Dense porous monolithic carbon from Indonesian daikon leaf waste via NaOH–CO2 physicochemical activation for binder-free volumetric supercapacitors

Apriwandi (aDepartment of Physics, University of Riau, 28293 Indonesia)
Widya Sinta Mustika (Department of Machine Maintenance and Repair, Politeknik Kampar, Riau 28412 Indonesia)
Julnaidi (Department of Mechanical Engineering, Sekolah Tinggi Teknologi Pekanbaru, Pekanbaru 28291 Indonesia)
Della Puspita (Department of Mechanical Engineering, Sekolah Tinggi Teknologi Pekanbaru, Pekanbaru 28291 Indonesia)
Rika Taslim (Department of Industrial Engineering, Universitas Islam Negeri Sultan Syarif Kasim, Riau 28293 Indonesia)
Erman Taer (Department of Mechanical Engineering, Sekolah Tinggi Teknologi Pekanbaru, Pekanbaru 28291 Indonesia)

Communications in Science and Technology
Vol 11 No 1 (2026)

High-performance volumetric supercapacitors are often limited by the trade-off between porosity and electrode density in biomass-derived carbon. Herein, binder-free monolithic carbon electrodes were produced from Indonesian daikon leaf waste through 0.5 M NaOH pretreatment followed by integrated N2–CO2 thermal processing. NaOH activation increased the surface area from 236 to 570 m2 g−1, increasing carbon purity from 88.34 to 95.45 wt% while retaining 4–5 wt% oxygen functionalities. The activated monolithic carbon electrode (VSAC) delivered a volumetric capacitance of 169 F cm−3 at 1 mV s−1 and 185 F cm−3 at 10 mA cm−2 in a symmetric two-electrode configuration without binders or conductive additives. VSAC also retained 83.2% capacitance at high scan rates and achieved 16.01 Wh L−1 at 838 W L−1. The improved performance arises from interconnected ion pathways, preserved electrode density, and surface oxygen functionalities, demonstrating a sustainable strategy for practical volumetric supercapacitors.