Study On Preparation And Properties Of Vanadium Compound/Graphene Electrodes For In-Plane Micro-Supercapacitors

In recent years,in-plane micro-supercapacitors have attracted extensive attention,with the development of electrochemical energy storage devices towards flexibility,miniaturization,integration and intelligence.Micro-supercapacitors exhibit high power density and long lifetime,and hold the potential for replacing or complementing microbatteries to power microelectronic devices.However,the low energy density and lack of large-scale fabrication methods limit the application of micro-supercapacitors.As a new nanocarbon material,graphene exhibits a unique two-dimensional structure,large specific surface area,high mechanical strength and excellent electrical conductivity,and hold great potential for high-performance micro-supercapacitors.Currently,graphene-based micro-supercapacitors exhibit good power performance.However,graphene is prone to stacking due to the π-π interaction between sheets,resulting in a much lower available specific surface area than the theoretical value.And the doublelayer charge storage process limits the specific capacitance and energy of graphene.As an important class of pseudocapacitive materials,transition metal vanadium compounds hold the advantages of various oxidation states,unique physical and chemical properties,low cost and easy availability,and can deliver high specific capacitance and energy.However,the electronic conductivity of vanadium compounds represented by vanadium oxides is poor,which limits their power performance.Therefore,this thesis aims to design high-performance electrodes by using highspecific-power graphene as the conductive framework and high-specific-energy vanadium compounds including vanadium oxide,vanadium nitride and vanadium carbide.The electrodes are fabricated by efficient and scalable methods,and the energy density of micro-supercapacitors is greatly enhanced,to advance the practical application of micro-supercapacitors.The main results are as follows:(1)Vanadium oxide(VO2)nanobelts were mixed with electrochemically exfoliated graphene(EG)nanosheets to utilize the synergistic effect of VO2 and EG,and interdigital VO2/EG electrodes were constructed by mask-assisted filtration method.VO2 nanobelts and EG nanosheets were alternately stacked in the VO2/EG electrode.It effectively inhibited the stacking of EG nanosheets,beneficial to increase their available specific surface area.At the same time,fast electron transport channels were formed at the interface of VO2 and EG.VO2/EG micro-supercapacitors exhibited areal capacitance of 36.1 mF cm-2,5.2 times higher than that of EG micro-supercapacitors,and 3.6 times higher than that of VO2 micro-supercapacitors.The energy and power density of VO2/EG micro-supercapacitors were 7.5 mWh cm-3 and 41 mW cm-3,respectively.Under the bending deformation of 180°,VO2/EG micro-supercapacitors could maintain stable electrochemical performance.And the output voltage or capacitance of the VO2/EG micro-supercapacitors could be improved by series or parallel connection.(2)Highly conductive vanadium nitride(VN)were fabricated by nitridation of VO2,while a large number of mesopores were introduced on VN nanobelts.Those nanobelts were mixed with high-quality EG nanosheets to construct interdigital VN/EG electrodes.The mesoporous VN nanobelts and EG nanosheets were in close contact and evenly mixed in the electrode,beneficial to improve the electron and ion transport capability of the electrode.VN/EG micro-supercapacitors exhibited operating voltage of 1.6 V,higher than that of symmetric VN-based micro-supercapacitors reported so far.VN/EG micro-supercapacitors delivered energy density of 1.8 mWh cm-3,power density of 296 mW cm-3,and capacitance retention of 91%after 10,000 charge/discharge cycles.VN/EG micro-supercapacitors exhibited good mechanical flexibility and series-parallel connection performance.Under various bending deformations from 0°to 180°,VN/EG micro-supercapacitors held almost 100%capacitance retention.The output voltage/capacitance could be further improved by series/parallel connection of VN/EG micro-supercapacitors.(3)Efficient fabrication of interdigital vanadium carbide/reduced graphene oxide(V8C7/rGO)electrodes was achieved by combining continuous centrifugal coating with laser scribing.Twenty-two micro-supercapacitors could be obtained by laser scribing within half an hour.Under the photothermal effect of the laser,the as-prepared V8C7/rGO exhibited porous micro-nanostructure due to the rapid gas generation and release during the reduction of graphene oxide(GO).V8C7 nanoparticles with diameters in the range of 10-30 nm were uniformly distributed on the rGO nanosheets.Such a structure was beneficial to improve the accessibility of charge carriers and the utilization of active sites inside the V8C7/rGO electrode.The areal capacitance of V8C7/rGO micro-supercapacitors was 49.5 mF cm-2,10 times higher than that of rGO micro-supercapacitors.V8C7/rGO micro-supercapacitors exhibited high energy density(3.4 mWh cm-3)and high power density(401 mW cm-3).V8C7/rGO microsupercapacitors also exhibited excellent flexibility,good series-parallel connection performance,and long cycling life,promising for powering microelectronic devices.