High performance graphene composite microsphere electrodes for capacitive deionisation

Graphene oxide/resorcinol–formaldehyde microsphere (GORFM) was synthesised by a simple-inverse suspension polymerisation under ambient pressure drying and the graphene composite microsphere (GCM) was obtained by carbonisation in a nitrogen atmosphere. The morphologies and microstructures of GORFM and GCM samples were characterised by scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and surface area analysis. The results showed that the diameters of GORFM were controlled in the range of 5–10 μm with perfect sphericity. The inner layer of GORFM was composed of resorcinol–formaldehyde (RF) particles, and the outer layer was the wrinkled GO sheets intercalated with RF through chemical and physical interactions. The GCM-1.5 sample achieved the highest specific surface area of 1128 m²/g and the mesopore area was 699 m²/g. The electrochemical behaviours of GCM as an electrode were investigated by cyclic voltammograms, electrochemical impedance spectroscopy and capacitive deionisation (CDI). The specific capacitance and electrosorption capacity of GCM-1.5 was 290.2 F/g and 33.52 mg/g, respectively. The current efficiencies were in the range of 77.75–81.41%. These results implied that GCM had a potential application in CDI due to its low-cost and high performance.     

Influence of multi-walled carbon nanotubes on the electrochemical performance of graphene nanocomposites for supercapacitor electrodes

In this work, multi-walled carbon nanotube (MWNT) bonded graphene (M-GR) composites were prepared using the chemical reduction of graphite oxide (GO) and acid treated MWNTs with different ratios. The M-GR/polyaniline (PANI) nanocomposites (M-GR/PANI) were prepared using oxidation polymerization. The effect of the M-GR ratio on the electrochemical performances of the M-GR/PANI was investigated. It was found that the substrate 2D graphene was coated with 1D MWNTs by chemical reduction and the M-GR was further coated with PANI, leading to increased electrical properties by the π–π interaction between the M-GR and PANI. In addition, the electrochemical performances, such as the current density, charge–discharge, and specific capacitance of the M-GR/PANI were higher than those of graphene/PANI and the highest specific capacitance (1118 F/g) of the composites was obtained at a scan rate of 0.1 A/g for the PANI containing a 0.5 M-GR ratio compared to 191 F/g for the graphene/PANI. The dispersion of the MWNTs onto the graphene surface and the ratio of M-GR had a pronounced effect on the electrochemical performance of the PANI-based composites, which was attributed to the highly conductive pathway created by the M-GR incorporated in the PANI-based composites and the synergistic effect between M-GR and PANI.     

High power density electrodes for Carbon supercapacitor applications

This paper presents results obtained with 4 cm² Carbon/Carbon supercapacitors cells in organic electrolyte. In the first approach, a surface treatment for Al current collector foil via the sol-gel route has been used in order to decrease the Al/active material interface resistance. Performances obtained with this original process are: a low equivalent series resistance (ESR) of 0.5 Ω cm² and a specific capacitance of 95 F g⁻¹ of activated carbon.Then, supercapacitors assembled with treated Al foil and active material containing activated carbon/carbon nanotubes (CNTs) with different compositions have been studied. Galvanostatic cycling measurements show that when CNTs content increases, both ESR and specific capacitance are decreased. Fifteen percent appears to be a good compromise between stored energy and delivered power with an ESR of 0.4 Ω cm² and a specific capacitance of 93 F g⁻¹ of carbonaceous active material.Finally, cells frequency behaviour has been characterized by Electrochemical Impedance Spectroscopy. The relaxation time constant of cells decreases when the CNTs content increases. For 15% of CNTs, the time constant is about 30% lower as compared to a cell using pure activated carbon-based electrodes leading to a higher delivered power.     

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.     

High performance of a solid-state flexible asymmetric supercapacitor based on graphene films

Solid-state flexible energy storage devices hold the key to realizing portable and flexible electronic devices. Achieving fully flexible energy storage devices requires that all of the essential components (i.e., electrodes, separator, and electrolyte) with specific electrochemical and interfacial properties are integrated into a single solid-state and mechanically flexible unit. In this study, we describe the fabrication of solid-state flexible asymmetric supercapacitors based on an ionic liquid functionalized-chemically modified graphene (IL-CMG) film (as the negative electrode) and a hydrous RuO(2)-IL-CMG composite film (as the positive electrode), separated with polyvinyl alcohol-H(2)SO(4) electrolyte. The highly ordered macroscopic layer structures of these films arising through direct flow self-assembly make them simultaneously excellent electrical conductors and mechanical supports, allowing them to serve as flexible electrodes and current collectors in supercapacitor devices. Our asymmetric supercapacitors have been optimized with a maximum cell voltage up to 1.8 V and deliver a high energy density (19.7 W h kg(-1)) and power density (6.8 kW g(-1)), higher than those of symmetric supercapacitors based on IL-CMG films. They can operate even under an extremely high rate of 10 A g(-1) with 79.4% retention of specific capacitance. Their superior flexibility and cycling stability are evident in their good performance stability over 2000 cycles under harsh mechanical conditions including twisted and bent states. These solid-state flexible asymmetric supercapacitors with their simple cell configuration could offer new design and fabrication opportunities for flexible energy storage devices that can combine high energy and power densities, high rate capability, and long-term cycling stability.     

