Effect of reducing system on capacitive behavior of reduced graphene oxide film: Application for supercapacitor

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Effect of reducing system on capacitive behavior of reduced graphene oxide film: Application for supercapacitor Hamdane Akbi, Lei Yu, Bin Wang, Qi Liu, Jun Wang, Jingyuan Liu, Dalei Song, Yanbo Sun, Lianhe Liu

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S0022-4596(14)00443-5 http://dx.doi.org/10.1016/j.jssc.2014.10.004 YJSSC18656

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Journal of Solid State Chemistry

Received date: 28 May 2014 Revised date: 26 September 2014 Accepted date: 5 October 2014 Cite this article as: Hamdane Akbi, Lei Yu, Bin Wang, Qi Liu, Jun Wang, Jingyuan Liu, Dalei Song, Yanbo Sun, Lianhe Liu, Effect of reducing system on capacitive behavior of reduced graphene oxide film: Application for supercapacitor, Journal of Solid State Chemistry, http://dx.doi.org/10.1016/j. jssc.2014.10.004 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 galley proof before it is published in its final citable 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.

Effect of reducing system on capacitive behavior of reduced graphene oxide film: application for supercapacitor. Hamdane Akbia, Lei Yua, Bin Wanga, Qi Liua, Jun Wang∗,a,b, Jingyuan Liua, Dalei Songa Yanbo Sunc and Lianhe Liua,b a

Key Laboratory of Superlight Material and Surface Technology, Ministry of Education, Harbin Engineering University, 150001, P. R. China.

b

Institute of Advanced Marine Materials, Harbin Engineering University, 150001, P. R. China. c

State Key Laboratory of Theoretical and Computational Chemistry,

Institute of Theoretical Chemistry, Jilin University Changchun, 130023, P. R. China.

Abstract To determine the best chemical reduction of graphene oxide film with hydriodic acid that gives maximum energy and power density, we studied the effect of two reducing systems, hydriodic acid/water and hydriodic acid/acetic acid, on the morphology and electrochemical features of reduced graphene oxide film. Using acetic acid as solvent results in high electrical conductivity (5195 S.m-1), excellent specific capacitance (384 F.g-1) and good cyclic stability (about 98% of its initial response after 4000 cycles). Using water as a solvent, results in an ideal capacitive behavior and excellent cyclic stability (about 6% increase of its initial response after 2100 cycles).

Keywords Capacitive behavior; Reducing system; Reduced graphene oxide film; Drop-casting technique. ∗

Corresponding author. Tel.: +86 451 8253 3026 Fax: +86 451 8253 3026.

E-mail address: [email protected] (Jun Wang).

1

1- Introduction Although high energy densities can reach as high as 180 Wh.kg

−1

, Lithium batteries

generally suffer from slow power delivery or uptake. Certain applications need faster and higherpower energy storage systems, a role given to the supercapacitor [1]. Supercapacitors have attracted much attention owing to their excellent power density, high rate capability, fast charging/discharging, long cycle life, ease of operation and low maintenance cost [2,3]. With a higher power density than batteries and a greater energy storage than conventional capacitors, supercapacitors are capable of bridging the power/energy gap between traditional dielectric capacitors and batteries/fuel cells. However, they are still limited by their low energy densities and slow rate capabilities [3,4]. Thus, major research on supercapacitors is directed at increasing the energy density without sacrificing other desirable properties. The energy E stored in the supercapacitor is dependent directly on its specific capacitance and cell voltage. Consequently, enhancing the energy density can be realized by improving both the cell voltage and specific capacitance. For supercapacitors, cell voltage can be enlarged by utilizing ionic liquid or organic electrolyte. Another way of increasing cell voltage is to develop asymmetric supercapacitors [5,6]. In other words, enhancing the specific capacitance can be achieved through increasing specific surface area and optimizing pore size by developing hierarchically porous structures, while maintaining good electrical conductivity [7]. It is well known that the electrode is the heart of the cell, and all electrochemical features, such as capacitance and charge storage of supercapacitors, essentially depend on the type of electrode material [8]. With its exceptional features, such as ultra-high electrical conductivity,

