A simple route to prepare free-standing graphene thin film for high-performance flexible electrode materials

Abstract

A free-standing graphene thin film is prepared by a simple electrochemical method applying positive and negative pulse electric signal, followed by air drying and being peeled off from the electrodes. During the process, formation and reduction of graphene oxide film have been simultaneously achieved. The free-standing graphene thin film obtained is characterized by X-ray diffraction, X-ray photoelectron and electrochemistry. The results show that the electrical conductivity and reduction of free-standing graphene film are influenced by pulse characteristic duty ratio and time of reaction. Their capacitive behavior is investigated by cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy using the two-electrode symmetric capacitor test. The free-standing flexible graphene film prepared under duty ratio 60% for 3 h exhibits a specific capacitance of 157 F g⁻¹ and good cycling stability. In addition, energy density and power density can reach to 3.36 W h kg⁻¹ and 20.5 kW kg⁻¹, respectively, at a discharge of 100 A g⁻¹. This approach opens up the possibility of fabrication of high-performance flexible electrode materials.

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1. Introduction

There is currently a strong demand for flexible energy storage devices, based on supercapacitors, to meet the various requirements of modern gadgets.^1–10 Free-standing paper-like carbon-based materials, characterized by their light weight and high conductivity, have been actively investigated for producing flexible electrodes.^11–14 Graphene, as a new kind of carbon material, is of particular importance as it possesses a 2D geometry consisting of sp²-bonded carbon atoms in a hexagonal lattice and as such is an ideal flexible electrode material for applications.^15–18 Many approaches, such as molecular templates,¹⁹ Langmuir–Blodgett assembly,^20,21 and direct chemical vapor deposition (CVD),^22–25 have been employed to obtain graphene thin film. For example, Zhu et al. reported that the graphene thin film prepared by CAD technique serving as an electrode material exhibited remarkable electrochemical performance. Most importantly, they could be readily transformed into diverse shapes.²⁴ Another approach is using a flow-directed filtration of graphene dispersion, which is considered to be simple and does not need special equipment.²⁶ However, graphene dispersion tends to aggregate in a disorderly fashion, which makes the formation of the film difficult. Some works start from a dispersion of graphene oxide (GO) instead of graphene.^27,28 GO is well-known as a chemically exfoliated derivative. It could be easily dispersed into a variety of solvents because of the presence of carboxylic and hydroxyl groups.²⁹ Mechanical tests have indicated that GO papers exhibit superior stiffness and strength, which surpass the properties of most carbon-based, paper-like materials such as buckypaper and flexible graphite foil.³⁰ Nevertheless, a lack of electrical conductivity limits its use. It needs to be further treated by chemical,³¹ electrochemical³² or thermal³³ reduction to regain conductivity. But post-reduction treatment would destroy the structure of film due to the decomposition of oxygen-containing functional groups. Moreover, the size of film from time-consuming membrane filtration method is limited by the size of the membrane. It is not suitable for large-scale production.

Electrochemical method is an effective technique that is used for preparing graphene films or graphene-coated material.^34–37 Most of the currently proposed methods typically involve two steps. The first step: a graphene oxide film electrode is formed by a coating process or electrophoretic deposition. The second step: the obtained graphene oxide film that serves as a negative electrode was reduced in a normal electrochemical cell. As a result, the process of fabricating graphene film is obviously time-consuming and tedious. In addition, it has rarely been reported that the obtained graphene film could be peeled off from the electrode. Consequently, exploring a simple electrochemical method to prepare free-standing graphene thin film still remains a challenge.

In this paper, a special electrical signal—a positive and negative pulse signal—was applied in GO aqueous dispersion. The alternation signal of positive and negative leads to the synchronization of the formation and reduction of GO film. The method in our study is simple and easy to scale up. The obtained graphene film can be peeled off like “paper” from the electrode after air drying. We refer to it as graphene paper (GP) in the following. GP thus obtained is freestanding and highly flexible. The size of such a paper can be precisely controlled by the effective area of the electrode. It served as a flexible electrode material and exhibited good capacitive performance. This approach opens up the possibility for the fabrication of high-performance flexible electrode materials.

