Kraft lignin as a raw material of activated carbon for supercapacitor electrodes

Kraft lignin is obtained from Kraft pulp, and it is mainly used as fuel for heat recovery, resulting in low-value utilization. In this work, a series of activated carbons were prepared from Kraft lignin and analyzed as electrodes of supercapacitors. Chemical activation was carried out with three activating agents (H₃PO₄, K₂CO_3, and KOH) at two impregnation ratios of activating agent to lignin of 1:1 and 2:1 and fixed temperature. The activated carbons showed a specific surface area ranging between 458 and 1515 m² g⁻¹. The electrochemical characterization of the supercapacitors was carried out in an aqueous medium using a two-electrode cell in a symmetrical configuration. The specific capacitance varied between 76 and 236 F g⁻¹, and interfacial capacitance was between 11.1 and 37.9 µF cm⁻² at 0.156 A g⁻¹. The highest value of specific capacitance and interfacial capacitance correspond to samples prepared with KOH and K₂CO₃, respectively, with capacitance retention percentages of 83% and 43%, respectively. Furthermore, the ANOVA results show that only the activating agent factor had a statistical effect on the specific surface area, micropores–mesopores volume ratio, and the specific capacity. These results suggested that KOH is the most appropriate activated agent for lignin due to the high gravimetric and interfacial capacitance values related to the highest specific surface area and porous diameter close to 1 nm. In addition, these samples showed a maximal energy density of 18.47 Wh kg⁻¹ and power density of 49.48 W kg⁻¹, and excellent cyclability performance after 2000 cycles.