2

high surface area (2620 m2.g−1), good theoretical capacitance (550 F/g), excellent thermal stability and outstanding mechanical properties, graphene-based materials derived from graphene oxide (GO) have aroused a great deal of interest in various applications [9–13], including solar cells [14], transparent conductors [15–17], gas sensors [18,19], and supercapacitors [4,20–22]. Among various graphene-based materials, graphene film or paper has attracted a lot of attention due mainly to its excellent mechanical properties, high electrical conductivity and high surface area [4,23–24]. Furthermore, the dispersed GO can be easily assembled into films by simple techniques such as drop-casting [4], vacuum filtering [4,25], spray coating [26], and dipcoating [27]. Because of good mechanical flexibility and high electrical conductivity the graphene film can act as both the active material and current collector in the supercapacitor, leading to simplified and lightweight supercapacitors [4]. Moreover, the oxygen-containing GO makes it practically insulated. Thus, an effective and suitable process of deoxygenation must be carried out to achieve the desired features. The epoxy and hydroxyl groups are the main oxygencontaining functional groups attached to GO [28,29]. The chemical reduction of GO film using halogen acid, including hydriodic (HI) and hydrobromic acid (HBr) as reducing agents, results in far higher efficiency compared with other agents [23,30]. Here, we focus our attention on the effect of the reducing systems, hydriodic acid/water and hydriodic acid/acetic acid, on the structure, morphology and electrochemical properties of GO film prepared using a drop-casting technique. Using HI/acetic acid as a reducing system results in a high specific capacitance as well as high electrical conductivity. Using HI/water, on the other hand, as the solvent results in an ideal capacitive behavior with excellent cyclic stability.

3

2. Experimental 2.1 Materials and method All chemicals were of analytical grade and were used without further purification. Graphite oxide was synthesized from natural graphite using a modified Hummers method [31]. Exfoliation of GO was achieved by ultrasonication of the dispersion in an ultrasonic bath (KQ500DB, 250 W). GO film was made by drop-casting GO dispersions (5 mg.ml-1) onto the mold constructed from smooth paper [4]. The films were then allowed to dry for 48 hours under ambient conditions, then for 8 hours at 60oC. The film thickness was controlled by varying two parameters: the concentration and the amount of the dispersing GO. The reduction of GO film was carried out by immersing GO films into 30% HI/water and 30% HI/Ac-OH at 70oC. 2.2 Characterization The crystallographic structures of the materials were determined by a powder X-ray diffraction (XRD) system (Rigaku TTR-III) equipped with Cu KR radiation (λ = 0.15406 nm). X-ray photoelectron spectroscopy (XPS) measurements were performed using a PHI 5700 ESCA spectrometer with monochromatic Al KR radiation (hν = 1486.6 eV). All XPS spectra were corrected by the C1s line at 284.4 eV. Raman spectroscopy measurements were taken using a Horiba HR800 micro-Raman spectrometer. The microstructure of the samples was investigated by scanning electron microscopy (SEM, JEOL JSM-6480A microscope). The sheet resistance of the reduced graphene oxide film (r-GO) was measured by a four-probe method. The thickness of the r-GO film was measured by a digital micrometer.

4

2.3 Preparation of electrodes and electrochemical measurement The working electrodes were prepared by pressing the r-GO film onto nickel foam as the current collector with a pressure of 20 MPa. All electrochemical measurements were done in a three-electrode setup: Ni foam/r-GO film as the working electrode, platinum foil and SCE electrode as the counter and reference electrodes, respectively. The measurements were carried out in a 6 M KOH aqueous electrolyte at room temperature. Typically, cyclic voltammogram (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS) were measured by a CHI 660C electrochemical workstation. CV tests were done between -1 and 0V at different scan rates. Galvanostatic charge/discharge curves were measured in the potential range of -1 to 0 V at different current densities, and EIS measurements were also carried out in the frequency range from 100 kHz to 0.005 Hz at open circuit potential with an ac perturbation of 5 mV.

3. Results and discussion GO was prepared from natural graphite by the modified Hummers methods [31]. The GO film was prepared by using a drop-casting technique (Fig. S1). The GO film was reduced chemically using hydriodic acid as a reducing agent in two different solvents (Fig. S2), water and acetic acid, at low temperature (70oC). The sheet resistance (R, Ω.sq-1) of the r-GO films was measured by a four-probe method and the corresponding volume conductivity (σ, S.m-1) was calculated using the formula σ = 1/(Rt), where t (unit: cm) is the film thickness. The resulting r-GO film possesses excellent properties in terms of electrical conductivity. The r-GO film reduced by hydriodic acid/acetic acid (r-GOHA) displays a high electrical conductivity (σHA = 5194.80 S.m-1) which is nearly twice

5

the electrical conductivity of r-GO film (σHW = 1984.13 S.m-1) reduced by hydriodic acid/water (r-GOHW). 3.1 Reducing mechanism and selection of solvents The main purpose of this work is to remove oxygen-containing groups from GO film without destroying its structure and acquisition of other desired features, a role given to some halogen acids. It has been proved that HI can catalyze epoxy group and transform it into hydroxyl group. In addition, the HI reducing agent would replace hydroxyl group with iodide, which is good leaving group, resulting in a highly-effective deoxygenation [23]. Moreover, the selection of water and acetic acid as solvents is attributed to several facts. Typically, the high proportion of functional groups in graphene oxide gave it a hydrophilic character, hence the GO film is dissolvable in water [32,33]. This feature causes an expansion in thickness of the film during the reduction process (Fig. S3). In addition, the use of water gives a desirable accessibility of HI into GO film layers, resulting in a fast and complete reduction. On the other hand, the use of acetic acid as a solvent with HI can eliminate the oxygen functional groups and replace them with organohalides, which are easily removed. As a consequence, this property leads to a high graphitization [30]. Fig. 1 clarifies the effect of each solvent on the reduction process of GO film. 3.2 Microstructure characterization The chemical reducing process causes a remarkable change in the morphology of the GO film. Fig. 2 shows the SEM images of GO films before and after reduction. As shown, the reduction process changes the morphology of the GO film. In addition, changing the reducing system has a pronounced effect. Regarding the surface (Fig. 2a, Fig. 2b and Fig. 2c), chemical treatment causes a significant change in the morphology of GO film, from a smooth surface, prior to reduction, to a wrinkled and corrugated surface after reduction. In addition, changing the

6

reducing system shows differences in the morphology with a greater wrinkled-surface when HI/water is used as the reducing system. To further understand the morphology of as-prepared GO film, a cross section of GO film was investigated with SEM before and after reduction. As shown in Fig. 2d, Fig. 2e and Fig. 2f, using water as a solvent result in an expansion in thickness of the film (Fig. 2e), while using acetic acid result in a shrinkage, owing to the van der Waals force between graphene sheets. More importantly, the reduction process gives the desired structure, resulting in a high specific capacitance, energy density and excellent rate performance. The XRD patterns (Fig. 3a) indicate that the natural graphite is totally transformed to GO. It can be seen that XRD spectrum of GO exhibits a sharp diffraction peak around 2θ = 10o and the interlayer distance increases from 3.4 Å for the natural graphite to 8.8 Å, resulting from the introduction of oxygen-containing functional groups. After the reduction of GO film, the interlayer distance of the r-GO films is decreased to 3.64 Å (2θ = 24.4o), which may be caused by the elimination of the oxygen-containing groups on the graphene sheets. According to the XRD pattern, the r-GOHA peak is relatively higher than the r-GOHW peak; therefore, the reducing system, HI/acetic acid, results in high graphitization compared with that of HI/water. The XPS analysis further reveals the surface compositions of GO and r-GO films. To verify the elimination of the oxygen-containing groups from GO film after reduction XPS analysis was employed. The percentages of carbon, oxygen, nitrogen and sulfur are shown in Table 1. The XPS spectra of GO, r-GOHA and r-GOHW films are shown in Fig. 3b.The binding energies in the XPS experiment are corrected by referencing the C1s peak to 284.5 eV. According to the spectrum, two prominent peaks are seen on the GO film samples, revealing the presence of a large number of functional groups on the GO surface, including the non-oxygenated ring C(C–C),

7

the C in C–O bonds (C–O), and the carboxylate carbon (O–C=O). Carbon content of GO is 73.3% while oxygen content is 26.2%. After reduction with HI, using water as a solvent, the carbon content increases to 78%, and oxygen content decreases to 20%. The small bumps at about 288.5 eV, 287.6 eV and 286.6 eV indicate that there are still a small number of oxygen groups in sample r-GOHW. On the other hand, using acetic acid as a solvent leads to an increase of carbon content to 89.4%, and a decrease in oxygen content to 9%. In addition, r-GOHA exhibits a clean spectrum with an increase in the carbon peak, indicating complete reduction. Fig. 4 shows the Raman spectra of GO and r-GO films. After reduction, the (ID/IG) ratio of both r-GO films did increase notably, inferring that more defects form when some oxygen atoms are removed. The intensity of the two-dimensional (~2670 cm-1) and S3 (~2930 cm-1) peaks of r-GOHA increased more than that of r-GOHW, indicating better graphitization. 3.3. Electrochemical behavior The performance of the as-prepared r-GO films electrode was tested using cyclic voltammetry (CV), galvanostatic charge/discharge, and electrical impedance spectroscopy (EIS). The specific capacitance was calculated from the slope of the charge-discharge curves. The EIS data was analyzed using Nyquist plots in which each data point on the Nyquist plot represents a different frequency [3]. Fig. 5a and Fig. 5c show the CV curves of r-GO films electrodes at different scan rates in the range of 0 to -1 V using 6 M KOH aqueous solution as electrolyte. It can be clearly seen that the CV curves of r-GOHW electrode displays a quasi-rectangular shape without obvious redox peaks, indicating excellent capacitive behavior. Moreover, both r-GO film electrodes exhibit excellent mirror images with respect to the zero-current line and symmetric I–E response at both

