Microwave Exfoliated Graphite Oxide Research Articles

Microwave exfoliated graphite oxide (MEGO) heat treatment: Transformation and stability

Graphene oxide (GO) is actively used in modern material science as a substitute for graphene, which is due to the presence of graphene regions in the GO structure. The setting and tuning of graphene unique properties are largely determined by the type of GO processing. Here, the composition, and microstructure of graphene oxide MEGO (microwave exfoliated graphite oxide) and products of MEGO thermal treatment (MEGO/573 and MEGO/1073) are determined. MEGO family particles are considered as consisting of bundles of oxidized graphene connected by numerous deformation defects of the layers – bends, folds, twists. For prolonged annealing of MEGO in vacuum, the disordered state of the GO bundles is preserved, partial exfoliation of graphene oxide bundles occurs, and holes are formed in the graphene oxide layers. The joint analysis of Raman spectroscopy data for MEGO's and for GO samples studied in a number of works was carried out. The stable minor crystallinity of MEGO samples is determined by small blocks of coherent diffraction (BCD), in which the interlayer distance deviates in both the larger and smaller sides from the characteristic value of graphite. The found features give reasons to believe that the graphene regions availability increases with a given MEGO annealing mode, and GO with similar properties can be used to develop “dry” methods for the synthesis of composites.

Preparation of polypyrrole-decorated MnO2/reduced graphene oxide in the presence of multi-walled carbon nanotubes composite for high performance asymmetric supercapacitors

The present work demonstrates the preparation of polypyrrole-decorated MnO2/rGO by oxidative polymerization method as well as in the presence of CNTs composites. The as-prepared nanocomposites are investigated by XRD, ATR-IR, Raman, TGA, and TEM techniques. Results illustrate polypyrrole with spherical nanoparticles covering the surface of MnO2/rGO and CNTs in which CNTs are intercalated between MnO2/rGO layers (Py@MnG2). In the meantime, the electrochemical performances for the obtained samples are investigated by cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS) experiments at 2 M KCl aqueous electrolyte. Owing to the presence of CNTs as well as the synergistic effect between polypyrrole and MnO2/rGO layers, the electrochemical performance of Py@MnG2 shows high specific capacitance of 295.83 F g−1 in a three-electrode system. The asymmetric supercapacitor (ASC) is assembled with Py@MnG2 and microwave exfoliated graphite oxide (MrGO) as positive and negative electrode material respectively. The as-fabricated ASC device exhibits a high energy density of 11.22 Wh Kg−1 and power density of 807.67 W kg−1 at 1 A g−1 in the potential window of 0–1.6 V. Moreover, the ASC device shows good cycling stability (78.42%) after 1000 cycles at 200 mV s−1.

High Interfacial Charge Storage Capability of Carbonaceous Cathodes for Mg Batteries.

A rechargeable Mg battery where the capacity mainly originates from reversible reactions occurring at the electrode/electrolyte interface efficiently avoids the challenge of sluggish Mg intercalation encountered in conventional Mg batteries. The interfacial reactions in a cell based on microwave-exfoliated graphite oxide (MEGO) as the cathode and all phenyl complex (APC) as electrolyte are identified by quantitative kinetics analysis as a combination of diffusion-controlled reactions involving ether solvents ( esols) and capacitive processes. During magnesiation, esols in APC electrolytes can significantly affect the electrochemical reactions and charge transfer resistances at the electrode/electrolyte interface and thus govern the charge storage properties of the MEGO cathode. In APC-tetrahydrofuran (THF) electrolyte, MEGO exhibits a reversible capacity of ∼220 mAh g-1 at 10 mA g-1, while a reversible capacity of ∼750 mAh g-1 at 10 mA g-1 was obtained in APC-1,2-dimethoxyethane (DME) electrolyte. The high capacity improvement not only points to the important role of the esols in the APC electrolytes but also presents a Mg battery with high interfacial charge storage capability as a very promising and viable competitor to the conventional intercalation-based batteries.

Retraction Statement: "High-Volumetric Performance Aligned Nano-Porous Microwave Exfoliated Graphite Oxide-based Electrochemical Capacitors" and "Aligned Nano-Porous Microwave Exfoliated Graphite Oxide Ionic Actuators with High Strain and Elastic Energy Density".