2. Experimental

2.1. Materials and instrument

Graphite oxide was purchased from Sixth Element Ltd., and all other chemicals used in this study were of analytical grade. Signal generator (DG1022) and oscilloscope (DS1052E) were purchased from Beijing Puyuan Technologies Co.; power amplifier (HVP-300A) was bought from Nanjing Fonan Co.

2.2. Preparation of GP

A suspension of graphene oxide (GO, 1 mg ml⁻¹) was prepared by dispersing graphite oxide powder into water with the help of ultrasonication for 2 h. Then it was loaded into a glass container. Copper electrodes were used in the experiment. Electrode separation was 10 millimeters. Positive and negative pulse signal whose frequency and peak-to-peak voltage (V_pp) were 5 Hz and 60 V, respectively, were applied on the electrodes in GO aqueous dispersion. This signal was produced by the signal generator and amplified by the power amplifier. SFig. 1a† shows the setup of the electrochemical reaction process. A schematic illustration of the procedure for fabricating GP is displayed in SFig. 1b.† GO platelets migrated toward the working electrode as soon as a positive pulse was applied. Then these deposited platelets were reduced when a negative pulse was applied next. This process was repeated until a thick film was formed. At last, the electrode, which adsorbed the film was taken out and then dried in the air. GP was obtained by peeling it off from the electrodes and then cut by a razor blade into rectangular strips for testing without further modification. The GP samples prepared were defined as GP-60%-1 h, GP-60%-2 h, GP-60%-3 h and GP-80%-3 h according to different duty ratios in SFig. 2† (60% and 80%) and reactive time (1 h, 2 h, 3 h). A digital image of the Cu electrode that adsorbed thick GO film under 60% duty ratio for 3 h is shown in SFig. 1c.†

2.3. Characterization

The X-ray diffraction (XRD) patterns of samples were obtained using a D8 Cu Kα radiation (D/max 2550VB3+/PC, Rigaku, Japan). The microstructures and morphology of graphene paper were observed with a field emission scanning electron microscopy (Quanta 200FEG, FEI). X-ray photoelectron spectroscopy (XPS) analysis was performed with an ESCALAB 250Xi spectrometer (Thermo Electron) using a monochromic Al Kα source at 1486.6 eV. Electrical conductivities were measured using the standard four-probe technique.

2.4. Fabrication of flexible supercapacitor

The fabrication of the flexible supercapacitors is described as follows: two nearly identical (in weight and size) graphene papers were separated by a filter paper soaked with 1.0 mol L⁻¹ KCl aqueous solution. Before electrochemical measurements, the slices of graphene paper were also immersed in KCl aqueous solution under vacuum in order to exchange their interior water with electrolyte. Two foam nickels were used as flexible current collectors. All components were assembled into a sandwiched structure between the two flexible substrates. A schematic image of the flexible GP supercapacitor devices is shown in Fig. 1.

Fig. 1 Design and fabrication of a flexible GP electrochemical capacitor.

2.5. Electrochemical measurements

Electrochemical performance of the cells was tested by cyclic voltammetry (CV), galvanostatic charge–discharge and electrochemical impedance spectroscopy (EIS) on a CHI660E electrochemistry workstation (Chenhua, Shanghai). Potential windows for the CV measurements and galvanostatic charge–discharge tests ranged from −0.5 to 0.5 V. EIS tests were carried out in the frequency range of 10⁵ to 0.1 Hz at the amplitude of 5 mV, referring to open circuit potential.

In order to analyze the variation of capacitance with varying scanning rates, specific capacitance (C_sc) of the electrodes can be calculated based on CV curves³⁸ according to the following equation:

C_sc = (∫IdV)/(vmΔV), (1)

where I is the response current (A), ΔV the difference of potential during CV tests (V), v the potential scan rate (V s⁻¹), and m the mass of one electrode (g).

