Holey Graphene Nanosheets Research Articles

Metal cation cross-linked 3D frameworks of Fe3O4 nanospheres encapsulated in graphene nanomesh with enhanced anode performance for lithium-ion batteries

Transition metal oxide (TMO) particles encapsulated in graphene shell has been reported to be an ideal lithium-ion batteries (LIBs) anode due to its well TMO particles volumetric adaptability upon cycling. In this paper, to further enhance the structural stability as well as electrochemical performance of Fe3O4-based electrodes for LIBs, 3D composite structures of Fe3O4 nanospheres (NSs) encapsulated in interconnected holey graphene nanosheets (Ca2+/p-rGO@Fe3O4) were fabricated by facile hydrothermal process using Ca2+ as crosslinker and hydrogen peroxide (H2O2) as etching agent. The composite was characterized by X-ray diffractometer (XRD), Scanning electron microscope (SEM), Transmission electron microscope (TEM), Fourier Transform Infrared Spectrometer (FTIR), and other test methods. The morphological and structural tests show that Fe3O4 NSs are homogenously embedded in the 3D framework of interconnected graphene nanomesh aerogels. Based on its unique structure, the Ca2+/p-rGO@Fe3O4 composite electrode delivers an initial discharge and charge capacities of 1288 and 783 mA h g−1, respectively, remarkable rate capability at current densities from 0.1 to 2 A g−1 and excellent cyclic stability with respect to that of its counterparts Ca2+/rGO@Fe3O4 electrode, p-rGO@Fe3O4 electrode and rGO@Fe3O4 electrode, which are prepared using a similar synthetic procedure but without the addition of H2O2 or CaCl2 or neither. It is proved that the synergistic effects of Ca2+-induced crosslinking reaction at the edge of GO nanosheets and in-plane pores on graphene coated on the surface of Fe3O4 can enhance the whole electrochemical performance of anode for LIBs.

Nanofiber-interlocked holey graphene composite aerogel with self-adapting mechanical resilience for a highly reversible dendrite-free lithium metal anode

Lithium (Li) metal, with the highest theoretical capacity, is considered the most promising anode in lithium metal batteries (LMBs) despite the undesirable growth of Li dendrites and large volume change during long-term cycling. Although three-dimensional (3D) hosts such as graphene aerogels are usually used to accommodate the Li metal, pure graphene aerogels can barely withstand large volume changes during the charge/discharge process. Herein, a polyimide (PI) nanofiber-interlocked holey graphene (HG) composite aerogel (CA) with self-adapting mechanical resilience, denoted as PI-HGCA, is prepared as a hierarchically porous host to accommodate Li metal. The HG nanosheets with in-plane nanopores can effectively regulate Li-ion flux and provide highly directed ion transfer pathways. Furthermore, the 3D CA host, constructed with horizontally aligned HG layers and interlocked PI nanofibers, exhibits highly reversible self-adapting compression resilience to large volume change during long-term cycling. Consequently, the lithiated PI-HGCA anode shows a relatively low overpotential of 46 mV with a long lifespan of 1300 cycles in the symmetrical cell and displays an ultrahigh capacity retention of 81.2% after 800 cycles at 1 C coupled with a LiFePO4 cathode, demonstrating the great potential of the interlocked aerogels in high-energy-density LMBs.

Optimized single-step synthesis of graphene-based carbon nanosheets from palm oil fuel ash

Recycling palm oil fuel ash (POFA) into lucrative holey graphene nanosheets will not only save resources but will also prove to be a great solution to avoid the hazards of environmental pollution. The present research aims on studying the effects of temperature and ratio of potassium hydroxide (KOH) during the chemical activation reaction of POFA, resulting in porous graphene sheets with great scope in a variety of industrial applications. These two parameters were studied because of their major influence on the yield, surface area, porosity, and quality of the produced graphene sheets. Porous graphene sheets were prepared by optimizing different activation temperatures (700 °C, 800 °C, and 900 °C) and POFA to KOH impregnation ratios (1:2, 1:3, 1:4, and 1:5). The optimal conditions for producing the best POFA-derived holey graphene in terms of the number of layers, surface area, and porosity were found with impregnation ratio of 1:3 and at temperature of 800 °C. From scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HR-TEM) analyses, it was found that the resulting few layer nanosheets have more uniform porosity at these optimized conditions. We found Brunauer-Emmett-Teller (BET) surface area, pore volume and carbon percentage of 1506.64 m2/g, 0.88 cm3/g and 99% respectively which are comparable to the previous findings using other biomass sources. It was also found that a higher temperature and higher KOH ratio resulted into pore widening and increasing the surface area, but at temperatures beyond the optimized conditions, the surface area and pore volume were reduced due to more vigorous reactions, and volatilization, which led to pore destruction. The present work emphasizes that these parameters have major impact on POFA activation, and their optimisation resulted into good quality graphene nanosheets in a greener and sustainable way, which can be used for many applications including inexpensive energy-storage and water purification applications.

