Preparation And Supercapacitive Performance Of Super-short Carbon Nanotubes Based Composites

Due to environmental pollution and energy depletion,the increasing demand for renewable energy sources has stimulated intense research on the continuous development of energy storage and conversion devices.Supercapacitors,also known as a novel kind of energy storage component,have high energy density,power densiy and excellent cycle life.They have many potential applications in the fields such as new kinds of electricity generation,power system,electric vehicles and electromagnetism equipment.Studies on supercapacitors are mainly focused on the preparation of electrode materials.In general,supercapacitors are divided into two parts concerning charge storage mechanism:EDLC which store charge in electrical double layer;and pseudocapacitors which are used for energy storage because of the faradic process.In particular,the critical element of supercapacitors is the electrode material,which directly determine their specific capacitances,cycling stability and rates capability.The common electrode materials for supercapacitors are mainly split into three categories:carbon materials,conducting polymers and metal oxides.The carbon materials such as graphene and carbon nanotubes（CNTs）possess high electrical and thermal conductivities,excellent mechanical properties and extremely large specific surface area,all of which are important attributes to satisfy the functional requirements of supercapacitors.However,graphene and CNTs have inherently low capacitance,limiting their practical applications when used alone;and easy agglomeration and stacking of graphene sheets would also restrict their application in the supercapacitors.In contrast,metal oxides and conducting polymers are two typical pseudocapacitive materials,which have higher energy storage capacity.Unfortunately,they suffer from poor stability and poor electrical conductivity,respectively.Therefore,the hybrid materials composed of any two electrode materials mentioned above can greatly improve the electrochemical performance of supercapacitors compared with the single electrode material.In this paper,multilayer super-short carbon nanotubes（SSCNTs）were prepared from the raw multiwalled carbon nanotubes（MWCNTs）through the mechanical-stirring and ultrasonic oxidation-cut approach.PANI was wrapped in the SSCNTs by in-situ polymerization for the synthesis of SSCNTs/PANI nanocable.Then SSCNTs/PANI nanocable were used as matrix to prepare ternary composites with metal ions,metal and graphene respectively.In addition,with the help of electrostatic interaction and hydrogen bonding,Ni（OH）2/SSCNTs composites were synthesized by the deposition of SSCNTs into the nanostructures of Ni（OH）2.At the same time,The PANI/Ag and RGO/PANI composites were also synthesized to study the properties of the electrode materials in the supercapacitors.These composites were characterized by Infrared spectra,UV spectra,Nitrogen absorption test,Transmission electron microscopy,Scanning electron microscopy,Raman spectroscopy,X diffraction spectroscopy and the electrochemical evaluation methods,which including cyclic voltammetry,galvanostatic charge/discharge,impedance test,four-point probe method and so on.Furthermore,they are used in the supercapacitors to investigate the electrochemical properties of the composites,study the relationship between the structure and properties of the composites,and explore the mechanism of the reactions.This paper mainly includes the four aspects:1.The present study represents a facile method for the preparation of FeCl₃ doped PANI/SSCNTs(PANI/SSCNTs/Fe³⁺)composites as the supercapacitor electrode material with noteworthy performance.The materials were fabricated through in situ polymerization of ferric chloride doped aniline in the presence of SSCNTs in HCl medium,and were characterized by FTIR and Raman spectroscopy and the XRD study.Field emission scanning electron microscopy（FESEM）and transmission electron microscopy（TEM）analyses confirmed the successful coating of Fe³⁺doped PANI on the SSCNTs surface.The electrochemical characterizations were carried out by a three-electrode probe method,with 3 M KOH as the electrolyte.Galvanostatic charge-discharge test established the superiority of the PANI/SSCNTs/Fe³⁺nanocomposite as the supercapacitor electrode material.2.The silver nanoparticles with high dispersion on the surface of polyaniline（PANI/Ag）was acquired by reduction of silver nitrate with the assistance of vitamin C acting as an environmentally friendly reducing agent.Specifically,this study could open an avenue for environmentally friendly,simple and cost effective methods in the surface functionalization and generation of silver nanoparticles into the PANI chains.The surface morphology of the PANI/Ag