Study On Construction Of Lignin Carbon Nanospheres Based On Hydrothermal Covalent Stabilization And Their Capacitance Performance

As one of the vital members of the carbon materials family,carbon nanospheres have attracted much attention in many applications due to their unique characteristics such as high specific surface area,adjustable particle diameter and porosity.However.various un-renewable carbon precursors,including petrochemical products and their derivatives,have been employed in the fabrication of porous carbon nanospheres.With the attention of the environmental pollution caused by the utilization of the petrochemical products,the exploration of bio-based carbon nanospheres has become a focus.Lignin is the renewable and biodegradable biomass material with abundant carbon content and is regarded as an ideal carbon source.However,it is difficult to use due to its complex amorphous structure.It is not only relaxed environmental pollution,but also provide a new pathway to utilize lignin resources in high value to construct lignin carbon nanospheres with controllable structure by regulating the morphology and structure.In this paper,enzymatic hydrolysis lignin which is the most in accord with natural lignin structure was selected as feedstock.Constructing covalent stabilizing lignin nanospheres by taking advantage of the characters of lignin,which including easy to self-assembly,condensation and high carbon content.The molecular mechanism of lignin in the formation process was analyzed,and the microstructure,physicochemical properties,and electrochemical behaviors of various lignin carbon nanospheres were comprehensively analyzed.The main research contents and results are as follows:(1)Based on lignin nanospheres obtained from self-assembly,the hydrothermal treatment method was chosen to accelerate crosslink between lignin molecules via using the natural character of self-condensation in the stimulation of temperature.The covalent stabilization lignin nanospheres with high structural strength,excellent thermal stabilization and solubility resistance were prepared,and its mechanism was revealed.It demonstrates that the chemical bonds or/and functional groups with a lower bond dissociation energy(for example β-O-4 bond,methoxy group,etc.)cleavage to form free-radicals.Simultaneously,these free-radicals generated intramolecular and intermolecular chemical cross-linking via higher bond dissociation energy such as α-5,β-5,β-β.In addition,the diameter and maximum decomposition temperature of lignin nanospheres decreased from 597 to 477nm and increased from 367.7 to 395.1℃,respectively,with hydrothermal temperature increased from room temperature to 190℃.The covalent stabilization lignin nanospheres exhibited a significantly improved solubility resistance in ethanol and tetrahydrofuran.The successful fabrication of covalent stabilization lignin nanospheres further promoted the potential application of lignin nanospheres.(2)Based on covalent stabilization lignin nanospheres,solid lignin carbon nanospheres were prepared by direct carbonization.The effect of various initial lignin content,carbonization temperature on the size and electrochemical performance of carbon nanospheres were explored.The results show that the particle size and specific surface area of solid lignin carbon nanospheres can be regulated in the range of 256416 nm and 652.2-736 m2/g by regulating the above factors.The capacitance of the solid lignin carbon nanosphere derived electrode material reaches 148 F/g at a current density of 0.5 A/g,and the electrochemical performance of the solid lignin carbon nano sphere derived electrode material remains above 85%after 10000 galvanostatic chargedischarge cycles.In addition,by considering the safety performance of electrode materials,lignin carbon nanospheres derived capacitors can maintain relatively high capacitive performance under the condition of temperature change and electrode bending,which provides the possibility to develop high performance,safe and stable capacitors.(3)In order to further improve the application of lignin carbon nanospheres in energy storage,porous lignin carbon nanospheres(ACS)were prepared by a simple carbonization-activation method based on solid lignin carbon nanospheres by introducing chemical activator potassium hydroxide.The specific surface area of lignin carbon nanospheres was 2238 m2/g,which was 3 times higher than that of solid lignin carbon nanospheres,and the specific capacitance of ACS in 6 M KOH electrolyte was 254.2 F/g,nearly 1 time higher than that of solid lignin carbon nanospheres.The specific capacitance of the ACS electrode can still be more than 80%after 10000 cycles using galvanostatic charge-discharge at 5 A/g.In order to further improve the energy density of the ACS electrode,the test results in 1 M Na2SO4 aqueous solution shows that the voltage window can be expanded to 2V,and the maximum energy density is 22.4 Wh/kg,which is about 3.6 times higher than that of 6 M KOH aqueous solution.The development of lignin porous carbon nanospheres provides potential for the application of lignin nanospheres in high-performance electrochemical energy storage.