Conversion of pencil graphite to graphene/polypyrrole nanofiber composite electrodes and its doping effect on the supercapacitive properties

Graphene platelets were synthesized from pencil flake graphite and commercial graphite by chemical method. The chemical method involved modified Hummer's method to synthesize graphene oxide (GO) and the use of hydrazine monohydrate to reduce GO to reduced graphene oxide (rGO). rGO were further reduced using rapid microwave treatment in presence of little amount of hydrazine monohydrate to graphene platelets. Chemically modified graphene/polypyrrole (PPy) nanofiber composites were prepared by in situ anodic electropolymerization of pyrrole monomer in the presence of graphene on stainless steel substrate. The morphology, composition, and electronic structure of the composites together with PPy fibers, graphene oxide (GO), rGO, and graphene were characterized using X‐ray diffraction (XRD), laser‐Raman, and scanning electron microscopic (SEM) methods. From SEM, it was observed that chemically modified graphene formed as a uniform nanocomposite with the PPy fibers absorbed on the graphene surface and/or filled between the graphene sheets. Such uniform structure together with the observed high conductivities afforded high specific capacitance and good cycling stability during the charge–discharge process when used as supercapacitor electrodes. A specific capacitance of supercapacitor was as high as 304 F g^?1 at a current density of 2 mA cm^?1 was achieved over a PPy‐doped graphene composite. POLYM. ENG. SCI., 55:2118–2126, 2015. © 2014 Society of Plastics Engineers     

Graphene/Polyaniline Aerogel with Superelasticity and High Capacitance as Highly Compression-Tolerant Supercapacitor Electrode

Superelastic graphene aerogel with ultra-high compressibility shows promising potential for compression-tolerant supercapacitor electrode. However, its specific capacitance is too low to meet the practical application. Herein, we deposited polyaniline (PANI) into the superelastic graphene aerogel to improve the capacitance while maintaining the superelasticity. Graphene/PANI aerogel with optimized PANI mass content of 63 wt% shows the improved specific capacitance of 713 F g^?1 in the three-electrode system. And the graphene/PANI aerogel presents a high recoverable compressive strain of 90% due to the strong interaction between PANI and graphene. The all-solid-state supercapacitors were assembled to demonstrate the compression-tolerant ability of graphene/PANI electrodes. The gravimetric capacitance of graphene/PANI electrodes reaches 424 F g^?1 and retains 96% even at 90% compressive strain. And a volumetric capacitance of 65.5 F cm^?3 is achieved, which is much higher than that of other compressible composite electrodes. Furthermore, several compressible supercapacitors can be integrated and connected in series to enhance the overall output voltage, suggesting the potential to meet the practical application.     

CoO/rGO composite prepared by a facile direct-flame approach for high-power supercapacitors

In this study, CoO nanoparticles (NPs) measuring approximately 20?nm in size are successfully grown on reduced graphene oxide (rGO) layers through a facile direct-flame approach. The obtained CoO/rGO nanocomposites are applied as electrode materials and show a high specific capacitance, reaching 1615.0?F?g^?1 at a current of 1?A?g^?1 (737.5?F?g^?1 at 50?A?g^?1), and good cycling stability (88.12% retention after more than 15,000 cycles at 5?A?g^?1), which are outstanding characteristics compared with those of recently reported pseudosupercapacitors. Furthermore, an asymmetric supercapacitor (ASC) produced using CoO/rGO as a positive electrode material and activated graphene (AG) as a negative electrode achieves a high cell voltage of 1.6?V and delivers a maximum energy density of 62.46?Wh?kg^?1 at a power density of 1600?W?kg^?1. The fabrication technique is facile and represents a promising means of obtaining metal oxide/graphene composites for high-performance supercapacitors.     

Multiwalled carbon nanotubes/polypyrrole/graphene/nonwoven fabric composites used as electrodes of electrochemical capacitor