Green Approach for Fabrication of Holey Graphene Based Electrode for Supercapacitor Application

Holey graphene, also known as perforated graphene is formed generating in-plane holes in basal planes of graphene based materials. By combining the advantages of holes and graphene, holey graphene based materials have attained significant research interest in energy storage systems due to the high surface and high electrical conductivity. In the present work, the holey graphene nanosheets are synthesized using ‘metal catalyst activation strategy’ using a green chemistry approach. The synthesized holey graphene nanosheets are studied in detail using various comprehensive characterization and electrochemical analysis techniques. The green chemistry based fabricated holey graphene nanosheets are directly used as an electrode material in the supercapacitor application. In the supercapacitive study, the fabricated electrode exhibits an efficient performance (high capacitance, rate capacity and stability) than its pristine form (graphene) due to the formation of highly pores morphology which provides stupendous electroactive sites and resistance free pathways for ionic interaction and transport. The presented results provide a new platform for employing green approach for fabricating holey graphene based materials as an electrode in energy storage systems. Keywords: Holey Graphene, Pores, Nanosheets, Green Chemistry, Supercapacitors

An electrochemical route to holey graphene nanosheets for charge storage applications

Holey graphene nanosheets are potentially useful in several relevant technological applications, including electrochemical energy storage and molecular separation. Access to this material is mostly accomplished by resorting to standard graphene oxides obtained by common routes (e.g., the Hummers method). However, such a type of highly oxidized graphenes may not be the best option as a precursor to holey graphene on account of their chemical/structural heterogeneity and harsh synthesis conditions. Here, we report the use of highly oxidized graphene nanosheets derived by an electrochemical exfoliation/oxidation strategy as an alternative precursor to holey graphene. Compared to a standard graphene oxide with the same extent of oxidation, the electrochemically derived precursor exhibited larger aromatic domains, which provided a structural basis for its higher electrical conductivity, as well as smaller and denser oxidized regions, associated to a higher chemical homogeneity and lability of its oxygen-containing functional groups. Through selective chemical etching of the oxidized domains, the latter feature was exploited to afford holey graphene nanosheets having smaller and more uniform holes. When used as an electrode material for electrochemical charge storage, the electrochemically derived holey graphene outperformed its standard graphene oxide-based counterpart in terms of capacity and energy density. Overall, boasting distinct structural and chemical characteristics, highly oxidized graphene obtained by electrochemical means can be regarded as a prospective advantageous precursor to many graphene-based materials whose preparation has traditionally relied on the processing of graphene oxides.

Flexible core/shelled PPy@PANI nanotube porous films for hybrid supercapacitors

Flexibility of the films and the limited ion transport in the vertical direction of film highly restrict the development of flexible supercapacitors. Herein, we have developed hybrid porous films consisting of N-doped holey graphene nanosheets (NHGR) with abundant in-plane nanopores and the vertically aligned polyaniline nanowires arrays on polypyrrole nanotubes (PPy@PANI) via a two-step oxidative polymerization strategy and vacuum filtration. The rational design can efficiently shorten the diffusion path of electrons/ions, alleviate volume variation of electrodes during cycling, enhance electric conductivity of the hybrids, and while offer abundant active interfacial sites for electrochemical reaction. Benefiting from the distinctive structural and compositional merits, the obtained PPy@PANI/NHGR film electrode manifests an excellent electrochemical properties in terms of specific capacity (1348 mF cm−2 at a current density of 1 mA cm−2), rate capability (81.2% capacitance retention from 1 to 30 mA cm−2), and cycling stability (capacitance retention of 73.7% at 20 mA cm−2 after 7000 cycles). Matched with NHGR negative electrode, the assembled flexible all-solid-state asymmetric supercapacitor displays a remarkable areal capacitance of 359 mF cm−2 at 5 mA cm−2, maximum areal energy density of 112.2 μWh cm−2 at 3.747 mW cm−2, and good flexibility at various bending angles while preserving stable cycling performance. The result shows the PPy@PANI/NHGR film with high flexibility and 3D ions transport channels is highly attractive for flexible energy storage devices.

