Hierarchical porous sulfur self-doped lignin carbon derived from full component utilization of black liquor for high-performance supercapacitors.

Black liquor, primarily consisting of lignin, polysaccharides, and inorganic substances, is a potential precursor of porous carbon materials for high-performance supercapacitors. However, the laborious purification of black liquor lignin and the introduction of exogenous heteroatoms have hindered their practical applications. Herein, the full components of black liquor were utilized to synthesize hierarchical porous sulfur self-doped lignin carbons (S-LCs) through a self-activation process aimed at improving the performance of supercapacitors. Benefiting from the intensified reactivity and crosslinking degree of the polysaccharide component and the sulfur self-doping and self-activation effect of inorganic substances, the resulting S-LCs exhibit a high specific surface area (SSA), abundant porous structure, and enhanced defect activity, all contributing toward increasing the energy storage capacity of supercapacitors. The as-obtained S-LC-G250/700 features a high SSA of 892.94 m² g^-1 and a sulfur content of 3.3 at.%. The S-LC-G250/700 demonstrates excellent specific capacitance (e.g., 405.06 F g^-1 at 0.5 A g^-1), remarkable stability (103 % capacity retention after 10,000 cycles), and high energy density of 30.4 Wh kg^-1. Density functional theory calculations verified the advantages of the high-content sulfur self-doping of black liquor, suggesting that self-doped sulfur contributes to charge adsorption on porous carbon surfaces and promotes electron transfer in the electrolyte.