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 (C_elec 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 (R_s), charge transfer resistance (R_ct), 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.