Studies On Preparation Of Graphene-based Supercapacitors And Transparent Conductive Films

As an emerging two-dimensional carbon nanomaterial,graphene possesses excellent electrical,optical and mechanical properties and a wide range of potential applications in energy storage and optoelectronic materials and devices.However,due to various reasons,it has not yet been applied in practice.In this thesis,two most promising applications of graphene,supercapacitor and transparent conductive film,are studied as follows::1.Preparation Characterization and Properties of High Performance Graphene-based SupercapacitorsGraphene is often used to fabricate supercapacitors because of its high specific surface area,good electrical conductivity and light flexibility.Among many methods that have been developed,graphene prepared by chemical oxidation and reduction of graphite is the most likely way to achieve large-scale production and application as electrode material for supercapacitors.However,graphene sheets obtained by conventional solution reduction method are prone to aggregate,lowering the specific surface area and resulting in a small mass specific capacitance.In recent years,various graphene materials with three-dimensional structure and high specific surface area have been developed to improve mass specific capacitance.However,due to their loose and low-density structures,their volumetric specific capacitances were not ideal,which limits their practical application.In this thesis,a novel strategy has been developed for preparing self-supporting graphene films for supercapacitors with both high mass specific capacitance and high volume specific capacitance.Firstly,graphene oxide was prepared by the modified Hummers method and dispersed in an aqueous solution.After castintg from this aquous solution and drying at low temperature,graphene oxide films were obtained.Then,the films were subjected to heat treatment at different temperatures in air,affording a serious of self-supporting graphene films.It was found that such heat treatment in air condition can significantly improve the capacitive properties.When heat treatment temperature was 300?C,the resulted film displayed the best capacitive behaviors among the family.Its mass specific capacitance reached 423 F/g at a current density of 0.5 A/g.Since its density was 0.9 g/cm~3,the volume specific capacitance of this graphene film was 370 F/cm~3,among the best values in literature.In order to check its practical application,a symmetrical supercapacitor was assemblied and obtained a mass specific capacitance of 300 F/g.Meanwhile,the device showed a relatively high energy density of 60 Wh/kg and power density of 1200 W/kg.This is of great significance for the practical application of graphene-based supercapacitors.2.Preparation Characterization and Properties of Graphene/Metallic Nanowires Transparent Conductive FilmsTransparent conductive films(TCFs)are important part of many optoelectronic devices which are mainly prepared from indium tin oxide(ITO)currently.However,ITO materials are short of resources,expensive and not suitable for flexible devices.Because of its low cost and solution-processability,graphene has been considered as one of ideal ITO alternatives.However,since graphene sheets prepared by the chemical reduction method have a large number of lattice defects,the so-based transparent conducting films generally have a high surface resistance,and thus they can not meet the practical needs.To address this issue,we propose a new strategy to reduce surface resistance of graphene-based TCFs by hybriding one-dimensional copper nanowires and two-dimensional graphene.In our experiment,we firstly introduced copper nanowires into the graphene dispersion prepared by chemical reduction method.Then,by using membrane filtration-transfer technique,a series of TCFs based on graphene/copper nanowires composite were prepared.It was found that the transparency and conductivity of so-prepared TCFs can be adjusted by the ratio of graphene to copper nanowires.Moreover,compared with the TCFs prepared by graphene alone,the hybrid TCFs have much lower surface resistance under the same transmittance.It was also found that two-dimensional graphene and one-dimensional copper nanowires have synergistic effects on the film conductivity and transparency.Moreover,the graphene-copper nanowire composite not only improves the flexural and folding resistance of copper nanowires,but also greatly improves their oxidation resistance.The graphene/copper nanowire composite film can maintain good stability even under the extreme conditions of 80?C and 80%relative humidity.This is of great significance to its practical application.