Easy synthesis of porous graphene nanosheets and their use in supercapacitors

We report the easy synthesis of porous graphene nanosheets (PGNs) using the etching of graphene sheets by MnO₂. An electrode made from PGNs exhibits a specific capacitance of 154 F g^?1 at 500 mV s^?1 in 6 M KOH compared to a value of 67 F g^?1 for graphene nanosheets, and a low capacitance loss of 12% after 5000 cycles. Interestingly, PGN electrode material shows an excellent rate capability due to its open layered and mesopore structures that facilitate the efficient access of electrolytes to the electrode material and shorten the ion diffusion pathway through the porous sheets. This approach offers the potential for cost-effective, environmentally friendly and large-scale production of PGNs.     

An efficient method of producing stable graphene suspensions with less toxicity using dimethyl ketoxime

A highly stable graphene suspension has been prepared using dimethyl ketoxime (DMKO) as reductant. Nitrogen was doped into the graphene plane at the same time as the graphene oxide (GO) sheets were reduced. X-ray photoelectron spectroscopy indicated that the C/O ratio of graphene was significantly increased after GO was treated with DMKO and the quantity of nitrogen incorporated into the graphene lattice was 3.67 at.%. The electrical conductivity of the graphene paper was found to be ～10² S m^?1, which was 5 orders of magnitude better than that of GO, and this demonstrated the effective chemical reduction of GO. The mechanism of the chemical reaction of GO with DMKO was also discussed. The as-produced graphene material showed good capacitive behavior and long cycle life with a specific capacitance of ～140 F g^?1.     

Effect of graphene nanosheets on electrochemical performance of Li4Ti5O12 in lithium-ion capacitors

In order to improve the electrochemical performance of lithium titanium oxide, Li₄Ti₅O₁₂ (LTO), for the use in the lithium-ion capacitors (LICs) application, LTO/graphene composites were synthesized through a solid state reaction. The composite exhibited an interwoven structure with LTO particles dispersed into graphene nanosheets network rather than an agglomerated state pristine LTO particles. It was found that there is an optimum percentage of graphene additives for the formation of pure LTO phase during the solid state synthesis of LTO/graphene composite. The effect of graphene nanosheets addition on electrochemical performance of LTO was investigated by a systemic characterization of galvanostatic cycling in lithium and lithium-ion cell configuration. The optimized composite exhibited a decreased polarization upon cycling and delivered a specific capacity of 173 mA h g⁻¹ at 0.1 C and a well maintained capacity of 65 mA h g⁻¹ even at 20 C. The energy density of 14 Wh kg⁻¹ at a power density of 2700 W kg⁻¹ was exhibited by a LIC full cell with a balanced mass ratio of anode to cathode along with a superior capacitance retention of 97% after 3000 cycles at a current density of 0.4 A g⁻¹. This boost in reversible capacity, rate capability and cycling performance was attributed to a synergistic effect of graphene nanosheets, which provided a short lithium ion diffusion path as well as facile electron conduction channels.     

High-performance supercapacitor electrodes based on highly corrugated graphene sheets

Highly corrugated graphene sheets (HCGS) have been prepared by a rapid, low cost and scalable approach through the thermal reduction of graphite oxide at 900 °C followed by rapid cooling using liquid nitrogen. The wrinkling of the graphene sheets can significantly prevent them from agglomerating and restacking with one another face to face and thus increase the electrolyte-accessible surface area. The maximum specific capacitance of 349 F g^?1 at 2 mV s^?1 is obtained for the HCGS electrode in 6 M KOH aqueous solution. Additionally, the electrode shows excellent electrochemical stability along with an approximately 8.0% increase of the initial specific capacitance after 5000 cycle tests. These features make the present HCGS material a quite promising alternative for next generation of high-performance supercapacitors.     

Carbon-coated SnO2/graphene nanosheets as highly reversible anode materials for lithium ion batteries

A simple approach is reported to prepare carbon-coated SnO₂ nanoparticle–graphene nanosheets (Gr–SnO₂–C) as an anode material for lithium ion batteries. The material exhibits excellent electrochemical performance with high capacity and good cycling stability (757 mA h g^?1 after 150 cycles at 200 mA g^?1). The likely contributing factors to the outstanding charge/discharge performance of Gr–SnO₂–C could be related to the synergism between the excellent conductivity and large area of graphene, the nanosized particles of SnO₂, and the effects of the coating layer of carbon, which could alleviate the effects of volume changes, keep the structure stable, and increase the conductivity. This work suggests a strategy to prepare carbon-coated graphene–metal oxide which could be used to improve the electrochemical performance of lithium ion batteries.     

