Graphene has attracted extensive interest in the field of supercapacitor research due to its 2D structure which grants it exceptional properties such as superior electrical conductivity and mechanical properties as well as an extensive surface area better than that of carbon nanotubes (CNTs). Furthermore, unlike other carbon materials, graphene is particularly optimal for supercapacitor applications as its surface area does not vary with pore size distribution and grants electrolyte access to both its surfaces. This article aims to review the advances in recent research and development of the use of graphene for supercapacitor use. The focus would mainly be on the areas of graphene synthesis, graphene modification, graphene–nanoporous carbon composites, graphene–polymer composites and graphene–metal oxides and their potential use in both asymmetric and symmetric supercapacitors. Lastly, the article aims to identify optimal testing methods for electrode performance and choice of electrolytes. It will then stress the increasing need to standardise electrode testing to ensure that test results are as relevant to real life applications as possible.

Graphene for supercapacitor applications

Epitaxy-the growth of a crystalline material on a substrate-is crucial for the semiconductor industry, but is often limited by the need for lattice matching between the two material systems. This strict requirement is relaxed for van der Waals epitaxy, in which epitaxy on layered or two-dimensional (2D) materials is mediated by weak van der Waals interactions, and which also allows facile layer release from 2D surfaces. It has been thought that 2D materials are the only seed layers for van der Waals epitaxy. However, the substrates below 2D materials may still interact with the layers grown during epitaxy (epilayers), as in the case of the so-called wetting transparency documented for graphene. Here we show that the weak van der Waals potential of graphene cannot completely screen the stronger potential field of many substrates, which enables epitaxial growth to occur despite its presence. We use density functional theory calculations to establish that adatoms will experience remote epitaxial registry with a substrate through a substrate-epilayer gap of up to nine angstroms; this gap can accommodate a monolayer of graphene. We confirm the predictions with homoepitaxial growth of GaAs(001) on GaAs(001) substrates through monolayer graphene, and show that the approach is also applicable to InP and GaP. The grown single-crystalline films are rapidly released from the graphene-coated substrate and perform as well as conventionally prepared films when incorporated in light-emitting devices. This technique enables any type of semiconductor film to be copied from underlying substrates through 2D materials, and then the resultant epilayer to be rapidly released and transferred to a substrate of interest. This process is particularly attractive in the context of non-silicon electronics and photonics, where the ability to re-use the graphene-coated substrates allows savings on the high cost of non-silicon substrates.

Remote epitaxy through graphene enables two-dimensional material-based layer transfer

We investigated the reduction of current fluctuations in few-layer black phosphorus (BP) field-effect transistors resulting from Al2O3 passivation. In order to verify the effect of Al2O3 passivation on device characteristics, measurements and analyses were conducted on thermally annealed devices before and after the passivation. More specifically, static and low-frequency noise analyses were used in monitoring the charge transport characteristics in the devices. The carrier number fluctuation (CNF) model, which is related to the charge trapping/detrapping process near the interface between the channel and gate dielectric, was employed to describe the current fluctuation phenomena. Noise reduction due to the Al2O3 passivation was expressed in terms of the reduced interface trap density values Dit and Nit, extracted from the subthreshold slope (SS) and the CNF model, respectively. The deviations between the interface trap density values extracted using the SS value and CNF model are elucidated in terms of t...

Few-Layer Black Phosphorus Field-Effect Transistors with Reduced Current Fluctuation

Graphene is a two-dimensional atomic crystal, and since its development it has been applied in many novel ways in both research and industry. Graphene possesses unique properties, and it has been used in many applications including sensors, batteries, fuel cells, supercapacitors, transistors, components of high-strength machinery, and display screens in mobile devices. In the past decade, the biomedical applications of graphene have attracted much interest. Graphene has been reported to have antibacterial, antiplatelet, and anticancer activities. Several salient features of graphene make it a potential candidate for biological and biomedical applications. The synthesis, toxicity, biocompatibility, and biomedical applications of graphene are fundamental issues that require thorough investigation in any kind of applications related to human welfare. Therefore, this review addresses the various methods available for the synthesis of graphene, with special reference to biological synthesis, and highlights the biological applications of graphene with a focus on cancer therapy, drug delivery, bio-imaging, and tissue engineering, together with a brief discussion of the challenges and future perspectives of graphene. We hope to provide a comprehensive review of the latest progress in research on graphene, from synthesis to applications.

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Synthesis, toxicity, biocompatibility, and biomedical applications of graphene and graphene-related materials.

