Facile synthesis of reduced graphene oxide/CeO2 nanocomposites and their application in supercapacitors

The combination of the attractive properties of graphene with excellent characteristics of other functional nanomaterials has become a popular pathway for achieving applications in multiple fields. Herein, reduced graphene oxide (RGO)/CeO₂ nanocomposites with enhanced capacitive performance were designed and synthesized by a facile two-step approach with a self-assembly method followed by thermal treatment. The structure, morphology and composition of the resulting RGO/CeO₂ nanocomposites were systematically investigated. The presence of RGO can prevent the aggregation and control the structures of the CeO₂ nanocrystals in the annealing process. The nanocomposites as electrode materials for supercapacitor exhibited an enhanced capacitive performance due to the synergic effect between RGO nanosheets and CeO₂ nanocrystals. The excellent capacitive performance of the RGO/CeO₂ nanocomposites offer great promise for supercapacitor applications.     

Scalable preparation of three-dimensional porous structures of reduced graphene oxide/cellulose composites and their application in supercapacitors

Controlling the assembled structures of graphene has recently attracted enormous attention due to intriguing properties of the resultant structures. In this study, three-dimensional (3D) porous structures of reduced graphene oxide (RGO) with various ratios of RGO to cellulose have been fabricated by a scalable, but simple and efficient, approach that consists of ball milling assisted chemical reduction of GO, template shaping, coagulating, and lyophilization. The efficient mechanical shearing of ball milling and the hydrogen bond interactions between RGO and cellulose molecules contribute to the formation of a homogeneous RGO/cellulose hydrogel, improved thermal stability of the resultant composites, and enhanced crystallinity of the cellulose in the composites. The coagulation effect of cellulose maintains the RGO sheets in the 3D structures of cellulose; on the other hand, the RGO sheets facilitate the preservation of the 3D structures during freeze-drying, leading to the formation of 3D porous structures of RGO/cellulose composites. Benefiting from the continuous RGO network in the composites, the 3D porous structures of RGO(70)/cellulose(100) (GO:cellulose = 70:100 in weight) show an electrical conductivity of 15.28 S m⁻¹. Moreover, the 3D porous structures show potential application in supercapacitors due to the fact that they provide high specific surface area and fast charge propagation.     

Preparation of hierarchically porous carbon nanosheets by carbonizing resol resin for supercapacitors

Journal of Porous Materials - Hierarchically porous carbon nanosheets were synthesized by using homogeneous mixture of soluble phenolic resin and KOH activator, which was carbonized and activated...     

Ideal asymmetric supercapacitors consisting of polyaniline nanofibers and graphene nanosheets with proper complementary potential windows

Polyaniline (PANI) nanofibers are synthesized via a chemical method of rapid mixing for the application of asymmetric supercapacitors. The diameter and aspect ratio of PANI nanofibers is found to be controllable by varying the aniline/oxidant concentration ratio. The ideal capacitive responses of PANI nanofibers between 0.2 and 0.7 V (vs. Ag/AgCl) in concentrated acidic media are demonstrated by cyclic voltammetric (CV) and electrochemical impedance spectroscopic (EIS) analyses coupled with a schematic equivalent-circuit model. The morphologies and textures of nanofibers are examined by scanning electron microscopic (SEM), transmission electron microscopic (TEM) and Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopic analyses. An aqueous asymmetric supercapacitor, consisting of a PANI nanofiber cathode and a graphene anode, with proper complementary potential windows is demonstrated in this work, which shows the device energy and power densities of 4.86 Wh kg⁻¹ and 8.75 kW kg⁻¹, respectively.     