These articles first published on 15 August 2013 and 21 August 2013 on the Wiley Online Library have been retracted at the request of the Research Integrity Officer (RIO) of The Pennsylvania State University, in agreement with the corresponding authors, the journal's Editor-in-Chief, and Wiley-VCH Verlag GmbH & Co. KGaA, because portions of the reported results cannot be considered reliable or reproducible. Following an investigation by the RIO of The Pennsylvania State University, it was found that the data in Figure 2a,b and Figure S1a,b (Supporting Information) of the article with DOI: 10.1002/adma.201301243, and Figure S3 (Supporting Information) of the article with DOI: 10.1002/adma.201301370 were falsified. Data regarding the carbon electrode material, A-aMEGO, reported to have a density of 1.15 g cm-3 , in the article with DOI: 10.1002/adma.201301243, were falsified. The RIO of The Pennsylvania State University confirms that the investigation found that the mentioned data were falsified by the first author. No findings of research misconduct were made against the co-authors of these publications. [1] M. Ghaffari, Y. Zhou, H. Xu, M. Lin, T. Y. Kim, R. S. Ruoff, Q. M. Zhang, Adv. Mater. 25: 2013, 4879. doi:10.1002/adma.201301243 [2] M. Ghaffari, W. Kinsman, Y. Zhou, S. Murali, Q. Burlingame, M. Lin, R. S. Ruoff, Q. M. Zhang, Adv. Mater. 25: 2013, 6277. doi:10.1002/adma.201301370.

Novel Cathode Materials for Rechargeable Magnesium Ion Batteries

Over the past few years, considerable effort has been devoted to the development of new energy storage devices with high power and energy density to substitute Lithium-based batteries, because the predicted high demand associated with many large-scale applications has generated serious concerns regarding the safety and sustainability of metallic Li.[1] In particular, Mg ion battery (MIB) has been regarded as the most promising candidate because of its ‘green’ character, high natural abundance in the Earth’s crust (13.9% as compared to 7*10-4 % for Li), and chemical stability.[2] In comparison with Li, Mg is inherently much safer due to its stability in air and lack of dendritic formation during electrochemical cycling.[3] Also, compared to Li, Mg has a theoretical volumetric capacity of 3833 mAh cm-3, nearly twice that of Li (2061 mAh cm-3), indicating that MIB has a good potential for reaching a high volumetric energy density.[4,5] However, it is still in high demand to find the electrode materials that unites with metallic Mg and fulfil requirements such as high specific capacity, power density, rate performance and good cyclability.[2,6,7] Here, we report several novel cathode materials, Mn3O4 nanoparticles, Mn3O4/graphene nanocomposite, Fe3O4 nanoparticles and microwave exfoliated graphite oxide (MEGO). The studies of Mg-battery performances were carried out in a standard coin cell which the synthesized materials as cathode, magnesium ribbon as anode, glass fibre as separator and (PhMgCl)2-AlCl3/THF as electrolyte. The electrochemical performance was evaluated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge-discharge techniques.

Influence of the content on properties of microwave-exfoliated graphite oxide and Ni(OH)2 composites

Composites of Ni(OH)2 and microwave-exfoliated graphite oxide (MEGO) with component ratios of 20:80, 35:65, and 50:50 have been synthesized by treating a water mixture of MEGO with NiSO4 × 7H2O in a KOH solution. The structure and properties of the composites obtained have been studied using IR spectroscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy. MEGO is known to possess high stability during the charge–discharge process and relatively low specific capacitance. On the contrary, Ni(OH)2 possesses a high specific capacitance and a low stability during the charge–discharge process. Our experimental results show that the addition of MEGO to Ni(OH)2 increases the stability of the composite electrode under a charge–discharge process. Some increase in the specific capacitance during the cycling have been observed for the composites with the 35:65 and 50:50 ratios. Moreover, the specific capacitance of the 35:65 composite matches the specific capacitance of pure Ni(OH)2 after 100 cycles.

Creating Pores on Graphene Platelets by Low‐Temperature KOH Activation for Enhanced Electrochemical Performance

KOH activation of microwave exfoliated graphite oxide (MEGO) is investigated in detail at temperatures of 450-550 °C. Out of the activation temperature range conventionally used for the preparation of activated carbons (>600 °C), the reaction between KOH and MEGO platelets at relatively low temperatures allows one to trace the structural transition from quasi-two-dimensional graphene platelets to three-dimensional porous carbon. In addition, it is found that nanometer-sized pores are created in the graphene platelets at the activation temperature of around 450 °C, leading to a carbon that maintains the platelet-like morphology, yet with a specific surface area much higher than MEGO (e.g., increased from 156 to 937 m(2) g(-1) ). Such a porous yet highly conducting carbon shows a largely enhanced electrochemical activity and thus improved electrochemical performance when being used as electrodes in supercapacitors. A specific capacitance of 265 F g(-1) (185 F cm(-3) ) is obtained at a current density of 1 A g(-1) in 6 m KOH electrolyte, which remains 223 F g(-1) (156 F cm(-3) ) at the current density of 10 A g(-1) .

