Ionic Liquids for the Electric Double Layer Capacitor Applications

Abstract

The properties of ionic liquids (ILs) include non-volatility, non-flammability, and relatively high ionic conductivity (Rogers & Seddon, 2001). As novel green, reusable solvents that can substitute for organic solvents, ionic liquids have attracted much attention as good media in organic synthesis and other chemical processes (Zhao et al., 2009, 2011). At the same time, some studies have been reported which aim to improve the high-temperature safety and durability of such electrochemical devices as lithium rechargeable batteries (Sato et al., 2004), electric double layer capacitors (EDLC) (Sato et al., 2004), and titanium oxide dyesensitized solar cells (Papageorgiou et al., 1996). The electric double layer capacitor (EDLC) is an energy storage device based on the operating principle of the electric double-layer that is formed at the interface between an activated carbon material and an electrolyte. Various solvents and salts (solutes in otherword) are available, offering specific advantages such as high capacitance and low temperature performances. Generally, an organic electrolyte that is a solid quaternary ammonium salt, such as N,N,N,N-tetraethylammonium tetrafuluoroborate (TEA-BF4), dissolved in the high dielectric constant solvent propylene carbonate (PC) has been used for high voltage EDLCs of 2V or more. This device stores electricity physically, and lacks the chemical reactions found in rechargeable batteries during charging and discharging (Zheng et al., 1997). Therefore, compared to rechargeable batteries, the EDLC has a remarkably long cycle life and high power density. Such devices are now widely used in power electronics for peak power saving and back up memories, and in electronic power supplies for automated guided vehicle systems and construction equipment. One of their most promising applications is for use in transportation, especially in hybrid electric vehicles (HEVs). However, some issues in the development of EDLCs remain. 1. A lower energy density compared with lithium ion secondary batteries. 2. Flammable electrolytes raise safety concerns. 3. Low cycle durability in the high temperature region. 4. Poor charge-discharge properties at low temperatures. In order to overcome these challenges, we are actively pursuing research and development in the use of ionic liquids for the electrolyte of EDLCs. The energy E stored in an EDLC is proportional to its capacity, C, as well as the voltage V applied between the positive and the negative electrodes: