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Fei Liu, Shuyan Song, Dongfeng Xue,* and Hongjie Zhang*

Folded Structured Graphene Paper for High Performance Electrode Materials

TION

There is currently a strong demand for flexible energy storage

devices, based on batteries and supercapacitors, to meet the various requirements of modern gadgets.[1–5] The free-standing paper-like carbon-based materials, characterized by their light weight, flexibility, and high conductivity, are promising for producing wearable or rolling-up electrodes.[6,7] Flexible elec-trode materials made of carbon nanotubes (CNTs) and their composites have been extensively studied.[8–11] Although these binder-free electrodes afford increased capacity, the relatively high production cost of CNTs and the difficulty in making stable CNT dispersions have limited their practical application as electrodes.[12] Recently, graphene oxide (GO) and graphene paper have been successfully fabricated by the flow-directed assembly of individual GO/graphene sheets.[13–15] Graphene paper is mechanically strong and electrically conductive, which is promising for flexible energy storage devices. The lithium-ion battery (LIB) performance of graphene paper was first investi-gated by Wallace and coworkers.[16] The discharge capacity of graphene paper was measured to be 680 mA h g−1 during the first discharge process, which decreased to only 84 mA h g−1 at the second cycle. Annealing and functionalization seem to be effective strategies for improving the performance, but the capacity of graphene paper is still unsatisfactory.[17,18] When used as supercapacitor electrodes, ultrathin graphene paper (e.g., 25 nm) can show a moderate specific capacitance (SC) of 111 F g−1, which was further decreased by increasing its thick-ness.[19] A recent work has shown that separating graphene sheets by solvent molecules can significantly enhance the elec-trochemical performance of graphene paper.[20] It is suggested that the closely stacked structure of graphene paper can create barriers to electrolyte or Li ion diffusion, as such aggregation and restacking are major hurdles that limit individual graphene sheets from realizing their full potential in an assembled bulk form.

Because of its atomically thin two-dimensional structure, graphene has a high in-plane Young’s modulus but is easily warped in the out-of-plane direction.[21] Folds in single/several layers of graphene and edge folds in suspended graphene have been observed,[22–25] however, most studies have focused on the individual behaviors of graphene sheets. Graphene folded struc-tures may be more complex and intriguing, since the folding

© 2012 WILEY-VCH Verlag Gm

F. Liu, Dr. S. Song, Prof. D. Xue, Prof. H. Zhang State Key Laboratory of Rare Earth Resource UtilizationChangchun Institute of Applied ChemistryChinese Academy of Sciences5625 Renmin Street, Changchun 130022, P. R. ChinaE-mail: dongfeng@ciac.jl.cn; hongjie@ciac.jl.cn

DOI: 10.1002/adma.201104691

Adv. Mater. 2012, 24, 1089–1094

of a structure can change its form/phase and functionality, which may induce new and distinct properties in graphene.[26] In this work, we report a novel approach to fabricate graphene paper with folded structured graphene sheets, as illustrated in Figure 1. The folded structured graphene paper is made from a graphene aerogel, which is prefabricated by freeze-drying a GO aqueous dispersion and subsequent thermal reduction. This kind of paper-like carbon material is free-standing and flexible, in which the layer folding and stacking exist. When used as electrodes for a LIB and supercapacitor, this kind of graphene paper can show significantly improved performances compared to available carbon materials, for example, it is more flexible, binder-free, free-standing, and suitable for mass production.

A typical GO aerogel was prepared by freeze-drying a homo-geneous GO aqueous dispersion (5 mg mL−1) in a Petri dish. The as-prepared GO aerogel is golden in color (Figure 2a) and ultra-light in weight (about 5 mg cm−3). The aerogel has a well-defined and interconnected three-dimensional (3D) porous network as imaged by scanning electron microscopy (SEM) (Figure 2b). The pore size of the network is in the range of submillimeter to several micrometers, the pore wall consists of thin layers of GO sheets. The partial overlapping or coalescence of flex-ible GO sheets resulted in the formation of cross-linking sites in the framework of the aerogel. Therefore, the inherent flex-ibility of the graphene sheets is a crucial factor for constructing the 3D macrostructures. An X-ray diffraction (XRD) pattern of the GO aerogel is shown in Figure 3a. A strong and exclu-sive peak can be observed at 2θ = 11.5°, which is similar to GO solids and GO paper reported previously.[12,17] Considering the GO sheets in the aerogel are freestanding, it is reasonable that these sheets are composed of multilayer GO, and the strong dif-fraction derived from the interspacing between these stacking layers corresponds to a layer-to-layer distance (d-spacing) of about 0.77 nm. Such a distance can be attributed to a one water molecule-thick layer that is presumably hydrogen bonded among the GO sheets. A graphene aerogel with the same struc-ture can be obtained by a facile thermal reduction of the GO aerogel at 200 °C in air for 1.5 h. Figure 2d shows the SEM image of a graphene aerogel, from which we can see that the 3D macrostructure is well preserved during the thermal reduc-tion process, and the density of the aerogel further decreases to about 3 mg cm−3 because of the loss of water molecules and oxygen-containing groups from GO. From enlarged images in Figure S1, Supporting Information, ripples and wrinkles can be observed in the graphene sheets of the aerogel. Different from graphene nanosheets dispersed in solution or on a substrate, these self-standing 2D planes possess a higher surface energy, which combines with the strain formed in the thermal reduc-tion process, and causes the formation of ripples and wrinkles. The XRD pattern of the graphene aerogel (Figure 3a) shows no

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Figure 1. Illustration of the formation process of graphene paper. a) GO aqueous dispersion. b) GO dispersion frozen at −50 °C. c) GO aerogel obtained by freeze drying (b) under vacuum. d) Graphene aerogel obtained by treating (c) at 200 °C in air. e) Mechanical pressing of the graphene aerogel to form graphene paper.

recognizable peak, indicating that the orderly stacking structure between single layers has been disturbed. During the thermal reduction process, most of the interlayer water molecules and

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Figure 2. Digital image (a) and SEM image (b) of a GO aerogel. Digital image (c) and SEM imagSEM images (h) of graphene paper.

oxygen-containing groups on the plane can be removed, which may lower the interac-tion between layers and cause expansion, resulting in a random d-spacing.

By pressing the graphene aerogel at 10 Mpa, graphene paper with a uniform thickness of about 10 μm can be obtained. As shown in Figure 2e−g, this paper-like product is freestanding and highly flexible. An elec-tronic conductivity of 18 S cm−1 has been measured using a four-probe method. SEM images of this kind of graphene paper in Figure 2h show a flat surface which is similar to graphene paper fabricated by flow-directed assembly, where well-packed layers can be observed in the whole paper sample. The fracture edge of a graphene paper sample is revealed in the inset of Figure 2h, from which well stacked graphene sheets with closed edges can be observed, different from the open edges in previously reported graphene paper.[14–17] The layering in our graphene paper is evident from its XRD pattern (Figure 3a), the broad peak centered at 2θ =

23.4° corresponding to the stacked graphene layers. This is mainly attributable to the recovery of a π–π conjugated system from freestanding graphene sheets upon mechanical pressing. The

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e (d) of a graphene aerogel. Digital images (e−g) and

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Figure 3. a) XRD patterns of GO aerogel, graphene aerogel, and graphene paper. b) Illustration of folded graphene sheets. c−e) SEM and f, h) TEM images of the peeled-off sheets from graphene paper. g, i) HRTEM images of two folded edges.

interlayer spacing of the graphene paper was calculated to be 0.38 nm, which is much lower than that of GO paper (0.77 nm) while slightly higher than that of flow-directed assembled graphene paper (0.37 nm).[12] These results indicate the existence of π–π stacking between graphene sheets and also the presence of residual oxygenated functional groups on the reduced GO sheets (see Supporting Information). The presence

© 2012 WILEY-VCH Verlag GAdv. Mater. 2012, 24, 1089–1094

of a small quantity of such hydrophilic oxygenated groups can act as a glue in the process of mechanical pressing. This factor together with the π–π stacking of graphene sheets result in the successful construction of graphene paper.

