<p>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>i.e.,</i> tetrabutylphosphonium ([P<sub>4444</sub>]<sup>+</sup>),
trihexyl(tetradecyl)phosphonium ([P<sub>66614</sub>]<sup>+</sup>), and
1-methyl-3-hexylimidazolium ([C<sub>6</sub>C<sub>1</sub>IM]<sup>+</sup>). We
explored the thermal and electrochemical properties, <i>i.e.</i>, 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 [C<sub>6</sub>C<sub>1</sub>IM][EHS] shown
high specific capacitance (<i>C<sub>elec</sub></i> in F g<sup>-1</sup>), a high energy density (<i>E</i> in Wh kg<sup>-1</sup>),
and a high power density (<i>P</i> in kW kg<sup>-1</sup>) when compared to those for the other SAILs <i>i.e.</i> [P<sub>4444</sub>][EHS],
[P<sub>66614</sub>][EHS], and [N<sub>8888</sub>][EHS] at all temperatures. The supercapacitor with an
MWCNT-based electrode and [C<sub>6</sub>C<sub>1</sub>IM][EHS], [P<sub>4444</sub>][EHS],
and [P<sub>66614</sub>][EHS] as an electrolyte showed a specific capacitance of
148, 90, and 47 F g<sup>-1</sup> (at the scan rate of 2 mV s<sup>-1</sup>) with
an energy density of 82, 50, and 26 Wh kg<sup>-1</sup> (at 2 mV s<sup>-1</sup>)
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>i.e.</i>, 290, 198, and 114 F g<sup>-1</sup> (at 2 mV s<sup>-1</sup>)
and 161, 110, and 63 Wh kg<sup>-1</sup> (at 2 mV s<sup>-1</sup>), respectively,
at 373 K. This study reveals that these new SAILs specifically [C<sub>6</sub>C<sub>1</sub>IM][EHS]
and [P<sub>4444</sub>][EHS] can potentially be used as electrolytes in the wide
range of temperature. The solution resistance (<i>R<sub>s</sub></i>),
charge transfer resistance (<i>R<sub>ct</sub></i>), 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. </p>
High Energy-Density Capacitor Based on Ammonium Salt Type Ionic Liquids and Their Mixing Effect by Propylene Carbonate
