Preparation And Performances Of Flexible Graphene Composite Fiber Based Supercapacitor

With the rapid growth of wearable,portable and implantable electronic,designing of miniature and high-performance energy storage and power supply systems has become an important research direction in the field of energy materials.Recently,fiber-based supercapacitors have shown great potential for development in the field of wearable electronic devices due to their high-power densities,long cycle life,and flexibility in three dimensions.As a typical carbon-based material electrode,graphene fiber demonstrates many advantages such as good conductivity,light weight,and structural designability.In this paper,a novel oriented graphene/carbon nanotube composite fiber is designed.By adjusting the distribution of mesopores and the degree of reduction of graphene oxide(GO),the capacitance of carbon-based fiber electrodes is significantly improved.Based on graphene/carbon nanotube hybrid fibers,a flexible asymmetric supercapacitor with widen voltage window is prepared to further enhance the energy density of fiber-based supercapacitor;In addition,an integrated anti-freezing graphene-based composite gel fiber is synthesized to cope with the problem of sharp decline of electrochemical performance and flexibility deteriorates under low temperature of all-gel-state fibrous supercapacitor.A spring-like stretchable supercapacitor is also prepared to content more functional requirements of wearable devices.The main works of the paper are as follows:(1)For the carbon-based fiber electrode,the electrochemical performance is affected by its specific surface area and internal pore structure distribution.To solve the problem of low capacitance caused by the re-stack of graphene sheets and uneven pore size distribution in graphene fibers,herein,graphene/carbon nanotube hybrid fibers with oriented carbon nanotube are successfully prepared by using non-liquid crystal wet spinning and capillary-assisted methods.Firstly,the oriented carbon nanotubes(CNT)not only build a conductive path,but also make the pore size distribution of hybrid fibers concentrated at?4 nm.The narrow mesoporous structure not only provides an electric double layer,but also facilitates the transport of electrolyte ions.Secondly,by adjusting reduction degree,the oxygen-containing functional groups retained on reduced graphene oxide(RGO)component could increase the reactive site.In the sulfuric acid electrolyte,the residual carboxyl groups improve the capacitance of the fiber through redox reaction.The all-solid-state supercapacitor assembled from RGO/CNT hybrid fibers demonstrates the capacitance of 354.9 F cm-3 and an energy density of 12.3 mWh cm-3.(2)The energy density of graphene fiber-based supercapacitor could be enhanced by constructing asymmetric supercapacitor.However,few attentions have been paid to the problem of the slow reaction rate and decreased cycle life for asymmetric supercapacitor caused by the weak interaction between the active material and the fiber substrate.Herein,using the graphene/carbon nanotube hybrid fiber(GCF)as the substrate,the surface of GCF is wrapped with a nitrogen-doped carbon layer(NC)by simple dopamine treatment,which effectively enhances the interaction between the NiCo2O4 nanoarray and the fiber substrate.Then,the positive electrode of GCF@NC@NiCo2O4(named as GCF@NC@NCO)fiber is obtained.The composite fiber demonstrates rich electrochemically active sites and stable interface,which can smoothly transfer electrons and electrolyte ions,so it shows good rate performance and excellent cycle stability.For the negative electrode,a simple carbothermal reaction is used to prepare porous graphene/carbon nanotube hybrid fiber with both axial and radial through channels.The porous fiber contains plenty of in-plane and out-of-plane pores,and it demonstrates continuous ion diffusion channel and rapid electrochemical reaction kinetics.Thanks to the ingenious design of the positive and negative electrodes,the assembled asymmetric flexible supercapacitor exhibits power density of 472.1 ?W cm-2 and high energy density of 11.2 ?Wh cm-2,and 93%of capacitance retention after 10000 cycles.(3)To cope with the impact of low temperature on the electrochemical performance of flexible energy equipment,a low-temperature resistant RGO/PEDOT-PVA composite gel fiber electrode with double-network structure is firstly prepared by the space-confined self-assembly and simple solvent replacement method.Based on anti-freezing system of the electrode,the low-temperature resistant electrolyte is also prepared,and an integrated anti-freezing all-gel-state supercapacitor is assembled.The integrated device possesses an enhanced interface and accessible surface area between the active micro-units of the electrode and electrolyte,exhibiting a high specific capacitance of 281.2 F g-1 under the current density of 0.1 A g-1,which is more than twice higher than that assembled with freeze-dried aerogel fibers(114.4 F g-1).Even at-20?,it can still maintain a specific capacitance of 212.6 F g-1.The low temperature-resistant device also demonstrates excellent flexibility and stable capacitance when tested under bending conditions at low temperatures.Due to the excellent mechanical properties of the device,a spring-like stretchable fiber-based supercapacitor is further designed,and demonstrating good electrochemical stability under a long-term stretching cycle(92%of capacitance retention after 5000 loading-unloading cycles at a strain of 500%).