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Optimization of ionic-liquid based electrolyte concentration for high-energy density graphene supercapacitors
Applied Materials Today 18 (2020) 100522
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Optimization of ionic-liquid based electrolyte concentration for high-energy density graphene supercapacitors Shao Ing Wong a,b , Han Lin a , Jaka Sunarso b,∗ , Basil T. Wong b , Baohua Jia a,∗ a
Centre for Translational Atomaterials, Faculty of Science, Engineering, and Technology, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia Research Centre for Sustainable Technologies, Faculty of Engineering, Computing and Science, Swinburne University of Technology, Jalan Simpang Tiga 93350, Kuching, Sarawak, Malaysia b
a r t i c l e
i n f o
Article history: Received 27 August 2019 Received in revised form 24 November 2019 Accepted 27 November 2019 Keywords: Ionic liquid concentration viscosity ionic conductivity specific capacitance energy density graphene supercapacitor
a b s t r a c t Electrolyte properties play an important role to determine the capacitive performance of electric doublelayer capacitors (EDLCs), especially the specific capacitance and energy density. In general, electrolytes with a high electrochemical stability window (ESW) can offer both higher specific capacitance and energy density, which explains why ionic liquid (IL)-based electrolytes have been extensively studied. The concentration of IL is a critical parameter to control its viscosity, ionic conductivity, and potential window that is reflected in EDLC working voltage, which has yet to be systematically studied. In this paper, we presented a systematic approach to determine the optimum IL concentration for graphene-based EDLCs by measuring the viscosity and ionic conductivity of IL electrolyte containing different amounts of organic solvent and the corresponding maximum working voltages (MWV) of EDLCs via cyclic voltammetry (CV), as well as the associated specific capacitances. Such a systematic study fills in the missing knowledge on the optimum ionic liquid-based electrolyte concentration for graphene-based supercapacitors applications. We found that the electrolyte viscosity increases exponentially with increasing IL concentration, while the ionic conductivity decreases with an increase in IL concentration beyond its maximum at 2 M EMIMBF4 /IL. The specific capacitance shows a strong dependence not only on the electrolyte viscosity and ionic conductivity, but also the MWV where electrode specific capacitance increases with the MWV of EDLCs. Therefore, despite the highest viscosity and the lowest ionic conductivity, the neat IL (i.e., EMIMBF4 in this work) offers the largest specific capacitance and energy density among all IL concentrations for graphene-based EDLCs due to the largest MWV offered by the neat ILs. However, if the EDLC is not required to operate at the ESW of the IL, diluted IL electrolytes with an optimized concentration/IL viscosity can be used instead to achieve the most economic EDLC performance. In conclusion, the concentration of IL electrolyte should be optimized according to the working voltage required. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction Over the past two decades, supercapacitors (SCs) have attracted intensive attention given their high-power density, long-cycle lifetime, and environmental-friendly nature [1,2]. Most of the commercial SCs are symmetric with both electrodes made of porous carbon materials [3,4]. Such SCs are called electric double-layer capacitors (EDLCs) due to their physical charge storage mechanism that generates electric double layers (EDL) at both electrodes’ surfaces [5]. While EDLCs outperform batteries in terms of power density, they have relatively low energy density (2.3 to 10 Wh kg-1 ),
∗ Corresponding authors. E-mail addresses: [email protected] (J. Sunarso), [email protected] (B. Jia). https://doi.org/10.1016/j.apmt.2019.100522 2352-9407/© 2019 Elsevier Ltd. All rights reserved.
which is about one order of magnitude lower than that of lithiumion batteries [6,7]. Enhancing the energy density of EDLCs holds a great promise to realize these devices’ large-scale utilization as either a stand-alone or an integrated energy storage system in highenergy application such as consumer electronics, electric vehicles, and renewable energy storage. The energy density (E) of a SC is given by E = ½ C V2 , which is mainly determined by its specific capacitance (C) and maximum working voltage (MWV) (V). The normalized capacitance is closely related to the electrode and the electrolyte material properties, while the MWV greatly depends on the electrolyte characteristics. Since energy density increases quadratically with MWV, developing electrolytes with a wide electrochemical-stability window (ESW) becomes crucial. Recently, ionic liquids (ILs) (i.e., molten salts) have been intensively studied as a potential high performance electrolyte for EDLCs given their high ESW [8–10]. In most
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applications, ILs are diluted with organic solvents to decrease viscosity of neat ILs and to increase ionic transport rate [11], which can be characterized by diffusion coefficient [12]. It is believed that the associated specific capacitance can be increased due to these effects [13,14]. Paradoxically, the introduction organic solvent in neat ILs reduces ion association and affect its redox potential, which inevitably decreases electrolyte ESW and thus limits MWV of EDLCs. Therefore, the determination of an optimum IL concentration that can keep MWV as high as possible while maximizing specific capacitance of EDLCs is necessary to eventually boost energy density. However, systematic studies to determine such an optimum IL concentration are currently lacking in the literature. As a result, the selection of most of the IL concentrations in the previous works did not come from clear rationale or optimization [15–17], which led to compromised device performance or lower cost efficiency considering the high cost of neat IL. Here, we developed a systematic approach to identify the critical link between the IL concentration and MWV. The IL studied in this work was 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4 ) since it is one of the most widely used ILs in the experimental studies of SCs given its low viscosity (35 mPa s at 25 ◦ C) [18] and high electrochemical stability [19]. On the other hand, acetonitrile (AN) was chosen as the organic solvent given its excellent oxidative stability and high dielectric constant that makes it miscible with electrolyte salts [20]. We optimized the EMIMBF4 concentration by measuring the viscosity and ionic conductivity of EMIMBF4 containing different amount of AN using a microviscosity meter and ionic conductivity meter, respectively, and the resultant MWV of EDLCs via cyclic voltammetry (CV). In this way, we established the relationship between viscosity, ionic conductivity, and the MWV versus IL concentration, as well as the relationship between viscosity and ionic conductivity versus EDLC capacitive performance at a particular voltage by using EMIMBF4 /AN or neat EMIMBF4 as the electrolyte in flash-reduced graphene-oxide (FRGO)-based EDLCs. Although electrolyte viscosity was found to increase exponentially with increasing IL concentration, we determined 4 M EMIMBF4 /AN as the optimum IL concentration if the highest electrode specific capacitance at a given voltage is desired while neat EMIMBF4 is more preferable to achieve the highest maximum electrode specific capacitance and the highest EDLC energy density. 2. Experimental methods 2.1. Preparation of FRGO electrodes A simple lab-scale doctor blade was used to prepare the GO film. A highly concentrated GO aqueous solution (10 mg ml-1 ) was spread uniformly over a polymer substrate and then dried at 60 ◦ C in a fan oven. The dried GO films have a thickness of 8 m. The GO film was then cut into circular discs with a diameter of 15 mm. The as-obtained GO discs were reduced by a flash unit with an energy of 211.1 Ws in a glovebox with <0.5 ppm H2 O and <0.5 ppm O2 . 2.2. Preparation of EMIMBF4 /acetonitrile electrolyte Electrolyte mixtures with molar concentrations of 1 M, 2 M, 3 M, 4 M, 5 M, 6 M, and 6.47 M were prepared by mixing EMIMBF4 with AN together at different volume ratios in the glovebox. Neat EMIMBF4 (or 6.54 M EMIMBF4 /AN) was also prepared. 2.3. Electrolyte characterization The viscosity and ionic conductivity of each electrolyte mixtures and neat EMIMBF4 at room temperature (23 ◦ C) was measured using a micro-viscosity meter and conductivity meter (equipped
