Effective Reduction of Graphene Oxide for Energy

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provides better reduction of graphene oxide than the chemical or the thermal reduction only, but ... needed to reduce oxygen functional groups for recovering.
New Physics: Sae Mulli, Vol. 65, No. 3, March 2015, pp. 240∼244

DOI: 10.3938/NPSM.65.240

Effective Reduction of Graphene Oxide for Energy-storage Devices Minh-Hai Tran · Hae Kyung Jeong∗ Department of Physics, Institute of Basic Science, Daegu University, Gyeongsan 712-714, Korea (Received 9 December 2014 : revised 5 February 2015 : accepted 11 February 2015)

Three kinds of reduction processes (chemical, thermal, and both) of graphene oxide for future energy-storage devices are investigated. The chemical reduction followed by the thermal reduction provides better reduction of graphene oxide than the chemical or the thermal reduction only, but the effective surface area after both processes is smaller than that after only the thermal reduction process. The smaller surface area has negative effects on the performance of the electrochemical capacitors. We, therefore, conclude that the reduction level, the graphene structure, and the effective surface area should be concerns for future energy-storage applications. PACS numbers: 81.05.±t, 81.05.Uw, 81.40.±z Keywords: Graphene, Graphene oxide, Reduced graphene oxide

I. INTRODUCTION

of the bigger grain size [8]. Electrochemical properties of thermally reduced GO (grain size of 70 µm) has also been investigated in an aqueous electrolyte, resulting in high capacitance due to the low impedance and the large surface area [9]. In this paper, we will compare the electrochemical performance of the chemically and/or thermally reduced GO (grain size of 5 µm). The chemical reduction followed by the thermal reduction provides the best reduction of graphene oxide compared to the chemical or thermal reduction only, but the effective surface area after both processes results in rather smaller than that of the only thermal reduced sample. The smaller surface area turns out to give negative effects on the performance of the electrochemical capacitors.

Graphene is a two-dimensional allotrope of carbon with the honeycomb structure, and it has the precious properties such as high thermal conductivity (∼5000 Wm−1 K−1 ) and skyscraping electron mobility (200,000 cm2 V−1 s−1 ) [1]. Moreover, graphene can be made from graphite which is cheap and has a large reserve in nature so that it has become the “rising star” for high technology research work [2]. Among many routes to produce graphene, the chemical method through graphene oxide (GO) is considered as an easy and economical way [3]. GO, however, is an insulator unfortunately so it is needed to reduce oxygen functional groups for recovering the graphene structure of high conductivity. Chemical or thermal reduction processes of GO have been investigated [4–6]. Each pathway has certain ad-

II. EXPERIMENTAL

vantages and disadvantages, resulting in different degrees GO was synthesized by the modified Brodie method as described elsewhere [10], and the process is briefly as following. Natural graphite (Sigma Aldrich, 5 µm) of 1 g was mixed with sodium chlorate (Sigma Aldrich) of 6.5 g, and the mixture was stirred in the nitric acid for 24 h. After the oxidation process, the solution was filtered many times with deionized (DI) water, and GO on the filter was dried in an oven at 65 ◦ C for overnight.

of regeneration of the graphene structure. Our recent research has been investigated the grain size effect on the electrochemical performance of the chemically reduced GO [7], finding that the smallest grain size, 5 µm, provided the best electrochemical performance due to better reduction compared to the other chemically reduced GO ∗ E-mail:

[email protected]

240 This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Effective Reduction of Graphene Oxide for Energy-storage Devices – Minh-Hai Tran · Hae Kyung Jeong

