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.