Additive-free hydrogelation of graphene oxide by ultrasonication

Graphene oxide hydrogels have been prepared by ultrasonication of precursor aqueous dispersions. The ultrasonication fractures the nanosheets, reducing their dimensions and exposing new sheet edges that do not possess the stabilizing carboxyl functional groups found along the edge of the as-prepared material. Ultrasonication does not affect the overall chemical functionality of the graphene oxide nanosheets, as spectra (carbon-13 nuclear magnetic resonance spectroscopy, infrared spectroscopy, and X-ray photoelectron spectroscopy) of samples before and after ultrasonication are nearly identical. Gelation is induced after only 30 min of ultrasonication to achieve a relatively weak gel with a shear modulus of 0.3 kPa; however, extension of ultrasonic treatment to 120 min yields a more robust hydrogel with a shear modulus of 1.6 kPa. Such enhancement in the gel’s physical properties can be attributed to the lack of stabilizing carboxyl groups on newly generated nanosheet fragments from the interior regions of the original nanosheets. As prepared, these hydrogels exhibit exceptionally low critical gelation concentrations ranging from ～0.050 to ～0.125 mg mL^?1 that can be tuned according to the extent of ultrasonic treatment.     

Three-dimensional graphene layers prepared by a gas-foaming method for supercapacitor applications

Inspired by baking bread, our research group demonstrates a novel method for baking three-dimensional (3D) graphene layers with an open porous network, pore size in the range of dozens of nanometers to several hundred nanometers, and a pore wall thickness of about 10 nm. Such continuously cross-linking structures not only effectively overcome the restacking and agglomeration of graphene nanosheets but also possess more channels between nanosheets to lower the resistance for electron access to the inter-space. Compared with reduced graphene oxide (rGO) prepared at the same temperature, the unique 3D porous-structured graphene layers also contain 4.3 at.% nitrogen. When the 3D graphene layers are employed as an active electrode material for a supercapacitor, a high specific capacitance (SC) of 231.2 F g⁻¹ at 1 A g⁻¹ is displayed after electrochemical activation, approximately two times that of rGO. Only <1.0% of the capacitance degrades after 8000 cycles, exhibiting its excellent cycle stability; furthermore, it liberates a high energy density of 32.1 Wh kg⁻¹ at a power density of 500 W kg⁻¹. The attractive performances of 3D graphene layers make them a promising candidate as an electrode material for supercapacitors.     

Synthesis of a graphene–carbon nanotube composite and its electrochemical sensing of hydrogen peroxide

Graphene, whose structure consists of a single layer of sp²-hybridized carbon atoms, provides an excellent platform for designing composite nanomaterials. In this study, we have demonstrated a facile process to synthesize graphene–multiwalled carbon nanotube (MWCNT) composite. The graphene–MWCNT composite material is endowed with a large electrochemical surface area and fast electron transfer properties in Fe(CN)₆^3?/4? redox species. A graphene–MWCNT composite modified electrode exhibits good performance in terms of the electrocatalytic reduction of H₂O₂; a sensor constructed from such an electrode shows a good linear dependence on H₂O₂ concentration in the range of 2 × 10^?5 to 2.1 × 10^?3 mol L^?1. The detection limit is estimated to be 9.4 × 10^?6 mol L^?1. This study provides a new kind of composite modified electrode for electrochemical sensors.     

Graphene nanosheets based on controlled exfoliation process for enhanced lithium storage in lithium-ion battery

A facile and rapid approach was used for the fabrication of chemically derived graphene nanosheets based on the reduction of graphite oxide (GO) in tube furnace assembly at different temperatures. The morphologies, microstructures, specific surface areas and other features of GO and graphene nanosheets were characterized. Structure characterization indicates that the platelet thickness of graphene nanosheets obtained at 300 °C was 1.62 nm, which corresponds to an approximately 5 layers stacking of the monoatomic graphene nanosheets. Electrochemical performances of the as-prepared graphene nanosheets were performed, the result of which could prove the above observation that graphene nanosheets (5 layers) obtained at 300 °C actually displayed the most remarkable electrochemical performances: the first discharge and charge capacities of graphene nanosheets were as high as 2137 mAh/g and 994 mAh/g, respectively, and after 100 cycles graphene nanosheets still possessed a high capacity of 478 mAh/g.     

