full device because it showed superior electrochemical results as compared to other mixtures. The CV curves of the devices assembled with different electrolytes at a scan rate of 20 mV s^-1 are displayed in Figure 6(a). The 2.5 M KNO₃ demonstrated the highest current response at a cell voltage of 1.6 V. This electrolyte shows a voltage window limitation as it is a water-based electrolyte. The EmimTFO device displayed the advantage of a water-free electrolyte which reached 2.6 V. Yet, its low conductivity led to a low current response as compared to the other two electrolytes. The [EmimTFO]_0.8[2.5 M KNO₃]_0.2 device illustrated EDLC behaviour up to a voltage window of 2.1 V while displaying a higher current response compared to the EmimTFO device. This result of the [EmimTFO]_0.8[2.5 M KNO₃]_0.2 electrolyte mixture is promising as it enabled voltage extension compared to KNO₃ electrolyte with a better current response as compared to EminTFO electrolyte which indicates the improvement in ionic conductivity within the device. Furthermore, the voltage extension will also be beneficial for enhancing the specific energy (E_s). Figure 6(b) shows the linear EDLC behaviour for the GCD patterns of the devices in these three different electrolytes. A higher charge-discharge time for the [EmimTFO]_0.8[2.5 M KNO₃]_0.2 MS-AC//MS-AC device is observed. This is due to the improved viscosity and conductivity which contributed toward having a higher number of ions diffusing within the porous material. The EIS Nyquist plot for the three devices is displayed in Figure 6(c). The addition of 2.5 M KNO₃ to the [EmimTFO]_0.8[2.5 M KNO₃]_0.2 mixture lowers the semicircle that is observed when the EmimTFO electrolyte is used alone. Also, the device displayed a short diffusion length, with its slope being closer to the vertical axis mirroring the ideal supercapacitor. Despite its value of 2.68 Ω, the overall EIS results show that the [EmimTFO]_0.8[2.5 M KNO₃]_0.2 electrolyte yielded a low charge transfer resistance, promoted by the good ionic conductivity within the device.

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Figure 7 provides the full electrochemical results of the [EmimTFO]_0.8[2.5 M KNO₃]_0.2 MS-AC//MS-AC symmetric device. The device’s CV curves at multiple scan rates are shown in Figure 7(a). Typical EDLC behaviour can be noticed from the curves having almost rectangular shapes. The GCD triangles are displayed in Figure 7(b). The figure illustrated a reversible path which confirms the EDLC behaviour. A slight bend is observed at approximately 2.1 V with the curve running at 0.5 A g^-1. This was used as an indication to determine the maximum voltage at which the device could operate as the undulation may indicate electrolyte deterioration taking place in the device. The CVs and GCDs obtained in Figures 7(a) and 7(b) further confirm the EDLC behaviour in the assembled device. The specific capacitance of the single electrode in the device is illustrated in Figure 7(c) and calculated using an equation (3). At a current density of 0.5 A g^-1, it exhibits a specific capacitance of 138.62 F g^-1. Even when the current density reached 5 A g^-1, the device could still maintain 40% of its original value with a specific capacitance of 55.08 F g^-1. The Nyquist plot as well as the equivalent circuit (inset to the figure) for the [EmimTFO]_0.8[2.5 M KNO₃]_0.2 devices is presented in Figure 7(d). The fitting was accomplished using a complex nonlinear least square program using the equivalent circuit. The device’s Nyquist plot demonstrates a low semicircle that is brought due to small charge transfer resistance () at the electrode and electrolyte interface. The obtained and values were 2.7 Ω and 3.4 Ω, respectively, for the experimental/fitting curve. The slow adsorption process, which occurs because of the complex mixture of ions, is suggested to be the root cause of this resistance. [34]. The element in the circuit is linked to the conductivity of the electrolyte mixture toward the electrode materials. Furthermore, the Warburg element (Z_w) which is also present in the circuit represents the mass-transport process of ions that takes place at the electrode/electrolyte interface [35]. This process is also associated with the mass capacitance, , in the circuit. The element in the circuit indicates that the device behaves like a capacitor. The symmetric device bode plot is illustrated in Figure 7(e). The figure indicates a device’s phase angle of -70.1° which is close to the ideal capacitor phase angle of -90°. The inset to Figure 7(e) illustrates a photograph whereby the [EmimTFO]_0.8[2.5 M KNO₃]_0.2 device powered 4 light-emitting diodes (LED) connected in parallel which requires at least 1.8 V. This illustrates the extent to which the assembled device can be applied. Figure 7(f) presents the real (C) and imaginary (C) capacitance values of the [EmimTFO]_0.8[2.5 M KNO₃]_0.2 device as a function of frequency. It was observed that the imaginary capacitance value, C, reaches its peak value at a frequency of approximately 0.02 Hz, which corresponds to a relaxation time of 3.54 seconds. This relaxation time is the minimum duration for the device to be completely charged. These results suggest that this device can attain full charge in a notably short time.