8

positive and negative polarizations, indicating easy diffusion of electrolyte ions and excellent adsorption behavior [34]. Galvanostatic cycling tests of the r-GO electrode were performed in the potential range of 1 to 0 V at different current densities. As seen in Fig. 5b and Fig. 5d, the discharge curves of rGOAW film electrode are linear in the total potential range, displaying an ideal capacitive behavior. On the other hand, the discharge curves of r-GOHA film electrodes show a very small curved line in the range of -1 to -0.8 V. Furthermore, CV curves and galvanostatic charge/discharge curves shows the effect of solvent on the capacitive behavior of r-GO films using. The r-GOHA film electrode exhibits larger CV curve area and a longer discharging time, resulting in higher specific capacitance. The r-GOHW film electrode CV curve shows a relatively good quasi-rectangular shape; in addition, charge/discharge curve shows a typical isosceles triangular shape, with the discharge curves nearly symmetric with their corresponding charge counterparts in the total potential range, indicating excellent capacitive behavior. From the galvanostatic charge/discharge curves, an imperfect linearity and redox peaks at low current densities are clearly seen, which can be attributed to the pseudocapacitance effect. To verify the source of this effect, the nickel foam was investigated as an electrode [35]. The results show that the use of nickel foam as current collector has a negligible effect on the capacitance. Otherwise, the pseudocapacitance may be affected by the contained-oxygen groups and other residual elements in the r-GO films [36,37]. The specific capacitance was calculated according to the discharge curves at different current densities (Fig. 6), based on the following equation: ∆

C  i ∆V

(1) 9

where i is discharged current (A), ∆t is discharged time (s), ∆V is the potential window (V), and m is the mass of electroactive material. It can be seen that the specific capacitance for the r-GO film electrodes decreases with the increase of scan rate, which is caused by insufficient time available for ion diffusion and adsorption inside the smallest pores within large particles due to the diffusion limit at high scan rates [38]. Furthermore, the r-GOHA film electrode shows relatively high capacitance compared with r-GOHW for the same current density, which is mainly due to high electrical conductivity that ensures sufficient electrolyte ions and electrons participating in reactions.

In the case of r-GOHW film electrode, the maximum specific

capacitance is calculated as 187 F.g-1, corresponding to a current density 0.1 A.g-1. For r-GOHA film electrode, the maximum specific capacitance is calculated as 384 F.g-1, corresponding to a current density 0.4 A.g-1. The cycling performance of the r-GO film electrodes was carried out by repeating the cyclic voltammetry test between 0 to -1V at a scan rate of 0.1 V/s for 4000 cycles (Fig. 7). Surprisingly, the capacitance of r-GOHW film electrode does not deteriorate but increases continuously until the 2100th cycle when it reaches 106% of the initial capacitance and is still above 96% after 4000 cycles. The increase of the specific capacitance is probably due to the activation process of the electroactive material [39,40]. On the other hand, r-GOHA film electrode shows a good cyclic stability with 98% retention of its initial capacitance after 4000 cycles. EIS analysis is a key method in diagnosing the fundamental behavior of electrode materials for supercapacitors [3,41]. Fig. 8a and b present the Nyquist plots of as-prepared electrodes. As seen they show a straight line in the low frequency region and an inconspicuous arc in the high frequency region. This high frequency loop is related to the electronic resistance between the graphene sheets [42]. The inconspicuous arc in the high frequency region proves that the

