Hierarchically Porous Carbon Nanosheets from Waste Coffee Grounds for Supercapacitors

The nanostructure design of porous carbon-based electrode materials is key to improving the electrochemical performance of supercapacitors. In this study, hierarchically porous carbon nanosheets (HP-CNSs) were fabricated using waste coffee grounds by in situ carbonization and activation processes using KOH. Despite the simple synthesis process, the HP-CNSs had a high aspect ratio nanostructure (∼20 nm thickness to several micrometers in lateral size), a high specific surface area of 1945.7 m² g^–1, numerous heteroatoms, and good electrical transport properties, as well as hierarchically porous characteristics (0.5–10 nm in size). HP-CNS-based supercapacitors showed a specific energy of 35.4 Wh kg^–1 at 11250 W kg^–1 and of 23 Wh kg^–1 for a 3 s charge/discharge current rate corresponding to a specific power of 30000 W kg^–1. Additionally, the HP-CNS supercapacitors demonstrated good cyclic performance over 5000 cycles