with a conductivity sensor) placed in the glovebox. The cell constant of the conductivity sensor was determined by calibration after each sample measurement using an aqueous 0.01 M KCl solution. 2.4. Fabrication of FRGO EDLCs The two-electrode symmetrical EDLCs were assembled to evaluate the electrochemical performance of the FRGO samples in different concentrations of EMIMBF4 -based electrolyte. The twoelectrode cell was assembled in a nitrogen glovebox. The separators (Celgard 3501), which were soaked with the as-prepared electrolytes, were placed between the two FRGO electrodes. Carbon coated aluminium foil was used as the current collector, on which the FRGO electrodes pressed. 2.5. Electrochemical measurements in two-electrode configuration Electrochemical measurements of the assembled EDLCs were performed at the room temperature using a multichannel electrochemical station (EC-Lab, VMP-300). The MWV of each EDLC with different IL electrolyte concentrations was first measured via cyclic voltammetry (CV), which was performed between 3.4 to 3.8 V at a scanning rate of 5 mV s-1 . The same process was repeated using galvanostatic charge/discharge (GCD) tests at a current density of 0.5 A g-1 to calculate maximum electrode specific capacitance. The CV tests at scanning rates of 5, 10, 20, 50, and 100 mV s-1 and GCD tests at current densities of 0.5, 1, 2, 5, and 10 A g-1 were then carried out from 0 V to the MWV of each EDLC, which varies depending on the IL electrolyte concentration. Electrochemical impedance spectroscopy (EIS) plots were also obtained from the EC-Lab electrochemical station in a frequency range of 10 mHz to 100 kHz at an open circuit potential with AC voltage of 5 mV. The gravimetric capacitance of one electrode was calculated from GCD curves using the following equation: CS = 4
I t m V
(1)
where CS (F g-1 ) is the specific capacitance of the single electrode, m (g) is total mass of two electrodes, I⁄m (A g-1 ) is the current density, t (s) is the discharge time, and V (V) is the cell voltage during the discharge process after the IR drop. The gravimetric energy density and power density of EDLC were evaluated according to the following equations: ES =
1 CS V2 8 × 3.6
(2)
PS =
ES × 3600 t
(3)
where ES (Wh kg-1 ) is gravimetric energy density of the SC, PS (W kg-1 ) is gravimetric power density of the SC, CS (F g-1 ) is the specific capacitance normalized to one electrode, V (V) is the cell voltage during the discharge process after the IR drop, and t (s) is the discharge time. 3. Results and Discussion The structures and sizes of cation and anion of EMIMBF4 as well as molecules of AN are presented in Fig. S1(a), (b), and (c). The principle of how IL concentrations can affect charge storage in an EDLC is illustrated in Fig. 1(a), (b) and (c). At the large amount of AN addition (or low IL concentration), charge-carrying ions are not sufficiently available for adsorption/desorption inside the porous electrodes (Fig. 1(a)). Under this circumstance, the high surface area of the porous electrodes cannot be fully utilized, which results in
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Fig. 1. Schematic diagram displaying the principle of how (a) low IL concentration, (b) optimum IL concentration, and (c) neat IL can affect charge storage in EDLC. Graphs showing (d) the AN content in each IL concentration and (e) electrolyte viscosity and ionic conductivity at room temperature as a function of IL concentration.
relatively low specific capacitance. If IL concentration is at its optimum, adequate ions become available to fill in all the accessible pores (Fig. 1(b)). The surface area of the porous electrodes can thus be fully exploited to form electric double-layer capacitance. Additionally, compared to neat IL, the increased ion mobility that stems from the presence of organic solvent molecules decreases the electrolyte viscosity and increases ionic conductivity, which are two of several factors that help to improve the specific capacitance of SC. However, the organic solvent-containing IL exhibits a lower ESW, which in turn reduces MWV of SC. Therefore, for EDLC applications which do not require high energy density, diluted IL electrolyte with optimum concentration is preferred. Finally, in the case of high IL concentration (e.g., neat IL), an excessive amount of non-solvated ions exists in the electrolyte. These inherently large cations and/or cations and ion clustering make ionic diffusion rate slower, thus leading to some pores remain unused (Fig. 1(c)). Theoretically, an increase in IL concentration beyond the optimum does not improve the specific capacitance and the energy density of the EDLCs. To understand the effect of organic solvent on electrolyte viscosity, we measured the viscosity and ionic conductivity of IL electrolyte containing different amount of AN (Fig. 1(d)) using a micro viscosity meter (Fig. S2) and ionic conductivity meter. The viscosity of the organic solvent (AN) is 0.357 mPa s at 23 ◦ C, which is one order of magnitude lower than that of the neat IL. Consequently, increasing the amount of organic solvent in IL decreases the electrolyte viscosity. The relationship between the electrolyte viscosity at 23 ◦ C and the IL concentration is depicted in Fig. 1(e). As anticipated, the electrolyte viscosity decreases with a decrease of IL concentration. Adding AN into the neat EMIMBF4 induces ion solvation in EMIMBF4 and therefore reduces EMIMBF4 ion pairing association [13]. The resultant weaker Coulombic interactions between ions in EMIMBF4 increases the ion mobility [21], which
then reduces the viscosity of IL. It is generally understood that electrolyte viscosity decreases with decreasing IL concentration. The electrolyte viscosity appears to increase exponentially with increasing IL concentration (Fig. 1(e)). This finding suggests the possibility of a dramatic reduction of IL viscosity by slightly adding the solvent without significantly compromising the MWV of the neat IL. In addition, it is possible to tune the viscosity according to the actual requirements and achieve any value between the viscosities of the organic solvent and the neat IL. The diffusion coefficient of different IL concentrations at 23 ◦ C was also estimated (Fig. S3). As demonstrated in Fig. 1(e), diluting IL with AN in general increases the ionic conductivity of neat IL due to the better ionic transport rate (lower viscosity). In addition, the ionic conductivity also depends on the ion density and free volume of the electrolyte mixture [22,23]. The maximum ionic conductivity is achieved by 2 M EMIMBF4 /AN, because the drastic increase of IL amount in the electrolyte mixture relative to 1 M EMIMBF4 /IL creates additional charge-carrying cation and anion concentrations via the formation of ion pairs, which in turn enhances ion density of the electrolytes [23]. Such a high conductivity is also achieved due to a high mobility of aprotic solvent molecules [11]. However, higher IL concentration tends to induce ion clustering or aggregation, which in turn decreases the free volume in the system that hinders ion diffusion, hence a relatively lower ionic conductivity is observed in 4 M EMIMBF4 /AN and neat IL [11]. In this work, reduced graphene oxide (RGO) porous materials have been employed as the electrode materials for SCs given its large specific surface area and high conductivity [24,25]. We fabricated the RGO porous material using the flashlight reduction method [26], which can simultaneously reduce graphene oxide (GO) and create highly porous structures. The scanning electron microscopic (SEM) image of the flash-reduced graphene oxide
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Fig. 2. (a) Cross sectional SEM image (left inset: digital image of free-standing film taken by camera; right inset: top-view SEM image), (b) C 1s XPS spectrum, (c) Raman spectrum, and (d) nitrogen adsorption/desorption isotherms of a FRGO electrode.
