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To investigate the degradation mechanisms of electric double-layer capacitor ͑EDLC͒ components using 1.0 M triethylmethylammonium ͑TEMA͒ tetrafluoroborate ͑BF 4 ͒ in propylene carbonate ͑PC͒, the failure-mode processes of positive and negative electrodes were characterized as a function of the applied voltage ͑2.5-4.0 V͒. When the cell voltage ranges below 3.0 V, no impedance spectra or surface morphology changes were observed, indicating that no side reactions occur in this case. In the voltage range from 3.0 to 3.7 V, the exfoliation of graphene layers in activated carbon ͑AC͒ and the formation of cracks were observed in the positive electrode over 4.9 V vs Li/Li + possibly due to the gasification of surface functional groups with adsorbed water. On the negative electrode, the adsorbed water is electrochemically reduced to H 2 gas and OH − . The generated OH − induces the Hoffman elimination of TEMA + and activates the hydrolysis of PC. These water-induced side reactions could be the most critical factors for higher voltage operation. In the higher voltage range ͑over 3.7 V͒, the accumulation of solid electrolyte interface films by electrochemical oxidation and the reduction of PC were observed for both electrodes, indicating that the electrochemical oxidation and the reduction of PC on the AC surfaces occur above 5.2 V and below 1.5 V vs Li/Li + , respectively.Electric double-layer capacitors ͑EDLCs͒ are attracting much attention as one of the potential devices for energy regeneration of strong hybrid or plug-in hybrid electric vehicles because of their higher power densities ͑1-10 kW kg −1 ͒ compared to other energy devices, such as Li-ion batteries ͑0.1-1 kW kg −1 ͒. However the weakness of the EDLC is its low energy density ͑10 Wh kg −1 ͒, which cannot reach the target values ͑ca. 20 Wh kg −1 ͒ required of the hybrid vehicles by the Department of Energy and others. EDLCs can store electric charge in an electrical double layer at the electrode-electrolyte interface. Because the energy density is a product of capacitance and voltage, increasing the capacitance or voltage is necessary to enhance energy density. Increasing the voltage is more effective because the energy density increases in proportion to voltage squared.Activated carbons ͑ACs͒ have been widely used as electrode material because of their high specific surface area ͑1000-2000 m 2 g −1 ͒, immediate availability, and low cost. 1 Nonaqueous electrolytes consisting of acetonitrile ͑AN͒ or propylene carbonate ͑PC͒ solutions with 1 M tetraethylammonium ͑TEA͒ tetrafluoroborate ͑BF 4 ͒ 2-5 are used in commercial EDLCs because they permit a wide operating voltage ͑2.5-2.7 V͒.Recently, new approaches have been suggested by several researchers to achieve the target values of the hybrid vehicles. These approaches can be classified into three types according to the method as follows:1. Multifunctional nanoarchitectures. To optimize the ionic and electronic conductivities of electrode materials with charge-storage functionality, nanoscale structuring ͑nanostr...

Nanosized hydrous RunormalO2 /Ketjen Black (KB) composites were prepared using an in situ sol-gel process induced by ultracentrifugal mechanical force for supercapacitors. The hydrous RunormalO2 in the prepared samples were nanosized particles ca. 2nm and were highly dispersed on conductive carbon (KB) even at high hydrous RunormalO2 content (50wt%) . The composite annealed at 150°C exhibited the high specific capacitance of 821Fnormalg−1 , which corresponds to a charge utilization as high as 96%. Such a high charge utilization could be because the nanoparticles have both outer and inner hydrous channels which facilitate ionic transport. A model capacitor assembled using the composite exhibited energy and power densities as high as 12Whkg−1 and 6kWkg−1 , respectively.