Proceedings of the 3rd Applied Science for Technology Innovation, ASTECHNOVA 2014

International Energy Conference

Yogyakarta, Indonesia, 13-14 August 2014

Performance of Electric Double Layer Capacitor Electrode from Reduced Graphene Oxide Material Prepared by Zn Reduction

Following by Hydrothermal Process

Haniffudin Nurdiansah, Diah Susanti,

Hariyati Purwaningsih, Lukman Noerochiem

Materials and Metallurgical Engineering Department, Institut Teknologi Sepuluh Nopember

Kampus ITS, Keputih, Sukolilo, Surabaya, Indonesia

santiche@mat-eng.its.ac.id

Dah Shyang Tsai

Chemical Engineering Department, National Taiwan University of Science and Technology

43, Section 4, Keelung Road, Taipei, Taiwan

ABSTRACT

Reduced graphene oxide (rGO) has been succesfully synthesized from graphite powder via

Hummer's method, followed by ultrasonication at different time (1.5, 2, and 2.5 h ), continued by Zn

reduction in acidic conditions, and finally thermally treated by hydrothermal process at 1600C. These

synthesized materials have been characterized by XRD, SEM, FTIR, EDS, and Raman

Spectroscopy. From the XRD result, it appears that the peak of Graphite Oxide (GO) (~9.80) has fully

disappeared and broad peak at ~23-240 has appeared. FTIR spectra indicate that the intercalated H2O

molecules and most of the oxide groups of GO are removed. The % weight of C and O around

81.98% and 18.02 %, concluded from EDS Testing. From Raman Spectroscopy Testing, shows that

the curve consists of two dominant peaks at 1350 cm-1

and 1590 cm-1

along with a wide band

extending from about 26003300 cm-1

. Furthermore, the electrochemical performance investigated by

using electrode prepared by dipping a piece of Nickel Foam into a rGO solution, and then dried it and

press it. The CV Testing shows a rectangular shape and a high capacitance up to 350.37 F/gr at scan

rate 2mV/s in the Na2SO4 solution 1M. Galvanostatic charge discharge shows the capacitance 368

F/gr at the current density 2 A/g. We concluded that the best result was obtained by sample with

ultrasonication time 1.5 hour.

KEYWORDS: Reduced graphene oxide, Zn reduction, Hydrothermal, Capacitance

1 INTRODUCTION

The traditional synthesis of rGO from GO involves harmful chemical reducing agents and is

undesirable for practical applications. Using Zn as a reductor to acquire rGO from GO under acidic

condition has been carried out, and it offers a potential for cost-effective, environmentally friendly,

and large-scale production of rGO (Panbo Liu, et al., 2013).

Several methods have recently been reported to synthesize rGO, such as chemical vapor

deposition, thermal annealing graphene oxide with NH3, and arc discharge method, but this process

need rigorous conditions or special instruments. Compared with these methods, the hydrothermal

method has merits of mild conditions and scale-up synthesis (Ping Chen, et al., 2013).

Electric double-layer capacitors (EDLCs), are used in a wide range of energy capture and

storage applications, which are believed to provide clean energy with almost zero waste emission.

Carbon materials can meet the request of EDLC electrodes. The commonly material used was

activated carbon. Although activated carbons always have high BET surface areas, only parts of the

surface can contribute to the specific.capacitance. Hence the specific surface capacitance was lower

than graphite (Xian Du, et al., 2010).

mailto:santiche@mat-eng.its.ac.id

In recent years, graphene has been extensively explored as an electrode material for EDLC

because of its high conductivity, large specific surface area (SSA), and excellent electrochemical

stability. For this purpose, reduced graphene oxide (rGO) has been paid the most intensive

attention, mainly because it can be cheaply produced on a large scale from graphite oxide (GO).