(FRGO) material is shown in Fig. 2(a), where the pores and cracks created from the reduction can be clearly observed. The presence of crack-derived edges can enhance specific capacitance [27]. The reduction degree of FRGO was also characterized by X-ray photoelectron spectroscopy (XPS). The resultant FRGO has a low C-to-O atomic ratio of 9.25 (Fig. 2(b)) with C = C content as high as 75.4%, which confirms an efficient removal of oxygen-containing functional groups. The Raman spectrum displays a clear signature of Dand G-peaks in FRGO (Fig. 2(c)). The high D-band corresponds to a high-defect density, which allows easy penetration of the ions from the electrolyte. The FRGO also has an electrical conductivity of 7.17 S m-1 . The nitrogen sorption result reveals an average pore size of 12.45 nm for the FRGO (Fig. 2(d)), confirming the formation of nanoscale pores. Such a RGO platform was used in this work to systematically study the concentration of IL. Reducing the viscosity of IL by adding organic solvent is considered as an effective route to achieve good capacitive performance in EDLC [14,28]. The 1 M- and 2 M-EMIMBF4 /AN and neat IL are often used as EDLC electrolytes in previous work [24,29–31], with specific capacitance calculated at inconsistent MWVs. Although the capacitive performance is highly dependent on the IL concentration, the relationship and the optimization of the IL concentration have not been studied. To investigate how the concentration of IL-based electrolyte can affect the actual EDLC performance, the electrolytes were assembled into FRGO-based sandwich devices for electrochemical test. The MWV of each EDLC with different IL concentration was first determined via cyclic voltammetry (CV). In this work, the MWV of EDLC is defined as the voltage before the presence of redox peaks, which is used to indicate electrolyte decomposition and/or redox reaction between electrolyte and electrode functional
groups. These conditions are not desired in EDLC that is solely based on electric double-layer mechanism. Herein the existence of a redox peak is defined if r > 6.5 % (exceeding the measurement error range of the system), where r is the difference of the current density magnitude between two consecutive cell voltage cycles at the same voltage position in CV run at the same scanning rate (Fig. 3(a)(i)). r I −I is represented by n+1In n × 100 %, where I is the current density of a cell voltage cycle at a particular voltage position and n is an integer number of cell voltage cycle. Accordingly, during the CV test, each EDLC with different IL concentrations was tested within a voltage range of 3.4 V to 3.8 V at a low scanning rate of 5 mV s-1 (Fig. 3(a)(i), (b)(i), (c)(i), and (d)(i)) to determine the voltage at which the redox peaks are present. Galvanostatic charge-discharge (GCD) measurement was also performed on every FRGO-based EDLC at the same voltage range with a current density of 0.5 A g-1 (Fig. 3(a)(ii), (b)(ii), (c)(ii), and (d)(ii)) to determine the maximum electrode specific capacitance. The MWV of FRGO-based EDLCs increases with increasing IL concentrations (Fig. 4(a)). Neat EMIMBF4 provides EDLC with the largest MWV (3.8 V), followed by 4 M EMIMBF4 /AN (3.5 V), 2 M EMIMBF4 /AN (3.4 V) and 1 M EMIMBF4 /AN (3.4 V). Upon a large amount of AN addition in EMIMBF4 , AN behaves like a co-solvent, instead of an additive, where huge amount of AN molecules can move freely to electrodes and in turn results in higher ionic conductivity. Nonetheless, more free AN molecules are susceptible to irreversible redox reactions at the electrodes. Such a phenomenon reflects why highly diluted EMIMBF4 /AN exhibits lower MWV [13,32]. The maximum electrode specific capacitances offered by different IL concentrations are closely associated to the MWVs, because capacitance (and charge) of SC increases with an increase of voltage across SC electrodes [33,34]. There are two possible expla-
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Fig. 3. (i) CV curves (scanning rate of 5 mV s-1 ) and (ii) GCD curves (current density of 0.5 A g-1 ) of FRGO based EDLC with (b) 1 M EMIMBF4 /AN, (c) 2 M EMIMBF4 /AN, (d) 4 M EMIMBF4 /AN, and (e) neat EMIMBF4 for a cell working voltage range from 3.4 V to 3.8 V.
nations for the increase in capacitance with voltage, which include: (1) the increase of electrolyte dielectric constant or the decrease of the charge separation distance at the electrode/electrolyte interface [33], and (2) the increase of the series-connected capacitance inside the electrode due to the space charge created by the displacement of charges [35]. Our experimental result as shown in Fig. 4(a) shows that maximum electrode specific capacitance increases with increase in IL concentration due to an increase in MWV, despite higher electrolyte viscosity (Fig. 4(b)) or lower ionic conductiv-
ity (Fig. 4(c)) in high-concentration IL electrolytes. FRGO with neat EMIMBF4 exhibited the largest maximum specific capacitance (165.7 F g-1 ), followed by the 4 M EMIMBF4 /AN- (158.3 F g-1 ), 2 M EMIMBF4 /AN- (147.8 F g-1 ) and then 1 M EMIMBF4 /AN-based EDLC (94.4 F g-1 ). The IL concentration significantly affect the MWV, which influences the maximum electrode specific capacitance and eventually the energy density of the EDLCs. The fact that higher ionic conductivity or lower viscosity does not always correlate with higher specific capacitance is also proven
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Fig. 4. Plot of maximum working voltage (MWV) and maximum FRGO electrode specific capacitance as a function of (a) IL concentration or (b) electrolyte viscosity at 23 ◦ C or (c) electrolyte ionic conductivity at 23 ◦ C. Plot of FRGO electrode specific capacitance at 3.4 V as a function of (d) IL concentration or (e) electrolyte viscosity at 23 ◦ C or (f) electrolyte ionic conductivity at 23 ◦ C.
when the electrode specific capacitance at a certain voltage (i.e., 3.4 V in this work) was compared among EDLCs assembled with different IL concentrations (Fig. 4(d)). The comparison between specific capacitance of the as-prepared FRGO electrodes was made at 3.4 V because it was the MWV that can be offered by the lowest IL concentration (1 M EMIMBF4 /AN). The highest FRGO electrode specific capacitance at 3.4 V is achieved at 4 M EMIMBF4 /AN even though 4 M EMIMBF4 /AN does not exhibit either the lowest viscosity (Fig. 4(e) or the highest ionic conductivity (Fig. 4(f)). Therefore, the general understanding that specific capacitance can be improved by reducing electrolyte viscosity is only true when the charge-carrying ions are sufficiently available. As explained in Fig. 1(d), the low availability of charge carrier in 1 M EMIMBF4 /AN makes it the least ideal electrolyte for FRGO-based EDLC if high specific capacitance is required. Although 2 M EMIMBF4 /AN possesses the highest ionic conductivity, the presence of substantial fractions of single ion resulting from ion pair dissociation by AN might be less favorable for charge storage. Instead, the comparable amount of ion pairs and free ions in 4 M EMIMBF4 /AN is likely to be
more beneficial for maximum utilization of electrode surface area and pore volume [11]. The optimum IL electrolyte concentration at 3.4 V is 4 M EMIMBF4 /AN, which corresponds to the situation in Fig. 1(b). When the concentration is further increased to beyond 4 M (i.e., 6 M EMIMBF4 /AN and neat EMIMBF4 ), the FRGO specific capacitance at 3.4 V decreases due to the existence of excess large non-solvated ions and/or ion clustering, by which the ion mobility towards the electrode/electrolyte interface was suppressed, as schematically shown in Fig. 1(c). Under such a circumstance, the IL is rendered wasted. We can deduce that in addition to electrolyte transport properties e.g., viscosity and ionic conductivity, ion availability and ion pair dissociation degree also play an important role in achieving high specific capacitance. Therefore, optimizing IL concentration for EDLC according to the required MWV is important to achieve the best capacitive performance and cost-effectiveness. Since IL material is expensive, dilution of IL electrolyte helps to reduce the overall cost of the electrolyte, which can be economically beneficial for large-scale industrial applications.
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Fig. 5. (a) Nyquist plots of EDLCs with 1 M, 2 M, 4 M, 6 M EMIMBF4 /AN and neat EMIMBF4 (inset: magnified view of high and medium frequency regions). (b) FRGO specific capacitance with different IL concentrations at their respective maximum stable cell voltage. (c) Specific capacitance retention of EDLCs assembled with 1 M, 2 M, 4 M EMIMBF4 /AN and neat EMIMBF4 after 1000 GCD cycles. (d) Ragone plot showing the energy density and power density of EDLCs with different IL concentrations.