Preparation of scale-like nickel cobaltite nanosheets assembled on nitrogen-doped reduced graphene oxide for high-performance supercapacitors

Ultrathin scale-like nickel cobaltite (NiCo₂O₄) nanosheets supported on nitrogen-doped reduced graphene oxide (N-rGO) are successfully synthesized through a facile co-precipitation of Ni²⁺ and Co²⁺ in the presence of sodium citrate and hexamethylenetetramine and subsequent calcination treatment. The composition and morphology of NiCo₂O₄ nanosheets@nitrogen-doped reduced graphene oxide (denoted as NiCo₂O₄ NSs@N-rGO) were characterized by Scanning electron microscope, Transmission electron microscope, X-ray diffraction, Raman spectra, X-ray photoelectron spectroscopy, Brunauer–Emmett–Teller and thermogravimetric analysis. The thickness of NiCo₂O₄ nanosheets anchored on the reduced graphene oxide is around 4 nm. The capacitance of NiCo₂O₄ NSs@N-rGO is evaluated by cyclic voltammogram and galvanostatic charge/discharge with the result that the NiCo₂O₄ NSs@N-rGO could deliver a specific capacitance of 1540 F g⁻¹ after 1000 cycles at 10 A g⁻¹.     

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.     

Easy synthesis of amorphous graphene and related hybrids for cold cathode application

Owing to enormous growing interest regarding the graphene based field emission display device, we have reported a very simple route to synthesis amorphous graphene (a-Gs) by ultra-sonication assisted unzipping of low-temperature synthesized amorphous carbon nanotubes. Also different hybrid systems have been developed by spinning assisted coating of the as prepared a-Gs on plasma enhanced chemical vapor deposited carbon nanoparticles or chemically synthesized vertically aligned silicon nanowires in order to develop hybrid cold cathode materials. Also the as synthesized a-Gs have been deposited in carbon cloth substrate by electrophoretic deposition process and thus making it usable as flexible field emitter. It has been seen that all the, pure and hybrid samples give good field emission characteristics and the best result comes from the a-Gs deposited on carbon cloth. The turn field in this case becomes as low as 0.52 V/μm thus comparable or even better than the existing graphene based materials. So there should be a possibility of substituting some graphene based devices with this a-Gs having further advantage of its simple synthesis procedure as well as gram level higher yield.     

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.     

One-pot hydrothermal synthesis of nitrogen-doped hierarchically porous carbon monoliths for supercapacitors

The nitrogen-doped hierarchically porous carbon monoliths (N-HPCMs) were successfully synthesized by using dicyandiamide (DCDA) as nitrogen source, phenolic resol as carbon precursor and mixed triblock copolymers as templates via a one-pot hydrothermal approach. The obtained carbon monoliths possess tunable mesopore size (4.3–11.4 nm), large surface area (552–660 m²/g), and high nitrogen content (up to 12.1 wt%). Ascribed to the nitrogen-doped frameworks and hierarchical porosity, N-HPCMs exhibit good electrochemical performance as the supercapacitor electrode with specific capacitance of 268.9 F/g (in 6 M KOH) at a current density of 1 A/g, and a 4.1 % loss of the specific capacitance after 5,000 charge–discharge cycles, indicating a long-term cycling stability. Such unique features make N-HPCMs promising electrode materials for high performance supercapacitors.     

Easy preparation of nitrogen-enriched carbon materials from peptides of silk fibroins and their use to produce a high volumetric energy density in supercapacitors

A novel nitrogen-rich carbon material, silk carbon, was prepared from Bombyx mori silk fibroins by simple heat-treatment under inert atmosphere. The nitrogen atoms were originated from copious peptide bonds contained in the silk fibroins. The adequate heat-treatment temperature turned the nitrogen atoms of the peptide bonds to the silk carbons in the form of nitrogen-containing functional groups. Furthermore, the activation of the silk carbons with steam and potassium hydroxide provided nitrogen-containing activated carbons (ACs) with different pore-size distribution. In the application to electric double layer capacitors, the silk carbon-based ACs showed a higher capacitance and an excellent electrochemical stability in the high voltage region, compared with a typical AC prepared from phenolic resin. More significantly, the steam-activated silk carbon showed the energy density that was comparable to that of the phenolic resin-based AC prepared by chemical activation. This indicates the superiority of steam-activated silk carbons to conventional KOH-activated carbon materials in supercapacitor application, due to the low production cost by the facilities of steam activation, and the relevant capacitance comparable with that of chemical activation.     

