Producing large-area, foldable graphene paper from graphite oxide suspensions by in-situ chemical reduction process

Accepted Manuscript Producing large-area, foldable graphene paper from graphite oxide suspensions by in-situ chemical reduction process Xingke Ye, Qianlong Zhou, Chunyang Jia, Zhonghua Tang, Yucan Zhu, Zhongquan Wan PII:

S0008-6223(16)31066-1

DOI:

10.1016/j.carbon.2016.11.081

Reference:

CARBON 11520

To appear in:

Carbon

Received Date: 2 August 2016 Revised Date:

25 November 2016

Accepted Date: 29 November 2016

Please cite this article as: X. Ye, Q. Zhou, C. Jia, Z. Tang, Y. Zhu, Z. Wan, Producing large-area, foldable graphene paper from graphite oxide suspensions by in-situ chemical reduction process, Carbon (2017), doi: 10.1016/j.carbon.2016.11.081. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Producing Large-area, Foldable Graphene Paper from Graphite Oxide Suspensions by In-situ Chemical Reduction Process Xingke Ye, Qianlong Zhou, Chunyang Jia∗, Zhonghua Tang, Yucan Zhu, Zhongquan Wan

RI PT

State Key Laboratory of Electronic Thin Films and Integrated Devices, School of

Microelectronics and Solid-State Electronics, University of Electronic Science and

SC

Technology of China, Chengdu 610054, P. R. China.

AC C

EP

TE D

M AN U

Graphical abstract

∗ Corresponding author. Tel.: +86 28 83201991; Fax: +86 28 83202569. Email: [email protected] (C. Y. Jia)

ACCEPTED MANUSCRIPT

Producing Large-area, Foldable Graphene Paper from Graphite Oxide Suspensions by In-situ Chemical Reduction Process Xingke Ye, Qianlong Zhou, Chunyang Jia∗, Zhonghua Tang, Yucan Zhu, Zhongquan Wan

State Key Laboratory of Electronic Thin Films and Integrated Devices, School of

RI PT

Microelectronics and Solid-State Electronics, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China. Abstract

SC

Graphene paper has attracted considerable attention because of its impressive

M AN U

electrical and mechanical properties, which endow it with the great potential for applications in future flexible electronics. Therefore, developing a high-efficiency route to fabricate large-area and foldable graphene paper with good electrical and mechanical properties is quite necessary. In this work, a facile in-situ chemical reduction method for the preparation of graphene paper was successfully developed.

TE D

The in-situ chemical reduction process involves simultaneous film formation and reduction, and the produced graphene paper has high integrity and good foldability. The thickness and area of graphene paper can also be readily regulated by adjusting

EP

the experimental parameters. Furthermore, the dissipation of reductant air stream

AC C

(HI/CH3COOH air stream) can “push up” the graphene nanosheets to form a loose layered structure. This inside-out reduction process gives the produced graphene paper a sheet resistance as low as 17.8 Ω sq-1, verifying the advantages of this in-situ chemical reduction method. For the application of the obtained graphene paper, a flexible solid-state supercapacitor based on the graphene paper was fabricated, which had a device areal capacitance of 44 mF cm-2 (device volumetric capacitance of 858 mF cm-3) and excellent electrochemical stability. ∗ Corresponding author. Tel.: +86 28 83201991; Fax: +86 28 83202569. Email: [email protected] (C. Y. Jia) 1

ACCEPTED MANUSCRIPT 1. Introduction Highly conductive graphene-based paper has attracted considerable attention because of its unique two-dimensional nanostructures, excellent electrical properties [1-4], and potential applications in various fields [5-9]. Producing graphene paper

RI PT

from chemically exfoliated graphene oxide is a low-cost and accessible method, which is suitable for large-scale production. For obtaining paper-like graphene, graphene oxide paper should be generally prepared beforehand. Graphene oxide can

SC

be readily assembled into films from its dispersions by vacuum-assisted filtration [2, 10], evaporation-induced self-assembly [11], and wet spinning [12]. The obtained

M AN U

graphene oxide paper is almost an insulator because of abundant oxygen-containing functional groups [13]. Therefore, conductive graphene paper can be obtained through postreduction of graphene oxide paper.

Generally, the approaches for the reduction of graphene oxide paper include high-

TE D

temperature thermal annealing [14-17] and chemical reduction [4, 18-21]. Thermal annealing with high temperature involves large energy consumption and limits the graphene oxide films on substrates with low melting point [22]. In addition, the

EP

graphene paper fabricated by thermal annealing is easily crumpled after heating to

AC C

300 °C, because of an amount of gases derived from the pyrolysis of oxygencontaining functional groups [23]. Therefore, the thermal annealing-obtained graphene paper has less mechanical integrity and flexibility [22]. For chemical reduction, the graphene oxide paper is usually immersed into a reductant solution [4, 18, 19] or sealed in reducing atmosphere [20, 21], which results in the outside-in reduction process. The reaction period of the outside-in reduction process may be prolonged to achieve excellent electrical conductivity when the thickness of graphene oxide paper is increased. Inevitably, the graphene paper produced from chemical 2

ACCEPTED MANUSCRIPT reduction can agglomerate [24] to different degrees because of the decrease in oxygen-containing functional groups and intercalated water, which is adverse to the specific surface area and quality of graphene paper. In addition, the preparation methods of graphene paper in two stages – prefilm formation combined with

RI PT

postreduction or prereduction combined with postfilm formation – are relatively tedious, which may limit the scale up to mass production. Therefore, developing a more effective route to produce large-area and foldable graphene paper without

SC

degrading its electrical conductivity and integrity is quite necessary.

In this paper, we have produced large-area and foldable graphene paper from

M AN U

graphite oxide (GO) suspensions by an in-situ chemical reduction process that involves simultaneous film formation and reduction. The inside-out and soft reduction process endows graphene paper with high electrical conductivity and high integrity. The obtained graphene paper has a loose layered structure, which can alleviate the

TE D

agglomeration of graphene nanosheets. In addition, the area and thickness of graphene paper can be readily controlled by regulating the area of substrate and the dose of GO suspensions. Electrochemical measurements for a flexible solid-state supercapacitor

EP

(FSSC) based on graphene paper indicate that the obtained graphene paper has great

AC C

capacitive performance.