One-pot synthesis of pearl-chain-like manganese dioxide-decorated titanium grids as advanced binder-free supercapacitors electrodes

Pearl-chain-like MnO2 arrays on Ti grid have been fabricated by a facile hydrothermal approach and further investigated as the binder-free electrode for high-performance supercapacitors. This unique well-designed binder-free electrode exhibits a high specific capacitance (313.6Fg−1 at a current density of 0.5Ag−1), good rate capability (66.8% retention), and excellent cycling stability (92.4% capacitance retention after 2000 cycles). Moreover, an asymmetric supercapacitor is developed using the as-prepared MnO2 arrays as the positive electrode and activated microwave exfoliated graphite oxide (MEGO) as the negative electrode. The optimized asymmetric supercapacitor displays excellent electrochemical performance with an energy density of 24.9Whkg−1 at power density of 224.7Wkg−1. These impressive performances suggest that the pearl-chain-like MnO2 array on Ti grid is a promising electrode material for supercapacitors.

Fluorinated microwave exfoliated graphite oxide: structural features and double layer capacitance

ABSTRACTFluorinated microwave exfoliated graphite oxide (MEGO) samples containing 5.7–29.3 at.% of fluorine were prepared by gas phase treatment of MEGO with XeF2. According to X-ray photoelectron spectroscopy and infrared spectroscopy data, fluorination of MEGO occurs at C=C bonds while leaving the oxygenated domains intact, and increases the O/C ratio. Cyclic voltammetry studies of fluorination effect on double layer capacitance of MEGO shows 67% growth of its specific capacitance, making such and related carbon materials prospective for supercapacitor applications.

Synthesis and Characterization of Microwave-Exfoliated Graphene Oxide-Wrapped Silicon Nanowire via Hydrosilylation.

Single-crystalline silicon nanowires (SiNWs) were fabricated by using an electroless metal-assisted etching of bulk silicon wafers with silver nanoparticles obtained by wet electroless deposition. The etching of SiNWs is based on sequential treatment in aqueous solutions of silver nitrate followed by hydrofluoric acid and hydrogen peroxide. Free-standing SiNWs were then obtained using ultra-sono method in toluene. Graphene oxide was prepared using the modified Hummers' process. Activated microwave-exfoliated graphite oxide (MEGO) was prepared and used for composition of silicon nanowires and graphene oxide via hydrosilylation. The silicon nanowire-graphene composite materials were characterized using XPS and FE-SEM.

Facile synthesis of ultrathin manganese dioxide nanosheets arrays on nickel foam as advanced binder-free supercapacitor electrodes

Ultrathin MnO2 nanosheets arrays on Ni foam have been fabricated by a facile hydrothermal approach and further investigated as the binder-free electrode for high-performance supercapacitors. This unique well-designed binder-free electrode exhibits a high specific capacitance (595.2 F g−1 at a current density of 0.5 A g−1), good rate capability (64.1% retention), and excellent cycling stability (89% capacitance retention after 3000 cycles). Moreover, an asymmetric supercapacitor is constructed using the as-prepared MnO2 nanosheets arrays as the positive electrode and activated microwave exfoliated graphite oxide (MEGO) as the negative electrode. The optimized asymmetric supercapacitor displays excellent electrochemical performance with an energy density of 25.8 Wh kg−1 and a maximum power density of 223.2 kW kg−1. These impressive performances suggest that the MnO2 nanosheet array is a promising electrode material for supercapacitors.

High Surface Area Graphene-Based Materials for Electrochemical Energy Storage

We present an overview of work in the Ruoff group on graphene-based and graphene-derived materials in energy storage systems (EES).With high electrical conductivity and surface area, graphene-based materials are being intensively studied as electrode material or support material in ultracapacitors and batteries. Graphene-based materials with different physicochemical properties have been studied including chemically reduced graphene oxide,[1] thermally reduced graphene oxide,[2] microwave exfoliated graphite oxide (MEGO),[3] and activated microwave-expanded graphite oxide (‘a-MEGO’).[4]Our recent work based on a highly porous graphene-derived carbon material showed that extremely high specific surface area can be obtained by an activation process, which allows for extensive formation of an electrochemical double layer (EDL) and a high gravimetric capacitance in a symmetric ultracapacitor. In addition, activated graphene-based materials showed significantly improved performance in terms of energy density approaching that of conventional lead-acid batteries.[4, 5]Other studies of highly porous graphene-derived materials in EES including Li-ion capacitors and Li-S batteries will also be presented, along with current efforts in our group on graphene-based and graphene–derived materials for electrical energy storage. REFERENCES 1. Stoller, M.D., et al., Graphene-Based Ultracapacitors. Nano Letters, 2008. 8(10): p. 3498-3502.2. Zhu, Y.W., et al., Exfoliation of Graphite Oxide in Propylene Carbonate and Thermal Reduction of the Resulting Graphene Oxide Platelets. ACS Nano, 2010. 4(2): p. 1227-1233.3. Zhu, Y.W., et al., Microwave assisted exfoliation and reduction of graphite oxide for ultracapacitors. Carbon, 2010. 48(7): p. 2118-2122.4. Zhu, Y.W., et al., Carbon-Based Supercapacitors Produced by Activation of Graphene. Science, 2011. 332(6037): p. 1537-1541.5. Kim, T., et al., Activated graphene-based carbons as supercapacitor electrodes with macro- and mesopores. ACS Nano, 2013. 7(8): p. 6899-905.

