Nonhalogenated Surface-Active Ionic Liquid as an Electrolyte for Supercapacitors

We report a nonhalogenated surface-active ionic liquid (SAIL) that consists of the surface-active anion 2-ethylhexyl sulfate and the tetraoctylammonium cation ([N_8,8,8,8]­[EHS]). We explored the thermal and electrochemical properties, i.e., degradation, melting and crystallization temperatures, ionic conductivity, and electrochemical potential window of neat SAIL and its binary mixture with acetonitrile. This SAIL was tested as an electrolyte in a multiwalled carbon nanotube (MWCNT)-based supercapacitor at various temperatures from 298 to 373 K. In addition, we also tested the binary mixture of SAIL with acetonitrile as an electrolyte at lower temperatures (253–298 K). The electrochemical performance of SAIL and the SAIL/acetonitrile binary mixture as a function of temperature was compared with that of a standard electrolyte, an aqueous solution of 6 M KOH, in the same MWCNT-based supercapacitor. The solution resistance (R_s), charge transfer resistance (R_ct), and equivalent series resistance (ESR) decreased with an increase in temperature for all SAIL-based electrolytes. We found that the supercapacitor cell with SAIL as an electrolyte has a high specific capacitance (C_elec in F g^–1), a high energy density (E in Wh kg^–1), and a high power density (in W kg^–1) compared to those for the binary mixture of SAIL with acetonitrile and for the 6 M KOH aqueous electrolytes, particularly at elevated temperatures. For the SAIL/MWCNT-based supercapacitor, C_elec increased from 75 F g^–1 at 298 K to 169 F g^–1 at 373 K, whereas the energy density increased from 42 Wh kg^–1 (at 298 K) to 94 Wh kg^–1 (at 373 K) and the power density increased from 75 kW kg^–1 (at 298 K) to 169 kW kg^–1 (at 373 K) at a scan rate of 2 mV s^–1 (potential window = 4 V). This study reveals that SAIL can potentially be used as an electrolyte for high-temperature electrochemical applications for energy storage devices.