Scalable fabrication of holey graphene nanosheets by electrochemical intercalation and microwave-assisted expansion of graphite

Holey graphene nanosheets (HGNS) contain nanoholes on the surface of graphene sheets and these nanoholes can provide the rich mass transfer channel for the electrons and the ions, thus induce more hole edge defects, controllable band gap and better mechanical stability. Here, HGNS were massive production by dual-electrochemical intercalation and microwave-assisted expansion of graphite. The final obtained HGNS contain few layer graphene (5–10 layers), with ultralow defects (ID/IG < 0.07) and higher electrical conductivity (691S/cm). The holey anodic graphene nanosheets demonstrate a narrow peak of pore size distribution centering at around 1.13 nm, 3 nm and a broad range from 3 nm to 252 nm centered at 68 nm, which denoting the characters of micropore, mesoporous and macropore structure. The holey cathodic graphene nanosheets with the feature of mesoporous and macropore structure in which the pore size in the range of 2 nm-252 nm centered at 3.18 nm, 68 nm.

Superior volumetric capacitance of nitrogen and fluorine Co-doped holey graphene/PANI composite film for supercapacitor electrodes

For the development of small portable energy storage devices with limited space, high volumetric capacitance which representing the chance of reducing the volume is becoming the most critical factor to the real industrial applications. Herein, we introduce a novel simple strategy to fabricate a free-standing binder-free N and F co-doped holey graphene/PANI (NFHG/PANI) composite film, in which the doping of nitrogen and fluorine in holey graphene nanosheets boosts the electron transfer efficiency and facilitates the polymerization and growth of aniline on the graphene surface in the form of nanoparticles. The NFHG/PANI performed a gravimetric capacitance of 580 F g−1 at a current density of 0.5 A g−1 and long-term cycling stability, more importantly, it delivered 828 F cm−3 superior volumetric capacitance at a high packing density. Apparently, such a free-standing NFHG/PANI composite film with excellent performance is of great significance for the development of miniaturized portable energy storage devices.

One-Step Exfoliation/Etching Method to Produce Chitosan-Stabilized Holey Graphene Nanosheets for Superior DNA Adsorption.

Holey graphene (HG) features universal applications in adsorption because of the large surface areas and the abundant active sites across the nanopores, but it is difficult to produce HG nanosheets straightforwardly from bulk graphite with current etching methods. Herein, for the first time, we developed a one-step sonication-assisted liquid-phase exfoliation/etching method to produce HG nanosheets from bulk graphite by taking advantage of chitosan for stabilization. With the cavitation bubble collapse stress during the intense sonication, the graphite powders were exfoliated and nanopores of tunable diameters from 40 to 200 nm were generated across the graphene nanosheets. Importantly, with chitosan as the stabilizing agent to reduce the fluid collapse stress transferred onto the graphene nanosheets, the lateral size of HG could be as large as 30 μm. Using this approach, several holey layered crystals (graphite, hexagonal boron nitride, and tungsten disulfide) were fabricated with adequate nanostructures, including lateral size, nanosheet thickness, and nanopore size. Notably, the nanoporous structure endowed the graphene nanosheets with superior high double-stranded DNA adsorption (1253 μg/mg, the highest until now) and excellent DNA protection capacity. Based on this, the HG nanosheets were developed for the surface-mediated reversal gene transfection, displaying appreciable efficiency with the traditional methods.

Graphene quantum dots induced porous orientation of holey graphene nanosheets for improved electrocatalytic activity

