There is intense interest in graphene in fields such as physics, chemistry, and materials science, among others. Interest in graphene's exceptional physical properties, chemical tunability, and potential for applications has generated thousands of publications and an accelerating pace of research, making review of such research timely. Here is an overview of the synthesis, properties, and applications of graphene and related materials (primarily, graphite oxide and its colloidal suspensions and materials made from them), from a materials science perspective.

/pdf/graphene-and-graphene-oxide-synthesis-properties-and-2q80uaujok.pdf

Graphene and Graphene Oxide: Synthesis, Properties, and Applications

In this critical review, metal oxides-based materials for electrochemical supercapacitor (ES) electrodes are reviewed in detail together with a brief review of carbon materials and conducting polymers. Their advantages, disadvantages, and performance in ES electrodes are discussed through extensive analysis of the literature, and new trends in material development are also reviewed. Two important future research directions are indicated and summarized, based on results published in the literature: the development of composite and nanostructured ES materials to overcome the major challenge posed by the low energy density of ES (476 references).

/pdf/a-review-of-electrode-materials-for-electrochemical-2cok5em0bo.pdf

A review of electrode materials for electrochemical supercapacitors

As a fascinating conjugated polymer, graphitic carbon nitride (g-C3N4) has become a new research hotspot and drawn broad interdisciplinary attention as a metal-free and visible-light-responsive photocatalyst in the arena of solar energy conversion and environmental remediation. This is due to its appealing electronic band structure, high physicochemical stability, and “earth-abundant” nature. This critical review summarizes a panorama of the latest progress related to the design and construction of pristine g-C3N4 and g-C3N4-based nanocomposites, including (1) nanoarchitecture design of bare g-C3N4, such as hard and soft templating approaches, supramolecular preorganization assembly, exfoliation, and template-free synthesis routes, (2) functionalization of g-C3N4 at an atomic level (elemental doping) and molecular level (copolymerization), and (3) modification of g-C3N4 with well-matched energy levels of another semiconductor or a metal as a cocatalyst to form heterojunction nanostructures. The constructi...

Graphitic Carbon Nitride (g-C3N4)-Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer To Achieving Sustainability?

Since the discovery of mechanically exfoliated graphene in 2004, research on ultrathin two-dimensional (2D) nanomaterials has grown exponentially in the fields of condensed matter physics, material science, chemistry, and nanotechnology. Highlighting their compelling physical, chemical, electronic, and optical properties, as well as their various potential applications, in this Review, we summarize the state-of-art progress on the ultrathin 2D nanomaterials with a particular emphasis on their recent advances. First, we introduce the unique advances on ultrathin 2D nanomaterials, followed by the description of their composition and crystal structures. The assortments of their synthetic methods are then summarized, including insights on their advantages and limitations, alongside some recommendations on suitable characterization techniques. We also discuss in detail the utilization of these ultrathin 2D nanomaterials for wide ranges of potential applications among the electronics/optoelectronics, electrocat...

Recent Advances in Ultrathin Two-Dimensional Nanomaterials

Although electrochemical capacitors (ECs), also known as supercapacitors or ultracapacitors, charge and discharge faster than batteries, they are still limited by low energy densities and slow rate capabilities. We used a standard LightScribe DVD optical drive to do the direct laser reduction of graphite oxide films to graphene. The produced films are mechanically robust, show high electrical conductivity (1738 siemens per meter) and specific surface area (1520 square meters per gram), and can thus be used directly as EC electrodes without the need for binders or current collectors, as is the case for conventional ECs. Devices made with these electrodes exhibit ultrahigh energy density values in different electrolytes while maintaining the high power density and excellent cycle stability of ECs. Moreover, these ECs maintain excellent electrochemical attributes under high mechanical stress and thus hold promise for high-power, flexible electronics.

/pdf/laser-scribing-of-high-performance-and-flexible-graphene-2b5d3mxb2h.pdf

Laser Scribing of High-Performance and Flexible Graphene-Based Electrochemical Capacitors

A composite of graphene oxide supported by needle-like MnO2 nanocrystals (GO−MnO2 nanocomposites) has been fabricated through a simple soft chemical route in a water−isopropyl alcohol system. The formation mechanism of these intriguing nanocomposites investigated by transmission electron microscopy and Raman and ultraviolet−visible absorption spectroscopy is proposed as intercalation and adsorption of manganese ions onto the GO sheets, followed by the nucleation and growth of the crystal species in a double solvent system via dissolution−crystallization and oriented attachment mechanisms, which in turn results in the exfoliation of GO sheets. Interestingly, it was found that the electrochemical performance of as-prepared nanocomposites could be enhanced by the chemical interaction between GO and MnO2. This method provides a facile and straightforward approach to deposit MnO2 nanoparticles onto the graphene oxide sheets (single layer of graphite oxide) and may be readily extended to the preparation of othe...