To confirm the mechanism of achieving high specific capacitance using 4 M EMIMBF4 /AN, we further studied the properties of IL electrolyte with different concentrations via electrochemical impedance spectroscopy (EIS). The Nyquist plots are shown in Fig. 5(a). Compared with other concentrations, the 4 M EMIMBF4 /AN shows the highest ion diffusion capability across the FRGO electrodes because the curve at the low frequency region is a straight line that is nearest to the imaginary axis, which confirms good capacitive behavior of the electric double layers [36]. Noticeably, the 6 M EMIMBF4 /AN shows a longer Warburg curve (the slope of the 45◦ portion in medium frequency region) relative to the neat EMIMBF4 (the inset of Fig. 5(a)). Such a phenomenon indicates a less effective ion propagation in 6 M EMIMBF4 /AN electrolyte [37], which corresponds to the comparatively lower electrode specific capacitance provided by 6 M EMIMBF4 /AN despite its lower viscosity and higher ionic conductivity relative to the neat EMIMBF4 , as depicted in Fig. 4(e) and (f). The rate response is another important performance parameter for EDLCs, which can be studied by charging and discharging the ELDCs at different current density. Although the neat EMIMBF4 exhibits the largest overall specific capacitance at a low current density, the specific capacitance decreases dramatically with the increase of current density (Fig. 5(b)). Such a behavior can be attributed to the comparatively higher viscosity of the neat EMIMBF4 . In comparison, the 4 M EMIMBF4 /AN electrolyte shows better specific capacitance at a high current density, confirming the fact that viscosity plays a more dominant role in the rate response. In addition, to confirm the performance stability of the EDLCs with IL electrolytes, all the assembled EDLCs were tested up to 1000 charge-discharge cycles as shown in Fig. 5(c). EDLCs assembled with 1 M, 2 M, 4 M EMIMBF4 /AN and neat EMIMBF4 can retain specific capacitance up to 77 %, 83.5 %, 86.9 %, and 81.5 %, respectively, which indicates the long-term stability of FRGO-
based EDLCs. Finally, the energy density and the power density of all EDLCs with different IL concentrations were compared and shown in Fig. 5(d). Owing to the highest MWV and highest specific capacitance, the EDLC assembled with neat EMIMBF4 achieved the highest energy density among all devices, i.e., 74.2 Wh kg-1 with a maximum power density of 17.5 kW kg-1 . EDLC based on 4 M EMIMBF4 /AN exhibited the second highest energy density (69.8 Wh kg-1 with a maximum power density of 16.5 kW kg-1 ), followed by those with 2 M EMIMBF4 /AN (62.6 Wh kg-1 with a maximum power density of 16.4 kW kg-1 ) and then 1 M EMIMBF4 /AN (37.7 Wh kg-1 with a maximum power density of 16.5 kW kg-1 ). 4. Conclusion In this work, we studied the effect of concentration of IL-based electrolyte on the performance of EDLCs. It was found that viscosity increases exponentially with the concentration of the IL-based electrolyte, while ionic conductivity decreases with an increase IL concentration beyond its maximum at 2 M EMIMBF4 /IL. Diluting the IL electrolyte with a small amount of organic solvent is possible to dramatically increase ionic conductivity and reduce electrolyte viscosity, which helps to achieve high specific capacitance without significantly compromising the MWV. An optimum amount of ion concentration and ion pair dissociation degree in IL-based electrolyte was found to ensure not only better cations and anions mobility, but also adequate ion availability for charge transport in the porous carbon electrodes. Under such a condition, highest specific capacitance can be achieved. Additionally, the maximum specific capacitance can be increased with an increase in the MWV. Thus, the neat IL can be utilized to obtain the highest maximum specific capacitance and EDLC energy density. However, when the devices are not required to operate under the high working voltage, the diluted IL electrolyte with an opti-
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mum concentration can be used. Our approach provides a general method to determine the best IL electrolyte concentration under different circumstances and is applicable to a wide range of ILbased electrolytes, i.e., imidazolium-, pyridinium-, pyrrolidinium-, piperidinium-, and ammonium-based IL with varying viscosity and ionic conductivity. The findings are critical to achieve the best EDLC performance and cost-effectiveness in different devices, in particular when large-scale manufacturing of graphene EDLCs is desired. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Shao Ing Wong: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Data curation, Writing - original draft, Visualization, Project administration. Han Lin: Conceptualization, Methodology, Validation, Writing - review & editing, Supervision, Project administration. Jaka Sunarso: Formal analysis, Writing - review & editing, Supervision, Project administration, Funding acquisition. Basil T. Wong: Writing - review & editing, Supervision, Project administration, Funding acquisition. Baohua Jia: Conceptualization, Methodology, Validation, Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition. Acknowledgement The authors wish to express gratitude to the Swinburne Melbourne and Swinburne Sarawak for funding this project under the ‘Melbourne-Sarawak Research Collaboration Scheme’ (MSRSC) grant. Also, SI Wong is thankful to Swinburne Sarawak for the awarded Ph.D. scholarship. B Jia acknowledges the support from the Australia Research Council through the Industrial Transformation Training Centres scheme (Grant IC 180100005). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apmt.2019. 100522. References [1] B. Zhao, D. Chen, X. Xiong, B. Song, R. Hu, Q. Zhang, B.H. Rainwater, G.H. Waller, D. Zhen, Y. Ding, Y. Chen, C. Qu, D. Dang, C.-P. Wong, M. Liu, A high-energy, long cycle-life hybrid supercapacitor based on graphene composite electrodes, Energy Storage Materials 7 (2017) 32–39. [2] P. Simon, Y. Gogotsi, B. Dunn, Where do batteries end and supercapacitors begin? Science 343 (2014) 1210–1211. [3] J. Zhao, Y. Li, G. Wang, T. Wei, Z. Liu, K. Cheng, K. Ye, K. Zhu, D. Cao, Z. Fan, Enabling high-volumetric-energy-density supercapacitors: designing open, low-tortuosity heteroatom-doped porous carbon-tube bundle electrodes, J. Mater. Chem. A 5 (2017) 23085–23093. [4] S.I. Wong, J. Sunarso, B.T. Wong, H. Lin, A. Yu, B. Jia, Towards enhanced energy density of graphene-based supercapacitors: Current status, approaches, and future directions, J. Power Sources 396 (2018) 182–206. [5] C. Merlet, B. Rotenberg, P.A. Madden, P.-L. Taberna, P. Simon, Y. Gogotsi, M. Salanne, On the molecular origin of supercapacitance in nanoporous carbon electrodes, Nat. Mater. 11 (2012) 306–310. [6] Tecate Group, Ultracapacitor & supercapacitor frequently asked questions, 2019, n.d. (Accessed 1 June 2019) https://www.tecategroup.com/ ultracapacitors-supercapacitors/ultracapacitor-FAQ.php. [7] A. Burke, M. Miller, Testing of electrochemical capacitors: capacitance, resistance, energy density, and power capability, Electrochim. Acta 55 (2010) 7538–7548.