Complex electrolyte diffusion through the stacked graphene nanosheets limits their electrochemical performance. As a potential solution, this study explored the potential of nitrogen-doped graphene quantum dots (NGQDs) to induce 3D porous orientation of holey graphene oxide (hGO) nanosheets. The sizes of NGQDs and antisolvent for phase separation assisted assembly were optimized to achieve a 3D nanoporous network. This nano-network serves as a soft template for the porous orientation of hGO, forming a 3D hierarchically porous carbon architecture. Benefiting from the porosity of the 3D framework, π-π restacking was radically avoided, providing high electrolyte transport rates. In addition, doped nitrogen and J-type aggregation of NGQDs effectively tuned the band structure to realize charge transfer at low overpotential. The enhanced electrocatalytic activity and exceptionally low charge transfer resistance of the composite structure were attributed to the enhanced electrode/electrolyte interface and multidimensional charge & electrolyte transport. Porous composite structure based counter electrode showed 78% enhanced photovoltaic performance (compared to unmodified graphene) in the dye-sensitized solar cell, which is comparable to the performance of Pt electrode. The proposed 3D porous orientation can be utilized in emerging electrocatalytic applications, such as supercapacitors, water splitting, and battery electrodes.

Green chemistry based fabrication of holey graphene electrodes for high-performance supercapacitors

Holey graphene, formed by generating in-plane pores in its basal planes can deliver efficient electrochemical performance due to low resistant pathways and short ion diffusion channels. However, generating holes in graphene’s basal planes is a challenging task due to its exceptional mechanical properties which often require toxic and expensive processing methods. In this work, a facile, cost-effective, scalable, and environmentally friendly green approach is adopted to produce holey graphene nanosheets for the supercapacitor application. The fabricated holey graphene electrode exhibits the highest specific capacitance of 226 F/g in the three-electrode system and a specific capacitance of 62 F/g, an energy density of 10.37 Wh/kg, and a power density of 1388 W/kg in the solid-state symmetric device configuration.

A Holey Graphene Additive for Boosting Performance of Electric Double-Layer Supercapacitors.

We demonstrate a facile and effective method, which is low-cost and easy to scale up, to fabricate holey graphene nanosheets (HGNSs) via ultrafast heating during synthesis. Various heating temperatures are used to modify the material properties of HGNSs. First, we use HGNSs as the electrode active materials for electric double-layer capacitors (EDLCs). A synthesis temperature of 900 °C seems to be optimal, i.e., the conductivity and adhesion of HGNSs reach a compromise. The gravimetric capacitance of this HGNS sample (namely HGNS-900) is 56 F·g−1. However, the volumetric capacitance is low, which hinders its practical application. Secondly, we incorporate activated carbon (AC) into HGNS-900 to make a composite EDLC material. The effect of the AC:HGNS-900 ratio on the capacitance, high-rate performance, and cycling stability are systematically investigated. With a proper amount of HGNS-900, both the electrode gravimetric and volumetric capacitances at high rate charging/discharging are clearly higher than those of plain AC electrodes. The AC/HGNS-900 composite is a promising electrode material for nonaqueous EDLC applications.

Microwave absorption properties of holey graphene/silicone rubber composites

Abstract The complex permittivity and return loss (RL) for the composites of silicone rubber filled with holey graphene nanosheets (HGNS, prepared by ultra-rapid heating during the step of thermal reduction/exfoliation of graphite oxide) were measured in the 3–18 GHz range. HGNS-based composites were found to have significantly higher microwave absorption than composites incorporating other types of graphene reduced at lower heating rates. Even with only 1 wt.% loading, its experimentally measured RL reached −32.1 dB at 13.2 GHz with a thickness of 2 mm, and simulation suggested that at a thickness of 3 mm its RL can be as low as −45.3 dB at 7.8 GHz. Material characterization indicated that the density of the holes increased with the temperature ramp rate, and the hole sizes ranged from 5 to 300 nm. Compared to other graphene samples, HGNS possessed significantly larger specific surface area and higher density of defects, suggesting that defect-induced losses, interfacial polarization, and multiple reflection/scattering at the interfaces are the major loss mechanisms. Our simple and low-cost process as well as the very low loading ratio of HGNS are advantageous for cost reduction in future applications.

Facile synthesis of holey graphene-supported Pt catalysts for direct methanol electro-oxidation

A simple synthetic method was developed to produce holey graphene with in-plane nanopores by a fast thermal expansion of graphene oxide (GO) in air and further thermal reduction in N2 flow at 900 °C. The as-synthesized holey graphene nanosheets (HGN) shows meso-macroporous structure and higher surface area than chemically reduced graphene oxide (CRGO) by using hydrazine hydrate. The catalysts of HGN-900 supported Pt nanoparticles (Pt/HGN-900) were further prepared through in-situ chemical co-reduction and applied in the electro-oxidation of methanol. The electrocatalytic performance of catalysts was investigated by cyclic voltammetry (CV) and chronoamperometry (CA) analysis. The results indicate that the catalytic activity of Pt/HGN-10min-900 (377.5 mA mg−1pt) is bout 1.83 and 2.77 times higher than that of Pt/CRGO-900 (206.1 mA mg−1pt) and Pt/XC-72 (136.2 mA mg−1pt) catalysts in 0.5 mol L−1 H2SO4 and 1.0 mol L−1 CH3OH, and Pt/HGN-900 catalysts show higher stable current density compared to that of Pt/XC-72 and Pt/CRGO-900 catalysts.

