Dissolution of alkali lignin in ionic liquid to fabricate nitrogen-doped carbon microspheres for electrochemical supercapacitors

Ionic liquids (ILs) have been extensively sought after for green solvents, catalysts, adsorbents, and carbon precursors. Herein, 1-ethyl-3-methylimidazolium acetate ([Emim][Ac])–lignin nitrogen-doped carbon microspheres (ECLMs) were easily synthesized through pre-oxidation and carbonization processes using a solution containing [Emim][Ac] and alkali lignin. The pre-oxidation process is analogous to the ionothermal carbonization method, wherein [Emim][Ac] serves as a reaction medium for preparing the ELCMs precursor. During the carbonization process, [Emim][Ac] served as a templating agent and heteroatom source to facilitate nitrogen doping and formation of micropores and mesopores. The as-synthesized ELCMs exhibits a considerably high specific surface area of 1267.2 m2/g and an optimal nitrogen-doping level of 2.21 %. This reveals its potential as a promising candidate for electrode materials in supercapacitor. The ELCMs demonstrate a remarkable specific capacitance of 262.5 F/g at 0.5 A/g and excellent rate capability in a three-electrode system. Additionally, the symmetric supercapacitor fabricated with electrode materials of ELCMs and a cellulose separator exhibits excellent cycling stability, with an impressive capacitance retention rate of 91.0 % after undergoing 5000 cycles at 5 A/g. Moreover, it achieves an ideal energy density of 4.17 Wh/kg at power density of 126 W/kg. This study investigates an effective strategy for designing and synthesizing different heteroatom-doped carbon materials from IL-biomass resources to enhance their applicability in energy storage devices.