Electrochemically reduced graphene oxide sheets for use in high performance supercapacitors

Graphene oxide (GO) was reduced by a rapid, effective and eco-friendly electrochemical method of repetitive cathodic cyclic potential cycling, without using any reducing reagents. The electrochemically reduced graphene oxide (ERGO) was characterized by UV–vis, EIS and zeta-potential measurements. Most of the oxygen functional groups in ERGO were successfully removed resulting in smaller charge transfer resistance. However, some electrochemically stable residuals still remained, enabling ERGO to facilitate electrolyte penetration and pseudocapacitance. Since ERGO was readily stabilized by cathodic potential cycling, it exhibited an outstanding stability in cycle life, nearly with no capacitive loss from the second cycle on. A specific capacitance of 223.6 F g⁻¹ was achieved at 5 mV s⁻¹, which makes the ERGO a competitive material for electrochemical energy storage.     

Multiscale patterning of graphene oxide and reduced graphene oxide for flexible supercapacitors

A simple and facile method for multiscale, in-plane patterning of graphene oxide and reduced graphene oxide (GO–rGO) was developed by region-specific reduction of graphene oxide (GO) under a mild irradiation. The UV-induced reduction of graphene oxide was monitored by various spectroscopic techniques, including optical absorption, X-ray photoelectron spectroscopy (XPS), Raman, and X-ray diffraction (XRD), while the resultant GO–rGO patterned film morphology was studied on optical microscope, scanning electron microscope (SEM), and atomic force microscope (AFM). Flexible symmetric and in-plane supercapacitors were fabricated from the GO–rGO patterned polyethylene terephthalate (PET) electrodes to show capacitances up to 141.2 F/g.     

Covalently-grafted polyaniline on graphene oxide sheets for high performance electrochemical supercapacitors

A simple route to achieve covalently-grafted polyaniline (PANI)/graphene oxide (GO) nanocomposites has been developed. The synthesized composites showed a uniform hierarchical morphology of the PANI thin film and short rod-like nanostructures that had densely grown on the GO sheets, in contrast to the nonuniform morphology of noncovalently-grafted PANI/GO. Compared to pure PANI and noncovalently-grafted PANI/GO composites, the covalently-grafted PANI/GO composites possessed a much larger specific surface area and pore volume, which increased the accessible surface area for the redox reaction and allowed faster ion diffusion. This unique hierarchical morphology maximized the synergistic effect between PANI and GO, resulting in excellent electrochemical performance (capacitance 442 F/g of PANI/GO (6:1) vs. 226 F/g of pure PANI) and improved cycling stability (83% @ 2000 cycles of PANI/GO (6:1) vs. 54.3% @ 1000 cycles of pure PANI). The enhanced electrochemical performance demonstrates the advantage of the PANI/GO composites prepared via this covalent grafting method.     

Functionalized three-dimensional graphene networks for high performance supercapacitors

Three-dimensional (3D) thermal reduced graphene network (TRGN) deposition on Ni foam without any conductive agents and polymer binders was successfully synthesized by dipping Ni foam into graphene oxide (GO) suspension and subsequent thermal reduction process. The direct and close contact between thermal reduced graphene and Ni foam is beneficial to the enhanced conductivity of the electrode, as well as the improvement of ion diffusion/transport into the electrode. Additionally, low-temperature reduction of GO possesses a large amount of stable oxygen-containing groups that can provide high pseudocapacitance. As a result, the TRGN electrode delivers a high specific capacitance of 442.8 F g⁻¹ at 2 mV s⁻¹ in 6 mol L⁻¹ KOH. Moreover, symmetric supercapacitor based on TRGN exhibits a maximum energy density of 30.4 Wh kg⁻¹ based on the total mass of the two electrodes in 1 mol L⁻¹ Na₂SO₄ electrolyte, as well as excellent cycling stability with 118% of its initial capacitance after 5000 cycles.     

