Mechanical Ball Milling For Fabricating Graphene-based Composites And Their Supercapacitive Properties

With the advantages of high power density,safety,stability,long service life,wide working temperature and good cycle stability,Among them,carbon-based materials mainly contribute double-layer capacitance,which has excellent rate performance and cyclic stability,but low specific capacity;transition metal oxides and conductive polymers mainly contribute pseudo-capacitance,its specific capacity is high(usually up to several times to dozens of times of carbon-based materials),the synergistic effect can be exerted and the comprehensive properties of composite electrode materials can be improved.Graphene is usually prepared by oxidation-reduction method and vapor deposition method in advance,and then graphene composite electrode materials are prepared by in-situ growth or out-of-situ recombination.These methods may involve strong acids,strong oxidants,or cumbersome synthesis steps and high preparation cost.The industrial application of supercapacitors is limited.The preparation of graphene by mechanical ball milling stripping method is a green,low-cost and easy-to-operate method.In this paper,using natural graphite as raw material,aniline,cobalt salt or block copolymer as multifunctional reagents(intercalation agent and precursor),graphene was prepared by mechanical ball milling,and Polyaniline(PANI),cobalt tetroxide(Co₃O₄)or porous carbon(MC)were loaded in situ.A series of graphene-based composite electrode materials were synthesized in one step.The work of this paper includes the following aspects:(1)graphene/polyaniline composite electrode material(Graphene/PANI)was prepared by one-step mechanical milling process using graphite powder as raw material and aniline as multi-functional reagent.Aniline not only acts as graphite intercalation agent to promote graphite peeling off to graphene,but also takes place as a monomer for in-situ polymerization.Polyaniline(PANI),on graphene surface can not only contribute pseudo-capacitance,but also help to stimulate synergistic effect.Improve the conductivity and ion diffusion of the composite electrode,so as to improve the specific capacitance and magnification ability of the supercapacitor.The composite electrode has excellent electrochemical performance and can provide a high specific capacitance of 886 F g^-1 at 5 m V s^-1,and the capacitance retention is73.4%when the sweep speed is increased to 100 m V s^-1,while the specific capacitance of pure PANI is 649 F g^-1murl at 5 m V s^-1 and 66.6%at 100 m V s^-1.In order to explore its application in practical equipment,the symmetrical supercapacitor composed of Graphene/PANI was tested,and the specific capacitance of the capacitor reached 294.4 and 256 F g^-1 at 0.5,10 A g^-1,respectively.(2)Graphene/Co₃O₄ composite electrode materials(Graphene/Co₃O₄)were prepared by one-pot ball milling using graphite as raw materials,(NH₄)₂CO₃ and Co(CH₃COO)₂ as intercalation agents and Co₃O₄ precursors.(NH₄)₂CO₃ as graphite intercalation agent can decompose and produce gas to promote graphite exfoliation.At the same time,it can also react with cobalt salt to form Co₃O₄ nanocrystals.The prepared Graphene/Co₃O₄ electrode shows high specific capacitance of 570 F g^-1 at 1A g^-1.When the current density is increased by 20times,the capacity retention is as high as 93%.Its excellent electrochemical performance can be attributed to the synergistic effect between highly conductive graphene lamellae and nanostructured Co₃O₄ to accelerate electron conduction and ion transport.In order to further improve the energy density of supercapacitors,asymmetric supercapacitors(Graphene//Graphene/Co₃O₄)were assembled using graphene and Graphene/Co₃O₄ as negative and positive electrodes respectively.The working voltage window of the asymmetric supercapacitor is widened to 1.6 V,so the high specific capacitance of 190 Fg^-1 is realized at1A g^-1.(3)graphene/mesoporous carbon/manganese tetroxide composite electrode material(G/MC/Mn₃O₄)was prepared in situ by mechanical ball milling using graphite as raw material,polyoxyethylene polyoxypropylene block copolymer(F127)as pore-forming agent and tannin and manganese acetate as precursors.The specific capacitance of the composite electrode material is 219 F g^-1 at 0.5 A g^-1,and the specific capacitance retention is 68%when the current density is increased by 20 times.Using graphene/mesoporous carbon(G/MC)as negative electrode and G/MC/Mn₃O₄ as positive electrode,an asymmetric supercapacitor is assembled.The operating voltage window of the device can reach 1.8 V,the high specific capacitance of138 F g^-1 is obtained at 0.5 A g^-1,and the energy density is as high as 62.1 Wh kg^-1 when the power density is 0.45 Kw kg^-1.