Non-halogenated Imidazolium and Phosphonium-based Surface-Active Ionic Liquids as Electrolytes for Supercapacitors

We report a comparative analysis of three novel non-halogenated surface-active ionic liquids (SAILs), which consist of a surface-active anion, 2-ethylhexyl sulfate ([EHS]-), and phosphonium or imidazolium cations: tetrabutylphosphonium ([P4,4,4,4]+), trihexyl(tetradecyl)phosphonium ([P6,6,6,14]+), and 1-methyl-3-hexylimidazolium ([C6C1Im]+). We explored thermal properties (degradation, melting and crystallisation temperatures) of these novel SAILs and their electrochemical properties (ionic conductivity and electrochemical potential window, ECW). These SAILs were tested as electrolytes in a multi-walled carbon nanotubes (MWCNTs)-based supercapacitor at various temperatures from 253 to 373 K and their electrochemical performance as a function of temperature was compared. We found that the supercapacitor cell with [C6C1Im][EHS] as an electrolyte has shown a higher specific capacitance (Celec in F g-1), a higher energy density (E in W h kg-1), and a higher power density (P in kW kg-1) as compared to the other studied SAILs, [P4,4,4,4][EHS], [P6,6,6,14][EHS] and [N8,8,8,8][EHS] (from previous our study) at a temperature range from 253 to 373 K. The supercapacitor with an MWCNT-based electrode and [C6C1Im][EHS], [P4,4,4,4][EHS] and [P6,6,6,14][EHS] as electrolytes 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 W h kg-1 (at 2 mV s-1), respectively, all at 298 K. For latter three SAILs, the temperature effect can be seen by a two to three-fold increase in the specific capacitance of the cell and the energy density values: 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. The solution resistance (Rs), charge transfer resistance (Rct), and equivalent series resistance (ESR) also decreased with an increase in temperature for all SAILs in this study. These new SAILs can potentially be used for high-temperature electrochemical applications, such as supercapacitors for high energy storage due to a reasonably high specific capacitance and enhanced energy and power density, and wider ECWs as compared to molecular organic and aqueous electrolytes. Specifically, [C6C1Im][EHS] and [P4,4,4,4][EHS], are the best candidates among other “EHS”-based SAILs in this and our previous study, as electrolytes in supercapacitors.