| More and more attention has been attracted in materials chemistry and nano techno logy by three-dimensional (3D) nano materials with hierarchical superstructures or complex architecture. It is expected that the 3D hierarchical structures could inherit the superior characteristics of the building blocks and also gain additional benefits from the unique secondary architecture. So far, there have been many studies about potential applications of 3D nanomaterials with novel properties. Among the various applications, supercapacitors as one of the most promising electrochemical energy storage, have aroused great concern due to their high power-density, rapid charging/discharging capacity and long life-cycle. Supercapacitors store the energy via two operating mechanisms:(1) Electrochemical double-layer capacitance (EDLC), which results from the electrical double-layer surrounding the surface of electrode. (2) Pseudo-capacitance, which is originated from the redox reaction of the electrode material with the electrolyte.Graphene is one of the most advanced carbon materials, due to its large surface area (beneficial to transfer electrolyte ions), high electrical conductivity (no need adding any additional conductive agents or auxiliary conductive materials during the process of preparing electrode), superior flexibility (for preparation of flexible electrode) and relatively wide potential window. However, EDLC is still not sufficient for most energy storage devices. To address this problem, we choose the graphene-based metal nanocomposites, which not only full play the cycling stability of electrochemical double-layer capacitance from graphene with excellent physical properties, but also full play the relatively high power density of pseudo-capacitance from metal materials. The composite material can also avoid low conductivity and poor stability of metal oxide/hydroxide.As is known, solid phase pyrolysis is currently one of the most common mthods to prepare multi-graphene. Taking advantage of the method, graphene can be obtained at low temperature with the catalyst. The process of preparation is simple, production is larger, and the purity of the product is higher. However, we have not seen relevant reports about simple preparation graphene-based metal electrode materials with 3D hierarchical structures. Herein, we combine solid phase pyrolysis (the formation of multi-layer graphene and metal nanoparticles at the same time), electrospinning (one of the most powerful techniques to fabricate nanoscale continuous ultralong fibers) and hydrothermal method (a simple process that controls crystal morphology by controlling the reaction temperature, time and solution component) to built three-dimensionally structural graphene-based nanocomposites. We all know that the porous unique architecture is favorable for efficient ion and electron transport and the outer graphene shells can better accommodate the structural change in the electrochemical reaction.The main content in the article is as follows:(1) Preparation and electrochemical properties ofNi@graphene:We prepared hierarchical nickel hydroxide/polyacrylonitrile (PAN) fiber, namely Ni(OH)2@PANF, by hydrothermal method. To obtain Ni(OH)2@PANF with ideal hierarchical structure, we set two groups of hydrothermal treatment conditions to investigate the influence of reaction time and temperature on structures. After comparing the morphologies of hydrothermal products with different times and temperatures, the product obtained at 170℃ for 15 h was selected for further analysis. After being immersed into poly(methyl methacrylate) (PMMA) solution, Ni(OH)2@PANF was annealed at 900℃ in Ar (5% H2) atmosphere for 3 min to obtain nickel@graphene structural fibers, and nickel@graphene was made into electrode material without requiring any additional conductive agents or auxiliary conductive materials. The specific capacitance of electrode material can reach 288.3 F/g at the current density of 0.5 A/g. And after 5000 cycles at the current density of 1 A/g, the specific capacitance can still remain 84% of that of the original.The results indicate that nickel@graphene shows exellent electrochemical stability. The electrochemical behaviors of nickel@graphene are further studied via cyclic voltammetry. Nickel hydroxides have two different crystallographic forms designated as a-Ni(OH)2 and β-Ni(OH)2. And the oxidation of nickel hydroxide has two other varieties of oxy-hydroxides (β-NiOOH and γ-NiOOH). CV curves indicate continuous phase transformation in the circulation process.(2) Preparation and electrochemical properties ofNi-Co@graphene:The different precursor fibers, such as PAN fibers, PMMA fibers and PMMA/Ni(AC)2, were treated at the same hydro thermal conditions, which shows that building secondary structure on the surface of different composition fibers need to match the conditions. We also chose different calcinations time for products of PAN precursor after hydrothermal treatment, which proves that the graphere layers coated on metal nanoparticles can realized the controllable growth.NiCo2O4/[PMMA/Ni(AC)2] was annealed at 700℃ in Ar atmosphere for 30 min to obtain nickel-cobalt@graphene structural fibers without adding PMMA as solid carbon sources. The CV curves recorded at different scan rates indicate that ratio performance is very good. Experiments confirm that the graphene coating on metal nanoparticles not only maintain the unique three-dimensional structure to increase the surfece area, but also make contribution to specific capacitance due to EDLC. The specific capacitance of electrode material can reach 419 F/g at the current density of 0.5 A/g. |
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