Facile synthesis of sandwich-like polyaniline/boron-doped graphene nano hybrid for supercapacitors

The physicochemical property of chemically prepared graphene can be significantly changed due to the incorporating of heteroatoms into graphene. In this article, boron-doped graphene sheets are used as carbon substrates instead of graphene for loading polyaniline by in situ polymerization. Compared with the individual component and polyaniline/non-doped graphene, the sandwich-like polyaniline/boron-doped graphene exhibits remarkably enhanced electrochemical specific capacitance in both acid and alkaline electrolytes. In a three-electrode configuration, the hybrid has a specific capacitance about 406 F g⁻¹ in 1 M H₂SO₄ and 318 F g⁻¹ in 6 M KOH at 1 mV s⁻¹. In the two-electrode system of a symmetric supercapacitor, this hybrid achieves a specific capacitance about 241 and 189 F g⁻¹ at 0.5 A g⁻¹ with a specific energy density around 19.9 and 30.1 Wh kg⁻¹, in the acid and alkaline electrolytes, respectively. The as-obtained polyaniline/boron-doped graphene hybrid shows good rate performance. Notably, the obtained electrode materials exhibit long cycle stability in both acid and alkaline electrolytes (∼100% and 83% after 5000 cycles, respectively). The improved electrochemical performance of the hybrid is mainly attributed to the introduction of additional p-type carriers in carbon systems by boron-doping and the well combination of pseudocapacitive conducting polyaniline.     

Green reduction of graphene oxide by aqueous phytoextracts

The reduction of graphene oxide (GO) by phytochemicals was investigated using aqueous leaf extracts of Colocasia esculenta and Mesua ferrea Linn. and an aqueous peel extract of orange (Citrus sinensis). The prepared GO and phytoextract reduced GO (RGO) were characterized by ultraviolet–visible spectroscopy, Raman spectroscopy and Fourier transform infrared analyses to provide a clear indication of the removal of oxygen-containing groups from the graphene and the formation of RGO. The extent of reduction was determined from elemental analysis. Formation of few layers of graphene was indicated by transmission electron microscopy. The obtained RGO exhibited good specific capacitance (17–21 Fg^?1), high electrical conductivity (3032.6–4006 Sm^?1) and high carbon to oxygen ratio (5.97–7.11).     

Influence of processing on fracture toughness of Si3N4 + graphene platelet composites

Silicon nitride + 1 wt% graphene platelet composites were prepared using various graphene platelets (GPL) and two processing routes; hot isostatic pressing (HIP) and gas pressure sintering (GPS). The influence of the processing route and graphene platelets’ addition on the fracture toughness has been investigated. The matrix of the composites prepared by GPS consists of Si₃N₄ grains with smaller diameter in comparison to the composites prepared by HIP. The indentation fracture toughness of the composites was in the range 6.1–9.9 MPa m^0.5, which is significantly higher compared to the monolithic silicon nitride 6.5 and 6.3 MPa m^0.5. The highest value of K_IC was 9.9 MPa m^0.5 in the case of composite reinforced by the smallest multilayer graphene nanosheets, prepared by HIP. The composites prepared by GPS exhibit lower fracture toughness, from 6.1 to 8.5 MPa m^0.5. The toughening mechanisms were similar in all composites in the form of crack deflection, crack branching and crack bridging.     

Application of nitrogen-doped graphene nanosheets in electrically conductive adhesives

Nitrogen-doped graphene nanosheets (N-GNSs) were used as a conductive filler for a polymer resin adhesive and as a performance improver for a silver-filled electrically conductive adhesive (ECA). The N-GNS samples were prepared by the chemical-intercalation/thermal-exfoliation of graphite followed by a thermal treatment in NH₃. Only 1 wt.% of N-GNSs was required for the adhesive to reach a percolation threshold, and the performance using N-GNSs was much better than that obtained using carbon black or multi-walled carbon nanotubes (MWCNTs). The effect of N-GNS or MWCNT additives on reducing the electrical resistivity of Ag-particle filled ECAs at low Ag loading ratios was also investigated. With 30 wt.% of Ag filler, the polymer resin was still non-conducting, while a resistivity of 4.4 × 10⁻² Ω-cm was obtained using an Ag/N-GNS hybrid filler fortified with only 1 wt.% of N-GNSs due to large specific surface area, high aspect ratio, and good electrical conductivity of the doped graphene.     

