Hydrazine-reduced graphite oxide and graphene oxide were synthesized to compare their performances as anode materials in lithium-ion batteries and sodium-ion batteries. Reduced graphite oxide inherits the layer structure of graphite, with an average spacing between neighboring layers (d-spacing) of 0.374 nm; this exceeds the d-spacing of graphite (0.335 nm). The larger d-spacing provides wider channels for transporting lithium ions and sodium ions in the material. We showed that reduced graphite oxide as an anode in lithium-ion batteries can reach a specific capacity of 917 mA h g−1, which is about three times of 372 mA h g−1, the value expected for the LiC6 structures on the electrode. This increase is consistent with the wider d-spacing, which enhances lithium intercalation and de-intercalation on the electrodes. The electrochemical performance of the lithium-ion batteries and sodium-ion batteries with reduced graphite oxide anodes show a noticeable improvement compared to those with reduced graphene oxide anodes. This improvement indicates that reduced graphite oxide, with larger interlayer spacing, has fewer defects and is thus more stable. In summary, we found that reduced graphite oxide may be a more favorable form of graphene for the fabrication of electrodes for lithium-ion and sodium-ion batteries and other energy storage devices.

We have designed and synthesized a unique structured graphene/SnO2 composite, where SnO2 nanoparticles are inserted in between interconnected graphene sheets which form hollow spherical multilayers. The hollow spherical multilayered structure provides much flexibility to accommodate the configuration and volume changes of SnO2 in the material. When it is used as an anode material for lithium-ion batteries, such a novel nanostructure can not only provide a stable conductive matrix and suppress the mechanical stress, but also eliminate the need of any binders for constructing electrodes. Electrochemical tests show that the unique graphene/SnO2 composite electrode as designed could exhibit a large reversible capacity over 1000 mA h g−1 and long cycling life with 88% retention after 100 cycles. These results indicate the great potential of the composite for being used as a high performance anode material for lithium-ion batteries.

The dramatically increased demand for electric devices such as electric vehicles and consumer electronics prompted us to explore new ideas in fabricating novel energy storage devices. In this work, we designed, constructed, and studied an asymmetric hybrid lithium-ion capacitor (LIC) by combining an electric double-layer capacitor cathode and a lithium-ion battery anode. Both electrodes were made of a single-wall carbon nanotube and graphene (SG) composite to reduce restacking of the graphene nanosheets, to improve the energy storage capacity, and to improve the electrical conductivity of the electrodes. One of the electrodes was pre-lithiated electrochemically. After pre-lithiation, the lithiated-SG (Li-SG) electrode showed excellent capacity in lithium intercalation and de-intercalation and was used as the anode of the LIC device. The advantage of the hybrid LIC with asymmetric graphene electrodes is that it maximizes the operable voltage between the two electrodes, thus increasing the energy density of the device. The hybrid LIC fabricated in this work exhibited an energy density of 222 W h kg−1 at a power density of 410 W kg−1.

The graphene material prepared by the chemical reduction method usually has oxygenic functional groups in it and such functional groups often result in interactions between the graphene electrode and the electrolyte in supercapacitors. We have examined the existential form of interactions between graphene as the electrode and three kinds of ionic liquid, 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide (EMI-TFSI), 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4), and 1-methyl-1-propylpiperidinium bis (trifluoromethyl sulfonyl) imide (MPPp-TFSI), as the electrolyte of a supercapacitor. Mass spectroscopy (MS) and Fourier transform infrared spectroscopy (FT-IR) analyses confirmed that the residual hydroxyl groups in graphene were transferred to EMI+ and TFSI− lost oxygen atoms to graphene, while little reaction took place in BF4− or MPPp+, during the process of charging. The chemical reactions are suggested to contribute to the device capacitance while it is also one of the reasons for the decreased electrochemical stability window. In this study the highest energy density achieved using the graphene electrode is 169 Wh kg−1 in MPPp-TFSI electrolyte charged to 4.4 V.

