The Study On The Vanadium-based Electrode Materials For Supercapacitors

Supercapacitors generally have charge storage properties that complement those of batteries,and it has the advantages of large power density and long cycle life,but the low specific energy restricts the development of supercapacitors.Pseudocapacitive materials are considered to be a better energy storage material,because it stores charge through a battery-like redox reaction,and its charge and discharge rate is comparable to that of electrochemical double-layer capacitors.With the development of nanomaterials,a lot of pseudocapacitive materials have been researched successively.Among them,vanadium-based materials have many advantages such as different structure,chemical composition,various valence states and high energy storage capacitance,which provide possibilities for the development of new supercapacitors.However,the disadvantages of poor conductivity,low reactivity and unstable structure limit the application of most vanadium-based compounds in supercapacitors.Therefore,it is particularly important to improve the energy storage performance of vanadium-based materials in supercapacitors through such modification strategies as conductive carbon composite,rare earth metal ion doping,construction of multiple heterogeneous vanadium-based compounds,and controllable synthesis of micro and nano structures.In this dissertation,the simple and low-cost preparation methods of vanadium-based materials with high energy storage performance are explored.Moreover,It focuses on the researches of microstructure,composition and corresponding electrochemical properties of vanadium-based materials.In addition,the energy storage mechanism of various types of vanadium-based materials is discussed,and the influence of material modification strategies on energy storage performance is explored.The applications of supercapacitor energy storage devices are further evaluated.The main work and results are as follows:(1)Binder-free and layered wrinkle V₂O₅@GO thin film electrode(VG)is successfully prepared directly on conductive glass ITO by a simple sol-gel method and drop coating method.After the ratio of composite material is optimized,the VG8composite thin film electrode displays an outstanding specific capacitance of 397 F g^-1at a current density of 1 A g^-1 and good cycling stability with 98%capacitance retention over 2000 cycles,which is much higher than the performance of pure V₂O₅ thin film electrode.This mainly benefits from the fact that the size of about 2?5 nm V₂O₅nanoparticles are evenly embedded in the two-dimensional GO nanosheet layer,exposing more active sites.Moreover,GO can prevent the volume expansion of V₂O₅during charging and discharging process.In addition,the symmetrical supercapacitor VG8//VG8 has an energy density of 17.7 Wh kg^-1 at a power density of 250 W kg^-1.(2)V₂O₅·n H₂O with large layer spacing is adopted as the research object.La³⁺doping is used to further increase the interlayer distance of V₂O₅·n H₂O and stabilize the layered structure.Oxidized acetylene black(OAB)is used to improve the outer conductivity of the material.The influences of the doping amount of La³⁺and the introduction amount of OAB on the microstructure and electrochemical properties of the composite are analyzed.The results have shown that the 20%La-VOH@OAB-4exhibits an ultrahigh specific capacitance of 612 F g^-1 at the current density of 1 A g^-1.When the current density increases by tenfold,the capacity retention rate is 81.1%.The Fe₂O₃@rGO negative electrode material is prepared by the similar sol-gel method,it displays the discharge capacitance of 366 F g^-1.The energy storage mechanism and capacitance contribution distribution of 20%La-VOH@OAB-4 and Fe₂O₃@rGO are discussed by kinetic analysis respectively.The results have shown that the total capacitance is more controlled by the surface capacitance contribution.The asymmetric supercapacitor 20%La-VOH@OAB-4//Fe₂O₃@rGO within a voltage window of 1.7 V delivers a relatively high energy density of 56.3 Wh kg^-1 at a power density of 850 W kg^-1.(3)Binder-free VOPO₄ thin film electrode is prepared by simple sol-gel method,the phytic acid,GO and polystyrene are used as phosphorus source,carbon source and template respectively.the honeycomb multiporous MP-VOPO₄@rGO binder-free composite film electrode can be obtained after calcining at the proper temperature.The reasonable dual strategy design of accelerating ion diffusion and electron transfer enables the composite material to realize the battery-like energy storage capacity and the rate performance of carbon-based capacitor.MP-VOPO₄@rGO displays the discharge capacitance of 672 F g^-1 at the current density of 1 A g^-1.When the current density increases tenfold,the capacitance retention rate is 76.8%.Additionally,based on kinetics analysis,it is found that the capacitance of MP-VOPO₄@rGO thin film electrode mainly come from the surface capacitance process.The symmetrical supercapacitor MP-VOPO₄@rGO//MP-VOPO₄@rGO can release the discharge specific capacitance of 172 F g^-1at current density of 1 A g^-1.When the power density is 250 W kg^-1,the energy density can reach 26.3 Wh kg^-1.(4)According to ostwald's maturation theory,V₂O₅ microspheres morphology with time-evolving are prepared by solvothermal methode.The in-situ coating of V₂O₅microspheres with rGO significantly improved the conductivity and stability of the composite material.The results show that the electrochemical properties of the synthesized V₂O₅ materials are affected significantly reaction time and rGO content.After optimizing the preparation conditions,the Vr G-5 flexible electrode has an discharge specific capacity of 575 C g^-1 at the current density of 1 A g^-1.In addition,the flexible Vr G-5//rGO asymmetric supercapacitor is assembled using Vr G-5 and rGO as positive and negative electrodes respectively,which has a working voltage of 1.4 V,discharge capacity of 237 C g^-1 and energy density of 46.1 Wh kg^-1.Moreover,the flexible supercapacitor exhibit a superior mechanical flexibility(even 82.7%capacity retention after 5000 cycles under mechanical bending 180°).(5)Ni Mo O₄@Co₃V₂O₈ nanorods/nanospheres(NMO@CVO)are synthesized by simple secondary hydrothermal method.Due to the unique nanorods/nanospheres microstructure and the synergistic effect of composite materials,the discharge capacity and cyclic stability of the optimized composite material(NMO@CVO-8)is improved significantly,displaying a maximum specific capacity of 357 C g^-1 and remarkable cycling stability(the capacity retention rate is 89.7%after 5000 cycles).In addition,an aqueous asymmetric supercapacitor is assembled based on the NMO@CVO-8 hybrid nanorod/nanosphere and activated carbon.The NMO@CVO-8//AC supercapacitor with1.6 V shows an ultrahigh energy density of 46.4 Wh kg^-1 at a power density of 800 W kg^-1.