Electrochemical double layer capacitors (EDLCs), a type of energy storage device, are currently receiving considerable attention. They have a high power density and good cycle ability. Furthermore, they operate on a simple mechanism where electrical charges in an electrochemical double layer are accumulated at the interface between the electrode and the electrolyte [1-3]. For these reasons, capacitors are used in a wide range of applications such as mobile phones, electrical vehicles, and industry power supplies. Capacitors generally consist of an electrode and an electrolyte. The electrode is prepared using carbon materials such as activated carbon, graphene, or graphite fibers [4-7]. Among the carbon materials, activated carbon, which has a high specific surface area and a large number of pores, is suitable for the capacitor electrode. In particular, it readily absorbs or desorbs electric charge. The electrolyte, meanwhile, can be divided into aqueous and non-aqueous electrolytes. Non-aqueous electrolytes have a wide electrochemical stability of operative voltage compare to aqueous electrolytes. Because the window potential is related to the energy density (E = 1/2V2), the voltage limit of the electrolyte is an important factor for the characteristics of a capacitor [2,8-10]. The ionic conductivity, capacitance, and impedance of electrochemical stability are also significant parameters of the electrolyte. Recently, ionic liquids (IL), which are also known as room temperature molten salts, have attracted attention for use in electrolytes due to their unique characteristics such as non-flammability, high ionic conductivity, low melting points, and wide electrochemical stability window. ILs have large cations and organic or inorganic anions in a liquid state at room temperature [11-15]. Among various ILs, imidazolium based ILs are used most widely because the hydrogen of imidazolium can be easily substituted. In particular, imidazolium, which has 1-methyl-3-butyl side chains, is widely used in many fields of study due to its relatively low viscosity, high conductivity, and low melting point. A capacitor using 10% 1-butyl-3-methyl imidazolium tetrafluoroborate (BMImBF4) was found to have the best electrochemical performance in our previous study [16]. Although imidazolium cation with 1,2,3-alkyl side chains has high viscosity, it presents excellent electrochemical stability [17]. However, the influence of 1,2,3-alkyl substituted imidazolium salts on the ion conducting property of organic electrolytes is not fully understood. The objective of this study was to find the optimal proportion of 1-butyl-2,3-dimethyl imidazolium tetrafluoroborate (BMMImBF4), which is a tri-alkyl substituted imidazolium salt, as an additive in an organic electrolyte and to confirm the electrochemical performance of the resultant electrolyte for capacitor applications. BMMImBF4 is an attractive candidate as an additive in organic electrolytes of EDLCs. The electrodes were composed of the activated materials, conductive agents, binders, and substrate. Activated carbon (MSP-20; Kuraray Chemical, Osaka, Japan) and carbon black (Super P; Alfa Aesar, Ward Hill, MA, USA) are used as an activated material and conductive agent, respectively. Carboxymethylcellulose (CMC; Aldrich, St. Louis, MO, USA) / styrene butadiene rubber (SBR) serves as a binder. These materials are well mixed with N-methylpyrrolidone (NMP; Junsei Chemical, Tokyo, Japan) using agate mortar for 30 minutes in a glove box. The mixed slurry (activated carbon : carbon blacks : CMC : SBR = 85 : 5 : 6 : 4 [wt%]) was then coated on a Ni foam substrate (1 cm × 1 cm) and dried at 100℃ in a vacuum oven for 12 h. This process is carried out to remove and evaporate the NMP. The prepared electrodes were pressed. The average loading weight of the active materials was about 10 mg/cm2 . The final film electrode was applied to an ELDC. The electrolytes were prepared using 0.1 M tetraethyl ammonium tetrafluoroborate (TEABF4; Aldrich) salts dissolved in ethylene carbonate (EC; Aldrich) and dimethoxyethane (DME; Daejung, Busan, Korea) (5/5 vol%) in a glove box. Different volume ratios (5, 10, 15, and 20 vol%) of BMMImBF4 were added to the prepared electrolyte, which was then stirred for 12 h. The chemical structures of BMImBF4 and BMMImBF4 are shown in Fig. 1. These final organic electrolytes solutions were used as electrolytes for EDLCs. Fig. 1. The structure of (a) 1-butyl-3-methyl imidazolium tetrafluoroborate (BMImBF4) and (b) 1-butyl-2,3-dimethyl imidazolium tetrafluoroborate (BMMImBF4). The ionic conductivities of the electrolytes were examined using AC impedance spectroscopy at a frequency range from 1000 kHz to 1 Hz at 0.1 V amplitude at room temperature. The electrochemical stability of the electrolytes was confirmed by linear sweep voltammetry at a scan rate of 10 mV·s–1. Pt wires served as counter electrodes and an Ag/Ag+ electrode was used as a reference electrode. The thermal stability of the electrolyte with IL additives was measured using a themogravimetric analyzer (TGA). The mass of the electrolyte sample was varied about 20mg. For the dynamic method, samples were heated from 30℃ to 500℃ at a heating rate of 10℃/min. The characterizations of the electrochemical perfomance including the capacitance, operative voltage, resistance, and cycle ability were carried out using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The evaluations were carried out in a three electrode system using an Iviumstat (Ivium Technologies, Eindhoven, the Netherlands). The working electrode was the prepared electode. AnAg/AgCl electrode and platinum wire were used as reference and counter electrodes, respectively. The EIS was performed by AC impedance spetroscopy over a frequency range from 100 kHz to 0.01 Hz with 5 mV amplitude modulation. The ion conducting property of an electrolyte for capacitor applications is one of the most important factors for the capacitive behavior of the capacitor. Among the various properties, the ionic conductivity of a solution has a critical effect on the electrochemical