Synthesis of bio-based xerogels from lignin precipitated from the black liquor of the paper industry for supercapacitors electrodes

In this paper, lignin-activated carbon xerogels from the black liquor of the paper industry were synthesized by the sol-gel polycondensation of resorcinol and lignin with formaldehyde, NaOH, and water. The sol-gel was prepared at three Lignin/(Lignin+Resorcinol) mass ratios of 0, 13, and 27%. A portion of sol-gel was functionalized with a no-common crosslinking agent, cyanuric chloride (CC) saturated in 1,4-dioxane, to introduce some nitrogen functional groups on the surface. The sol-gels non- and functionalized were dried and activated with H₃PO₄ at a mass ratio of 1:1 at 450 °C. After the functionalization with the CC and activation process, the superficial nitrogen content percentage increased to 7 wt%, where the most prevalent functionality was N-pyrrole. The sample with a high percentage of nitrogen groups did not show the best electrochemical behavior (112 F g⁻¹ at 0.125 A g⁻¹). However, increasing lignin content up to 27% without functionalization improved the gravimetric capacitance value of 149.5 F ⁻¹. These results are mainly attributed to electrostatic effects and the high presence of phosphorus functionalities introduced after the activation process that enhance the surface wettability of the electrodes. Furthermore, the analysis of the variance confirmed that the CC impregnation process did not favor gravimetric capacitance. Lignin can be considered as an excellent substitute for resorcinol partially to produce bio-based activated carbon xerogels and use them as electrodes for supercapacitors and reduce production costs.