Unusually, two sharp and strong XRD peaks at 2θ = 11.8° and 24.3° are observed in our graphene paper, which is sim-ilar to multiwalled carbon nanotubes (MWCNTs). As shown in

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Figure 3b, these two peaks come from the folding edge of the

multilayer graphene sheets (Str. I). As reported previously, the strong (002) reflection (2θ ≈ 25°) is a result of wave interference between scattering by different graphene layers at the curved edge, which is a feature of a multilayered folded edge.[22] The presence of a strong (002) reflection in the case of multilay-ered graphene can be used to distinguish a folded edge from an open edge. Moreover, the peak at 2θ = 11.8° originates from the relatively uniform spacing of the channel at the folding axis, which is similar to the intertube space of MWCNTs. The XRD pattern indicates that in addition to layer stacking, the folding of graphene layers is also a common structural feature in the current graphene paper. The folded structure was fur-ther confirmed by SEM and transmission electron microscopy (TEM) observation of peeled-off sheets from graphene paper, which are shown in Figure 3c−i. Folds with random size are quite common in the graphene paper, and high resolution TEM (HRTEM) images of the folded edge in Figure 3g and i reveal the multilayer structure of the graphene sheet; folded edges with ∼10 layers of graphene can be identified. In this work, the

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Figure 4. Electrochemical characterizations. a) Discharge/charge profiles (1sdensity of 100 mA g−1 as a LIB anode. b) Cycle performance of the graphengraphene paper as a supercapacitor electrode at different scan rates. d) Sperate. e) Cycling test for the graphene paper as supercapacitor electrode at t

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folding can be explained by the preexistence of wrinkles and the random curving of the sheets in the graphene aerogel. Expe-riencing a fast deformation under a mechanical press, wrin-kles and curves can be transformed into folds in the graphene paper.

Graphene paper has been utilized as binder-free electrode materials in LIBs, however, the primary limitation in the per-formance of graphene paper anodes is the significant irrevers-ible capacity during the initial charge/discharge cycle where a Coulombic efficiency as low as 12.4% was reported.[16] Such poor reversibility can be attributed to the formation of a solid-electrolyte interface (SEI) on the paper surface, which stabi-lizes the battery during charge and discharge at the expense of battery performance.[17] We hypothesize that in the folded structured graphene paper anodes, where the folding may provide slightly increased intersheet spacing and nucleation sites, the SEI formation would be facilitated and better con-trolled, leading to a higher reversible capacity. To this end, we tested the performance of our graphene paper as an anode in a LIB. Figure 4a presents the discharge/charge profiles of the

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t, 2nd, 5th, and 100th cycle) of the as-prepared graphene paper at a current e paper as a LIB anode at different current densities. c) CV curves of the cific capacitance of the graphene paper as a function of charge/discharge

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as-prepared graphene paper at a current density of 100 mA g−1. It can be seen that the first discharge and charge capacities of graphene paper are as high as 1091 and 864 mA h g−1, corre-sponding to a Coulombic efficiency of 79.2%. The second dis-charge and charge capacities are 815 and 806 mA h g−1. From the second cycle, the Coulombic efficiencies are all over 98%. These results indicate that the reversible capacity of the cur-rent graphene paper is much higher than that of previously reported graphene paper.[16–18] The main reason is that the folded structure of the graphene sheets with fewer layers can provide more lithium insertion active sites, such as edge-type sites and nanopores. The cyclic voltammogram (CV) curves of the as-prepared graphene paper are shown in Figure S10, Sup-porting Information. The shape of the CV curves matches well with the discharge/charge profiles. Figure 4b presents the cycle performance of graphene paper at different current densities of 200, 500, 1000, and 1500 mA g−1, the corresponding reversible specific capacity of the as-prepared graphene paper can reach 557, 268, 169, and 141 mAhg−1, respectively. After 100 cycles, the reversible capacity is still maintained at 568 mA h g−1 at 100 mA g−1. These results demonstrate that the as-prepared graphene paper is a potential candidate for anode materials with high reversible capacity, good cycle performance, and high rate discharge/charge capability.

