Study On Controllable Preparation And Performance Of Flexible PAN-Based Carbon Nanofibers

As the portable and wearable electronics have found their way,the flexible energy storage devices have become a spurt demand for people.Flexible supercapacitors（SCs）are highly favored in the field of energy storage due to their excellent mechanical flexibility,fast storage/release capability,long cycle life,high safety,and environmental friendliness.The lower energy density of SCs in comparison to Li-ion batteries severely limits the prospects for SC application,despite their prominent power density.Among the many materials,carbon nanofibers（CNFs）,with large specific surface area and outstanding mechanical properties,are widely used in the field of SCs electrodes.However,carbon-based electrodes prepared from a single organic polymer are often difficult to achieve the desired capacitance level,and need to improve the electrochemical properties of carbon-based electrodes by means of morphology design,structure optimization and elemental doping.Dopamine（DA）and polyacrylonitrile（PAN）were used as raw materials to prepare dopamine/acrylonitrile nanofiber films（x DA/PAN,x represents the mass percentage of DA to PAN）by electrostatic spinning technique and interfacial modification method,then flexible self-supporting nitrogen-doped carbon nanofibers（x-CNFs）were successfully prepared by DA self-polymerization,pre-oxidation and high-temperature carbonization.Furthermore,the flexible self-supported composite carbon nanofibers（GO-x CNFs）were obtained under the same carbonization conditions by enhancing the electrical conductivity of carbon nanofibers with the combination of poly（dopamine）（PDA）and graphene oxide（GO）with the help of its good adhesion properties.The prepared x-CNFs and GO-x CNFs have a rough surface morphology and tunable pore structure with substantially improved electrochemical properties,which can be applied as carbon-based electrode materials for flexible supercapacitors.x-CNFs and GO-x CNFs can also provide a self-supporting effect,reducing the use of additional binders and conductive agents,and can support bending or folding to a certain extent.The specific work consists of the following three parts.1.The effects of electrostatic spinning parameters（such as spinning solution concentration,ambient humidity,spinning voltage and receiving roller speed）,carbonization conditions（including pre-oxidation temperature and carbonization temperature）and doping methods（for example,DA addition method,DA addition amount and DA polymerization time）on the morphology and properties of carbon nanofibers were optimized.2.Flexible self-supported nitrogen-doped carbon nanofibers（10%CNFs）prepared under optimal conditions were characterized and tested.The contribution of PDA to enhance the capacitance level of 10%CNFs was investigated by measuring the elemental composition and structural changes on the surface of carbon nanofibers.The doping of carbon substrates and the modulation of microscopic morphology by PDA improved the wettability of electrode materials,enhanced the response performance,and the flexible self-supporting structure was maintained.The prepared 10%CNFs have a specific surface area of 404.7 m²g^-1,which facilitates the charge exchange between the electrolyte and the electrode material and further increases the micropores,exposing more active sites and exhibiting excellent electrochemical properties.In comparison with pure CNFs,it was found that the hydrophilicity and conductivity of 10%CNFs were greatly improved,and the mass specific capacitance reached 228 F g^-1(current density of 1 A g^-1),and the capacitance retention rate could still reach 68.6%even at 20 A g^-1 current density.3.The flexible self-supporting composite electrode materials（GO-10%CNFs）were prepared by compounding 10%PDA/PAN with GO,and the application potential of GO-10%CNFs in flexible supercapacitors was initially evaluated.It was found that GO facilitates the construction of fast electron migration paths and the conductivity of GO-10%CNFs can reach up to 1.374 S cm^-1.Electrochemical tests show that the specific capacitance of GO-10%CNFs can reach 265 F g^-1(current density of 1 A g^-1),which is nearly four times higher relative to pure CNFs and also about 40 F g^-1 more than that of 10%CNFs.10%CNFs as positive and negative electrodes were assembled into supercapacitors with an energy density of 16.1 W h kg^-1 and a power density of 700 W kg^-1,still maintaining a high level of energy storage.