Preparation Of Graphene Composite Materials And Their Applications In Supercapacitors And Electrochemical Sensors

Graphene,can regard as "single-layerd graphite",is a new nano-material followed by the discovery of carbon nanotubes and fullerenes.The appearance of graphene promotes the construction of a complete carbon paradigm from zero-dimensional to three-dimensional.Graphene has been received much attention due to its distinctive properties,which is widely used in the fields of supercapacitors,lithium ion battery,catalytic oxidation,electrochemical biosensors,and so on.However,the two-dimensional(2D)graphene sheets easily tend to aggregate or restack,which has greatly hindered the practical applications of graphene.It is possible to assemble 2D graphene sheets into three-dimensional(3D)gels,such as graphene hydrogels(GHs)with 3D structure,which can prevent the occurrence of stacking phenomena and retain the advantages of graphene.GHs can prepared by graphene oxide(GO)with self-assembled via functional interactions force,hydrogen bonds,and π-π conjugate bond.At present,some pure GHs,hydrogel composite based on graphene(such as composite of graphene with transition metal oxides/hydroxides or conductive polymer)as electrode material of supercapacitor.The composites have many advantages compared with the traditional carbon materials.The GHs were prepared by a modified hydrothermal method and G-Ni-Co ternary composites were obtained by the hydrothermal method and used them as supercapacitor electrodes materials.The morphologies and electrochemical performances were tested.Furthermore,on the basis of the previous work,the pristine graphene(PG)was also prepared via a liquid-phase method in low-boiling point mixed-solvent system such as IPA-H2O with addition of salts.The as-prepared graphene could be directly applied to the electrochemical sensor.The paper mainly includes the following three parts:(1)The GHs-Glu was prepared via a one-step hydrothermal used the glucose as the reductant.The Glu not only conductive the reduction of GO,but also enrich the pore structure and increase the specific surface area of GHs.The dried GHs-Glu has a good electrical conductivity of 95.3 S/m.In a three-electrode system,the specific capacitance of the GHs-Glu is 362.8 F/g at the scan rate of 10 mV/s and 384.6 F/g at the current density of 1 A/g.In a two-electrode system,at the scan rate of 10 mV/s and the current density of 1 A/g,the specific capacitance of the GHs-Glu is 263.2 and 202.4 F/g,respectively.The cyclic stability of GHs-Glu was tested.The specific capacitance can still retain 88%after 5000 cycles at a scan rate of 100 mV/s.(2)The G-Ni-Co ternary composite with surpior performances was prepared by adjusting the mass ratio of Ni(NO3)2·6H2O toCo(NO3)2·6H2O(Ni-Co),the mass ratio of GO to Ni-Co,and pH of reaction system.The G-Ni-Co composite was assembled into a symmetric supercapacitor(G-Ni-Co//G-Ni-Co)and its electrochemical performances were tested.In a two-electrode system,at the scan rate of 10 mV/s and the current density of 1 A/g,the specific capacitance of the G-Ni-Co is 454.5 and 400.0 F/g,respercively.The specific capacitance still retains 82%after 1000 cycles at a scan rate of 100 mV/s.The power density and energy density of G-Ni-Co//G-Ni-Co were also calculated.The energy density is 16.8 and 11.0 W·h/kg at the power density is 1154.2 and 5839.4 W/kg,respercively.(3)The pristine graphene(PG)can be easily prepared in low boiling-point system such as IPA-water mixture with or without salts as assistant via liquid-phase exfoliation method.The highest exfoliation efficiency was appared in IPA-water mixture(IPA volume fraction is～30%)without salts.However,the proper volume fraction of IPA was changed if salts were added.The concentration of PG dispersion reaches as high as 0.56 mg/mL in the IPA-water mixtures containing 80%IPA with Na4P2O7·10H2O.The exfoliation efficacy was effectively promoted by Na4P2O7·10H2O.The PG film prepared by drop-casting method shows an excellent electrical conductivity(7.095 × 104 S/m).Furthermore,an electrochemical biosensor based on the PG shows high selectivity and sensitivity for the determination of dopamine.The linear detection range for dopamine is 2.5-1500 μM with detection limit of 1.5 μM.