Wade MILLAR1, Rob ATKIN2, Debbie SILVESTER-DEAN1, Yunxiao ZHANG2, Hua LI2
1Curtin University, Perth, Australia
2University of Western Australia, Perth, Australia
Surface-active ionic liquids (SAILs) are ionic liquids that contain at least one amphiphilic ion with a charged polar head group and one (or more) long, non-polar alkyl chain tails. The amphiphilic structuring of the SAIL 1-butyl-3-methylimidazolium dioctyl sulfosuccinate [BMIm][AOT] results in self-assembly of the ions into non-polar and charged domains (see Figure), and unique structuring at electrode surfaces. This results in high capacitance, which is beneficial for supercapacitor applications. However, the high viscosity of SAILs at room temperature inhibits ion movement during electrode polarization, impeding the formation of ordered interfacial structures. In this work, we investigate the change in capacitive charging properties at different temperatures caused by lowering viscosity when the SAIL is mixed with a polar organic solvent, propylene carbonate (PC). PC is a common solvent used as a capacitor and battery electrolyte due to its low volatility, low flammability, and high voltage window (>4 V). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) is used to measure the capacitance of the electric double layer (EDL) at glassy carbon (GC) electrodes in [BMIm][AOT] and is compared to the non-amphiphilic ionic liquid (NAIL) [BMIm][TFSI], where [TFSI]– = bis(trifuloromethylsulfonyl)imide. We reveal that lowered SAIL viscosity allows easy formation of charged and non-polar domains at the interface, resulting in a strong, densely charged EDL structure with reduced undesirable overscreening effects. Optimal capacitance is observed at high PC concentrations as the polar solvent stabilizes the polar region of the charged domain, thereby increasing order in both domains for a denser EDL with enhanced capacitance. Our results demonstrate the possibility of employing SAILs as electrolytes for capacitive devices, with enhanced capacitance achieved via dilution in an organic solvent.