Direct Microwave Conversion from Lignin to Micro/Meso/Macroporous Carbon for High‐Performance Symmetric Supercapacitors

Abstract

AbstractEnzymatic hydrolysis lignin (EHL) is a common biomass that is difficult to use for the synthesis of porous carbon (PC) via direct microwave heating because it absorbs very little microwave energy. In this study, a novel method was developed to synthesize high‐performance PC directly from EHL using only 8 min of microwave heating. The resulting PC exhibits a high specific surface area of 2482 m2 g−1 and a hierarchical pore distribution with a high mesopore content of 35.6 % and oxygen content of 17.1 %. Nitrogen (as a protecting gas) passes through water molecules provided by steam to absorb microwave energy and acts as a physical porogen to form mesopores. Using solid KOH allows one to bypass the time‐consuming drying process and generate sufficient micropores. Additionally, fast self‐pyrolysis and a high heating rate lead to the generation of both mesopores and macropores in a PC framework. Furthermore, multiple active centers can be formed during microwave heating and subsequently stabilized by oxidation once the PC is exposed to an air atmosphere, resulting in the attachment of oxygen functional groups to the PC surface. Based on these desirable physicochemical characteristics, the PC electrodes fabricated in this study exhibit a high specific capacitance of 338 F g−1 at 1 A g−1 and high capacitance retention of 86 % at 10 A g−1. The final PC‐based supercapacitor delivers a maximum energy density of 17.1 Wh kg−1 in a Na2SO4 electrolyte and 8.6 Wh kg−1 in a KOH electrolyte.