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

Nanostructured integrated electrodes with binder‐free design show great potential to solve the ever‐growing problems faced by currently commercial lithium‐ion batteries such as insufficient power and energy densities. However, there are still many challenging problems limiting practical application of this emerging technology, in particular complex manufacturing process, high fabrication cost, and low loading mass of active material. Different from existing fabrication strategies, here using a CoP alloy foil as a precursor a simple neutral salt solution‐mediated electrochemical dealloying method to well address the above issues is demonstrated. The resultant freestanding mesoporous np‐Co(OH)x/Co2P product possesses not only active compositions of high specific capacity and large electrode packing density (>3.0 g cm−3) to meet practical capacity requirements, high‐conductivity and well‐developed nanoporous framework to achieve simultaneously fast ion and electron transfer, but also interconnected ligaments and suitable free space to ensure strong structural stability. Its comprehensively excellent electrochemical energy storage (EES) performances in both lithium/sodium‐ion batteries and lithium‐ion capacitors can further illustrate the effectiveness of the integrated electrode preparation strategy, such as remarkable reversible specific capacities/capacitances, dominated pseudo‐capacitive EES mechanism, and ultra‐long cycling life. This study provides new insights into preparation and design of high‐performance integrated electrodes for practical applications.

Engineering CoP Alloy Foil to a Well‐Designed Integrated Electrode Toward High‐Performance Electrochemical Energy Storage

Two-dimensional (2D) materials with mono or few layers have wide application prospects, including electronic, optoelectronic, and interface functional coatings in addition to energy conversion and storage applications. However, the exfoliation of such materials is still challenging due to their low yield, high cost, and poor ecological safety in preparation. Herein, a safe and efficient solid suspension-improving method was proposed to exfoliate hexagonal boron nitride nanosheets (hBNNSs) in a large yield. The method entails adding a permeation barrier layer in the solvothermal kettle, thus prolonging the contact time between the solvent and hexagonal boron nitride (hBN) nanosheet and improving the stripping efficiency without the need for mechanical agitation. In addition, the proposed method selectively utilizes a matching solvent that can reduce the stripping energy of the material and employs a high-temperature steam shearing process. Compared with other methods, the exfoliating yield of hBNNSs is up to 42.3% at 150 °C for 12 h, and the strategy is applicable to other 2D materials. In application, the ionic conductivity of a PEO/hBNNSs composite electrolytes reached 2.18 × 10−4 S cm−1 at 60 °C. Overall, a versatile and effective method for stripping 2D materials in addition to a new safe energy management strategy were provided.

High-efficiency 2D nanosheet exfoliation by a solid suspension-improving method

Amphiphilic carbonaceous material (ACM), which could be dispersed in alkaline aqueous solutions in nano-scale, was prepared by oxidation of green needle coke. Mesoporous activated carbon was then obtained by further KOH activation, named as AC-A. It was showed that AC-A was a mesoporous material with surface area and pore volume up to 3347 m2/g and 1.81 cm3/g. For its excellent pore structure and surface characteristics, AC-A was selected as electrode material for Electric double-layer capacitor (EDLC). According to electrochemical tests, AC-A electrode exhibited specific capacitance as high as 348 F/g and also excellent performance under large discharging currents, which was partly attributed to the large specific surface area, high mesoporosity and good surface wettability of the material.

Mesoporous activated carbon from amphiphilic carbonaceous material and its application in EDLC

Li-rich layered oxides (LLOs) are one of the most attractive next-generation positive electrode materials as a result of their high energy density and low cost. However, the deterioration of cycling stability observed in LLOs remains one of the fundamental obstacles to commercialization. Carbonate-based electrolytes reacting with oxygen radicals evolved from the lattice of LLOs is the chief cause of their poor cyclability. Herein, we construct no carbonyl group, trimethyl phosphate (TMP)-based electrolytes with a fluorinated ether co-solvent and apply them to investigate the electrochemical behaviors of LLO batteries. These electrolytes can capture active oxygen species; the initial reversible capacity of cells reaches 295.5 mAh g-1; and the capacity retention remains 96.7% after 100 cycles. In contrast, the capacity retention of cells using carbonate-based electrolytes is only 54.7% after 60 cycles. These results would provide the scientific basis and theoretical support for building electrolytes of LLOs with high properties in the future.

Mingming Chen

Papers

Engineering CoP Alloy Foil to a Well‐Designed Integrated Electrode Toward High‐Performance Electrochemical Energy Storage

High-efficiency 2D nanosheet exfoliation by a solid suspension-improving method

Mesoporous activated carbon from amphiphilic carbonaceous material and its application in EDLC

Exploring Trimethyl-Phosphate-Based Electrolytes without a Carbonyl Group for Li-Rich Layered Oxide Positive Electrodes in Lithium-Ion Batteries.

Urea-assisted Strategy Controlling The Pore Structure And Chemical Composition Of The Porous Carbon For High-performance Supercapacitors