Hierarchical porous carbons with high performance for supercapacitor electrodes

A series of hierarchical porous carbons (HPCs) were prepared by a combination of self-assembly and chemical activation. Pore-structure analysis shows that micropores can be generated within the mesopore wall of mesoporous carbon in a controllable manner during activation. As evidenced by cyclic voltammetry, galvanostatic charge/discharge cyclings and frequency response measurements, HPCs show superior capacitive performances to hard-templated ordered mesoporous carbons, which can be attributed to the generated pore surfaces that play most important role in the formation of double-layer capacitance and to their unique hierarchical porous structure that favors the fast diffusion of electrolyte ions into the pores. Of special interest is the fact that HPCs maintains 180 F/g at high-frequency of 1 Hz.     

High performance binder-free reduced graphene oxide nanosheets/Cu foam anode for lithium ion battery

Binder-free combination of reduced graphene oxide with Cu foam (RGO/Cu foam) anode for lithium ion battery was designed and achieved via one-step facile electro-reduction. The as-prepared composite RGO/Cu foam anode were studied in terms of scanning electron microscope (SEM), field emission scanning electron microscope (FESEM), transmission electron microscope (TEM), energy dispersive X-ray spectroscopy (EDX) and Fourier transform infrared spectroscopy (FTIR), Raman, galvanostatic charge/discharge, cyclic voltammogram and AC impedance. As expected, graphene oxide nanosheets were indeed successfully electro-reduced to large degree and tightly combined with Cu foam without any additional polymer binder. Moreover, the integrated RGO/Cu foam electrode delivered high reversible capacity of 1196.2 mAh/g at 0.25 A/g, indicating satisfactory electrochemical performances. High Li-storage activity, large surface area, high conductivity of RGO nanosheets and the binder-free combination with porous Cu foam should be jointly responsible for high electrochemical 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.     

The effect of reduction time on the surface functional groups and supercapacitive performance of graphene nanosheets

Graphene nanosheets were prepared by reducing graphite oxide with hydrazine hydrate. The effects of reduction time on the structure and morphology of graphene nanosheets have been investigated. Their electrochemical performance in aqueous and organic electrolytes was also analyzed. With an increase of reduction time, the C and N contents of graphene nanosheets increased, while the specific surface areas and the specific capacitances decreased. Changes in reduction time produced a significant effect on the numbers as well as the types of oxygen and nitrogen functionalities. The graphene nanosheets, prepared by using a reduction time of 30 min have the highest specific capacitance of 192 F g^?1 in a 6 mol L^?1 KOH electrolyte. All prepared graphene nanosheets have a good rate performance and cycle stability.     

Far-infrared reduced graphene oxide as high performance electrodes for supercapacitors

We report a novel far-infrared (FIR) thermal reduction process to effectively reduce graphene oxide films for supercapacitor electrode applications. The binder-free graphene oxide films used in this study were produced by electro-spray deposition of a graphene oxide colloidal solution onto stainless steel current collectors. The reduction of graphene oxide was performed using a commercial FIR convection oven that is ubiquitous in homes for cooking and heating food. The reduction process incorporated a simple, one-step FIR irradiation carried out in ambient air. Further, the FIR irradiation process was completed in ∼3 min, wherein neither special atmosphere nor high temperature was employed, resulting in an economic, efficient and simplified processing technique. The as-produced FIR graphene electrode gave a specific capacitance of ∼320 F/g at a current density of ∼0.2 A/g with less than 94% loss in specific capacitance over 10,000 charge/discharge cycles. This is one of the best specific capacitances reported for all-carbon electrodes without any additives. Even at ultrafast charge/discharge rates (current densities as high as ∼100 A/g), the FIR graphene electrode still delivered specific capacitances in excess of 90 F/g. The measured energy and power densities of the FIR supercapacitors were found to be ∼3–6 times higher than commercial (activated carbon) supercapacitor devices. This excellent electrochemical performance of the FIR graphene coupled with its ease of production (in air at low temperatures) using a commercial home-use FIR convection oven indicates the significant potential of this concept for large-scale commercial electrochemical supercapacitor applications.     

