Synthesis And Electrochemical Performance Of The All-carbon Flexible Electrodes Based On Nitrogen-doped Graphene Quantum Dots

In recent years,with the application of wearable,portable flexible electronic products,the development of flexible energy storage devices with high power density and high energy density has attracted much attention.As one of the front research aspects in flexible energy storage devices,flexible supercapacitors have a broad application prospects in the field of emerging electronic intelligent equipment and other advanced technology due to their high power density,fast charge and long cycle life.In order to obtain high-performance supercapacitors,it is particularly important to develop electrode materials with excellent performance.In this dissertation,the carbon nanotube/carbon cloth?CNT/CC?composite materials with porous network structure are prepared as flexible carbon-based skeleton.By means of optimizing its spatial structure,enhancing its surface area,forming porous-connected multidimensional structure to be good for electronic transmission and ion diffusion,the electric double layer capacitor performance of flexible carbon base is improved.On this basis,highly nitrogen-doped monocrystalline graphene quantum dots are deposited on the surface of flexible carbon skeletons by electrophoretic deposition method respectively.Regulating the loading amount of quantum dots and the contributions of pseudocapacitance properly,the specific capacitance of active material can be improved dramaticlly and long cycle life of carbon based flexible electrode can be retained as well.Mechanical properties and electrochemical properties of these flexible capacitors are researched under the conditions of bending and folding deformation.Through optimizing the hierarchical composite structure of these various carbon materials,it is expected to develop novel flexible supercapacitors with high energy density,high power density and long cycle life even under multiple deformation.The main contents are described as follows:1.Firstly,the flexible carbon-based skeleton CNT/CC composite material with high specific surface area and high conductivity was prepared by chemical vapor deposition?CVD?.Secondly,the water-soluble single crystal NGQDs were prepared by alkali-catalyzed water phase molecular fusion method.Finally,the three-dimensional composite material N-doped graphene quantum dot/carbon nanotube/carbon cloth?NGQD/CNT/CC?was successfully prepared by the deposition of NGQDs on the flexible carbon-based skeleton.The structure and morphology of the composite materials were characterized by TEM?AFM?SEM?XRD?XPS?Raman and FTIR analysis respectively.Dense,brushy CNTs were first grown to form a 3D CNT/CC mesh network on the surface of the carbon fibers with high surface area.Then,colloidal GQDs in aqueous solution were uniformly deposited on the CNT/CC matrix,forming the ternary N-O-GQD/CNT/CC flexible electrode materials.2.Three kinds of NGQD/CC,CNT/CC,NGQD/CNT/CC flexible electrodes were assembled.According to controlling the loading of NGQDs,the prepared flexible electrode materials show the high capacitance performance of both the electric double layer capacitance and the pseudo capacitance.Supercapacitor was assembled from the NGQD/CC,CNT/CC,NGQD/CNT/CC electrodes with 1 M H₂SO₄ aqueous solution.When the current density is 0.5 mA cm^-2,the areal capacitance of the prepared three flexible composite electrodes are 44.6 mF cm^-2?163.9 mF cm^-2?554.4 mF cm^-2.The optimized NGQD/CNT/CC supercapacitors can deliver the energy density of 41.7?Wh cm^-2,and 89.7%of the original capacitance is retained after 5000 cycles.3.Using H₂SO₄/PVA as gel electrolyte,the all-solid-state flexible supercapacitors were assembled by the NGQD/CNT/CC electrodes.When the current density is 0.5 m A cm^-2,the areal capacitance of the all-solid-state flexible electrode is461 mF cm^-2.The solid device keeps a high retention ratio of 87.5%after 2000charge-discharge cycles.It can be observed that the high bending degree has almost no adverse effects on the capacitive behavior,further demonstrating the good stability,high flexibility,and good mechanical properties of the all-solid-state symmetric flexible supercapacitors based on NGQD/CNT/CC electrodes.4.Nitrogen doped carbon-based flexible electrode materials not only show excellent capacitance performance,due to their excellent electrochemical performance,flexible electrodes applied in the field of lithium ion battery have also been explored.A high reversible specific capacity 1287 mAh g^-1 is obtained after annealing at a current density of 100 mA g^-1.Moreover,The NGQD/CNT/CC flexible electrode shows high capacity of 856 m Ah g^-1(at a current density of 200 mA g^-1)after 50 cycles,which is much higher than the theoretical capacity of commercial graphite(372 mAh g^-1).More strikingly,the flexible electrode exhibits excellent rate capability,even at a high current density of 2000 mA g^-1,and the specific charge capacity is still as high as 460 mAh g^-1.The high reversible specific capacity and good cycle stability of the NGQD/CNT/CC electrodes are related to the stable structure,high conductivity,high specific surface area and low charge transfer resistance of the prepared composite material.On the other hand,after annealing,the oxygen-containing groups in the composites are further reduced,which improves the conductivity of the composites and is conducive to the diffusion of lithium ions and the transport of electrons.