Synthesis And Electrochemical Performance Of Transition Metal Chalcogenides Nanostructure Arrays As Electrode Materials For Supercapacitors

As an efficient energy-storage device that lies between batteries and traditional electrostatic capacitors,supercapacitor(SC)has many advantages such as high power density,quick charge and discharge capability,and long cycle life.Supercapacitors have been widely used in mobile communication,consumer electronics,and aerospace.Recently,supercapacitors also have exhibit huge application value and market potential in electric vehicles and attracted much research attention.Electrode material is the key factor of supercapacitor and directly determines its electrochemical performance.Nanostructure transition metal chalcogenides have a high theoretical specific capacitance and relative low resistance,making them become a kind of promising electrode material for supercapacitor.In this paper,several unique three-dimensional(3D)nanostructure electrodes have been successfully constructed by directly growing transition metal chalcogenides on high conductive 3D porous Ni foam and directly acted as electrodes for SCs.Compared to traditional powder electrode materials,this kind of nanostructure arrays have unique geometric/morphologic characteristics and superior electrochemical properties.On one hand,the nanostructure arrays with a robust contact on high conductive Ni foam have larger active surface areas and can provide more fastly pathway for electrons transport.On the other hand,the nanostructure arrays integrated electrodes can avoid the subsequent complex electrode fabrication process and realize the binder-free structure with lower resistance.The current thesis aims to design and develop simple and efficient methods to synthesize several transition metal chalcogenides nanostructure arrays on Ni foam with different composition,morphology,and dimension.These nanostructure arrays are directly used as electrodes of supercapacitors and the electrochemical performances and energy storage mechanism have been also studied in detail.Main research contents are as follows:(1)One-dimensional Co9S8 nanoneedle bundle arrays and Co0.85Se nanotube bundle arrays have been successfully synthesized by a hydrothermal reaction and following ion-exchange process.These two nanostructure array electrode materials all exhibit high specific capacitances(the specific capacitances of Co9S8 Co0.85Se are 1400 F g-1 and 1394 F g-1 at the current density of 4 A g-1;the areal capacitances of Co9S8 Co0.85Se are 3.34 F cm-2 and 3.28 F cm-2 at the current density of 2 mA cm-2,respectively.)and excellent cycle stability.(2)The Mn/Co co-doped two-dimensional nickel sulfide(Mn,Co-Ni3S2)nanosheet arrays have been prepared by a two-step hydrothermal reaction,also using Ni foam as the conductive substrate and reactive material.The main component of the nanosheet arrays are Ni3S2 and the electrochemical performance can be effectively improved by adjusting the doping contents of Mn2+ and Co2+ ions.The optimized nanosheet arrays electrode represents an impressive areal capacitance of 4.0 F cm-2 at 2 mA cm-2 and excellent cycling stability,which above 90%of the areal capacitance can be maintained after 2000 cycles).(3)Based on the above two transition single metal sulfide nanostructure arrays research,we continue designed and prepared two-dimensional bimetallic Mn element surface decorated NiCO2S4 nanosheet arrays on Ni foam.This integrated and composite electrode has a strong structural stability.The electrochemical performance also can be modified by changing the amount of Mn2+ ion in reaction system.When the amount of Mn2+ was 1 mmol,the obtained electrode exhibited the optimal electrochemical performance,with a high areal capacitance of 4.6 F cm-2 and good cycle stability.(4)The 1 M-NiCo2S4 nanosheet arrays on Ni foam was used as electrode material,a symmetrical supercapacitor with Faraday pseudocapacitance as its main body was designed and fabricated.The supercapacitor has excellent electrochemical performance with a specific capacitances 560 F g-1(2.0 F cm-2)at 4 A g-1 current density.Based on the MATLAB software,the starting starter model with combining the supercapacitor and battery in parallel as the composite power was established.The results show that the parallel connection of supercapacitor is beneficial to protect the battery and enhance the starting efficiency.