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Pandolfo, Ionic liquid-solvent mixtures as supercapacitor electrolytes for extreme temperature operation, RSC Adv. 2 (2012) 5591–5598. [14] E. Frackowiak, Supercapacitors based on carbon materials and ionic liquids, J. Braz. Chem. Soc. 17 (2006) 1074–1082. [15] G. Wu, P. Tan, D. Wang, Z. Li, L. Peng, Y. Hu, C. Wang, W. Zhu, S. Chen, W. Chen, High-performance supercapacitors based on electrochemical-induced vertical-aligned carbon nanotubes and polyaniline nanocomposite electrodes, Sci. Rep. 7 (2017) 43676. [16] C. Yu, C. Masarapu, J. Rong, B. Wei, H. Jiang, Stretchable supercapacitors based on buckled single-walled carbon-nanotube macrofilms, Adv. Mater. 21 (2009) 4793–4797. [17] Z. Lei, Z. Liu, H. Wang, X. Sun, L. Lu, X.S. Zhao, A high-energy-density supercapacitor with graphene-CMK-5 as the electrode and ionic liquid as the electrolyte, J. Mater. Chem. A 1 (2013) 2313–2321. [18] T. Nishida, Y. Tashiro, M. Yamamoto, Physical and electrochemical properties of 1-alkyl-3-methylimidazolium tetrafluoroborate for electrolyte, J. Fluorine Chem. 120 (2003) 135–141. [19] S. Kazemiabnavi, Z. Zhang, K. Thornton, S. Banerjee, Electrochemical stability window of imidazolium-based ionic liquids as electrolytes for lithium batteries, J. Phys. Chem. B 120 (2016) 5691–5702. [20] Y. Yamada, K. Furukawa, K. Sodeyama, K. Kikuchi, M. Yaegashi, Y. Tateyama, A. Yamada, Unusual stability of acetonitrile-based superconcentrated electrolytes for fast-charging lithium-ion batteries, J. Am. Chem. Soc. 136 (2014) 5039–5046. [21] A. Deshpande, L. Kariyawasam, P. Dutta, S. Banerjee, Enhancement of lithium ion mobility in ionic liquid electrolytes in presence of additives, J. Phys. Chem. C 117 (2013) 25343–25351. [22] W. Hongmei, L. Suqin, H. Kelong, Y. Xiongge, L. Younian, P. Sanjun, BMIMBF4 ionic liquid mixtures electrolyte for Li-ion batteries, Int. J. Electrochem. Sci. 7 (2012) 1688–1698. [23] H.-N. Kwon, S.-J. 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Maat, High-voltage operation of binder-free CNT supercapacitors using ionic liquid electrolytes, J. Mater. Res. 33 (2018) 1179–1188. [29] A. Izadi-Najafabadi, S. Yasuda, K. Kobashi, T. Yamada, D.N. Futaba, H. Hatori, M. Yumura, S. Iijima, K. Hata, Extracting the full potential of single-walled carbon nanotubes as durable supercapacitor eectrodes operable at 4 V with high power and energy density, Adv. Mater. 22 (2010) E235–E241. [30] Z. Lei, S. Bai, Y. Xiao, L. Dang, L. An, G. Zhang, Q. Xu, CMK-5 mesoporous carbon synthesized via chemical vapor deposition of ferrocene as catalyst support for methanol oxidation, J. Phys. Chem. C 112 (2008) 722–731. [31] W.-W. Liu, Y.-Q. Feng, X.-B. Yan, J.-T. Chen, Q.-J. Xue, Superior micro-supercapacitors based on graphene quantum dots, Adv. Funct. Mater. 23 (2013) 4111–4122. ´ [32] A. Lewandowski, M. Galinski, General properties of ionic liquids as electrolytes for carbon-based double layer capacitors, in: I.V. Barsukov, C.S. Johnson, J.E. 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Applied Materials Today journal homepage: www.elsevier.com/locate/apmt
Optimization of ionic-liquid based electrolyte concentration for high-energy density graphene supercapacitors Shao Ing Wong a,b , Han Lin a , Jaka Sunarso b,∗ , Basil T. Wong b , Baohua Jia a,∗ a
Centre for Translational Atomaterials, Faculty of Science, Engineering, and Technology, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia Research Centre for Sustainable Technologies, Faculty of Engineering, Computing and Science, Swinburne University of Technology, Jalan Simpang Tiga 93350, Kuching, Sarawak, Malaysia b
a r t i c l e
i n f o
Article history: Received 27 August 2019 Received in revised form 24 November 2019 Accepted 27 November 2019 Keywords: Ionic liquid concentration viscosity ionic conductivity specific capacitance energy density graphene supercapacitor
a b s t r a c t Electrolyte properties play an important role to determine the capacitive performance of electric doublelayer capacitors (EDLCs), especially the specific capacitance and energy density. In general, electrolytes with a high electrochemical stability window (ESW) can offer both higher specific capacitance and energy density, which explains why ionic liquid (IL)-based electrolytes have been extensively studied. The concentration of IL is a critical parameter to control its viscosity, ionic conductivity, and potential window that is reflected in EDLC working voltage, which has yet to be systematically studied. In this paper, we presented a systematic approach to determine the optimum IL concentration for graphene-based EDLCs by measuring the viscosity and ionic conductivity of IL electrolyte containing different amounts of organic solvent and the corresponding maximum working voltages (MWV) of EDLCs via cyclic voltammetry (CV), as well as the associated specific capacitances. Such a systematic study fills in the missing knowledge on the optimum ionic liquid-based electrolyte concentration for graphene-based supercapacitors applications. We found that the electrolyte viscosity increases exponentially with increasing IL concentration, while the ionic conductivity decreases with an increase in IL concentration beyond its maximum at 2 M EMIMBF4 /IL. The specific capacitance shows a strong dependence not only on the electrolyte viscosity and ionic conductivity, but also the MWV where electrode specific capacitance increases with the MWV of EDLCs. Therefore, despite the highest viscosity and the lowest ionic conductivity, the neat IL (i.e., EMIMBF4 in this work) offers the largest specific capacitance and energy density among all IL concentrations for graphene-based EDLCs due to the largest MWV offered by the neat ILs. However, if the EDLC is not required to operate at the ESW of the IL, diluted IL electrolytes with an optimized concentration/IL viscosity can be used instead to achieve the most economic EDLC performance. In conclusion, the concentration of IL electrolyte should be optimized according to the working voltage required. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction Over the past two decades, supercapacitors (SCs) have attracted intensive attention given their high-power density, long-cycle lifetime, and environmental-friendly nature [1,2]. Most of the commercial SCs are symmetric with both electrodes made of porous carbon materials [3,4]. Such SCs are called electric double-layer capacitors (EDLCs) due to their physical charge storage mechanism that generates electric double layers (EDL) at both electrodes’ surfaces [5]. While EDLCs outperform batteries in terms of power density, they have relatively low energy density (2.3 to 10 Wh kg-1 ),
∗ Corresponding authors. E-mail addresses: [email protected] (J. Sunarso), [email protected] (B. Jia). https://doi.org/10.1016/j.apmt.2019.100522 2352-9407/© 2019 Elsevier Ltd. All rights reserved.
which is about one order of magnitude lower than that of lithiumion batteries [6,7]. Enhancing the energy density of EDLCs holds a great promise to realize these devices’ large-scale utilization as either a stand-alone or an integrated energy storage system in highenergy application such as consumer electronics, electric vehicles, and renewable energy storage. The energy density (E) of a SC is given by E = ½ C V2 , which is mainly determined by its specific capacitance (C) and maximum working voltage (MWV) (V). The normalized capacitance is closely related to the electrode and the electrolyte material properties, while the MWV greatly depends on the electrolyte characteristics. Since energy density increases quadratically with MWV, developing electrolytes with a wide electrochemical-stability window (ESW) becomes crucial. Recently, ionic liquids (ILs) (i.e., molten salts) have been intensively studied as a potential high performance electrolyte for EDLCs given their high ESW [8–10]. In most
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applications, ILs are diluted with organic solvents to decrease viscosity of neat ILs and to increase ionic transport rate [11], which can be characterized by diffusion coefficient [12]. It is believed that the associated specific capacitance can be increased due to these effects [13,14]. Paradoxically, the introduction organic solvent in neat ILs reduces ion association and affect its redox potential, which inevitably decreases electrolyte ESW and thus limits MWV of EDLCs. Therefore, the determination of an optimum IL concentration that can keep MWV as high as possible while maximizing specific capacitance of EDLCs is necessary to eventually boost energy density. However, systematic studies to determine such an optimum IL concentration are currently lacking in the literature. As a result, the selection of most of the IL concentrations in the previous works did not come from clear rationale or optimization [15–17], which led to compromised device performance or lower cost efficiency considering the high cost of neat IL. Here, we developed a systematic approach to identify the critical link between the IL concentration and MWV. The IL studied in this work was 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4 ) since it is one of the most widely used ILs in the experimental studies of SCs given its low viscosity (35 mPa s at 25 ◦ C) [18] and high electrochemical stability [19]. On the other hand, acetonitrile (AN) was chosen as the organic solvent given its excellent oxidative stability and high dielectric constant that makes it miscible with electrolyte salts [20]. We optimized the EMIMBF4 concentration by measuring the viscosity and ionic conductivity of EMIMBF4 containing different amount of AN using a microviscosity meter and ionic conductivity meter, respectively, and the resultant MWV of EDLCs via cyclic voltammetry (CV). In this way, we established the relationship between viscosity, ionic conductivity, and the MWV versus IL concentration, as well as the relationship between viscosity and ionic conductivity versus EDLC capacitive performance at a particular voltage by using EMIMBF4 /AN or neat EMIMBF4 as the electrolyte in flash-reduced graphene-oxide (FRGO)-based EDLCs. Although electrolyte viscosity was found to increase exponentially with increasing IL concentration, we determined 4 M EMIMBF4 /AN as the optimum IL concentration if the highest electrode specific capacitance at a given voltage is desired while neat EMIMBF4 is more preferable to achieve the highest maximum electrode specific capacitance and the highest EDLC energy density. 