Controllable Preparation And Electrochemical Properties Of Graphene-based Composites

Supercapacitor,also known as electrochemical capacitor,has attracted great attention,due to their high power density,fast charge-discharge process and long life cycles.The performances of supercapacitor are mainly determined by the electrode materials.Therefore,to develop high-performance electrode materials is significant.Graphene,as a new type of two-dimensional materials with ultra-high theoretical specific surface area and outstanding physical and chemical properties,has been regarded as an ideal material for supercapacitor.However,the poor electroconductivity,easy agglomeration and finite specific capacitance of graphene,prepared by chemical method,limited its application for supercapacitor.Polyaniline(PANI),as a typical pseudocapacitance material,has attracted widely research due to ease of synthesis,good chemical stability and high pseudocapacitance.However,PANI is repeatedly effected by the doping and dedoping,as a result,the structure is easy to change or even collapse,causing the decay of specific capacitance.Consequently,this paper is aimed at the preparation and characterization of graphene-based composites and their application in supercapacitors in order for the high specific capacitance and good cycle stability.Chemical doping with heterogeneous atoms is an effective method to improve the intrinsic properties of graphene.In this work,a competitive type of highly nitrogen-doped graphene(HNG)and its composites hybridized with Cobaltosic oxide(Co3O4)nanoparticles were synthesized via simple pre-mixing followed by a hydrothermal method.The highest nitrogen content achieved was 11.7%,which greatly improved the electrical conductivity and inhibited the reunion of graphene.And thus the graphene exhibited superior electrochemical performance.Co3O4 nanoparticles could be obtained with an optimal size of approximately 80-100 nm and were homogenously anchored on the graphene nanosheets.Because of the synergy between the highly nitrogen-doped graphene and uniform Co3O4 nanoparticles,the HNG/Co3O4 hybrids exhibited enhanced rate capability and cycling stability,which were considerably higher than those of bare HNG and Co3O4.As the current density increased from 0.2 to 5 A g-1,80.6%of the specific capacitance was retained.Moreover,over 84.5%of the original specific capacitance was maintained.This study has demonstrated the significance of nitrogen doping and modifying on the performance of graphene-based materials for supercapacitor electrodes.In order to explore the effect of graphene surface chemistry on electrochemical performance based on polyaniline-graphene hybrid material electrodes,four different polyaniline-graphene nanocomposites were fabricated through selecting graphene oxide,reduced graphene oxide,aminated graphene and sulfonated graphene as carries,respectively.The nanocomposites feature variant structure and morphology,which could be served to more deeply understand the morphology effect and structure effect caused by surface chemistry on the electrochemical performance.The experimental results reveal that the functionalized graphene with electronegativity were conducive to the vertical and neat growth of PANI nanorods.The array architecture endowed the PANI-GS nanocomposite with largest ion-accessible surface area and highest-efficiency electron and ion transport pathways.Meanwhile,the introducing of sulfonic acid functional groups accelerated the redox reaction with doping and dedoping of the PANI.Therefore,the PANI-GS nanocomposite exhibited the highest specific capacitance of 863.2 F g-1 at a current density of 0.2 A g-1 and the best rate capability of 67.4%(581.6 F g-1 at 5 A g-1),which were much better than the other three nanocomposites.An effective design and fabrication of more steady structure for high-performance electrodes applications still remains a challenge.Herein,we have designed and fabricated a hierarchical heterostructure of graphene@polyaniline@graphene sandwich consisting of the hollow polyaniline spheres as the sandwich layer,and graphene both as an internal skeleton shell and a cladding layer.The special sandwich configuration not only enlarged the specific surface area,but also improved the electrical conductibility.Most importantly the graphene double shells could prevent the structural breakdown(swelling or shrinkage)of polyaniline.Therefore,as a supercapacitor electrode,the hybrid exhibited excellent performance with a specific capacitance of 682.75 F g-1 at 0.5 A g-1 and a remarkable cycling stability with capacitance retentions of 92.8%after 1 000 cycles and even 87.6%after 10 000 cycles,which were better than those of pure polyaniline.In addition,the specific capacitance could reach 217.11 F g-1 at a high current density of 20Ag-1.Using covalent chemical grafting is an effective method to improve the cyclic stability of PANI.A novel route was introduced to synthesize hierarchical polyaniline-grafted reduced graphene oxide(PANI-rGO)hybrid materials by polyaniline nanorods covalently bonded on the surface of rGO.Aminophenyl groups were initially grafted on rGO via diazonium treatment.Then the PANI nanorods were aligned vertically on the rGO to construct a three-dimensional(3D)structure.The 3D structure could shorten the electronic transmission path and form abundant space for electrolyte ions.Most important,the structural stability of PANI was improved,and thus led to excellent cyclic performance.The PANI-rGO hybrid materials fabricated as supercapacitor electrode exhibited a maximal specific capacitance of 1045.51 F g-1,and good rate capability,and after cycling 1000 times,the specific capacitance could maintain 94.8%of the initial value.The energy density(E)could achieve an upper value of 8.3 Wh kg-1 at the current density of 0.2 A g-1 simultaneously.