Supercapacitive Properties of Micropore- and Mesopore-Rich Activated Carbon in Ionic-Liquid Electrolytes with Various Constituent Ions

Ionic-liquid (IL) electrolytes, characterized by large potential windows, intrinsic ionic conductivity, low environmental hazard, and high safety, are used for micropore- and mesopore-rich activated-carbon (AC _micro and AC _meso ) supercapacitors. IL electrolytes consisting of various cations [1-ethyl-3-methylimidazolium (EMI ⁺ ), N-propyl-N-methylpyrrolidinium (PMP ⁺ ), and N-butyl-N-methylpyrrolidinium (BMP ⁺ )] and various anions [bis(trifluoromethylsulfonyl)imide (TFSI ⁻ ), BF ₄ ⁻ , and bis(fluorosulfonyl)imide (FSI ⁻ )] are investigated. The electrolyte conductivity, viscosity, and ion transport properties at the AC _micro and AC _meso electrodes are studied. In addition, the capacitance, rate capability, and cycling stability of the two types of AC electrodes are systematically examined and post-mortem material analyses are conducted. The effects of IL composition on the charge–discharge capacitances of the AC _micro electrodes are more pronounced than those for the AC _meso electrodes. The FSI-based IL electrolytes, for which electrochemical properties are cation dependent, are found to be promising. Incorporating EMI ⁺ with FSI ⁻ results in a low electrolyte viscosity and a fast ion transport, giving rise to optimized electrode capacitance and rate capability. Replacing EMI ⁺ with PMP ⁺ increases the cell voltage (to 3.5 V) and maximum energy density (to 42 Wh kg ⁻¹ ) of the AC _micro cell at the cost of cycling stability.