Research On Controlled Preparation And Applications Of Graphene Fibers

Graphene is a two-dimensional carbon nanomaterial with only one atomic thickness.Due to its unique structure,graphene has excellent mechanical,thermal,electrical and optical properties,which has been favored by researchers since its discovery.Graphene fibers(GF)are one-dimensioned assembled from graphenes or functionalized graphene nanosheets by wet spinning and other technologies.The GF has excellent properties which inherited from the graphene at the nanoscale.So it has became the most attractive new carbon-based fibers.Due to its high strength,high electrical conductivity,high thermal conductivity,low density,flexible weavability and simplicity of processing,GF have shown promising applications in multi-functional fabrics,lightweight conductors,wearable supercapacitors,flexible batteries and sensors.However,the research on GF in epoxy resin composite materials,flexible piezoresistive sensors,and hollow structures produced by wet spinning is still blank,it is necessary to expand and develop their applications in these areas.In this thesis,the high-quality graphene oxide(GO)was prepared by the modified Hummers method with sulfuric acid-potassium permanganate system.The high-speed centrifugation strategy was used to obtain GO dispersions with different solvents and different concentrations forcontrollable preparation of graphene oxide fibers(GOF)by means of the wet spinning.After reduction,GF were obtained and a series of research work was carried out on this basis.The main research results are listed as follows:The GOF was prepared by wet spinning method using a concentrated GO solution of large size graphene oxide sheets(20 mg m L^-1)with DMF solvent as the spinning solution and ethyl acetate as the coagulation bath.The"two-step method"has been developed to obtain short GOF.Compared with the traditional"one-step method"to obtain short GOF,the"two-step method"has obvious advantages.The tensile strength and modulus of a GOF were 81.9 MPa and 3.69 GPa,respectively,and the elongation at break was 2.22%.The tensile strength and modulus of the GF obtained by thermal reduction at 1000 ^oC were increased to 135.6 MPa and 7.71 GPa,respectively,and the elongation at break was 1.76%.GF also has excellent conductivity,and the conductivity at room temperature is 6.9×10⁴ S m^-1.The tensile strength and electrical properties of GOF and GF were higher than the test values of short fibers obtained by the researchers using the traditional"one-step method".Then,the 3D reduced graphene oxide fiber fabric(GFF)with excellent mechanical,thermal and electrical properties was prepared by introducing hydrogen bonds between the short fibers through"re-dispersion"strategy.After thermal reduction at 1000 ^oC,the electrical conductivity of 3D GFFwas up to5028.16 S m^-1,the thermal conductivity was up to 153.63 W m^-1K^-1,the tensile strength and modulus were 1.29 MPa and 49.62 MPa,respectively,and the elongation at break was 2.60%.In order to make full use of the excellent properties of GFF,the GFF epoxy resin(GFF/E)composites were prepared for the first time.When the mass fraction of GFF in the composites was as low as 0.8%,the thermal conductivity of epoxy resin composites were significantly improved(190%increase).A flexible resistive pressure sensor based on a hierarchical 3D porous GFF and a PDMS elastomer was presented for the first time.The internal conductive network is formed by the GF which is mutually contacted by interfused or overlapped fiber-to-fiber interfaces,further endowing good conductivity and unique structural characters.Finally,the as-designed flexible pressure sensor can provide applied strain from low to high(0.24-70.0%)with an ultrahigh Gauge Factor(gf)up to 1686.48 at an applied compression of 66.0%,a fast response time(30 ms),a wide frequency range(0.01-1.0Hz),great stability and durability,and a limit of detection as low as 1.17 Pa,which can be used to detect a millet particles as light as 6.6 mg.The sensor can also be employed in wearable electronic devices for monitoring the motion of fingers and arms,the pulse,breathing during exercise and movements of robots.At last,a method of synthesizing the pore enriched and hollow CoNiO₂@GF hybrid fibers by wet spinning and hydrothermal was firstly developed by using the spontaneous diffusion behavior of GO dispersion in electrolyte solution and the flocculation effect of GO colloid solution in electrolyte.Due to the strong interface effect conferred by the in-situ nucleation growth of porous and ultrathin CoNiO₂nanosheets,as well as the good conductive properties and hollow structure of GF,the electrode materials showed a fast redox reaction and mass transfer kinetics.The electrochemical test results shown that the as-assembled CoNiO₂@GF electrode with binder-free characteristics exhibits a large specific capacity of 645.8 C g^-1 at a current density of 2 A g^-1;even with a 25-fold increase in current density(50 A g^-1),the capacity remains as high as 460.0 C g^-1,showing extraordinary rate capabilities;after50000 charge and discharge cycles,the capacity of the electrode increased by 42.9%instead of decreasing.By using pure GF as anode materials,the CoNiO₂@GF//GF Supercapacitor-battery(SC-Battery)hybrid device could achieve an energy density of about 43.99 Wh kg^-1 at a power density of 1.70 k W kg^-1;even at an ultrahigh power density of 21.61 k W kg^-1,the energy density is still as high as 38.41 Wh kg^-1.In addition,in order to expand the application of SC-Battery in the field of flexible wearables,we also assembled flexible all-solid SC-Battery hybrid energy storage devices,and evaluated the cyclic stability of flexible hybrid energy storage devices under three-point bending load-unload test.In summary,based on wet spinning in this thesis,we have developed and expanded the applications of GF in epoxy resin composite materials,flexible piezoresistive sensors,multi-level structures and their electrochemical energy storage devices.Some research results have been obtained,which are expected to have a certain role in promoting the research on controlled preparation and applications of GF and its composite materials.