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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 ([P4,4,4,4]+), trihexyl(tetradecyl)phosphonium ([P6,6,6,14]+), 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 (Celec 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, [P4,4,4,4][EHS], [P6,6,6,14][EHS] and [N8,8,8,8][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], [P4,4,4,4][EHS] and [P6,6,6,14][EHS] as electrolytes has the specific capacitance, Celec = 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: Celec = 290, 198 and 114 F g−1 and E = 161, 110 and 63 Wh kg−1, respectively. The solution resistance (Rs), charge transfer resistance (Rct) 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 [P4,4,4,4][EHS] are the most appropriate candidates for such applications.

Non-halogenated boron-based ionic liquids (ILs) composed of phosphonium cations and chelated orthoborate anions have high hydrolytic stability, low melting point and exceptional properties for various applications. This study is focused on ILs with the same type of cation, trihexyltetradecylphosphonium ([P6,6,6,14]+), and two orthoborate anions, such as bis(salicylato)borate ([BScB]−) and bis(oxalato)borate ([BOB]−). We compare the results of this study with our previous studies on ILs with bis(mandelato)borate ([BMB]−) and a variety of different cations (tetraalkylphosphonium, dialkylpyrrolidinium and dialkylimidazolium). The ion dynamics and phase behavior of these ILs is studied using 1H and 11B pulsed-field-gradient (PFG) NMR. PFG NMR is demonstrated to be a useful tool to elucidate the dynamics of ions in this class of phosphonium orthoborate ILs. In particular, the applicability of 11B PFG NMR for studying anions without 1H, such as [BOB]−, and the limitations of this technique to measure self-diffusion of ions in ILs are demonstrated and discussed in detail for the first time.

In this work, we investigate the ion pair tetramethylphosphonium cation, [P1,1,1,1]+, and bis(oxalato)borate anion, [BOB]−, as a model system for the study of ionic liquids interacting with both hydroxylated and oxygen terminated α-SiO2 (001) surfaces, using first-principles electronic structure theory. We use a single ionic pair and clusters of ion pairs, in order to have exclusively neutral supercell slab models. We use dispersion-corrected density functional theory (DFT) to ascertain that both the strong physical binding between the ions, dominated by ionic binding, and the weaker physical binding of ions to the different surfaces are correctly described. We have found that the binding of ion pairs is stronger to the hydroxylated α-SiO2 (001) surface compared to the oxygen terminated surface, which is attributed to the formation of H-binding with the oxygen atom(s) of the [BOB]− anion. Through rotation of ionic pair(s), we estimate the surface-ions energy barrier for translational movement and, thus, the strength of H-binding of the ions. At the surface of hydroxylated α-SiO2 (001), we have studied how water molecules form a network of H-binding with the OH groups of the surface and the [BOB]− anion, which offers an explanation for the reduction in the friction of ionic liquids on the inclusion of water. We suggest modeling protocols for simulation of ion pairs on surfaces, which can open up the possibility to use DFT to aid in designing and understanding the physicochemical mechanism of interactions of ionic materials (including ionic liquids) in various technological applications.