Preparation And Electrochemical Properties Of Lignin-Based Porous Carbon Materials

Porous carbon materials are widely used in supercapacitors due to their advantages of simple preparation,low cost,and abundant resources.The traditional carbon precursors mostly use coal,asphalt,coal tar and other non-renewable raw materials,which are restricted by the exhaustion of petrochemical resources.Lignin has a natural three-dimensional aromatic hydrocarbon structure,and the porous carbon material synthesized by lignin has the advantage of not forming porous morphology collapse at high temperature,which is very suitable as the raw material of carbon electrode material for supercapacitor.Lignin is abundant in nature,according to statistics,the lignin contributed by plant growth in the world has 0.5-360 million tons every year,however,about 95% of industrial lignin is directly burned as a low-value fuel,and only 2% of lignin is used as a substitute material for chemicals or materials,resulting in a serious lack of utilization of lignin.In this paper,an electrode material with high specific surface area,excellent stability and good electrochemical performance was prepared by using sodium lignosulfonate,a by-product of papermaking pulp,as raw material,doping it with heteroatoms,and adjusting its surface.The research contents are as follows:（1）Preparation and electrochemical properties of N/S co-doped porous carbon materials.N/S co-doped porous carbon materials were synthesis via simple carbonization activation after carbon precursors were prepared by free-drying using chitosan and sodium lignosulfonate as nitrogen source and sulfur source,respectively.The effect of the synthesis temperature and the mass ratio of raw materials on the structure of carbon materials and their electrochemical properties were explored.The results show that when the temperature is too high,the pore structure will collapse,the specific surface area will be reduced,and the oxygen-containing functional groups will be decomposed into CO and CO2 at high temperature,which will reduce the stability of the electrode material.When the carbonization temperature is 700 oC and the mass percentage of chitosan and sodium lignosulfonate is 30:70,after KOH activation,a layered porous carbon structure with a size of tens of microns to tens of microns stacked on each other is obtained.The specific surface area can reach 1104 m2 g-1.In the threeelectrode system,CNS30:70 exhibited a specific capacitance of 320 F g-1 at 0.5 A g-1,and the capacitance retention rate was 98.94% after 10,000 cycles,showing excellent electrochemical performance and cycle stability.（2）Preparation and electrochemical properties of N/S/P co-doped porous carbon materials.Porous carbon materials were prepared with chitosan,sodium lignosulfonate,and sodium dihydrogen phosphate as nitrogen source,sulfur source and phosphorus source,respectively.The effects of sodium dihydrogen phosphate content on the pore structure,heteroatom composition and graphitization degree of porous carbon materials were investigated.The N content（2.28%）,S content（3.01%）,P content（0.46%）,O content（17.99%）,and high specific surface area（1974 m2 g-1）were achieved on CNSP0.8.Meanwhile,CNSP0.8 shows large specific capacitance（528 F g-1 at 0.5 A g-1）in three-electrode system with 1 M H2SO4 electrolyte.High energy density（7.5 Wh kg-1,power density of 27.86 W kg-1）and good cycling stability（89.30%,10000 cycles at a current density of 10 A g-1）were obtained in a two-electrode system with the assembled symmetric supercapacitor.（3）Preparation and electrochemical properties of carbon/graphene oxide composites materials.Abandoning the potassium hydroxide activation method to prepare high-performance carbon materials,the use of hydrothermal reaction to composite the unactivated carbon materials with graphene oxide can effectively inhibit the accumulation of graphene oxide and form honeycomb-like three-dimensional hierarchical pores structure.This unique porous structure facilitates the rapid diffusion of ions to its inner surface,enhancing the electrochemical performance.The specific surface area is also effectively increased,so the active sites are also enhanced,which has a certain promotion effect on the improvement of electrochemical performance.In a three-electrode system,C/GO1:1 shows high mass specific capacitance of 418 F g-1 at0.5 A g-1.The retention rate of capacitance is about 66.51%.