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.

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Graphene and Graphene Oxide: Synthesis, Properties, and Applications

Supercapacitors, also called ultracapacitors or electrochemical capacitors, store electrical charge on high-surface-area conducting materials. Their widespread use is limited by their low energy storage density and relatively high effective series resistance. Using chemical activation of exfoliated graphite oxide, we synthesized a porous carbon with a Brunauer-Emmett-Teller surface area of up to 3100 square meters per gram, a high electrical conductivity, and a low oxygen and hydrogen content. This sp 2 -bonded carbon has a continuous three-dimensional network of highly curved, atom-thick walls that form primarily 0.6- to 5-nanometer-width pores. Two-electrode supercapacitor cells constructed with this carbon yielded high values of gravimetric capacitance and energy density with organic and ionic liquid electrolytes. The processes used to make this carbon are readily scalable to industrial levels.

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Carbon-based Supercapacitors Produced by Activation of Graphene

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.

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

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 based materials: Past, present and future

The reasonable design of metal compound electrocatalysts is proved to be a good approach to accelerate the sluggish conversion kinetics of polysulfides for high performance lithium–sulfur (Li–S) batteries. Since metal cations usually serve as the active sites to interact with polysulfides, optimizing their electronic and valence states is vital to enhance catalytic activity, especially for electrochemical reactions involving multiple electrons. Herein, the electronic and valence states of metal active sites in hollow carbon-encapsulated Ni3ZnC0.7 (denoted as C/Ni3ZnC0.7) nanospheres are modulated by introducing either donor defects (P dopants) or acceptor defects (Ni vacancies) via P-doping or NaBH4-etching, respectively. Such P dopants and Ni vacancies not only endow P-doped C/Ni3ZnC0.7 (denoted as C/Ni3ZnC0.7-Pm) and NaBH4-etched C/Ni3ZnC0.7 (denoted as C/Ni3ZnC0.7-Bn) nanospheres with abundant defects and distortions, but also modulate the valence states of metal active sites in different ways. P dopants and accompanied Zn vacancies remarkably decrease the electron density of Zn active sites in C/Ni3ZnC0.7-P5 while the Ni vacancies contribute to the dramatic increase of Ni2+ species in C/Ni3ZnC0.7-B1 nanospheres. The modulated Zn sites could anchor polysulfides and serve as reservoirs to regulate the valence of Ni sites. However, the modulated Ni active sites with more Ni2+ species enable fast electron transfer between the Ni2+/Ni(0) pair and polysulfides, playing a more significant role in catalyzing the conversion of polysulfides. As a result, compared with C/Ni3ZnC0.7 and C/Ni3ZnC0.7-P5, C/Ni3ZnC0.7-B1 with more Ni2+ species exhibits stronger chemical adsorption ability toward polysulfides and catalytic ability to accelerate the conversion kinetics of polysulfides. When C/Ni3ZnC0.7-B1 nanospheres are used to modify commonly used PP separators for Li–S batteries, remarkable rate performance up to 4.0C (525.6 mA h g−1) and excellent cycling stability (a decay of 0.0179% over 1400 cycles at 1.0C) are achieved.

Rational valence modulation of bimetallic carbide assisted by defect engineering to enhance polysulfide conversion for lithium–sulfur batteries

CoB and BN composites enabling integrated adsorption/catalysis to polysulfides for inhibiting shuttle-effect in Li-S batteries

Effects of carbonization temperature on microstructure and electrochemical performances of phenolic resin-based carbon spheres

MnO2/C composite electrodes free of conductive enhancer for supercapacitors

In response to the concept of "compact energy storage", research on electrolyte dosage dwindling is definitely efficient owing to present electrolyte usage up to 70 wt % in a cell. While less electrolyte usage leads to slow reaction kinetics. Herein, a heterojunction, MoP/MoS2 core with much defects and vacancies coated by porous carbon shell, is synthesized. Besides, the small particle size of MoP/MoS2@C facilitates a close packing to form a dense and porous modified layer on PP-based (F-PP) separator. The heterojunction with defects exposes abundant interfaces and assures an adequate local electrolyte availability and an improved electrolyte affinity that are beneficial for Li+ transfer. When using F-PP separator, Li-S cell performs well in the lean electrolyte. Apart from a high discharging capacity of 517.1 mAh g-1 at 5 C in E/S = 10 (only half benchmark dosage), the cell realizes a favorable stability at C/2 over 500 cycles even in E/S = 7 (0.065% decay per cycle), demonstrating an effective polysulfides (PS) shuttling relief and reversibility of PS-relating chemical conversion. All these enhanced electrochemical behaviors in lean electrolyte result from a three-in-one strategy realized by defects-included MoP/MoS2@C heterojunction, including incorporating the lithiuphilic and sulfophilic sites for PS confinement and electrocatalysis triggered by abundant S vacancies and Lewis and Bronsted acid sites.

Mingming Chen

Papers

Rational valence modulation of bimetallic carbide assisted by defect engineering to enhance polysulfide conversion for lithium–sulfur batteries

CoB and BN composites enabling integrated adsorption/catalysis to polysulfides for inhibiting shuttle-effect in Li-S batteries

Effects of carbonization temperature on microstructure and electrochemical performances of phenolic resin-based carbon spheres

MnO2/C composite electrodes free of conductive enhancer for supercapacitors

Abundant Defects-Induced Interfaces Enabling Effective Anchoring for Polysulfides and Enhanced Kinetics in Lean Electrolyte Lithium-Sulfur Batteries.