Merging of Kirkendall Growth and Ostwald Ripening: CuO@MnO2 Core-shell Architectures for Asymmetric Supercapacitors

Fabricating hierarchical core-shell nanostructures is currently the subject of intensive research in the electrochemical field owing to the hopes it raises for making efficient electrodes for high-performance supercapacitors. Here, we develop a simple and cost-effective approach to prepare CuO@MnO2 core-shell nanostructures without any surfactants and report their applications as electrodes for supercapacitors. An asymmetric supercapacitor with CuO@MnO2 core-shell nanostructure as the positive electrode and activated microwave exfoliated graphite oxide (MEGO) as the negative electrode yields an energy density of 22.1 Wh kg−1 and a maximum power density of 85.6 kW kg−1; the device shows a long-term cycling stability which retains 101.5% of its initial capacitance even after 10000 cycles. Such a facile strategy to fabricate the hierarchical CuO@MnO2 core-shell nanostructure with significantly improved functionalities opens up a novel avenue to design electrode materials on demand for high-performance supercapacitor applications.

Aligned Nano‐Porous Microwave Exfoliated Graphite Oxide Ionic Actuators with High Strain and Elastic Energy Density

A high-density aligned nanoporous activated microwave exfoliated graphite oxide (aMEGO) ionic actuator is studied. Before applying an external electric field, the cations and anions are randomly distributed in the composite. After applying the electric field, ions ingress in between the aligned aMEGO sheets through the nanopores to compensate the charges on the electrodes, resulting in the separation of neighboring sheets and unidirectional electro actuation.

Fabrication of microwave exfoliated graphite oxide reinforced thermoplastic polyurethane nanocomposites: Effects of filler on morphology, mechanical, thermal and conductive properties

Different formulations of microwave-exfoliated graphite oxide (MEGO) based thermoplastic polyurethane (TPU) nanocomposites were successfully prepared via melt blending followed by injection molding. The spectroscopic study indicated that a strong interfacial interaction had developed between the MEGO and the TPU matrix. The microscopic observations showed that the MEGO layers were homogeneously dispersed throughout the TPU matrix. Thermal analysis indicated that the glass transition temperatures (Tg) of the nanocomposites increased with increasing MEGO content and their thermal stability improved in comparison with pure TPU matrix. The mechanical properties of nanocomposites improved substantially by the incorporation of MEGO into the TPU matrix. Electrical conductivity test indicated that a conductivity of 10−4Scm−1 was achieved in the nanocomposite containing only 4.0wt.% of MEGO.

Preparation of activated graphene and effect of activation parameters on electrochemical capacitance

Activation parameters such as temperature and the amount of potassium hydroxide (KOH) were varied during the synthesis of activated microwave-exfoliated graphite oxide (a-MEGO) and the effects of these parameters on the specific surface area of a-MEGO and electrochemical capacitance of a-MEGO electrodes were investigated. At 800°C and a KOH/MEGO mass ratio of 6.5, a maximum specific surface area of 3100m2/g was obtained and a high specific capacitance of 172F/g (at 1A/g constant current and 3.5V maximal voltage) was measured in a two-electrode cell with a-MEGO electrodes in an organic electrolyte.

Microwave-Exfoliated Graphite Oxide/Polycarbonate Composites

We present the first report of polymer composites using microwave-exfoliated graphite oxide (MEGO) as filler, a high surface area carbon material that resembles graphene on a local scale. MEGO has a “wormlike” layered structure which can be sheared apart during melt mixing with a polymer host. In this study, we produced MEGO/polycarbonate (PC) composites at various loadings and evaluated their morphology and properties. Transmission electron microscopy and X-ray scattering studies suggested an exfoliated morphology, with wrinkled platelets of approximately 4–5 nm thickness evenly dispersed throughout the PC matrix. Frequency scans of composite melts using shear rheology showed an onset of frequency-independent terminal behavior around 2.1 wt %, suggesting an effective aspect ratio of nearly 50 for the dispersed platelets, in agreement with TEM analysis. The composites showed significant increases in electrical conductivity, with an onset of electrical percolation around 1.3 wt %, but only exhibited modest...

Carbon-Based Supercapacitors Produced by Activation of Graphene

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.