Ion conducting properties of imidazolium salts with tri-alkyl chains in organic electrolytes against activated carbon electrodes

Abstract

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][2][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][5][6][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 nonaqueous 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 = 1/2V 2 ), the voltage limit of the electrolyte is an important factor for the characteristics of a capacitor [2,[8][9][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][12][13][14][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 (BMImBF 4 ) 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 (BMMImBF 4 ), 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. BMMImBF 4 is an attractive candidate as an additive in organic electrolytes of EDLCs.The electrodes were composed of the activated materials, c...