Binder-free polyaniline interconnected metal hexacyanoferrates nanocomposites (Metal = Ni,Co) on carbon fibers for flexible supercapacitors

Improvement of the electrical conductivity, specific capacitance and binder-free polyaniline (PANI) interconnected with metal(II) hexacyanoferrate(III) (MHCF) nanocomposites (M?=?Ni, Co) on flexible carbon fibers (CF) were designed in our present research goal. PANI/MHCF/CF nanocomposites were prepared by one-step co-polymerization method. Electrochemical studies like cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy were analyzed. Under the optimized conditions, the nanocomposites demonstrated remarkable electrochemical performances as supercapacitor electrode with outstanding specific capacitances of ~725 F g^?1 at a current density of 1 A g^?1, and retained ~325 F g^?1 even at a high current density of 20 A g^?1 in 0.5 M H₂SO₄?+?0.5 M Na₂SO₄ solution. The excellent cycling stability with capacitance retention of 80% after 1000 cycles may be a potential electrode material for future supercapacitor when its cycling stability and rate performance are addressed.     

Synthesis and characterization of nanotree-like polyaniline electrode material for supercapacitors

In this work, polyamidoamine (PAMAM) dendrimers are used as templates to synthesize nanotree-like polyaniline (PANI) materials, which are further used as electrode materials for supercapacitors. Effects of PAMAM content on PANIs’ structural characteristics and electrochemical properties are investigated detailedly. SEM images show that at 0.1 % PAMAM content, the PANI presents a stable self-assembled dendritic structure composed of many nanorods. Compared with pure PANI, nanotree-like PANI has better crystallinity, higher doping degree, larger high surface area and better reactivity. At a current density of 1 A g^?1, the PANI nanotree electrode displays a high specific capacitance value of 812 F g^?1, which is 119 % higher than that of pure PANI. Even after 1000 cycles, it still maintains about 69 % of the initial capacitance, showing good electrochemical stability. Thus the PANIs with nanotree structures are promising electrode materials for high-performance electrical energy storage devices. Moreover, the possible electrochemical enhancement mechanism of PANI nanotree electrode for supercapacitor is also discussed.     

Preparation of amorphous cobalt/carbon nanofibers composite as binder-free and conductive-free electrode materials for high supercapacitor

One dimensional carbon nanofibers embedded amorphous cobalt oxide with high electrochemical performances were successfully prepared by electrospinning Co(NO₃)₂ in PAN/DMF solution followed by a high-temperature heat treatment process. The different molar ratio of AN/Co(NO₃)₂ were synthesis. The optimized Co/CNFs(30), in which the molar ratio of AN/Co(NO₃)₂ was 30/1, exhibited a specific capacitance of 1096 F g^?1 at 1 A g^?1 and almost no decay in specific capacitance after cycling 2500 times at 5 A g^?1. The Co/CNFs were characterized by scanning electron microscopy, X-ray diffraction, Raman, transmission electron microscopy, X-ray photoelectron spectroscopy, thermal-gravity-analysis and the N₂ adsorption–desorption. The result showed that cobalt element was successfully dispersed in the carbon nanofibers with an amorphous state.     

Nitrogen-doped multiwalled carbon nanotubes decorated with copper(I) oxide nanoparticles with enhanced capacitive properties

We report on the enhanced capacitive properties of a copper(I) oxide nanoparticle (Cu₂O NP)-decorated multiwalled carbon nanotube (MWCNT) forest with nitrogen (N) doping. A careful in situ solid-state dewetting and plasma doping method was developed that ensured homogeneous decoration and contamination-free Cu₂O NPs with N doping on the nanotube sidewalls. The morphology and structure of the hybrid materials were characterised by scanning electron microscopy, transmission electron microscopy, energy-dispersive spectroscopy, Raman spectroscopy and X-ray photoemission spectroscopy. The electrochemical performance of the hybrid materials was investigated by cyclic voltammetry and galvanostatic charge/discharge tests in a 0.1 M Na₂SO₄ electrolyte. The electrochemical tests demonstrated that the Cu₂O NP/N-MWCNT electrode exhibits a specific capacitance up to 132.2 F g^?1 at a current density of 2.5 A g^?1, which is 30% higher than that of the pure MWCNT electrode. Furthermore, the electrode could retain the specific capacitance at 85% stability over 1000 cycles. These observations along with the simple assembly method for the hybrid materials suggest that the Cu₂O NP/N-MWCNT could be a promising electrode for supercapacitor applications.     