Microwave exfoliated reduced graphene oxide epoxy nanocomposites for high performance applications

Graphene has captured the attention of scientific community due to recently emerging high performance applications. Hence, studying its reinforcing effects on epoxy resin is a significant step. In this study, microwave exfoliated reduced graphene oxide (MERGO) was prepared from natural graphite for subsequent fabrication of epoxy nanocomposites using triethylenetetramine (TETA) as a curing agent via in-situ polymerization. Thermogravimetric analysis (TGA), X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), C¹³ NMR spectroscopy, X-ray photoelectron spectroscopy (XPS) and ultraviolet–visible (UV–vis) spectroscopy were employed to confirm the simultaneous reduction and exfoliation of graphene oxide. The reinforcing effect of MERGO on epoxy resin was explored by investigating its static mechanical properties and dynamic mechanical analysis (DMA) at MERGO loadings of 0 to 0.5 phr. The micro-structure of epoxy/MERGO nanocomposites was investigated using scanning electron microscope (SEM), transmission electron microscope (TEM) and XRD techniques. The present work reports an enhancement of 32%, 103% and 85% in tensile, impact and flexural strength respectively of epoxy by the addition of even 0.25 phr MERGO. At this loading elastic and flexural moduli also increased by 10% and 65%, respectively. Single-edge-notch three-point-Bending (SEN-TPB) fracture toughness (K_IC) measurements were carried out where a 63% increase was observed by the introduction of 0.25 phr MERGO. The interfacial interactions brought about by graphene also benefited the dynamic mechanical properties to a large extent in the form of a significant enhancement in storage modulus and slightly improved glass transition temperature. Considerable improvements were also detected in dielectric properties. The epoxy nanocomposite also attained an ac conductivity of 10⁻⁵ S/m and a remarkable increase in dielectric constant. The simple and cost effective way of graphene synthesis for the fabrication of epoxy/MERGO nanocomposites may be extended to the preparation of other MERGO based polymer nanocomposites. This remarkable class of materials has thrown open enormous opportunities for developing conductive adhesives and in microelectronics.     

Tubular graphene nanoribbons with attached manganese oxide nanoparticles for use as electrodes in high-performance supercapacitors

Graphene nanoribbons (GNRs) with tubular shaped thin graphene layers were prepared by partially longitudinal unzipping of vapor-grown carbon nanofibers (VGCFs) using a simple solution-based oxidative process. The GNR sample has a similar layered structure to graphene oxide (GO), which could be readily dispersed in isopropyl alcohol to facilitate electrophoretic deposition (EPD). GO could be converted to graphene after heat treatment at 300 °C. The multilayer GNR electrode pillared with open-ended graphene tubes showed a higher capacitance than graphene flake and pristine VGCF electrodes, primarily due to the significantly increased surface area accessible to electrolyte ions. A GNR electrode with attached MnO₂ nanoparticles was prepared by EPD method in the presence of hydrated manganese nitrate. The specific capacitance of GNR electrode with attached MnO₂ could reach 266 F g⁻¹, much higher than that of GNR electrode (88 F g⁻¹) at a discharge current of 1 A g⁻¹. The hydrophilic MnO₂ nanoparticles attached to GNRs could act as a redox center and nanospacer to allow the storage of extra capacitance.     

Restacking-inhibited nitrogen-incorporated mesoporous reduced graphene oxides for high energy supercapacitors

