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 ZnCl₂ activation and FeCl₃ 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 m² 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 H₂SO₄ 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 Na₂SO₄ electrolyte.