Graphene oxide doped polyaniline for supercapacitors

A novel high-performance electrode material based on fibrillar polyaniline (PANI) doped with graphene oxide sheets was synthesized via in situ polymerization of monomer in the presence of graphene oxide, with a high conductivity of 10 S cm^?1 at 22 °C for the obtained nanocomposite with a mass ratio of aniline/graphite oxide, 100:1. Its high specific capacitance of 531 F/g was obtained in the potential range from 0 to 0.45 V at 200 mA/g by charge–discharge analysis compared to 216 F/g of individual PANI. The doping and the ratio of graphene oxide have a pronounced effect on the electrochemical capacitance performance of the nanocomposites.     

Constructing a hierarchical graphene–carbon nanotube architecture for enhancing exposure of graphene and electrochemical activity of Pt nanoclusters

Stacking of individual graphene sheets (GS) is effectively inhibited by introducing one-dimensional carbon nanotubes to form a 3-D hierarchical structure which enhances the utilization of GS-based composites. From SEM images, CNTs are useful nanospacers for diminishing the face-to-face aggregation of GS. The specific electrochemically active surface area (SECSA) and specific double-layer capacitance (C_S,DL) of Pt/GS–CNTs (127.9 m²/g, 171.3 F/g) is much higher than that of Pt/GS (105.4 m²/g, 104.7 F/g) and Pt/CNTs (51.5 m²/g, 37.1 F/g), revealing the synergistic effects between GS and CNTs on enhancing the electrochemical activity of Pt nanoparticles and electrolyte-accessible surface area.     

A strategy for scalable synthesis of Li4Ti5O12/reduced graphene oxide toward high rate lithium-ion batteries

Li₄Ti₅O₁₂/reduced graphene oxide (RGO) composites were prepared via a simple strategy. The as-prepared composites present Li₄Ti₅O₁₂ nanoparticles uniformly immobilized on the RGO sheets. The Li₄Ti₅O₁₂/RGO composites possess excellent electrochemical properties with good cycle stability and high specific capacities of 154 mAh g^− 1 (at 10C) and 149 mAh g^− 1 (at 20C), much higher than the results found in other literatures. The superior electrochemical performance of the Li₄Ti₅O₁₂/RGO composites is attributed to its unique hybrid structure of conductive graphene network with the uniformly dispersed Li₄Ti₅O₁₂ nanoparticles.     

Multilayer graphene nanoribbons exhibit larger capacitance than their few-layer and single-layer graphene counterparts

This article demonstrates that it is not always beneficial to exfoliate graphitic structures to single-layer graphene to achieve maximum electrochemical performance. Using electrochemical impedance spectroscopy, we show that multilayer graphene nanoribbons with cross sections of 100 × 100 nm provide larger capacitance (15.6 F/g) than do few-layer graphene nanoribbons (14.9 F/g) and far larger capacitance than single-layer graphene nanoribbons (10.9 F/g) with the same cross section.     

Potentiostatically deposited nanostructured α-Co(OH)2: A high performance electrode material for redox-capacitors

A high specific capacitance was obtained for α-Co(OH)₂ potentiostatically deposited onto a stainless-steel electrode in 0.1 M Co(NO₃)₂ electrolyte at −1.0 V vs. Ag/AgCl. The structure and surface morphology of the obtained α-Co(OH)₂ were studied by using X-ray diffraction analysis and scanning electron microscopy. A network of nanolayered α-Co(OH)₂ sheets was obtained; the average thickness of individual α-Co(OH)₂ sheets was 10 nm, and the thickness of the deposit was several micrometers. The capacitive characteristics of the α-Co(OH)₂ electrodes were investigated by means of cyclic voltammetry and constant current charge–discharge cycling in 1 M KOH electrolyte. A specific capacitance of 860 F g⁻¹ was obtained for a 0.8 mg cm⁻² α-Co(OH)₂ deposit. The specific capacitance did not decrease significantly for the active mass loading range of 0.1–0.8 mg cm⁻² due its layered structure, which allowed easy penetration of electrolyte and effective utilization of electrode material even at a higher mass. This opens up the possibility of using such materials in supercapacitor applications.     