The reduced graphene oxide/nonwoven fabric (rGO/NWF) composites have been fabricated through heating the NWF coated with the mixture of GO and HONH₂·HCl at 130°C, during which the GO is chemically reduced to rGO. Then the composites of polypyrrole (PPy)/rGO/NWF have been prepared through chemically polymerizing pyrrole vapor by using the FeCl₃·6H₂O adsorbed on rGO/NWF substrate as oxidant. Finally, multiwalled carbon nanotubes (MWCNTs) are used as conductive enhancer to modify PPy/rGO/NWF through dip‐dry process to obtain MWCNTs/PPy/rGO/NWF. The prepared composites have been characterized and their capacitive properties have been evaluated in 1.0M KCl electrolyte by using two‐electrode symmetric capacitor test. The results reveal that MWCNTs/PPy/rGO/NWF possesses a maximum specific capacitance (C_sc) of about 319 F g^?1 while PPy/rGO/NWF has a C_sc of about 277.8 F g^?1 at the scan rate of 1 mV s^?1 and that optimum MWCNTs/PPy/rGO/NWF retains 94.5% of initial C_sc after 1000 cycles at scan rate of 80 mV s^?1 which is higher than PPy/rGO/NWF (83.4%). Further analysis reveals that the addition of MWCNTs can increase the charger accumulation at the outer and inner of the composites, which is favorable to improve the stability and the rapid charge‐discharge capacity. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41023.     

Synthesis of rGO/nanoclay/PVK nanocomposites,electrochemical performances of supercapacitors

ABSTRACT

In this study, graphene oxide (GO) was chemically reacted with sodium borohydride (NaBH₄) to form reduced graphene oxide (rGO). rGO, Montmorillonite nanoclay, and polyvinylcarbazole (PVK) were used to form a ternary nanocomposite via chemical reaction. These nanocomposite qualities were described via scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX), Fourier transform infrared spectroscopy-attenuated transmission reflectance (FTIR-ATR). In addition, these materials were used in supercapacitor device as an active material to test electrochemical performances via cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS). The rGO/nanoclay/PVK nanocomposite shows significantly improved specific capacitance (C_sp = 168.64 Fg^?1) compared to that of rGO (C_sp = 63.26 Fg^?1) at the scan rate of 10 mVs^?1 by CV method. The enhanced capacitance results in high power density (P = 5522.6 Wkg^?1) and energy density (E = 28.84 Whkg^?1) capabilities of the rGO/nanoclay/PVK nanocomposite material. The addition of nanoclay and PVK increased the specific capacitance of rGO material due to a dopant effect for supercapacitor studies. Ragone plots were drawn to observe energy and power density of supercapacitor devices. The C_sp of rGO/nanoclay/PVK nanocomposite has only 86.4% of initial capacitance for charge/discharge performances obtained by CV method for 5000 cycles.     

One-step electrosynthesis of MnO2/rGO nanocomposite and its enhanced electrochemical performance

We present a facile one-step electrochemical approach to generate MnO₂/rGO nanocomposite from a mixture of Mn₃O₄ and graphene oxide (GO). The electrochemical conversion of Mn₃O₄ into MnO₂ through potential cycling is expedited in the presence of GO while the GO is reduced into reduced graphene oxide (rGO). The MnO₂ nanoparticles are evenly distributed on the rGO nanosheets and act as the spacer to prevent rGO nanosheets from restacking. This unique structure provides high electroactive surface area (1173?m² g^?1) that improves ions diffusion within the MnO₂/rGO structure. As a result, the MnO₂/rGO nanocomposite exhibits high specific capacitance of 473?F?g^?1 at 0.25?A?g^?1, which is remarkably higher (3 times) than the Mn₃O₄/GO prior conversion. In addition, the electrosynthesized nanocomposite shows higher conductivity and excellent potential cycling stability of 95% at 2000 cycles.     

Graphite oxide/polypyrrole composite electrodes for achieving high energy density supercapacitors

Fabrication and characterization of high energy density supercapacitor based on graphite oxide/polypyrrole (GO/PPy) composites is reported. Improvement in charge storage has been obtained by exfoliation of graphite oxide sheets via intercalation of polypyrrole. The formation of composite has been shown by the analysis of X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, and Fourier transfer of infrared spectroscopy data. Scanning electron and transmission electron microscopy clearly show sheet-like layered structure of graphite oxide surrounded by polypyrrole. Supercapacitors fabricated using this composite system result in a reduced equivalent series resistance value ~1.85 Ω. Such low value can be attributed to the intercalation of conducting polypyrrole into the graphite sheets. A specific capacitance of ~181 F g^?1 in 1 M Na₂SO₄ aqueous electrolyte with a corresponding specific energy density of ~56.5 Wh kg^?1 could be achieved. These values make GO-based materials suitable for their use as electrodes in high performance supercapacitors.     

