Preparation And Electrochemical Performance Of Functionalized Hierarchical Porous Carbon Materials

Supercapacitors have become a favorable competitor of energy storage devices in the future,due to have the advantages of fast charging and discharging,long service life,high power density,safety and environmental protection.Carbon materials with hierarchical pore structure and high ion accessible specific surface area are used as electrode materials for supercapacitors.However,the low energy density severely limits the development and application of carbon-based supercapacitors.This thesis improves the energy density of carbon-based supercapacitors from two aspects:the structure optimization of electrode materials and the energy storage mechanism of supercapacitors.The main research contents of the thesis are summarized as follows:（1）From the perspective of structure design of electrode material,a nitrogen-doped porous carbon composite with a three-dimensional（3D）conductive network was prepared by carbonized carbon nanotube（CNT）strung zeolite imidazolate framework 8（ZIF-8）composite material（N-PC/CNT）.The ZIF-8 derived nitrogen-doped porous carbon with a rich pore structure in the material acts as a"reservoir",providing sufficient storage space and developed ion transmission channels for electrolyte ions,while CNT serves as a"wire"to form an internal 3D conductive network,which is benefical to the rapid transmission of electrons.N-PC/CNT offers a high nitrogen doping content of 12.1 at%,which contributes to increase the specific capacitance of the material.It has a large specific surface area of 980.2 m² g^-1 that provides sufficient ion storage space,which is conducive to electric double layer capacitance.The hierarchical pore structure of micropores,mesopores and macropores simultaneously achieves high specific capacitance and excellent performance during rapid charge and discharge.In the three electrode system with 6 mol L^-1KOH as electrolyte,the N-PC/CNT electrode material exhibits a high specific capacitance of334.5 F g^-1 at current density of 1 A g^-1.Even at a high currenr density of 100 A g^-1 it still can maintain a specific capacitance of 195.5 F g^-1,showing outsanding excellent rate properties.It delivers up to 98%of the capacitance retention after 20000 cycles charging and discharging.In addition,N-PC/CNT-based symmetrical supercapacitors can output a maximum energy density of17 Wh kg^-1 in 1 mol L^-1 Na₂SO₄ electrolyte.（2）From the point of the energy storage mechanism about supercapacitors to prepare K₃Fe（CN）₆modified porous carbon/MnO₂ composite material（OPHC-MnO₂-K₃Fe（CN）₆）,firstly,the hollow spherical porous carbon material was synthesized by the Si O₂ template method,and then oxygen-containing functional groups were introduced on the surface of the carbon material.Secondly,MnO₂ was grown in-situ on the surface of porous carbon,finally modified with electrochemically active substance K₃Fe（CN）₆.The porous carbon in OPHC-MnO₂-K₃Fe（CN）₆exhibits a hollow sphere with rich pore structure on the outer wall,which can be used as a"reservoir"about the electrolyte,to greatly shorten the diffusion distance of electrolyte ions in the material,and facilitate the rapid transmission of ions.The oxygen-containing functional groups on the surface of porous carbon not only provide abundant active sites for the growth of MnO₂,but also firmly bind K₃Fe（CN）₆ to the surface of OPHC-MnO₂-K₃Fe（CN）₆,so that these pseudocapacitance materials can increase the specific capacitance of OPHC-MnO₂-K₃Fe（CN）₆through a rapid oxidation-reduction reaction.In 1 mol L^-1 Na₂SO₄ three electrode system,the specific capacitance of OPHC-MnO₂-K₃Fe（CN）₆ up to 332.7 F g^-1 at current density of 0.5 A g^-1.Moreover,the capacitance retention is as high as 90%after 20000-cycle charging and discharging.OPHC-MnO₂-K₃Fe（CN）₆-based asymmetric supercapacitor shows an energy density of 56.7 Wh kg^-1 at a power density of 832.9 W kg^-1 in aqeous electrolyte.Furthermore,OPHC-MnO₂-K₃Fe（CN）₆-based quasi-solid asymmetric supercapacitor presents an energy density of 29.8 Wh kg^-1 when the power density is 119.1 W kg^-1.OPHC-MnO₂-K₃Fe（CN）₆-based quasi-solid asymmetric device shows 100%of capacitance retention and coulombic efficiency after 10000 cycles.More importantly,the quasi-solid asymmetric supercapacitor assembled with OPHC-MnO₂-K₃Fe（CN）₆ can be used as power source to make the device work.