2. Materials and experimental procedure 2.1 Materials

Graphite powder was purchased from Shanghai HuaYi Group HuaYuan Chemical Industry Co. Ltd. (Shanghai, China). Polyvinyl alcohol (PVA, 98–99% hydrolyzed, medium molecular weight) was purchased from Alfa Aesar (USA). Other chemical

3

ACCEPTED MANUSCRIPT reagents of reagent grade were purchased from China and used without further purification. 2.2 Experimental details 2.2.1

Preparation of GO suspensions

RI PT

Graphite oxide was synthesized from natural graphite powder by the modified Hummers method [25]. The concentration of GO suspensions was ~18 mg ml-1. 2.2.2

Scrape coating GO/HI/CH3COOH mixed suspensions onto Teflon block

SC

A certain amount of HI acid (57 wt% in H2O) and acetic acid (≧99.5 wt%) (volume ratio:1:2) was added into GO suspensions with thorough stirring to form

M AN U

GO/HI/CH3COOH mixed suspensions. Then, the GO/HI/CH3COOH mixed suspensions was uniformly coated onto a freshly polished (sand paper 2000 grit) Teflon block by the scrape coating method, and the different layers of tapes were used as the spacers to obtain the desired thickness [26]. Preparation of graphene paper

TE D

2.2.3

The GO/HI/CH3COOH mixed suspensions coated onto Teflon block was heated in a sealed oven at 60 °C for 1 h. After the heating process, the free-standing graphene

EP

paper could be readily peeled off from the Teflon block by tweezers (the GO paper

AC C

was prepared by the above process without adding reductant). 2.2.4

Preparation of FSSC based on graphene paper

PVA-KOH gel electrolyte was prepared through a solution-casting method. In brief,

1 g PVA was added into 10 ml deionized water, and heated at ~90 °C. It was stirred until solution became clear. After cooling, 1 g KOH solution (6 M) was added into the clear solution and stirred until mixed thoroughly. The resulting homogeneous solution was poured into a Petri dish, and the excess water was evaporated in a fume hood at room temperature. The stable PVA-KOH gel electrolyte film with the thickness of 4

ACCEPTED MANUSCRIPT ~0.13 mm was peeled off from Petri dish for further use. The electrode based on graphene paper was prepared by pressing the graphene paper onto a rectangular nickel foam strips current collector with a pressure of 20 MPa. The FSSC based on graphene paper was established by placing two rectangular strips with identical sample sides

RI PT

face to face. A piece of PVA-KOH gel electrolyte film was placed between the strips as a separator. The final FSSC device with the thickness of 0.5 mm was obtained after encapsulation.

SC

2.3 Characterization methods

The surface morphology and microstructure of the samples were observed by a

M AN U

scanning electron microscope (SEM, JEOL JSM-7600F). The surface roughness and microstructure of samples were investigated by an atomic force microscope (AFM, Bruker instruments Dimension Edge) with a silicon tip in the tapping mode. The porous structure of graphene paper was investigated by N2 adsorption/desorption

TE D

isotherms measurement (V-Sorb 2800, Gold APP Instrument). The crystallographic structures of the samples were examined by X-ray diffraction (SHIMADZU XRD7000) using Cu Kα radiation. The X-ray photoelectron spectrum (XPS)

EP

characterization of the samples was carried out with a spectrometer (Kratos XSAM800) operated at 1486.60 eV, and all the binding energies were referenced to

AC C

the C1s neutral carbon peak at 284.8 eV. The changes in microstructure of samples were analyzed by Raman spectroscope (Princeton Instruments PIX100BRX-SF-Q-FA) using 633 nm laser excitation (R-30qa5 17.0 mW-25.0 mW). The sheet resistance (RS, Ω sq-1) of graphene paper was investigated by a standard four-point probe (MODEL 280) method and the corresponding volume conductivity (σ, S m-1) was calculated according to Equation (1), where t (m) is the film thickness. All the samples for mechanical testing were cut into rectangular strips with a width of 10 mm 5

ACCEPTED MANUSCRIPT and length of 30 mm by a razor blade, and mechanical tensile tests were conducted with an INSTRON Universal Testing Machine with a constant loading rate of 0.5 mm min-1.

1 ( S m-1 ) RS t

(1)

RI PT

σ=

The electrochemical properties of FSSC based on graphene paper, including cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical

SC

impedance spectroscopy (EIS), were carried out in a two-electrode system at room temperature by an electrochemical working station (CHI660 E). The gravimetric

M AN U

capacitance (CM) of graphene paper was calculated from the GCD curves based on Equation (2), the areal capacitance (CA) and volumetric capacitance (CV) of FSSC were calculated from the GCD curves based on Equations (3) and (4), and the energy density (E) and power density (P) were defined in Equations (5) and (6), respectively.

I∆t (F cm-2 ) (3) A∆V 2 CV Vmax E= (Wh cm-3 ) (5) 7200

CA =

EP

3600E (W cm-3 ) (6) ∆t

AC C

P=

TE D

4I∆t (F g -1 ) (2) M∆V I∆t CV = (F cm-3 ) (4) V0 ∆V CM =

where I is the constant discharge current (A), ∆t is the discharge time (s), and ∆V is

the potential window (V). M is the mass of active materials in the device (g). A (4.0 cm × 1.5 cm) and V0 (~0.3 cm3) are the area and volume of the FSSC device, respectively. Vmax is the operation potential (V).

6

ACCEPTED MANUSCRIPT 3. Results and discussion 3.1 Preparation procedures of free-standing graphene paper The preparation procedures of graphene paper are shown in Fig. 1a. First, the GO/HI/CH3COOH mixed suspensions were uniformly coated onto a freshly polished

RI PT

Teflon block by the scrape coating method. Second, the uniformly coated GO/HI/CH3COOH mixed suspensions were heated in a sealed oven at 60 °C for 1 h. Specifically, the HI acid and acetic acid (HI/CH3COOH) air stream dissipated from

SC

the inside of GO/HI/CH3COOH mixed suspensions to the oven; this process caused the inside-out reduction for GO. The HI/CH3COOH vapor, which pervaded in the

M AN U

inner space of oven, continuously provided vapor reduction for GO during the heating process (Fig. 1b). Finally, the free-standing graphene paper could be readily peeled off from the Teflon block by a pair of tweezers (Fig. S1f, supplementary information(SI)). With the graphene paper loosely adhered to the Teflon block with

TE D

many macroscopic wrinkles (Fig. S1c, SI), the cross-sectional microscope images show that gaps were formed between the graphene paper and the Teflon block after the reduction process (Fig. S1d and e, SI). The formed gaps, which are presumably

EP

derived from the vaporization of reagents (as illustrated by Fig. 1c), are conducive to

AC C

the peeling procedure. In contrast, the GO paper firmly adhered to the Teflon block and no obvious gaps between the GO paper and the Teflon block was observed (Fig. S1a and b, SI). In addition, the obtained graphene paper has a sheet resistance as low as 17.8 Ω sq-1 by the above-mentioned reduction process.

7

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig. 1 (a) Schematic illustration of preparation procedures of graphene paper. (b) Schematic illustration of reduction process for GO. (c) Schematic illustration of formation of gaps between the graphene paper and the Teflon block. 3.2 Regulation of the thickness and area of graphene paper The thickness of graphene paper can be regulated by producing graphene paper with different areal densities, which depend on the area of Teflon block and the dose of GO suspensions. The GO paper with different areal densities are presented in Fig. 8

ACCEPTED MANUSCRIPT 2a; typically, the GO paper with the areal densities of ~1.44, ~2.16, ~2.88, and ~3.60 mg cm-2 has corresponding thicknesses of ~0.82, ~5.02, ~6.77, and ~8.85 µm, respectively. The cross-sectional SEM images of GO paper with different areal densities are shown in Fig. S2a–d (SI), indicating that the thickness of film can be

RI PT

controlled by changing the dose of GO suspensions. Additionally, the area of graphene paper can be controlled by using Teflon blocks with different areas. The

M AN U

SC

obtained graphene paper with different areas is shown in Fig. 2b.