Synthesis and characterization of a nanocomposite of goethite nanorods and reduced graphene oxide for electrochemical capacitors

We report a one-step synthesis of a nanocomposite of goethite (α-FeOOH) nanorods and reduced graphene oxide (RGO) using a solution method in which ferrous cations serve as a reducing agent of graphite oxide (GO) to graphene and a precursor to grow goethite nanorods. As-prepared goethite nanorods have an average length of 200 nm and a diameter of 30 nm and are densely attached on both sides of the RGO sheets. The electrochemical properties of the nanocomposite were characterized by cyclic voltammetry (CV) and chronopotentiometry (CP) charge–discharge tests. The results showed that goethite/RGO composites have a high electrochemical capacitance of 165.5 F g^?1 with an excellent recycling capability making the material promising for electrochemical capacitors.     

A Co(OH)2−graphene nanosheets composite as a high performance anode material for rechargeable lithium batteries

A Co(OH)₂?graphene nanosheets (Co(OH)₂?GNS) composite as a high performance anode material was firstly prepared through a simultaneous hydrothermal method. The structure, morphology and electrochemical performance of the obtained samples were systematically investigated by X-ray diffraction (XRD), transmission electron microscope (TEM) and electrochemical measurements. According to the TEM analysis, the surface of the Co(OH)₂ is surrounded with GNS in the Co(OH)₂?GNS composite. The specific discharge (lithiation) and charge (delithiation) capacities of Co(OH)₂?GNS attain to 1599 and 1120 mAh/g at a current density of 200 mA/g in the first cycle, respectively. After 30 cycles, the reversible capacity of Co(OH)₂?GNS is still 910 mAh/g with the retention of 82%. The particular structure of Co(OH)₂ particles surrounded by the GNS could limit the volume change during cycling and provide an excellent electronic conduction pathway, which could be the main reason for the remarkable improvement of electrochemical performance.     

Electrochemical characterization on cobalt sulfide for electrochemical supercapacitors

High capacitance at a high charge–discharge current density of 50 mA/cm² for a new type of electrochemical supercapacitor cobalt sulfide (CoS_x) have been studied for the first time. The CoS_x was prepared by a very simply chemical precipitation method. The electrochemical capacitance performance of this compound was investigated by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge–discharge tests with a three-electrode system. The results show that CoS_x has excellent electrochemical capacitive characteristic with potential range −0.3 ∼ 0.35 V (versus SCE) in 6 M KOH solution. Charge–discharge behaviors have been observed with the highest specific capacitance values of 475 F/g at the current density of 5 mA/cm², even at the high current density of 50 mA/cm², CoS_x also shows the high specific capacitance values of 369 F/g.     

Hollow Co9S8 from metal organic framework supported on rGO as electrode material for highly stable supercapacitors

We demonstrate a hydrothermal method to fabricate a composite of reduced graphene oxide (rGO) with hollow Co₉S₈ derived from metal organic framework (MOF), which exhibits a high specific capacitance of 575.9 F/g at 2 A/g and 92.0% capacitance retention after 9000 cycles.     

Preparation of hexagonal nanoporous nickel hydroxide film and its application for electrochemical capacitor

Nanoporous nickel hydroxide film has been successfully electrodeposited on titanium substrate from nickel nitrate dissolved in the aqueous domains of the hexagonal lyotropic liquid crystalline phase of Brij 56. Low-angle X-ray diffraction (XRD), transmission electron microscopy (TEM), and atomic force microscopy (AFM) studies show that the film has a regular nanostructure consisting of a hexagonal array of cylindrical pores with a repeat center-to-center spacing of about 7 nm. Preliminary electrochemical studies are carried out using cyclic voltammetry (CV) and chronopotentiometry technology. A maximum specific capacitance of 578 F g⁻¹ could be achieved for the nanoporous Ni(OH)₂ film electrode, suggesting its potential application in electrochemical capacitors.     

Nanocomposite of manganese ferrocyanide and graphene: A promising cathode material for rechargeable lithium ion batteries

A nanocomposite of potassium manganese ferrocyanide and graphene (12% C, 88% K_1.8Mn_1.1Fe(CN)₆∙ 0.27H₂O) was prepared by ball milling of graphene oxide powder and nanoparticles of manganese–iron Prussian Blue. It exhibits enhanced electrochemical performance compared to pure Prussian Blue with a specific capacity of 150 mAhg^− 1 at average 3.8 V vs. Li⁺/Li and a good cyclability. The nanocomposite can be considered as competitive to standard cathode materials of present rechargeable lithium ion batteries like cobalt oxide, iron phosphate or NMC.     