One-step electrochemical deposition of nickel sulfide/graphene and its use for supercapacitors

In this current work, the electrochemical co-deposition of nickel sulfide/electrochemically reduced graphene oxide(ERGO) nanocomposites is presented. During the electrochemical process, the graphene oxide nanosheets loose their hydrophilicity and precipitate onto the electrode. In the meantime, nickel sulfide is also electrochemically deposited on the electrode. The porous structure with ERGO covered by nickel sulfide, which facilitates the charge and ion transport in the electrode, has been observed by a scanning electron microscope. The cycle voltammetry curves as well as the galvanostatic charge/discharge curves of the nickel sulfide/ERGO nanocomposites exhibit distinct pseudocapacitive characteristic. The nanocomposites maintain 66.8% of the initial specific capacitance for the first 500 cycles, and only 4.6% loss of the specific capacitance is experienced for the further 1500 cycles, evidently showing a relatively high cycling stability. The results suggest that the nickel sulfide/ERGO is a promising electrode material for supercapacitors.     

Nano graphene porous/conductive polymer as a composite material for energy storage in supercapacitors

This research studies the improving effects of graphene porous (GP) on the supercapacitive performance of a polyaniline/graphene porous (PANI/GP) nanocomposite. GP nanosheets were synthesized via chemical vapor deposition, and PANI/GP was electrochemically composited through successive cyclic voltammetry. The samples were characterized by fast Fourier transform infrared (FTIR), x-ray diffraction (XRD), and scanning electron microscopy (SEM), and energy-dispersive x-ray spectrometry (EDS) techniques. Porous GP nanosheets were uniformly dispersed in the composite structure. Furthermore, the electrochemical performances of the synthesized samples were compared using galvanostatic charge/discharge, electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV). Incorporating GP into the PANI significantly increased specific capacitance from 276 (in PANI) to 577 F/g (in PANI/GP). The electrochemical stability of electrodes was compared during 1000 successive charge/discharge cycles. After 1000 cycles, PANI/GP kept 90% of its initial capacitance, and only 25% of the charge storage capacitance of bare PANI remained.     

多孔碳纳米片的制备及其在电化学领域的应用研究进展

介绍了二维多孔碳纳米片(PCNs)材料在能源领域的应用优势,综述了PCNs的制备方法以及在锂离子电池、超级电容器和电催化氧还原反应等电化学领域的应用研究进展。PCNs的制备方法为硬模板法(包括空间、盐粒表面和其他二维材料表面模板)、软模板法(包括使用两亲性小分子和两亲性块状共聚物(BCPs)制备PCNs)、无模板法(包括小分子、聚合物和生物质作为前驱体来制备PCNs)。其中软模板法相比硬模板法,其稳定性较差,还需进一步提高。指出未来应致力于开发制备PCNs的新方法和新型PCNs基材料,同时研究此类PCNs材料在分离膜、生化传感器、量子器件等方面的应用。     

Exfoliation of graphene nanosheets in aqueous media

Graphene has attracted much attention and holds great promise in various applications due to its extraordinary properties. To realize applications of graphene in large scale, developing a facile, green and cost-effective method for mass production of high-quality graphene is highly desired. Relative to expensive and complicated bottom-up approaches, top-down methods for graphene production are promising owing to their low cost and simplicity. Specifically, exfoliation of graphene nanosheets in liquid phase is favorable for their dispersion, functionalization and processing. Instead of highly toxic organic solvents, using water as the liquid medium makes exfoliation process eco-friendly and sustainable. In this review, recent progress on exfoliation of graphene nanosheets in water is discussed, with a particular focus on exfoliation and stabilizing mechanism in various aqueous media. Different water-based exfoliation methods, such as liquid-phase exfoliation and electrochemical exfoliation, are surveyed.     

