The performance of electrochemical capacitors (ECs) is determined by the properties of their main components electrodes and the electrolyte. The energy density of EC is directly related to specific capacitance of the electrodes and the working potential of the device. In recent years, Ionic liquids (ILs) have attracted a lot of attention as electrolytes for ECs as they are stable at higher potentials compared to conventional aqueous and organic electrolytes, leading to higher energy density of the device.
However, neat ionic liquids have much higher viscosities compared to electrolytes based on organic solvents such as acetonitrile (AN) or propylene carbonate (PC) and they are also more expensive. In this study, we introduce mixtures of ionic liquids and PC as effective substitutes for conventional organic electrolytes. We show that by using these mixtures as the electrolyte instead of a typical organic electrolyte (tetraethylammonium tetrafluoroborate (TEA BF4) in PC), we can extend the working potential and even cycle life of ECs without sacrificing the rate performance. The ionic liquids all contain the tetrafluoroborate anion just like the organic electrolyte used for comparison.
The three different cations are 1-Butyl-1-methylhomopiperidinium, 1-Ethyl-1-methylpyrrolidinium, and 1,1-Dimethylpyrrolidinium. In all cases, multilayered graphene sheets are used as the electrode material. Electrochemical performance is assessed through galvanostatic cycling, cyclic voltammetry, and impedance spectroscopy. Results indicate better capacitive performance of all three IL electrolytes compared to the conventional organic electrolyte as well as an impressive capacitive stability over 10000 cycles. These electrolytes find applications in ECs with high energy density and long lifetime requirements.