Controllable Preparation, After-treatment, And Electrochemical Capacitive Properties Of Graphene

Graphene is composed of six-carbon-atom rings with single layer asthe basic structural unit for graphitic materials. Since the free standinggraphene monolayer was verified in2004, the electronic, optical,mechanical and thermal properties of graphene have been graduallydeveloped, and graphene has the potential quality to be applied inelectronic devices, composites and energy storage, which have beenwidely investigated nowadays. Graphene is supposed to be the maximumexfoliation of graphite, and holds large theoretical specific surface area(2630m²g^-1) and electronic mobility (200,000cm²Vs^-1), which makesgraphene an excellent candidate for electrode of supercapacitors andlithium ion batteries. The capacitive performances of graphene electrodehave been studied mainly from2008. However, most of the publishedspecific surface area (400^-1000m²g^-1) and capacitance (120-300F g^-1) of graphene are largely far less than its corresponding theoretical values.The high intrinsic specific surface area of graphene can hardly be takenfull use, which weakens the significance by electrochemical research ofgraphene aggregates. According to the ultra-thin2D structure, graphenesheets have a very high tendency to restack themselves during all phasesof synthesis and subsequent electrode preparation, resulting in lowspecific surface area and absent from the excellent intrinsic characteristicsof graphene. Therefore, it is important to detect the intrinsicelectrochemical properties of graphene by the optimization of graphenedesign. In addition, the capacitance contribution of key factors ongraphene sheets can help us to find targeted measures for enhancedcapacitance properties, which will make actual guiding significance toelectrode design.In this paper, largely controllable preparation of graphene nanosheets（GNSs） were realized by chemical exfoliation with changed raw graphitematerials, different oxidation addition and rising centrifugation rate. Theelectrochemical performances of GNS electrodes with certainmorphology and structure were researched in detail. To prevent theagglomeration of GNSs, a kind of capacitance-inert small-powdergraphite （SG） was introduced as “spacers” intercalated into the GNSs,which can be used to investigate the intrinsic capacitance behavior ofGNS electrode. To detect the capacitance contribution of each key factor, GNSs were ball-mlled for different time in Ar atmosphere, which canprovide guidelines for electrode design. The morphology and structure ofGNSs were studied by SEM, TEM, HRTEM, AFM, XRD, Raman and N2adsorption/desorption analysis, and the electrochemical performances ofGNS electrodes were studied by cyclic voltammetry, galvanostaticcharge-discharge cycling, and electrochemical impedance spectroscopyrespectively in two-electrode supercapacitor, three-electrodesupercapacitor, and lithium ion battery.The results indicate that chemical exfoliation can realize controllablepreparation of graphene with certain lateral size and pore structure（mesopores with diameters of3-6nm）. Different addition of oxidantagent in the preparation of graphite oxide （GO） can result in GNSs withdifferent disorder degree and defect density. GNSs from GO with higheroxidation degree will lead to small lateral dimension, whose crystal latticestructure has been severely damaged. GNS55electrode with plenty ofdefects can maintain150F g^-1capacitance under specific current densityof0.1A g^-1for500cycles with superior rate ability and cycling stability.Thus, the defect sites on GNSs are contributing factor for the capacitanceenhance.By accelerating the centrifugation rate in the deionizing process of GO,GNSs with different lateral size and layer thickness were effectivelyextracted with different density under the increasing power of centrifugation. With all ice bath for144h, GO has been fully oxidizedand chemical exfoliated with ultra-thin sheets. With increasingcentrifugation rate from4000r min^-1to15000r min^-1, the sheets ofobtained GNSs are gradually smaller and thinner. The lateral dimensionof10k-GNSs is almost1/6of that of4k-GNSs, while exhibits doublecapacitance as electrode for supercapacitors. Therefore, the GNSsobtained by ultra-high centrifugation rate (10000r min^-1) fully exploit theelectrochemical advantage of edge plane and perform good capacitancebehavior.Raw graphite materials with decreasing particle size （20-2μm） werechosen to prepare GO, the resultant GNS dimensions of which wereeffectively limited by raw materials. Meantime, we investigated thescattering condition of GNSs with varied sheet size under differentultrasonication time （15min and30min）. It is found that15minultrasonication can exactly disperse the GNSs, while time of30min istoo long to destroy the graphene structure with14%capacitance decline.The electrode of GNS with small sheet size （5μm） has plenty of gap sites,which are benefit for the accessibility of electrolyte into electrode, whilethe edge plane also contribute to enhance the electrochemicalperformance.A kind of capacitance-inert small-powder graphite （SG,0.5-2μm） wastaken as “spacers” intercalated into the GNS layers （10-30μm） to prevent agglomeration. To disperse the GNSs as much as possible, we increasedthe content of SG in GNS/SG composite with mass ratio from1:10to1:300. When the mass ratio of GNS/SG comes to1:200, the compositeelectrode maintains a capacitance platform of856F g^-1, which is theintrinsic capacitance of the chemical exfoliated GNSs in our experiment.Based on the theoretical specific capacitance and specific surface area, weelicited and confirmed the relationship between the layer number andintrinsic capacitance of GNSs, which provide an electrochemical methodfor the measure of GNS thickness.Based on the above study, we can conclude that specific surface areaand defect density are the two key factors for electrochemicalperformance of GNS electrode. Here, we ball-milled GNS powder for0.5-21h in Ar atmosphere and obtained ball-milled GNSs with extremelydecreased surface area and large plenty of defects, such as broken carbonrings, edge plane and vacancies. The ordered lattice degree was seriouslydestroyed by long-time ball-milling, resulting in fuzzy carbon sheetswithout clear lattice fringes. The morphology and specific surface areavalue (26m²g^-1) of GNS21are almost equal to those of the raw graphitematerial, but the specific capacitance of GNS21electrode maintain¹80F g^-1, even higher than that (665m²g^-1,165F g^-1) of the original GNS.The large specific capacitance of ball-milled GNS electrode can onlycome from the large plenty of defects on GNSs. Therefore, we quantify the capacitive contribution of specific surface area and defect density forGNS electrode, and provide new strategy for the design of GNS electrodefor energy storage.