Thermochemical Preparation Of Lignin-based Activated Carbons And Graphene Quantum Dots And Their Applications

Although lignin has a high carbon content and widespread production,its effective utilization rate is low.With its low operating cost and high conversion efficiency,thermochemical conversion is one of the most effective ways to lignin valorization.This thesis explored how to use thermochemical conversion methods to produce lignin-based,high-performance carbon materials,and consequently,studied their specific high value-added applications.This research mainly includes two aspects:1)The carbonization and activation processes were optimized to produce lignin activated carbons（LAC）with superior electrochemical properties,and a LAC-based composite was prepared as an electrode material to significantly improve its specific capacitance and energy density due to the synergistic effect.2)Lignin-based graphene quantum dots（GQDs）were successfully prepared via the hydrothermal method,characterized for their structural morphology,chemical composition,and fluorescence properties.Their applications in fluorescence detection of Fe³⁺ions and ascorbic acid（AA）as well as cell imaging were investigated.The main results of this study are as follows:（1）In a newly designed bubbling fluidized bed pilot plant,lignin carbonization was performed using fast pyrolysis rather than slow pyrolysis,and the chemical activation parameters were adjusted to regulate the structural morphology and physicochemical properties of the resulting LAC,resulting in the optimal process conditions for the preparation of premium LAC electrode materials.The specific surface area and total pore volume of lignin char（LC）obtained from fast pyrolysis at 550°C were 123.9 m² g^–1 and 0.09 cm³ g^–1,respectively,which were 73 and 30 times higher than those of slow pyrolysis.In the subsequent chemical activation process,using fast-pyrolysis biochar be used as a precursor,KOH:LC of 2,activation temperature of 800°C,and activation time of 2 h for the chemical activation process gives a high specific surface area(2149.5 m² g^–1),high specific capacitance(300 F g^–1 at a current density of 0.5 A g^–1 for the three-electrode system),excellent energy density(19.15 W h kg^–1 at a power density of 250 W kg^–1for the two-electrode system),and long/stable cycle life（98.2%capacitance retention after 10,000 charge/discharge cycles）.This work shows that fast-pyrolysis lignin char is an excellent precursor for high-performance activated carbons,and when combined with chemical activation,the resulting LAC electrode material outperforms previously reported biomass-based carbon materials.（2）The"one-pot"method was applied to prepare LAC-based composite materials,and the mass ratios were adjusted to regulate the structural morphology and physicochemical properties of the resulting composite materials,resulting in the optimal process conditions for the preparation of premium LAC-based composite electrode materials.LAC/r GO/PANI_0.03,the composite material prepared by optimizing the mass ratio of LAC,GO and PANI to 8:4:3,has a high specific surface area(2036.5 m² g^-1),superior specific capacitance(668 F g^-1 at a current density of 0.5 A g^-1 for the three-electrode system),superior energy density(51.58 W h kg^-1 at a power density of 345.8 W kg^-1 for the two-electrode system),and fairly long/stable cycle life（94%capacitance retention after 10,000 charge/discharge cycles）.This work shows that the LAC/r GO/PANI ternary composite can overcome the shortcomings of carbon-based electric double layer capacitors and greatly improve the electrochemical properties such as specific capacitance and energy density.（3）A two-step method was used to prepare lignin-based graphene quantum dots（GQDs）,followed by characterizations of their morphological features,optical properties,cytotoxicity as well as an exploration of their application in cell imaging in vitro.The GQDs were quasi-cylindrical with a 0.21 nm lattice spacing,and the fluorescence quantum yield was as high as 28%.They presented excellent fluorescence stability for 12 months at room conditions,or with<3%loss in photoluminescence intensity under 30 W UV irradiation for12 h or within an aqueous solution of p H 3-10 or Na Cl concentration up to 1 mol L^-1.In terms of cytotoxicity,He La cells maintained more than 95%viability after a 48-h incubation period at GQDs concentrations up to 50 mg L^-1.This work shows that lignin GQDs have a high fluorescence quantum yield and excellent optical properties,implying that they can be used to replace conventional semiconductor quantum dots as a novel,sustainable material for bioimaging applications.（4）Based on the fluorescence"off"or"on"response,the sensitivity,selectivity,accuracy,and reproducibility of the GQDs(for Fe³⁺detection)or GQDs/Fe³⁺（for AA detection）fluorescence sensing systems were thoroughly investigated.The detection limits were as low as 1.49μmol L^-1and 1.62μmol L^-1,respectively.Despite the strong interference of 13 metal ions similar to Fe³⁺and eight chemicals like AA,the two fluorescence sensing systems demonstrated good accuracy and reproducibility for Fe³⁺and AA detection,even in environmental water samples or cells in vitro.This work shows that lignin GQDs can function as a sensitive,accurate,and facile fluorescence probe with great potential for Fe³⁺and AA detection as well as cell tracking.In summary,using lignin as the raw feedstock,this thesis prepared high-performance activated carbons and composite materials for supercapacitors,as well as graphene quantum dots for Fe³⁺and AA fluorescence detection and cellular imaging,and systematically investigated their structure-effect relationships and regulatory mechanisms.The results in this thesis will provide data support and theoretical basis for lignin-based,high-value applications.