Supercapacitor device performances of polycarbazole/graphene and polycarbazole/nanoclay/graphene nanocomposites

In this study, graphene oxide (GO) is chemically reacted with sodium borohydride (NaBH₄) to form reduced graphene oxide (rGO). rGO, polycarbazole (PCz)/rGO and PCz/nanoclay/rGO materials were obtained by chemical polymerisation method. These three materials were characterised by Fourier-transform infra-red spectroscopy-attenuated transmission reflectance, scanning electron microscopy, energy-dispersive X-ray analysis, cyclic voltammetry (CV), galvanostatic charge–discharge and electrochemical impedance spectroscopy. The PCz/nanoclay/rGO nanocomposite shows significantly improved capacitance (C_sp?=?187.78?F?g^?1) compared to that of PCz/rGO (C_sp?=?74.18?F?g^?1) and rGO (C_sp?=?20.78?F?g^?1) at the scan rate of 10?mV?s^?1 by CV method. The supercapacitor device performance results show high power density (P?=?1057.81?W?kg^?1) and energy density (E?=?1.7?Wh?kg^?1) obtained from Ragone plot for PCz/nanoclay/rGO material. Stability tests were also examined by the CV method for 1000 cycles.     

Facile synthesis of graphene/polyaniline composite hydrogel for high-performance supercapacitor

The graphene/polyaniline (PANI) composite hydrogel was successfully prepared by a one-step hydrothermal method. The morphology and structure of the sample were characterized by digital camera, scanning electron microscopy, and Fourier transform infrared spectroscopy spectra. By combining the advantages of high conductivity of graphene and high pseudocapacitance of PANI, the composite hydrogel was taken as supercapacitor electrode material. Cyclic voltammetry and galvanostatic charge/discharge experimental results show that the composite has excellent electrochemical performance. The specific capacitance value is 258.5 F g^?1 at a scan rate of 2 mV s^?1 and the specific capacitance value is up to 307 F g^?1 at a current density of 0.2 A g^?1. The specific capacitance value can still maintain 90 % of the initial value after repeating the galvanostatic charge–discharge for 1000 cycles at a current density of 1.0 A g^?1 showing good cycle stability.     

Electrical conductivity of silicon carbonitride‐reduced graphene oxide composites

Silicon carbonitride ceramics–reduced graphene oxide (SiCN–rGO) composites are synthesized using polyvinylsilazne (PVSZ) and GO as precursors and N‐dimethylformamide (DMF) as a solvent. We find that the electrical conductivity of SiCN–rGO composites exhibits nonmonotonic changes as a function of GO concentrations, in which the conductivity increases by six orders of magnitude from 7.41E‐09 (Ω/cm)^?1 to 4.35E‐03 (Ω/cm)^?1 after the integration of 0.2 wt% GO, followed by three orders of magnitude drop to 3.34E‐06 (Ω/cm)^?1 when 0.3 wt% GO is integrated. Based on the energy‐dispersive spectroscopy and Raman spectroscopy analysis, we conclude that the conductive behavior of SiCN–rGO composites is controlled by both the concentration and the distribution of “free‐carbon” in the composites.     

Nanodiamond particles/reduced graphene oxide composites as efficient supercapacitor electrodes

The paper reports on the preparation of reduced graphene oxide (rGO) modified with nanodiamond particles composites by a simple solution phase and their use as efficient electrode in electrochemical supercapacitors. The technique relies on heating aqueous solutions of graphene oxide (GO) and nanodiamond particles (NDs) at different ratios at 100 °C for 48 h. The morphological properties, chemical composition and electrochemical behavior of the resulting rGO/NDs nanocomposites were investigated using UV/vis spectrometry, Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, transmission electron microscopy (TEM) and electrochemical means. The electrochemical performance, including the capacitive behavior of the rGO/NDs composites were investigated by cyclic voltammetry and galvanostatic charge/discharge curves at 1 and 2 A g⁻¹ in 1 M H₂SO₄. The rGO/ND matrix with 10/1 ratio displayed the best performance with a specific capacitance of 186 ± 10 F g⁻¹ and excellent cycling stability.     

Preparation and capacitance performance of polyaniline/titanium nitride nanotube hybrid

A polyaniline/titanium nitride (PANI/TiN) nanotube hybrid was prepared and used for an electrochemical supercapacitor application. Firstly, the well-aligned TiN nanotube array was prepared by anodization of titanium foil and subsequent nitridation through ammonia annealing. Then, PANI was deposited into TiN nanotube through the electrochemical polymerization process. The obtained PANI/TiN nanotube hybrid had an ordered porous structure. A high specific capacitance of 1,066 F g^?1 was obtained at the charge–discharge current density of 1 A g^?1 when only the mass of PANI was considered. The specific capacitance can even achieve 864 F g^?1 at 10 A g^?1 and still keep 93 % of the initial capacity after 200 cycles. An aqueous supercapacitor, consisting of two symmetric PANI/TiN nanotube hybrid electrodes and 1.0 M H₂SO₄ electrolyte solution, showed the specific capacitance of 194.8 F g^?1, energy density of 9.74 Wh kg^?1, and power density of 0.3 kW kg^?1.     