Graphene is considered a promising active electrode material due to a large surface area, high electronic conductivity, and chemical and mechanical stabilities for supercapacitor (SC) applications. However, the current bottleneck is the fabrication of restacking-inhibited graphene on an electrode level which otherwise loses the capability to achieve the aforementioned properties. Herein, we demonstrate the synthesis of restacking-inhibited nitrogen (N)-incorporated mesoporous graphene for high energy SCs. The melamine-formaldehyde acts as a restacking inhibitor by forming a bonding with reduced graphene oxide (RGO) through a condensation reaction and as an N precursor to be decomposed to create open pores and N sources upon heat treatment. The d-spacing increases up to 0.352 nm and the surface area is as high as 698 m² g^?1 with high mesoporosity, confirming restacking inhibition by N incorporation decomposed by melamine-formaldehyde. The restacking-inhibited RGO-based SC cells in organic electrolyte show the specific capacitance of 25.8 F g^?1, the energy density of 21.8 kW kg^?1 and 85% of capacitance retention for 5000 cycles, which are better than those of pristine RGO-based cells. These improved SC performances are attributed to the fast ion transport through a mesoporous channel in crumpled structure and the doping effect of N incorporation. This work provides a simple yet effective chemical approach to fabricate restacking-inhibited RGO electrodes for improved SC performances.     

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.     

High performance supercapacitors based on reduced graphene oxide in aqueous and ionic liquid electrolytes

Partially reduced graphene oxide (RGO) has been fabricated using hydrobromic acid. Since hydrobromic acid is a weak reductant, some oxygen functional groups which are relatively stable for electrochemical systems remain in RGO. Therefore, RGO can be re-dispersed in water and 2–3 layers of graphene can be observed by transmission electron microscopy, showing excellent affinity with water. RGO facilitates the penetration of aqueous electrolyte and introduces pseudocapacitive effects. Moreover, its capacitive nature is enhanced after cycling measurements. It is concluded that the increase of capacitance is due to the reduction of the oxygen functional groups by the cyclic voltammetry and electrochemical impedance spectroscopy analysis. The electrochemical properties in the ionic liquid electrolyte, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIPF₆), are also investigated. At a current density of 0.2 A g⁻¹, the maximum capacitance values of 348 and 158 F g⁻¹ are obtained in 1 M H₂SO₄ and BMIPF₆, respectively.     

Simultaneously morphology and phase controlled synthesis of cobalt manganese hydroxides/reduced graphene oxide for high performance supercapacitor electrodes

Cobalt manganese hydroxides with well-defined nanowire morphology (CoMn-HW) is scalable fabricated by adjusting solution contents, Mn/Co ratio and alkaline species. To further improve the conductivity of CoMn-HW, GO is introduced during fabrication process and reduced to rGO according to the high temperature and alkali atmosphere. By optimizing the adding mass of rGO, CoMn-HW/rGO with sandwiched like structure is successfully synthesized for supercapacitor electrode. The composite delivers a high specific capacitance of 784 F g⁻¹ at current density of 0.5 A g⁻¹, good rate capability (84.2% capacitance retention after current density increase 10 times). Moreover, an asymmetric supercapacitor with CoMn-HW/rGO10 as the positive electrode and active carbon as the negative electrode, is assembled and delivers a maximum energy density of 38.3 Wh kg⁻¹ and power density of 8000 W kg⁻¹, representing its potential in energy storage and conversion systems.     

Rapid and controllable synthesis of nitrogen doped reduced graphene oxide using microwave-assisted hydrothermal reaction for high power-density supercapacitors

Nitrogen doped reduced graphene oxide (N-RGO) is synthesized using microwave-assisted hydrothermal (MAHA) reaction. The proper configurations of nitrogen atoms in graphene sheets considerably increase the intrinsic electrical properties of N-RGO resultantly improving its capacitance and other kinetic properties in supercapacitor. Here, under the controlled MAHA reaction, we adjusted the ratio of nitrogen configurations (pyridinic-N, pyrrolic-N and quaternary-N) for the most optimum supercapacitor performances of N-RGOs in the shortest time ever reported, and clarified that its enhanced electrical conductivity and supercapacitor performances are attributed to its enlarged concentration of quaternary-N. With this MAHA reaction, we present a supercapacitor based on N-RGO, which is capable of displaying the promising electrochemical properties.     