Fig. 2 (a) Optical images of GO paper with different areal densities. (b) Optical

TE D

images of graphene paper with different areas. 3.3 The foldability of graphene paper

The free-standing graphene paper (15 cm × 15 cm) was folded twice into a small

EP

area of graphene paper (7.5 cm × 7.5 cm); subsequently, the folded graphene paper

AC C

could be unfolded without significant damage (Fig. 3a–c), and the graphene paper was flexible enough to be shaped in desired structures (Fig. 3d and e). The SEM images at the folding regions of graphene paper are presented in Figs. S3 and S4 (SI) to further explore the changes of graphene paper after folding. The surface and cross-section at the slight crease region (Figs. S3a and b and S4a and b, SI) of graphene paper did not show conspicuous cracks; however, some cracks and damages emerged at the surface and cross-section of the sharp crease region (Figs. S3c and d and S4c and d, SI). In addition, the tensile strength measurements for graphene paper after different folding 9

ACCEPTED MANUSCRIPT cycles were also conducted. The stress–strain curves of graphene paper after different folding cycles are shown in Fig. S5b, SI. The fracture strength of graphene paper decreased after different folding cycles; typically, the fracture strength of graphene paper could be maintained to 55.34% of its initial value after continuous 100 folding

RI PT

cycles (Fig. S5c, SI). The degraded mechanical performance of graphene paper resulted from the structural destruction after the folding process, which can be observed through the SEM images (Figs. S3c and d and S4c and d, SI). These results

SC

indicate that the fabricated graphene paper is a pliable macroscopic material composed of stiff (in-plane) but slightly compliant (out-of-plane) graphene sheets.

M AN U

This graphene paper with good foldability and flexibility holds a potential for

EP

TE D

applications in future flexible electronics.

Fig. 3 (a–c) Optical images of folding procedures of the large-area graphene paper. (d

AC C

and e) Optical images of folded graphene paper with desired structures. 3.4 Morphology analysis of GO paper and graphene paper Fig. 4a and b shows the SEM images of GO paper (areal density: ~1.44 mg cm-2

and thickness: ~0.82 µm) whose surface has numerous protuberant wrinkles and ripples. Moreover, the GO paper is composed of two-dimensional graphene oxide nanosheets, which were tightly stacked in a layer-by-layer pattern. Fig. 4c and d shows the SEM images of graphene paper prepared from the GO suspensions with an 10

ACCEPTED MANUSCRIPT areal density of ~1.44 mg cm-2. The wrinkles and ripples are hardly observed on the surface of graphene paper, which resulted from the restoration of the π-π conjugated network after removing the oxygen-containing functional groups on graphene oxide nanosheets [27]. The surface roughness of GO paper and graphene paper was

RI PT

quantitatively characterized by AFM, and the corresponding discussion is given in SI. It is noteworthy that the graphene paper is composed of fluctuant graphene nanosheets with a loose layered structure, which resulted from the vaporization of reductant.

SC

Specifically, the HI/CH3COOH air stream dissipated from the inside of GO/HI/CH3COOH mixed suspension to the external environment during the heating

M AN U

process; thus, the air stream can “push-up” the graphene oxide nanosheets to produce interlayer galleries among graphene nanosheets (Fig. 4e). Therefore, the obtained graphene paper has a larger thickness of ~7.0 µm than that of the corresponding GO paper because of the produced interlayer galleries. The porous structure of fabricated

TE D

graphene paper was studied by the N2 adsorption/desorption analysis, as shown in Fig. S9, SI. The adsorption increases slowly at low relative pressure but rapidly at high relative pressure, and the N2 adsorption/desorption isotherms show typical hysteresis

EP

(Fig. S9a, SI), both of which suggest that the presence of mesopores in the graphene

AC C

paper [28, 29]. The specific surface area of graphene paper was measured to have a relatively low value (SI), corresponding to a typical macroporous material [30]. It is noteworthy that the pore size distribution (Fig. S9b, SI) of graphene paper shows a wide mesopore distribution combined with some macropores (2–400 nm). This porous structure, especially the macropores, of graphene paper may result from the dissipation of HI/CH3COOH air stream, which induced the loose layered structure with some porous structures. In addition, the graphene paper has a lower areal density

11

ACCEPTED MANUSCRIPT of 1.20 mg cm-2 than that of the corresponding GO paper because of the removal of oxygen-containing functional groups and intercalated water. In contrast, the graphene paper, which was produced by immersing GO paper into HI/CH3COOH solution, has no loose layered structure (Fig. S10, SI). This difference

RI PT

further elucidates that the inside-out reduction method is crucial to achieve the loose

AC C

EP

TE D

M AN U

SC

layered structure in graphene paper.

Fig. 4 (a and b) SEM images of GO paper (top and cross-sectional view). (c and d) SEM images of graphene paper (top and cross-sectional view). (e) Schematic illustration of the formation process of the loose layered structure in graphene paper. 12

ACCEPTED MANUSCRIPT 3.5 XRD analysis of GO paper and graphene paper Fig. 5a shows the XRD patterns of graphite, GO paper, and graphene paper. The distinct peak (002) of natural graphite at 26.5° has a corresponding d-spacing of ~3.4 Å. After oxidation for graphite, a distinct peak at 11.2° corresponding to a d-spacing

RI PT

of ~7.88 Å was observed for GO paper, which was larger than the d-spacing of natural graphite because of the heavy functionalized oxygen-containing groups and intercalated water tapped between hydrophilic graphene oxide nanosheets [31, 32].

SC

After the reduction process, the peak of graphene paper showed a dramatic shift close to the natural graphite peak at ~25°, and the decrease in interlayer spacing among

M AN U

graphene nanosheets was attributed to the elimination of oxygen-containing functional groups and intercalated water [33]. Furthermore, the slightly larger dspacing in graphene paper than that of natural graphite is presumably attributed to the produced multilayered structure of graphene by the reduction process.

TE D

3.6 XPS analysis of GO paper and graphene paper

Fig. 5b shows the XPS survey spectra of GO paper and graphene paper. Apparently, the intensity of O1s peak in graphene paper significantly decreased in comparison to

EP

that of GO paper, indicating a pronounced decrease in oxygen-containing groups on

AC C

graphene oxide nanosheets. The I3d and I4d peaks emerged in graphene paper, which were derived from the residual reductant in graphene paper after reduction. Fig. 5c and d shows the high-resolution XPS C1s core level spectra of GO paper and graphene paper, respectively. The C1s XPS spectra of GO paper distinctly indicate a considerable degree of oxidation with four components that correspond to carbon atoms in different functional groups, including C-C/C=C (284.6 eV) for sp2 carbon in graphite, C-O (286.89 eV), C=O (288.57 eV), and C(O)O (289.22 eV) [27, 34]. After the reduction process, the intensity of C-C/C=C peak significantly increased, while 13

ACCEPTED MANUSCRIPT the intensities of oxygen peaks dramatically decreased. The above results demonstrate the successful deoxygenation and the restoration of conjugated network within the graphene paper. In addition, the surface C/O atomic ratio detected by XPS is ~1.96 for the original GO paper and is increased to ~9 after the reduction process, indicating

3.7 Raman spectra analysis of GO paper and graphene paper

RI PT

a high-level graphitization of graphene paper produced by our method.