2. Experimental methods 2.1. Preparation of FRGO electrodes A simple lab-scale doctor blade was used to prepare the GO film. A highly concentrated GO aqueous solution (10 mg ml-1 ) was spread uniformly over a polymer substrate and then dried at 60 ◦ C in a fan oven. The dried GO films have a thickness of 8 m. The GO film was then cut into circular discs with a diameter of 15 mm. The as-obtained GO discs were reduced by a flash unit with an energy of 211.1 Ws in a glovebox with <0.5 ppm H2 O and <0.5 ppm O2 . 2.2. Preparation of EMIMBF4 /acetonitrile electrolyte Electrolyte mixtures with molar concentrations of 1 M, 2 M, 3 M, 4 M, 5 M, 6 M, and 6.47 M were prepared by mixing EMIMBF4 with AN together at different volume ratios in the glovebox. Neat EMIMBF4 (or 6.54 M EMIMBF4 /AN) was also prepared. 2.3. Electrolyte characterization The viscosity and ionic conductivity of each electrolyte mixtures and neat EMIMBF4 at room temperature (23 ◦ C) was measured using a micro-viscosity meter and conductivity meter (equipped
with a conductivity sensor) placed in the glovebox. The cell constant of the conductivity sensor was determined by calibration after each sample measurement using an aqueous 0.01 M KCl solution. 2.4. Fabrication of FRGO EDLCs The two-electrode symmetrical EDLCs were assembled to evaluate the electrochemical performance of the FRGO samples in different concentrations of EMIMBF4 -based electrolyte. The twoelectrode cell was assembled in a nitrogen glovebox. The separators (Celgard 3501), which were soaked with the as-prepared electrolytes, were placed between the two FRGO electrodes. Carbon coated aluminium foil was used as the current collector, on which the FRGO electrodes pressed. 2.5. Electrochemical measurements in two-electrode configuration Electrochemical measurements of the assembled EDLCs were performed at the room temperature using a multichannel electrochemical station (EC-Lab, VMP-300). The MWV of each EDLC with different IL electrolyte concentrations was first measured via cyclic voltammetry (CV), which was performed between 3.4 to 3.8 V at a scanning rate of 5 mV s-1 . The same process was repeated using galvanostatic charge/discharge (GCD) tests at a current density of 0.5 A g-1 to calculate maximum electrode specific capacitance. The CV tests at scanning rates of 5, 10, 20, 50, and 100 mV s-1 and GCD tests at current densities of 0.5, 1, 2, 5, and 10 A g-1 were then carried out from 0 V to the MWV of each EDLC, which varies depending on the IL electrolyte concentration. Electrochemical impedance spectroscopy (EIS) plots were also obtained from the EC-Lab electrochemical station in a frequency range of 10 mHz to 100 kHz at an open circuit potential with AC voltage of 5 mV. The gravimetric capacitance of one electrode was calculated from GCD curves using the following equation: CS = 4
I t m V
(1)
where CS (F g-1 ) is the specific capacitance of the single electrode, m (g) is total mass of two electrodes, I⁄m (A g-1 ) is the current density, t (s) is the discharge time, and V (V) is the cell voltage during the discharge process after the IR drop. The gravimetric energy density and power density of EDLC were evaluated according to the following equations: ES =
1 CS V2 8 × 3.6
(2)
PS =
ES × 3600 t
(3)
where ES (Wh kg-1 ) is gravimetric energy density of the SC, PS (W kg-1 ) is gravimetric power density of the SC, CS (F g-1 ) is the specific capacitance normalized to one electrode, V (V) is the cell voltage during the discharge process after the IR drop, and t (s) is the discharge time. 3. Results and Discussion The structures and sizes of cation and anion of EMIMBF4 as well as molecules of AN are presented in Fig. S1(a), (b), and (c). The principle of how IL concentrations can affect charge storage in an EDLC is illustrated in Fig. 1(a), (b) and (c). At the large amount of AN addition (or low IL concentration), charge-carrying ions are not sufficiently available for adsorption/desorption inside the porous electrodes (Fig. 1(a)). Under this circumstance, the high surface area of the porous electrodes cannot be fully utilized, which results in
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Fig. 1. Schematic diagram displaying the principle of how (a) low IL concentration, (b) optimum IL concentration, and (c) neat IL can affect charge storage in EDLC. Graphs showing (d) the AN content in each IL concentration and (e) electrolyte viscosity and ionic conductivity at room temperature as a function of IL concentration.
relatively low specific capacitance. If IL concentration is at its optimum, adequate ions become available to fill in all the accessible pores (Fig. 1(b)). The surface area of the porous electrodes can thus be fully exploited to form electric double-layer capacitance. Additionally, compared to neat IL, the increased ion mobility that stems from the presence of organic solvent molecules decreases the electrolyte viscosity and increases ionic conductivity, which are two of several factors that help to improve the specific capacitance of SC. However, the organic solvent-containing IL exhibits a lower ESW, which in turn reduces MWV of SC. Therefore, for EDLC applications which do not require high energy density, diluted IL electrolyte with optimum concentration is preferred. Finally, in the case of high IL concentration (e.g., neat IL), an excessive amount of non-solvated ions exists in the electrolyte. These inherently large cations and/or cations and ion clustering make ionic diffusion rate slower, thus leading to some pores remain unused (Fig. 1(c)). Theoretically, an increase in IL concentration beyond the optimum does not improve the specific capacitance and the energy density of the EDLCs. To understand the effect of organic solvent on electrolyte viscosity, we measured the viscosity and ionic conductivity of IL electrolyte containing different amount of AN (Fig. 1(d)) using a micro viscosity meter (Fig. S2) and ionic conductivity meter. The viscosity of the organic solvent (AN) is 0.357 mPa s at 23 ◦ C, which is one order of magnitude lower than that of the neat IL. Consequently, increasing the amount of organic solvent in IL decreases the electrolyte viscosity. The relationship between the electrolyte viscosity at 23 ◦ C and the IL concentration is depicted in Fig. 1(e). As anticipated, the electrolyte viscosity decreases with a decrease of IL concentration. Adding AN into the neat EMIMBF4 induces ion solvation in EMIMBF4 and therefore reduces EMIMBF4 ion pairing association [13]. The resultant weaker Coulombic interactions between ions in EMIMBF4 increases the ion mobility [21], which
then reduces the viscosity of IL. It is generally understood that electrolyte viscosity decreases with decreasing IL concentration. The electrolyte viscosity appears to increase exponentially with increasing IL concentration (Fig. 1(e)). This finding suggests the possibility of a dramatic reduction of IL viscosity by slightly adding the solvent without significantly compromising the MWV of the neat IL. In addition, it is possible to tune the viscosity according to the actual requirements and achieve any value between the viscosities of the organic solvent and the neat IL. The diffusion coefficient of different IL concentrations at 23 ◦ C was also estimated (Fig. S3). As demonstrated in Fig. 1(e), diluting IL with AN in general increases the ionic conductivity of neat IL due to the better ionic transport rate (lower viscosity). In addition, the ionic conductivity also depends on the ion density and free volume of the electrolyte mixture [22,23]. The maximum ionic conductivity is achieved by 2 M EMIMBF4 /AN, because the drastic increase of IL amount in the electrolyte mixture relative to 1 M EMIMBF4 /IL creates additional charge-carrying cation and anion concentrations via the formation of ion pairs, which in turn enhances ion density of the electrolytes [23]. Such a high conductivity is also achieved due to a high mobility of aprotic solvent molecules [11]. However, higher IL concentration tends to induce ion clustering or aggregation, which in turn decreases the free volume in the system that hinders ion diffusion, hence a relatively lower ionic conductivity is observed in 4 M EMIMBF4 /AN and neat IL [11]. In this work, reduced graphene oxide (RGO) porous materials have been employed as the electrode materials for SCs given its large specific surface area and high conductivity [24,25]. We fabricated the RGO porous material using the flashlight reduction method [26], which can simultaneously reduce graphene oxide (GO) and create highly porous structures. The scanning electron microscopic (SEM) image of the flash-reduced graphene oxide
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Fig. 2. (a) Cross sectional SEM image (left inset: digital image of free-standing film taken by camera; right inset: top-view SEM image), (b) C 1s XPS spectrum, (c) Raman spectrum, and (d) nitrogen adsorption/desorption isotherms of a FRGO electrode.