Fabrication of manganese oxide/three-dimensional reduced graphene oxide composites as the supercapacitors by a reverse microemulsion method

Manganese oxide (MnO₂)/three-dimensional (3D) reduced graphene oxide (RGO) composites were prepared by a reverse microemulsion (water/oil) method. MnO₂ nanoparticles (3–20 nm in diameter) with different morphologies were produced and dispersed homogeneously on the macropore surfaces of the 3D RGO. Scanning electron microscopy and transmission electron microscopy were applied to characterize the microstructure of the composites. The MnO₂/3D RGO composites, which were annealed at 150 °C, displayed a significantly high specific capacitance of 709.8 F g⁻¹ at 0.2 A g⁻¹. After 1000 cycles, the capacitance retention was measured to be 97.6%, which indicates an excellent long-term stability of the MnO₂/3D RGO composites.     

Synthesis of reduced graphene oxide/tungsten trioxide nanocomposite electrode for high electrochemical performance

A facile and well-controllable reduced graphene oxide/tungsten trioxide (rGO/WO₃) nanocomposite electrode was successfully synthesized via an electrostatic assembly route at 350 rpm for 24 h. In this study, hexagonal-phase WO₃ (h-WO₃) nanofiber was well distributed on rGO sheets by applying optimal processing parameters. The as-synthesized rGO/WO₃ nanocomposite electrode was compared with pure h-WO₃ electrode. A maximum specific capacitance of 85.7 F g⁻¹ at a current density of 0.7 A g⁻¹ was obtained for the rGO/WO₃ nanocomposite electrode, which showed better electrochemical performance than the WO₃ electrode. The incorporation of WO₃ into rGO could prevent the restacking of rGO and provide favourable surface adsorption sites for intercalation/de-intercalation reactions. The impedance studies demonstrated that the rGO/WO₃ nanocomposite electrode exhibited lower resistance because of the superior conductivity of rGO that improved ion diffusion into the electrode. To evaluate the contribution of WO₃ to the rGO/WO₃ nanocomposite, the influence of mass loading of WO₃ on the capacitance was investigated.     

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.     

Hierarchical MnNiCo ternary metal oxide/graphene nanoplatelets composites as high rated electrode material for supercapacitors

For achieving a higher supercapacitor performance, electrode material with high surface area and conductivity such as graphene, graphene nanoplatelets (GNP), and carbon nanotubes along with Transition Metal oxides (TMO) can be used. Herein, the composite of graphene nanoplatelets with ternary metal oxide of manganese, nickel, and cobalt (MNC) is synthesized through a facile cost-effective hydrothermal process and its compositional, morphological, and electrochemical properties are investigated. As-synthesized MNC-GNP composite showed excellent electrochemical properties owing to the high porosity offered by graphene nanoplatelets and synergistic effects produced by individual components of the composite. For comparative studies, ternary oxide MNC was prepared by the same hydrothermal route. The cubic structure of the MNC-GNP composite is confirmed by X-ray diffraction (XRD). Scanning Electron Microscope (SEM) showed distinct hierarchical dendritic structures which showed an increase in density by the addition of graphene nanoplatelets. Electrochemical testing revealed that MNC-GNP exhibited an enhanced specific capacity of 605 mAh g^-1 which is higher compared to MNC which exhibited 243 mAh g^-1 at a current density of 2 mV s^-1. GCD also depicted an increased charge-discharge time in the case of MNC-GNP as compared to its counterpart. MNC-GNP has also shown charge stability up to 99.5 % of capacity retention up to 1000 cycles. Hence, synthesized composite shows to be an effective electrode material for supercapacitors owing to enhanced electrochemical properties.     