The first pathway is to reduce GO with hydrazine. GO of 100 mg dispersed in the DI water of 100 µl is reduced by hydrazine hydrate (80%, Aldrich) with the ratio (1 µl N2 H4 / 1 mg GO) at 90 ◦ C for 24 h. It was then cooled down to room temperature followed by the vacuum filtration with a lot of DI water several times and dried in an oven at 60 ◦ C for overnight. The chemically reduced GO by the hydrazine treatment was referred as CRGO. CRGO was then heated at the low temperature (350 ◦ C) for 2 h in an argon atmosphere to remove the residue related to the oxygen functional groups of GO. The chemically reduced GO followed by the thermal process was referred as T-CRGO. The second pathway is to reduce GO by the thermal treatment only at the high temperature. GO of 50 mg was placed in a round calcination boat and heated at 800 ◦ C for 1 h in the argon atmosphere. The thermally reduced GO was named as TRGO. The interlayer distance of the samples was analyzed by powder X-ray diffraction (XRD, D/MAX2500/PC, Rigaku, Japan), and the surface morphology was examined by field emission scanning electron microscope system (FESEM, S-4300, Hitachi, Japan). Fourier transformed infrared spectroscopy (FTIR, Nicolet 380, Thermo Scientific) was used to investigate functional groups. The content of carbon, nitrogen, oxygen, and hydrogen of the samples was investigated by elemental analyzer (EA, Vario EL, US). The surface area were measured at 77 K using surface area and porosity analyzer (ASAP 2420, USA). The electrochemical performance was measured using a Potentiostat/Galvanostat with the impedance spectroscopy (Bio-Logic SAS, SP-150). Each sample of 1 mg was dispersed in isopropyl alcohol solution of 1 µl by the sonication for 3 h. The mixture of 5 µl was then dropcasted onto the glassy carbon electrode (GCE, BAS Inc.) as a working electrode, and silver-silver chloride and platinum electrodes were used as a reference and a counter electrode, respectively, for a 3-electrode cell with the acid electrolyte (1 M H2 SO4 ). The cyclic voltammetry (CV) were performed at the scan rate of 50 mVs−1 , and the charge/ discharge curves were measured at 1 Ag−1 . The impedance measurement was carried with the frequency from 100 mHz to 50 kHz with the voltage amplitude of 10 mV.

241

Fig. 1. (Color online) (a) XRD and (b) FTIR results of GO, CRGO, T-CRGO, and TRGO.

III. RESULTS AND DISCUSSION Figure 1(a) shows the XRD result of GO, CRGO, TRGO, and T-CRGO. The peak near 15 degree of GO manifests that the interlayer distance of GO increased to ˚ from the characteristic interlayer distance (3.4 ˚ 6.1 A A) of the graphite after the oxidation. The peak then shifts to the higher angles, around 26 degree, after the reduction processes, indicating regeneration of the graphite structure through the reduction processes. The T-CRGO peak is the most prominent among the other peaks of the reduced samples, meaning that T-CRGO recovered the graphite structure best. Figure 1(b) presents FTIR spectra of the samples. The epoxy (C-O ∼1050 cm−1 ) peak of GO disappeared in CRGO and T-CRGO due to the oxygen interaction with the hydrazine [4] while the carbonyl (C-O ∼ 1394 cm−1 ) and carboxyl (C=O ∼ 1650 cm−1 ) peaks of GO became smaller in TRGO and T-CRGO. The amine (C-N ∼ 1250 cm−1 ) and amide (N-H ∼1565 cm−1 ) [11] peaks of CRGO indicates existence of the nitrogen functional residues after the chemical reduction. It is concluded

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Table 1. The elemental analysis (EA) results of GO, CRGO, T-CRGO, and TRGO. Element

Carbon (at %) 58.97 78.70 91.88 89.63

GO CRGO T-CRGO TRGO

Oxygen (at %) 26.44 9.14 4.13 4.29

Hydrogen (at %) 14.60 11.69 3.63 6.08

Nitrogen (at %) 0.00 0.46 0.38 0.00

C/O ratio 2.2 8.6 22.2 20.8

Table 2. The surface area analysis results of T-CRGO, and TRGO. Surface area (m2 /g) 1-3 BET Langmuir t-Plot Smicropore /S BET Pore Volume (cm3 /g) Single point (at P/Po=0.1) t-Plot (micropore) BJH adsorption BJH desortion Vmicropore /V BET Pore Size (˚ A) BET BJH desorption

Fig. 2. FE-SEM results of (a) GO, (b) CRGO, (c) TCRGO, (d) TRGO in 2 k magnification, (e) T-CRGO, and (f) TRGO in 20 k magnification.

that T-CRGO has less oxygen functional groups among the samples and consistent with the XRD result.

