A Nanoporous Carbon/Exfoliated Graphite Composite For Supercapacitor Electrodes Memoria Rosi, Muhamad P. Ekaputra, Ferry Iskandar, Mikrajuddin Abdullah, and Khairurrijal# Physics of Electronic Materials Research Division Faculty of Mathematics and Natural Sciences, Institut Teknologi Bandung Jalan Ganesa 10, Bandung 40132, Indonesia # Corresponding author. E-mail address:
[email protected] Abstract. Nanoporous carbon was prepared from coconut shells using a simple heating method. The nanoporous carbon is subjected to different treatments: without activation, activation with polyethylene glycol (PEG), and activation with sodium hydroxide (NaOH)-PEG. The exfoliated graphite was synthesized from graphite powder oxidized with zinc acetate (ZnAc) and intercalated with polyvinyl alcohol (PVA) and NaOH. A composite was made by mixing the nanoporous carbon with NaOH-PEG activation, the exfoliated graphite and a binder of PVA solution, grinding the mixture, and annealing it using ultrasonic bath for 1 hour. All of as-synthesized materials were characterized by employing a scanning electron microscope (SEM), a MATLAB’s image processing toolbox, and an x-ray diffractometer (XRD). It was confirmed that the composite is crystalline with (002) and (004) orientations. In addition, it was also found that the composite has a high surface area, a high distribution of pore sizes less than 40 nm, and a high porosity (67%). Noting that the pore sizes less than 20 nm are significant for ionic species storage and those in the range of 20 to 40 nm are very accessible for ionic clusters mobility across the pores, the composite is a promising material for the application as supercapacitor electrodes. Keywords: composite, exfoliated graphite, nanoporous carbon, supercapacitor. PACS: 82.45.Gj, 82.45.Mp, 82.45.Wx, 82.47.Uv
previous study reported elsewhere [8]. The graphite layer space was increased by introducing NaOH [9]. A composite that will be used as an electrode of supercapacitor was obtained by mixing the nanoporous carbon and exfoliated graphite.
INTRODUCTION Nanoporous carbon (activated carbon) is a widely studied material for use as an electrode in a supercapacitor because of its high specific surface area, chemical stable, easy to produce, and less cost [12]. However, nanoporous carbon still have a limitation concerning to the poor of electrical conductivity [3]. As an alternative, we prefer to combine the nanoporous carbon with exfoliated graphite. The exfoliated graphite was obtained from the exfoliation of graphite intercalates by chemical agent such as polymer or alkali oxides [4-5]. In this present study, the activated carbon was produced from coconut shells by utilizing the chemical activation of sodium hydroxide (NaOH) impregnation [6] and polyethylene glycol (PEG) template method [7]. The exfoliated graphite was synthesized from graphite oxidized by zinc acetate (Zn(CH3CO2)2 or ZnAc) and intercalated with polyvinyl alcohol (PVA) and NaOH. ZnAc could oxidize the PVA as in our
EXPERIMENTS Coconut shells as a raw material were taken from Indonesia and treated by carbonizing in furnace without carrier gas at 600°C for 2 hours. The produced carbon was immediately ground to obtain refined carbon (sample A). Nanoporous carbon synthesized with a template method was prepared by dissolving sample A to PEG solution (10 wt. %; with a molecular weight of 20,000 g/mol purchased from Wako Pure Chemicals, Japan). The mixture was pyrolized at 800°C for 2 hours. The experimental procedure of template method was described in our previous study
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[10]. The nanoporous carbon obtained from this stage is labeled as sample B. The activation method using NaOH and PEG was prepared by impregnating coconut shells to NaOH solution (70 wt.%; purchased from Bratachem Indonesia) for 24 hours. The coconut shells was subsequently carbonized in furnace without a carrier gas at 600°C for 2 hours and followed by activation at 800°C for 2 hours. The carbonized coconut shells were washed with DI water to remove alkali metal and treated similar to the treatment of sample B. The sample obtained from this activation stage is marked as sample C. Exfoliated graphite was synthesized by dissolving 1.5 g of PVA to 4.5 g of ZnAc (Wako Pure Chemicals, Japan) in 50 ml of DI water. The mixture was stirred for 3 hours and followed by dissolving 6 g of graphite powder (Bratachem, Indonesia). About 0.01 M of NaOH was also added into the solution. The precipitation of this mixture was filtered and heated at 60°C for 1 hour to acquire exfoliated graphite (sample D). The nanoporous carbon with NaOH-PEG activation (60 wt.%) was mixed with the exfoliated graphite (30 wt.%) and a binder (10 wt.%) of PVA solution. The mixture were ground and annealed using ultrasonic bath for 1 hour to obtain a uniform paste of
(a)
nanoporous carbon with NaOH-PEG activation/exfoliated graphite composite. All samples were characterized by employing a JEOL JSM-6360 LA scanning electron microscope (SEM) to examine their morphologies and their pore distributions were analyzed by using a MATLAB’s image processing toolbox. Crystallinities of the samples were investigated by utilizing a Philips PW1710 x-ray diffractometer (CuKα, λ = 0.154 nm).
