Preparation Of Lignin-based Biochar Materials And Its Application In Electrode Materials For Supercapacitors

Lignin is the second largest biomass resource in the plant world,second only to cellulose,which has the characteristics of wide source and low cost.In this paper,different lignin was used as raw materials to prepare lignin-based carbon materials by high temperature carbonization in nitrogen environment.The effects of pretreatment process,activator content,carbonization process and foaming agent on the structure and properties of the prepared lignin-based carbon materials were studied.The lignin and its carbon materials were characterized by relative molecular weight（GPC）,infrared spectroscopy（FT-IR）,thermogravimetric analysis（TG）,X-ray diffraction（XRD）,Raman spectroscopy（SEM）,specific surface area and pore distribution（BET）.The prepared lignin-based carbon materials were prepared into electrode sheets,and the electrochemical properties of the electrode materials were tested in 6M KOH electrolyte by using a three-electrode system.The findings:（1）Alkali lignin（JL）,sodium lignosulfonate（NL）and calcium lignosulfonate（GL）all had major decomposition reactions in the temperature range of 200-450℃,and condensed into amorphous carbon in the temperature range of 450-900℃The final carbon residues of JL,NL and GL were 38.5%,respectively.43.8%and 37.2%.After carbonization,the carbon content of the three lignin materials increased obviously,the loss of sulfur element in NL and GL was about 1%,the relative content of sodium element in NLC decreased from 8.81%to3.74%,while the relative content of Ca element in GLC remained preserved after carbonization increased from 8.78%to 15.01%.Raman spectra showed that the graphitization degree of the three groups from high to low was GLC>NLC>JLC.The specific surface area analysis of the three carbon materials showed that the maximum value of GLC was194 m²/g,and the contribution of GLC micropores was the highest,up to73%.（2）The graphitization degree of carbon materials（MLC）prepared by laccase activated lignin with GL as carbon source increased.The S_BETof MLC increased by 307 m²/g,and the contribution rate of micropores did not change much.The specific capacitance of MLC was significantly increased from 61 F/g（GLC）to 142 F/g.KOH activation of GL can effectively promote the formation of more regular carbonization structure,the degree of graphitization of the prepared carbon materials KGL from the largest to the least:KGL-d>KGL-c>KGL-b>KGL-a.Under the same carbonization process,KGL-b has the highest specific surface area of1771 m²/g,and KGL-b has dense micropores of 1-2 nm observed by electron microscopy.The maximum specific capacitance of KGL-d is 91F/g at the current density of 10 A/g.（3）With GL as the carbon source,NHL2 lignin-based carbon materials prepared by NaHCO₃ foaming had the highest graphitization and the maximum specific surface area of NHL2 was up to 1078.85m²/g.NHL2 observed many mesoporous structures in the carbon sphere under electron microscope.It shows that after lignin and inorganic foaming agent are mixed,the foaming agent in the carbon sphere decomposes to form gas in the sintering process,resulting in the formation of more pores in NHL.Among them,NHL2 specific capacitance is the largest,reaching 125 F/g（10 A/g）.ACL lignin-based carbon materials were prepared using AC foaming agent,and the maximum current density was 83 F/g at 10 A/g.At the current density of10 A/g,the specific capacity of MPL3 was the highest,up to 110 F/g.