Preparation Of Porous Carbon Nanocomposites And Their Application In Supercapacitors

The accelerated development of human civilization has brought the global challenge of energy crisis.New energy sources such as lithium-ion batteries,fuel cells,solar cells and supercapacitors have attracted extensive attention.Supercapacitors are considered as the most potential energy storage equipment and clean energy,also known as electrochemical capacitors.They have the advantages of traditional batteries and capacitors,such as high capacity,high power density,good multiplier performance and long life.The key factor affecting the performance of supercapacitors is electrode material,which must have high specific surface area,reasonable aperture distribution and good conductivity,so as to meet the requirements of high material utilization rate and high capacitance.Therefore,the preparation of materials with high capacitance,high energy density and high stability is the research focus.Porous carbon materials are often used as electrode materials for supercapacitors due to their good conductivity,large specific surface area,developed pore structure and high chemical stability.Although they have the advantages of large current,rapid charge and discharge capacity and ultra-long cycle stability,their energy density is relatively low,which limits the practical application of porous carbon materials in the field of energy storage.In order to improve the energy storage density of porous carbon materials,the following attempts were made.The porous carbon nanofiber mats were prepared based on electrospun technology by adjusting the composition of spinning solution which had a self-supporting structure and could be directly used as an electrode without the binder,which was a kind of electrode material with excellent capacitance performance.To further improve the capacitance performance,we loaded the Ni₂P nanosheets on the electrospun carbon nanofiber mats and tested its capacitance performance.In addition,we took biomass juncus as the raw material and carbonized it in different ways to obtain three-dimensional hierarchical porous carbon,which was used as a pollutant adsorbent,supercapacitor electrode material and oxygen reduction reaction catalyst for research.Finally,MnO₂ nanosheets and polyaniline（PANI）nanorods were modified to prepare composite materials based on the porous carbon derived from juncus and used as electrode for supercapacitors.The specific work content includes the following five aspects:In the first part,porous carbon nanofiber mats from electrospun polyacrylonitrile/polymethylmethacrylate composite nanofibers for supercapacitor electrode materials.The polyacrylonitrile/polymethylmethacrylate composite nanofiber mats were prepard by an electrospinning methode.The nanofiber composite mats were peroxided,carbonized to obtain porous carbon nanofiber mats.The surface morphology、properties、and BET and pore character of composite mat materials were characterized by SEM、TEM、FT-IR、Raman、XRD and N₂ adsorption/desorption analysis.Cyclic voltammetry（CV）and chronopotentiogram（CP）methods were used to study the electrochemical properties of the porous carbon nanofiber composite mat materials.Results showed that the porous carbon nanofiber mats have a good capacitance performance and the specific capacitance of PAN/PMMA=7:3 was 140.8F g^-1.At a current density of 10 A g^-1,the specific capacitance of PAN/PMMA=7:3only decreased by 4.6%after 10000 charge/discharge cycles.Therefore,the porous carbon nanofiber mats were excellent materials for use in supercapacitors.In the second part,flexible Ni₂P nanosheet/electrospun carbon nanofiber mats composites were prepared and applied in supercapacitor.The polyacrylonitrile（PAN）nanofiber mats were prepard by an electrospinning methode.The nanofiber mats were peroxided,carbonized to obtain carbon nanofiber mats（CNF）.The Ni₂P nanosheet arrays were grown on the surface of flexible electrospun carbon nanofibers（CNF）via a phosphorization reaction of Ni-LDH nanosheet arrays that were synthesized by a facile hydrothermal reaction.Others,the obtained CNF loaded by Ni₂P nanosheet arrays（Ni₂P NSAs-CNF）can be used as a self-supported、binder-free flexible electrode.Electrochemical testing was used to research the electrochemical performances of the Ni₂P NSAs-CNF as supercapacitors electrode.Results showed that the Ni₂P NSAs-CNF electrode displayed remarkable specific capacitance(1057.5F g^-1 at the current density of 0.5 A g^-1),good rate capability(387.5 F g^-1 at the current density of 50 A g^-1)and prominent