Compressible, Dense, Three-Dimensional Holey Graphene Monolithic Architecture.

By creating holes in 2D nanosheets, tortuosity and porosity can be greatly tunable, which enables a fast manufacturing process (i.e., fast removal of gas and solvent) toward various nanostructures. We demonstrated outstanding compressibility of holey graphene nanosheets, which is impossible for pristine graphene. Holey graphene powder can be easily compressed into dense and strong monoliths with different shapes at room temperature without using any solvents or binders. The remarkable compressibility of holey graphene, which is in sharp contrast with pristine graphene, not only enables the fabrication of robust, dense graphene products that exhibit high density (1.4 g/cm3), excellent specific mechanical strength [18 MPa/(g/cm3)], and good electrical (130 S/cm) and thermal (20 W/mK) conductivities, but also provides a binder-free dry process that overcomes the disadvantages of wet processes required for fabrication of three-dimensional graphene products. Fundamentally different from graphite, the holey graphene products are both dense and porous, which can enable possible broader applications such as energy storage and gas separation membranes.

Holey graphene nanosheets with surface functional groups as high-performance supercapacitors in ionic-liquid electrolyte.

Pores and surface functional groups are created on graphene nanosheets (GNSs) to improve supercapacitor properties in a butylmethylpyrrolidinium-dicyanamide (BMP-DCA) ionic liquid (IL) electrolyte. The GNS electrode exhibits an optimal capacitance of 330 F g(-1) and a satisfactory rate capability within a wide potential range of 3.3 V at 25 °C. Pseudocapacitive effects are confirmed using X-ray photoelectron spectroscopy. Under the same conditions, carbon nanotube and activated carbon electrodes show capacitances of 80 and 81 F g(-1) , respectively. Increasing the operation temperature increases the conductivity and decreases the viscosity of the IL electrolyte, further improving cell performance. At 60 °C, a symmetric-electrode GNS supercapacitor with the IL electrolyte is able to deliver maximum energy and power densities of 140 Wh kg(-1) and 52.5 kW kg(-1) (based on the active material on both electrodes), respectively, which are much higher than the 20 Wh kg(-1) and 17.8 kW kg(-1) obtained for a control cell with a conventional organic electrolyte.

Correlations between electrochemical Na+storage properties and physiochemical characteristics of holey graphene nanosheets

Na+storage mechanisms of graphene, involving a double-layer charging/discharging reaction, a surface redox reaction, and an intercalation/deintercalation reaction, are disclosed.

A facile approach to produce holey graphene and its application in supercapacitors

We demonstrate a low-cost and simple method to prepare holey graphene nanosheets (HGNSs) by ultra-rapid heating during the process of thermal reduction/exfoliation of graphite oxide. The number density of the holes increased with the heating rate, and the size ranged between 10 and 250nm. In addition, supercapacitors (SCs) using HGNSs as the electrode material were fabricated and their performances were evaluated. Compared to common graphene, HGNSs offered much higher capacitance values and better capability at high rates due to the much shorter cross-plane ion transport paths in the graphene stack through the large amount of holes on graphene sheets. The SC with HGNS electrodes exhibited an excellent high-rate capacitance of 170F/g at 50A/g in a 6M KOH aqueous electrolyte. In the low-rate limit, the HGNS SC showed a very large specific capacitance of 350F/g at 0.1A/g.

Holey graphene nanosheets: large-scale rapid preparation and their application toward highly-effective water cleaning.

In this communication, we demonstrate the rapid large-scale preparation of holey graphene nanosheets (h-G NSs) via low-temperature thermal treatment of oxidized graphite powder obtained by a modified Brodie method in air. The h-G NSs have nanopores with a high specific surface area and large pore volume. When used as a water cleaning agent, they exhibit excellent absorption performances for oils, solvents and dyes.