Enhanced activity and stability of Pt catalysts on functionalized graphene sheets for electrocatalytic oxygen reduction

Electrocatalysis of oxygen reduction using Pt nanoparticles supported on functionalized graphene sheets (FGSs) was studied. FGSs were prepared by thermal expansion of graphite oxide. Pt nanoparticles with average diameter of 2 nm were uniformly loaded on FGSs by impregnation methods. Pt-FGS showed a higher electrochemical surface area and oxygen reduction activity with improved stability as compared with the commercial catalyst. Transmission electron microscopy, X-ray photoelectron spectroscopy, and electrochemical characterization suggest that the improved performance of Pt-FGS can be attributed to smaller particle size and less aggregation of Pt nanoparticles on the functionalized graphene sheets.     

Phase transfer catalysis: Synthesis of monodispersed FePt nanoparticles and its electrocatalytic activity

Using dibenzo-24-crown-8-ether (DB24C8) as phase transfer catalyst, the monodispersed iron–platinum (FePt) alloy nanoparticles with size of ∼17 nm were synthesized by reduction of H₂PtCl₆·6H₂O and FeCl₂·4H₂O in the solvothermal system. The structure, magnetic property and electrocatalytic activity of FePt nanoparticles were characterized by transmission electron microscopy (TEM), X-ray diffraction system (XRD), vibration sample magnetometer (VSM) and CHI 820 electrochemical analyser (three electrodes system, the reference electrode is saturated calomel electrode (SCE), the counter electrode is platinum electrode and the glassy carbon electrode is used as working electrode (GCE)), respectively. The results show that the as-synthesized FePt nanoparticles have a chemically disordered fcc structure and can be transformed into chemically ordered fct structure after annealing treatment above 400 °C, simultaneously accompanying with the coercivity changed from 5 to 2400 Oe. CVs of 0.5 M H₂SO₄/0.5 M CH₃OH on GCE modified with FePt nanoparticles monolayer illustrate that the as-synthesized FePt nanoparticles have strong electrocatalytic activity toward the oxidation of CH₃OH in aqueous solution.     

Nitrogen enriched mesoporous carbon spheres obtained by a facile method and its application for electrochemical capacitor

A kind of mesoporous carbon spheres (MCS) containing in-frame incorporated nitrogen has been prepared by a facile polymerization-induced colloid aggregation method. As the electrode material for electric double layer capacitor (EDLC) in 5 mol/L H₂SO₄, the MCS products present excellent specific capacitance as 211 F/g much larger than that of the most popularly applied activated carbon at a high discharge current density of 1 A/g. Its specific capacitance can still remain 200 F/g at 20 A/g. The superior electrochemical performance of MCS is associated with the following characteristics: high specific surface area (∼1330 m²/g) contributed mainly by the mesopores, uniform pore size as large as 29 nm and moderate content of nitrogen (10 wt%), which are the requirements for ideal supercapacitors.     

Arrayed CNx NT–RuO2 nanocomposites directly grown on Ti-buffered Si substrate for supercapacitor applications

Significant enhancement in supercapacitor performance has been achieved via a new RuO₂ nanocomposite materials prepared by direct ruthenium sputtering on arrayed multi-walled carbon nanotubes supported by Ti-buffered Si wafer. XPS, HRTEM and SAED analyses reveal that as-prepared nanoparticles have a crystalline Ru metal core with RuO₂ oxide coating. The nanocomposites convert to RuO₂–CN_x NTs with subsequent electrochemical cycling. At present, well-dispersed and strongly adhered RuO₂ NPs have been densely populated on CN_x NTs to obtain the overall specific capacitance (1380 F/g-RuO₂), charging–discharging rate (up to 600 mV/s) and operation stability (5000 cycles). Thus, RuO₂–CN_x NTs nanocomposites would make a promising candidate for use in next-generation high efficiency miniaturized supercapacitors directly fabricated on Si substrate.     