cycle stability(the capacitance retention is 89.7%after 3000 cycles at 5 A g^-1).The asymmetric supercapacitors based on Ni₂P NSAs-CNF and CNF were fabricated.The fabricated device showed energy density of8.5 Wh kg^-1 at power density of 353.3 W kg^-1 and the capacitance was maintained at92.5%after 3000 cycles at 2 A g^-1.Therefore,the self-supported Ni₂P NSAs-CNF is an excellent electrode for high-performance supercapacitors.In the third part,juncus-derived three-dimensional hierarchically porous carbon material was prepared and used for versatile application.The biomass juncus derived hierarchically porous carbon（HPC）was fabricated by a low-cost,simple and environment friendly one-step method.The juncus has been carbonized,their structures were well kept.HPC-800 was obtained by one step carbonization at 800°C,which showed high absorption capacity of 44?113.8 times its own weight for different oils and organic solvents and could be recycled via simple combustion.The nitrogen-doped porous carbon（NDPC）was prepared by mixing juncus and ZnCl₂through one step pyrolysis and activation,it had most of porous structures and high specific surface area,and was applied for supercapacitors and oxygen reduction reaction（ORR）.The resulted NDPC-3-800 was used as electrode material of supercapacitor,it exhibited excellent specific capacitance of 290.5 and 175 F g^-1 in alkaline electrolyte at the current densities of 0.5 and 50 A g^-1,respectively,with a good cycle stability,the capacitance was maintained at 94.5%after 10000 cycles.NDPC-5-800 exhibited outstanding ORR catalytic activity and stability attributing to their high specific surface area and a lot of actives sites.Therefore,juncus is a renewable materials,it can derive various materials and apply to different fields widely.In the fourth part,the preparation and characterization of MnO₂/hierarchically hollow porous carbon composites and their application in supercapacitors.The MnO₂-HHPC composite materials was prepared by the reaction of biomass juncus derived hierarchically hollow porous carbon（HHPC）with KMnO₄ solution.Because HHPC had three-dimensional hierarchically porous structure,it was favorable for KMnO₄ solution to enter into the HHPC and react.The morphology of MnO₂/HHPC was characterized by SEM,and MnO₂ nanosheets were found growing on the inner and outer surfaces of HHPC.The MnO₂-HHPC composite materials still maintained the three-dimensional hierarchically porous structure,the electrolyte could effectively enter into the material,so that the electrolyte could fully and contact effectively with the internal MnO₂ nanosheets,thus improving the specific capacitance of MnO₂-HHPC composite materials.The results of electrochemical test showed that the specific capacitance of MnO₂-HHPC-24h was 236.4 F g^-1 at the current density of 0.5A g^-1.In 1 M Na₂SO₄ electrolyte,MnO₂-HHPC-12h as positive electrode and HHPC as negative electrode were assembled into MnO₂-HHPC-12h//HHPC asymmetric supercapacitor,and the working voltage window could reach 0?2.0 V.The specific capacitance was 67.1 F g^-1 at the current density of 0.25 A g^-1.The energy density could reach 36.6 Wh kg^-1 at the power density of 247.8 W kg^-1.After 1500 cycles of charge and discharge at 2 A g^-1,the capacitance retention rate was 99.5%.In the fifth part,the activated hierarchically hollow porous carbon/polyaniline composites were prepared from juncus and applied in supercapacitors.PANI-AHPC composite materials were synthesized by in-situ chemical oxidation polymerization with active hierarchically porous carbon（AHPC）derived from biomass juncus.AHPC provided a large number of active sites for PANI growth due to its three-dimensional hierarchically hollow porous structure.In addition,PANI-AHPC had high conductivity and porous structure,which could alleviate the damage to the PANI structure.The PANI-AHPC composite materials were used as the electrode of the supercapacitor.In the three-electrode system,PANI-AHPC-3 had a high specific capacitance(439.5 F g^-1 at the current density was 1 A g^-1)and a good rate capability.In addition,in 1 M H₂SO₄ electrolyte,the PANI-AHPC-3//PANI-AHPC-3symmetrical supercapacitor was assembled with PANI-AHPC-3 electrode,and the specific capacitance was 197.7 F g^-1 at the current density of 0.5 A g^-1.The energy density could reach 27.5 Wh kg^-1 at the power density of 1123 W kg^-1.And after 2000cycles of charge and discharge tests at 1 A g^-1,the capacitance retention rate was96.3%.It indicated that PANI-AHPC composite had stable structure and excellent electrochemical performance.