Preparation Of Porous Carbon From Fractional Components Of Shengli Lignite For Supercapacitors

Lignite is rich in heteroatoms and has high ash yield.The way of traditional chemical utilization for lignite will cause serious pollution to the environment.Therefore,efficient conversion organic matters in lignite into functional carbon materials are a clean and efficient utilization method.In this thesis,the soluble organic matters（SOMs）in Shengli lignite（SL）were successfully separated by combining with supercritical alcoholysis and ultrasonic extraction using acetone/CS₂,and extraction with n-heptane was used to further separate the SOMs into light components（LC）and heavy components（HC）.Meanwhile,insoluble extraction residue（ER）was obtained.Gas chromatograph/mass spectrometer（GC/MS）and quadrupole/Orbitrap MS（Orbitrap MS）analyses show that LC containsmore small molecular aromatic compounds and HC has a higher average molecular weight and more aromatic compounds with higher condensation degree.ER retains the three-dimensional（3D）macromolecular structure of SL.Porous carbon was prepared by different processes using the fractional components of SL for supercapacitors,and nitrogen doping was used to improve the wettability of the material,further improving the electrochemical properties of the porous carbon.Hierarchically porous carbon was prepared by adjusting the mass ratio of carbon precursors using HC and LC as the carbon precursors.The results show that LC tended to form an ordered graphite structure,and HC could avoid the transition polymerization of LC while forming a suitable hierarchical porous structure.The prepared porous carbon（denoted as HL₄-700-2）has the optimal electrochemical performance when the LC:HC mass ratio is 4:1,the activation temperature is 700℃,and the template ratio is 2:1.HL₄-700-2 has the highest micropore ratio（81%）and short-range ordered graphite structure.The specific capacitance value is 320 F g^-1at the current density of 0.5 A g^-1in the three-electrode system.The capacitance retention rate is 95.7%after 10000 cycles in the two-electrode system,showing high cycle stability.Highly stable porous carbon was prepared by a two-step pre-carbonization-activation using the ashless extraction residue（AER）of lignite as the carbon precursor.The results show that pre-carbonization could effectively improve the condensation degree of AER and increase the stability of the porous carbon structure.The prepared porous carbon(denoted as AER₅₀₀-700-3)has the optimal electrochemical performance when the pre-carbonization temperature is 500℃,the mass ratio of KOH/AER₅₀₀is 3:1,and the activation temperature is 700℃.AER₅₀₀-700-3 has the highest specific surface area(2779.58 m²g^-1),the highest micropore volume(1.30 cm³g^-1),and a lower degree of graphitization.The specific capacitance value is 300 F g^-1at the current density of 0.5 A g^-1in the three-electrode system.The assembled symmetrical supercapacitor(AER₅₀₀-7003//AER₅₀₀-700-3)show a good energy density of 8.055 Wh kg^-1(power density is 250 W kg^-1),and the capacitance was almost unchanged after 10000 cycles of charging and discharging,showing excellent electrochemical stability.The electrochemical performance of the porous carbon was improved by doping nitrogen based on the above preparation process of porous carbon.Nitrogen-doped porous carbon was successfully prepared through heating impregnation method using AER₅₀₀as the carbon precursor,nano-ZnO as the templating agent,KOH as the activator,and dicyandiamide or melamine or urea as nitrogen source.The results show that nitrogen doping could effectively improve the wettability and electrochemical performance of porous carbon.The prepared nitrogen-doped porous carbon(denoted as D₁AER₅₀₀-700-2)has the optimal electrochemical performance when dicyandiamide is used as nitrogen source,the mass ratio of nitrogen source to carbon source is 1:1,the activation time is 2 h,and the activation temperature is700℃.The microscopic morphology of D₁AER₅₀₀-700-2 show a 3D porous layered stacking structure,with the largest surface area(2648.18 m²g^-1),the highest N content（3.42%,mainly pyrrolic-N and pyridinic-N）and higher wettability than AER₅₀₀-700-3.The specific capacitance value is 336 F g^-1at the current density of 0.5A g^-1in the three-electrode system.The capacitance retention rate of assembled symmetrical supercapacitor(D₁AER₅₀₀-700-2//D₁AER₅₀₀-700-2)is 92.6%after 10000cycles of charging and discharging.And the symmetrical supercapacitor has certain potential in practical applications.The thesis has 54 figures,13 tables and 146 references.