Furthermore, C_sc, power density, and energy density can also be calculated from the galvanostatic charge–discharge curves.³⁹

For example, C_sc can be obtained using the equation:

C_sc = 2IΔt/ΔV_m (2)

where I represents the constant discharge current, Δt the discharging time, m the mass of one electrode, and ΔV the voltage drop upon discharging. Then, energy density (E) and power density (P) of the flexible supercapacitor depicted in the Ragone plot can be calculated using the equations:

E = (1/8)C_scΔV² (3)

P = E/Δt (4)

3. Results and discussion

3.1. Characterizations of GP

A flat and uniform macroscopic paper was obtained under 60% for 3 h, as shown in Fig. 2a. This paper is freestanding and highly flexible, that it is a promising candidate for rolling-up electrode (Fig. 2b). A layered nanostructure is clearly identified for this paper, based on a cross-sectional scanning electron microscope (SEM) observation with different magnifications (Fig. 2c and d). The thickness of such a paper can be precisely controlled by adjusting the volume of container and GO concentration. In our study, the thicknesses of GP-60%-1 h, GP-60%-2 h, GP-60%-3 h and GP-80%-3 h are 33, 35, 29, and 32 μm respectively. The size of GP is solely determined by the area of electrode used, and a large-area GP was easily obtained so long as a large electrode was employed.

Fig. 2 Digital image of a large-area (about 40 mm × 30 mm) GP (a) a flexible GP (b) SEM images with different magnifications of PM prepared under a 60% duty ratio for 3 h (c and d).

Fig. 3a displays the XRD patterns of GO and the obtained GP at different times. It is clear the XRD pattern of GO shows one sharp diffraction peak at 11.2°, which corresponds to an interplanar spacing of 0.79 nm according to the Bragg equation (2d sin θ = nλ). The number is consistent with the apparent thickness of a GO's single layer observed by AFM (SFig. 3†). After 1 h of reaction, peak intensities became weak at 2θ = 11.2°. Meanwhile, a new peak of GP at 22.7° with d-spacing of 0.39 nm, weak and broad, was observed. The interplanar spacing of GP showed drastic decreases to 0.380 nm (23.4°) for 2 h, 0.37 nm (23.9°) for 3 h, indicating that the oxygen-containing group of GO had been gradually removed with increasing time. Fig. 3b displays the XRD patterns of the obtained GP at different duty ratios (60% and 80%) for 3 h. GO had a better reductive effect when duty ratio was 60% and the obtained sample showed better lattice structure.

Fig. 3 XRD patterns of GO and obtained samples GP-60%-1 h, GP-60%-2 h, GP-60%-3 h and GP-80%-3 h.

As a powerful tool for identifying element states in material,⁴⁰ XPS is used to characterize the elemental composition of samples prepared under various conditions. The bands centered at 284.7 and 531.0 eV are associated with C 1s and O1s, respectively (Fig. 4a). The intensity of O 1s in GP-60%-2 h, GP-60%-3 h and GP-80%-3 h significantly decreases compared with that in GO; Fig. 4b–f show the C 1s XPS spectra of GO and reaction product (GP) separately. Four different peaks centred at 284.0 eV (C=C), 284.5 eV (C–C), 286.3 eV (C–O) and 288.1 eV (O=C) were detected in the GO sample (Fig. 4b). After reaction, the intensities of all C 1s peaks of carbons binding to oxygen decreased gradually with the time increased or duty ratio reduced.

Fig. 4 General XPS spectra of GO, GP-60%-1 h, GP-60%-2 h, GP-60%-3 h and GP-80%-3 h (a) and high resolution C 1s XPS spectra of GO (b), GP-60%-1 h (c), GP-60%-2 h (d), GP-60%-3 h (e) and GP-80%-3 h (f).

Since the purpose of reduction is mainly to restore the high conductivity of graphene, the electrical conductivity of samples can be a direct criterion to judge the effect of reduction. It can be calculated from eqn (5), where σ is conductivity (unit: S cm⁻¹), t is sample thickness and R_s (unit: Ω sq⁻¹) is electrical resistance of a sheet:

σ = 1/tR_s (5)