RESULTS AND DISCUSSION Figure 1 shows SEM images of as-synthesized samples. The carbonized coconut shell without activation (sample A) provides few pores as given in Figure 1.(a). Figure 1.(b) explains that the nanoporous with PEG activation (sample B) creates excessive number of pores. The porous structure seen in the nanoporous carbon with NaOH-PEG activation (sample C), as illustrated in Figure 1.(c), is more complicated than that in the nanoporous carbon with PEG activation (sample B). The porous structure obtained here is irregular with various shapes and sizes. The smaller pores certainly originate from the NaOH impregnation.
(b)
(d)
(c)
(e)
FIGURE 1. SEM images of inactivated carbon (a), nanoporous carbon with PEG activation (b), nanoporous carbon with NaOHPEG activation (c), exfoliated graphite (d), nanoporous carbon/exfoliated graphite composite (e).
The exfoliated graphite has particles distributed slightly homogeneously as a consequence of the graphite and PVA polymer mixing as shown in Figure 1.(d). The PVA polymer is seen as small circles filled by the graphite matrix. However, the number of pores of the exfoliated graphite is lesser than that of the
activated carbon. It might be due to the weak oxidative agent (ZnAc) that cleaves the PVA. The composite of the nanoporous carbon with NaOH-PEG activation and the exfoliated graphite, as demonstrated in Figure 1.(e), has a high particle density, which stems from the various particle sizes of the composite.
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FIGURE 2. Pore size distribution of the five samples. Samples A to C are the inactivated carbon, the nanoporous carbon activated with PEG, the nanoporous carbon activated with NaOH-PEG, respectively. Samples D and E are the exfoliated graphite and the composite of nanoporous carbon/exfoliated graphite, respectively.
FIGURE 3. X-ray diffraction patterns of inactivated carbon (a), nanoporous carbon with PEG activation (b), nanoporous carbon with NaOH-PEG activation (c), exfoliated graphite (d), nanoporous carbon/exfoliated graphite composite (e)
Pores of the SEM images were analyzed by employing the MATLAB’s image processing toolbox to obtain pore size distributions and porosities. Note that the pore sizes less than 20 nm are significant for ionic species storage [11]. From the histogram in Figure 2, the composite of nanoporous carbon/exfoliated graphite (sample E) has a high distribution of pore sizes less than 20 nm. It implies that the composite is promising for supercapacitor
electrodes. As a comparison, the inactivated carbon (sample A) has no pore sizes within the range. It tells that the activation process is very important to create small pores. The middle class pores in the range between 20 to 40 nm are very accessible for ionic clusters mobility across the pores [12]. Again, from Figure 2, the composite has the highest count for the middle class pores. The pore sizes higher than 40 nm is dominated by the nanoporous carbon activated with
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NaOH-PEG (sample C). The composite has limited pore distribution over this range. In addition, the calculated porosities are about 30, 33, 70, 43 and 67% for the samples A to E, respectively. X-ray diffraction patterns of all samples are displayed in Figure 3. The inactivated carbon (sample A) has an amorphous structure as shown in Figure 3.(a). The broad reflections of (002) between 20° and 30° in the inactivated carbon, the nanoporous carbon with PEG activation, and the nanoporous carbon with NaOH-PEG activation (samples A to C, respectively), which are given in Figures 3.(a)-3.(c), demonstrate the small domain of coherent and parallel stacking of graphene sheets. This is in agreement with the study reported in Ref. 13. The exfoliated graphite (sample D) has a crystalline structure with (002) and (004) orientations as illustrated in Figure 3.(d). Other peaks correspond to ZnAc and NaOH. The nanoporous carbon/exfoliated graphite composite (sample E) is also crystalline with (002) and (004) orientations (Figure 3.(e)). This is not surprising due to the exfoliated graphite in the composite. In addition, the number of diffraction peaks of ZnAc in the sample E is fewer and their intensities are also weakly detected compared to those in the sample D. These may come from the partial binding of ZnAc to the PVA in the composite. It verifies that ZnAc is a weak oxidative agent. This is in agreement with the SEM image in Figure 1.(d). Indeed, the diffraction peaks of NaOH in the sample E do not appear as a result of good binding of NaOH to the PVA.
species storage and those in the range of 20 to 40 nm are very accessible for ionic clusters mobility across the pores, the nanoporous carbon/exfoliated graphite composite is a good candidate for supercapacitor electrodes.
ACKNOWLEDGMENTS This research was financially supported by Institut Teknologi Bandung through the “Riset KK 2010” and “Outstanding Doctor Program” Research Grants.
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CONCLUSIONS A nanoporous carbon/exfoliated graphite composite has been successfully produced by using a simple heating method and characterized by employing a scanning electron microscope (SEM), an image processing toolbox, and an x-ray diffractometer (XRD). From SEM images, it has been found that the composite has a high particle density, which indicates that the composite has a high surface area. Using the image processing toolbox, it has been obtained that the composite owns a high distribution of pore sizes less than 40 nm and a high porosity (67%). Employing XRD, it has been observed that the composite is crystalline with (002) and (004) orientations. Since the pore sizes less than 20 nm are significant for ionic
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