T-CRGO 26.06 ± 0.08 32.12 ± 0.44 Micropore 0.52 External 25.54 2.00%

TRGO 654.95 ± 4.53 892.16 ± 34.48 Micropore 27.89 External 627.06 4.26%

0.11 0.000043 0.11 0.11 0.04%

2.82 0.016 2.78 2.83 0.55%

173.80 60.25

172.17 68.89

The EA results of the samples are shown in Table 1. The C/O ratio of GO was 2.2 and the ratio increased to 8.6, 20.8, and 22.2 after each reduction process, indicating different reduction degree of each process. T-CRGO has the highest carbon atomic percentage and C/O ratio, demonstrating the highest degree of the reduction among the others, and it is consistent with the XRD and FTIR results. The SEM results of the samples are displayed in Fig. 2. GO and CRGO show the platelet-like structure while TCRGO and TRGO have the sponge-like structure. It seems that the thermal treatment expanded the grain size of the samples like a pop-corn. It is obvious that the grain of TRGO expanded to c-axis direction through the thermal process, leading it has the worm-like structure. In the larger magnification (Figs. 2(e) and (f)), T-CRGO and TRGO show the different layer cleavages, indicating different surface area and pore structure. The surface area and pore volume are summarized in Table 2. The surface area of TRGO (655 m2 g−1 ) is considerably larger than that of T-CRGO (26 m2 g−1 ), and

Effective Reduction of Graphene Oxide for Energy-storage Devices – Minh-Hai Tran · Hae Kyung Jeong

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Fig. 3. (Color online) (a) CV, (b) galvanostatic charge and discharge, (c) impedance, and (d) impedance as a function of frequency results of GO, CRGO, T-CRGO, and TRGO in the acid electrolyte.

the ratio of the surface area between micro-pore and total surface area is 2 and 4% for T-CRGO and TRGO sample, respectively, indicating that TRGO has more micro-pores than that of T-CRGO. The pore volumes of TRGO and T-CRGO were 2.82 and 0.11 cm3 g−1 , proving that more pores existed in TRGO than in T-CRGO. The thermal treatment at the high temperature, compared to the chemical treatment, results in the larger surface area and pore volumes which are positive factors for the high performance of electrochemical capacitors. Figure 3 shows the electrochemical properties of the samples in 1M H2 SO4 . TRGO presents the largest CV area in which the rectangular shape, representative of the electric double layer capacitor (EDLC), of the highest current density was shown in Fig. 3(a). The specific capacitance of the samples could be calculated from the charge and discharge curves in Fig. 3(b) and are 1.5, 8.0, 10.0, and 70.2 Fg−1 for GO, CRGO, T-CRGO, and TRGO, respectively. TRGO again has the highest capacitance among the others due to the highest surface area. The following impedance measurement proved that TRGO has the lowest impedance, and it increases in order of T-CRGO, CRGO, and GO as shown

in Figs. 3(c) and (d). The impedance of TRGO was the lowest through all frequency regimes followed by it of TCRGO, CRGO, and GO. The better reduction of GO, therefore, is not the only key factor for the high performance of the energy storage devices, and it should be concerned with the surface area and pore structure as well.

IV. CONCLUSIONS The role of thermal and/or chemical reduction of GO was investigated. The thermal treatment increased the surface area and pore volume, but it destroyed the graphitic structure. While the chemical treatment provided quite reasonable regeneration of the graphitic structure but the smaller surface area and pore volume. It is, therefore, concluded that the degree of the reduction, the graphite structure, as well as the effective surface area should be concerned for future energy storage applications

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