Solvent-Free Mechanochemical Preparation of Hierarchically Porous Carbon for Supercapacitor and Oxygen Reduction Reaction.

Hierarchically porous carbon (HPC) derived from fallen flowers was prepared by a green method of combining solvent-free ball milling with the safe activating agent potassium bicarbonate. By regulating the mass ratio of KHCO3 to biochar, the as-prepared HPC materials possess high specific surface areas of up to 2147.9 m2  g-1 and abundant hierarchical pores comprised of micro-, meso-, and macropores. The specific capacitances of the HPC materials are able to reach 302.7 F g-1 at a current density of 0.5 A g-1 in 6 m KOH, and the symmetric supercapacitor composed of two identical HPC electrodes shows good stability because 100 % of the specific capacitance is retained after 5000 charge/discharge cycles and 90.5 % of the specific capacitance remains after 12 000 cycles. Nitrogen self-doped HPC materials also show higher electrocatalytic activity and stronger methanol tolerance for the oxygen reduction reaction than that of the commercial Pt/C catalyst. This preparation method can be extended to the green conversion of other varieties of biomass waste into useful carbon materials.