Electric double layer capacitors of high volumetric energy based on ionic liquids and hierarchical-pore carbon

Abstract

In this study, we analyze the high-voltage charge-storage behavior of electric double-layer capacitors in which two ionic-liquid electrolytes are used, 1-ethyl-3-methylimidazolium and 1-methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imides (EMIm- and MPPy-TFSIs), and are operated at 3.5 and 4.1 V, respectively. Symmetric two-electrode capacitor cells assembled using micropore-rich activated mesophase pitch (aMP) and activated carbon fiber (aCF) carbons show a standard capacitive behavior in cyclic voltammetry analysis, whereas cells featuring templated mesoporous carbon (tMC) show ion-intercalating peaks in high-voltage scans. Impedance analysis performed at high voltages reveals that the aMP and aCF cells show lower charge-storage resistance than the tMC, although tMC facilitates ion transport more efficiently than aMP and aCF. The experimental results indicate that micropore-rich aMP and aCF accommodate single ions at high voltages, whereas the carbon structure is destroyed in micropore-deficient tMC because of graphitic-layer intercalation. The aMP carbon, which contains hierarchically connected micropores and mesopores, is effective in storing charge at a high rate at high voltages. Because of the compact feature of aMP, incorporating ionic liquids with aMP represents a very promising strategy for assembling capacitors of ultrahigh volumetric energy densities.