Nanoscale Micellization Of Lignin And Its Electrochemical Properties After Carbonization

As the most abundant bio-mass material containing aromatic structure in nature,lignin was used as low additional value products after being collected from pulping liquor.In recent years,researchers did many studies on separating,depolymerizing,modifying,recombinating or carbonizing lignin,which would explore the application to functional composite materials,platform chemicals,energy development,etc.Controlling the morphology of lignin to prepare lignin micellar microspheres will make lignin acquire bigger specific surface area and surface energy,with better stability in solution.Depending on these properties,lignin nanospheres had the potential to be employed in ceramics,chemical industry,microelectronics,bioengineering,medical science.In this study,lignin was separated from wood powder by organosolv pulping,combined with mechanical treatment to prepare lignin micellar microspheres.Then,lignin was composited with formaldehyde/pyrrole to form N-doped aerogel and carbonized to N-doped carbon aerogel.Finally,N-doped carbon aerogel was used as supercapacitor electrode.The lignin molecular weight,morphology,functional groups,intramolecular linkage,the diameter size of lignin particles,electrochemical properties were characterized and analyzed by gel permeation chromatography,scanning electron microscope,transmission electron microscope,atom force microscope,Fourier transform infrared spectroscopy,¹H–¹³C correlation two-dimensional heteronuclear single quantum coherence（2D HSQC）nuclear magnetic resonance（NMR）spectroscopy,particle-size analyzer,ultraviolet-visible spectroscopy,electrochemical workstation,etc.According the testing and data analysis,the best formulation of lignin micellar microspheres was confirmed,the mechanism of the formation of lignin micellar microspheres and the potential application of lignin N-doped carbon aerogel supercapacitor electrode were tested.The main research work and results were listed as follows.（1）Organosolv treatment were utilized to prepare ethanol lignin,which was compared with acetone lignin and Klason lignin.The results showed that their productivity was 15.78%、4.8%and 20.03%,respectively.Acetone lignin had higher molecular weight and lower productivity,Klason lignin was environmentally unfriendly and sulfuric acid was hard to be recycled.Thus,ethanol lignin was used as the subsequent experimental material because of its productivity and recoverability.（2）The classic cross-signals of methoxyl structure andβ-O-4′linkages of ethanol lignin were showed in 2D HSQC NMR.And small amount signals of typical xylan were also detected.Due to the hydrophobic feature of lignin and hydrophilic feature of carbohydrates,ethanol lignin was a nature polymer with amphiphilic properties,which served as assembly building blocks and formed the foundation of preparation of lignin micellization.（3）Ethanol lignin was later treated by high speed blending and high pressure homogenizing.The diameter size of ethanol lignin particles was distributed unevenly between190⁴60 nm after high-speed blending,which didn’t reflect the effect of physical processing.But the number-average molecular weight（M_n）of ethanol lignin was 2570 after high-speed blending,less than the original ethanol lignin（M_n=3011）.（4）Morphology and particle size of ethanol lignin were controlled by the ratio of ethanol/water and the homogenizing time when processed by high pressure homogenizer.After homogenized 45 min,ethanol lignin formed uniform nanospheres,which average diameter was100²00 nm and the least reached 80 nm.The ethanol lignin spheres became more stable and the diameter was smaller with the added homogenizing time when the ethanol concentration was 50%.At the same time,ethanol lignin couldn’t form intact nanospheres when the ethanol concentration was higher or lower than 50%.High pressure homogenization wouldn’t influence the chemical structure and properties of ethanol.（5）High pressure homogenization distributed the ethanol lignin molecules at first,and hydroxyl groups served as hydrophilic parts,aliphatic segmer and aliphatic skeleton as hydrophobic parts.When the ethanol lignin molecules aggregated again after homogenization,hydrophobic parts of lignin tended to form the cores of micellar microspheres as hydrophilic parts formed the shells.Moreover,the appropriate ethanol concentration would control the activity the lignin molecules under water/ethanol system and ensure the distance and free volume between lignin molecules.（6）N-doped carbon aerogels with steady morphology,three dimensional cross-linked network and numerous pores were composited by ethanol lignin,formaldehyde and pyrrole,of which the nitrogen content was about 8%.The supercapacitor assembled by N-doped carbon aerogels in three electrode system exhibited the specific capacitance of 103.5 F/g when the current density was 0.25 A/g.The supercapacitor modeled itself on electrical double-layer supercapacitor and showed good reversibility,inferred from cyclic voltammetry（CV）,galvanostatic charge–discharge（GCD）curves and electrochemical impedance spectroscopy.（7）N-doped carbon aerogels prepared by ethanol lignin assembled two solid symmetrical supercapacitors and tested in two electrode system.The CV and GCD curves in different scanning potential windows and 1¹000 charge-discharge cycles demonstrated the tailorable working potential windows and the cycling stability.And the retention ratio of specific capacitance was 93.6%after the 1000th cycle.Supercapacitors were applied in series circuit or parallel circuit.The voltage was double when supercapacitors were in series,and the current was double when supercapacitors were in parallel.It proved that the supercapacitors followed the basic rules in series circuit or parallel circuit.