Fig. 5e shows the Raman spectra of GO paper and graphene paper. The D band at

SC

1335.88 cm−1 represents the breathing modes of sp2 atoms, and the G band at 1593.04 cm−1 represents the E2g phonon at the Brillouin zone center [35]. Corresponding

M AN U

changes in the Raman spectrum of GO paper occurred after the reduction process; the intensities of D and G bands increased remarkably and their intensity ratio (ID/IG) also increased. These results indicate that the GO paper was successfully reduced into graphene paper [36, 37]. In addition, the single sharp D peak, which is a feature of

TE D

graphene [38], emerged in the Raman spectrum of graphene paper, further indicating a high degree of reduction for GO paper.

Intensity (a.u.)

Graphene paper

AC C

GO paper

10

Graphite

20 30 2θ (degree)

GO paper Graphene paper

b Intensity (CPS)

EP

a

I

3d

C O

1s

1s

I

40

1200

14

900 600 300 Binding energy (eV)

4d

0

ACCEPTED MANUSCRIPT C-C/C=C C-O C=O C(O)O

291 288 285 282 Binding energy (eV)

e

Intensity (CPS)

279

294

291 288 285 282 Binding energy (eV)

GO paper Graphene paper

D

Intensity (a.u.)

500

Graphene paper

G

1000 1500 2000 -1 Raman shift (cm )

279

SC

294

C-C/C=C C-O C=O C(O)O

M AN U

Intensity (CPS)

GO paper

d

RI PT

c

2500

Fig. 5 (a) XRD patterns of graphite, GO paper, and graphene paper. (b) XPS survey

TE D

spectra of GO paper and graphene paper. (c and d) High-resolution XPS C1s corelevel spectra of GO paper and graphene paper. (e) Raman spectra of GO paper and graphene paper.

EP

3.8 The electrical conductivity analysis of graphene paper The graphene paper reduced by different types of reductants by the aforementioned

AC C

method is shown in Fig. 6a (the fabrication details of the corresponding graphene paper are listed in Table 1). The graphene paper reduced by HI/CH3COOH, KOH, and Vitamin C were designated as G(HI/CH3COOH), G(KOH), and G(Vitamin

C),

respectively.

G(HI/CH3COOH) is pale black with metallic luster, while G(KOH) and G(Vitamin C) are atrous without metallic luster. The thickness of G(HI/CH3COOH) is larger than those of G(KOH) and G(Vitamin

C),

and the increased thickness of G(HI/CH3COOH) results from the

HI/CH3COOH air stream dissipation-induced interlayer galleries among graphene 15

ACCEPTED MANUSCRIPT nanosheets. In addition, the G(HI/CH3COOH) has a volume conductivity of 4400 S m-1, which is much higher than those of other two pieces of graphene paper (3.3 × 10-3 S m-1 for G(KOH) and 2.5 × 10-3 S m-1 for G(Vitamin C)). The graphene paper reduced with different doses of HI/CH3COOH is shown in Fig.

RI PT

6b (the fabrication details of the corresponding graphene paper are listed in Table 2). The graphene paper was produced with the weight ratio of HI:GO at 6.73, 13.46, and 26.92 (the weight of GO is 72 mg uniformly), which were designated as G(6.73), G(13.46),

SC

and G(26.92), respectively. On the surfaces of graphene paper, macroscopic wrinkles emerged when the dose of reductant was increased. The macroscopic wrinkles

M AN U

resulted from the dissipation of an amount of HI/CH3COOH air stream. The increased dissipation of HI/CH3COOH air stream contributed to the slightly increased thickness of G(13.46) and G(26.92) in comparison to that of G(6.73). Moreover, the volume conductivities of G(6.73), G(13.46), and G(26.92) were 4600, 6400, and 6200 S m-1,

TE D

respectively. These results indicate that the electrical conductivity of graphene paper can be improved by increasing the dose of reductant, and the corresponding changes in electrical conductivity are unapparent with further increase in the dose of reductant.

EP

In addition, it is noteworthy that the fluidity of GO/HI/CH3COOH mixed suspensions

AC C

enhanced with the increased dose of HI/CH3COOH, and the GO/HI/CH3COOH mixed suspensions are too fluid to control with the weight ratio of HI:GO beyond 26.92 in this work, implying that the GO/HI/CH3COOH mixed suspensions with high fluidity are not applicable to the scrape coating process. Additionally, the graphene paper reduced from GO suspensions with different areal densities is shown in Fig. 6c (the fabrication details of the corresponding graphene paper are listed in Table 3). The produced graphene paper with the areal densities of 0.99, 2.42, 3.71, and 5.38 mg cm-2 were designated as G[0.99], G[2.42], G[3.71], and G[5.38], 16

ACCEPTED MANUSCRIPT respectively. The thicknesses of G[0.99], G[2.42], G[3.71], and G[5.38] are ~6.2, ~13.2, ~20.9, and ~27.9 µm, respectively. The cross-sectional SEM images of G[0.99], G[2.42], G[3.71], and G[5.38] are shown in Fig. S11 (SI), and all the graphene papers produced by the in-situ chemical reduction process had loose layered structures. It is noteworthy

RI PT

that many bumps appeared on the surfaces of G[3.71] and G[5.38] (Fig. S12c and d, SI); however, the bumps were not observed on the surfaces of G[0.99] and G[2.42] (Fig. S12a and b, SI). The emerged bumps resulted from the dissipation of HI/CH3COOH air

SC

stream in the thick graphene paper. Specifically, the graphene paper with large thickness was prepared based on considerable GO, which can partly hinder the

M AN U

dissipation of HI/CH3COOH air stream during the heating process; however, the HI/CH3COOH air stream could “break” the impediment derived from GO to some degree during the dissipation process, and this process produced the bumps on the surfaces of G[3.71] and G[5.38]. Furthermore, the volume conductivities of G[0.99], G[2.42],

TE D

G[3.71], and G[5.38] were 6500, 6400, 6800, and 6700 S m-1, respectively. These results indicate that the electrical conductivity of the produced graphene paper was not decreased with the increase in the thickness of graphene paper. In contrast, the

EP

volume conductivities of the outside-in graphene paper (the preparation details of the outside-in graphene paper are presented in SI) decreased from 6393 S m-1 to 3875 S

AC C

m-1 with the increase in the thickness of graphene paper (Table S4, SI), indicating inexhaustive reduction for thick graphene paper under the same reduction condition (with temperature of 60 °C for 1 h). This characteristic of the inside-out reduction method is also superior to the HI-based vapor reduction method [20, 21], which can hardly reduce the graphene oxide located at the inside of thick films in a short time. In addition, the CM of fabricated graphene paper was calculated from the GCD curves, and the dependences of CM on current densities for G[0.99], G[2.42], G[3.71], and G[5.38] are 17

ACCEPTED MANUSCRIPT shown in Fig. S13a–d (SI) (measured in 6 M KOH). Typically, the obtained highest CM is 90.2 F g-1 at 1.0 A g-1 of G[0.99], which is moderate among the CM of graphene paper-based electrodes reported by other works [39-42]. The moderate CM of fabricated graphene paper is mainly caused by the following reasons. First, the

RI PT

fabricated graphene paper with high areal densities (0.99–5.38 mg cm-2) and large thicknesses (6–30 µm) was not favorable to decrease the weight of graphene paper with a certain area. As discussed by Gogotsi and Simon [43, 44], the excellent

SC

gravimetric performance such as CM is usually reported for thin electrodes because of the decreased dead weight. Second, the surfaces of fabricated graphene paper have no

M AN U

special morphologies (e.g., crumpled and porous structures) to facilitate the permeation of electrolyte and increase the ion adsorption/desorption sites in the electrode materials.