Fig. 5a shows the relationship between electrical conductivity of GP and reaction time under 60%. It is found that electrical conductivity increased from 18 S m⁻¹ for 0.5 h to 490 S m⁻¹ for 4 h. This gives evidence that with increasing time, the graphene oxide was gradually reduced to graphene and as such, the ordered crystal structure was restored and regained the high conductivity of graphene. The influence of duty ratio on the electrical conductivity of GP is shown in Fig. 5b. It was found that the conductivity of GP prepared become increasingly larger with a decreased duty ratio. This could be explained by the fact that a smaller duty ratio means a longer negative pulse time for reducing GO and restoring the structure within the graphene nanosheets. The electroformation and electroreduction of the GO film may proceed by mechanism (6)–(8):

Positive pulse: 4OH⁻ − 4e⁻ = 2H₂O + O₂ (6)

Negative pulse: 4H⁺ + 2GO + 4e⁻ = 2G + 2H₂O (7)

Cell reaction: 4OH⁻ + 4H⁺ + 2GO = 4H₂O + O₂ + 2G (8)

Fig. 5 Plot of electrical conductivity of GP prepared under 60% versus time (a) and for 3 h versus duty ratio (b) of reaction.

The GO film was formed due to electrophoretic deposition when positive pulse was applied. Meanwhile, bubbles, which have an adverse effect on the formation of smooth film, were produced on the surface of the electrode. Negative pulse applied in the following process not only caused GO film to be reduced but also provided time for the elimination of bubbles. This may explain why we could not obtain smooth film when direct current (DC) voltage was applied in the GO dispersion (SFig. 4†). If a GO film is not smooth, we cannot peel it off from the electrode as a free-standing paper. We also found that the duty ratio cannot be less than 50%, which is the critical condition for GO film formation. The electroformation and electroreduction mechanism suggested herein is significant. It not only gives an opportunity for fundamental research on GO and GP, but it also demonstrates a promising simple method for the environmentally friendly production of GP.

3.2. Electrochemical studies on GP supercapacitors

The samples GP-60%-3 h and GP-80%-3 h had a better reductive effect compared with the other samples and were used as flexible electrode materials in electrochemical performances tests. Cyclic voltammetry (CV) curves for GP-60%-3 h- and GP-80%-3 h-based cells are shown in Fig. 6(A and B). Both cases yielded very small internal resistance as reflected by sharp charge and discharge processes. In addition, the shapes of the curves are all rectangular-like at all scan rates from 100 to 800 mV s⁻¹, which indicates a rapid current response on voltage reversal at each end potential. From another perspective, the almost ideal rectangular CV curves from GP electrodes reflect small contact resistance. The reason may be that GP has a parallel 2D structure in itself, which is beneficial to fast charge storage and transport.

Fig. 6 CV behavior of capacitor cells assembled by GP-60%-3 h (A), GP-80%-3 h (B) and plots of specific capacitances of capacitor cell prepared by GP-60%-3 h (C-a), GP-80%-3 h (C-b).

Furthermore, the capacitor assembled by the GP-60%-3 h exhibit smaller deviation from the rectangle-like shape than that of GP-80%-3 h at the same scan rates. C_sc of the materials can be calculated based on the CV curves and are shown in Fig. 6C. At a given scan rate, e.g., 10 mV s⁻¹, the specific capacitance of electrodes derived from the CV curves was 157 F g⁻¹ for GP-60%-3 h, larger than 145 F g⁻¹ for GP-80%-3 h. This may be because that the smaller duty ratio means a longer negative reductive time to restore the high conductivity. The high conductivity is helpful for fast charge transfer. The C_sc of the capacitor assembled at GP-60%-3 h is also higher than some graphene-based supercapacitors previously reported by other groups.^41–43

Fig. 7(A and B) shows charge–discharge curves of cells prepared by GP-60%-3 h and GP-80%-3 h at a current density of 2.0 A g⁻¹; the symmetrical triangles of the charge–discharge plots with small IR drops indicate an ideal double layer capacitor behavior. In addition, the cell prepared by GP-60%-3 h displays a much lower IR drop at the beginning of the discharge process and a longer discharge time than GP-80%-3 h, which may be due to a better reductive effect for GP-60%-3 h. It is also found (Fig. 7C) that the C_sc values calculated from the discharged curves decrease slightly when current densities increase and that the electrode of GP-60%-3 h possesses relatively higher C_sc than the other GP-80%-3 h electrodes.