One‐pot synthesis,characterization, and field emission investigations of composites of polypyrrole with graphene oxide,reduced graphene oxide,and graphene nanoribbons

Pyrrole monomer was polymerized by a chemical oxidative route in the presence of graphene oxide (GO), reduced GO (rGO), and graphene nanoribbons (GNR) separately to prepare composites of polypyrrole (PPy) as PPy–GO, PPy–rGO, and PPy–GNR, respectively. The morphological, chemical, and structural characterization of the as‐synthesized products was carried out using scanning electron microscopy, Raman spectroscopy, and Fourier transform infrared spectroscopy. Field emission studies of the PPy–GO, PPy–rGO, and PPy–GNR emitters were performed at the base pressure of 1 × 10^?8 mbar in a planar “diode” configuration. The turn‐on field values, corresponding to an emission current density of 1 µA/cm², are observed to be 1.5, 2.2, and 0.9 V/µm for the PPy–GO, PPy–rGO, and PPy–GNR emitters, respectively. The maximum emission current density of 2.5 mA/cm² is drawn from PPy–GO at an applied electric field of 3.2 V/µm, 1.2 mA/cm² at 3.6 V/µm from the PPy–rGO, and 8 mA/cm² at 2.2 V/µm from the PPy–GNR emitters. All of the composites exhibit good emission stability over more than 2 h. The results indicate the potential for a facile route for synthesizing composites of conducting polymers and graphene‐based materials, with enhanced functionality. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134 , 45170.     

The effect of halide ion concentration on capacitor performance

The effect of halide ion concentration on the capacitor performance was considered during this study. Iodide anion has been selected as the most profitable halide taking into account its electrochemical properties and environmental impact. Several concentrations of NaI were tested (from 0.25 to 5 mol L^?1 aqueous solutions) using as electrodes two commercial activated carbons and one KOH-activated carbon. Detailed electrochemical investigation by galvanostatic charging/discharging, cyclic voltammetry, and impedance spectroscopy confirmed the significant impact of iodide concentration on the supercapacitor behavior. The higher concentration of iodide affected especially the performance of positive electrode; increase of iodide concentration changed the potential range of positive electrode and its capacitance increased from 119 F g^?1 for 0.25 mol L^?1 NaI to 475 F g^?1 for 2 mol L^?1 NaI solution. The electrode capacitance measured in two-electrode system at current density of 2 A g^?1 ranged from 198 F g^?1 for 0.25 mol L^?1 NaI to 272 F g^?1 for 2 mol L^?1 NaI solution (capacitance expressed as average of the positive and negative electrode capacitances). It has been proved that 2 mol L^?1 alkali metal iodide solution is an optimal electrolyte for the capacitor based on KOH-activated carbon. High capacitance values and perfect stability (100 % retention) of such systems have been observed during long-term galvanostatic charging/discharging (15,000 cycles). In addition, satisfactory floating tests at extended voltage range (1.2 V) were performed.     

Simultaneous reduction and surface functionalization of graphene oxide for enhancing flame retardancy and thermal conductivity of mesogenic epoxy composites

Graphene oxide (GO ) is reduced and surface functionalized by 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide simultaneously. This functional reduced graphene oxide (F‐rGO ) with better thermal stability can be used as a nano‐filler to improve the flame retardancy, mechanical properties and thermal conductivity of mesogenic epoxy (EO ). Due to the presence of an oriented structure, EO is an intrinsic highly thermal conductive polymer compared with common polymer. After being filled with F‐rGO , the ordered domains in the EO matrix are connected by F‐rGO . As a result, the thermal conductivity coefficient of F‐rGO /EO composite is increased by 30.8% compared with pure EO . The dynamic mechanical analysis results indicate that E ' of F‐rGO /EO is 26.7% higher than that of EO . Because of the stable structure of F‐rGO , F‐rGO /EO is self‐extinguishing. The total heat release of F‐rGO /EO ‐15 is 24.1 kJ g^?1, which is 5.6 kJ g^?1 lower than that of EO . © 2016 Society of Chemical Industry     

Electrochemical reduction of graphene oxide in organic solvents

Graphene oxide (GO) cast on conductive substrates was electrochemically reduced in some organic solvents. The amount of electricity required for the almost complete reduction of GO was 2.0 C for 1 mg GO, corresponding to attaching of a one-electron reducible species to each benzene ring in graphene. The electrochemically reduced GO film gave an electrical conductivity of about 3 S cm⁻¹ and exhibited a relatively high specific capacitance of 147.2 F g⁻¹ in propylene carbonate. The electrochemical reduction of GO was feasible on Al foils as well.     

