Controlled Fabrication Of One-Dimensional Composite Nanostructures As Electrode Materials For Supercapacitors

Energy crisis and environmental issues are two major challenges facing human society at present.With the growth of world population and the development of global economy,there is an ever-increasing demand for the efficient,clean and sustainable power sources as well as the advanced energy storage and conversion devices.Supercapacitors,also known as electrochemical capacitors,have the advantages of high power density,fast charge-discharge processes,long cycle lifespan,good safety and relatively low cost,which can fill the gap between rechargeable batteries and traditional dielectric capacitors.It is widely known that electrode materials play an important role in determining the property of supercapacitors.Therefore,numerous attempts have been made to the research and development of high-performance electrode materials in recent years.One-dimensional?1D?nanostructures with a large aspect ratio not only increase the electrode-electrolyte contact area,but also shorten the electron/ion transport length,which is beneficial to improve the electrochemical property of the electrode.Carbon materials,metal oxides?MOs?and conducting polymers are three types of frequently-used electrode materials for supercapacitors.However,the single-component nanosystems usually suffer from a low theoretical specific capacitance?carbon materials?,poor conductivity?MOs?,weak cycle stability?conducting polymers?,and undesired surface nature,leading to their incompetence to meet the application requirements.It is generally acknowledged that the design and construction of composite nanostructures is an effective strategy to enhance the performance of supercapacitor electrodes by using the synergistic effect between the components and taking full advantages of them.In this thesis,we choose the above three electrode materials as the research objects,and engage in the design and regulation of the composition and morphology of 1D composite nanostructures to improve the property of supercapacitor electrodes,which may lay a foundation for the application of 1D composite nanostructures in the field of energy storage.The details are shown as follows:1.1D conducting polymers-based electrode materials: Conducting polymer,especially polyaniline?PANi?,is a typical pseudocapacitive material.With PANi as the substrate or shell,the inorganic species were introduced into the systems via a simple and facile template method,resulting in the enhanced electroactivity of the 1D composite electrode materials in neutral electrolyte by improving the performance of PANi or preventing the dissolution of MOs.?1?A novel PANi thorn/Bi OCl flake?BPB?heterostructure was prepared at low temperature for the first time by a one-pot approach using Bi₂S₃ nanowire as both the sacrificial template and the Bi source for Bi OCl,HCl as the dopant for PANi and the Cl source for Bi OCl.The empty space between adjacent PANi thorns was favorable for the propagation of electrolyte.At the same time,the stabilizing effect of Bi OCl on the doping H+ could maintain the electroactivity of BPB in neutral condition,thus contributing to the optimization of the electrochemical performance.Compared with the HCl-doped PANi electrode,the as-synthesized BPB electrode exhibited a larger specific capacitance?169.9 F g-1 at 0.5 A g-1?and a higher rate capability?15.6%,4.0 A g-1?with elevated cycle stability.?2?Conducting PANi was deposited in situ as a homogeneous shell on the surface of the monocrystalline VO2 nanobelt which acted as a reactive template to obtain the 1D VO2@PANi coaxial nanostructure.This unique method without the assistance of any surfactant can be extended to the construction of core/yolk-shell or hollow inorganic/organic functional nanoarchitectures with various shapes by tuning the p H or reaction time.At 0.5 A g-1,the specific capacitance of the prepared VO2@PANi electrode was calculated to be 246.0 F g-1,which was much higher than those of both VO2 nanobelts?160.9 F g-1?and HCl-doped PANi nanofibers?139.4 F g-1?.When the current density was increased by 10 times to 5.0 A g-1,the VO2@PANi electrode had a better rate capability with a capacitance retention of 27.3% in comparison with that of pristine VO2?11.3%?.After 1000 cycles,the specific capacitance of the VO2@PANi electrode was 28.6% of the initial value,higher than that of HCl-doped PANi?2.8 %?.2.1D MOs-based electrode materials: In addition to conducting polymers,inorganic metal oxide is another frequently-used pseudocapacitive material.?-Fe_O3has been considered as a promising candidate due to its high theoretical specific capacitance,good corrosion resistance,nontoxicity,and abundant natural resource.Nevertheless,the poor intrinsic conductivity of ?-Fe_O3results in the unsatisfactory specific capacitance that is far below the expected value.In order to address the problem above,we integrated ?-Fe_O3with other MOs,that is,the electrospun ?-Fe_O3nanotubes as the main body was doped with V2O5 or coated with MnO₂ shells.The electrochemical property was improved by regulating the structure and composition of the hybrid materials.?1?A series of V2O5/?-Fe_O3nanotubes were fabricated by electrospinning technique and thermal treatment.When used as electrode materials,the composite nanotubes with the mass ratio of V2O5/Fe_O3at 1.0% displayed a larger specific capacitance?183 F g-1 at 1.0 A g-1?,a good rate capability?> 60%,5.0 A g-1?,and a higher cycle stability?81.5%,200 cycles?as compared to the individual ?-Fe_O3nanotube electrode.?2?The ?-Fe2O3@MnO₂ core-shell heterostructures with various MnO₂ contents were synthesized by the chemical bath deposition of MnO₂ nanosheets on the electrospun ?-Fe_O3nanotubes.Compared with the pure MnO₂-based electrode,the hierarchical composite nanostructure,especially FM10020 containing 60.1 wt% of MnO₂,has a larger specific capacitance of 289.9 F g-1 at a current density of 1.0 A g-1,a better rate capability of 40.8% at 5.0 A g-1,and a higher cycle performance of 85.3% after 1200 cycles.3.Modified carbon fibers-based electrode materials: For the hybrids of binary MOs,the electrochemical dissolution and conductivity issues caused their capacitive property largely unsatisfactory.Another feasible way to enhance the performance of MOs electrode materials is to combine them with conductive substrates.The electrospun carbon nanofibers?CNFs?with tunable pore structures,which are regarded as good supporting scaffolds,are convenient to process besides the inherent advantages of carbon materials.The incorporation of metal salts into the polymer spinning solution can effectively improve the graphitization degree of CNFs and create abundant pores on/in the matrix,thus generating the modified C-MOx composite nanofibers.Herein,we selected C-MOx as the core to improve the electrochemical performance of the multicomponent system by the deposition of N-containing carbon layer and MnO₂ shell on their surface.?1?Porous C-Co Ox-C composite nanofibers with N-doped surface were obtained by electrospinning,chemical vapor-phase polymerization and thermal treatment.The specific capacitance of C-Co Ox-C electrode was increased faintly owing to the higher carbon content and the larger Co Ox particles.However,the good conductivity and unique protection mechanism of the N-containing carbon envelope resulted in the higher rate capability and better cycle stability of C-Co Ox-C electrode than those of C-Co Ox electrode.?2?The MnO₂ shells were coated on the electrospun C-MOx?M = Mn,Cu,Co?composite nanofibers for the preparation of a series of 1D C-MOx@MnO₂ core-shell heterostructures.The presence of low-valence MOx enhanced the conductivity and porosity of the nanocomposites to some extent through expanding graphitic domains or mixing metallic Cu into the CNFs substrates.Furthermore,the C-MOx core could not only serve as an active material to participate in the charge storage process,but also act as a conductive matrix to make the most of the MnO₂ shell with synergistic effect.In comparison with the CNFs@MnO₂ core-shell electrode,the obtained C-MOx@MnO₂ electrode exhibited larger specific capacitance,higher rate capability and good cycling performance.It is believed that these results will provide an alternative way to further improve the capacitive properties of CNFs-or MOs-based electrode materials.