Improved electrochemical performances of reduced graphene oxide based supercapacitor using redox additive electrolyte

Reduced graphene oxide (rGO) is prepared by simple and eco-friendly hydrothermal reduction method. X-ray photoelectron spectroscopy and Ultraviolet–visible analysis corroborated the reduction of graphene oxide into rGO in basic medium. The flexibility of the prepared rGO is inferred from transmission electron micrographs. Further, the identification of suitable electrolyte is carried out using different anions (SO₄²⁻, Cl⁻, OH⁻) and cations (K⁺, Na⁺) for the superior performance of the rGO based supercapacitors. The electrochemical performance revealed that K⁺ and OH⁻ ions are more active species in aqueous solutions. Subsequently, an effort was taken to improve the specific capacitance in the optimized 1 M KOH electrolyte by KI as redox additive at different concentrations (0.025, 0.05, 0.075 and 0.1 M). The calculated specific capacitance and energy density of rGO electrode in the optimized 1 M KOH + 0.05 M KI electrolyte is 500 F g⁻¹ and 44 Wh kg⁻¹, respectively. On the other hand, it exhibited the specific capacitance of 298 F g⁻¹ at 0.83 A g⁻¹ in non-aqueous polymer gel (PVA + KOH + KI) electrolyte. Finally, the charged aqueous device is utilized to glow the light emitting diode.     

Structural,magnetic, and textural properties of iron oxide-reduced graphene oxide hybrids and their use for the electrochemical detection of chromium

Superparamagnetic Fe₃O₄ nanoparticles were anchored on reduced graphene oxide (RGO) nanosheets by co-precipitation of iron salts in the presence of different amounts of graphene oxide (GO). A pH dependent zeta potential and good aqueous dispersions were observed for the three hybrids of Fe₃O₄ and RGO. The structure, morphology and microstructure of the hybrids were examined by X-ray diffraction, transmission electron microscopy (TEM), Fourier transform infrared spectroscopy, Raman and X-ray photoelectron spectroscopy. TEM images reveal lattice fringes (d₃₁₁ = 0.26 nm) of Fe₃O₄ nanoparticles with clear stacked layers of RGO nanosheets. The textural properties including the pore size distribution and loading of Fe₃O₄ nanoparticles to form Fe₃O₄–RGO hybrids have been controlled by changing the concentration of GO. An observed maximum (～10 nm) in pore size distribution for the sample with 0.25 mg ml^?1 of GO is different from that prepared using 1.0 mg ml^?1 GO. The superparamagnetic behavior is also lost in the latter and it exhibits a ferrimagnetic nature. The electrochemical behavior of the hybrids towards chromium ion was assessed and a novel electrode system using cyclic voltammetry for the preparation of an electrochemical sensor platform is proposed. The textural properties seem to influence the electrochemical and magnetic behavior of the hybrids.     

Effects of reduction process and carbon nanotube content on the supercapacitive performance of flexible graphene oxide papers

The effects of the reduction process and carbon nanotube (CNT) content on the supercapacitive behavior of electrodes made from flexible, binder-free thick graphene oxide (GO) papers are studied. It is found that the supercapacitive performance depends on several factors, including the presence of oxygenated functional groups after reduction, the interlayer spacing of the GO papers and their wettability with electrolyte. A moderate reduction of GO papers using hydrazine or annealing at a low temperature of 220 °C in air is proven to be more beneficial to achieve a high capacitance than the heavy reduction using a hydrazine vapor or a high temperature thermal treatment. The addition of a small amount of CNT, typically 12.5 wt.%, to form thick GO/CNT sandwich papers gives rise to an excellent specific capacitance of 151 F g^?1 at a current density of 0.5 A g^?1, as well as a retention ratio of 86% of the initial value after 6000 charge/discharge cycles at 5 A g^?1. These improvements arise from the synergistic effects of the increased electronic conductivity and effective surface area associated with large electrochemical active sites due to the presence of intercalated CNT.     