•Co(OH)2 nano-flakes are coated on flexible carbon fiber cloth.•Highest gravimetric and areal specific capacitance of 3404 F g−1 and 3.3 F cm−2 are obtained.•Structural and electrochemical characterization is performed.We describe preparation and characterization of nanostructured electrodes using Co(OH)2 nano-flakes and carbon fiber cloth for supercapacitors. Nanostructured Co(OH)2 flakes are produced by electrodeposition and they are coated onto the electro-etched carbon fiber cloth. A highest specific capacitance of 3404.8 F g−1 and an area-normalized specific capacitance of 3.3 F cm−2 have been obtained from such electrodes. Morphology and structure of the nanostructured electrodes have been characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The electrochemical properties have been studied by cyclic voltammetry (CV), constant-current charge and discharge, electrochemical impedance spectroscopy (EIS), and long-time cycling.

We have successfully assembled graphene nanosheets into spherical shells using polystyrene spheres as templates. Compared with stacked planar graphene, the as-prepared graphene spherical shells have more free space in between the spheres, which results in a larger accessible surface area for adsorption of electrolyte ions in supercapacitors. Electrochemical tests show that the graphene hollow spheres exhibit a high specific capacitance of 273 F g−1 and excellent electrochemical stability.

•Fabrication and characterization of graphene composite structure as supercapacitor electrodes.•Energy density of 188 Wh kg−1 has been obtained.•Graphene composite with carbon nanotubes and PANI components leads to high device performance.Graphene and single-walled carbon nanotube (CNT) composites are explored as the electrodes for supercapacitors by coating polyaniline (PANI) nano-cones onto the graphene/CNT composite to obtain graphene/CNT–PANI composite electrode. The graphene/CNT–PANI electrode is assembled with a graphene/CNT electrode into an asymmetric pseudocapacitor and a highest energy density of 188 Wh kg−1 and maximum power density of 200 kW kg−1 are achieved. The structure and morphology of the graphene/CNT composite and the PANI nano-cone coatings are characterized by both scanning electron microscopy and transmission electron microscopy. The excellent performance of the assembled supercapacitors is also discussed and it is attributed to (i) effective utilization of the large surface area of the three-dimensional network structure of graphene-based composite, (ii) the presence of CNT in the composite preventing graphene from re-stacking, and (ii) uniform and vertically aligned PANI coating on graphene offering increased electrical conductivity.

GdB44Si2 is an excellent thermoelectric material, and nanostructured GdB44Si2 makes it possible to potentially improve its properties further. GdB44Si2 nanowires and nanobelts have been fabricated by chemical vapor deposition and characterized by electron diffraction and high-resolution electron microscopy. These nanostructures are of the YB50 structure and grew in the [010] direction. The nanowires have thickness of less than 100 nm and length of several tens of microns. The nanobelts have thickness of about a few tens of nanometers. Morphological and compositional analyses confirmed that the nanowire growth followed the vapor–liquid–solid mechanism and the nanobelts were formed by a subsequent vapor–solid process of condensation.

Graphene-based materials are promising electrodes for supercapacitors, owing to their unique two-dimensional structure, high surface area, remarkable chemical stability, and electrical conductivity. In this paper, graphene is explored as a platform for energy storage devices by decorating graphenes with flower-like MnO2 nanostructures fabricated by electrodeposition. The as-prepared graphene and MnO2, which were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), have been assembled into an asymmetric supercapacitor. The specific capacitance of the graphene electrode reached 245 F/g at a charging current of 1 mA after an electro-activation process. This value is more than 60% larger than the one before electro-activation. The MnO2 nano-flowers which consisted of tiny rods with a thickness of less than 10 nm were coated onto the graphene electrodes by electrodeposition. The specific capacitance after the MnO2 deposition is 328 F/g at the charging current of 1 mA with an energy density of 11.4 Wh/kg and 25.8 kW/kg of power density. This work suggests that our graphene-based electrodes are a promising candidate for the high-performance energy storage devices.

We describe a graphene and single-walled carbon nanotube (SWCNT) composite film prepared by a blending process for use as electrodes in high energy density supercapacitors. Specific capacitances of 290.6 F g−1 and 201.0 F g−1 have been obtained for a single electrode in aqueous and organic electrolytes, respectively, using a more practical two-electrode testing system. In the organic electrolyte the energy density reached 62.8 Wh kg−1 and the power density reached 58.5 kW kg−1. The addition of single-walled carbon nanotubes raised the energy density by 23% and power density by 31% more than the graphene electrodes. The graphene/CNT electrodes exhibited an ultra-high energy density of 155.6 Wh kg−1 in ionic liquid at room temperature. In addition, the specific capacitance increased by 29% after 1000 cycles in ionic liquid, indicating their excellent cyclicity. The SWCNTs acted as a conductive additive, spacer, and binder in the graphene/CNT supercapacitors. This work suggests that our graphene/CNT supercapacitors can be comparable to NiMH batteries in performance and are promising for applications in hybrid vehicles and electric vehicles.

