The increasing demand for sustainable and cost-effective energy storage solutions has driven interest in biomass-derived carbon materials for supercapacitor electrodes. This study explores the valorization of coconut residue (CR), an abundant agricultural waste, as a carbon precursor for nanoporous carbon (NPC) production. NPC was synthesized via hydrothermal carbonization (HTC) of CR, followed by chemical activation using potassium hydroxide (KOH) at varying temperatures (700, 800, and 900 °C). The effects of activation temperature on the structure and electrochemical performance of the NPC were systematically investigated. The activated materials exhibited amorphous, highly porous structures, with surface areas increasing alongside activation temperature—reaching a maximum of 1969 m² g⁻¹ at 900 °C. Electrochemical characterization was conducted using a three-electrode setup through cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) in a 1 M Na₂SO₄ electrolyte. The sample activated at 900 °C with a CR:KOH weight ratio of 1:2.5 achieved the highest specific capacitance of 52 F g⁻¹ at a specific current of 1 A g⁻¹. These findings underscore the potential of CR as a low-cost and sustainable raw material for fabricating efficient electrode materials in energy storage applications.

This study investigates the co-hydrothermal carbonization (Co-HTC) of polystyrene (PS) and brewery waste (BW) to produce high-quality hydrochar with enhanced fuel properties. Experiments were carried out with different PS-to-BW ratios and reaction temperatures to assess their impact on hydrochar yield, physicochemical properties, and combustion performance. The results showed that higher PS content notably enhanced both the hydrochar yield and higher heating value (HHV), demonstrating increased energy recovery potential. FTIR analysis revealed a gradual reduction in peak intensities with increasing temperature, indicating enhanced decomposition and structural transformation of functional groups. Examination of combustion properties demonstrated improved ignition and firing behavior, evidenced by reduced combustion durations and elevated combustion indices. Van Krevelen analysis showed that the co-hydrochars migrated toward the coal compositional region, supporting their potential as a sustainable alternative to conventional solid fuels. These findings underscore Co-HTC as a promising waste-to-energy strategy for the concurrent valorization of plastic and biomass waste streams.