Preparation Of Lignin Porous Carbon Materials Via A Gas-Exfoliation And Template Method And Their Structural Regulation And Capacitive Performances

Lignin is the most abundant renewable aromatic macromolecule in plants and a high-quality carbon material precursor with a high carbon content of up to 60%.A large amount of industrial lignin can be produced from the papermaking and biorefinery industries,and lignin valorization is in line with the concept of sustainable development and the strategic goal of"carbon peak and neutrality".Supercapacitors,as one of the new energy storage development plans of the"14th Five-Year Plan",show good market prospects but the high cost and low capacity of carbon electrode materials limit their large-scale applications.Therefore,preparing high-performance carbon electrode materials from industrial lignin is of great significance to the development of energy saving and emission reduction and capacitive energy storage technology.However,the disordered aggregation of lignin structure and poor compatibility with activators lead to disordered structure and fewer mesopores in lignin porous carbon,resulting in low specific capacitance and poor rate performance.At the same time,the traditional chemical activation process is highly corrosive and polluting,which further limits the application of lignin porous carbon in supercapacitor electrodes.To solve these problems,this dissertation aims to construct high-performance lignin carbon electrode materials.Based on the disordered aggregation and pyrolysis mechanism of lignin,innovative strategies such as green gas-exfoliation and template activation technology,solvent-induced driven self-assembly,compositing polyaniline,and high-efficiency nitrogen fixation with zinc cyanamide intermediates were adopted to regulate the ordered microstructure and energy storage active sites of lignin porous carbon.The microstructural regulation and capacitance strengthening mechanism of lignin porous carbon were revealed,and the specific capacitance,rate characteristics,and cycle stability of lignin porous carbon were significantly improved.The main contents and conclusions of the dissertation are as follows:（1）To address the issues of the strong corrosiveness of the traditional chemical activation and lack of mesopores in lignin porous carbon,three activators（Zn Cl₂,Zn CO₃,and Zn C₂O₄）were utilized to activate sodium lignosulfonate to prepare porous carbons with different ratio of mesopores.The results of TG-MS and TG-IR confirm that different from the chemical etching type activator Zn Cl₂,Zn CO₃ and Zn C₂O₄ have gas-exfoliation and template functions,and their pyrolysis mechanism is the key to determining the pore size distribution of lignin carbon.The lower decomposition temperature of Zn CO₃ leads to weak gas-exfoliation and significant template effects.The porous carbon activated by Zn CO₃（LSC-Zn CO₃）showed a honeycomb structure with a mesoporosity as high as 85.1%.The decomposition temperature of Zn C₂O₄ is higher than that of sodium lignosulfonate,which leads to synergistic thermochemistry and promotes the gas-exfoliation process.The porous carbon activated by Zn C₂O₄（LSC-Zn C₂O₄）had a microporous-mesoporous structure with a mesoporosity of 67.9%and was rich in wrinkles,enhancing the mass transfer kinetic.When applied as the supercapacitor electrode,LSC-Zn C₂O₄ had a specific capacitance of 306 F/g at 0.5 A/g,superior to LSC-Zn CO₃（220 F/g）,and the honeycomb microporous carbon activated by Zn Cl₂（165 F/g）.（2）To solve the problem of disordered structure caused by severe lignin aggregation and poor compatibility between lignin and activators,a solvent-induced cation-π-driven self-assembly technique was proposed to fabricate an order sodium lignosulfonate/zinc oxalate complex precursor in ethanol/water solvents.Following carbonization at a high temperature and acid treatment,lignin porous carbons with continuous nanosheet conductive units and a reasonable hierarchical micropore-mesoporous structure was constructed.The carbonation parameters were optimized,and the specific capacitance of LSC-650-2 at 0.5 A/g was up to 365 F/g in 6 M KOH when the carbonation temperature was 650℃and the dosage ratio of zinc oxalate to sodium lignosulfonate was 2,higher than the commercial activated carbon YP-50F prepared through steam activation（270 F/g）.LSC-650-2 showed a specific capacitance of 260 F/g at 20.0 A/g with 71.2%retention,demonstrating good rate capability.Furthermore,the LSC-650-2-based solid-state symmetric supercapacitor delivered a high energy density of 13.7 Wh/kg.The analysis of microstructure and energy storage kinetics shows that the excellent electrochemical performance of LSC-650-2 is attributed to the integrated structure of conducting sheet,micropore for adsorption,and mesopore for mass transfer,which accelerates the mobility of electrons and ions and improves the utilization of capacitive active sites.