We report a facile strategy to synthesize the nanocomposite of Co3O4 nanoparticles anchored on conducting graphene as an advanced anode material for high-performance lithium-ion batteries. The Co3O4 nanoparticles obtained are 10−30 nm in size and homogeneously anchor on graphene sheets as spacers to keep the neighboring sheets separated. This Co3O4/graphene nanocomposite displays superior Li-battery performance with large reversible capacity, excellent cyclic performance, and good rate capability, highlighting the importance of the anchoring of nanoparticles on graphene sheets for maximum utilization of electrochemically active Co3O4 nanoparticles and graphene for energy storage applications in high-performance lithium-ion batteries.

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Flexible energy storage devices 1Ϫ3 have many potential applications in portable electronic devices, 4Ϫ6 including roll-up display, electronic paper, stretchable integrated circuits, and wearable systems for personal multimedia, computing, or medical devices. Flexible supercapacitors are available with large power density, moderate energy density, good operational safety, and long cycling life and hence are highly desirable as a modern energy storage system. 7 A freestanding binder-free electrode with favorable mechanical strength and large capacitance is a vital component of a flexible supercapacitor. Although transition metal oxides and conducting polymers have been widely studied as supercapacitor electrode materials, only carbon-based materials have shown favorable flexibility and hence been promising as freestanding soft electrodes. Papers, films, and/or clothes made from carbon nanotubes/fibers have been demonstrated to be suitable as freestanding electrodes.2,8Ϫ13 Nevertheless, the less active surface of carbon materials always prevents them from high capacitance performance. The incorporation of an electrochemically active second phase in a carbonbased freestanding electrode can dramatically enhance the electrode capacitance.14 Graphene is an intriguing twodimensional carbon material and has attracted much research attention due to several breakthroughs in fundamental research and promising practical applications. 15Ϫ30Chemical modified graphene exhibits enormous active edges and oxygen functional groups. It has extraordinary electrochemical and mechanical properties comparable to or even better than carbon nanotubes. 21,26,27 Flexible papers with graphene sheet or graphene oxide sheet as sole building block have already been fabricated by flow-directed assembly. 16,25,31,32 Graphene paper presents excellent tensile modulus up to 35 GPa and room temperature electrical conductivity of 7200 S m Ϫ1. 25 These intriguing characteristics enable graphene paper as a freestanding electrode. Various conducting polymers have been widely studied as electrode materials for supercapacitors because of their high capacitance, easy production, and low cost. However, poor conductivity and weak flexibility of conducting polymers limit them from usage in high-performance flexible supercapacitors. It has been confirmed that graphene can enhance not only the electric conductivity of silica 18 but especially the mechanical strength of polymer composites. 21 This work is aimed to prepare graphene-conducting polymer composite paper as a flexible electrode combining the advantages of graphene paper (high

Hydrous ruthenium oxide (RuO2)/graphene sheet composites (ROGSCs) with different loadings of Ru are prepared by combining sol–gel and low‐temperature annealing processes. The graphene sheets (GSs) are well‐separated by fine RuO2 particles (5–20 nm) and, simultaneously, the RuO2 particles are anchored by the richly oxygen‐containing functional groups of reduced, chemically exfoliated GSs onto their surface. Benefits from the combined advantages of GSs and RuO2 in such a unique structure are that the ROGSC‐based supercapacitors exhibit high specific capacitance (∼570 F g−1 for 38.3 wt% Ru loading), enhanced rate capability, excellent electrochemical stability (∼97.9% retention after 1000 cycles), and high energy density (20.1 Wh kg−1) at low operation rate (100 mA g−1) or high power density (10000 W kg−1) at a reasonable energy density (4.3 Wh kg−1). Interestingly, the total specific capacitance of ROGSCs is higher than the sum of specific capacitances of pure GSs and pure RuO2 in their relative ratios, which is indicative of a positive synergistic effect of GSs and RuO2 on the improvement of electrochemical performance. These findings demonstrate the importance and great potential of graphene‐based composites in the development of high‐performance energy‐storage systems.

Large-area sheets are highly desirable for fundamental research and technological applications of graphene. Here we introduce a modified chemical exfoliation technique to prepare large-area graphene oxide (GO) sheets. The maximum area of the GO sheets obtained can reach ∼40000 μm(2). We found that the GO area is strongly correlated with the C-O content of the graphite oxide, which enables the area of the synthesized GO sheets to be controlled. By simply changing oxidation conditions, GO sheets with an average area of ca. 100-300, ca. 1000-3000, and ∼7000 μm(2) were selectively synthesized. For transparent conductive film applications, thin GO films were fabricated by self-assembly on a liquid/air interface and reduced by HI acid. We found that the sheet resistance of the reduced GO (rGO) films decreases with increasing sheet area at the same transmittance because of the decrease in the number of intersheet tunneling barriers. The rGO film made from GO sheets with an average area of ∼7000 μm(2) shows a sheet resistance of 840 Ω/sq at 78% transmittance, which is much lower than that (19.1 kΩ/sq at 79% transmittance) of a rGO film made from small-area GO sheets of ca. 100-300 μm(2), and comparable to that of graphene films grown on Ni by chemical vapor deposition.