High yield conversion of biowaste coffee grounds into hierarchical porous carbon for superior capacitive energy storage.

Recently great efforts have been focused on converting biowastes into high-valued carbon materials. However, it is still a great challenge to achieve high carbon yield and controllable porous distribution in both industrial and academic research. Inspired by the multi-void structure of waste coffee grounds, herein we fabricated hierarchical porous carbon via the combination of catalytic carbonization and alkali activation. The catalytic carbonization process was applied to obtain well-defined mesoporous carbon with carbon yield as high as 42.5 wt%, and subsequent alkali activation process produced hierarchical porous carbon with ultrahigh specific surface area (3549 m g) and large meso-/macropores volume (1.64 cm g). In three-electrode system, the electrode exhibited a high capacitance of 440 F g at 0.5 A g in 6 M KOH aqueous electrolyte, superior to that of many reported biomass-derived porous carbons. In two-electrode system, its energy density reached to 101 Wh kg at the power density of 900 W kg in 1-Ethyl-3-Methylimidazolium Tetrafluoroborate (EMIMBF). This work provided a cost-effective strategy to recycle biowastes into hierarchical porous carbon with high yield for high-performance energy storage application.