Synthesis of surface-functionalized graphene nanosheets with high Pt-loadings and their applications to methanol electrooxidation

Surface-functionalized graphene nanosheets (GNSs) were synthesized from natural graphite via chemical oxidation and subsequent thermal exfoliation. High Pt metal loadings, up to 80 wt.%, were deposited over the synthesized GNS supports and the Pt particle size was maintained at less than 3 nm. The current densities of methanol electrooxidation with these Pt/GNS catalysts were at least twice as large as those observed with conventional Pt/C catalysts. Furthermore, these Pt/GNS catalysts maintained high Pt mass activities with increased Pt loading on the working electrode, ranging from 0.2 to 2.0 mg/cm². This improved mass activity indicates that our electrocatalysts are able to provide more efficient Pt utilization for alcohol electrooxidation compared to conventional catalysts.     

RESS制备石墨烯纳米片及防止其再团聚的新方法

本文以天然鳞片石墨为原料,利用超临界状态下二氧化碳的快速膨胀（RESS）来剥离石墨产生石墨烯纳米片。电子显微镜（SEM）表征证实RESS可有效地实现石墨的剥离,并产生了一些石墨烯纳米片层。同时,为了解决再团聚难题,提出利用碳纳米管在产生石墨烯纳米片间穿层的方法和利用小分子包覆法来防止其再团聚,实验证实都起到良好的效果。     

Furfuryl alcohol functionalized graphene nanosheets for synthesis of high carbon yield novolak composites

Graphene oxide and furfuryl alcohol modified graphene nanosheets (G‐FA) were used to prepare graphene/novolak composites. Effect of graphene compatibilization on the properties of the composites especially carbon yield value is evaluated. Both types of graphene nanosheets were dispersed uniquely in the novolak matrix as proved by X‐ray diffraction analysis. However, modification of graphene sheets by furfuryl alcohol results in more improved dispersions. Thermogravimetric analysis confirms the elevated thermal stability of the nanocomposites in comparison with the neat novolak. In addition, G‐FA containing composites have higher carbon yield values. A shift in the wave number of characteristic bonds of graphene after oxidation and modification with furfuryl alcohol, O? H, C?O, and C? O bonds, are seen in the Fourier transform infrared spectroscopy spectra. Raman results and scanning electron microscopy images show that graphene nanosheets reduced in size and wrinkled by oxidation and functionalization. Transmission electron microscopy image of the composite with 0.2 wt % of G‐FA reveals the presence of nanosheets with curvature. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40273.     

Facile synthesis and strongly microstructure-dependent electrochemical properties of graphene/manganese dioxide composites for supercapacitors

Graphene has attracted much attention since it was firstly stripped from graphite by two physicists in 2004, and the supercapacitor based on graphene has obtained wide attention and much investment as well. For practical applications of graphene-based supercapacitors, however, there are still many challenges to solve, for instance, to simplify the technological process, to lower the fabrication cost, and to improve the electrochemical performance. In this work, graphene/MnO₂ composites are prepared by a microwave sintering method, and we report here a relatively simple method for the supercapacitor packaging, i.e., dipping Ni-foam into a graphene/MnO₂ composite solution directly for a period of time to coat the active material on a current collector. It is found that the microwave reaction time has a significant effect on the microstructure of graphene/MnO₂ composites, and consequently, the electrochemical properties of the supercapacitors based on graphene/MnO₂ composites are strongly microstructure dependent. An appropriately longer microwave reaction time, namely, 15 min, facilitates a very dense and homogeneous microstructure of the graphene/MnO₂ composites, and thus, excellent electrochemical performance is achieved in the supercapacitor device, including a high specific capacitance of 296 F/g and a high capacitance retention of 93% after 3,000 times of charging/discharging cycles.

PACS

81.05.ue; 78.67.Sc; 88.80.fh