Preparation And Supercapacitive Properties Of Nanostructured Vanadium Oxides

Supercapacitors（SCs）,one kind of the energy storage devices,have attracted great attentions due to their high power density,fast charge-discharge characteristics and long cycling life.Vanadium oxides have been noticed to be a promising candidate electrode material for SCs owing to their large specific capacity,wide potential window,high energy density and abundant source.However,the low electrical conductivity,slow ion diffusion,fast capacity decay of vanadium oxides restrict their application in SCs.Aiming to improve the electronic conductivity and the cycling stability of vanadium oxides,the thesis is focused upon the architecture design,preparation technology,performance optimization and storage mechanism of vanadium oxides.The details are summarized briefly as follows:1.Ru doped V₂O₅ hierarchical hollow microspheres were prepared via solvothermal process followed by heat treatment.The doping of Ru improves the electronic conductivity and the crystallinity of V₂O₅.Electrochemical results demonstrated that the best properties can be achieved when the doping ratio of Ru is 1%.The specific capacitance was 369 F g^-1 at 0.5 A g^-1;remaining 73%capacitance at a high current density of 8 A g^-1 and only decaying 33.2%after 1000 charge/discharge cycles.2.CNTs/V₂O₅ hierarchical hollow microspheres were prepared via solvothermal process followed by calcination.The influence of the ratio for CNTs to V₂O₅ on the morphologies,structures and electrochemical performances were systemically investigated.The results indicated that CNTs dramatically enhance the rate performances of the composite electrodes.When the ratio of CNTs is 7.1%,the composite electrode exhibits the best electrochemical performances,which delivers a specific capacitance of 346 F g^-1 at 0.5 A g^-1 and maintains 75%at 8 A g^-1 in 5 mol L^-1 LiNO₃.The electrode also shows good cycling stability which retains 73.5%of its initial capacitance after 1000 cycles at a current density of 1 A g^-1.A Hybrid capacitor was assembled using commercial activated carbon as the negative electrode and the composite as the positive electrode.The hybrid capacitor with the working potential range of 0～1.6 V exhibits an energy density of 12.5 Wh Kg^-1 at a power density of 350 W Kg^-1.3.One pot synthesis of V₂O₅ microsheets with hierarchical structure grown on carbon textiles without templates was used as self-supported electrode for SCs directly.The resultant electrode shows good rate capability(243 F g^-1 at 1 A g^-1 and 162 F g^-1 at 8 A g^-1)and cycling stability(67%retention after 2000 cycles at 1 A g^-1)in 1 mol L^-1 Na₂SO₄ compared to that of the traditional powder electrode.4.Mixed-valence vanadium oxides（VO_x）/porous carbon composites were prepared by chemical vapor deposition（CVD）method,in which acetylene was used as carbon source for carbon coated V₂O₅ nanobelts.The thickness of the porous carbon layer can be controlled by changing the reaction time.The best electrochemical performance can be achieved when the reaction time is 20 min.A specific capacitance of 377 F g^-1 and 302 F g^-1 at 0.5 A g^-1 and maintain 39%and 45%at 8 A g^-1 could be obtained in 5 mol L^-1 LiNO₃ and 1 mol L^-1 Na₂SO₄,respectively.The higher retention in 1 mol L^-1 Na₂SO₄ could be attributed to the smaller hydrated ion radius of Na+,which could insert/extract VO_x more easily.5.Mixed-valence vanadium oxides（VO_x）@N-doped C composites were in situ synthesis through molten salt method.The best electrochemical properties can be obtained when the ratio of vanadium source（vanadium（IV）oxy acetylacetonate）and carbon source（1,4-phthalaldehyde and melamine）is 2:5.The composites delivers a specific capacitance of 310 F g^-1 at 0.5 A g^-1 and maintain 57.1%at 8 A g^-1 in 5 mol L^-1 LiNO₃.Hybrid capacitors were assembled using the composite as the positive electrode and negative electrode with commercial activated carbon,respectively.The hybrid capacitor exhibited higher energy density at the same power density when the composites were used as negative electrode.6.Mixed-valence vanadium oxide（VO_x）/highly ordered mesoporous carbon（CMK-3）composites were synthesized through a facile liquid-phase method followed by calcination.The relevant results showed that vanadium oxide nanoparticles with mixed valence were successfully embedded in mesoporous channels in the conductive matrix and dispersed on the short-rod like CMK-3 surface to form the interwoven composite.The introduction of the CMK-3 framework not only improves electron transfer but also prevents the structure collapsing during cycling.As expected,the composite exhibits excellent electrochemical properties.It delivered a specific capacitance of 257 F g^-1 at 0.5 A g^-1 and maintained 80%at 8 A g^-1 in 5 mol L^-1 LiNO₃.After 5000 cycles,the capacitance only decreased 20%.A hybrid capacitor was assembled using commercial activated carbon as the negative electrode and the composite as the positive electrode.The hybrid capacitor with the working potential range of 0～1.9 V exhibited an energy density of 17.3 Wh Kg^-1 at power density of 475 W Kg^-1.7.V₃O₇·H₂O@N-doped C nanosheets composites were prepared by a facile freeze-drying followed by calcination strategy.Two-dimensional V3O7·H2O nanosheets grown on N-doped two-dimensional carbon materials,which could improve the electronic conductivity and ionic conductivity of the composites,raise the electrochemical utilization of vanadium oxides.Besides,the introduction of N into carbon could enhance the electronic conductivity and increase the affinity of the composites with the electrolyte.The electrochemical performances showed that the composites exhibit a specific capacitance of 295 F g^-1 at 0.5 A g^-1 and maintain 67.8%at 8 A g^-1 in 5 mol L^-1 LiNO₃.After 3000 cycles,the specific capacitance retention was 48.2%.