From Waste to Energy Storage: Moderately Porous, N-doped Biochar from Microwave Pyrolysis of Watermelon Peel for Supercapacitor Applications

The microwave pyrolysis of watermelon peel at 250 °C yielded an extremely low solid residue (4.0 wt.%), indicating that the applied thermal conditions favored devolatilization rather than carbon conservation, making them suboptimal for targeted biochar production. Nonetheless, the resulting carbonaceous material exhibited a moderate specific surface area (~388 m² g⁻¹) and predominantly microporous structure with an average pore width of 1.64 nm, features favorable for charge storage. Its low pore volume (0.096 cm³ g⁻¹), however, limits immediate applicability as a high-capacitance electric double-layer capacitor (EDLC) electrode. To unlock its potential, post-synthesis activation—particularly chemical treatments to enhance surface area and pore volume while retaining nitrogen-derived heteroatom doping is recommended. Such optimization could introduce additional pseudocapacitance and broaden applicability to supercapacitors, adsorption, soil amendment, and catalysis. Overall, the study validates watermelon peel as a promising feedstock for functional porous carbons and highlights the need for process intensification and targeted activation to improve yield and electrochemical performance.