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nanoneedle MnO2 reduction
Microelectronics Reliability 54 (2014) 587–594
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Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Capacitance behavior of composites for supercapacitor applications prepared with different durations of graphene/nanoneedle MnO2 reduction Myeongjin Kim a, Myeongyeol Yoo a, Youngjae Yoo b, Jooheon Kim a,⇑ a b
School of Chemical Engineering & Materials Science, Chung-Ang University, Seoul 156-756, Republic of Korea Division of Advanced Materials, Korea Research Institute of Chemical Technology, Daejeon 305-600, Republic of Korea
a r t i c l e
i n f o
Article history: Received 25 September 2013 Received in revised form 5 November 2013 Accepted 11 November 2013 Available online 7 December 2013
a b s t r a c t Graphene/MnO2 composites were prepared by hydrazine hydrate-mediated reduction of graphene oxide (GO)/MnO2 at various reduction times to determine the optimal conditions for obtaining materials with excellent electrochemical performance. Variations in the oxygen-containing surface functional groups were observed as the reduction time was varied. These changes were found to affect the electrical conductivity and density of nanoneedle MnO2, which influence the surface area and significantly affect the supercapacitive performance of the composites. Morphological and microstructural characterizations of the as-prepared composites demonstrated that MnO2 was successfully formed on the GO surface and indicated the efficacy of hydrazine hydrate as a reducing agent for GO. The capacitive properties of the graphene/MnO2 electrodes prepared at a reduction time of 28 h (rGO(28)/MnO2) exhibited a low sheet-resistance value as well as a high surface area, resulting in a GO/MnO2 composite with excellent electrochemical performance (371.74 F g1 at a scan rate of 10 mV s1). It is anticipated that the formation of MnO2-based nanoneedles on GO surfaces by the demonstrated 28-h hydrazine-reduction protocol is a promising method for supercapacitor electrode fabrication. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The increasing global demands for energy along with growing concerns regarding environmental impact (air pollution, climate change, etc.) has prompted significant research on novel methods and materials for energy storage and conversion from alternative energy sources [1–4]. Supercapacitors have rapidly emerged as attractive power sources for various energy applications currently under investigation because of their high power density, moderate energy density, good operational safety, and long cycling life [5–7]. Generally, supercapacitors can be classified into one of the following two categories on the basis of their energy storage mechanism: electric double-layer capacitors (EDLCs) and pseudocapacitors employing faradic redox reactions [8,9]. The chemical composition of electrode materials plays a dominant role in the design and development of supercapacitors. Typical electrode materials for EDLCs include high-surface-area carbon, which is convenient for storing energy in double-layer structures formed on the surface. Conversely, in the case of pseudocapacitors, most electrode materials consist of conducting polymers and metal oxides, which transfer faradic charges between an electrolyte and
⇑ Corresponding author. Tel.: +82 2 820 5763; fax: +82 2 812 3495. E-mail address: [email protected] (J. Kim). 0026-2714/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.microrel.2013.11.005
electrode. Numerous materials have been investigated as possible supercapacitor electrodes, including carbon-based materials, conductive polymers, and transition metal oxides [10,11]. Graphene has attracted tremendous attention as an electrode material for EDLCs because of its large surface area (2630 m2 g1), excellent electronic conductivity, outstanding chemical stability, and good physical properties [12–19]. In contrast, metal oxide electrode materials are highly desired for pseudocapacitor applications owing to their large capacitance and fast redox kinetics. Manganese oxide (MnO2) is generally considered to be the most promising transition metal oxide for next-generation supercapacitor materials because of its high energy density, low cost, environmental friendliness, and natural abundance [20,21]. Some studies have