Transformation of Plant Biomass Waste into Resourceful Activated Carbon Nanostructures for Mixed-Assembly Type Electrochemical Capacitors

Activated carbon (AC) was obtained from three different plant biomass wastes sources (coconut shell, pine cones and rice husk) via hydrothermal treatment followed by carbonization at 800 °C for different times. The morphological and structural characteristics of the transformed carbon material revealed a highly disordered graphitic carbon composed of a porous network with energy storage capability. The mixed-assembly type cells fabricated from the best samples based on specific capacitance from the single electrode tests exhibited electric double layer capacitance (EDLC) behaviour in all sample combinations using all the three transformed activated carbon materials. The mixed-assembly device worked comfortably in a voltage window of 1.5 V in neutral aqueous electrolyte. A specific capacitance ( C s ) of ∼ 110 F g −1 was obtained with a corresponding energy density of 8.5 W h kg −1 and power density of 380 W kg −1 at a current density of 0.5 A g −1 for the PC_RH device An excellent stability was exhibited with a coulombic efficiency of a 99.7% and capacitance retention of 80% after 10,000 continuous cycling at 5.0 A g −1 . Furthermore, subjecting the PC_RH mixed device to a floating test for ∼ 48 h (2 days) at its optimum voltage (1.5 V) revealed retention in the capacitance value to more than 50% its initial value with still no recorded device failure. Remarkably, the asymmetric design showed a potential for adopting EDLC materials of different carbon sources in order to capture the entire properties for efficient and stable energy storage devices. Graphical Abstract