Graphene has attracted tremendous research interest in recent years, owing to its exceptional properties. The scaled-up and reliable production of graphene derivatives, such as graphene oxide (GO) and reduced graphene oxide (rGO), offers a wide range of possibilities to synthesize graphene-based functional materials for various applications. This critical review presents and discusses the current development of graphene-based composites. After introduction of the synthesis methods for graphene and its derivatives as well as their properties, we focus on the description of various methods to synthesize graphene-based composites, especially those with functional polymers and inorganic nanostructures. Particular emphasis is placed on strategies for the optimization of composite properties. Lastly, the advantages of graphene-based composites in applications such as the Li-ion batteries, supercapacitors, fuel cells, photovoltaic devices, photocatalysis, as well as Raman enhancement are described (279 references).

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Graphene-based composites

Graphene oxide: preparation, functionalization, and electrochemical applications.

The science and technology of ultracapacitors are reviewed for a number of electrode materials, including carbon, mixed metal oxides, and conducting polymers. More work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and an organic electrolytes. The energy density of these devices is 3¯5 Wh/kg with a power density of 300¯500 W/kg for high efficiency (90¯95%) charge/discharges. Projections of future developments using carbon indicate that energy densities of 10 Wh/kg or higher are likely with power densities of 1¯2 kW/kg. A key problem in the fabrication of these advanced devices is the bonding of the thin electrodes to a current collector such the contact resistance is less than 0.1 cm2. Special attention is given in the paper to comparing the power density characteristics of ultracapacitors and batteries. The comparisons should be made at the same charge/discharge efficiency.

Ultracapacitors: Why, How, and Where is the Technology

This paper presents a review of the research progress in the carbon–metal oxide composites for supercapacitor electrodes. In the past decade, various carbon–metal oxide composite electrodes have been developed by integrating metal oxides into different carbon nanostructures including zero-dimensional carbon nanoparticles, one-dimensional nanostructures (carbon nanotubes and carbon nanofibers), two-dimensional nanosheets (graphene and reduced graphene oxides) as well as three-dimensional porous carbon nano-architectures. This paper has described the constituent, the structure and the properties of the carbon–metal oxide composites. An emphasis is placed on the synergistic effects of the composite on the performance of supercapacitors in terms of specific capacitance, energy density, power density, rate capability and cyclic stability. This paper has also discussed the physico-chemical processes such as charge transport, ion diffusion and redox reactions involved in supercapacitors.

Nanostructured carbon–metal oxide composite electrodes for supercapacitors: a review

Graphene, a one-atom layer of graphite, possesses a unique two-dimensional (2D) structure, high conductivity and charge carrier mobility, huge specific surface area, high transparency and great mechanical strength. Thus, it is expected to be an ideal material for energy storage and conversion. During the past several years, a variety of graphene based materials (GBMs) have been successfully prepared and applied in supercapacitors, lithium ion batteries, water splitting, electrocatalysts for fuel cells, and solar cells. In this review, we will summarize the recent advances in the synthesis and applications of GBMs in these energy related systems. The challenges and prospects of graphene based new energy materials are also discussed.

Graphene based new energy materials

Graphene/manganese dioxide (MnO2) composite papers (GMCP) are fabricated via a simple three-step route: preparation of graphene oxide/MnO2 composite (GOMC) dispersion, subsequent vacuum filtration of GOMC dispersion to achieve graphene oxide/MnO2 composite paper (GOMCP), and finally thermal reduction of GOMCP to generate GMCP. The morphology and microstructure of the prepared samples are characterized by field-emission scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, Fourier transformation infrared spectroscopy, thermal gravimetric analysis and X-ray photoelectron spectroscopy. Moreover, as a binder-free and flexible electrode material for supercapacitors, the electrochemical properties of the prepared GMCP are evaluated by cyclic voltammetry and galvanostatic charge/discharge tests. As a result, the specific capacitance of the GMCP with the MnO2 weight ratio of 24% (GMCP-24) reaches 256 F g−1 at a current density of 500 mA g−1 and also shows good cycle stability, indicating a promising potential application as an effective electrode material for supercapacitors.

Flexible graphene/MnO2 composite papers for supercapacitor electrodes

Reduced graphene oxide (RGO) sheets were covalently assembled onto silicon wafers via a multistep route based on the chemical adsorption and thermal reduction of graphene oxide (GO). The formation and microstructure of RGO were analyzed by X-ray photoelectron spectroscopy (XPS), attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, Raman spectroscopy, and water contact angle (WCA) measurements. Characterization by atomic force microscopy (AFM) was performed to evaluate the morphology and microtribological behaviors of the samples. Macrotribological performance was tested on a ball-on-plate tribometer. Results show that the assembled RGO possesses good friction reduction and antiwear ability, properties ascribed to its intrinsic structure, that is, the covalent bonding to the substrate and self-lubricating property of RGO.

Tribology study of reduced graphene oxide sheets on silicon substrate synthesized via covalent assembly.

A new class of multilayer films was constructed by electrostatic layer-by-layer self-assembly, using poly(sodium 4-styrenesulfonate) mediated graphene sheets (PSS-GS), manganese dioxide (MnO2) sheets, and poly(diallyldimethylammonium) (PDDA) as building blocks. UV-vis spectroscopy, field-emission scanning electron microscopy and X-ray photoelectron spectroscopy were used to characterize the microstructures and morphologies of the multilayer films. Capacitive properties of the synthesized multilayer film electrodes were studied using cyclic voltammetry and galvanostatic charge/discharge in 0.1 M Na2SO4 electrolyte. The specific capacitance of the ITO/(PDDA/PSS-GS/PDDA/MnO2)10electrode reached 263 F g−1 at a discharge current density of 0.283 A g−1; moreover, this film electrode also shows a good cyclic stability and high Coulombic efficiency. Anticipatedly, the synthesized multilayer films will find promising applications as a novel electrode material in supercapacitors and other devices in virtue of their outstanding characteristics of controllable capacitance, good cycle stability, low cost and environmentally benign nature.

Electrostatic layer-by-layer self-assembly multilayer films based on graphene and manganese dioxide sheets as novel electrode materials for supercapacitors

Sheng Liu

Papers

Flexible graphene/MnO2 composite papers for supercapacitor electrodes

Tribology study of reduced graphene oxide sheets on silicon substrate synthesized via covalent assembly.

Electrostatic layer-by-layer self-assembly multilayer films based on graphene and manganese dioxide sheets as novel electrode materials for supercapacitors

Synthesis of hydrothermally reduced graphene/MnO2 composites and their electrochemical properties as supercapacitors

“Green” electrochemical synthesis of Pt/graphene sheet nanocomposite film and its electrocatalytic property