2-Ethylhexylsulfate Anion-based Surface-Active Ionic Liquids (SAILs) as temperature persistent electrolytes for supercapacitors

We report on a comparative study of three novel non-halogenated surface-active ionic liquids (SAILs), which contain a surface-active anion, 2-ethylhexyl sulfate ([EHS]⁻), and phosphonium or imidazolium cations: tetrabutylphosphonium ([P_4,4,4,4]⁺), trihexyl(tetradecyl)phosphonium ([P_6,6,6,14]⁺), and 1-methyl-3-hexylimidazolium ([C₆C₁Im]⁺). Thermal and electrochemical properties i.e., ionic conductivities at different temperatures and electrochemical potential windows of these SAILs were thoroughly studied. SAIL's electrochemical performance as electrolytes was also examined in a multi-walled carbon nanotubes (MWCNT)-based supercapacitor over a wide range of temperatures from 253 to 373 K. We observed that the electrode material in the supercapacitor cell with [C₆C₁Im][EHS] as an electrolyte has a higher specific capacitance (C_elec in F g⁻¹), a higher electric energy density (E in W h kg⁻¹), and a higher electric power density (P in kW kg⁻¹) as compared to the other studied SAILs, [P_4,4,4,4][EHS], [P_6,6,6,14][EHS] and [N_8,8,8,8][EHS] (from our preceding study) in a temperature range from 253 to 373 K: At the scan rate of 2 mV s⁻¹ a supercapacitor cell with a MWCNT-based electrode and [C₆C₁Im][EHS], [P_4,4,4,4][EHS] and [P_6,6,6,14][EHS] as electrolytes has the specific capacitance, C_elec = 148, 90 and 47 F g⁻¹ and the energy density, E = 82, 50 and 26 W h kg⁻¹, respectively, when measured at 298 K. For the named three SAILs at the scan rate of 2 mV s⁻¹, a two- to three-fold increase in the specific capacitance and the energy density values was measured at 373 K: C_elec = 290, 198 and 114 F g⁻¹ and E = 161, 110 and 63 Wh kg⁻¹, respectively. The solution resistance (R_s), charge transfer resistance (R_ct) and equivalent series resistance (ESR) all decreased two- to three-fold with an increase in temperature from 298 to 373 K. With the high specific capacitance and enhanced energy and power density and wider electrochemical potential window as compared to the molecular organic and aqueous electrolytes, these SAILs can be used for high-temperature electrochemical applications, such as high power and energy storage devices. In particular, up to now, [C₆C₁Im][EHS] and [P_4,4,4,4][EHS] are the most appropriate candidates for such applications.