Preparation And Performance Of Porous Or Doped Carbon Fibers Based Flexible Supercapactors

With the rapid development of the global economy,energy consumption is increasing,while the use of non-renewable petrochemical resources has brought serious social and environmental problem.Herce,it is imminent for us to develop efficient,clean and sustainable energy storage device which is required to have high energy density and power density,long service life,and be environmental friendly,safe,reliable and so on.With the progress of modern science and technology,as well as the popularity of micro electronic wearable or biological implants,energy storage system will be developed to be flexible and flimsy.On the other hand,because of good chemical stability,high mechanical strength,strong electrical conductivity,environmental friendliness and low price,carbon materials have been used as electrode materials for lithium-ion batteries and supercapacitors,which greatly improve the performance of these energy storage devices.At the same time,the fiber has better flexibility and weaving,so carbon fiber is considered to be one of the most promising electrode materials for flexible supercapacitors.In our study,electrospinning and centrifugal spinning were respectively used to prepare PAN-based fibers.By comparison,it was found that high concentration spinning solution is easy to plug the nozzle in electrospinning,and the fiber prepared by centrifugal spinning can withstand the maximum tensile stress of 6.95MPa.Moreover,the mechanical strength is 4 times higher than that of the electrospinning,and the centrifugal spinning rate is 240 times more than that of the electrospinning,so we chose centrifugal spinning as the preparation of fibers.Fibers were prepared form PAN based spinning solution with PEG as pore formation agent.Then the operations such as water-washing,stretching pre-oxidation,carbonization and so on,were carried out to get porous carbon fibers of which the specific area increased from 6.6 m²/g to250.5 m²/g.The use of scanning electron microscope,transmission electron microscopy and specific surface area test,characterization means that it is an essential step to remove PEG by water washing in the preparation of porous structure.The porous structure of carbon fiber will increase its specific surface area,but which can lead to reduce its stability and conductivity.To make the electrode material have high specific volume and high rate performance,it is necessary to keep the porous structure and conductivity of carbon fiber at a suitable equilibrium point.The experimental results show that the specific capacitance of porous carbon fibers electrode prepared by the solution of PAN/PEG mass ratio of 5/2 is the highest,reaching 194.4F/g（electrolyte 1M Na₂SO₄,current density 0.3A/g）,and its cycle life and bending resistance of the electrode are excellent.The specific capacitance is basically maintained after 6700charge-discharge cycles,and the specific capacitance can remain at 97.3%after 200 times of bending.This study provides a new route for the preparation of porous flexible carbon fibers.The specific capacitance of pure porous carbon fibers as the electrode material is very limited,and the rate performance needs to be improved.While the transition metal oxide is redox in the process of energy storage to produce the Faraday effect,the system has higher energy density and capacitance.Because cobalt oxide is rich in natural resources,low in cost,with a theoretical value up to 3560F/g,and has good power storage capacity,we planned to prepare cobalt oxide-doped porous carbon fibers which improve electrical properties of materials.In this paper,Co₃O₄ particles were directly added to to the spinning solution in the spinning additive method for spinning the fibers.The spinning additive method was used to get cobalt oxide-doped porous carbon fibers in this experiment.The internal defects of the material become larger and the mechanical strength obviously drops.Therefore,cobalt oxide-doped porous carbon fibers were prepared by direct spinning method,which of internal structure to some extent was undermined.However,cobalt oxide-doped porous carbon fibers prepared by immersion method were soaked in cobalt acetate solution after heat treatment.According to scanning electron microscope,energy spectrum analysis,X-ray diffractometer,Fourier infrared spectrum and other characterization means cobalt oxide successfully doped into the carbon fibers.In the soaking process,the influences of treatment of fibers soaked in different stages（raw silk stage,pre-oxidized fiber stage and post-carbonized fiber stage）and different soaking liquids（Cobalt acetate in aqueous solution and Cobalt acetate in ethanol solution）were investigated.The capacitance of cobalt oxide-doped porous carbon fibers is up to 300.4 F/g（1M Na₂SO₄ as electrolyte,current density1.0 A/g）,meanwhile the voltage window can be extended up to 1.4V,and higher current density of material can be withstand.When the scan rates is 1mV/s to 100 mV/s,the specific capacitance can still be retained by 80%.The method above can be expanded to the preparation and assembly of other metal oxides and porous carbon fibers for flexible capacitor.