Eco-Friendly Preparation of Biomass-Derived Porous Carbon and Its Electrochemical Properties

High-porosity activated carbon (AC) was prepared from low-cost coconut shells (CS) using KOH as an activating agent with different KOH/char mass ratios. To cut down the amount of KOH used for AC preparation, freezing pretreatment for a certain number of times was carried out on CS before carbonization, which resulted in the maximum increase in the specific surface area (SBET) and total pore volume of 92.8 and 44.4%, respectively, in the resultant biochar. For the sake of description, AC from CS undergoing x times of freezing pretreatment and with a KOH/char ratio of y is denoted as AC-xy. The SBET values of AC-13 and AC-24 are 193 and 166 m2 g-1 larger than that of AC-05 (2217 m2 g-1), respectively. At a current density of 0.25 A g-1, the specific gravimetric capacitance (Cg) values of AC-05, AC-13, and AC-24 are 386, 403, and 425 F g-1. Moreover, a symmetric supercapacitor based on AC-24 exhibits a high energy density of 14.7 Wh kg-1 at a power density of 120 W kg-1. The energy density retention rate of AC-24 is 71.1% with the power density increased by about 110 times, indicating excellent rate capability. Additionally, a capacitance retention rate of about 95% after 3000 cycles implies an outstanding cycle lifetime of an AC-24-based capacitor. The freezing strategy developed here provides a novel route for low-cost and eco-friendly production of AC from biomass wastes for high-performance supercapacitors.