reported the preparation of MnO2 particles with different morphologies: the results showed that by increasing the specific area of MnO2, specific capacitances can be enhanced. Chen et al. reported the synthesis of MnO2 particles with different crystallographic forms (a, c) and morphologies (needles, rods, and spindles), and investigated their electrochemical performance. They reported that the needle-like samples exhibited a higher specific capacitance than other examined forms did [22]. Likewise, Devaraj and Munichandraiah investigated the effect of the MnO2 crystallographic structure on its electrochemical capacitance properties and found that MnO2 with a and d crystallographic structures exhibited higher specific capacitances [23]. In addition,
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Chen et al. created electrode materials by combining graphene oxide (GO) with nanoneedle MnO2 structures and demonstrated that such structures can be homogeneously dispersed on GO sheets using various functional groups and that their edges act as reactive sites on GO surfaces [24]. Furthermore, they found that the specific capacitance of GO/MnO2 electrodes gradually increases with increasing MnO2 content. However, Chen et al. obtained a relatively lower specific capacitance value (197.2 F g1 at 0.2 A g1) compared to other reported graphene/MnO2 composites, because GO cannot form electronic conductive channels. To enhance the electrochemical performance of GO/MnO2 composite electrodes, Mao et al. suggested a new reaction method, whereby MnO2 nanoneedles were formed directly on graphene sheets in order to take advantage of their excellent electrical conductivity [25]. Improved electrochemical performances were expected owing to the outstanding electrical conductivity, EDL capacitance of graphene, and pseudocapacitive reactivity of MnO2. However, the specific capacitances of the graphene/MnO2 electrodes were found to decrease with increasing MnO2 content. This result was attributed to diffusion limitations, as a steeper vertical line was observed in the Nyquist plot at low frequencies with decreasing MnO2 content, primarily owing to the dispersion features of MnO2 nanoneedles. Moreover, MnO2 nanoneedles are unable to form the desired homogeneous dispersion owing to the absence of functional groups on graphene sheets. Therefore, graphene/MnO2 composites prepared with a high feed ratio of MnO2 exhibit the properties of graphene sheets with partly aggregated MnO2, both of which cannot participate in pseudocapacitive reactions. In order to obtain a graphene/MnO2 composite exhibiting the synergistic effects of EDL capacity, excellent electrical conductivity of graphene, and pseudocapacitive reactivity, we recently reported a procedure to fabricate a graphene/nanoneedle MnO2 composite as follows: First, MnO2 nanoneedles were formed on GO sheets using various functional groups to obtain a homogeneous dispersion of nanosized MnO2 particles. In the second stage, to compare the reducing effects of hydrazine hydrate and sodium borohydride, GO/MnO2 was reduced to graphene/MnO2 via an immersion method [26]. We found that the hydrazine reduction is more effective in increasing the hydrocarbon component because hydrazine reduction is known to form hydrocarbons from carbonyl groups, whereas sodium borohydride reduction forms residual hydroxyl functional groups. However, the optimum reduction conditions such as the concentration of reducing agent and reduction time were not evaluated. In this study, graphene/MnO2 composites exhibiting excellent electrochemical performances were prepared via reduction of GO/MnO2 at various reduction times using fixed concentrations of hydrazine hydrate to determine the optimal reduction time. Changes in the oxygen-containing functional groups were observed with variation in reduction times, resulting in marked effects on both electrical conductivity and MnO2 nanoneedle density, which impacts the surface area and can significantly affect the supercapacitive performance. The morphology and microstructural characteristics of graphene/MnO2 were investigated in detail. The electrochemical performances of the prepared electrodes were also investigated using cyclic voltammetry (CV), galvanostatic charge/discharge experiments, electrochemical impedance spectroscopy (EIS), and analysis of the specific capacitance and capacitance retention.