Fig. 7 Galvanostatic charge–discharge curves of cells prepared by GP-60%-3 h (A), GP-80%-3 h (B) at current density of 2.0 A g⁻¹. Specific capacitance of the GP-60%-3 h (C-a), GP-80%-3 h (C-b) at various discharge current densities. Nyquist plot of GP-60%-3 h (D-a), GP-80%-3 h (D-b), and inset show the high-frequency region of the plot.

From the complex-plane impedance plots in Fig. 7D, the values on the x-axis, i.e., the corresponding real impedance, reflected a very small equivalent series resistant (ESR) for the supercapacitors from both cases. ESR was 508 mΩ for GP-80%-3 h, slightly larger than 376 mΩ for the GP-60%-3 h samples, confirming the results in Fig. 7(A and B). The charge–discharge curves of GP-60%-3 h and GP-80%-3 h cells in Fig. 7(A and B) revealed the corresponding IR drops of 21 mV and 32 mV, respectively. The IR drop is usually caused by the overall internal resistance of the devices. Low internal resistance is of great importance in energy-storing devices as less energy will be wasted to produce unwanted heat during charging–discharging processes.

In addition, to validate the promising applications of various GP electrodes in electrochemical capacitors, the Ragone plot of symmetric capacitors based on the data of the charge–discharge curves is shown in Fig. 8A. The cells of GP-60%-3 h possess an energy density of 3.36 W h kg⁻¹ when the power density reaches about 24.99 kW kg⁻¹ at a discharging current density of 100 A g⁻¹. Nevertheless, the energy and power densities would be slightly decreased with increased thickness. This behavior is supposed to be related to the rate of mass transport from electrode/electrolyte interface to the interior of the electrodes, as diffusion and migration path becomes longer with increasing electrode thickness. But, if thickness of sample is too small, the obtained graphene film cannot be peeled off like ‘paper’ from the electrode after air drying.

Fig. 8 (A) Ragone plots of energy density versus power density for cells assembled by GP-60%-3 h (line a), GP-80%-3 h (line b). (B) CV curves of device assembled by GP-60%-3 h under different bending angles at a scan rate of 100 mV s⁻¹.

Flexible electronic devices are required to be bendable in practical application. Therefore, CV performance of these devices assembled by GP-60%-3 h (Fig. 8B), and GP-80%-3 h (SFig. 5†) were analyzed under different bending conditions. Both manifested that bending had almost no effect on the capacitive behavior; they could be bent from 0° to 180° without degrading performance. This performance can be attributed to the high flexibility of the obtained GP.

The service life is a very important factor for the electrode of an electrochemical capacitor, so the stabilities of the cells assembled by GP electrodes have been evaluated using CV method at a scan rate of 100 mV s⁻¹ (Fig. 9). As can be seen, C_sc of the cells prepared by GP-60%-3 h and GP-80%-3 h still remained about 91.3%, 88.5%, respectively, after 2000 CV charge–discharge processes, which illustrates that the material of GP exhibits good durability and may be developed as a suitable material for electrochemical capacitor applications.

Fig. 9 Relationships between specific capacitance and cycle number of cells assembled by GP-60%-3 h (line a), GP-80%-3 h (line b).

4. Conclusions

It is demonstrated for the first time that GP with a controllable size can be obtained by electrochemical method by applying positive- and negative-pulse electric signal, followed by air drying and peeling off from electrodes. The sample GP-60%-3 h is tested to be the ideal flexible electrode material. The GP-60%-3 h-based supercapacitor has a high C_sc of 157 F g⁻¹ at scan rate of 10 mV s⁻¹ and shows a long cyclic life along with about 91.3% C_sc retention after 2000 cycle tests at a scan rate of 100 mV s⁻¹. In addition, the energy and power densities can reach 3.36 W h kg⁻¹ and 20.5 kW kg⁻¹, respectively, at a discharge of 100 A g⁻¹. Therefore, this method is a promising candidate to provide GP for high-performance flexible electrode materials.

Acknowledgements

The authors thank the National High Technology Research and Development Program of China (no. 2012AA030303) and Basic Research Key Program of Shanghai (no. 12JC1408600).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra04031g

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