Efficient fabrication of ordered mesoporous carbon derived from lignin via deep eutectic solvent pretreatment for supercapacitors

Abstract

Lignin can be used as a green and sustainable substitute for phenol in the preparation of ordered mesoporous carbon using phenol resin prepolymer as precursor owing to the availability of aromatic ortho -positions in the lignin structure. To further improve the reactivity towards formaldehyde and increase the substitution degree of phenol, masson pine alkali lignin (MAL) was pretreated by an acidic deep eutectic solvent (DES). It was found that adding an appropriate amount of water to DES showed beneficial attributions to the reduction in pretreatment time and the upsurge in the phenolic hydroxyl content of regenerated lignin (DAL). The content of β - O -4 ether bond in MAL was almost cleaved. More importantly, the contents of syringyl and guaiacyl units were decreased. In contrast, the content of p -hydroxyphenyl (H) units was increased obviously, thereby facilitating lignin to react with formaldehyde. Results showed that the optimized DAL with higher content of H units was an appropriate candidate for substituting 100% of phenol in the preparation of highly-ordered lignin-derived mesoporous carbon (LDMC). Finally, the optimized LDMC exhibited high specific capacitance of 197.32 F g -1 at current density of 0.2 A g -1 , high capacitance retention of 84% at current density of 20 A g -1 , superior cyclic stability with capacitance retention rate of 99.09% after 10000 cycles and high energy density of 6.85 Wh∙kg −1 at power density of 100 W kg −1 . • Water addition improves the efficiency of acidic DES pretreatment on alkali lignin. • The reactivity of regenerated lignin towards formaldehyde was improved significantly. • DES-pretreated alkali lignin replaced 100% of phenol to synthesize OMC successfully. • Supercapacitor with OMC as electrode exhibited superior electrochemical performance.