Effects Of Ball-Milling On The Morphologies And Capacitive Performances Of Graphene Nanosheets

Supercapacitor is a new-generation energy storage device after lithium ion battery and attracts worldwide researchers'interests as a promising high-power energy source in many fields. Many porous carbon materials have been investigated for supercapacitor electrode materials for their high specific surface areas and chemical stability. However, their specific surface capacities are always much lower than the theoretical value, which should be attributed to the anfractuous pore-texture and poor electric conductibility. Graphene as a two-dimensional layer with one atomic thickness has been proposed a competitive material for supercapacitors applications for its high specific surface area, electric conductivity and simple layer structure.In this thesis, graphene nanosheets (GNSs) were synthesized by grinding the expanded graphite which was prepared from natural flake graphite and artificial graphite via the oxidation-thermal explosion method and were applied as the electrode materials for supercapacitor. The morphologies and structures of both expanded graphite and GNSs were characterized by Scanning Electron Microscope, Transmission Electron Microscope, X-ray Diffraction, nitrogen adsorption measurement, Fourier transform InfraRed spectrum and Raman spectrum mea. Electrochemical performances as electrode materials for supercapacitor were studied by constant current Galvanostatic charge/discharge test, cyclic voltammetry and alternating current impedance in 30wt.% KOH electrolyte.The results show that expanded graphites prepared from different raw materials all have a worm-like porous structure. Expanded artificial graphite has not only a larger specific surface area of 524m-g-1 than that of expanded natural flakes graphite,358m2·g-1 exactly, but also a higher expansion volume. Under the current density of 100mA·g-1, the specific capacity of expanded artificial graphite is obviously higher than expanded natural flake graphite,196F·g-1 and 157F·g-1 respectively. However, its capacity retention is lower than the latter as the current density increasing to 1000mA·g-1 gradually. After milling, the worm-like porous structures are broken into randomly-oriented graphene nanosheets with a size distribution from tens to hundreds nanometers. The stacking height of GNSs samples present a decreasing-increasing trend and reach a weakest point at 3h milling. Accordingly, the specific capacities of GNSs increase obviously after milling as the milling time increases from 2h to 6h, and GNS-3h electrode achieves a highest specific capacity of 211F·g-1 at a current density of 100mA·g-1. For GNS-4h and GNS-6h, although specific capacity values obviously fall back, 170F·g-1 and 167F·g-1 respectively, there is a significant improvement compared to that of expanded natural flakes graphite. The reasons should be attributed to the simple layer structures of GNSs samples, lattice defects on the GNSs surfaces produced during the ball-milling process and so on. At the same time, although the specific capacity slightly decreases with the increasing of the current load from 100 to 2000mA·g-1 for each sample, all the GNSs samples keep excellent capacity retentions.