performance of the capacitor. The variation of the ionic conductivity of the BMMImBF4-organic solvent mixture per volume percent of ILs is presented in Table 1. From the conductivity results, as more IL was added, the ionic conductivity increased at room temperature. It appears that BMMImBF4 as an additive can ion charges effectively migrate or diffuse. Table 1. Resistances, conductivity, and capacitance of organic electrolytes containing ionic liquid additives BMMImBF4, 1-butyl-2,3-dimethyl imidazolium tetrafluoroborate; R1, bulk resistance; R2, deposited film resistance; R3, charge transfer resistance; W, Warburg resistance. The CVs of EDLCs using the BMMImBF4 additive in an organic electrolyte at a scan rate of 5 mV·s–1 demonstrate the electrochemical performance, as presented in Fig. 2. Rectangular shapes were observed, corresponding with a typical EDLC. The specific capacitance (C) of EDLCs is defined by the following equation (1): Fig. 2. Cyclic voltammograms of capacitor containing EC/DME electrolyte with 0.1 M tetraethyl ammonium tetrafluoroborate (TEABF4) with different ratios of BMMImBF4 at a scan rate of 5mV·s–1. EC, ethylene carbonate; DME, dimethoxyethane; BMMImBF4, 1-butyl-2,3-dimethyl imidazolium tetrafluoroborate; BMImBF4, 1-butyl-3-methyl imidazolium tetrafluoroborate. I and V denote the response current density (Acm–2) and the scan rate of potential (V·s–1), respectively. m is the active material weight of the working electrode (g) and ΔV is the operative voltage (V) in a 3 electrode system [18,19]. The specific capacitances were calculated and are presented in Table 1. The capacitances increased to 133.7 F·g–1 with addition of BMMImBF4 ILs additives at 10%. It is speculated that ions were effectively adsorbed and desorbed between the electrolyte and the surface or pores of the electrode. However, the capacitances decreased when the amount of additive exceeded 10%. It appears that too many ion charges of ILs were saturated and interrupted the movement of ion charges, thereby having adverse effects on the interface between the electrode and the electrolyte. AC impedance spectra of all electrolytes are presented in Fig. 3, which shows two circles and vertical lines. The initial part of the plot and the diameter of the semi-circle indicate the bulk resistance (R1) and the deposited film resistance (R2), respectively, at high frequency. The diameter of the second semi-circle signifies the charge transfer resistance (R3) at intermediate frequency. The Warburg resistance (W), which presents a straight line with a slope of 45°, defines the ion diffusion in an activated carbon electrode. The character of a pure EDLC could be confirmed by bulk impedance at low frequency [20]. The estimated values of resistances are indicated in Table.1. As shown in Fig. 3, the EDLCs containing 10% BMMImBF4 in the organic electrolyte displayed the lowest charge transfer resistance and Warburg resistance. The results of this test indicated that the use of BMMImBF4 has a positive effect of increasing the capacitance of the EDLC. Fig. 3. Impedance spectra according to content of BMMImBF4 ionic liquids in organic electrolyte. EC, ethylene carbonate; DME, dimethoxyethane; BMMImBF4, 1-butyl-2,3-dimethyl imidazolium tetrafluoroborate. Fig. 4 shows the capacitance retention behavior at prolonged charge-discharge cycles. It was evaluated in a range of –0.5 V to 1.2 V for 1000 cycles at 100 mV·s–1. The cycle stability is a very important parameter to evaluate capacitor performance. As shown in Fig. 4, the cycle retention using 10% BMMImBF4 in the electrolyte decreased by only 15%, whereas a 20% decrease was observed in the case of using the 10% BMImBF4 additives. It is thought that the 1,2,3-tri-alkyl imidazolium cation is more stable than the 1,2-dialkyl imidazolium cation, because the disubstituted- imidazolium cation has low cathodic stability in terms of electrochemical performance and it undergoes some side reactions such as cathodic dimerization or di-alkylation reactions including the acidic proton in the two positions in Fig. 1. As a result, 1,2,3-tri-alkyl imidazolium cation based-ILs exhibited outstanding cycle ability, compared with the 1,2-dialkyl imidazolium ILs [21,22]. This is likely related with the tri-alkyl imidazolium cations having larger volume size than di-alkyl imidazolium cations. This cation volume size difference will affect the ion dissociation and the charge accumulation onto the activated carbons surfaces. By using molecular modification techniques, we could alter the ion mobility or charge capacitance, thereby affecting the electrochemical performance of the capacitor. Fig. 4. Capacitance retention of electrochemical double layer capacitor containing 10% imidazolium ionic liquids with di-alkyl and tri-alkyl chains at a scan rate of 100 mV·s–1. BMImBF4, 1-butyl-3-methyl imidazolium tetrafluoroborate; BMMImBF4, 1-butyl-2,3-dimethyl imidazolium tetrafluoroborate. In this study, the effect of imidazolium cations with tri-alkyl chain substitutions in the organic electrolyte of a capacitor was investigated by chemical and electrochemical characterization. Reduced resistance and enhanced capacitance of 133.7 F·g–1 indicated that the BMMImBF4 IL could function as an effective additive in the capacitor electrolyte. In particular, the capacitor containing a small amount (10%) of BMMImBF4 showed the maximum capacitance and low resistance at the interface between the electrode and the electrolyte resulting from the effective electric double layer thickness. Also, the cycle stability (85% retention after 1000 cycles) of the capacitor using tri-alkyl based imidazolium ILs was superior to that of the capacitor using di-alkyl based imidazolium. It could be concluded that the improved capacitance behaviors were dependent on the ion mobility and on the charge accumulation onto the surface of the activated carbon electrode by changing the molecular structure of additive salts. In conclusion, the BMMImBF4 IL is a promising additive for organic electrolytes for capacitor and other electrochemical applications.