Graphene materials for supercapacitors seem to be extremely attractive for the present. Stoller et al.,

(2008) fabricated a symmetric supercapacitor based on chemically reduced graphene oxide (rGO) to

give a specific capacitance of 135 F/gr in KOH and 99 F/gr in an organic electrolyte. Xian Du et al.,

(2010) fabricated a supercapacitor graphene nanosheet by coated into a piece of nickel foam, which

has a specific capacitance up to 150 F/gr in Na2SO4 electrolyte. In this work, rGO was prepared by Zn reduction under acidic conditions. The ultrasonication

was conducted at different time (1.5, 2, and 2.5 h). Compared to other synthesis methods, the

advantages of our strategy is that this reduction process is very fast, and environmentally friendly. In

addition, the hydrothermal process gave a rGO paper which has a good quality and can be obtained at a

lower temperature process (1600C). Therefore, the resulting rGO paper exhibits excellent

electrochemical capacitive properties in neutral electrolyte, Na2SO4.

2 EXPERIMENTAL PROCEDURE

2.1 Preparation of rGO

Graphite oxide was synthesized using graphite flakes by the Hummers method. In a typical

synthesis, 2.0 g of graphite powder was added to 80 mL of cold (00C) concentrated H2SO4 in an ice

bath. Then, NaNO3 (4.0 g) and KMnO4 (8.0 g) were added gradually under stirring and the

temperature of the mixture was kept to be below 100C. The reaction mixture was continually stirred

for 4 h at temperature below 100C. Next step, the mixture was stirred at 35

0C for 4 h, and then diluted

with 200 mL of deionized (DI) water. After adding all of the 200 mL of DI water, the mixture was

stirred for 20 h. The reaction was terminated by adding 15 mL of 30% H2O2 solution. The solid

product was separated by centrifugation, washed repeatedly with 5% HCl solution until sulfate could

not be detected when titrated with BaCl2. For further purification, the resulting solid was re-dispersed

in DI water and then was washed several times until the pH was neutral. Finally, the resulting solid

was dried at 1100C for 12 h to obtain graphite oxide (GO) paper. Graphene oxide was achieved by

ultrasonication of graphite oxide paper in water for different time (1.5, 2, and 2.5 h). For the reduction

process, 1.6 gr Zn powder and 10 mL HCl (35wt%) were added into 40 mL graphene oxide solution

(1 mg/mL) solution. The mixture was stirred for 30 min. After that, 10 mL HCl (35wt%) was added

into the above solution and then maintained for a period of time to remove excess Zn powder. Finally,

the resulting rGO was collected, washed with pure water several times, and put into an autoclave and

then dried at hydrothermal temperature (1600C) for 12 h in a muffle furnace.

2.2 Characterization of Graphite Oxide Paper and rGO paper

The morphologies and structures of the samples were observed by scanning electron

microscope (SEM, Inspect S50). The ratio of C/O measure by EDS testing on SEM Jeol JSM-

7001F with INCA software. Chemical compositions and crystallite structures of the samples were

determined by XRD (X Pert Pro PANalytcal, Philips) using Cu K radiation from 10 to 900 angles. The information of functional groups was measured by Fourier transform infrared spectroscopy

instrument (FTIR, Nicolet Nexus 670). Raman spectra were recorded on Renishaw Invia Raman

Microscope, with WiRE 2.0 software.

2.3 Electrode Preparation and Electrochemical Measurements

EDLCs electrode was prepared by dipping a piece of nickel foam (10cm x 1cm) in a

suspension of rGO (1 mg/ml) under stirring, without any binders . The area which contact with the

solution was 1cm2. After 30 minutes dipping, the bath was ultrasonication for 10 minutes. Nickel

foam rod then dried at 1100C for 12 h in a muffle furnace, and then pressed using pressure machine

for 1 minutes. Finally, the electrode was dipping into the electrolyte for 5 hours, prior to be used.

All electrochemical measurements were carried out in a conventional glass electrochemistry

cell with a three-electrode system in 1 M Na2SO4 aqueous solution. All measurements conducted

using set up 3 electrode system. A platinum wire used as a counter electrode and a saturated calomel

electrode (SCE) as reference electrode. The cyclic voltammetry (CV) measurements were conducted

at different scan rates ranging from 2 to 100 mV with a potential window from -0.2 V to -0.7 V in 1

M Na2SO4, respectively. Galvanostatic charge/discharge measurements were run on from -0.2 to -0.7

V (in 1 M Na2SO4 electrolyte) at different current densities. All measurements were carried out on a

Jiehan 5000 Electrochemical Workstation Instrument.

