Facile preparation of functionalized hierarchical porous carbon from bean dregs for high-performance supercapacitors

Functionalized hierarchical porous carbons have been successfully synthesized through simple hydrothermal treatment of biowaste bean dregs and succedent KOH-activation procedure. By adjusting the ratio of activation agent to the hydrochar, well-developed hierarchical porosity coupled with plentiful N/O containing functional groups was achieved in the resulting carbon. These features endowed the electrodes with high specific capacitances, maximum of 321.3 F g−1 at 1 A g−1 in 6 M KOH, as well as decent rate performance. Furthermore, symmetric supercapacitor system was fabricated using aqueous electrolytes. High-energy density of 9 Wh kg−1 with superb cyclic stability of 90.8% retention after 5000 cycles was achieved in 6 M KOH. By applying 1 M Na2SO4, the energy density reached 25.9 Wh kg−1. This work sheds light on the economical and environmentally friendly way of employing natural abundant biomass as precursor to prepare porous carbons for energy storage applications.