Preparation and characterization of reduced graphene oxide-reinforced boron carbide ceramics by self-assembly polymerization and spark plasma sintering

A self-assembly polymerization process was used to prepare graphene oxide/boron carbide (GO/B₄C) composite powders, spark plasma sintering (SPS) was used to fabricate reduced graphene oxide/boron carbide (rGO/B₄C) composites at 1800 °C and 30 MPa with a soaking time of 5 min. The effects of rGO addition on mechanical properties of the composites, such as Vickers hardness, flexural strength and fracture toughness, were investigated. The results showed that GO/B₄C composite powders were successfully self-assembled and a network structure was formed at high GO contents. The flexural strength and fracture toughness of rGO/B₄C composites were 643.64 MPa and 5.56 MPa m^1/2, respectively, at 1 and 2.5 wt.% rGO content, corresponding to an increase of 99.11% and 71.6% when compared to B₄C ceramics. Uniformly dispersed rGO in rGO/B₄C composites played an important role in improving their strength and toughness. The toughening mechanisms of rGO/B₄C composites were explained by graphene pull-out, crack deflection and bridging.     

Bio-inspired functionalization and redox charge transfer of graphene oxide sponges for pseudocapacitive electrodes

Bio-inspired and environmentally friendly chemical functionalization method for reducing of graphene oxide (GO) sponge using dopamine derivatives is developed. The GO sponge is readily modified by poly(norepinephrine) (p(Nor)) through a simple wet chemistry, and mechanical properties of the GO sponge is improved with structural stability. The oxidative polymerization of norepinephrine can considerably remove oxygen-containing species from the GO sponge surface as well as provide redox active quinone moieties for pseudocapacitive electrodes. Furthermore, three-dimensional (3D) macroporous networks of the reduced GO (rGO) sponge provide a large surface area and continuous ion pathways. As a result, p(Nor)-functionalized rGO (p(Nor)@rGO) sponge exhibits enhanced capacitive performance. The p(Nor)@rGO sponge shows that a specific capacitance of 232.1 F g⁻¹ (2.5 times higher than that of chemically converted rGO sponge) and good cyclic stability over 2000 charge/discharge cycling tests.     

Electrochemically synthesized graphene/polypyrrole composites and their use in supercapacitor

Composite films consisting of polypyrrole (PPy) and graphene oxide (GO) were electrochemically synthesized by electrooxidation of 0.1 M pyrrole in aqueous solution containing appropriate amounts of GO. Simultaneous chronoamperometric growth profiles and frequency changes on a quartz crystal microbalance showed that the anionic GO was incorporated in the growing GO/PPy composite to maintain its electrical neutrality. Subsequently, the GO was reduced electrochemically to form a reduced GO/PPy (RGO/PPy) composite by cyclic voltammetry. Specific capacitances estimated from galvanostatic discharge curves in 1 M H₂SO₄ at a current density of 1 A g^?1 indicated that values for the RGO/PPy composite were larger than those of a pristine PPy film and the GO/PPy composite. In the case of 6 mg mL^?1 GO for the preparation of GO/PPy, a high specific capacitance of 424 F g^?1 obtained at the electrochemically prepared RGO/PPy composite indicated its potential for use as an electrode material for supercapacitors.     

Rational design of electrochemically active polymorphic MnOx/rGO composites for Li+-rechargeable battery electrodes

Electrochemical behavior of different MnO_x @reduced graphene oxide (rGO) composites derived from a MnO₂/GO template are thoroughly investigated. As-prepared MnO₂/GO mixture is gradually converted to MnO₂/rGO and finally to Mn₃O₄/rGO composites under controlled post annealing conditions. The semispherical Mn₃O₄ crystalline compound anchored composite exhibits stable electrode performances, including both the Li⁺ anode and the Li⁺-air cathode catalyst, induced by the electrochemically favorable composite with an effective large contact area between the active materials and the electronic conductive rGO. It is such a meaningful to suggest the facile and controllable synthetic procedures for obtaining Li-rechargeable electrodes with a MnO_x nanoparticle-incorporated composites for the highly reactive lithiation/delithiation electrochemical reactions.