Table 1. The fabrication details of graphene paper reduced by different types of

TE D

reductants.

Areal density of

Samples

Reductant

GO (mg cm-2)

G(KOH)

σ

(Reductant:GO)

(Ω sq-1)

(S m-1)

paper (µm)

2.88

6.73

12.7

17.8

4400

KOH

2.88

6.73

4.2

7.3 × 107

3.3×10-3

Vitamin C

2.88

6.73

5.1

7.9 × 107

2.5×10-3

AC C

G(Vitamin C)

HI/CH3COOH

RS graphene

EP

G(HI/CH3COOH)

Thickness of

Weight ratio

Table 2. The fabrication details of graphene paper reduced with different doses of reductant. Samples

Areal density of

Weight ratio

Thickness of

RS

σ

GO (mg cm-2)

(HI:GO)

graphene paper (µm)

(Ω sq-1)

(S m-1)

Reductant

G(6.73)

HI/CH3COOH

2.88

6.73

12.7

17.2

4600

G(13.46)

HI/CH3COOH

2.88

13.46

13.2

11.9

6400

G(26.92)

HI/CH3COOH

2.88

26.92

13.3

12.1

6200

18

ACCEPTED MANUSCRIPT Table 3. The fabrication details of graphene paper reduced from GO suspensions with different areal densities. Samples

Areal density of

Weight ratio

Thickness of

RS

σ

GO (mg cm-2)

(HI:GO)

graphene paper (µm)

(Ω sq-1)

(S m-1)

Reductant

HI/CH3COOH

1.44

13.46

6.2

25

6500

G[2.42]

HI/CH3COOH

2.88

13.46

13.2

11.9

6400

G[3.71]

HI/CH3COOH

4.32

13.46

20.9

G[5.38]

HI/CH3COOH

5.76

13.46

27.9

RI PT

G[0.99]

6.99

6800

5.31

6700

SC

3.9 Tensile strength analysis of GO paper and graphene paper

The typical stress–strain curves of GO paper (areal density: ~1.44 mg cm-2) and

M AN U

G[0.99] are shown in Fig. 6d. The fracture strength of G[0.99] is ~61.57 MPa with elongation ~0.65%, and the mechanical performance of G[0.99] is superior to that of the corresponding GO paper (fracture strength: ~31.07 MPa and elongation: ~0.54%). The stress–strain curves of G(HI/CH3COOH), G(KOH), and G(Vitamin C) are presented in Fig.

TE D

6e. The fracture strength of G(HI/CH3COOH) is ~86.53 MPa with an elongation of ~1.15%; however, the mechanical performances of G(KOH) (the fracture strength of which is 10.33 MPa with an elongation of ~0.49%) and G(Vitamin C) (the fracture strength of

EP

which is 18.95 MPa with an elongation of ~0.57%) are relatively poor when

AC C

compared with that of G(HI/CH3COOH). The degraded mechanical performances of G(KOH) and G(Vitamin C) result from the residual reductants (KOH and Vitamin C) in graphene paper, which makes graphene paper friable and rigid. The stress–strain curves of G[0.99], G[2.42], G[3.71], and G[5.38] are also presented in Fig. 6f. The mechanical performance of graphene paper enhanced with the increase in areal density until 3.71 mg cm-2 (the fracture strength of which is ~106.23 MPa with an elongation of ~1.41%). However, the mechanical performance of graphene paper decreased when the areal density further increased to 5.38 mg cm-2 (the fracture strength of which is 19

ACCEPTED MANUSCRIPT ~78.74 MPa with an elongation of ~1.02%); this decreased mechanical performance is caused by the considerable defects in graphene paper. Specifically, the HI/CH3COOH air stream difficultly dissipated from the thick graphene paper, and many residual HI/CH3COOH remained in the thick graphene paper after the heating process, thus

RI PT

inducing considerable defects in graphene paper. Therefore, the graphene paper with a

100

GO paper G[0.99]

45

80

30 15 0 0.0

0.6 0.9 Strain (%)

1.2

1.5

120

G(HI/CH3COOH) G(KOH)

60 40 20

0.3

e

0 0.0

G(Vitamin C)

f G[0.99]

90

Stress (MPa)

d

Stress (MPa)

Stress (MPa)

60

M AN U

SC

large thickness had a decreased mechanical performance.

G[2.42] G[3.71]

60

G[5.38]

30

0.5

1.0 Strain (%)

1.5

2.0

0 0.0

0.5

1.0 1.5 Strain (%)

2.0

2.5

TE D

Fig. 6 (a) Optical images of G(HI/CH3COOH), G(KOH), and G(Vitamin C). (b) Optical images of G(6.73), G(13.46), and G(26.92). (c) Optical images of G[0.99], G[2.42], G[3.71], and G[5.38]. (d) Typical stress–strain curves of GO paper and the corresponding graphene paper

EP

(G[0.99]). (e) Stress–strain curves of G(HI/CH3COOH), G(KOH), and G(Vitamin C). (f) Stress–

AC C

strain curves of G[0.99], G[2.42], G[3.71], and G[5.38]. 3.10

Electrochemical performance of FSSC based on graphene paper

The electrochemical performance of FSSC based on graphene paper (the electrode

materials of FSSC are G[2.42]) was tested by CV, GCD, and EIS. Fig. 7a presents the CV curves of FSSC based on graphene paper at different scan rates with a potential range from 0 to 1.0 V. All the CV curves at different scan rates show the quasirectangular shapes, indicating that the produced graphene paper has the typical feature of electrochemical double-layer capacitors (EDLCs) [45]. In addition, the rapid 20

ACCEPTED MANUSCRIPT current response to voltage reversal at each end potential in CV curves indicates the fast charge propagation at the electrode–electrolyte interface [46, 47], which could be attributed to the high electrical conductivity of graphene paper, and the excellent conductivity of graphene paper can provide the ideal ballistic transport path length for

RI PT

electrolyte ions.