Graphene Oxide−MnO2 Nanocomposites for Supercapacitors

Graphene sheets, which possess unique nanostructure and a variety of fascinating properties, can be considered as promising nanoscale building blocks of new composites, for example, a support material for the dispersion of nanoparticles. Here, we present a general approach for the preparation of graphene−metal particle nanocomposites in a water−ethylene glycol system using graphene oxide as a precursor and metal nanoparticles (Au, Pt and Pd) as building blocks. These metal nanoparticles are adsorbed on graphene oxide sheets and play a pivotal role in catalytic reduction of graphene oxide with ethylene glycol, leading to the formation of graphene−metal particle nanocomposites. The typical methanol oxidation of graphene−Pt composites in cyclic voltammograms analyses indicated its potential application in direct methanol fuel cells, bringing graphene−particle nanocomposites close to real technological applications.

Graphene−Metal Particle Nanocomposites

A simple, bioinspired approach to effectively prevent the restacking of chemically converted graphene sheets in multilayered films is presented. The method enables the creation of a new generation of supercapacitors that combine high energy density, high power density, and high operation rates. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Bioinspired Effective Prevention of Restacking in Multilayered Graphene Films: Towards the Next Generation of High‐Performance Supercapacitors

The voltage limit for aqueous asymmetric supercapacitors is usually 2 V, which impedes further improvement in energy density. Here, high Na content Birnessite Na0.5 MnO2 nanosheet assembled nanowall arrays are in situ formed on carbon cloth via electrochemical oxidation. It is interesting to find that the electrode potential window for Na0.5 MnO2 nanowall arrays can be extended to 0-1.3 V (vs Ag/AgCl) with significantly increased specific capacitance up to 366 F g-1 . The extended potential window for the Na0.5 MnO2 electrode provides the opportunity to further increase the cell voltage of aqueous asymmetric supercapacitors beyond 2 V. To construct the asymmetric supercapacitor, carbon-coated Fe3 O4 nanorod arrays are synthesized as the anode and can stably work in a negative potential window of -1.3 to 0 V (vs Ag/AgCl). For the first time, a 2.6 V aqueous asymmetric supercapacitor is demonstrated by using Na0.5 MnO2 nanowall arrays as the cathode and carbon-coated Fe3 O4 nanorod arrays as the anode. In particular, the 2.6 V Na0.5 MnO2 //Fe3 O4 @C asymmetric supercapacitor exhibits a large energy density of up to 81 Wh kg-1 as well as excellent rate capability and cycle performance, outperforming previously reported MnO2 -based supercapacitors. This work provides new opportunities for developing high-voltage aqueous asymmetric supercapacitors with further increased energy density.

High-Performance 2.6 V Aqueous Asymmetric Supercapacitors based on In Situ Formed Na0.5MnO2 Nanosheet Assembled Nanowall Arrays

A composite of graphite oxide (GO)-supported Co3O4nanoparticles (NPs) was prepared through a facile chemical route. The lamellar GO sheets in this composite were exfoliated and decorated randomly by Co3O4 particles with an average size of 100 nm. The formation route to anchor Co3O4 NPs onto the exfoliated GO sheets was proposed as the intercalation and adsorption of cobalt ions into the layered GO sheets, followed by the nucleation and growth of Co3O4 crystals, resulting in the exfoliation of GO sheets. The nanocomposite reveals unusual catalytic effect for the decomposition of ammonium perchlorate due to the concerted effects of GO and Co3O4 NPs or their integrated properties. This methodology made the synthesis of GO-NP composites possible and may be further extended to prepare more complicated nanocomposites based on GO sheets for technological applications.

Junwu Zhu

Papers

Graphene Oxide−MnO2 Nanocomposites for Supercapacitors

Graphene−Metal Particle Nanocomposites

Bioinspired Effective Prevention of Restacking in Multilayered Graphene Films: Towards the Next Generation of High‐Performance Supercapacitors

High-Performance 2.6 V Aqueous Asymmetric Supercapacitors based on In Situ Formed Na0.5MnO2 Nanosheet Assembled Nanowall Arrays

Deposition of Co3O4nanoparticles onto exfoliated graphite oxide sheets