(FRGO) material is shown in Fig. 2(a), where the pores and cracks created from the reduction can be clearly observed. The presence of crack-derived edges can enhance specific capacitance [27]. The reduction degree of FRGO was also characterized by X-ray photoelectron spectroscopy (XPS). The resultant FRGO has a low C-to-O atomic ratio of 9.25 (Fig. 2(b)) with C = C content as high as 75.4%, which confirms an efficient removal of oxygen-containing functional groups. The Raman spectrum displays a clear signature of Dand G-peaks in FRGO (Fig. 2(c)). The high D-band corresponds to a high-defect density, which allows easy penetration of the ions from the electrolyte. The FRGO also has an electrical conductivity of 7.17 S m-1 . The nitrogen sorption result reveals an average pore size of 12.45 nm for the FRGO (Fig. 2(d)), confirming the formation of nanoscale pores. Such a RGO platform was used in this work to systematically study the concentration of IL. Reducing the viscosity of IL by adding organic solvent is considered as an effective route to achieve good capacitive performance in EDLC [14,28]. The 1 M- and 2 M-EMIMBF4 /AN and neat IL are often used as EDLC electrolytes in previous work [24,29–31], with specific capacitance calculated at inconsistent MWVs. Although the capacitive performance is highly dependent on the IL concentration, the relationship and the optimization of the IL concentration have not been studied. To investigate how the concentration of IL-based electrolyte can affect the actual EDLC performance, the electrolytes were assembled into FRGO-based sandwich devices for electrochemical test. The MWV of each EDLC with different IL concentration was first determined via cyclic voltammetry (CV). In this work, the MWV of EDLC is defined as the voltage before the presence of redox peaks, which is used to indicate electrolyte decomposition and/or redox reaction between electrolyte and electrode functional
groups. These conditions are not desired in EDLC that is solely based on electric double-layer mechanism. Herein the existence of a redox peak is defined if r > 6.5 % (exceeding the measurement error range of the system), where r is the difference of the current density magnitude between two consecutive cell voltage cycles at the same voltage position in CV run at the same scanning rate (Fig. 3(a)(i)). r I −I is represented by n+1In n × 100 %, where I is the current density of a cell voltage cycle at a particular voltage position and n is an integer number of cell voltage cycle. Accordingly, during the CV test, each EDLC with different IL concentrations was tested within a voltage range of 3.4 V to 3.8 V at a low scanning rate of 5 mV s-1 (Fig. 3(a)(i), (b)(i), (c)(i), and (d)(i)) to determine the voltage at which the redox peaks are present. Galvanostatic charge-discharge (GCD) measurement was also performed on every FRGO-based EDLC at the same voltage range with a current density of 0.5 A g-1 (Fig. 3(a)(ii), (b)(ii), (c)(ii), and (d)(ii)) to determine the maximum electrode specific capacitance. The MWV of FRGO-based EDLCs increases with increasing IL concentrations (Fig. 4(a)). Neat EMIMBF4 provides EDLC with the largest MWV (3.8 V), followed by 4 M EMIMBF4 /AN (3.5 V), 2 M EMIMBF4 /AN (3.4 V) and 1 M EMIMBF4 /AN (3.4 V). Upon a large amount of AN addition in EMIMBF4 , AN behaves like a co-solvent, instead of an additive, where huge amount of AN molecules can move freely to electrodes and in turn results in higher ionic conductivity. Nonetheless, more free AN molecules are susceptible to irreversible redox reactions at the electrodes. Such a phenomenon reflects why highly diluted EMIMBF4 /AN exhibits lower MWV [13,32]. The maximum electrode specific capacitances offered by different IL concentrations are closely associated to the MWVs, because capacitance (and charge) of SC increases with an increase of voltage across SC electrodes [33,34]. There are two possible expla-
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Fig. 3. (i) CV curves (scanning rate of 5 mV s-1 ) and (ii) GCD curves (current density of 0.5 A g-1 ) of FRGO based EDLC with (b) 1 M EMIMBF4 /AN, (c) 2 M EMIMBF4 /AN, (d) 4 M EMIMBF4 /AN, and (e) neat EMIMBF4 for a cell working voltage range from 3.4 V to 3.8 V.
nations for the increase in capacitance with voltage, which include: (1) the increase of electrolyte dielectric constant or the decrease of the charge separation distance at the electrode/electrolyte interface [33], and (2) the increase of the series-connected capacitance inside the electrode due to the space charge created by the displacement of charges [35]. Our experimental result as shown in Fig. 4(a) shows that maximum electrode specific capacitance increases with increase in IL concentration due to an increase in MWV, despite higher electrolyte viscosity (Fig. 4(b)) or lower ionic conductiv-
ity (Fig. 4(c)) in high-concentration IL electrolytes. FRGO with neat EMIMBF4 exhibited the largest maximum specific capacitance (165.7 F g-1 ), followed by the 4 M EMIMBF4 /AN- (158.3 F g-1 ), 2 M EMIMBF4 /AN- (147.8 F g-1 ) and then 1 M EMIMBF4 /AN-based EDLC (94.4 F g-1 ). The IL concentration significantly affect the MWV, which influences the maximum electrode specific capacitance and eventually the energy density of the EDLCs. The fact that higher ionic conductivity or lower viscosity does not always correlate with higher specific capacitance is also proven
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Fig. 4. Plot of maximum working voltage (MWV) and maximum FRGO electrode specific capacitance as a function of (a) IL concentration or (b) electrolyte viscosity at 23 ◦ C or (c) electrolyte ionic conductivity at 23 ◦ C. Plot of FRGO electrode specific capacitance at 3.4 V as a function of (d) IL concentration or (e) electrolyte viscosity at 23 ◦ C or (f) electrolyte ionic conductivity at 23 ◦ C.
when the electrode specific capacitance at a certain voltage (i.e., 3.4 V in this work) was compared among EDLCs assembled with different IL concentrations (Fig. 4(d)). The comparison between specific capacitance of the as-prepared FRGO electrodes was made at 3.4 V because it was the MWV that can be offered by the lowest IL concentration (1 M EMIMBF4 /AN). The highest FRGO electrode specific capacitance at 3.4 V is achieved at 4 M EMIMBF4 /AN even though 4 M EMIMBF4 /AN does not exhibit either the lowest viscosity (Fig. 4(e) or the highest ionic conductivity (Fig. 4(f)). Therefore, the general understanding that specific capacitance can be improved by reducing electrolyte viscosity is only true when the charge-carrying ions are sufficiently available. As explained in Fig. 1(d), the low availability of charge carrier in 1 M EMIMBF4 /AN makes it the least ideal electrolyte for FRGO-based EDLC if high specific capacitance is required. Although 2 M EMIMBF4 /AN possesses the highest ionic conductivity, the presence of substantial fractions of single ion resulting from ion pair dissociation by AN might be less favorable for charge storage. Instead, the comparable amount of ion pairs and free ions in 4 M EMIMBF4 /AN is likely to be
more beneficial for maximum utilization of electrode surface area and pore volume [11]. The optimum IL electrolyte concentration at 3.4 V is 4 M EMIMBF4 /AN, which corresponds to the situation in Fig. 1(b). When the concentration is further increased to beyond 4 M (i.e., 6 M EMIMBF4 /AN and neat EMIMBF4 ), the FRGO specific capacitance at 3.4 V decreases due to the existence of excess large non-solvated ions and/or ion clustering, by which the ion mobility towards the electrode/electrolyte interface was suppressed, as schematically shown in Fig. 1(c). Under such a circumstance, the IL is rendered wasted. We can deduce that in addition to electrolyte transport properties e.g., viscosity and ionic conductivity, ion availability and ion pair dissociation degree also play an important role in achieving high specific capacitance. Therefore, optimizing IL concentration for EDLC according to the required MWV is important to achieve the best capacitive performance and cost-effectiveness. Since IL material is expensive, dilution of IL electrolyte helps to reduce the overall cost of the electrolyte, which can be economically beneficial for large-scale industrial applications.