Carbon fiber clothes are a promising material for electrodes of supercapacitors owing to their unique 3-D structure, high surface area, remarkable chemical stability, and electrical conductivity. In this Article, electro-etched carbon fiber cloth is explored as an electrode for supercapacitors by coating polyaniline nanowires. The as-prepared electro-etched carbon fiber cloth and polyaniline nanowires, which were characterized by scanning electron microscopy and transmission electron microscopy, were assembled into supercapacitors. The polyaniline nanowires can reach a mass-normalized specific capacitance of 673 F/g and an area-normalized specific capacitance of 3.5 F/cm2. We also studied the etching and coating of single carbon fibers using the same processing method. The single fiber shows almost the same specific capacitance as the carbon fiber cloth of the same coating density, indicating good accessibility of etched carbon fiber cloth electrode. This work suggests that our polyaniline-based etched carbon fiber cloth electrodes can be a low-cost and scalable solution for the high-performance energy storage devices.

We have assembled single-wall carbon nanotubes into ultrathin long fibrils using a dielectrophoretic technique and studied the mechanical and electrochemical properties of carbon nanotube fibrils. The diameter of the fibrils can be controlled in the range of 200 nm to 2 μm, and the length can reach as long as 1 cm. The obtained fibrils have a tensile strength of about 65 MPa, electrical conductivity ranging from 80 to 200 S cm−1, and specific capacitance more than 200 F g−1. The results indicate that these ultrathin long carbon nanotube fibrils are of great potential for applications as conductive wires and probe electrodes.Keywords: carbon nanotube; conductivity; fibril; specific capacitance

The dramatically increased demand for electric devices such as electric vehicles and consumer electronics prompted us to explore new ideas in fabricating novel energy storage devices. In this work, we designed, constructed, and studied an asymmetric hybrid lithium-ion capacitor (LIC) by combining an electric double-layer capacitor cathode and a lithium-ion battery anode. Both electrodes were made of a single-wall carbon nanotube and graphene (SG) composite to reduce restacking of the graphene nanosheets, to improve the energy storage capacity, and to improve the electrical conductivity of the electrodes. One of the electrodes was pre-lithiated electrochemically. After pre-lithiation, the lithiated-SG (Li-SG) electrode showed excellent capacity in lithium intercalation and de-intercalation and was used as the anode of the LIC device. The advantage of the hybrid LIC with asymmetric graphene electrodes is that it maximizes the operable voltage between the two electrodes, thus increasing the energy density of the device. The hybrid LIC fabricated in this work exhibited an energy density of 222 W h kg−1 at a power density of 410 W kg−1.

We describe a graphene and single-walled carbon nanotube (SWCNT) composite film prepared by a blending process for use as electrodes in high energy density supercapacitors. Specific capacitances of 290.6 F g−1 and 201.0 F g−1 have been obtained for a single electrode in aqueous and organic electrolytes, respectively, using a more practical two-electrode testing system. In the organic electrolyte the energy density reached 62.8 Wh kg−1 and the power density reached 58.5 kW kg−1. The addition of single-walled carbon nanotubes raised the energy density by 23% and power density by 31% more than the graphene electrodes. The graphene/CNT electrodes exhibited an ultra-high energy density of 155.6 Wh kg−1 in ionic liquid at room temperature. In addition, the specific capacitance increased by 29% after 1000 cycles in ionic liquid, indicating their excellent cyclicity. The SWCNTs acted as a conductive additive, spacer, and binder in the graphene/CNT supercapacitors. This work suggests that our graphene/CNT supercapacitors can be comparable to NiMH batteries in performance and are promising for applications in hybrid vehicles and electric vehicles.

We have successfully assembled graphene nanosheets into spherical shells using polystyrene spheres as templates. Compared with stacked planar graphene, the as-prepared graphene spherical shells have more free space in between the spheres, which results in a larger accessible surface area for adsorption of electrolyte ions in supercapacitors. Electrochemical tests show that the graphene hollow spheres exhibit a high specific capacitance of 273 F g−1 and excellent electrochemical stability.