The preparation of novel hollow tetragonal starlike polyaniline and its electrochemical performance

Novel hollow tetragonal starlike polyaniline (HTS-PANI) doped with citric acid has been successfully synthesized by hydrothermal method for the first time. Scanning electron microscopy, transmission electron microscopy, UV–visible spectroscopy, Fourier transmission infrared spectroscopy, and X-ray diffraction were employed to analysis the morphology and structure of the obtained PANI. The results show that the HTS-PANI is in semi redox state and highly crystallized, accompanied with good thermal stability. According to the galvanostatic charge–discharge analysis, the specific capacitance of the sample is up to 460 F g^?1 at a current density of 0.2 A g^?1 in 1 M KCl electrolyte, and retains about 58 % after 1,000 charge–discharge processes at a current density of 5 A g^?1.     

Electrochemical co-deposition and characterization of MnO2/SWNT composite for supercapacitor application

Manganese oxide/single-wall carbon nanotubes (MnO₂/SWNT) composite was co-deposited by the potentiostatic method on a graphite slice. Morphological and structural performances for MnO₂/SWNT composite were characterized by means of scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The wall surface of SWNT was wrapped by ramsdellite MnO₂ nanoparticles to fabricate MnO₂/SWNT coaxial nanotubes, which further interconnected other MnO₂ particles to form the porous MnO₂/SWNT composite. The electrochemical properties were examined by cyclic voltammograms, galvanostatic charge and discharge and electrochemical impedance spectrum. A high specific capacitance of 421 F g^?1 was obtained for overall MnO₂/SWNT composite electrode at the constant current density of 1 A g^?1 in 3 mol L^?1 KCl solution.     

Synthesis and electrochemical performances of a novel two-dimensional nanocomposite: polyaniline-coated laponite nanosheets

A novel two-dimensional nanocomposite, polyaniline-coated laponite (polyaniline/laponite) nanosheets, has been prepared by in situ oxidative polymerization of aniline on the surface of laponite nanosheets. These sheets present a loosely stacked structure with the formation of a great number of pores, which it can provide a larger electrode/electrolyte contact surface area, shorten the path for ions transport in the active material, and alleviate the expansion and contraction of the electrode material during the charge/discharge processes, leading to an improved electrochemical performance. As an active material for supercapacitors, the specific charge/discharge capacitance of polyaniline/laponite nanosheets is 375 and 330 F g^?1 (based on the total working electrode mass) at a current density of 0.5 A g^?1, respectively, with a coulombic efficiency of 88 % which is higher than that of pure polyaniline (28 %). Moreover, polyaniline/laponite nanosheets also show a good rate capability with a growth of current density from 0.5 to 30 A g^?1, a specific discharge capacitance of 266 F g^?1 remained at 30 A g^?1. This work suggests a strategy to improve the electrochemical performances of polyaniline.     

Synthesis and characterization of polypyrrole nanotubes/multi-walled carbon nanotubes composites with superior electrochemical performance

Novel polypyrrole nanotubes/multi-walled carbon nanotubes (PPyNTs/MWCNTs) composites have been successfully synthesized via in situ chemical oxidation polymerization with methyl orange as soft template. Scanning electron microscopy and transmission electron microscopy images revealed that MWCNTs intertwined with the PPyNTs and PPyNTs/MWCNTs composites formed in water–ethanol solution. The obtained composites exhibited perfect electrochemical characteristic compared with PPyNTs and MWCNTs owing to the synergetic effect and the specific capacitance of the composites was strongly influenced by the mass ratio of pyrrole to MWCNTs. According to the galvanostatic charge/discharge analysis, the specific capacitance of PPyNTs/MWCNTs composites is up to 352 F g^?1 at a current density of 0.2 A g^?1 in 1 M KCl solution, much higher than that of the PPyNTs (178 F g^?1) and MWCNT (46 F g^?1), suggesting its potential application in supercapacitors.     

