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

Abstract

Enzymatic 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.

Electrochemical Zinc Ion Capacitors Enhanced by Redox Reactions of Porous Carbon Cathodes

“…Obviously, partial hydroxyl groups (COH located at 533.4 eV) [ 14,34 ] of PC 800 electrode take part in oxidizing reaction into carbonyl groups (CO) during the charge process and are reduced back to hydroxyl groups (COH) during the discharge process (Equations (1) and (2)). [ 35 ] On the other hand, SEM images suggest that some flocculent particles are generated on the cathode after PC800 is discharged from 0.76 to 0 V, and then disappear during the charge process from 0 to 1.9 V (Figure 3b). XRD analysis demonstrates that the flocculent particles are alkaline salts of Zn 4 ClO 4 (OH) 7 and Zn(OH) 2 (Figure 3d).…”

Lignin Derived Porous Carbons: Synthesis Methods and Supercapacitor Applications

“…KOH activation is also the most popular way for synthesizing LDPCs. [84][85][86]88,[125][126][127][128][129][130][131][132][133][134][135][136][137][138][139][140][141][142][143] KOH-activation method is essentially a carbonization-activation method. A carbonization process is used to stabilize the organic precursor and achieve a better activation effect with KOH activation because the carbon matrix is highly reactive with KOH.…”

Lignocellulosic Biomass-Derived Carbon Electrodes for Flexible Supercapacitors: An Overview

“…For instance, peanut shell activated by bimetallic activation with CO 2 at 800 °C yielded biochar with SSA of 1428 m 2 /g, total pore volume of 0.55 cm 3 /g, and a large proportion of micropores (73.91%) [ 102 ]. Lignin activated by KOH/H 2 O with microwave heating showed high SSA (2482 m 2 /g) and total pore volume (1.45 cm 3 /g) [ 103 ]. Microwave-assisted activation transforms the microwave energy with dipole rotation and ionic conduction within the particles [ 104 ] to yield biomass-based AC.…”