November 14, 2013

Koreans say graphene supercapacitors are ready for electric cars

Conventional batteries take so long to charge that they cannot efficiently store braking energy. But now graphene supercapacitors that store almost as much but charge in just 16 seconds could do the job instead.

Now Santhakumar Kannappan at the Gwangju Institute of Science and Technology in Korea and a few pals say they have a solution based on the wonder material of the moment–graphene. These guys have built high-performance supercapacitors out of graphene that store almost as much energy as a lithium-ion battery, can charge and discharge in seconds and maintain all this over many tens of thousands of charging cycles.

The trick these guys have perfected is to make a highly porous form of graphene that has a huge internal surface area. They create this graphene by reducing graphene oxide particles with hydrazine in water agitated with ultrasound.

The graphene powder is then packed into a coin-shaped cell, and dried at 140 degrees C and at a pressure of 300/kg/cm for five hours.

Arxiv - Graphene based Supercapacitors with Improved Specific Capacitance and Fast Charging Time at High Current Density

Kannappan and co have measured the performance of their supercapacitor and are clearly impressed with the results. They say it has a specific capacitance of over 150 Farrads per gram can store energy at a density of more than 64 Watt hours per kilogram at a current density of 5 Amps per gram.

That’s almost comparable with lithium-ion batteries which have an energy density of between 100 and 200 Watt hours per kilogram.

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.

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