  Graphene is a promising material for energy storage, especially for high
performance supercapacitors. For real time high power applications, it is
critical to have high specific capacitance with fast charging time at high
current density. Using a modified Hummer's method and tip sonication for
graphene synthesis, here we show graphene-based supercapacitors with high
stability and significantly-improved electrical double layer capacitance and
energy density with fast charging and discharging time at a high current
density, due to enhanced ionic electrolyte accessibility in deeper regions. The
discharge capacitance and energy density values, 195 Fg-1 and 83.4 Whkg-1, are
achieved at a current density of 2.5 Ag-1. The time required to discharge 64.18
Whkg-1 at 5 A/g is around 25 sec. At 7.5 Ag-1 current density, the cell can
deliver a specific capacitance of about 137 Fg-1 and maintain 98 % of its
initial value after 10,000 cycles, suggesting that the stable performance of
supercapacitors at high current rates is suitable for fast charging-discharging
applications. We attribute this superior performance to the highly porous
nature of graphene prepared with minimum restacking due to crimple nature
wrinkles and the improved current collecting method.
