The Preparation And Electrochemical Properties Of Nitrogen Doped Graphene Electrodes

Supercapacitor is a new type of energy storage device with fast charge and discharge, high power density and long cycle life, and its capacitive performance is between traditional capacitor and secondary battery. Supercapacitors have a broad application market in electrical or hybrid vehicles, telecommunications, military and other fields.Graphene hydrogel has three-dimensional porous structure with high specific surface area, low density, good thermal stability and electrical conductivity. Therefore it is very important to apply it in the field of supercapacitors. However, the specific capacity and energy density of the pure graphene hydrogel is far lower than the pseudocapacitance materials because of its main application in the electric double layer supercapacitor, which can not satisfy the actual needs. How to keep the high cycle stability and the ability of fast charge and discharge, and at the same time greatly enhance the specific capacity and energy density is the key scientific problem that this paper intend to solve. We deposite high nitrogen doped graphene quantum dots onto graphene hydrogel by electrophoretic deposition method to build nitrogen doped graphene hydrogel three-dimensional electrode materials. On the basis of without destroying the original porous and network structure of graphene hydrogels in favour of electric double layer capacitance, graphene quantum dots, as carbon materials, can play a role in the increase of the electric double layer capacitance. More importantly, the nitrogen activity site in the edge and on the surface of graphene quantum dots as pseudocapacitance active materials will greatly enhance the specific capacity and energy density of the electrode materials. In addition, we also explore to deposite GQDs onto the interdigital finger Au electrodes by electrophoretic deposition to prepare GQDs micro-supercapacitor. The main contents are described as follows:1. The preparation and characterization of GQDs@GH composite materials. High concentrations of nitrogen doped GQDs were loaded into graphene hydro gels by electrophoretic deposition method. The structure and morphology of GQDs, GH and GQDs@GH composite materials were characterized by TEM、AF M、SEM、XRD、XPS、Raman and FTIR analysis respectively.2. The electrochemical properties of GQDs@GH electrode and its application in symmetrical supercapacitor were studied. In a three-electrode system, using 1 M H2SO4 solution as the electrolyte, electrochemical performance of GQDs@GH that influenced by the deposition voltage and time was investigated. And we chose the 30 V, 20 min as the optimum deposition conditions.3. The GQDs@GH electrode has a high area specific capacitance of 1401 mF cm-2 at a current density of 1 mA cm-2,which is far higher than GH(599.9 mF cm-2) at the same current density. When the current density reaches 100 mA cm-2,the electrode still has an area specific capacitance of 1040 mF cm-2, and maintains 74% of 1 mA cm-2, which shows that the electrode has good properties of charge or discharge under large current. Two GQDs@GH electrodes is assembled to a symmetrical supercapacitor. The specific capacity of the symmetrical supercapacitor reaches 1025.6 mF cm-2(579 F g-1) at the current density of 0.2 mA cm-2, and the device has an energy density of 71.2 μWh cm-2(20.1 Wh kg-1). The three GQDs@GH supercapacitors in series can light up a red LED for 40 min after charging for 15.2 s at the current density of 15 mA cm-2, which shows that the GQDs@GH supercapacitor has actual application prospect. After 10000 cycles at 15 m A cm-2, the device can maintain 93% of its initial specific capacitance, which shows that the supercapacitor not only has high specific capacity and energy density, but also has excellent cycle stability.4. Nitrogen doped GQDs were applied in the micro-supercapacitor. We prepared nitrogen doped GQDs by the alkali-catalyzed molecular fusion method, and GQDs were charactered by AFM, TEM, XPS, Raman and so on. Then GQDs were deposited onto the interdigital finger Au electrodes by electrophoretic deposition method to prepare micro-supercapacitor. Using 1 M H2SO4 solution and H3PO4-PVA as electrolyte, electrochemical performances of GQDs micro-supercapacitor were studied respectively. 1 M H2SO4 solution as electrolyte, the specific capacity of the GQDs micro-supercapacitor reaches 41.8 mF cm-2 at the current density of 50 μA cm-2, the device has an energy density of 5.8 μWh cm-2. The micro-supercapacitor displays an approximately 91% retention of its initial specific capacitance after 5000 cycles at the scan rate of 1 V s-1. When H3PO4-PVA is as electrolyte, the specific capacity of the all-solid GQDs micro-supercapacitor reaches19.3 m F cm-2 at the current density of 10 μA cm-2, the device has an energy density of 2.7 μWh cm-2. The all-solid micro-supercapacitor displays an approximately 90% retention of its initial specific capacitance after 5000 cycles at the scan rate of 1 V s-1. All the results show that nitrogen doped graphene quantum dots micro-supercapacitor has excellent electrochemical properties.