The GCD curves of FSSC at different current densities within a potential window of 0–1.0 V are shown in Fig. 7b. All the GCD curves at different current densities

SC

show nearly symmetric triangle-shaped charge–discharge curves and ideal linear relationship between potential and time, indicating the rapid current-voltage response

M AN U

and pure electrochemical double-layer capacitive behavior from the charge separation at the electrode–electrolyte interface [48]. The CA of FSSC device at different current densities is shown in Fig. 7c; the highest CA of FSSC device based on graphene paper was 44 mF cm-2 obtained at a current density of 2.0 mA cm-2, which is excellent

TE D

among the reported values to date [42, 47, 49-55]. The volumetric performance is critical to evaluate the overall performance of energy storage systems [56]; therefore, the volumetric performance of FSSC based on graphene paper was tested. The

EP

dependences of CV at different current densities for FSSC device are presented in Fig. 7c, with the highest CV of FSSC device is 858 mF cm-3 at a current density of 39.0 mA

AC C

cm-3. The Ragone plot of FSSC device is shown in Fig. 7d (inset: a red LED was powered by the two FSSC devices in series); the obtained highest energy density was 0.12 mWh cm-3 with a power density of 19.64 mW cm-3, and the highest power density was 78.50 mW cm-3 with an energy density of 0.096 mWh cm-3. In addition, the cycling stability of FSSC device based on graphene paper is shown in Fig. 7e, and the capacitance value of FSSC could be maintained at 95.21% of its initial value even

21

ACCEPTED MANUSCRIPT after 10000 charge–discharge cycles at 200 mV s-1, demonstrating the remarkable electrochemical stability of FSSC based on graphene paper. The electrical resistance and ion-transport behavior of FSSC device were characterized by EIS in frequency range from 105 to 10-2 Hz with a potential

RI PT

amplitude of 5 mV. The corresponding Nyquist plot is shown in Fig. 7f. At low frequency, the approximately vertical line toward the Z′ axis demonstrates the fast ion diffusion/transportation rate in the electrode surface [57] because of the extraordinary

SC

conductivity of graphene paper. At high frequency, the equivalent series resistance (ESR) of FSSC, which can be found as the intercept on the Z′ axis at the frequency

M AN U

end [58], was only 1.06 Ω. Furthermore, no distinct semicircle shape was observed at high frequency, indicating the fast charge transfer behavior inside the graphene paperbased electrode during its charge–discharge process. The excellent charge transfer behavior is attributed to the high electrical conductivity of graphene paper and its

a

AC C

0

0.6 0.4 Potential (V)

39.0 mA cm -3 59.0 mA cm -3 78.0 mA cm -3 98.0 mA cm -3 117.5 mA cm -3 137.0 mA cm -3 157.0 mA cm

0.6 0.4 0.2 0.0

-200

0.8

-3

b

0.8

-100

1.0

1.0 Potential (V)

100

-1

10 mV s -1 20 mV s -1 50 mV s -1 100 mV s -1 200 mV s

EP

200

-3

Current density (mA cm )

electrode material.

TE D

loose layered structure, and it is easy for the electrolyte to be in contact with the

0.2

0.0

0

22

10

20 30 Time (s)

40

50

-3

Energy density (mWh cm )

c

60 44 mF cm-2 40 20 4 6 -2 Current density (mA cm )

8

1200 -3 858 mF cm 900 600 300 40

80 120 -3 Current density (mA cm )

0.00 -0.06 -0.12 -0.18

20 40 60 80 -3 Power density (mW cm )

160

80

e

100 1 st 10000 th

95.21%

0.00

-1

-0.06 1.0

0

40 20

-0.03

20 0

14

0.03

0.8

200 mV s 0.6 0.4 0.2 0.0 Potential (V)

7

M AN U

40

Current (A)

60

0.06

-Z″ (Ω )

80

f

60

SC

2

0.06

-Z″ (Ω )

0

FSSC device

d

0.12

RI PT

80

Capacitance retention (%)

-3

CV, device (mF cm )

-2

CA, device (mF cm )

ACCEPTED MANUSCRIPT

0 0

0

2000 4000 6000 8000 10000 Number of cycles

0

20

2

Z′ (Ω)

40 Z′ (Ω)

4

6

60

80

Fig. 7 (a) CV curves of FSSC based on graphene paper at scan rates ranging from 10

TE D

mV s-1 to 200 mV s-1. (b) GCD curves of FSSC based on graphene paper at current densities ranging from 39.0 mA cm-3 to 157.0 mA cm-3. (c) The relationship between CA (or CV) of FSSC device and current densities. (d) The Ragone plot of FSSC based

EP

on graphene paper. (e) The cycling performance of FSSC (inset: the CV curves at 200 mV s-1 of FSSC at different charge–discharge cycles). (f) The Nyquist plot of FSSC

AC C

based on graphene paper (inset: the enlarged high frequency region). In addition, the electrochemical performance of FSSC based on graphene paper

during and after multiple sharp bending was also studied. The optical images of FSSC under the different bending states and the corresponding CV curves are presented in Fig. 8a and b. The shapes of CV curves almost maintained the same shapes under the different bending states, indicating no noticeable structural destruction and performance degradation of the device under its bending states. Typically, the 23

ACCEPTED MANUSCRIPT capacitance value of FSSC could be maintained at 93.96% of its initial value even at bending state 2 (Fig. 8c). The shapes of CV curves (Fig. 8e) have no conspicuous differences after multiple sharp bending (the optical images of FSSC under its flat and bending states are shown in Fig. 8d). The capacitance value could be maintained at

RI PT

84.62% of its initial value after continuous 100 bending cycles (Fig. 8f). The good electrochemical stability of FSSC based on graphene paper is mainly ascribed to the following reasons. First, the macroscopic flexibility of graphene paper can effectively

SC

alleviate the structural destruction of FSSC under the bending state. Second, the PVAKOH gel electrolyte film, which adheres to graphene paper, serves as a buffer layer to

TE D

50

0

EP

-50

Without bending

1.0

0.8

0.6 0.4 Potential (V)

c

100 80 60 40 20

Bending state 2

0.2

0

0.0

AC C

-100

Bending state 1

Capacitance retention (%)

120

b

-3

Current density (mA cm )

100

M AN U

decrease the damage for graphene paper during the sharp bending process.

24

0

1 Bending state

2

ACCEPTED MANUSCRIPT 120

e

50

0 0 1 st 10 th 20 th 50 th 100 th

-50

-100

1.0

0.8

0.6

0.4

0.2

100 80 60 40 20 0

0.0

0

20

40

60

80

100

RI PT

Capacitance retention (%)

f

-3

Current density (mA cm )

100

Bending cycle number

Potential (V)

Fig. 8 (a) Optical images of FSSC with different bending states. (b) CV curves (50

SC

mV s-1) of FSSC at different bending states. (c) Capacitance retention at different bending states. (d) Optical images of FSSC under its flat and bending states. (e) CV

under different bending cycles.

4. Conclusion

M AN U

curves (50 mV s-1) of FSSC at different bending cycles. (f) Capacitance retention

TE D

In summary, we produced large-area and foldable graphene paper from GO suspensions by an in-situ chemical reduction process that involves simultaneous film formation and reduction. The soft reduction process endows graphene paper with high

EP

integrity and good flexibility, and the thickness and area of graphene paper can be readily regulated. In addition, the obtained graphene paper has high electrical

AC C

conductivity (the sheet resistance of which is ~17.8 Ω sq-1). It is noteworthy that the HI/CH3COOH air stream can “push up” the graphene nanosheets to form a loose layered structure, which can alleviate the agglomeration of graphene nanosheets. FSSC containing this in-situ chemical reduction-produced graphene paper had excellent capacitive performance (the CA and CV of which are 44 mF cm-2 and 858 mF cm-3, respectively) and electrochemical stability. This work not only provides a simple method for the preparation of large-area and foldable graphene paper but also conducts a comprehensive research on electrical conductivity and mechanical and 25

ACCEPTED MANUSCRIPT capacitive performance of the produced graphene paper, which may have implications for the potential applications in different types of wearable electronic devices.

Acknowledgments

References

SC

21572030, 21272033, and 21402023) for financial support.