（3）To improve the specific capacitance of lignin porous carbon,lignin porous carbons with different geometries were used as the hosts to anchor polyaniline for carbon/polyaniline composites via in-situ oxidative polymerization.It was determined the disordered honeycomb carbon host inhibits the penetration of the aniline monomer,resulting in the formation of PANI aggregates.However,the lamellar porous carbon host can provide a large accessible surface area and promote penetration and heterogeneous nucleation of aniline.Benefiting from the uniform network of the interpenetrating polyaniline nanofibers and the mesopore-strengthened interfacial anchoring effect,the composite prepared with the lamellar hierarchical porous carbon effectively alleviated the volume expansion of polyaniline during charging and discharging and greatly activated the pseudocapacitive activity of polyaniline.Consequently,it possessed a high capacitance of up to 643 F/g at 1.0 A/g and a sufficient capacitance of 390 F/g at 30.0 A/g.The asymmetric supercapacitor assembled with this composite electrode had an energy density of 36.3 Wh/kg.Moreover,it showed excellent cycling stability with high capacitance retention（88.0%）after 5.0 A/g of 5000 cycles,which was superior to the device assembled with polyaniline electrodes（34.5%）.（4）To address nitrogen loss of nitrogen-doped lignin porous carbons due to the excessive use of strong corrosive reagents in the traditional preparation process,nitrogen-enriched lignin carbon was prepared by carbonizing a mixture of sodium lignosulfonate/zinc oxalate complex and melamine at high temperature,where the zinc cyanamide intermediate formed from zinc oxalate and melamine effectively inhibits nitrogen loss.Zn C₂O₄ activation achieved a high nitrogen conversion rate of 13.5%at 800°C,much higher than that of Zn Cl₂ activation（1.9%）.The as-prepared N-LSC-800 had an extremely high nitrogen doping of 14.47 at%and a high bulk density of 1.26 g/cm³.Consequently,N-LSC-800 demonstrated a high gravimetric capacitance of 300 F/g and superior volumetric capacitance of 378 F/cm³ at 0.2 A/g,higher than the doped carbon by Zn Cl₂ activation（N-LSC-800-Zn Cl₂,252 F/g,and 267 F/cm³）.The assembled N-LSC-800-based coin-type symmetric supercapacitor had a high volumetric energy density of 5.1 Wh/L,and the specific capacitance retention rate of the device was 96.7%over 20000 charge/discharge cycles at 2.0 A/g.Energy storage kinetics analysis indicated that the excellent electrochemical performance of N-LSC-800 was attributed to the enhanced pseudocapacitive effect induced by the high nitrogen content.（5）Nitrogen-doped lignin porous carbons have drawbacks of disordered structures and low specific surface area due to the excessive filling of nitrogen species.To address these issues,cubic lignin/F127/zinc oxalate precursor was prepared by a solvent evaporation-induced hydrogen bond-driven self-assembly using alkali lignin as the carbon source,F127 as a structure-directing agent,and Zn C₂O₄ as a confined template,where the alkali lignin/F127micelles were confined to the Zn C₂O₄interlayer and surface.The lignin/F127/zinc oxalate precursor was then carbonized with melamine at a high temperature for nitrogen-rich accordion-like lignin porous carbon via the confinement of Zn C₂O₄ and the mild activation of zinc cyanamide.The as-prepared N-HPLC-700 retained a high specific surface area and exhibited a stable framework with interconnected sheets,hierarchical porous structure,and the ultrahigh edge-nitrogen doping level of up to 12.20 at%,which boosted electrical double-layer capacitance and pseudocapacitance.Consequently,N-HPLC-700 showed a high gravimetric capacitance of 354 F/g and an extraordinary surface area-normalized capacitance of 82.1μF/cm² at 0.2 A/g.Moreover,the fabricated N-HPLC-700-based coin-type symmetric supercapacitor displayed a high energy density of 12.9 Wh/kg at 161.9 W/kg and superior cycling stability with a 99.5%capacitance retention after 16000 cycles at 2.0 A/g.