10

electronic resistance interlayer in the r-GO films is low. Furthermore, the vertical shape (Fig. 8b) at lower frequencies represents a pure capacitive behavior, which is an indicator of ion diffusion and migration within the structure of the electrode [43]. The more vertical the curve, the more closely the supercapacitor behaves as an ideal capacitor. It can be clearly seen that r-GOHW film electrode exhibits a relatively more vertical shape than r-GOHA. The intersection of the high frequency region at the x-axis corresponds to the equivalent series resistance (ESR) of the electrode. ESR data determines the rate that the supercapacitor can be charged/discharged, which is important for the power density of a supercapacitor [44]. Accordingly, the r-GOHW film electrode (ESRHA=0.6 Ω) displays a lower ESR compared with rGOHA film electrode (ESRHA=0.7 Ω). The maximum power density of the electrode supercapacitor was determined (Table 2) from the low frequency data of the impedance spectrum by the following equation: P 

∆V

(2)

.ESR

where ∆V is the operating voltage window, R is the ESR and m is the mass of electrodes with a cell voltage of 1V. The Ragone plot (Fig. 9) shows the relationship between energy density and power density. The maximum storage energy is calculated with equation (2) [3,45]: 

   ∆ 

(3)

where C is specific capacitance and ∆V as the voltage window (∆V = 1).The power density was calculated based on following equation : 

 

(4)

where t is the discharge time.

11

The r-GOHA film electrode displays a higher energy density (EHA=53.3 Wh.Kg-1 at a power density of 200 W.kg-1) than r-GOHW film electrode (EHW= 26 Wh.Kg-1 at a power density of 50 W.kg-1) which results from the higher specific capacitance. The chemically reduced graphene properties depend largely on the reducing system. As illustrated before, the choice of reducing system determines the morphology and structure of the film and, as a result, affects largely electrical conductivity and capacitive behavior. Table 3 compares the properties for both r-GO electrodes and shows the advantages of each method. The r-GOHA film electrode possesses high specific capacitance as well as excellent electrical conductivity, leading to enhanced energy and power densities, while r-GOHW film electrode shows an ideal capacitive behavior and excellent cyclic stability.

4. Conclusion We have studied the effect of reducing systems, water/hydriodic acid and acetic acid/hydriodic acid, on the capacitive behavior of graphene oxide film. Consequently, benefits of each reducing solvent were highlighted. Using water as a reducing solvent results in an ideal capacitive behavior and

excellent cyclic stability with increased specific capacitance during

charging/discharging cycles, while using acetic acid results in a high electrical conductivity (5195 S.m-1) and excellent specific capacitance (384 F.g-1). We strongly believe that the chemically reduced graphene film materials have a great advantage in the development of a new generation of supercapacitors with high energy density.

Acknowledgements This work was supported by National Natural Science Foundation of China (21353003), Special Innovation Talents of Harbin Science and Technology (2013RFQXJ145), Fundamental Research Funds of the Central University (HEUCFZ), Natural Science Foundation of Heilongjiang 12

Province

(B201316),

Program

of

International

S&T

Cooperation

special

project

(2013DFA50480), the fund for Transformation of Scientific and Technological Achievements of Harbin (2013DB4BG011), and Research and Development of Industrial Technology Project of Jilin Province (JF2012C022-4)

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Table 1. Elemental analyses results of the GO, r-GOHW and r-GOHA showing the carbon, nitrogen, oxygen and sulfur contents. Table 2. ESRs and maximum power densities determined for r-GO film electrodes. Table 3. Comparison between r-GO films reduced by different reducing systems.

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Table 1 Table 1. Elemental analyses results of the GO, r-GOHW and r-GOHA showing the carbon, nitrogen, oxygen and sulfur contents GO(%) r-GOHW (%) r-GOHA (%) Film C

73.33

78.01

89.40

O

26.19

20.03

9.84

N

0

1.82

0.76

S

0.48

0.15

0

C/O

2.8

3.9

9.08

Table 2

Table 2. ESRs and maximum power densities determined for r-GO film electrodes r-GOHW r-GOHA Electrode ESR (Ω) Mass (mg) -1

Pmax (KW.Kg )

0.6

0.7

1.5

1.1

277.78

324.67

19

Table 3 Table 3. Comparison of r-GO films features reduced by different reducing systems Electrode Effect on the structure determined from SEM microscopy. The rapport (C/O) determined from XPS analysis.

r-GOHW An expansion on thickness.

r-GOHA A shrinking on thickness.

3.9

9.1

Conductivity (S.m-1)

1984.1

5194.8

Capacitive behavior

Excellent on the total potential range

Relatively pure at the range from -1 to -0.8 V

Specific capacitance (F.g-1)

187 at 0.1 A/g

384 at 0.4 A/g

Cyclic stability after 4000 cycles

Retaining 96% of initial capacitance

Retaining 98% of initial capacitance

ESR (Ω)

0.6

0.7

Energy density(Wh.Kg-1)

26

53.3

Power density(kW.Kg-1)

277.8

324.7

20

Graphical abstract legend:

Not only the reducing agent but also the solvent system plays a crucial role in the properties of the reduced GO-based materials.