Utilization of multiple graphene nanosheets in fuel cells: 2. The effect of oxidation process on the characteristics of graphene nanosheets

Structural properties of graphene nanosheets that will be used as electrode material in fuel cells were investigated at different oxidation times. As the oxidation time was increased, the strong bonding between graphene layers in graphite was reduced and graphene layers started to exfoliate forming clusters with a few number of graphene layers. The variations in interplanar spacings, layer number and percent crystallinity indicated how stepwise chemical procedure influenced the morphology of graphite. It was possible to produce relatively flat graphene clusters with definite number of layers by controlling the oxidation time. Graphene nanosheets were characterized in detail by scanning electron microscopy, atomic force microscopy, X-ray diffraction, Raman spectroscopy, and thermal gravimetric analyzer.     

Synthesis of carbon-coated graphene electrodes and their electrochemical performance

In this work, C-graphene composed of core graphene and carbon shells was prepared to obtain a new type of carbon electrode materials. Carbon shells containing nitrogen groups were prepared by coating polyaniline (PANI) onto graphene by in situ polymerization and subsequent carbonization at 850 °C. After carbonization, the C-graphene contained 6.5% nitrogen and showed a 2D plate structure and crystallinity like that of pristine graphene. In addition, the C-graphene exhibited electrochemical performance superior to that of pristine graphene, and the highest specific capacitance (170 F/g) of the C-graphene was obtained at a scan rate of 0.1 A/g, as compared to 138 F/g for pristine graphene. This superior performance was attributed to the synergistic effect of porous carbon layer and the graphene and the pseudocapacitive effect by the nitrogen groups formed on the carbon electrode after carbonization.     

Integration of tailored reduced graphene oxide nanosheets and electrospun polyamide-66 nanofabrics for a flexible supercapacitor with high-volume- and high-area-specific capacitance

Nanofiber fabric is firstly introduced to replace common microfiber fabrics as the platform for flexible supercapacitors. Nanofiber and microfiber electrodes can be simply fabricated using a dipping process that impregnates reduced graphene oxide (RGO) nanosheets into electrospun polyamide-66 (PA66) nanofiber and microfiber fabrics. RGO nanosheets are tailored to various sizes and only RGO with a medium diameter of 250–450 nm (denoted as M-RGO) can effectively penetrate the pores of nanofiber fabrics for constructing smooth conductive paths within PA66 nanofiber fabrics. The synergistic effect between suitable sizes of RGO nanosheets and nanofiber fabrics with a high specific area provides a symmetric supercapacitor composed of M-RGO/PA66 nanofiber fabric electrodes with high-volume and high-area specific capacitance (C_S,V and C_S,A, equal to 38.79 F cm⁻³ and 0.931 F cm⁻² at 0.5 A g⁻¹, respectively), which are much larger than that of a symmetric supercapacitor composed of RGO/PA66 microfiber fabric electrodes (8.52 F cm⁻³ and 0.213 F cm⁻² at 0.5 A g⁻¹). The effect of impregnating nanofiber fabrics with suitably sized RGO to promote C_S,V and C_S,A of flexible supercapacitors has been demonstrated.     

石墨烯/聚吡咯复合电极材料超电容性能研究进展

分析了目前石墨烯和聚吡咯(PPy)用作电极材料的不足,详细介绍了近年来超级电容器用石墨烯/PPy复合电极材料的研究进展,指出石墨烯/PPy复合材料在能量转换和存储领域的未来发展方向.     

Three-dimensional strutted graphene loading manganese oxide for supercapacitor

Graphene has shown superiority for advanced carbon electrodes in supercapacitors, characterized by high power density but limited energy density. Combining pseudocapacitive materials with graphene can alleviate the problem. This work synthesized the three-dimensional strutted graphene (SG) via the ammonium-salt-assisted sugar-blowing method, and the self-supporting MnO₂/SG porous monolith was then constructed via growing manganese oxide (MnO₂) nanorod array on the SG support in hydrothermal process. When tested in supercapacitor, the MnO₂/SG hybrid electrode achieved a high specific capacitance of 343.6 F·g^-1 at a current density of 0.5 A·g^-1, exhibiting excellent cycling stability with 83.8% capacitance retention after 5000 cycles. The symmetric supercapacitor further showed a high energy density of 11.93 W·h·kg^-1 at a power density of 500 W·kg^-1. The impressive result indicates a promising prospect of the excellent MnO₂/SG hybrid to be applied to electrochemical energy storage.     