RI PT

We are grateful to the National Natural Science Foundation of China (Grant Nos.

[1] Li D, Müller MB, Gilje S, Kaner RB, Wallace GG. Processable Aqueous Dispersions of Graphene

M AN U

Nanosheets. Nat. Nanotechnol. 2008; 3(2): 101-5.

[2] Chen HQ, Müller MB, Gilmore KJ, Wallace GG, Li D. Mechanically Strong, Electrically Conductive, and Biocompatible Graphene Paper. Adv. Mater. 2008; 20(18): 3557-61. [3] Cheng QF, Jiang L, Tang ZY. Bioinspired Layered Materials with Superior Mechanical Performance. Acc. Chem. Res. 2014; 47(4): 1256-66.

TE D

[4] Zhang M, Wang YL, Huang L, Xu ZP, Li C, Shi GQ. Multifunctional Pristine Chemically Modified Graphene Films as Strong as Stainless Steel. Adv. Mater. 2015; 27(42): 6708-13. [5] Wang DW, Li F, Zhao JP, Ren WC, Chen ZG, Tan J, et al. Fabrication of Graphene/Polyaniline

EP

Composite Paper via In Situ Anodic Electropolymerization for High-Performance Flexible Electrode. ACS Nano 2009; 3(7): 1745-52. [6] Robinson JT, Perkins FK, Snow ES, Wei ZQ, Sheehan PE. Reduced Graphene Oxide Molecular

AC C

Sensors. Nano Lett. 2008; 8(10): 3137-40.

[7] Blake P, Brimicombe PD, Nair RR, Booth TJ, Jiang D, Schedin F, et al. Graphene-Based Liquid Crystal Device. Nano Lett. 2008; 8(6): 1704-8.

[8] Bunch JS, Van der Zande AM, Verbridge SS, Frank IW, Tanenbaum DM, Parpia JM, et al. Electromechanical Resonators from Graphene Sheets. Science 2007; 315(5811): 490-3. [9] Chen PY, Sodhi J, Qiu Y, Valentin TM, Steinberg RS, Wang ZY, et al. Multiscale Graphene Topographies Programmed by Sequential Mechanical Deformation. Adv. Mater. 2016; 28(18): 3564-71.

26

ACCEPTED MANUSCRIPT [10] Dikin DA, Stankovich S, Zimney EJ, Piner RD, Dommett GH, Evmenenko G, et al. Preparation and Characterization of Graphene Oxide Paper. Nature 2007; 448(7152): 457-60. [11] Chen CM, Yang QH, Yang YG, Lv W, Wen YF, Hou PX, et al. Self-Assembled Free-Standing Graphite Oxide Membrane. Adv. Mater. 2009; 21(29): 3007-11. [12] Liu Z, Li Z, Xu Z, Xia ZX, Hu XZ, Kou L, et al. Wet-Spun Continuous Graphene Films. Chem.

RI PT

Mater. 2014; 26(23): 6786-95.

[13] Boukhvalov DW, Katsnelson MI. Modeling of Graphite Oxide. J. Am. Chem. Soc. 2008; 130(32): 10697-701.

SC

[14] Mattevi C, Eda G, Agnoli S, Miller S, Mkhoyan KA, Celik O, et al. Evolution of Electrical, Chemical, and Structural Properties of Transparent and Conducting Chemically Derived Graphene Thin Films. Adv. Funct. Mater. 2009; 19(16): 2577-83.

M AN U

[15] Wang X, Zhi LJ, Muellen K. Transparent, Conductive Graphene Electrodes for Dye-Sensitized Solar Cells. Nano Lett. 2008; 8(1): 323-7.

[16] Becerril HA, Mao J, Liu ZF, Stoltenberg RM, Bao ZN, Chen YS. Evaluation of SolutionProcessed Reduced Graphene Oxide Films as Transparent Conductors. ACS Nano 2008; 2(3): 463-70.

TE D

[17] Dong LY, Hu CG, Song L, Huang XK, Chen N, Qu LT. A Large-Area, Flexible, and FlameRetardant Graphene Paper. Adv. Funct. Mater. 2016; 26(9): 1470-6. [18] Pei SF, Zhao JP, Du JH, Ren WC, Cheng HM. Direct Reduction of Graphene Oxide Films into

4466-74.

EP

Highly Conductive and Flexible Graphene Films by Hydrohalic Acids. Carbon 2010; 48(15):

AC C

[19] Xu YX, Sheng KX, Li C, Shi GQ. Highly Conductive Chemically Converted Graphene Prepared from Mildly Oxidized Graphene Oxide. J. Mater. Chem. 2011; 21(20): 7376.

[20] Moon IK, Lee J, Ruoff RS, Lee H. Reduced Graphene Oxide by Chemical Graphitization. Nat. Commun. 2010; 1: 73.

[21] Liu HY, Wang HT, Zhang XW. Facile Fabrication of Freestanding Ultrathin Reduced Graphene Oxide Membranes for Water Purification. Adv. Mater. 2015; 27(2): 249-54. [22] Pei SF, Cheng HM. The Reduction of Graphene Oxide. Carbon 2012; 50(9): 3210-28. [23] Compton OC, Nguyen ST. Graphene Oxide, Highly Reduced Graphene Oxide, and Graphene: Versatile Building Blocks for Carbon-Based Materials. Small 2010; 6(6): 711-23. 27

ACCEPTED MANUSCRIPT [24] Park S, Ruoff RS. Chemical Methods for the Production of Graphenes. Nat. Nanotechnol. 2009; 4(4): 217-24. [25] Hummers WS, Offeman RE. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958; 80(6): 1339. [26] Cruz SR, Morelos GA, Kim HI, Jang HK, Tristan F, Vega DS, et al. Super-Stretchable Graphene

RI PT

Oxide Macroscopic Fibers with Outstanding Knotability Fabricated by Dry Film Scrolling. ACS Nano 2014; 8(6): 5959-67.

[27] Stankovich S, Dikin DA, Piner RD, Kohlhaas KA, Kleinhammes A, Jia YY, et al. Synthesis of

SC

Graphene-Based Nanosheets via Chemical Reduction of Exfoliated Graphite Oxide. Carbon 2007; 45(7): 1558-65.

[28] Xiong ZY, Liao CL, Han WH, Wang XG. Mechanically Tough Large-Area Hierarchical Porous

M AN U

Graphene Films for High-Performance Flexible Supercapacitor Applications. Adv. Mater. 2015; 27(30): 4469-75.

[29] Liu LY, Shen BS, Jiang D, Guo RS, Kong LB, Yan XB. Watchband-Like Supercapacitors with Body Temperature Inducible Shape Memory Ability. Adv. Energy Mater. 2016; 6(16): 1600763. [30] Gao HL, Zhu YB, Mao LB, Wang FC, Luo XS, Liu YY, et al. Super-Elastic and Fatigue Resistant

TE D

Carbon Material with Lamellar Multi-Arch Microstructure. Nat. Commun. 2016; 7: DOI: 10.1038/ncomms12920.

[31] Aboutalebi SH, Chidembo AT, Salari M, Konstantinov K, Wexler D, Liu HK, et al. Comparison

EP

of GO, GO/MWCNTs Composite and MWCNTs as Potential Electrode Materials for Supercapacitors. Energy Environ. Sci. 2011; 4(5): 1855.