Electrochemical properties of graphene nanosheet/carbon black composites as electrodes for supercapacitors

Graphene nanosheet/carbon black composites were prepared by the ultrasonication and in situ reduction methods. Microstructure measurements show that most carbon black particles deposit on the edge surfaces of nanosheets by the ultrasonication method, and on the basal surfaces of nanosheets by in situ reduction method. The electrochemical performances of hybrid materials are superior to pure graphene material, demonstrating that carbon black particles as spacers ensured the high electrochemical utilization of graphene layers as well as the open nano-channels provided by three-dimensional graphene nanosheet/carbon black hybrid material. Therefore, the resulting composite is a promising carbon material for supercapacitors.     

Chemically engineered graphene oxide as high performance cathode materials for Li-ion batteries

The development of environment-friendly electrode materials is highly desired for the clean and sustainable Li-ion batteries (LIBs) system. Organic cathode materials that involve conducting polymers, organic carbonyl/sulfur compounds are expected to be promising candidates for future LIBs with a concept of “green and sustainable”. However, their battery performances are relatively worse than that of inorganic counterparts due to their low electronic conductivity and unwanted dissolution reactions occurring in electrolytes. Aimed to alter their performances, we herein focuses on the preparation of upgraded organic materials by chemical engineering of graphene oxide (GO) and the systematic study of their electrochemical performance as positive electrodes for LIBs. The obtained decarboxylated GO and carbonylated/hydroxylated GO electrodes show significantly enhanced electrochemical performance compared with that of the GO electrode. Our results demonstrate that the manipulation of oxygen functional groups on GO is an effective strategy to greatly improve the Li storage property of GO-based materials for advanced LIBs cathodes.     

A flexible graphene/multiwalled carbon nanotube film as a high performance electrode material for supercapacitors

A flexible graphene/multiwalled carbon nanotube (GN/MWCNT) film has been fabricated by flow-directed assembly from a complex dispersion of graphite oxide (GO) and pristine MWCNTs followed by the use of gas-based hydrazine to reduce the GO into GN sheets. The GN/MWCNT (16 wt.% MWCNTs) film characterized by Fourier transformation infrared spectra, X-ray diffraction and scanning electron microscope has a layered structure with MWCNTs uniformly sandwiched between the GN sheets. The MWCNTs in the obtained composite film not only efficiently increase the basal spacing but also bridge the defects for electron transfer between GN sheets, increasing electrolyte/electrode contact area and facilitating transportation of electrolyte ion and electron into the inner region of electrode. Electrochemical data demonstrate that the GN/MWCNT film possesses a specific capacitance of 265 F g⁻¹ at 0.1 A g⁻¹ and a good rate capability (49% capacity retention at 50 A g⁻¹), and displays an excellent specific capacitance retention of 97% after 2000 continuous charge/discharge cycles. The results of electrochemical measurements indicate that the freestanding GN/MWCNT film has a potential application in flexible energy storage devices.     

Polydopamine-assisted formation of Co3O4-nanocube-anchored reduced graphene oxide composite for high-performance supercapacitors

Tailoring transition-metal oxide nanoparticles with two-dimensional carbon has become a favorite way to improve their electrochemical performance. In this study, a composite of reduced graphene oxide was anchored by Co₃O₄ nanocubes and easily prepared with the assistance of polydopamine (PDA), using a combination of hydrothermal reaction and pyrolysis (Co₃O₄@PDA-rGO). Polydopamine, which possesses abundant catechol and amine groups, could be easily grafted onto graphene oxide to reduce the aggregation of graphene particles. Furthermore, PDA provided active sites, i.e., catechol and amine groups, which coordinated with Co²⁺, enabling enrichment of metal ions on the surface of graphene. After the pyrolysis of Co²⁺-containing PDA-grafted graphene at 400 °C, the Co²⁺ ions were converted into Co₃O₄ nanocubes, while the PDA carbonized to form N-doped porous carbon on the surface of graphene. The resulting product, Co₃O₄@PDA-rGO, demonstrated extraordinary supercapacitive behavior with good cycling stability owing to its unique porous structure as well as the intimate contact between Co₃O₄ and the carbon matrix.