Citric/Acetic Acid-Assisted Hydrothermal Synthesis of Chitosan-Derived Porous Carbon for Supercapacitors

Utilization of abundant renewable resources for preparing green materials for energy storage, such as supercapacitors, is important and challenging. The organic-acid-assisted hydrothermal process has recently emerged to be an effective way to regulate the porous structure of the final activated carbon. In this work, citric/acetic acids are employed as hydrothermal assisting reagents for the carbonization of chitosan prior to KOH activation of the hydrochar, for conducting a comparative study to better understand the effects of these organic acids. The study systematically investigated the impact of the acetic acid/citric acid addition ratio on the products of hydrochar and KOH-activated porous carbon. The results demonstrated that the addition of acetic acid (AC) was more conducive to the development of mesopores, while the addition of citric acid (CA) was more favorable for the development of micropores. The hierarchical distribution of the pore structure resulted from AC hydrothermal exhibiting the optimal electrochemical and cycle life performance at the same concentration. The highest specific surface area (3336 m2/g) was achieved when 0.8 mol/L AC (08ACK3) was added, which was significantly higher compared to those of nonassisted conventional as well as CA hydrothermal. In the three-electrode system, the specific capacitance of 08ACK3 reached 431.5 F/g at 0.5 A/g, which was higher than those of most of the previously reported chitosan-based porous carbon materials. Furthermore, the specific capacity retention was even higher at 10 A/g, which was 96% of the original specific capacitance after 30,000 charge/discharge cycles. Compared to citric acid, which predominantly enhances the total specific surface area by increasing micropores (<2 nm), acetic acid shows a stronger tendency to augment the micro-mesopore (1.1–3 nm) specific surface area, consequently contributing to the overall surface area improvement.