Effects Of Ball-milling On The Electrochemical Performances Of Graphene Nanosheets

As a new energy storage device, supercapacitors are intermediatebetween traditional capacitors and batteries, and have a higher energydensity than conventional capacitors and greater power density thanbatteries. The supercapacitors combine the advantage of high energydensity, high power density, long service life, wide temperature range,fast charging speed in one. In addition to the assembly process of thesupercapacitors, the electrochemical performance of supercapacitors islargely determined by the quality of the electrode material. Due to highspecific surface area, high charge mobility and the lamellar structure,graphene is considered to be a competitive material for supercapacitors.On the basis of predecessors’ experiences and results of ourlaboratory, we ball milled the expanded graphite which was prepared bythe oxidation-thermal explosion method to prepare the graphenenanosheets （GNSs） and applied them as the electrode materials forsupercapacitors. And we modified the process of ball milling, used theliquid ball milling in stead of the air ball milling. The morphologies and structures of samples were characterized by SEM, XRD, nitrogenadsorption measurement, FT IR, Raman spectrum and electrochemicalmeasurement. Then we discussed the milling process for the structure ofthe graphene nanosheets and the surface area, the impact of functionalgroups, the regularity of the change of the electrochemical performance.The results of experiment show that the milling process led to thegraphene microstructure refinement, the increase of surface oxygenfunctional groups and the decrease of surface area and so on. With theincrease of milling time, the specific capacitance of the samples whichproduced by three milling methods first increased and then decreased.Among them, with the current density of100mA·g^-1, air ball millingsamples reached the greatest242.8F·g^-1in3h, raised by32.6%than thenon-milled sample. Moreover, the ethanol ball milling samples achieved181.1F·g^-1in4h, the cyclohexane ball milling achieved201.3F·g^-1in3h.Further we investigate the electrochemical properties of the sample.Combined with cyclic voltammetry and AC impedance test, we discussthe mechanism of the milling process of graphene structure, and how theball milling method improved electrochemical properties of GNSs.