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Figure 7

(a) CV curves measured at different scan rates, (b) GCD curves measured at various specific currents, (c) specific capacitance and specific current relationship, (d) fitting of EIS Nyquist plot (inset equivalent series circuit), (e) phase angle as a function of frequency (with a photograph of the device powering LEDs (light-emitting diodes) in the inset). (f) Real and imaginary plots of the capacitance vs. frequency for the device assembled with [EmimTFO]_0.8[2.5 M KNO₃]_0.2 as the electrolyte.

Figure 8(a) illustrates the Coulombic efficiency of all the devices with an overall performance above 99% up to 10,000 cycles. Figure 8(b) displays the retention in capacitance of the devices after 10,000 charge-discharge cycles with percentages of 94.7, 71.4, and 88.5 for the 2.5 M KNO₃, the [EmimTFO]_0.8[2.5 M KNO₃]_0.2, and EmimTFO electrolytes-based devices, respectively. The minor decline in capacitance retention occurring with the [EmimTFO]_0.8[2.5 M KNO₃]_0.2 device after performing 6000 charge-discharge cycles may be related to the water content within the mixture which disturbed the device’s electrochemical stability. Figure 8(c) presents the Ragone plot that shows the correlation between the specific power and specific energy of the device. The plot demonstrates the essence of this investigation as the [EmimTFO]_0.8[2.5 M KNO₃]_0.2 is seen to increase the range of the specific energy. The specific energies (E_s) for the [EmimTFO]_0.8[2.5 M KNO₃]_0.2 device obtained at various specific currents varied between 22.2 Wh kg⁻¹ and 8.4 Wh kg⁻¹ with specific powers (Ps) varying between 153.8 W·kg⁻¹ and 1458.3 Wkg⁻¹. The specific energies of the [EmimTFO]_0.8[2.5 M KNO₃]_0.2 device surpassed those obtained with the other two assembled devices which illustrated maximum E_s values of 11.2 Wh kg⁻¹ for 2 M KNO₃ and 4.7 Wh kg⁻¹ for EmiTFO when evaluated in the same specific currents. These results demonstrate the highly efficient reversible adsorption and desorption of electrolyte ions which can be achieved through the dilution of compatible ionic liquid electrolytes. A comparison of the performance of dual-electrolytes (ionic liquid-aqueous electrolytes) in symmetric configurations is shown in Table 2. The [EmimTFO]_0.8[2.5 M KNO₃]_0.2 electrolyte can be seen to outperform or match the previously reported devices based on numerous parameters. The outstanding specific energy obtained could be directly related to the wider cell voltage and improved transport properties obtained from merely mixing 2.5 M KNO₃ and EmimTFO in an optimised ratio. This electrolyte mixture could be easily reproduced and used with other capacitive materials to boost the specific energy which is a critical factor in supercapacitor applications.

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4. Conclusion

A successful investigation of the dilution effect on EmimTFO using 2.5 M KNO₃ toward higher electrochemical performances was conducted. The transport properties of EmimTFO as well as mixtures diluted with 2.5 M KNO₃ by molar fractions of 0.6, 0.7, 0.8, and 0.9 were investigated. Despite a minor decline in potential windows from -1.6 to -1.2 V vs. Ag on the negative potential and 0.8 to 1.0 V vs. Ag on the positive potential due to the presence of water, the addition of the 2.5 M KNO₃ solution generated higher specific capacities C_sp (~40.2 mAh g^-1 for C_Sp⁺ and ~85.8 mAh g^-1 for C_Sp^-) than the neat EmimTFO in a three-electrode measurement ran at 0.25 A g^-1, whereby activated carbon was used as the electrode material. These potential windows are wider than the ones previously obtained when using 2.5 M KNO₃ electrolyte. [EmimTFO]_0.8[2.5 M KNO₃]_0.2 was then used to build an MS-AC//MS-AC supercapacitor that could run at an operating voltage of 2.1 V. The device exhibited stable charge-discharge cycling over 10,000 cycles, with a capacitance retention of 71.35%. Additionally, the device demonstrated a high specific energy of 22.21 Wh kg^-1 and a specific power of 520 W kg^-1 at a current density of 0.5 A g^-1. This electrolyte mixing technique proved its advantages in enhancing the specific energy of the supercapacitor which is a critical factor for this application.

Data Availability

Data is available on request from Prof. Ncholu Manyala at email: ncholu.manyala@up.ac.za.

Conflicts of Interest

The authors declare that there is no conflict of interest.

Acknowledgments

The authors wish to extend their appreciation to the Department of Science and Technology (DST) and the National Research Foundation of South Africa (NRF) with grant no. 61056 for their generous financial support that enabled the completion of this research, Open Access funding enabled and organized by SANLiC Gold. The observations, findings, and conclusions outlined in this report are solely those of the authors, and the NRF does not assume any responsibility or liability in this regard. Vianney N. Kitenge would also like to acknowledge the financial support received from the University of Pretoria for his Ph.D. studies.

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Copyright © 2023 V. N. Kitenge et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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