Microwave-assisted KOH activation from lignin into hierarchically porous carbon with super high specific surface area by utilizing the dual roles of inorganic salts: Microwave absorber and porogen

Abstract

Searching an appropriate biomass that is capable of absorbing microwave energy is significant to realize its rapid microwave-assisted conversion into porous carbons (PCs). Herein, the lignin derived from waste pulping black liquor via crude extraction is directly converted into high-performance PC for a very short duration of 10–30 min by microwave heating. The inherent inorganic salts in crude lignin with a high content of 17.3% is innovatively used as initial microwave absorber to achieve rapid conversion from lignin into bio-char, and further as porogen combined with KOH to synergistically obtain well-developed porous texture. The resultant PC possesses a super high specific surface area of 3065 m 2 g −1 , hierarchical pore size distribution with abundant micropores (0.73 cm 3 g −1 ) and a remarkable meso-/macropores ratio of 64.4%, highly disordered and amorphous structure, as well as oxygen-enriched structure with a surface oxygen content up to 16.5%. These attractive characteristics contribute to excellent electrochemical properties of the resultant supercapacitor electrode which exhibits a high specific capacitance of 325 F g −1 at 0.5 A g −1 in 6 mol L −1 KOH electrolyte, still maintaining a high value of 258 F g −1 (79.4%) at 50 A g −1 . The prepared supercapacitor can remain 81.9% energy density when the power density was increased by 20 folds, suggesting its excellent energy-power synergetic outputting property. These attractive results pave a new way to utilize such a biomass waste, especially with high inorganic salts content, into value-added electrode material for advanced energy storage application. • Conversion from waste lignin into porous carbon only required 10–30 min. • Inorganic salts act as microwave absorber and also porogen. • Super high specific surface areas surpassing 3000 m 2 g −1 were achieved. • A remarkably high rate capability of 79.4% was achieved at 50 A g −1 .