Synthesis And Supercapacitive Performance Of Nano-sized Oxides And Carbon-based Composites

With the ongoing demands for increasing energy and environmental benignity,the development of energy storage device with sustainable,renewable and efficient properties has become a pressingly essential need in scientific area.supercapacitors(ECs,also known as electrochemical capacitors),which are considered as a promising candidate to power the next generation of energy storage device,have been widely investigated to meet the increasing requirements owing to their high power density,long cycling life and high rate capacity.The performance of the supercapacitor depends on the properties of the electrodes.This means that the electrode should not only have the features of low-cost and environmental friendly character,but also possess high electrochemical capability.However,single material cannot meet the requirements of practical application due to the intrinsic defects.Thus,the design of the composites,taking full advantage of their synergic effect,can achieve the purpose of developing high electrochemical electrode for supercapacitors.In this paper,we have designed and synthesize the composites combining carbon-based materials and transition metal oxides,and explore the electrical performance as supercapacitive electrode.Meanwhile,we believe that these synthetic routes can provide new ideas to prepare other materials with admirable electrochemical properties.The studying contents of this paper are summarized as follows:A novel core-shell structured Fe3O4@C@Ni-Al LDH composite containing carbon-coated Fe3O4 magnetic core and a layered double hydroxide(LDH)had been successfully prepared by a combination of the hydrothermal method and a facile in situ growth process.Owing to the unique layered feature,the composite displays core-shell structure with flower-like morphology,high surface area(792 m2/g)and specific pore size distribution.Moreover,the as-synthesized Fe3O4@C@Ni-Al LDH microsphere as an electrode material was fabricated into a supercapacitor and characterized by cyclic voltammetry(CV),electrochemical impedance spectroscopy(EIS),and galvanostatic charge-discharge measurements.It turned out that the Fe3O4@C@Ni-Al LDH exhibits specific capacitance of 767.6 F/g,good rate capability,and remarkable cycling stability(92% after 1000 cycling).Therefore,such novel synthetic route to assemble the high-performance electrochemical capacitor may open a new strategy to synthesize other materials with largely enhanced electrochemical property,which can be of great promise in energy storage device applications.We also present a facile and efficient process for the preparation of Fe3O4 nanoparticles(about an average diameter of 5 nm)grown on reduced graphene oxide(Fe3O4/r GO)with high supercapacitive properties.The novel composite with high surface area and mesoporous structure are prepared by a one-step hydrothermal method with the help of glucose,which can serve as a binder for the assembly of Fe3O4 nanoparticles(NPs)and a reducing agent for the reduction of graphene oxide simultaneously.Benefiting from the combined r GO and Fe3O4 in such a unique structure,the Fe3O4/r GO electrode material possesses a high specific capacitance of 241 F/g at 1 A/g within the potential range from-1 to 0 V and an excellent cycling stability of 79.2 % after 1000 cycles at a high current density of 10 A/g.These results demonstrate that such synthetic route may open a new pathway to design and fabricate other materials with largely enhanced electrochemical properties,which can be of great potential in the development of energy-storage systems.Binary metal oxides Mn Co2O4 nanosheets wrapped on hollow activated carbon shell(C@Mn Co2O4)has been successfully synthesized through a facile hydrothermal method followed by a calcination process.The novel flower-like C@Mn Co2O4 composite with good conductive carbon shell and well interconnected nanosheets can efficiently facilitate the electrolyte penetration and offer expedite transport path for ion and electron.Notably,the large surface(347 m2/g)of the hybrid composite can endow large amount of active sites,which evidently accommodate the strain during cycling.Benefited from this elegant combination and the effectively mesoporous structure,the specific capacitance of the C@Mn Co2O4 composites can be achieved as high as 728.4 F/g,which is,to the best of our knowledge,the highest value so far reported for Mn Co2O4 based electrode.In addition,C@Mn Co2O4 composite exhibits enhanced rate capability and an excellent cycling stability of 95.9% retention after 1000 cycles at high current density of 8 A/g.Therefore,the desirable integrated electrical performance enables it to be a promising electrode material for supercapacitor application.We present a novel and rational strategy for preparing hierarchical porous CNTs@NCS@Mn O2 core-shell composite via a facile in situ chemical polymerization coating method,followed by a hydrothermal process.The intermediate nitrogen-doped carbon shell(NCS)with mesoporous structure and favorable chemical durability is obtained by utilizing resorcinol-formaldehyde resin as carbon source and L-cysteine as nitrogen source,respectively.Benefiting from unique structure and considerable combination,the composites possess high comprehensive electrochemical performance: high specific capacitance(312.5 F/g at a current density of 1 A/g),good rate capability(76.8% retention with charge-discharge rate increasing from 1 A/g to 10 A/g)and superior reversibility.In order to increase the energy density and voltage window,an asymmetric supercapacitor(ASC)was assembled using CNTs@NCS@Mn O2 and activated carbon(AC)as the positive and negative electrodes,respectively.The as-fabricated asymmetric supercapacitor achieved a high specific capacitance with a stable operational voltage of 1.8 V and admirable energy density.Such a synthetic route to prepare the capacitor materials can thoroughly motivate the synergistic effect between electrical double layer capacitors and pseudocapacitors for obtaining high comprehensive performance electrode in energy storage fields.A porous hybrid g-C3N4/r GO(CNRG)material has been fabricated through a facile hydrothermal process with the help of glucose molecules,and serves as an efficient immobilization substrate to support ultrathin Ni(OH)2 nanosheets under an easy precipitation process.It was found that the g-C3N4 flakes can uniformly coat both sides of the r GO,forming sandwich-type composites with a hierarchical structure.It is worth noting that the induction of the g-C3N4 can effectively achieve the highly dispersion and avoid the agglomeration of the nickel hydroxide,and significantly enhance the synthetically capacitive performance.Owning to this unique combination and structure,the CNRG/Ni(OH)2 composite possesses large surface area with suitable pore size distribution,which can effectively accommodate the electrolyte ions migration and accelerate efficient electron transport.When used as electrode for supercapacitor,the hybrid material exhibits high supercapacitive performance,such as an admirable specific capacitance(1785 F/g at a current density of 2 A/g),desirable rate stability(retain 1106 F/g at 16 A/g)and favorable cycling durability(maintaining 87.6 % capacity after 1000 cycles at 8 A/g).Such desirable properties signify that the CNRG/Ni(OH)2 composites can be a promising electrode material in the application of the supercapacitor.