Sustainable and cost-effective energy generation has become crucial for fulfilling present energy requirements. For this purpose, the development of cheap, scalable, efficient, and reliable catalysts is essential. Carbon-based heteroatom-doped, 3D, and mesoporous electrodes are very promising as catalysts for electrochemical energy conversion and storage. Various carbon allotropes doped with a variety of heteroatoms can be utilized for cost-effective mass production of electrode materials. 3D porous carbon electrodes provide multiple advantages, such as large surface area, maximized exposure to active sites, 3D conductive pathways for efficient electron transport, and porous channels to facilitate electrolyte diffusion. However, it is challenging to synthesize and functionalize isotropic 3D carbon structures. Here, various synthesis processes of 3D porous carbon materials are summarized to understand how their physical and chemical properties together with heteroatom doping dictate the electrochemical catalytic performance. Prospects of attractive 3D carbon structural materials for energy conversion and efficient integrated energy systems are also discussed.

3D Heteroatom‐Doped Carbon Nanomaterials as Multifunctional Metal‐Free Catalysts for Integrated Energy Devices

This paper describes the fabrication and characterization of a hybrid nanostructure comprised of carbon nanotubes (CNTs) grown on graphene layers for supercapacitor applications. The entire nanostructure (CNTs and graphene) was fabricated via atmospheric pressure chemical vapor deposition (APCVD) and designed to minimize self-aggregation of the graphene and CNTs. Growth parameters of the CNTs were optimized by adjusting the gas flow rates of hydrogen and methane to control the simultaneous, competing reactions of carbon formation toward CNT growth and hydrogenation which suppresses CNT growth via hydrogen etching of carbon. Characterization of the supercapacitor performance of the CNT–graphene hybrid nanostructure indicated that the average measured capacitance of a fabricated graphene–CNT structure was 653.7 μF cm − 2 at 10 mV s − 1 with a standard rectangular cyclic voltammetry curve. Rapid charging–discharging characteristics (mV s − 1) were exhibited with a capacitance of approximately 75% (490.3 μF cm − 2). These experimental results indicate that this CNT–graphene structure has the potential towards three-dimensional (3D) graphene–CNT multi-stack structures for high-performance supercapacitors.

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Out-of-plane growth of CNTs on graphene for supercapacitor applications.

https://web.stevens.edu/ses/documents/fileadmin/documents/pdf/1-s2.0-S0008622313007227-main.pdf

The influence of thermal annealing to remove polymeric residue on the electronic doping and morphological characteristics of graphene

In most envisioned applications, the full utilization of a graphene-carbon nanotube (CNT) construct requires maintaining the integrity of the graphene layer during the CNT growth step. In this work, we exhibit an approach toward controlled CNT growth atop graphene substrates where the reaction equilibrium between the source hydrocarbon decomposition and carbon saturation into and precipitation from the catalyst nanoparticles shifts toward CNT growth rather than graphene consumption. By utilizing C2H4 feedstock, we demonstrate that the low-temperature growth permissible with this gas suppresses undesirable catalytic hydrogenation and dramatically reduces the etching of the graphene layer to exhibit graphene-CNT hybrids with continuous, undamaged structures.

Chemical Vapor Deposition of Carbon Nanotubes on Monolayer Graphene Substrates: Reduced Etching via Suppressed Catalytic Hydrogenation Using C2H4

In most envisioned applications, full utilization of a graphene-carbon nanotube (CNT) construct requires maintaining the integrity of the graphene layer during the CNT growth step. In this work, we exhibit an approach towards controlled CNT growth atop graphene substrates where the reaction equilibrium between source hydrocarbon decomposition and carbon saturation into and precipitation from the catalyst nanoparticles shifts towards CNT growth rather than graphene consumption. By utilizing C2H4 feedstock, we demonstrate that the low temperature growth permissible with this gas suppresses undesirable catalytic hydrogenation and dramatically reduces the etching of the graphene layer to exhibit graphene-CNT hybrids with continuous, undamaged structures.

Chemical vapor deposition of carbon nanotubes on monolayer graphene substrates: reduced etching via suppressed catalytic hydrogenation using C2H4

Graphene-carbon nanotube multi-stack three-dimensional architectures (graphene-CNT stacks) are formed by a “popcorn-like” growth method, in which carbon nanotubes are grown throughout the architecture in a continuous step. Alternating layers of graphene and a transition metal are grown by a vapor deposition process. The metal is fragmented and etched to form an array of catalytic sites. Carbon nanotubes grow from the catalytic sites in a vapor-solid-liquid process. The graphene-CNT stacks have applications in electrical energy storage devices, such as supercapacitors and batteries. The directly grown carbon nanotube array between graphene layers provides ease of ion diffusion and electron transfer, in addition to being an active material, spacer and electron pathway.

Kitu Kumar

Papers

Out-of-plane growth of CNTs on graphene for supercapacitor applications.

The influence of thermal annealing to remove polymeric residue on the electronic doping and morphological characteristics of graphene

Chemical Vapor Deposition of Carbon Nanotubes on Monolayer Graphene Substrates: Reduced Etching via Suppressed Catalytic Hydrogenation Using C2H4

Chemical vapor deposition of carbon nanotubes on monolayer graphene substrates: reduced etching via suppressed catalytic hydrogenation using C2H4

Popcorn-like growth of graphene-carbon nanotube multi-stack hybrid three-dimensional architecture for energy storage devices