Improvement of hydrothermally synthesized MnO2 electrodes on Ni foams via facile annealing for supercapacitor applications

Nanostructured manganese dioxide (MnO₂) is deposited on nickel foams by a hydrothermal synthesis route. As-deposited MnO₂ thin films are largely amorphous. Facile post-deposition annealing significantly improves the electrochemical performance of the MnO₂ thin films via changing their morphology, phase, and crystallinity. The specific capacitance of the MnO₂ electrode increases with the annealing temperature and reaches an optimal value of 244 F g^?1 (at the current density of 1 A g^?1) in a neutral 1 M Na₂SO₄ electrolyte for a specimen annealed at 500 °C. Furthermore, when an alkaline 5 M KOH electrolyte is used, an exceptionally high capacitance of 950 F g^?1 is achieved at the current density of 2 A g^?1. The cost-effective facile synthesis, high specific capacitance, and good cycle stability of these MnO₂-based electrodes enable their applications in high-performance supercapacitors.     

One-step preparation of graphene nanosheets via ball milling of graphite and the application in lithium-ion batteries

An improved method for mass production of good-quality graphene nanosheets (GNs) via ball milling pristine graphite with dry ice is presented. We also report the enhanced performance of these GNs as working electrode in lithium-ion batteries (LIBs). In this improved method, the decrease of necessary ball milling time from 48 to 24 h and the increase of Brunauer–Emmett–Teller surface area from 389.4 to 490 m²/g might be resulted from the proper mixing of stainless steel balls with different diameters and the optimization of agitation speed. The as-prepared GNs are investigated in detail using a number of techniques, such as scanning electron microscope, atomic force microscope, high-resolution transmission electron microscopy, selected area electron diffraction, X-ray diffractometer, and Fourier transform infrared spectroscopic. To demonstrate the potential applications of these GNs, the performances of the LIBs with pure Fe₃O₄ electrode and Fe₃O₄/graphene (Fe₃O₄/G) composite electrode were carefully evaluated. Compared to Fe₃O₄-LIBs, Fe₃O₄/G-LIBs exhibited prominently enhanced performance and a reversible specific capacity of 900 mAh g^?1 after 5 cycles at 100 and 490 mAh g^?1 after 5 cycles at 800 mA g^?1. The improved cyclic stability and enhanced rate capability were also obtained.     

Facile fabrication of activated carbonized horseweed-based biomaterials and their application in supercapacitors

Carbonized horseweed was prepared for the first time using KOH as activating agent and employed as an electrode material. Varying the KOH/C weight ratio had a dramatic effect on the electrochemical capacitance of this electrode material. The obtained results showed that the sample prepared using a KOH/C weight ratio of 5/1 exhibited the highest specific surface area of 1469 m² g^?1, with average pore diameter of 3.18 nm. Further, this sample also exhibited the highest specific capacitance (184.2 F g^?1) at a current density of 0.4 A g^?1 in 6 M KOH electrolyte. In addition, the sample retained 97.6 % of its initial specific capacitance even after 1000 cycles, owing to the formation of a microporous/mesoporous structure by the activation process, the structure which provided suitable sites for charge transport and electrolyte diffusion. Thus, activated microporous carbon materials derived from horseweed could be effective as electrode materials in supercapacitors.     

Flower-like hierarchical porous nitrogen-doped carbon spheres from a facile one-step carbonization method for supercapacitors

A facile one-step carbonization method was developed to fabricate flower-like hierarchical porous nitrogen-doped carbon sphere (FHPNCS) from polyimide using polyurethane foam as macroporous scaffold. The FHPNCS possessed flower-like spherical morphology, well-developed hierarchical porous structure, high specific surface area and nitrogen-containing functional groups. These advantages led to excellent electrochemical performance. The FHPNCS electrode exhibited a high specific capacitance of 251.6 F g^?1 at 1 A g^?1, a high rate capability of 76% capacitance retention at 5 A g^?1, and an outstanding cycling stability of only 4.4% loss in specific capacitance after 2000 cycles. Compared with previously reported multi-step templating methods, the present method only involves a facile thermal treatment procedure and avoids the use of hard templates and toxic raw materials, thus exhibiting great potential for large scale production of nitrogen-doped carbon materials for practical applications in supercapacitors.     