Synthesis of graphene/vitamin C template-controlled polyaniline nanotubes composite for high performance supercapacitor electrode

A graphene nanosheet/polyaniline nanotube (GPNT) composite is prepared for the first time by in-situ chemical oxidative polymerization of aniline using vitamin C as a structure directing agent. The vitamin C molecules lead to the synthesis of polyaniline (PANI) nanotubes through the development of rod-like assembly by H-bonding in an aqueous medium. The initially synthesized graphene oxide/polyaniline nanotubes composite is reduced to graphene using hydrazine monohydrate followed by re-oxidation and protonation of the PANI to produce the GPNT nanocomposite. This novel composite showed a high specific capacitance of 534.37 F/g and an excellent energy density of 74.27 Wh/kg at a constant current of 0.5 mA. Besides, the GPNT composite exhibited excellent cycle life with 91.4% specific capacitance retained after 500 charge-discharge cycles. The excellent performance is due to the synergistic combination of graphene which provides good electrical conductivity and mechanical stability, and PANI nanofiber which deals with good redox activity.     

Reduced chemically modified graphene oxide for supercapacitor electrode

An efficient active material for supercapacitor electrodes is prepared by reacting potassium hydroxide (KOH) with graphene oxide followed by chemical reduction with hydrazine. The electrochemical performance of KOH treated graphene oxide reduced for 24 h (reduced chemically modified graphene oxide, RCMGO-24) exhibits a specific capacitance of 253 F g^-1 at 0.2 A g^-1 in 2 M H₂SO₄ compared to a value of 141 F g^-1 for graphene oxide reduced for 24 h (RGO-24), and good cyclic stability up to 3,000 cycles. Interestingly, RCMGO-24 demonstrated a higher specific capacitance and excellent cycle stability due to its residual oxygen functional groups that accelerate the faradaic reactions and aid in faster wetting. This non-annealed strategy offers the potential for simple and cost-effective preparation of an active material for a supercapacitor electrode.     

The effect of heat treatment on formation of graphene thin films from graphene oxide nanosheets

Graphene thin films with very low concentration of oxygen-containing functional groups were produced by reduction of graphene oxide nanosheets (prepared by using a chemical exfoliation) in a reducing environment and using two different heat treatment procedures (called one and two-step heat treatment procedures). The effects of heat treatment procedure and temperature on thickness variation of graphene platelets and also on reduction of the oxygen-containing functional groups of the graphene oxide nanosheets were studied by atomic force microscopy and X-ray photoelectron spectroscopy. While formation of the thin films composed of single-layer graphene nanosheets with minimum thickness of 0.37 nm and nearly without any functional group bonds was observed at the high temperature of 1000 °C in the one-step reducing procedure, similar high quality graphene thin films were obtained at the lower temperature of 500 °C in our two-step reducing temperature. The results also indicated possibility of efficient reduction of the graphene oxide thin films at even lower heat treatment temperatures (?500 °C).     

SDS-stabilized graphene nanosheets for highly electrically conductive adhesives

We report sodium dodecyl sulfate (SDS) stabilization of graphene nanosheets, with two different sizes as auxiliary fillers inside the conventional electrically conductive adhesive (ECA) composite. Using this non-covalent modification approach we were able to preserve the single-layer structure of graphene layers and prevent their re-stacking inside the composite, which resulted in a significant electrical conductivity improvement of ECAs at noticeably low filler content. Addition of 1.5 wt% small and large SDS-modified graphene into the conventional ECAs with 10 wt% silver flakes led to low electrical resistivity values of 5.5 × 10³ Ω cm and 35 Ω cm, respectively, while at least 40 wt% of silver flakes was required for the conventional ECA to be electrically conductive. A highly conductive ECA with very low bulk resistivity of 1.6 × 10⁻⁵ Ω cm was prepared by adding 1.5 wt% of SDS-modified large graphene into the conventional ECA with 80 wt% silver flakes which is less than that of eutectic lead-based solders.     

The effect of graphene nanosheets on the mechanical properties of polyvinylchloride

In this work, excellent improvement in mechanical properties was observed by incorporating graphene nanofillers in polyvinyl chloride via solution blending. About 63% increase in modulus and 19% increase in ultimate tensile strength were measured at 1.5 wt% nanofiller content while almost no drop in elongation at break was observed. POLYM. COMPOS., 37:1572–1576, 2016. © 2014 Society of Plastics Engineers