order to synthesize GO/MnO2 at a ratio of 1:5, GO (0.05 g) and MnCl24H2O (0.34 g) were dispersed in isopropyl alcohol (50 mL) with ultrasonication for 30 min. Subsequently, the mixture was heated to approximately 85 °C in a water-cooled condenser with vigorous stirring. KMnO4 (0.18 g) dissolved in DI water (5 mL) was then added to the abovementioned boiling solution under vigorous stirring for 30 min. The obtained mixture was filtered and washed several times, and finally dried in a vacuum oven at 60 °C for 24 h. Hydrazine hydrate-reduced graphene/MnO2 (rGO/ MnO2) was obtained by an immersion method. In order to compare the reducing effect as a function of the reduction time, GO/MnO2 was immersed in an aqueous hydrazine hydrate (30 mM) solution for various reaction durations (20, 24, 28, 32, and 36 h). The resulting rGO/MnO2 composites were collected by filtration, washed with DI water several times, and dried in a vacuum oven at 90 °C for 24 h. The products are denoted as rGO(20)/MnO2, rGO(24)/ MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/MnO2, corresponding to the reduction times of 20, 24, 28, 32, and 36 h, respectively. 2.2. Characterization methods The X-ray crystallographic patterns of the as-prepared composites were collected using an X-ray diffractometer (XRD, New D8Advance/Bruker-AXS) at a scan rate of 1° s1 in the 2h range 5– 70° with Cu Ka1 radiation (k = 0.154056 nm). Field-emission scanning electron microscopy (FE-SEM, SIGMA, Carl Zeiss) was used to examine the morphology of the as-prepared samples. X-ray photoelectron spectroscopy (XPS) analysis was carried out using a VGMicrotech, ESCA2000 system, equipped with a spectrometer with an Mg Ka X-ray source (1253.6 eV) and a hemispherical analyzer. In the curve fitting, the Gaussian peak widths were constant in each spectrum. Thermal gravimetric analyses (TGA) were carried out on a TA instrument (TGA-2050) at a heating rate of 10 °C min1 in air. The sheet resistance was measured using a four-point probe method (ChangMin, LTD, CMT-SR2000N, Korea). Brunauer–Emmett–Teller (BET) surface areas were determined with an Autosorb-iQ 2ST/MP (Quantachrome) system using nitrogen as an adsorptive agent at 77 K. Prior to measurements, the samples were degassed in vacuum at 200 °C for 12 h. The fabrication of working electrodes was carried out as follows. The as-prepared materials, carbon black, and poly(vinylidene fluoride) (PVDF) were mixed at a mass ratio of 75:20:5 and dispersed in N-methylpyrrolidone (NMP). The resulting mixture was then coated onto a nickel foam substrate (1 cm 1 cm) and dried in a vacuum oven at 60 °C for 6 h. The loading mass of each electrode was ca. 2.7–3.2 mg. To investigate the electrochemical behavior of the as-prepared samples, cyclic voltammetry (CV), galvanostatic charge/discharge experiments, and electrochemical impedance spectroscopy (EIS) measurements were conducted in the three-electrode mode using the as-prepared samples, platinum foil, and Ag/AgCl (KCl-saturated) as the working, counter, and reference electrodes, respectively. The measurements were carried out in a 1 M Na2SO4 aqueous electrolyte at room temperature. The CV, galvanostatic charge/discharge characteristics, and EIS measurements were performed using a CHI 660C electrochemical workstation. The EIS measurements were recorded under AC voltage amplitude of 5 mV in the frequency range 105–0.1 Hz and an open circuit potential.
