Preparation And Supercapacitor Applications Of Graphene Based Nano-composites

Graphene is an ideal material for energy-storage applications owing to its unique two-dimensional geometry and excellent physical and chemical properties. The energy-storage field including batteries and supercapacitors will move forward continually as the development of the mass production of graphene materials. Supercapacitors can be used in a wide range of civil and military applications due to some advantages such as low cost, high power density, long cyclic life. The inferior energy density over traditional batteries still limits the development of supercapacitors. Accordingly, graphene based supercapacitors with high energy-storage capability attract tremendous interests. Oxidation-reduction synthesizing method is considered as a promising and important one towards scalability among so many approaches of synthesis of graphene attributed to its facility, efficiency and repeatability. The present work aims at the synthesis of graphite oxide and graphene nanosheets by using the oxidation-reduction approach, flexible graphene paper and graphene based nano-composites by various methods. Meanwhile, the supercapacitor electrochemical performances based on those graphene materials were investigated systematically, which can open up a pathway and offer a technical support on the mass production of graphene materials and the fabrication of ultrathin high power/energy densities electric double layer capacitors and graphene based pseudo-capacitors.Serving as one of the most important precursors for synthesizing graphene by oxidation-reduction method, the structural characteristics of graphite oxide strongly affect the structures and properties of the pre-synthesized graphene materials. In this work, the oxidation degree of graphite oxide was tailored by various oxidized reactions which were used for unzipping single wall carbon nanotube. As compared with the traditional Hummers method, high oxidation degree of graphite oxide can be obtained without the emission of toxic gases such as NOX and less exo-therm involved. The as-synthesized graphite oxide with various oxidation degrees was evaluated by a wide range of advanced characterization technologies including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV-vis spectroscopy, thermal gravimetric analysis (TGA) and Raman spectroscopy:the interlayer d-spacing of graphite oxide can be up to0.9nm, a large quantity of oxygen containing functional groups (hydroxyl, carboxyl, epoxy) were distributed on graphene basal plane. The sp3hybrid oxidized carbon corresponding to the high oxidation degree was65%relatively, which is favorable to form high quality graphene nanosheets. In addition, the reaction conditions of synthesizing graphene by thermal shock method were also investigated and the sp2carbon of reduced graphene was recovered at a large extent. The morphology and microstructures were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The surface area of porous graphene nanosheets measured by Brunauer-Emmett-Teller (BET) is550m2/g.To explore a kind of flexible high performance supercapacitor, the robust graphene paper can be produced by a vacuum filtration method based on the high dispersed graphene colloid synthesized from graphite oxide precursors through electrostatic repulsions stabilization. The individual graphene sheet existing in the graphene paper may self-restack easily due to the van der Waals attractions, which strongly affects the electrochemical behaviors of supercapacitors. By incorporated with carbon black nanoparticles, the pillared graphene paper with better performances remains three-dimensional network, leaving open and smooth diffusion paths for ions and enhancing the electrical connection and transport. The morphology and microstructures were characterized and the electrochemical performances were analyzed by cyclic voltammetry, galvanostatic charge/discharge, electrochemical impedance spectroscopy (EIS):The pillared graphene paper exhibits a more than700%improvement over the original graphene paper electrode at a fast scan rate of500mV/s and remains a specific capacitance of80F/g. The maximum specific capacitance of the pillared graphene paper is138F/g and83.2F/g in6M KOH aqueous electrolyte and1M LiPF6organic electrolyte at room temperature, respectively, with only3.85%and4.35%degradation at a current densisty of10A/g after2000cycles. The calculated maximum energy and power density is26Wh/kg and5.1kW/kg, respectively. The specific capacitance remains a linear relationship to the paper thickness, demonstrating the electrode is fully scalable with the addition of layers without degrading its superior performance, which is favorable to increase the volumetric specific capacitance and propel the progress of mass production.To further explore various kinds of graphene based pseudo-capacitors, a novel graphene/MnSn(OH)6nano-composite was synthesized by a co-precipitation method. A series of different crystallinities graphene/MnSn(OH)6nano-composite were prepared by tailoring conditions to evaluate the impacts on the electrochemical performances. The morphology, microstructures were characterized. As pseudo-capacitive electrode materials, the electrochemical performances were measured:The graphene/MnSn(OH)6nano-composite displayed much improved electrochemical performances (more than300%improvement) as compared to the monolithic MnSn(OH)6nanoparticles alone, demonstrating graphene nanosheets play an important role. The total specific capacitance depended strongly on the crystallinity of the MnSn(OH)6nanoparticles, where the materials with a poor crystallinity showed the maximum specific capacitance of59.4F/g at the scan rate of5mV/s based on the quality of MnSn(OH)6nanoparticles. The improved crystallinity leaded to increase the diffusion resistance and decrease the ionic, electrical transport capability of the electrode. There was no obvious capacitance degradation after500cycles, indicating this kind of MnSn(OH)6nanoparticles possess a good stability under electrochemical conditions.Serving as an example applied at the pseudo-capacitors, the graphene/TiO2nano-composite was synthesized by the advanced atomic layer deposition (ALD) technology under various cyclic conditions. The TiO2nanoparticles can be deposited uniformly on the surface of the porous graphene nanosheets. The morphology, microstructures were characterized. The pseudo-capacitive electrochemical performances results indicate:The100ALD cycles graphene/TiO2sample showed the improved specific capacitance of84and43F/g (106and54F/g contributed from the pure TiO2) at the scan rates of10and150mV/s, respectively, as compared with previous reports. There are no degradations by increasing the electrode mass loadings, indicating the ALD holds a promising in the field of the graphene based pseudo-capacitors and may propel the industrial progress.