| Summary: | 博士 === 國立交通大學 === 材料科學與工程學系所 === 107 === With the rapid depletion of fossil fuels and the rising controversy over their use, the development of energy storage devices is becoming increasingly necessary for the global community. Graphene, a 2D single-layered carbon material having honeycomb-like lattices, has drawn much attention for its extraordinary electrical and thermal conductivities, elasticity, transparency, and specific surface area so that it becomes one of the most feasible materials used in energy storage devices including batteries and supercapacitors. Here, we describe a production of graphene materials based on electrochemical cathodic plasma exfoliation method, and use different ions in the process to produce graphene nanosheets with different morphologies. The morphology of graphene nanosheets produced by using NaOH as the electrolyte is in the form of sheets; the onion-like graphene nanosheet can be produced by using H2SO4 as an electrolyte. The latter sample has a higher specific surface area, defects, and oxidation levels, that result in higher electrochemical performances. Furthermore, doping of metal-free heteroatoms (e.g., boron, nitrogen, oxygen, phosphorus, sulfur) into graphene has been used to enhance its electrochemical properties. Among them, nitrogen atoms have been among the most popular dopants because of their similar radii to the carbon atom, compatibility with carbon lattices, rich content of electrons, and ready accessibility from common chemicals. Herein, we described two ways to produce nitrogen-doped graphene based on electrochemical cathodic plasma exfoliation process and the energy-storage properties of the as-produced materials were investigated as the electrode materials of supercapacitors and lithium-ion batteries. For the first method, electrolytes that contain an appropriate concentration of NH4OH or urea in an alkaline solution (NaOH) can be used to dope nitrogen atoms into carbon lattices. Also, nitrogen-doped graphene nanosheets can be produced by using aqueous HNO3 as the electrolytes. X-ray spectroscopy revealed that the concentration of nitrogen atoms doped in the graphene nanosheets varied from 0.6 to 0.81 at%. Measurements of supercapacitive performance at a scan rate of 5 mV s–1 in 1 M NaCl indicated that graphene nanosheets produced via aqueous NaOH as electrolyte, has a higher specific capacitance of 21.6 F g–1, that is larger than that of graphite powder (11.4 F g–1); the calculated specific capacitance of N-doped graphene nanosheets produced via aqueous HNO3, NaOH+Urea, and NaOH+NH4OH were 54.2, 34.6, and 30.5 F g–1, respectively, ascribed to the nitrogen-doping and the higher specific surface area of those samples. In the second method, we used a home-made composite cathode, containing graphite powder, poly(methyl methacrylate) and melamine, to produce nitrogen-doped graphen nanosheets. Interestingly, higher and better tunable levels of nitrogen in graphene nanosheets were produced by tuning the concentration of melamine in the graphite/melamine composites. The specific capacitances were measured at the same conditions revealing even better supercapacitive performances than the as-produced materials by the first method. The calculated specific capacitance of the sample exfoliated from the composite electrode without melamine (27.3 F g–1), is larger than that of graphite powder (11.4 F g–1), which can be ascribed to the thinner morphologies and higher specific surface area of exfoliated graphene nanosheets by electrochemical cathodic plasma process. The calculated specific capacitance of the sample derived from the composite electrode with 5% and 9% of melamine were 64.1 and 74.2 F g–1, respectively, presumably because the more highly disordered graphene structure, nitrogen-doping active sites and the greater specific surface area, providing extra volume and area for the absorption of ions during charging and discharging. Besides supercapacitive applications, the nitrogen-doped graphene nanosheets produced by the second method further served as the anode materials of lithium-ions batteries and the sample with nitrogen 4.63 at% displayed excellent performance with a capacity of 488 mA h g–1 at 100 mA g–1 after 100 cycles that was better than that of the device incorporating pristine graphene sheets (344 mA h g–11 at 100 mA g–1 after 100 cycles). This method is a new and tunable approach for producing nitrogen-doped graphene nanosheets from graphite rod to powder at room temperature, and has potential for use in the industrial-scale mass production for energy storage applications.
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