High Performance Graphene Fiber-Based Flexible Supercapacitors

To meet the rapid developments of wearable electronics,considerable efforts have been devoted to flexible,reliable,efficient energy storage systems,including the lithium-ion batteries,solar cells,supercapacitors.Among them,supercapacitors show many advantages,including fast charge-discharge rate,long cycling life and ultrahigh power density,rendering them promising power source.Fiber-shaped supercapacitors are desirable for the wearable electronics,as the fibers are flexible,weavable and strong to meet the requirement of wearable electronics.Graphene fiber,as one of the assembled ordered materials in graphene field,consists many graphene sheets assembled in one dimension.Thus,graphene fiber retains a series of good properties of graphene,including the high electrical conductivity,thermal conductivity,mechanical strength and flexibility.Therefore,Graphene fiber is desirable for the fiber-shaped supercapacitors.However,the mechanical properties of graphene fibers are not satisfactory for the wearable electronics resulting from the fiber precursor,assembling method and post-treatment.Therefore,many researchers are dedicated to develop high performance graphene fibers?high tensile strength and electrical conductivity?.In addition,the?-?restacking of graphene dramatically decreased the solvated ion accessible surface area,resulting in the low electrochemical performances of graphene fibers.To improve the strength of graphene fibers,we selected the large size graphene oxide as the graphene fiber precursor,and utilized the wet spinning method,followed by high temperature thermal annealing.The high strength and electrically conductive graphene fibers were successfully fabricated because of the increased the crystallinity,orientation and decreased defects;To increase the specific capacitance and energy density of graphene fibers,we designed a core-sheath,porous graphene fibers with tunable specific surface area and pore size distribution,and systematically investigated the effects of electrical conductivity,surface area and pore size distribution on the electrochemical performance of graphene fibers.In addition,we also designed a core-sheath,porous polyaniline nanorods/graphene fibers by chemical deposition,and studied the electrochemical performance of the fabricated polyaniline nanorods/graphene fibers.Moreover,we also studied the application of graphene fiber-based flexible supercapacitors on the wearable electronics.The main results are as follows:?1?Microstructures and properties of thermally annealed and chemically reduced graphene oxide fibers.We used the large size graphene oxide aqueous dispersion as the spinning dope,and systemitally studied the effects of coagulation bath and reduction methods on the structure and performance of graphene fibers.Compared with the chemically reduced graphene oxide fibers,thermally reduced graphene oxide fibers show better electrical performance and mechanical properties.We investigated the effects of coagulation bath and reduction method on the structure and properties of graphene fibers through microscopic characterizations,and built the structure evolution model from the graphene oxide liquid crystal to long-range ordered graphene fibers.?2?Hierarchically porous core-sheath graphene-based fiber-shaped supercapacitors with high energy density.High strength graphene oxide fibers were firstly spun by using the giant graphene oxide in commercial wet spinning method.The mixture of nano-size graphene oxide and phenolic resin was evenly coated on the as-spun graphene oxide fibers through the“Dip-coating”method.The carbonized composite fibers showed high BET specific surface area?up to 416 m²/g?,micropore porosity?up to 96.9%?,rational pore size distribution,and high electrical conductivity and flexibility.The assembled fiber-shaped supercapacitors showed high areal specific capacitance of 391.2mF/cm²,high cell energy density?8.7?Wh/cm² for PVA/H₂SO₄ electrolyte and 66.4?Wh/cm² for organic electrolyte?at high power density,ultralong cycling life,and good flexibility and cycling stability.The result showed that the micropore-majority hierarchical porous structure and high electrical conductivity had the synergetic effects on improving the electrochemical performance of the graphene fibers.?3?Core-sheath porous polyaniline nanorods/graphene fibers?GF@PANI?for the flexible,high performance,fiber-shaped supercapacitors.To improve the rate capability of graphene fibers,we designed and fabricated a core-sheath porous PANI nanorods/graphene hybrid fibers by in-situ polymering PANI nanorods on GF.The GF@PANI supercapaicitors had high specific capacitance?357.1 mF/cm² at 0.1 mA/cm²?,and high areal and volumetric energy density of 7.93?Wh/cm²,5.7mWh/cm³,respectively.The GF@PANI supercapacitors showed excellent rate capability,flexibility and ultrahigh cycling life,as there was a 96.2%capacitance retention after 5000 charge-discharge cycles.