A facile synthetic method and electrochemical performances of nickel oxide/carbon fibers composites

The uniform and completed nanofilms of nickel oxide (NiO) were electrodeposited on the carbon fibers (CFs) by a facile method of cyclic voltammetric. The as-prepared NiO/CFs composites can be used as a flexible electrode for electrochemical supercapacitors. Electrochemical measurements showed that 1.0-NiO/CFs had a good redox process and reversibility, and displayed the specific capacitances as high as 929 F g^?1 at a current density of 1 A g^?1. After 5000 cycles of charge and discharge, the 1.0-NiO/CFs composite materials could retain more than 88% of initial capacitance and show an excellent cyclability. Meanwhile, this supercapacitor exhibited a higher energy density of 20.8 Wh kg^?1 at a power density of 200 W kg^?1. The carbon fibers acting as active substrate for the composite electrode are a good conductor and have a larger capacitance of electrical double layer. The nanofilm structure of NiO could facilitate the contact of the electrolyte with the active materials, thus increasing the Faradaic pseudo-capacitance.     

A promising mechanical ball-milling method to synthesize carbon-coated Co<Subscript>9</Subscript>S<Subscript>8</Subscript> nanoparticles as high-performance electrode for supercapacitor

A Co₉S₈/C nanocomposite has been prepared using a solid-state reaction followed by a facile mechanical ball-milling treatment with sucrose as the carbon source. The phases, morphology, and detailed structures of Co₉S₈/C nanocomposite are well characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy. Our experimental results show that not only a process of particle size reduction, the ball-milling treatment also promotes the carbon and Co₉S₈ combining with each other more effectively to form an ultrafine nanocomposite. When used as an electrode material in supercapacitor, Co₉S₈/C nanocomposite exhibits a high initial specific capacitance of 756.2 F g^?1 at 1 A g^?1 and excellent cycling stability with 73.4% retention after 2000 cycles. Its outstanding electrochemical properties are mainly attributed to the nanosize of particles and amorphous carbon layer coating on its surface.     

Samarium-doped Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanoparticles with improved magnetic and supercapacitive performance: a novel preparation strategy and characterization

Sm³⁺-doped magnetite (Fe₃O₄) nanoparticles were synthesized through a one-pot facile electrochemical method. In this method, products were electrodeposited on a stainless steel (316L) cathode from an additive-free 0.005 M Fe(NO₃)₃/FeCl₂/SmCl₃ aqueous electrolyte. The structural characterizations through X-ray diffraction, field-emission electron microscopy, and energy-dispersive X-ray indicated that the deposited material has Sm³⁺-doped magnetite particles with average size of 20 nm. Magnetic analysis by VSM revealed the superparamagnetic nature of the prepared nanoparticles (Ms = 41.89 emu g^?1, Mr = 0.12 emu g^?1, and H _Ci = 2.24 G). The supercapacitive capability evaluation of the prepared magnetite nanoparticles through cyclic voltammetry and galvanostat charge–discharge showed that these materials are capable to deliver specific capacitances as high as 207 F g^?1 (at 0.5 A g^?1) and 145 F g^?1 (at 2 A g^?1), and capacity retentions of 94.5 and 84.6% after 2000 cycling at 0.5 and 1 A g^?1, respectively. The results proved the suitability of the electrosynthesized nanoparticles for use in supercapacitors. Furthermore, this work provides a facile electrochemical route for the synthesis of lanthanide-doped magnetite nanoparticles.     

Preparation and electrochemical properties of coke powder activated carbon based electrode materials

Coke powder activated carbons (CPACs) were prepared using coke powder as raw materials. The as-prepared CPACs were characterized by energy dispersive spectrometer, scanning electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction. The BET surface area and porous properties of CPACs were evaluated by nitrogen adsorption at 77 K. The optimal preparation conditions for CPACs were optimized according to the results of a series of electrochemical performance measurements by using the as-prepared CPACs as electrode in 2 mol L^?1 electrolyte solution of KOH. The CPACs, which were prepared under optimal conditions, exhibit a BET surface area of 285.6 m² g^?1, a pore volume of 0.112 m³ g^?1, a statistical average pore diameter of 10 nm, and ratios of the sum of micro and meso pore volume to the total pore volume of about 80.4 %, respectively. The electrochemical measurement results show that the electrode prepared by CPACs under optimal preparation conditions exhibits a specific capacitance of 211 F g^?1. Moreover, it also exhibits a good electrochemical stability with a specific capacitance reaches up to 156 F g^?1 over consecutive 1,000 cycle numbers.     

