Preparation And Electrochemical Performance Of Porous Carbon Electrode Materials From Biomass With Different Structures

To cope with the huge demand for energy storage in portable electronic devices and electric vehicles,etc,it is crucial to design and develop efficient energy storage and conversion devices.High-performance electrode materials have a very important position in the process of energy storage and conversion,among which carbon-based materials,especially biomass-based carbon materials,have received much attention because of their good electrochemical performance,low cost and wide sources.Biomass in nature has diverse,ordered and complex morphological structures,which endow biomass carbon based materials with different force,thermal and electrical properties,however,the special internal structure of biomass and its application in energy storage still lacks reasonable attention.In this thesis,a variety of different types of biomass carbon electrode materials were prepared by high-temperature carbonization method using natural one-dimensional flat fibers,three-dimensional spiral fibers and complex mesh porous structures possessed in different biomass bodies,and microscopic modulation of the surface interface porous structure was carried out by KOH activation technique,and the electrochemical energy storage mechanisms of biomass-based porous carbon electrode materials in different dimensions and the key factors affecting the electrochemical performance were discussed.The main results of the study are as follows:（1）A bamboo-based porous carbon fiber electrode material BLCF was prepared by carbonization activation method using linear flat fibers in the body of bamboo leaves.This bamboo-based carbon fiber is mainly composed of carbon and oxygen elements,has a one-dimensional flat linear structure,and the single fiber diameter is about 5-8μm.The amount of KOH has a great influence on the formation of porous microstructure,and the surface of sample BLCF 1-5 has abundant oxygen-containing functional groups at the alkali-carbon ratio of 5.The specific surface area and oxygen-containing functional groups together improve the chemisorption of electrolyte ions on the surface of the electrode material,so that the specific capacitance of BLCF 1-5 can reach 154 F/g at a current density of 0.5 A/g.（2）Three biomass-based porous spiral carbon fibers:ZSCF,XSCF and HSCF were successfully prepared using spiral duct fibers from the bodies of Zizyphus,Western ivy and heather.These three biomass-based porous spiral carbon fibers have similar regular spiral structures with a spiral pore diameter of about 5μm and a fiber diameter of about 0.5μm.The oxygen-containing functional groups on the fiber surface are stable at the same activation ratio,and their specific surface areas after The specific surface areas after activation were 726.4 m²/g,629.8 m²/g and381.4 m²/g,and their electrochemical properties were compared and analyzed:the specific capacitances of ZSCF,XSCF and HSCF were as high as 188.2 F/g,146 F/g and 124.1 F/g,respectively,at a current density of 0.5 A/g.The results showed that the three electrode materials The advantages and disadvantages of energy storage performance are not only positively related to the size of specific surface area,but also may be related to the distribution condition of the helical conduit affecting water transport in the blade.（3）Water hyacinth stem pith was used as the carbon precursor,and water hyacinth-based porous carbon materials were obtained by different carbonization temperatures,and water hyacinth-based self-supporting electrode WHCF-1-x was successfully prepared.WHCF-1-x is composed of several vesicular air cavity structures closely arranged in a fish scale shape inside the material,which constitutes a three-dimensional structure of the network.The carbonization temperature has a great influence on the physical and chemical properties of WHCF-1-x:as the temperature increases,a small amount of mineral ash is precipitated on the surface,and the specific surface area increases while the graphitization degree is enhanced.The highest specific surface area of WHCF-1-1000 is 307.47m²/g,which contains a large number of micropores with an average pore size of 2.24 nm,and the surface is rich in oxygen-containing functional groups,which increases the wettability of the material with the electrolyte and makes WHCF-1-1000 have the best electrochemical properties.The electrochemical performance of WHCF-1-1000 electrode in different electrolytes was also discussed:the electrochemical performance gradually improved with the increase of electrolyte Na₂SO₄concentration（0.5M,0.75M,1M）,and the specific capacitance at 0.5 A/g was 416 F/g,467.5 F/g,661.5 F/g in order.but when the concentration reached 1.5 M,the specific capacitance at 0.5 A/g the specific capacitance is 622 F/g,indicating that the increase of electrolyte ion content in 0.5～1M Na₂SO₄is beneficial to the full utilization of the material and the improvement of energy storage performance.In 3M H₂SO₄,the specific capacitance at 0.5A/g was measured to be 564.5 F/g,while the specific capacitance at 5 A/g was still as high as 342 F/g.Moreover,it has a lower R_tas well as R_sin3M H₂SO₄than in 1M Na₂SO₄,and is able to perform efficient and fast charge/discharge functions with high capacitance retention.