| This thesis aims to explore new methods of high-value utilization of lignin and divides the main research content into three parts.The first part of the research is the preparation of lignin-based carbon fiber?LCF?,which provides a good substrate for the preparation of lignin-based supercapacitors.First,the commercial alkali lignin?AL?was used as the raw material to prepare the lignin fiber precursor?LF?by electrospinning,and the best process parameters?spinning solution concentration,quality ratio of lignin to polyvinyl alcohol solution?PVAaq??were explored.Then,three types of lignin-based carbon fibers?LCF-800,LCF-900 and LCF-1000?were prepared by pre-oxidation and carbonization at different temperatures?800°C,900°C and 1000°C?with LF prepared under optimal conditions.Through a scanning electron microscope?SEM?,X-ray diffraction?XRD?,raman spectroscopy?Raman?and X-ray photoelectron spectroscopy?XPS?characterization methods to explore the effect of carbonization temperature on LCF morphology and structure,clarify the influence mechanism of carbonization temperature.The research content of the second part is to prepare a lignin-based supercapacitor with pseudocapacitive properties.Using LCF-800,LCF-900 and LCF-1000 as raw materials,?-Mn O2/lignin-based carbon fiber electrode material??-Mn O2-LCF?was prepared by solution reaction method.Using XRD,specific surface area?BET?and XPS methods to characterize the crystal form,crystallinity,pore size distribution and valence state distribution of Mn O2obtained under different reaction time,and analyze the influence mechanism of?-Mn O2-LCF electrochemical performance.The research content of the third part is to prepare?-Mn O2/lignin-based carbon fiber composite material??-Mn O2-LCF?,to further improve the specific capacitance of lignin-based supercapacitors.Using LCF-800,LCF-900,and LCF-1000 as working electrodes,?-Mn O2-LCF was prepared by the electrodeposition method.The crystallinity of?-Mn O2 and the valence distribution of surface manganese elements obtained under different deposition times were explored to analyze the influence mechanism of Mn O2-LCF electrochemical performance.The specific research results are as follows:?1?The optimal spinning process conditions for lignin are 40%spinning solution concentration,7:3 mass ratio of lignin to PVAaq.The LF obtained by electrospinning under this preparation condition is more uniform and smooth than other preparation conditions.When investigating the effect of carbonization temperature on the LF morphology through SEM,it was found that the diameter of LF decreases with the increase of carbonization temperature,and the higher the temperature,the greater the degree of reduction.In addition,XRD and raman results show that as the carbonization temperature increases,the degree of graphitization of LCF increases,and the structural defects decrease.This shows that the carbonization temperature is an important factor affecting the preparation of LCF.To explore the structural changes of lignin during carbonization,this paper studied the bond state composition of carbon elements on the surface of LCF through XPS.The analysis found that with the increase of carbonization temperature,the content of R-OH/C-O-C and C-O in lignin molecules decrease,and the content of C-C/C=C increases.This means that the process of lignin carbonization is to form the precursor of graphite crystals?that is,polybenzene fused ring structure?by removing the branches of lignin molecules?aliphatic hydroxyl and methoxy groups?and the hydrogen element of the benzene ring structure.?2?Study the structure and electrochemical properties of Mn O2-LCF composites prepared by solution method.XRD results show that the composite materials?Mn O2-LCF-800-72 h,Mn O2-LCF-900-72 h,Mn O2-LCF-1000-72 h?obtained by reacting LCF-800,LCF-900 and LCF-1000 with potassium permanganate solution for 72 h is?-Mn O2 crystal,while the composite material obtained by reacting for 24 h and 48 h The surface-loaded substance is Mn3O4.BET analyzed the specific surface area and pore size distribution of Mn O2-LCF-800-72 h,Mn O2-LCF-900-72 h,Mn O2-LCF-1000-72 h,showing that the sample Mn O2-LCF-800-72 h has the most mesopores structure,and the sample Mn O2-LCF-1000-72 h has the largest number of micropores.It indicated that Mn O2-LCF-800-72 h can provide a larger diffusion channel for the insertion and extraction of electrolyte cations as an electrode material.Further analysis of the valence distribution of manganese on the surface of Mn O2-LCF-800-72 h,Mn O2-LCF-900-72 h and Mn O2-LCF-1000-72 h by XPS,revealed that the manganese elements on the surface of the three samples all contained Mn3+and occupy the vacancies in the crystal structure,thus hindering the insertion and extraction of electrolyte cation.Therefore,Mn O2-LCF-800-72 h can provide a larger number of ion diffusion channels as an electrode material.The electrochemical performance was tested under the three-electrode system.The results showed that the electrochemical performance of Mn O2-LCF-800-72 h is the most excellent.Details as follows:working voltage 0-0.9 V,at a current density of 0.3 A?g-1,the specific capacitance is 131.28 F?g-1,the specific energy is 14.77 Wh?kg-1,the specific power is 135.01 W?kg-1.?3?Study the structure and electrochemical performance of?-Mn O2-LCF.XRD research results show that the Mn O2 crystal types deposited on the LCF-800,LCF-900 and LCF-1000 surfaces by the electrodeposition method are all?-type,indicating that the electrodeposition time and the degree of graphitization of lignin can not affect the Mn O2 crystal type.However,SEM analysis found that for LCF-1000,the shape of?-Mn O2 gradually changed from spherical to thin film as the deposition time increased,while for LCF-800 and LCF-900,the deposition time did not affect the shape of?-Mn O2.All are spherical?-Mn O2 formed by interspersing thin slices.This result indicates that the electrodeposition time and the degree of graphitization of lignin will affect the morphology of?-Mn O2.Using XPS to further analyze the influence mechanism of the electrochemical performance of?-Mn O2,the results show that the manganese element on the surface of the composite material contains two content and the most Mn4+content.In the electrochemical performance test,the?-Mn O2-LCF-900-20 min performs bestly.The specific capacitance of the material is still as high as 255.74 F?g-1 at a current density of 0.5 A?g-1,which is higher than that of the second part of the research content.The specific capacitance of the Mn O2-LCF-800-72 h sample is increased by 95%. |
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