Highly conductive and porous activated reduced graphene oxide films for high-power supercapacitors.

We present a novel method to prepare highly conductive, free-standing, and flexible porous carbon thin films by chemical activation of reduced graphene oxide paper. These flexible carbon thin films possess a very high specific surface area of 2400 m(2) g(-1) with a high in-plane electrical conductivity of 5880 S m(-1). This is the highest specific surface area for a free-standing carbon film reported to date. A two-electrode supercapacitor using these carbon films as electrodes demonstrated an excellent high-frequency response, an extremely low equivalent series resistance on the order of 0.1 ohm, and a high-power delivery of about 500 kW kg(-1). While higher frequency and power values for graphene materials have been reported, these are the highest values achieved while simultaneously maintaining excellent specific capacitances and energy densities of 120 F g(-1) and 26 W h kg(-1), respectively. In addition, these free-standing thin films provide a route to simplify the electrode-manufacturing process by eliminating conducting additives and binders. The synthetic process is also compatible with existing industrial level KOH activation processes and roll-to-roll thin-film fabrication technologies.

Activated graphene as a cathode material for Li-ion hybrid supercapacitors.

Chemically activated graphene ('activated microwave expanded graphite oxide', a-MEGO) was used as a cathode material for Li-ion hybrid supercapacitors. The performance of a-MEGO was first verified with Li-ion electrolyte in a symmetrical supercapacitor cell. Hybrid supercapacitors were then constructed with a-MEGO as the cathode and with either graphite or Li(4)Ti(5)O(12) (LTO) for the anode materials. The results show that the activated graphene material works well in a symmetrical cell with the Li-ion electrolyte with specific capacitances as high as 182 F g(-1). In a full a-MEGO/graphite hybrid cell, specific capacitances as high as 266 F g(-1) for the active materials at operating potentials of 4 V yielded gravimetric energy densities for a packaged cell of 53.2 W h kg(-1).

Mesoporous carbon capsules as electrode materials in electrochemical double layer capacitors.

The performance of mesoporous carbon capsules as electrode materials in electrochemical double layer capacitors (EDLCs) was evaluated in the presence of a variety of electrolytes, including room temperature ionic liquids (ILs).

Lyotropic liquid crystalline self-assembly in dispersions of silver nanowires and nanoparticles.

We report demixed nematic lyotropic liquid crystalline phase formation in dispersions of silver nanowires and spherical nanoparticle aggregates in ethylene glycol and water. This phase is observed in samples in spite of the high density, large aspect ratio, and long relaxation times of the nanowires which have an average length of 6.8 microm. Remarkably, in the biphasic region, the nanowire-rich liquid crystalline phase exhibits a strandlike morphology which has only previously been reported for single-walled carbon nanotube liquid crystals. Shearing predominantly liquid crystalline dispersions results in both significant nanowire alignment and nanowire-aggregate demixing. The results of this research suggest that the nanoparticle contaminants common to many synthesis schemes facilitate liquid crystalline phase formation and that these dispersions can be processed into aligned coatings.

Incorporation of manganese dioxide within ultraporous activated graphene for high-performance electrochemical capacitors.

Manganese dioxide (MnO(2)) particles 2-3 nm in size were deposited onto a porous "activated microwave expanded graphite oxide" (aMEGO) carbon scaffold via a self-controlled redox process. Symmetric electrochemical capacitors were fabricated that yielded a specific capacitance of 256 F/g (volumetric: 640 F/cm(3)) and a capacitance retention of 87.7% after 1000 cycles in 1 M H(2)SO(4); when normalized to MnO(2), the specific capacitance was 850 F/g. Asymmetric electrochemical capacitors were also fabricated with aMEGO/MnO(2) as the positive electrode and aMEGO as the negative electrode and had a power density of 32.3 kW/kg (for an energy density of 20.8 Wh/kg), an energy density of 24.3 Wh/kg (for a power density of 24.5 kW/kg), and a capacitance retention of 80.5% over 5000 cycles.

Simultaneous transfer and doping of CVD-grown graphene by fluoropolymer for transparent conductive films on plastic.

Chemical doping can decrease sheet resistance of graphene while maintaining its high transparency. We report a new method to simultaneously transfer and dope chemical vapor deposition grown graphene onto a target substrate using a fluoropolymer as both the supporting and doping layer. Solvent was used to remove a significant fraction of the supporting fluoropolymer, but residual polymer remained that doped the graphene significantly. This contrasts with a more widely used supporting layer, polymethylmethacrylate, which does not induce significant doping during transfer. The fluoropolymer doping mechanism can be explained by the rearrangement of fluorine atoms on the graphene basal plane caused by either thermal annealing or soaking in solvent, which induces ordered dipole moments near the graphene surface. This simultaneous transfer and doping of the graphene with a fluoropolymer increases the carrier density significantly, and the resulting monolayer graphene film exhibits a sheet resistance of ∼320 Ω/sq. Finally, the method presented here was used to fabricate flexible and a transparent graphene electrode on a plastic substrate.

Nanostructured reduced graphene oxide/Fe2O3 composite as a high-performance anode material for lithium ion batteries.

Reduced graphene oxide/Fe(2)O(3) composite was prepared using a facile two-step synthesis by homogeneous precipitation and subsequent reduction of the G-O with hydrazine under microwave irradiation to yield reduced graphene oxide (RG-O) platelets decorated with Fe(2)O(3) nanoparticles. As an anode material for Li-ion batteries, the RG-O/Fe(2)O(3) composite exhibited discharge and charge capacities of 1693 and 1227 mAh/g, respectively, normalized to the mass of Fe(2)O(3) in the composite (and ∼1355 and 982 mAh/g, respectively, based on the total mass of the composite), with good cycling performance and rate capability. Characterization shows that the Fe(2)O(3) nanoparticles are uniformly distributed on the surface of the RG-O platelets in the composite. The total specific capacity of RG-O/Fe(2)O(3) is higher than the sum of pure RG-O and nanoparticle Fe(2)O(3), indicating a positive synergistic effect of RG-O and Fe(2)O(3) on the improvement of electrochemical performance. The synthesis approach presents a promising route for a large-scale production of RG-O platelet/metal oxide nanoparticle composites as electrode materials for Li-ion batteries.

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.

Graphene and graphene oxide: synthesis, properties, and applications.

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.