Self-limited growth of large-area monolayer graphene films by low pressure chemical vapor deposition for graphene-based field effect transistors

During the last decade, fabrication of high-quality graphene films by chemical vapor deposition (CVD) for nanoelectronics and optoelectronic applications has attracted increasing attention. However, processing of large-area monolayer and defect-free graphene films is still challenging. In this work, we have studied the effect of processing conditions on the self-limited growth of graphene monolayers on copper foils during low pressure CVD both experimentally and theoretically based on thermokinetics and kinetics of Langmuir adsorption. The effect of copper pre-treatment, growth time, and carbon potential of the atmosphere (indicated by the methane-to-hydrogen gas ratio, r) on the quality of graphene nanosheets (number of layers, surface roughness and the lateral size) were studied. Microscopic studies show that careful pre-treatment of the copper foil by electropolishing provides a suitable condition for the self-limited growth of graphene with minimum surface roughness and defects. Raman spectroscopy and atomic force microscopy determine that the number of graphene sheets decreases with increasing the carbon potential while smother surfaces are attained. Large-area monolayer graphene films are obtained at relatively high carbon potential (r=1) and controlled growth time (10 min) at 1000 °C. Measurement of the electrical response of the prepared monolayer graphene films on SiO₂ (300 nm)/Si substrates in a field effect transistor (FET) device shows a high mobility of 2780 cm² V⁻¹ s⁻¹. Interestingly, the device exhibits p-type semiconducting behavior with the Dirac point at a gate voltage of 25 V. The finding show a great promise for graphene-based FET devices for future nanoelectronics.     

Ag nanoparticles anchored NiO/GO composites for enhanced capacitive performance

Hierarchical nickel oxide/graphene oxide (NiO/GO) and nickel oxide/graphene oxide/silver (NiO/GO/Ag) heterostructures were sucessfully fabricated as high-performance supercapacitors electrode materials by using a hydrothermal process and a photoreduction process. The experimental results showed that the NiO/GO/Ag heterostructure electrodes showed better electrochemical performance than those of NiO/GO and bare NiO nanosheets. The NiO/GO/Ag electrode exhibited a higher specific capacitance of 229 F g⁻¹ at a current density of 1 A g⁻¹, higher than that of 161 F g⁻¹ for NiO/GO composites. Furthermore, NiO/GO/Ag electrode also showed good rate capability (still 200 F g⁻¹ at 6 A g⁻¹) and cycling stability (24% loss after 2000 repetitive cycles at a scan rate of 20 mV s⁻¹). The enhanced capacitive performance of the NiO/GO/Ag composites was mainly attributed to the introduction of Ag nanoparticles, which increased the electrical conductivities of the composites, and promoted the electron transfer between the active components. This study suggested that NiO/GO/Ag composites were a promising class of electrode materials for high performance energy storage applications.     

Synthesis of graphene-CoS electro-catalytic electrodes for dye sensitized solar cells

A large CoS-implanted graphene (G-CoS) film electrode was prepared using chemical vapor deposition followed by successive ionic layer absorption and reaction. HRTEM and AFM show that CoS nanoparticles are uniformly implanted on the graphene film. Furthermore, the G-CoS electro-catalytic electrode is characterized in a dye sensitized solar cells (DSSC) and is found to be highly electro-catalytic towards iodine reduction with low charge transfer resistance (R_ct ～5.05 Ω cm²) and high exchange current density (J₀～2.50 mA cm^?2). The improved performance compared to the pristine graphene is attributed to the increased number of active catalytic sites of G-CoS and highly conducting path of graphene. The comprehensive G-CoS synthesis process is a simple and scalable process which can easily adapt for large scale electro-catalytic film fabrication for several other electro-chemical energy harvesting and storage applications.     

Synthesis and supercapacitor performance studies of N-doped graphene materials using o-phenylenediamine as the double-N precursor

N-doped graphene (NG) materials have been prepared through a one-step solvothermal reaction by using o-phenylenediamine as a double-N precursor. N-doping and reduction of graphene oxide (GO) are both achieved simultaneously during the solvothermal reaction. The results of scanning electron microscopy and high resolution transmission electron microscopy measurements indicate that NG is highly crumpled. And the N-doping is confirmed by elemental analysis, X-ray photoelectron spectroscopy, Raman spectroscopy, Fourier transformed infrared spectroscopy and ultraviolet–visible spectroscopy. The doping level of nitrogen reaches up to 7.7 atom% and the types in NG are benzimidazole-N and phenazine-N. The NG materials exhibit excellent electrochemical performance for symmetric supercapacitors with a high specific capacitance of 301 F g⁻¹ at a current density of 0.1 A g⁻¹ in 6 M KOH electrolyte, which is remarkably higher than the solvothermal products of pristine GO (210 F g⁻¹ at 0.1 A g⁻¹). The NG materials also exhibit superior cycling stability (97.1% retention) and coulombic efficiency (99.2%) after 4000 cycles, due to the high content of nitrogen atoms, unique types of nitrogen and improved electronic conductivity.