Design And Performance Research Of High Specific Capacitance Micro-nano Carbon Fiber-based Electrodes

With the development of science and technology and the progress of human civilization,people’s awareness of ecological environment protection has gradually increased.In the past decades,electrochemical energy storage devices such as batteries,fuel cells,and supercapacitors have played an important role in human life.In practical applications,supercapacitors have become a research hotspot because of their high power density,rapid charge-discharge capability,long service life,and higher safety.The electrode material determines the ultimate performance of the supercapacitor.The energy density of the supercapacitor can be improving by increasing the specific capacitance.For carbon-based electrode materials,the methods to increase the specific capacitance mainly include increasing the specific surface area,adjusting the pore size and distribution,doping elements,and compounding with pseudo-electrode.Carbon nanofiber and carbon fabric are important electrode materials for supercapacitors,but their specific capacitance is limited,and the energy density of their devices is relatively low,which limits the application of supercapacitor.In this paper,carbon nanofiber and carbon fabric electrodes are designed and studied to improve the specific capacitance of the electrode and achieve the improvement of energy density.The specific surface area and pore volume of carbon nanofibers were increased by sacrificial activation method,and the pore forming mechanism was explored.At the same time,the carbon nanofibers prepared by different sacrificial polymers have different porous structures,which also affect the electrochemical performance of the electrode.The composite of carbon fabric and polypyrrole with higher theoretical specific capacitance can not only increase the specific capacitance of carbon fabric,but also improve the cycle stability of polypyrrole electrode.However,the surface treatment of carbon fabric is needed to improve the interface adhesion between carbon fabric and polypyrrole,and a supercapacitor with high specific capacitance and long service life is designed.The main research contents of this paper are as follows:(1)The hierarchically porous carbon nanofiber is prepared by using polyacrylonitrile(PAN)as precursor and high-amylose starch as sacrificial polymer.The addition of high-amylose starch favors the formation of carbon nanofibers with microporous structure,thus favoring the increase of electrode specific capacitance.The specific surface area of the prepared carbon nanofibers is 637 m~2/g,the total pore volume is 0.42 cm~3/g,and the micropore content is 59%.At 1 A/g,the specific capacitance of the carbon nanofibers electrode is 282 F/g,the rate capability is 67%at50 A/g.The energy density of the device made from carbon nanofibers is 8.5 wh/kg at the power density of 0.5 k W/kg.(2)The carbon nanofiber with hierarchically porous and interconnected structure is prepared by using PAN as precursor and polysulfone with high decomposition temperature as sacrificial polymer.The interconnected structure accelerated the rapid conduction of electrons,increased the rate capability of the electrode,and reduced the internal resistance of the electrode.When the mass ratio of polyacrylonitrile to polysulfone is 4:1,the specific surface area of carbon nanofibers is 687 m~2/g,the total pore volume is 0.52 cm~3/g,and the micropore content is 58%.At 1 A/g,the specific capacitance of the carbon nanofibers electrode is 300 F/g,the rate capability is 70%at50 A/g.The energy density of the device made from carbon nanofibers is 9.9 Wh/kg at the power density of 0.5 k W/kg.(3)The carbon nanofiber with high specific surface area,hierarchically porous and interconnected structure is prepared by PAN as precursor and linear novolac resin with low melting point,high carbonization yield,high decomposition temperature and low molecular weight as sacrificial polymer.The increase of specific surface area further improves the specific capacitance of the electrode.When the mass ratio of polyacrylonitrile to linear novolac is 7:3,the specific surface area is 1468 m~2/g,the total pore volume is 0.89 cm~3/g,and the micropore content is 81%.At 1 A/g,the specific capacitance of the carbon nanofibers electrode is 394 F/g,the rate capability is 71%at50 A/g.The energy density of the device made from carbon nanofibers is 14.2 Wh/kg at the power density of 0.5 k W/kg.(4)The carbon fabric is treated by plasma and acrylic acid to obtain functional carbon fabric,and then polypyrrole is deposited on the functional carbon fabric to prepare the composite electrode.This treatment method increases the interface binding force between polypyrrole and carbon fabric,improves the conductivity of polypyrrole,and forms a looser structure.These excellent structural characteristics also improve the electrochemical performance of the composite electrode.The areal specific capacitance of the supercapacitor device made from polypyrrole/functional carbon fabric composite electrode is as high as 417 m F/cm~2 at 20 m V/s,and the mass specific capacitance of the device is as high as 190 F/g at 0.25 A/g.When the power density is 100 W/kg,the energy density of the device is 16.9 Wh/kg,and the initial capacitance of 96%can be maintained after charging and discharging for 4000 times at 0.25 A/g.