Incorporation of surface-based capacitances (C/S) simulated by Helmholtz models with pore size distribution obtained from the non-local density functional theory precisely predicts the double-layer capacitance of distinct forms of carbon.
Graphene sheets are an ideal carbon material with the highest area available for electrolyte interaction and can be obtained by reducing graphite oxide (GO). This study presents the photocatalytic reduction of GO in water with mercury-lamp irradiation. The specific capacitance of the reduced GO in an H 2 SO 4 aqueous solution reached levels as high as 220 F g À1 . This is because of the double layer formation and the reversible pseudocapacitive processes caused by oxygen functionalities at the sheet periphery. The rate capability for charge storage increases with irradiation time due to the continued reduction of oxygenated sites on the graphene basal plane. Alternating current impedance analysis shows that prolonged light irradiation promotes electronic percolation in the electrode, significantly reducing the capacitive relaxation time. With a potential widow of 1 V, the resulting symmetric cells can deliver an energy level of 5 Wh kg À1 at a high power of 1000 W kg À1 . These cells show superior stability, with 92% retention of specific capacitance after 20 000 cycles of galvanostatic chargeÀdischarge.
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.
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