Aqueous supercapacitors based on carbonized silk electrodes.

Graphitic nitrogen-doped hierarchical porous carbon nanosheets for supercapacitor application were derived from an easily obtained and green silk by simultaneous ZnCl 2 activation and FeCl 3 graphitization at different heating temperatures. By increasing the heating temperature from 700 to 850 °C, the degree of graphitization and BET surface area rose to their highest levels, while the nitrogen doping content was maintained at 2.24 wt%. Carbonized silk at 850 °C displays a nanosheet morphology and a considerable specific surface area (1285.31 m 2 g -1 ), and it was fabricated into a supercapacitor as an electrode material, exhibiting superior electrochemical performance with a high specific capacitance of 178 F g -1 at 0.5 A g -1 and an excellent rate capability (81% capacitance retention ratio even at 20 A g -1 ) in 1 mol L -1 H 2 SO 4 electrolyte. A symmetric supercapacitor using carbonized silk at 850 °C as the electrodes has an excellent specific energy of 14.33 W h kg -1 at a power density of 251 W kg -1 operated over a wide voltage range of 2.0 V in aqueous neutral Na 2 SO 4 electrolyte.