Functional-Group Modification of Kraft Lignin for Enhanced Supercapacitors

Recent efforts to improve the capacitances of Kraft lignin (KL) in supercapacitors have mainly focused on screening KL substrates, which could either compensate the poor conductivity of KL or directly contribute to the capacitance. However, increasing the pseudocapacitance contributed by KL itself, through hydroquinone/quinone redox cycles, remains a challenge, owing to the roughly fixed content of hydroquinone species in natural KL. In this study, the capacitance of KL is greatly improved by using a functional-group modification strategy in which methoxy groups in KL are selectively converted into phenolic hydroxy groups, which facilitate the formation of additional hydroquinone moieties and thus lead to higher pseudocapacitances. The oxidized KL materials show up to 25.6 % enhancement of the phenolic hydroxy content in comparison to raw KL, which results in 21.9 % capacitance improvement from 322 to 390 F g^-1 at 0.5 A g^-1 in an acidic system.