Vertically-aligned Mn(OH)<Subscript>2</Subscript> nanosheet films for flexible all-solid-state electrochemical supercapacitors

The arrangement of the electrode materials is a significant contributor for constructing high performance supercapacitor. Here, vertically-aligned Mn(OH)₂ nanosheet thin films were synthesized by cathodic electrodeposition technique on flexible Au coated polyethylene terephthalate substrates. Morphologies, microstructures, chemical compositions and valence state of the nanosheet films were characterized systematically. It shows that the nanosheets arranged vertically to the substrate, forming a porous nanowall structures and creating large open framework, which greatly facilitate the adsorption or diffusion of electrolyte ions for faradaic redox reaction. Electrochemical tests of the films show the specific capacitance as high as 240.2 F g^?1 at 1.0 A g^?1. The films were employed to assemble symmetric all-solid-state supercapacitors with LiCl/PVA gel severed as solid electrolyte. The solid devices exhibit high volumetric capacitance of 39.3 mF?cm^?3 at the current density 0.3 mA cm^?3 with robust cycling stability. The superior performance is attributed to the vertically-aligned configuration.     

Building an interpenetrating network of Ni(OH)<Subscript>2</Subscript>/reduced graphene oxide composite by a sol–gel method

Herein, a facile sol–gel strategy for building the ordered interpenetrating network of Ni(OH)₂ and reduced graphene oxide (rGO) was proposed. In this strategy, rGO nanosheets were homogeneously fixed inside composite utilizing the pores of Ni(OH)₂ gel as template, forming rGO-interpenetrated gel network. It was found that the rGO nanosheets could effectively reduce the internal resistant of composites and provide mechanical support for the gel network of Ni(OH)₂. Therefore, the composite presented high electrochemical performance, especially high-rate performance, due to the interpenetrating of rGO nanosheets plus the supplementary role of acetylene black. It had high specific capacitance of 2163 F g^?1 at low current density of 2.9 A g^?1 and 733 F g^?1 at high current density of 86.8 A g^?1.     

Porous nanocrystalline TiO2 with high lithium-ion insertion performance

Porous nanocrystalline anatase TiO₂ was prepared by a modified hydrolytic route coupled with an intermediary amorphization/recrystallization process. The phase structure and morphology of the products were analyzed by X-ray diffraction, transmission electron microscopy, and field-emission scanning electron microscopy. The electrochemical properties were investigated by cyclic voltammetry, constant current discharge–charge tests, and electrochemical impedance techniques. Applied as an anode in a lithium-ion battery, the material exhibited excellent specific capacities of 130 mAh g^?1 (at the rate of 2000 mA g^?1) and 96 mAh g^?1 (at the rate of 4000 mA g^?1) after 100 cycles; the coulombic efficiency was ~99.5 %, indicating excellent rate capability and reversibility. Furthermore, the electrochemical impedance spectra showed improved electrode kinetics after cycling. These results indicate that the porous nanocrystalline TiO₂ synthesized by this improved synthesis route might be a promising anode material for high energy and high power density lithium-ion battery applications.     

Super-capacitive performance depending on different crystal structures of MnO2 in graphene/MnO2 composites for supercapacitors

In the present study, we synthesize nanoneedle structures of MnO₂/graphene nanocomposites (N-RGO/MnO₂) and birnessite-type MnO₂/graphene nanocomposites (B-RGO/MnO₂). The morphologies and microstructures of as-prepared composites are characterized by X-ray diffractometry, field-emission scanning electron microscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis. Our characterizations indicate that nanoneedle structures of MnO₂ and birnessite-type MnO₂ are successfully formed on graphene surfaces. Capacitive properties of the N-RGO/MnO₂ and B-RGO/MnO₂ electrodes are measured using cyclic voltammetry, galvanostatic charge/discharge tests, and electrochemical impedance spectroscopy in a three-electrode experimental setup using a 1 M Na₂SO₄ aqueous solution as the electrolyte. The N-RGO/MnO₂ electrode displays a specific capacitance as high as 327.5 F g^?1 at 10 mV s^?1, which is higher than that of a B-RGO/MnO₂ electrode (248.5 F g^?1). It is believed that the nanoneedle structure of MnO₂ shows excellent electrochemical properties than birnessite-type MnO₂ for the electrode materials for supercapacitors.