On Preparation And Electrochemical Performance Of Hierarchical Porous Carbon

As the advanced energy storage devices,supercapacitor（SCs）is widely used in transportation,instrumentation,aerospace,photovoltaic energy storage and other fields due to its fast charging and discharging,high power density and long cycle life.With the development of science and technology,the energy density requirement for energy storage devices is getting increasingly higher.However,the low energy density of SCs(≤5 Wh kg^-1)limits their application severely.Therefore,in order to improve the energy density of supercapacitors,the current research focus mainly on the electrode materials with high capacitance,high-voltage electrolyte and the optimization of device structure.The specific capacitance and specific energy of supercapacitors are mainly determined by the relationship between the pore structure of activated carbon electrode material,the size and surface property of electrolyte ions and the decomposition voltage of electrolyte.While,the matching relationship of the double electrode structure is also very important.In this research,the symmetric supercapacitor with high energy density was assembled by the ionic liquid electrolyte and the cotton based hierarchical porous activated carbon with high specific surface area prepared by acid-thermal activation method.Aniline-pyrrole copolymer hollow spherical hierarchical porous activated carbon prepared by chemical polymerization and high temperature alkali activation was used as the cathode material for asymmetric Zn-BSH and Li-BSH respectively,which further improved their specific capacitance and specific energy.The research results are as follows:（1）Cotton based hierarchical porous activated carbon with high specific surface area for ionic liquid supercapacitors with high energy densityA new method for preparing hierarchical porous activated carbon materials with high specific surface area which can significantly increase the specific energy of supercapacitors in the ionic liquid electrolyte was proposed.Activated carbon material prepared from cotton by acid thermal activation method has high specific surface area(1235.3 m²g^-1),hierarchical porous structure with rich mesopores and pseudocapacitive effect caused by N/O co-doped elements.The electrochemical performance of this material was excellent in the electrolyte of ionic liquid EMIMBF₄.When the current density was 0.5 A g^-1,the specific capacitance and energy density were 162 F g^-1and 84.4 Wh kg^-1respectively.（2）Polymer-based hierarchical hollow carbon spheres for high specific energy hybrid capacitorsA method for preparing hollow carbon nanospheres with high specific surface area of hierarchical porous activated carbon was proposed:the hierarchical hollow carbon spheres with high specific surface area(2428.7 m²g^-1)and mainly mesoporous（69.1%）were prepared from aniline-pyrrole conjugated hollow spheres by chemical polymerization-high temperature alkali activation method,which could improve the energy density of hybrid capacitors.The electrochemical properties and energy storage mechanism of hierarchical porous hollow carbon spheres in three kinds of capacitors were studied.Firstly,the specific capacitance and energy density of this carbon material in the symmetric supercapacitor were 127.3 F g^-1and 66.9 Wh kg^-1respectively,and its capacitance retention rate after 10,000 cycles was above 91.5%.Secondly,in the Zinc ion hybrid capacitor,when the electrolyte was 2 M Zn SO₄aqueous solution and the current density was 0.1 A g^-1,the specific capacitance and energy density were 306.6 F g^-1and 106.7 Wh kg^-1(79.2 W kg^-1)respectively,and the capacitance retention rate after 10,000 cycles was above 95.1%.Finally,when the capacitor was Lithium ion hybrid capacitor and the current density was 0.1 A g^-1,the specific capacitance and energy density of this material were 188.8 F g^-1and 116.6Wh kg^-1respectively.and the capacitance retention rate after 3,000 cycles was above92%.The study on the electrochemical properties of porous hollow carbon spheres in different capacitors provides a new way to research and develop high performance cathode materials for hybrid capacitors.