| Supercapacitors have the advantages of high power density,fast charging and discharging speed,long cycle life,wide working temperature range,and high safety.They have been regarded as a kind of electrochemical energy storage device between traditional dielectric capacitors and secondary batteries.Supercapacitors have a very broad application prospect in the fields of new energy and electric power transportation.According to the energy storage mechanism,supercapacitors can be divided into electric double-layer capacitors?EDLCs?and pseudocapacitors.EDLCs have high power density but low energy density.However,pseudocapacitors combine both high energy density and high power density,which is of great significance for the construction of hierarchical electrochemical energy storage system to meet different requirements.The common materials for pseudocapacitor electrodes include transition metal oxides,transition metal nitrides,and conductive polymer.However,there are some problems existing in the direct application of these pseudocapacitive active materials for energy storage:?1?pure pristine materials are easily agglomerated which hinders the diffusion of ions and lowers their utilization efficiency;?2?most of the materials have a low conductivity;?3?the process of charging and discharging can easily cause the deformation and phase transformation of the active materials,which shortens their service life.In order to solve these problems,the common solution is to build their composite electrode with carbon materials which have a large specific surface area,good conductivity,and stable chemical properties.Vertically oriented graphene?VG?grown by plasma method is considered to be an ideal substrate material for pseudocapacitors due to a series of excellent properties.The specific performances are as follows:?1?the large specific surface area is favorable to the dispersion of the active material;?2?the open channel is supportive to the ion transmission;?3?the good conductivity is favorable to the electron transmission;?4?VG can be grown directly on the current collector by the plasma method,reducing their contact resistance.In this thesis,high-performance pseudo-capacitor based on VG is systematically studied.Firstly,the electrostatic adsorption mechanism of VG was studied by using electrochemical quartz crystal microbalance?EQCM?and molecular dynamics simulation?MD?.The results show that the ion transport laws in the process of electrostatic adsorption for VG electrode with different heights are similar,which indicates that the energy storage mainly occurs in the edge region,that is,the charge accumulation in the edge region.Anions with different electronegativity and size have different effects on the energy storage process.This conclusion will provide theoretical guidance for the construction of the pseudocapacitor electrode based on VG and the selection of electrolyte.Based on the above nanoscale and microsale energy storage mechanism,a three-dimensional hierarchical composite electrode structure was constructed by depositing manganese dioxide?Mn O2?with a low conductivity onto the edge of VG.By moderately oxidizing VG,the utilization efficiency of the substrate was increased and the thickness of the Mn O2 was reduced and the uniformity was improved.The electrode exhibited high specific capacitance,good ratio performance,and good cycle performance.The specific capacitance of the electrode is 612 F g-1 at the scan rate of 2m V s-1,and the capacitance retention is 51.3%when the scan rate is increased to 200m V s-1.At the scan rate of 100 m V s-1,after 5000 cyclic voltammetry?CV?cycles,the capacitance retention rate is as high as 109%,and the mechanical properties are still good.An energy density of 30.4 Wh kg-1 and a power density of 27.8 k W kg-1 have been obtained by assembling it into an asymmetric supercapacitor,which is among the highest energy storage properties of composite pseudocapacitor electrodes with similar structure.These excellent energy storage performance can be attributed to the following reasons:?1?VG between the Mn O2 and current collector sets up a multi-site electronic transmission channel by serving as a bridge;?2?VG has an open structure,a large specific surface area,and dense,rich edges which help disperse Mn O2 nanostructures and facilitate the diffusion of ions;?3?the hierarchical structure acts as a buffer during charging and discharging,hence effectively increasing the service life.In addition,a new system based on VG with commercial titanium nitride?Ti N?nanoparticles as active material has been constructed by a simple coating method,which realized high-performance pseudocapacitance.The results showed that the energy storage performance of commercial Ti N coated on VG substrate was 6 times higher than that directly coated on nickel foil.The good performance was mainly attributed to the following reasons:on one hand,the huge specific surface area of VG was favorable for the dispersion of active materials and the contact efficiency between Ti N and ions was improved;on the other hand,the excellent conductivity of VG built a fast electron transport channel between the collector and the active material.However,commercial Ti N nanoparticles have low crystallinity and low purity.The preparation by traditional chemical methods has some disadvantages such as complex steps,high energy consumption,and introduction of impurity atoms.In this work,a one-step,efficient,and safe transferred arc method was used to realize the large-scale and low-cost preparation of high-purity Ti N nanoparticles.Two kinds of Ti N nanoparticles with different morphologies were obtained by using different growth parameters.Compared with the commercial Ti N,they have higher crystallinity,smaller size,and more metal bonds,which could lead to higher conductivity,more electrochemical active sites,and faster ion transport paths.The results showed that the energy storage capacity of the composite electrode composed of Ti N nanoparticles by transfer arc method and VG was 3-4 times higher than that of commercial Ti N on VG.The coin cell based on the transferred arc method prepared Ti N on VG exhibited good specific capacitance,energy density,power density,and cycle life at a voltage window of 1.8 V,which are among the best reported pseudocapacitors based on Ti N nanostructures. |
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