Hydrothermal co-processing of plastic waste with lignocellulosic biomass and its application as a supercapacitor material

Designing and optimizing a continuous process for hydrothermal conversion of biomass is critical in increasing the production capacity of valuable products. This study investigates the use of a continuous high-pressure single screw reactor for converting a mixed feedstock comprising sawdust and polypropylene wastes into a carbon-rich solid product that can be used in energy storage devices, especially as supercapacitor material. The process parameters for the continuous operation were optimized using the Box-Behnken design, the maximum yield obtained after the optimization was to be 50.15 % at 305.3 °C. Subsequently, the produced solid (biochar) was further activated using potassium hydroxide (KOH) and sulfuric acid (H₂SO₄) to improve the surface properties which would eventually improve the energy storage capability of the material. Electrochemical tests conducted on pristine biochar, KOH, and H₂SO₄ activated biochar showed that the specific capacitance values of 196.8 F/g for non-activated biochar, 403.67 F/g for KOH-activated biochar, and 325.96 F/g for H₂SO₄-activated biochar at a current density of 1 A/g. Energy density analysis indicated that the alkali and acid-activated biochar had energy densities of 45.53 Wh/kg and 36.0 Wh/kg, with corresponding power densities of 427.5 W/kg and 423.0 W/kg, respectively. These findings highlight the feasibility of utilizing hydrothermal co-processing along with a continuous process as a sustainable and efficient approach for waste management and the production of high-performance energy storage materials.