3 RESULT AND DISCUSSION

3.1 Morphologies and Structures of rGO

The morphologies of the graphite, graphite oxide and obtained rGO were observed by SEM

and their images are shown in Figure 1. Figure 1 exhibits the different morphologies of graphite (A),

graphite oxide (B), and rGO (C-D). Graphite has a flakes structure, and opaque. On the other hands,

graphite oxide and rGO exhibits almost transparent paper structure. rGO at the high magnification

shows the structure of layer by layer, someplace shows almost single layer structure, which is very

thin and transparent. It can be seen the thin wrinkled structure and folding structure that graphene

owns intrinsically.

Figure 1: SEM images of Graphite (A), graphite oxide (B), and rGO (C-D)

The structural changes from graphite to graphite oxide and finally to rGO were investigated by

XRD measurement, and the patterns are shown in Figure 2 (a). The interlayer distance in graphite is

about 0.336356 nm, in 2= 26.50030. This is markedly expanded to 0.830268 nm, 2= 10.6556

0 in

GO due to the formation of intercalated moieties and oxygen functional groups between the layer of

GO. In rGO, at 2= 23.71180, with the interlayer distance about 0.370995 nm peak has appeared,

which indicates that most of intercalated water and oxygen fuctionalities are removed. Figure 2 (b)

show the typical XRD pattern of rGO at different ultrasonication time. It clearly seen that the broad

peak of (002) decreasing with the increasing of ultrasonication time. Also, the value of intensity also

increasing with the increasing of ultrasonication time, as seen on Figure 2 (c).

Figure 2: (a) XRD pattern of graphite, graphite oxide and rGO. (b) XRD pattern of rGO at different

ultrasonication time. (c) Intensity pattern of rGO at different ultrasonication time.

Raman technology was performed to indicate the structures of graphite, graphite oxide and rGO

by the resulting characteristic G and D bands sensitive to defects and disorder, respectively. From the

result, it clearly seen at the figure 3 (a) that all of the material consists of two dominant peaks at 1350

cm-1

and 1590 cm-1

along with a wide band extending from about 26003300 cm-1

. The peak at

around 1350 cm-1

is the D band. The D band is attributed to the presence of defects, like disruption in

the sp2 bonding (because of vacancies, heptagon and pentagon rings, edge effect, etc.), wrinkle

formation and the presence of functional groups.

The peak at around 1590 cm-1

is the G band. The G band is a characteristic of all graphitic

structures, arising due to the inplane bond-stretching motion of sp2 hybridized carbon atoms. The peak

at around 2600 cm-1

is known as the 2D (or G) band since it is an overtone of the D band. The peak

around 2930 cm-1

is a combination mode of G and D bands, often referred to as the D + G band. The

hump at about 3170 cm-1

is due to an overtone of the D band (2D).

Figure 3: (a) Raman pattern of graphite, graphite oxide and rGO. (b) Raman pattern of rGO at

different ultrasonication time

Graphite has a nearly perfect structure, indicates by small weak peak of D band, and the sharp

peak of 2D. Its also supported by the ID/IG ratio value, which is 0.16, nearly 0. Graphite oxide and

rGO has similar pattern, large D band and small weak 2D band. The ID/IG ratio is increased from 0.74

in graphite oxide to 1.70 in rGO, which suggests that more sp2 domains are formed, indicating that

reduction of GO takesplace. It also shows that the defect on the structure of rGO is increasing. Figure

3 (b) shows raman pattern of rGO at different ultrasonication time. All sampel shows the same

pattern. The value of ID/IG is increasing from the ultrasonication time 1.5 hour to 2 hour, and then

decrease again at 2.5 hour. Increasing of ID/IG value corresponding to the increasing level of defect on

material.

Figure 4: (a) FTIR pattern of graphite oxide and rGO. (b) FTIR pattern of rGO at different

ultrasonication time

Another confirmation of the reduction mechanism is obtained by the FTIR spectra. FTIR

spectra indicate that the intercalated H2O molecules and most of the oxide groups of graphite oxide

are succesfully removed at the structure of rGO. However, a small number of the epoxide groups

remain after reductions, as shown in the FTIR spectra at Figure 4 (a). Figure 4 (b) shows the FTIR

pattern at different ultrasonication time. Increasing the ultrasonication time caused almost all of the

oxide group has been succesfully removed.