21

Fig. 1. Schematic illustration for the reduction mechanism of r-GO films. (a) Dissolution of GO film before complete reduction, preventing the restacking of graphene sheets. (b) After reduction, r-GOHA film volume will decrease, owing to restacking of graphene sheets. Fig. 2. SEM image of the GO film surface before reduction (a); after reduction with HI/water (b); after reduction with HI/acetic acid (c); cross sectional SEM images of GO film before (d); after reduction with HI/water (e); and after reduction with HI/acetic acid (f). Fig. 3. Characterization of the GO films before and after 1 h reduction. (a) XRD patterns; (b) XPS C1s peaks. Fig. 4. Raman spectra of the GO films before and after 1 h reduction. Fig.

5. Cyclic voltammograms under different scan rates of (a) r-GOHA, (c) r-GOHW.

Galvanostatic charge/discharge under different current densities of (b) r-GOHA, (d) r-GOHW. Fig. 6. Specific capacitance for different current densities obtained from galvanostatic charge/discharge curves of r-GO film electrodes. Fig. 7. Cyclic performances of r-GO electrode films. Fig. 8. Nyquist plots for r-GO film electrode in the frequency range of 100 kHz to 0.005 Hz measured during the life cycle tests. Fig. 9. Ragone plots related to energy and power densities of r-GO film electrodes

22

Fig. 1

23

Fig. 2

24

(a) 10

(b)

GO r-GOHA

o

24.4

5

10

15

20

o

25

290

30

288

Fig. 3

GO r-GOHW

G

r-GOHA

Intensity (a.u.) 1000

1500

286

284

Binding energy (eV)

2θ(degree)

D

284.4o

r-GOHW

Intensity (a.u.)

Intensity (a.u)

r-GOHW

GO r-GOHA

2000

2D

S3

2500

3000

-1

Raman shift (cm ) Fig. 4

25

282

40 30

Current (A/g)

(c)

0.05 V/s 0.1 V/s 0.2 V/s 0.5 V/s

20 10 0 -10 -20

-0.2

-0.4

-0.6

-0.8

0 -5 -10

Potential (V) 0.0

0.4 A/g 0.5 A/g 1 A/g 2 A/g 4 A/g 5 A/g 10 A/g

-0.2 -0.4 -0.6

-20

-1.0

0.0

-0.2

-0.4

-0.6

-1.0

0.1 A/g 0.2 A/g 0.4 A/g 1 A/g 2 A/g 4 A/g 10 A/g

-0.2

-0.8

-0.8

Potential (V)

(d) 0.0 Potential (V)

0.0

Potential (V)

5

-15

-40

-1.0

0.005 V/s 0.01 V/s 0.02 V/s 0.05 V/s 0.1 V/s

10

-30

(b)

20 15

Current (A/g)

(a)

-0.4 -0.6 -0.8

0

500

1000

1500

2000

-1.0

0

1000

2000

Time (s)

Time (s)

Fig. 5

26

3000

Specific capacitance (F/g)

400

r-GOHW r-GOHA

300

200

100

0

0

2

4

6

8

Current density (A/g)

10

Retention (%)

Fig. 6

140

r-GOHW

120

r-GOHA

100 80 60 40 20 0

0

500 1000 1500 2000 2500 3000 3500 4000

Cycle number Fig. 7

27

(a) 300

(b) 15

r-GOHW

r-GOHA

r-GOHA

250

10

Z'' (Ohm)

200

Z'' (Ohm)

r-GOHW

150 100

5

50 0

0

50

100

150

200

250

300

0

0

5

10

Z' (Ohm)

Z' (Ohm)

Fig. 8

4

Power density (W/Kg)

10

r-GOHW r-GOHA

3

10

36 ms

2

10

0.36 s

1

10

0

3.6 s

10 -2 10

36 s

1h

360 s -1

10

0

10 h 1

10

10

100 h 2

10

Energy density (Wh/Kg) Fig. 9

28

3

10

15

Highlights 1/ The structure of the graphene film has a pronounced effect on capacitive behavior. 2/ The use of water/HI as reducing system results in an ideal capacitive behavior. 3/ The use of acetic acid/HI as reducing system results in a high specific capacitance.

29