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Fig. 5. (a) Nyquist plots of EDLCs with 1 M, 2 M, 4 M, 6 M EMIMBF4 /AN and neat EMIMBF4 (inset: magnified view of high and medium frequency regions). (b) FRGO specific capacitance with different IL concentrations at their respective maximum stable cell voltage. (c) Specific capacitance retention of EDLCs assembled with 1 M, 2 M, 4 M EMIMBF4 /AN and neat EMIMBF4 after 1000 GCD cycles. (d) Ragone plot showing the energy density and power density of EDLCs with different IL concentrations.
To confirm the mechanism of achieving high specific capacitance using 4 M EMIMBF4 /AN, we further studied the properties of IL electrolyte with different concentrations via electrochemical impedance spectroscopy (EIS). The Nyquist plots are shown in Fig. 5(a). Compared with other concentrations, the 4 M EMIMBF4 /AN shows the highest ion diffusion capability across the FRGO electrodes because the curve at the low frequency region is a straight line that is nearest to the imaginary axis, which confirms good capacitive behavior of the electric double layers [36]. Noticeably, the 6 M EMIMBF4 /AN shows a longer Warburg curve (the slope of the 45◦ portion in medium frequency region) relative to the neat EMIMBF4 (the inset of Fig. 5(a)). Such a phenomenon indicates a less effective ion propagation in 6 M EMIMBF4 /AN electrolyte [37], which corresponds to the comparatively lower electrode specific capacitance provided by 6 M EMIMBF4 /AN despite its lower viscosity and higher ionic conductivity relative to the neat EMIMBF4 , as depicted in Fig. 4(e) and (f). The rate response is another important performance parameter for EDLCs, which can be studied by charging and discharging the ELDCs at different current density. Although the neat EMIMBF4 exhibits the largest overall specific capacitance at a low current density, the specific capacitance decreases dramatically with the increase of current density (Fig. 5(b)). Such a behavior can be attributed to the comparatively higher viscosity of the neat EMIMBF4 . In comparison, the 4 M EMIMBF4 /AN electrolyte shows better specific capacitance at a high current density, confirming the fact that viscosity plays a more dominant role in the rate response. In addition, to confirm the performance stability of the EDLCs with IL electrolytes, all the assembled EDLCs were tested up to 1000 charge-discharge cycles as shown in Fig. 5(c). EDLCs assembled with 1 M, 2 M, 4 M EMIMBF4 /AN and neat EMIMBF4 can retain specific capacitance up to 77 %, 83.5 %, 86.9 %, and 81.5 %, respectively, which indicates the long-term stability of FRGO-
based EDLCs. Finally, the energy density and the power density of all EDLCs with different IL concentrations were compared and shown in Fig. 5(d). Owing to the highest MWV and highest specific capacitance, the EDLC assembled with neat EMIMBF4 achieved the highest energy density among all devices, i.e., 74.2 Wh kg-1 with a maximum power density of 17.5 kW kg-1 . EDLC based on 4 M EMIMBF4 /AN exhibited the second highest energy density (69.8 Wh kg-1 with a maximum power density of 16.5 kW kg-1 ), followed by those with 2 M EMIMBF4 /AN (62.6 Wh kg-1 with a maximum power density of 16.4 kW kg-1 ) and then 1 M EMIMBF4 /AN (37.7 Wh kg-1 with a maximum power density of 16.5 kW kg-1 ). 4. Conclusion In this work, we studied the effect of concentration of IL-based electrolyte on the performance of EDLCs. It was found that viscosity increases exponentially with the concentration of the IL-based electrolyte, while ionic conductivity decreases with an increase IL concentration beyond its maximum at 2 M EMIMBF4 /IL. Diluting the IL electrolyte with a small amount of organic solvent is possible to dramatically increase ionic conductivity and reduce electrolyte viscosity, which helps to achieve high specific capacitance without significantly compromising the MWV. An optimum amount of ion concentration and ion pair dissociation degree in IL-based electrolyte was found to ensure not only better cations and anions mobility, but also adequate ion availability for charge transport in the porous carbon electrodes. Under such a condition, highest specific capacitance can be achieved. Additionally, the maximum specific capacitance can be increased with an increase in the MWV. Thus, the neat IL can be utilized to obtain the highest maximum specific capacitance and EDLC energy density. However, when the devices are not required to operate under the high working voltage, the diluted IL electrolyte with an opti-
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mum concentration can be used. Our approach provides a general method to determine the best IL electrolyte concentration under different circumstances and is applicable to a wide range of ILbased electrolytes, i.e., imidazolium-, pyridinium-, pyrrolidinium-, piperidinium-, and ammonium-based IL with varying viscosity and ionic conductivity. The findings are critical to achieve the best EDLC performance and cost-effectiveness in different devices, in particular when large-scale manufacturing of graphene EDLCs is desired. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Shao Ing Wong: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Data curation, Writing - original draft, Visualization, Project administration. Han Lin: Conceptualization, Methodology, Validation, Writing - review & editing, Supervision, Project administration. Jaka Sunarso: Formal analysis, Writing - review & editing, Supervision, Project administration, Funding acquisition. Basil T. Wong: Writing - review & editing, Supervision, Project administration, Funding acquisition. Baohua Jia: Conceptualization, Methodology, Validation, Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition. Acknowledgement The authors wish to express gratitude to the Swinburne Melbourne and Swinburne Sarawak for funding this project under the ‘Melbourne-Sarawak Research Collaboration Scheme’ (MSRSC) grant. Also, SI Wong is thankful to Swinburne Sarawak for the awarded Ph.D. scholarship. B Jia acknowledges the support from the Australia Research Council through the Industrial Transformation Training Centres scheme (Grant IC 180100005). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apmt.2019. 100522. References [1] B. Zhao, D. Chen, X. Xiong, B. Song, R. Hu, Q. Zhang, B.H. Rainwater, G.H. Waller, D. Zhen, Y. Ding, Y. Chen, C. Qu, D. Dang, C.-P. Wong, M. Liu, A high-energy, long cycle-life hybrid supercapacitor based on graphene composite electrodes, Energy Storage Materials 7 (2017) 32–39. [2] P. Simon, Y. Gogotsi, B. Dunn, Where do batteries end and supercapacitors begin? Science 343 (2014) 1210–1211. [3] J. Zhao, Y. Li, G. Wang, T. Wei, Z. Liu, K. Cheng, K. Ye, K. Zhu, D. Cao, Z. Fan, Enabling high-volumetric-energy-density supercapacitors: designing open, low-tortuosity heteroatom-doped porous carbon-tube bundle electrodes, J. Mater. Chem. A 5 (2017) 23085–23093. [4] S.I. Wong, J. Sunarso, B.T. Wong, H. Lin, A. Yu, B. Jia, Towards enhanced energy density of graphene-based supercapacitors: Current status, approaches, and future directions, J. Power Sources 396 (2018) 182–206. [5] C. Merlet, B. Rotenberg, P.A. Madden, P.-L. Taberna, P. Simon, Y. Gogotsi, M. Salanne, On the molecular origin of supercapacitance in nanoporous carbon electrodes, Nat. Mater. 11 (2012) 306–310. [6] Tecate Group, Ultracapacitor & supercapacitor frequently asked questions, 2019, n.d. (Accessed 1 June 2019) https://www.tecategroup.com/ ultracapacitors-supercapacitors/ultracapacitor-FAQ.php. [7] A. Burke, M. Miller, Testing of electrochemical capacitors: capacitance, resistance, energy density, and power capability, Electrochim. Acta 55 (2010) 7538–7548.
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