Synthesis Of Graphene-based Composites And Its Application As Electrode Materials For Energy Storage Devices

Graphene is a new two-dimensional honeycomb-like carbonaceous material. Due to its outstanding mechanical properties (1060 GPa), large specific surface area (2600 m~2/g) and good electrical conductivity, graphene has been successfully applied to many areas of nanoelectronic devices, photonic sensors and gene sequencing etc.. The excellent energy storage performance of graphene is believed to be an important research direction. Compared to the commercial graphite electrode, the theoretical energy storage capacity of graphene is nearly three times of that, which may provides new train of thought in response to the growing energy needs. However, an obviously drawback of graphene is the structural defects which causes the crystallization, resulting in the high irreversible capacity and poor cycle performance. Recently, nanometer quantum dots have been researched extensively owing to its excellent optoelectronic properties. The deposition of nanometer quantum dots on graphene sheets is expected to prevent the aggregation of the single-layer graphene. Therefore, it is important to focus on the synthesis of graphene-based composites and its application as electrode materials for energy storage devices.Single-layer graphene oxide nanosheets were prepared by oxidating and exfoliating natural graphite using a Hummers method, and the graphene oxide and Zn(NO3)2 (Cd(NO3)2) were then refluxed in dimethylsulfoxide under heating condition and finally formed graphene/ZnS(CdS) quantum dot composites, the preparation process was optimized. It is found that graphene synthesized from imported graphite has larger specific surface area, attributed to the raw materials posses smaller crystal size and more disordered particles. Besides, quantum dots were better distributed in G-1% QdS with a uniform size.The microscopy morphology of composites was systematically investigated by SEM, TEM and AFM. The result of AFM analysis confirmed the single graphene sheet was successfully prepared by in-situ method. The IR, XRD and Raman analysis confirmed the structure and composition of graphene/ZnS(CdS) composites.The electrochemical performance of composites in Lithium-ion batteries was examined. Ac impendence spectra revealed that there is a fine solid electrolyte interphase film (SEI) on the surface of the composites, the fact shows that lithium ion can reversibly insert/extract the Electrode. The initial discharge capacities of composites and graphene are about to 1200 mAh/g, after 30 cycles, the reversible capacities are 900 mAh/g and 228.9 mAh/g , which attenuations of the initial capacity are 25% and 80.9%, respectively. Above data have demonstrated that addition of the quantum dots into the graphene layers obviously improves the electrochemical performances of the graphene. On the other hand, G-ZnS displayed better cycle performance, the reversible capacity is about 2 times of G-CdS at the current density of 200 mA/g. The time-resolved fluorescence spectroscopy analysis showed the electron transfer process from the excited ZnS to the graphene matrices only need a time of 5 picosecond, confirming the excellent conductivity of graphene layers.The electrochemical performance of composites in supercapacitors was examined. Galvanostatic charge/discharge curves displayed symmetrical triangular waves, indicating the excellent capacitance of graphene/quantum dot composites. The specific capacitance of G-1% ZnS and G-1% CdS are 93 F/g and 90 F/g at the current density of 0.5 A/g, higher than G (87 F/g).In summary, the good distribution of quantum dots not only prevent the aggregation effectively, but also improve the electronic conductivity of graphene. The energy storage capacity and cycle performance of the composites are significantly better than graphene and natural graphite. Therefore, this new class of the composites can be widely used in lithium-ion batteries and supercapacitors.