The performance of our graphene paper as a supercapac-itor electrode was measured in a two-electrode configuration. Figure 4c shows the CV curves of graphene paper at scan rates of 50–1000 mVs−1 over the voltage range from 0 to 1 V. All these curves display a quasirectangular shape, even at a scan rate as high as 1000 mV s−1, suggesting that the as-prepared graphene paper exhibits excellent capacitance characteristics. The redox currents of CV curves near 0 V are caused by a handful of oxygen containing groups including hydroxy and carbonyl groups.[27] These oxygen containing groups can be partially reduced in a highly reversible way and thus display pseudocapacitance. During the positive potential scan, the hydroxy groups are oxi-dized to carbonyl groups, with the reverse reaction taking place during the negative potential scan. The graphene paper gives a SC of up to 172 F g−1 at a charge/discharge rate of 1 A g−1, and a capacitance of 110 F g−1 can be obtained even when the superca-pacitor is operated at a fast rate of 100 A g−1 (Figure 4d), which are much higher than that of flow-directed assembled graphene paper,[19] and comparable with solvent-molecule-intercalated graphene paper, graphene powders and hydrogels.[20,28,29] In addition, the graphene paper exhibits excellent cyclability. It can retain over 99% of its capacitance over 5000 cycles even under a high operation current of 20 A g−1 (Figure 4e). Figure S11, Supporting Information, shows the Nyquist impedance plots of the as-prepared graphene paper and after 5000 cycles, both plots have small loops and second slopes of nearly 90°, which further implies that the current graphene paper has a low elec-tronic resistance and pure capacitance behavior. The excellent electrochemical properties can be attributed to the unique folding structure of these graphene sheets, which is helpful for electrode materials to contact the electrolyte and remarkably improve the capacitance characteristics.

In conclusion, a novel strategy has been demonstrated herein to make graphene paper by mechanically pressing a graphene aerogel, this kind of graphene paper shows a unique

© 2012 WILEY-VCH Verlag GmAdv. Mater. 2012, 24, 1089–1094

structure with folded graphene sheets, which was confirmed by XRD, SEM, TEM, and HRTEM characterizations. When used as electrodes for LIBs and supercapacitors, the graphene paper obtained in this work shows much higher performances com-pared to graphene paper fabricated by a flow-directed assembly method. The exceptional performance can be attributed to the existence of graphene sheet folding, which can enhance the accessibility of Li+ ions and electrolyte. This unique graphene paper is promising to act as a new kind of flexible electrode for wearable or rolling-up devices.

Experimental SectionMaterial Synthesis: GO was prepared from natural graphite as reported

elsewhere.[30] To prepare a GO aqueous dispersion, GO solid (150 mg) was dispersed in deionized water (30 mL) with the aid of ultrasonication. The as-formed GO dispersion was then transferred into a Petri dish with a diameter of 10 cm, and frozen at −50 °C for 2 h. The GO aerogel was formed by freeze-drying of the ice solid under vacuum for 24 h. The GO aerogel could be converted into a graphene aerogel by directly heating in a preheated electric oven at 200 °C in air for 1.5 h. Graphene paper could be obtained by pressing the graphene aerogel at 10 MPa using a compression machine.

Material Characterization: XRD measurements were performed on a Bruker D8 Focus diffractometer with Cu Kα radiation and a Lynx Eye detector at a scanning rate of 5 deg min−1. The microstructural characterizations were performed using a Hitachi S4800 field emission scanning electron microscope operated at 10 kV and a FEI Tecnai F20 transmission electron microscope operated at 200 kV.

Electrochemical Evaluation: For LIB tests, a small piece of graphene paper was used exclusively as the working electrode, coin cells (CR2025) were fabricated using lithium metal as the counter electrode, Celgard 2400 as the separator, and 1 m LiPF6 in ethylene carbonate/dimethyl carbonate/diethyl carbonate (EC/DMC/DEC, 1:1:1 vol%) as the electrolyte. Charge–discharge tests were performed using a CT2001A cell tester (LAND Electronic Co. China) over the potential range 0.01–3.5 V vs. Li/Li+, cyclic voltammetry tests were performed using a CHI660D electrochemical workstation (CH Instrument Co. China). For supercapacitor tests, two graphene papers with the same mass were attached to two titanium foils as working electrodes, the test was carried out in a two-electrode system. The titanium foils were first attached to glass slides, both glass slides were assembled with a filter paper sandwiched in between. The cell assemblies were then dipped in 1 m H2SO4 electrolyte. Cyclic voltammetry and galvanostatic charge–discharge were performed over the potential range of 0–1 V, electrochemical impedance spectroscopy was performed between 0.1 and 100 kHz, all tests were performed using a CHI660D electrochemical workstation.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsFinancial support from the National Natural Science Foundation of China (grant nos. 50872016, 20973033 and 51125009) and National Natural Science Foundation for Creative Research Group (grant no. 20921002) is acknowledged.

Received: December 8, 2011Published online: January 24, 2012

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