Iron (III) nanocomposites for enzyme-less biomimetic cathode: A promising material for use in biofuel cells

In this paper, we discuss the synthesis and electrochemical properties of a new material based on iron oxide nanoparticles stabilized with poly(diallyldimethylammonium chloride) (PDAC); this material can be used as a biomimetic cathode material for the reduction of H₂O₂ in biofuel cells. A metastable phase of iron oxide and iron hydroxide nanoparticles (PDAC–FeOOH/Fe₂O₃-NPs) was synthesized through a single procedure. On the basis of the Stokes–Einstein equation, colloidal particles (diameter: 20 nm) diffused at a considerably slow rate (D = 0.9 × 10^? 11 m s^? 1) as compared to conventional molecular redox systems. The quasi-reversible electrochemical process was attributed to the oxidation and reduction of Fe³⁺/Fe²⁺ from PDAC–FeOOH/Fe₂O₃-NPs; in a manner similar to redox enzymes, it acted as a pseudo-prosthetic group. Further, PDAC–FeOOH/Fe₂O₃-NPs was observed to have high electrocatalytic activity for H₂O₂ reduction along with a significant overpotential shift, ΔE = 0.68 V from ? 0.29 to 0.39 V, in the presence and absence of PDAC–FeOOH/Fe₂O₃-NPs. The abovementioned iron oxide nanoparticles are very promising as candidates for further research on biomimetic biofuel cells, suggesting two applications: the preparation of modified electrodes for direct use as cathodes and use as a supporting electrolyte together with H₂O₂.     

Flexible free-standing graphene-silicon composite film for lithium-ion batteries

Flexible, free-standing, paper-like, graphene-silicon composite materials have been synthesized by a simple, one-step, in-situ filtration method. The Si nanoparticles are highly encapsulated in a graphene nanosheet matrix. The electrochemical results show that graphene-Si composite film has much higher discharge capacity beyond 100 cycles (708 mAh g^? 1) than that of the cell with pure graphene (304 mAh g^? 1). The graphene functions as a flexible mechanical support for strain release, offering an efficient electrically conducting channel, while the nanosized silicon provides the high capacity.     

Electrodeposition of MnO2 nanowires on carbon nanotube paper as free-standing,flexible electrode for supercapacitors

MnO₂ nanowires were electrodeposited onto carbon nanotube (CNT) paper by a cyclic voltammetric technique. The as-prepared MnO₂ nanowire/CNT composite paper (MNCCP) can be used as a flexible electrode for electrochemical supercapacitors. Electrochemical measurements showed that the MNCCP electrode displayed specific capacitances as high as 167.5 F g⁻¹ at a current density of 77 mA g⁻¹. After 3000 cycles, the composite paper can retain more than 88% of initial capacitance, showing good cyclability. The CNT paper in the composite acted as a good conductive and active substrate for flexible electrodes in supercapacitors, and the nanowire structure of the MnO₂ could facilitate the contact of the electrolyte with the active materials, and thus increase the capacitance.     

Achieving high electrode specific capacitance with materials of low mass specific capacitance: Potentiostatically grown thick micro-nanoporous PEDOT films

Electrode materials for supercapacitors are at present commonly evaluated and selected by their mass specific capacitance (C_M, F g⁻¹). However, using only this parameter may be a misleading practice because the electrode capacitance also depends on kinetics, and may not increase simply by increasing material mass. It is therefore important to complement C_M by the practically accessible electrode specific capacitance (C_E, F cm⁻²) in material selection. Poly[3,4-ethylene-dioxythiophene] (PEDOT) has a mass specific capacitance lower than other common conducting polymers, e.g. polyaniline. However, as demonstrated in this communication, this polymer can be potentiostatically grown to very thick films (up to 0.5 mm) that were porous at both micro- and nanometer scales. Measured by both cyclic voltammetry and electrochemical impedance spectrometry, these thick PEDOT films exhibited electrode specific capacitance (C_E, F cm⁻²) increasing linearly with the film deposition charge, approaching 5 F cm⁻², which is currently the highest amongst all reported materials.     

Preparation of monodisperse carbon nanospheres for electrochemical capacitors

The efficiently hydrothermal route using sucrose without any catalysts is employed to prepare the uniform carbon spheres. The monodisperse 100–150 nm carbon spheres are obtained with the activation treatment in molten KOH. The carbon spheres are characterized by transmission electron microscope, X-ray diffraction, N₂ adsorption, Raman spectroscopy and electrochemical techniques. The relationships of specific capacitance and surface properties of carbon spheres are investigated. A single electrode of carbon nanosphere materials performs excellent specific capacitance (328 F g⁻¹), area capacitance (19.2 μF cm⁻²) and volumetric capacitance (383 F cm⁻³).