Comparative Investigation of Non-halogenated Imidazolium and Phosphonium-based Surface-Active Ionic Liquids as Electrolyte for Supercapacitors
We report a comparative analysis of non-halogenated surface-active ionic liquids (SAILs), which consists of the surface-active anion, 2-ethylhexyl sulfate, and the phosphonium, and imidazolium cations i.e., tetrabutylphosphonium ([P4444]+), trihexyl(tetradecyl)phosphonium ([P66614]+), and 1-methyl-3-hexylimidazolium ([C6C1IM]+). We explored the thermal and electrochemical properties, i.e., degradation, melting and crystallization temperatures, and ionic conductivity of this new class of IL. These SAILs were tested as an electrolyte in a multi-walled carbon nanotubes (MWCNTs)-based supercapacitor at various temperatures from 253 to 373 K. The electrochemical performance of different SAILs by varying the cationic core as a function of temperature were compared, in the same MWCNT-based supercapacitor. We found that the supercapacitor cell with [C6C1IM][EHS] shown high specific capacitance (Celec in F g-1), a high energy density (E in Wh kg-1), and a high power density (P in kW kg-1) when compared to those for the other SAILs i.e. [P4444][EHS], [P66614][EHS], and [N8888][EHS] at all temperatures. The supercapacitor with an MWCNT-based electrode and [C6C1IM][EHS], [P4444][EHS], and [P66614][EHS] as an electrolyte showed a specific capacitance of 148, 90, and 47 F g-1 (at the scan rate of 2 mV s-1) with an energy density of 82, 50, and 26 Wh kg-1 (at 2 mV s-1) respectively, at 298 K. The temperature effect can be seen by the three to four-fold increase in the specific capacitance of the cell and the energy density values, i.e., 290, 198, and 114 F g-1 (at 2 mV s-1) and 161, 110, and 63 Wh kg-1 (at 2 mV s-1), respectively, at 373 K. This study reveals that these new SAILs specifically [C6C1IM][EHS] and [P4444][EHS] can potentially be used as electrolytes in the wide range of temperature. The solution resistance (Rs), charge transfer resistance (Rct), and equivalent series resistance (ESR) also decreased with an increase in temperature for all SAILs as electrolytes. These new SAILs can explicitly be used for high-temperature (wide range of temperature) electrochemical applications, such as efficient supercapacitors for high energy storage due to enhanced specific capacitance, energy, and power density at elevated temperatures.