Preparation And Supercapacitive Performance Of Functional Graphite Nanosheet Array And Its Composites

Recently, there have many reports about graphene as ideal electrode materials.Although graphene with the theoretical surface areas of2630m2·g^-1is considered tobe a very competitive electrode material because of its good chemical stability,electrical conductivity, thermal conductivity, mechanical strength and flexibility, thesurface inert and serious agglomeration of usually limit its electrochemicalperformance. However, graphite oxide, a graphene derivative, has richoxygen-containing functional groups on the surface, which leads to highelectrochemical activity and surface hydrophile natures. If one can construct a uniquecarbon structure that the graphite nanosheets consist of outer layers like grapheneoxide and internal layers of pristine graphene, which gathers the conductivity ofgraphene and hydrophile and the electrochemical activity of graphite oxide both, weare able to obtain a new carbon material. In this paper, an electrochemical method isused to exfoliate graphite rod substrate to form graphite nanosheet array with oxygencontaining functional groups on the surface. On the basis of these graphite nanosheetarrays, several composites are designed and synthesized by using anodic oxidationand cathodic reduction electrodeposition and gas reduction of KMnO₄. The necessaryanalytical method and technique are used to characterize the morphology, crystalphase, structure and conponents of the resultant materials. Moreover, theelectrochemical performances of the as-prepared samples are investigated in details.The main contents are following:1. A novel graphite nanosheet array is prepared for supercapacitors byelectrochemically exfoliating the graphite rods. In practical sense, these graphitenanosheets are a multilayer graphene of which the number of layers is various.However, there are rich oxygen-containing functional groups on the surface of thenanosheets. The large surface area, electrochemical activity and wettability foraqueous electrolyte solutions as well as high overpotential for evolution of gases（hydrogen or oxygen） are the primary features of the graphite nanosheets, which isresponsible for superior electrochemical capacitance and wide potential windows.Furthermore, the space interval between the parallel graphite nanosheets provides an accessible micro-reservoir for electrolytes. Electrochemical measurements show thatthe resultant graphite nanosheet array electrode achieves a specific capacitance of5030F·m^-2within a potential window up to2.2V （ranging from-1.4to0.8V） at thescan rate of10mV·s^-1in0.5mol·L^-1aqueous Na₂SO₄. Moreover, the as-preparedarray electrode reaches specific power as high as653W·m^-2while holding an energydensity of1.99Wh·m^-2at the given conditions. In addition, the characterizationsabout the graphite nanosheets are performed to understand the crucial influence ofphysical and chemical properties and structures on capacitive behaviors deeply.2. The ZnO film with different morphologies is electrochemically deposited onthe graphite substrate with different structure and surface hydrophile natures. It isfound that the spindle-shaped ZnO nanoparticles arrange to the film on the graphitesurface with disordered microstructure and weak hydrophilic natures. After anelectrochemical exfoliation, the surface on the graphite rod form graphite nanosheetsarray with oxygen containing functional groups on the surface which results in thesuper hydrophilic natures of the surface. The electrochemical deposition of ZnO onthis unique graphite substract produces a3D network film with porous structurewhich facilitates fast transport of ion the electrolyte. The spindle-shaped and3Dnetwork ZnO film electrodes achieve specific capacitance of220.29and326.17F·g^-1within a potential window up to2.2V （ranging from-1.4to0.8V） at the scan rate of5mV·s^-1in0.5mol·L^-1Na₂SO₄, respectively.3. After an electrochemical exfoliation, the surface on the graphite rod formgraphite nanosheets which their layers are various in numbers and parallel to eachother in space arrangements and perpendicular to the carbon substrate, resulting in anopen2D nanosheet array. Manganese oxide is uniformly loaded on the surface ofgraphite nanosheets by anodic oxidation electrochemical deposition. As a result, theelectrochemical activated graphite carbon/manganese oxide composite membrane isobtained. Electrochemical measurements show that the composite membrane haveexcellent performance in ultra-capacitance and achieve a specific capacitance as highas7750F·m^-2within a potential window up to2.2V at the scan rate of5mV·s^-1in0.5mol·L^-1aqueous Na₂SO₄.