High energy density supercapacitors with hierarchical nitrogen-doped porous carbon as active material obtained from bio-waste

Supercapacitors (SCs) is a promising energy storage approach to solve the intermittent problems of most renewable energy sources. N-doped hierarchically activated porous carbon materials (ACBS) for supercapacitor applicaitons are obtained by pre-carbonization and KOH activation of N-rich sword bean shells. ACBS containing 1–2% of N show hierarchical porosity, very high surface area and pore volume (equal to 2917 m2/g and 1.73 cm3/g, respectively). These materials are incorporated into supercapacitors. The device containing ACBS obtained using 700 °C as a post-treatment temperature shows a very high specific capacity (equal to 264 F/g at 1 A/g), which remains high (∼180 F/g) even at 20 A/g current density. Corresponding symmetric coin-cell supercapacitor demonstrats 12.5 Wh/kg energy density at 100 W/kg power density. This cell is capable of maintaining its energy density at the 11.1 Wh/kg level at 5000 W/kg power density and demonstrats almost 100% capacity retention after one thousand 1 A/g charge/discharge cycles. Such superior electrochemical performance of devices fabricated using ACBS as active materials is possible because of synergy between electrical double-layer capacitance and faradaic pseudocapacitance, which strongly depend on the surface area, pore volume and size distribution as well as dopant of heteroatoams.