Ionic Liquids Containing Sulfonium Cations as Electrolytes for Electrochemical Double Layer Capacitors

In this work, we report on the behavior of ionic liquids (ILs) containing sulfonium cations as electrolytes for electrochemical double layer capacitors (EDLCs). Physical properties such as viscosity and ionic conductivity are reported over a range of temperatures for ILs containing the diethylmethyl sulfonium [S₂₂₁], triethyl sulfonium [S₂₂₂], and diethylpropyl sulfonium [S₂₂₃] cations paired with the bis­(trifluoromethanesulfonyl) imide [Tf₂N] anion. The size and structure of the cations are shown to influence the physical and electrochemical properties of the ILs, with a significant degree of ionic coordination being evident in [S₂₂₃]­[Tf₂N]. The electrochemical behavior of these ILs in EDLCs was compared with that of a fairly established IL electrolyte, N-butyl-N-methylpyrrolidinium bis­(trifluoromethanesulfonyl) imide ([Pyr₁₄]­[Tf₂N]), and it is shown that [S₂₂₁]­[Tf₂N] can perform better in terms of energy and power at room temperature, despite operating at a much reduced potential.