Toward high-performance supercapacitors: Electrodes from solvent-fractionated lignin with enhanced activity

The presence of lignin-rich oxygen-containing groups significantly increases the potential of supercapacitors. However, the heterogeneity of lignin limits their use in industry. In this study, eucalyptus wood was used as the raw material to obtain highly reactive fractionated lignin through solvent extraction and fractionation processes. After fractionation, the weight-average molecular weight decreased from 27,760 to 19,665, and the polydispersity decreased from 2.70 to 2.11. The fractionated lignin was subsequently used to partially replace phenol, synthesizing a series of lignin-based phenolic resin-derived carbon materials, which were then applied in supercapacitors. Electrochemical tests indicated that the material with a 40% substitution ratio exhibited the optimal performance. Accordingly, all subsequent investigations were focused on this optimal ratio. After carbonization, ALPFC (the 40% acetone-fractionated lignin-based phenolic resin carbon material) had a high specific surface area of 3210 m²·g^-1, outperforming carbon materials derived from phenolic resin (2142 m²·g^-1). Electrochemical tests reveal that ALPFC has a specific capacitance of 335.5 F/g at a current density of 0.5 A/g, which is significantly greater than that of phenol-formaldehyde resin (196.2 F/g). Moreover, ALPFC has excellent double-layer capacitance characteristics and outstanding cycling stability, with a capacitance retention rate of 110.88% after 10,000 cycles at 10 A/g. In a dual-electrode system, there is a minimal decrease in the specific capacitance, with a capacitance retention rate of 98.15%. These findings offer an effective pathway for the green and low-carbon preparation of supercapacitor electrode materials, while also verifying and establishing a feasible approach to efficiently utilize lignin without compromising the key properties of the resin.