AC C

[32] Uthaisar C, Barone V, Fahlman BD. On the Chemical Nature of Thermally Reduced Graphene Oxide and Its Electrochemical Li Intake Capacity. Carbon 2013; 61: 558-67.

[33] Han DD, Zhang YL, Jiang HB, Xia H, Feng J, Chen QD, et al. Moisture-Responsive Graphene Paper Prepared by Self-Controlled Photoreduction. Adv. Mater. 2015; 27(2): 332-8.

[34] Waltman RJ, Pacansky J, Bates CW. X-Ray Photoelectron Spectroscopic Studies on Organic Photoconductors:

Evaluation

of

Atomic

Charges

on

Chlorodiane

Blue

and

p-

(Diethylamino)benzaldehyde Diphenylhydrazone. Chem. Mater. 1993; 5(12): 1799-804. [35] Ferrari AC, Robertson J. Interpretation of Raman Spectra of Disordered and Amorphous Carbon. Phys. Rev. B 2000; 61(20): 14095-107. 28

ACCEPTED MANUSCRIPT [36] Yun YJ, Hong WG, Kim WJ, Jun Y, Kim BH. A Novel Method for Applying Reduced Graphene Oxide Directly to Electronic Textiles from Yarns to Fabrics. Adv. Mater. 2013; 25(40): 5701-5. [37] Sun GZ, Liu JQ, Zhang X, Wang XW, Li H, Yu Y, et al. Fabrication of Ultralong Hybrid Microfibers from Nanosheets of Reduced Graphene Oxide and Transition-Metal Dichalcogenides and Their Application as Supercapacitors. Angew. Chem. Int. Ed. 2014; 53(46): 12576-80.

RI PT

[38] Ferrari AC, Meyer JC, Scardaci V, Casiraghi C, Lazzeri M, Mauri F, et al. Raman Spectrum of Graphene and Graphene Layers. Phys. Rev. Lett. 2006; 97(18): 187401.

[39] Yoon Y, Lee K, Kwon S, Seo S, Yoo H, Kim S, et al. Vertical Alignments of Graphene Sheets

SC

Spatially and Densely Piled for Fast Ion Diffusion in Compact Supercapacitors. ACS Nano 2014; 8(5): 4580-90.

[40] Choi BG, Yang M, Hong WH, Choi JW, Huh YS. 3D Macroporous Graphene Frameworks for

M AN U

Supercapacitors with High Energy and Power Densities. ACS Nano 2012; 6(5): 4020-8. [41] Zang JF, Cao CY, Feng YY, Liu J, Zhao XH. Stretchable and High-Performance Supercapacitors with Crumpled Graphene Papers. Sci. Rep. 2014; 4: 6492.

[42] Maiti UN, Lim J, Lee KE, Lee WJ, Kim SO. Three-Dimensional Shape Engineered, Interfacial

2014; 26(4): 615-9.

TE D

Gelation of Reduced Graphene Oxide for High Rate, Large Capacity Supercapacitors. Adv. Mater.

[43] Gogotsi Y, Simon P. True Performance Metrics in Electrochemical Energy Storage. Science 2011; 334(6058): 917-8.

EP

[44] Beidaghi M, Gogotsi Y. Capacitive Energy Storage in Micro-Scale Devices: Recent Advances in Design and Fabrication of Micro-Supercapacitors. Energy Environ. Sci. 2014; 7(3): 867-84.

AC C

[45] Huang CW, Hsieh CT, Kuo PL, Teng H. Electric Double Layer Capacitors Based on a Composite Electrode of Activated Mesophase Pitch and Carbon Nanotubes. J. Mater. Chem. 2012; 22(15): 7314.

[46] Zhou QL, Ye XK, Wan ZQ, Jia CY. A Three-Dimensional Flexible Supercapacitor with Enhanced Performance Based on Lightweight, Conductive Graphene-Cotton Fabric Electrode. J. Power Sources 2015; 296: 186-96. [47] Pech D, Brunet M, Durou H, Huang PH, Mochalin V, Gogotsi Y, et al. Ultrahigh-Power Micrometre-Sized Supercapacitors Based on Onion-Like Carbon. Nat. Nanotechnol. 2010; 5(9): 651-4. 29

ACCEPTED MANUSCRIPT [48] Su FH, Lv XM, Miao MH. High-Performance Two-Ply Yarn Supercapacitors Based on Carbon Nanotube Yarns Dotted with Co3O4 and NiO Nanoparticles. Small 2015; 11(7): 854-61. [49] Yang XW, Zhu JW, Qiu L, Li D. Bioinspired Effective Prevention of Restacking in Multilayered Graphene Films: Towards the Next Generation of High-Performance Supercapacitors. Adv. Mater. 2011; 23(25): 2833-8.

RI PT

[50] Chen J, Sheng KX, Luo PH, Li C, Shi GQ. Graphene Hydrogels Deposited in Nickel Foams for High-Rate Electrochemical Capacitors. Adv. Mater. 2012; 24(33): 4569-73.

[51] Hahm MG, Leela Mohana Reddy A, Cole DP, Rivera M, Vento JA, Nam J, et al. Carbon

SC

Nanotube-Nanocup Hybrid Structures for High Power Supercapacitor Applications. Nano Lett. 2012; 12(11): 5616-21.

[52] Zhang LL, Zhao X, Stoller MD, Zhu YW, Ji HX, Murali S, et al. Highly Conductive and Porous

12(4): 1806-12.

M AN U

Activated Reduced Graphene Oxide Films for High-Power Supercapacitors. Nano Lett. 2012;

[53] Li X, Zang XB, Li Z, Li XM, Li PX, Sun PZ, et al. Large-Area Flexible Core-Shell Graphene/Porous Carbon Woven Fabric Films for Fiber Supercapacitor Electrodes. Adv. Funct. Mater. 2013; 23(38): 4862-9.

TE D

[54] Wang GK, Sun X, Lu FY, Sun HT, Yu MP, Jiang WL, et al. Flexible Pillared Graphene-Paper Electrodes for High-Performance Electrochemical Supercapacitors. Small 2012; 8(3): 452-9. [55] Shao YL, El-Kady MF, Lin CW, Zhu GZ, Marsh KL, Hwang JY, et al. 3D Freeze-Casting of

6719-26.

EP

Cellular Graphene Films for Ultrahigh-Power-Density Supercapacitors. Adv. Mater. 2016; 28(31):

AC C

[56] Zhang J, Terrones M, Park CR, Mukherjee R, Monthioux M, Koratkar N, et al. Carbon Science in 2016: Status, Challenges and Perspectives. Carbon 2016; 98: 708-32.

[57] Fan HL, Niu RT, Duan JQ, Liu W, Shen WZ. Fe3O4@Carbon Nanosheets for All-Solid-State Supercapacitor Electrodes. ACS Appl. Mater. Interfaces 2016: DOI: 10.1021/acsami.6b05415.

[58] Hastak RS, Sivaraman P, Potphode DD, Shashidhara K, Samui AB. All Solid Supercapacitor Based on Activated Carbon and Poly [2,5-benzimidazole] for High Temperature Application. Electrochim. Acta 2012; 59: 296-303.

30