Research On The Mechanical And Capacitive Properties Of Graphene Quantum Dot Reinforced Actived Electrospun Carbon Nanofiber Fabrics

As a new kind of carbon materials that can integrate structure and function,the electrospun carbon nanofiber fabrics show good applications in electrochemical energy storage,clean energy and other fields.The carbon nanofiber fabrics with large specific surface area,good electric conductivity,and high mechanical strength are widely used in supercapacitor electrodes.But there is a trade-off between the well-developed porous structure,strong carbon skeleton,and continuous conductive network.Based on the double layer energy storage mechanism,most carbon nanofiber fabrics with low specific surface area have poor capacity and rate performance.However,when the specific surface area increases,the mechanical strength and electric conductivity of the carbon nanofiber fabrics decrease greatly,which limite their practical application.Herein,we prepared the carbon nanofiber fabrics with large specific surface area,good mechanical strength,and electric conductivity through electrospinning,carbonization,and chemical activation using polyacrylonitrile and graphene quantum dots(GQDs)as precursors.Then the robust reinforcing effect of GQDs and the capacitive energy storage mechanism of the carbon nanofiber fabrics were deeply investigated.The main research content and the corresponding results of this paper are as follows:1.We investigated the influence on the mechanical properties,electric conductivity,and specific surface area of the fabrics by changing the amount of GQDs,heat treatment temperature,and activation ratio.We find that the addition of GQDs increase the tensile strength and electric conductivity of the fabrics by 5 times.The increase in heat treatment temperature also slightly increase the tensile strength and electric conductivity of the fabrics.Activation effectively increase the specific surface area of the fabrics,but reduce the tensile strength and electric conductivity with the rising of KOH dosage.The tensile strength(1.5 MPa)and electric conductivity(62.0S m^-1)of the GQDs reinforced fabric at the highest KOH dosage are much better than the tensile strength(1.2 MPa)and electric conductivity(12.2 S m^-1)of the fabric without GQDs,indicating that GQDs can effectively enhance the mechanical properties and electric conductivity of fabrics.2.After further investigating the enhancement mechanism of GQDs,we find that GQDs can fix polyacrylonitrile molecular chains around them by oxygen bridges during carbonization,thereby,forming a dense carbon skeleton and overall conductive network inside the fiber.This structure significantly improves the mechanical properties and electric conductivity of the fabrics,indicating that GQDs can be used as a new bi-functional reinforcing agent to overcome the contradictory relationship between the specific surface area,the mechanical properties,and electric conductivity of the fabrics.3.We find that the GQDs reinforced carbon nanofiber fabrics with large specific surface area and good electric conductivity can be used as freestanding supercapacitor electrode.Under the optimized condition,the specific surface area of the fabric is2242 m²g^-1,the tensile stress is 2.3 Mpa,and the electric conductivity can also reach65 S m^-1.When used in a symmetric supercapacitor,the fabric performs high capacitance of 335 F g^-1at 1 A g^-1and extremely high capacitance retentions of 77%at 100 A g^-1and 45%at 500 A g^-1.Importantly,the symmetric device can be charged to 80%capacitance within only 2.2 s,and cycle without obvious capacity fading after10,000 cycles at 50 A g^-1.The results show that the overall conductive network formed by the GQDs and abundant ion migration channels significantly improve the electrochemical kinetics,thus improving the capacitive and rate performance of the fabrics.In addition,the fabrics with large specific surface area also has good adsorption properties for dyes,oil stains,and organic reagents.In this work,the oxygen bridge bonds are formed between the GQDs and the polyacrylonitrile molecular chain during carbonization,leading to the formation of dense carbon skeleton and overall conductive network inside of the carbon nanofiber.Therefore,the carbon nanofiber fabrics can maintain good mechanical strength and conductivity after activation.This strategy not only overcomes the contradictory relationship between pore structure,mechanical strength,and conductivity,but also significantly improves the capacitive and adsorption performance.This work provides a new idea for the design of multi-functional carbon nanofiber fabrics.