EDS analysis at figure 5 also indicates that the contain smaller amounts of oxygen. The % weight of C and O around 81.98% and 18.02 %, as shows at figure 5. So, the ratio of C/O for rGO is 4.54.

Figure 5: EDS result of rGO, contain C and O element.

3.2 Electrochemical Performance of rGO

Figure 6 (a-c) shows CV curves of rGO sample with various scan rates in the range of -0.2 to -0.7 V in 1 M Na2SO4 aqueous solution , and rGO electrode exhibited fairly rectangular CV curves which is indicative of double layer capacitor behavior. Also, the current density response gradually increased with the increase of the voltage sweep rate.

Figure 6: CV curve pattern of rGO at scan rates varying from 2mV/s until 20mV/s for different

ultrasonication time (a) 1.5 hour (b) 2 hour (c) 2.5 hour and (d) comparison of capacitance values with

increasing scan rates

Element Weight

%

Atomic

%

C K 81.98 85.84

O K 18.02 14.16

Totals 100

The best value of capacitance is 350.37 F/gr at scan rate 2mV/s, higher than Xian Du, et al.,

(2010). Figure 6 (d) shows the value of capacitance at different scan rate. It appear that the capacitace

is decreasing with the increasing of scan rates. It also shows that the capacitance is decreasing with

the increasing of ultrasonication time.

Figure 7 (a-c) shows the galvanostatic charge/discharge curves of rGO tested at different

current densities for sampel from different ultrasonication time. At sampel with ultrasonication time

1.5 h, rGO exhibits a nearly linear and symmetric charge/discharge profile at different current

densities, giving capacitance as big as 368 F/gr. A small portion of IR drop happens at beginning of

discharge curve, IR drop occurs from the accumulation of DC internal resistance and the electric

current.

For sampel with ultrasonication time 2 (b) and 2.5 hour (c), the profile not really shows

triangle, indicating that charging and discharging process occurs with different time. From the curve,

its easy to look, the charging time is much longer than dicharging time, which indicate that the

performance is not so good, and it proven by the value of capacitance, which is much lower than

sampel with ultrasonication time 1.5 hour.

Figure 7: Charge-Discharge curve pattern of rGO at at different ultrasonication time (a) 1.5 hour (b) 2

hour (c) 2.5 hour and (d) comparison of capacitance values with increasing scan rates

The value of capacitance is almost contiguous with the result from CV testing. The behaviour

of decreasing capacitance with the increasing of current densities also occurs at charge discharge

testing for all sampel, as shows at figure 7 (d).

4 CONCLUSION

Reduced graphene oxide (rGO) has been succesfully synthesized from graphite powder via

Hummer's method, followed by ultrasonication at different time (1.5, 2, and 2.5 h ), continued by Zn

reduction in acidic conditions, and finally thermally treated by hydrothermal process at 1600C. It

result a transparent structure of rGO sheet, has structure of rGO from XRD test, with almost all of the

oxygen containing functional group was removed. The paper contain only C and O element, indicates

doesnt have any impurities. Raman spectra shows that rGO structure has two dominant peak, D band

and G band, with various value of ID/IG, indicate the level of defect on materials. Electrochemical test

shows that rGO exhibits high rate supercapacitive performance. The CV Testing shows a rectangular

shape as the characteristic of EDLC, shows a high capacitance up to 350.37 F/gr at scan rate 2mV/s in

the Na2SO4 solution 1M. Galvanostatic charge discharge shows the pattern almost triangular, with the

capacitance 368 F/gr at the current density 2 A/g. Both CV and Charge Discharge test shows that

increasing the ultrasonication time will decreased the capacitance of rGO. Hence, we concluded that

the best result was obtained by sample with ultrasonication time 1.5 hour.

5 ACKNOWLEDGEMENTS

This work was supported by Institut Teknologi Sepuluh Nopember and Prof Dah Shyang Tsai

from NTUST, Taiwan.

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