composite revealed the PANI nanofibers with bright spots of silver nanoparticles.The PANI/Ag composite exhibited the excellent capacitive performance with a specific capacitance as high as 553 F g^-1 at 1 A g^-1 because of the synergic effect of PANI and silver nanoparticles,which was much higher than that of PANI.Eventually,the greatly enhanced capacitive performance was mainly attributed to the silver nanoparticles,which could increase the electrical conductivity and promote the electron transfer between the active components.In order to further enhance the electrochemical properties of the composite material,the silver nanoparticles with high dispersion on the surface of the hybrid nanocomposites（SSCNTs/PANI/Ag）consisting of SSCNTs with the polyaniline was acquired by reduction of silver nitrate with vitamin C.It is found that it exhibits excellent capacitive performance with a specific capacitance as high as 615 F g^-1 at 1 A g^-1 which is much higher than that of PANI and PANI/Ag.The greatly enhanced capacitive performance of the nanocomposites is mainly attributed to the introduction of silver nanoparticles,which can increase the electrical conductivities of the nanocomposites.Furthermore,the open-ended pipes of SSCNTs provide abundant additional transport paths and short axial dimension so that it can shorten the transmission distance for the electrolyte ions and electrons in the electrode,and promote the electron transfer between the active components.This study suggests that the SSCNTs/PANI/Ag composite is a promising class of electrode materials for high performance energy storage applications.3.Nanostructured layered nickel hydroxide and SSCNTs have been successfully assembled to form Ni（OH）₂/SSCNTs composite by electrostatic force.Compared with pure Ni（OH）₂,the Ni（OH）2/SSCNTs composite exhibits the much better electrochemical performance with a specific capacitance of 1887 F g^-1 at 1 A g^-1,and demonstrates a good rate capability and excellent long-term cyclic stability（92%capacity retention after 3000 cycles）.It is the reason that the SSCNTs can form a conductive network onto the surface of Ni（OH）2 nanoflakes,and their excellent electric conductivity is advantaged to the charge transport on the electrode in discharge process and charge process.Therefore,the greatly enhanced capacitive performance of Ni（OH）₂/SSCNTs can be attributed to a synergetic effect of Ni（OH）₂ and SSCNTs.4.The graphene/polyaniline nanofibers（RGO/PANI）composite could be prepared by in situ polymerization of aniline in acid solution,followed by reduction of GO with hydrazine in the presence of well-dispersed graphene oxide（GO）and re-oxidation of PANI by strong oxidant.The microscope images indicated that the PANI nanofibers could adsorb on the surface of graphene sheets through a large van der Waal force and hydrogen bonding force.The maximum specific capacitance of RGO/PANI composite achieved 596 F g^-1,which was much higher than that of PANI.Moreover,the RGO/PANI composite also showed a good cycling stability,retaining over 85%of its initial capacitance after 1000 cycles.The good electrochemical performances of RGO/PANI composite were contributed to the modification of RGO sheets surface,the network structure and the synergic effects of the two components.Therefore,this study demonstrates the promising application of RGO/PANI composite as electrode materials for energy storage.In order to further enhance the capacitance of the whole material,the SSCNTs/polyaniline/reduced graphene oxide（SSCNTs/PANI/RGO）ternary hybrid composite was fabricated by reducing SSCNTs/PANI/GO precursor prepared by self-assembly from the complex dispersion of graphene oxide（GO）and the as-prepared SSCNTs/PANI nanocables,followed by redoping and reoxidation of the reduced PANI to restore the conducting structure of PANI in the ternary composite.The microscope images indicated that SSCNTs/PANI nanocables could uniformly distribute in the conductive network of graphene sheets and prevent the agglomeration of graphene.Such the hierarchical structure perfectly facilitated the contact between PANI for the pseudo-capacitance and electrolyte ions,and efficiently synergized the double-layer capacitance of SSCNTs and graphene sheets at the electrode-electrolyte interfaces.The maximum specific capacitance of the SSCNTs/PANI/RGO composite achieved 845 F g^-1,which was much higher than that of pure PANI and SSCNTs/PANI nanocables.Moreover,the ternary composite also showed good cycling stability,retaining about 96%of its initial capacitance after 1000 cycles because of the synergistic effect of SSCNTs/PANI nanocables and graphene sheets.Therefore,the combined effects between SSCNTs/PANI nanocables and graphene sheets could readily explain the excellent electrochemical performance for supercapacitors.