2. Experimental 3. Results and discussion 2.1. Synthesis of materials Graphene oxide (GO) was prepared from powdered flake graphite by a modified Hummers method as described previously [26]. In
The procedure for the formation of rGO/MnO2 composites comprises the following three steps: (i) synthesis of GO from graphite powder, (ii) formation of the MnO2 nanoneedles on GO surfaces
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and edges according to the reaction 2KMnO4 + 3MnCl2 + 2H2O ? 5MnO2 + 2KCl + 4HCl, and (iii) hydrazine hydrate-mediated chemical reduction of GO/MnO2 at varying immersion times to investigate the reducing effect as a function of reaction time. First, it was essential to confirm the introduction of MnO2 nanoneedles on the GO sheets prior to the reduction process. Fig. 1 shows the XRD patterns of the as-prepared GO and GO/MnO2, confirming the formation of nanoneedle-like structures of MnO2. The XRD pattern of GO revealed an intense peak around 2h = 11.5°, corresponding to the (0 0 1) reflection, which indicated that the interlayer spacing is much larger than that of pristine graphite owing to the introduction of oxygen-containing functional groups on the graphite sheets [27]. The diffraction peaks of the as-synthesized GO/MnO2 were similar to those of the nanotetragonal phase of a-MnO2 (JCPDS 44-0141, a = 9.7845 Å, c = 2.8630 Å), and the (0 0 1) reflection peak of layered GO is almost invisible, suggesting that homogeneous composites are indeed formed by the deposition of needle-like nanoscale MnO2 structures on the surface of graphene oxide sheets [22,24,25]. This result correlates well with the results of previous studies that showed that the diffraction peak heights decrease or the peaks disappear upon exfoliation of regular stacks of GO [28]. The formation of MnO2 nanoneedles can be attributed to a disruptive effect. As reported by Chen et al., in a DI water/isopropyl alcohol system, the coordination of H2O and isopropyl alcohol with the O atoms of MnO6 has a disruptive effect on the packing of the growing crystal species. The intermolecular hydrogen bonds formed among the coordinated H2O molecules favor the formation of highly ordered aggregates, whereas a shortage of such strong intermolecular interactions in isopropyl alcohol-coordinated species results in a disordered crystal. Therefore, the overall system has only one preferable crystal growth direction [24]. Surface areas and pore volumes are the most important factors determining the supercapacitive performance of graphene/MnO2 composites. High specific surface areas and the presence of mesopores are favorable for improving both the main pseudocapacitance of MnO2 nanoneedles and the EDL capacitance of graphene because the hydrated ions in the electrolyte are easily accessible to the exterior and interior pore surfaces [29]. Table 1 summarizes the BET surface area and pore volume behavior as a function of reduction time. In order to determine the surface area and porosity, liquid nitrogen cryosorption was performed on the as-prepared
Fig. 1. XRD patterns of GO and GO/MnO2.
Table 1 BET surface area and pore volume of GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/MnO2.
GO/MnO2 rGO(20)/MnO2 rGO(24)/MnO2 rGO(28)/MnO2 rGO(32)/MnO2 rGO(36)/MnO2
BET surface area (m2 g1)
Pore volume (cm3 g1)
114.26 112.41 109.74 105.23 74.75 38.63
0.72 0.71 0.70 0.68 0.47 0.32
rGO/MnO2 composites. The surface area and pore volume of rGO/ MnO2 decreased with increasing hydrazine reduction time. It can be assumed that removal of MnO2 nanoneedles during the reduction process decreases the surface area and pore volume because MnO2 nanoneedles are beneficial for pore formation and specific area enlargement. At reduction times greater than 28 h, the surface area and pore volume of the rGO/MnO2 composites decreased dramatically, indicating that significant changes in the composite MnO2 proportions occurred, likely because of the strong reduction conditions employed. The compositions of the as-prepared GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/MnO2 composites were further investigated by TGA (Fig. 2). The experiments were performed up to 700 °C in air at a heating rate of 10 °C min1. Under these conditions, the GO and rGO sheets were decomposed, while MnO2 was converted into Mn2O3 [30]. Fig. 2 shows the representative TGA curves of the nanoneedles of MnO2 (MnO2), GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, rGO(36)/MnO2, GO, and rGO. The typical weight losses of MnO2, GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, rGO(36)/MnO2, GO, and rGO were 12.5%, 21.9%, 23.1%, 23.9%, 24.6%, 31.2%, 34.1%, 92.1%, and 100%, respectively. Accordingly, the mass ratios of MnO2/(GO or rGO) for GO/MnO2, rGO(20)/MnO2, rGO(24)/ MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/MnO2 were 6.92:1, 6.74:1, 6.24:1, 5.84:1, 3.48:1, and 2.9:1, respectively. The mass ratios of MnO2/(GO or rGO) for rGO/MnO2 composites were observed to decrease with increasing reduction time, because hydrazine hydrate removed not only the functional groups on the GO sheets (i.e., hydroxyl, epoxy, and carboxylic groups), but
Fig. 2. TGA curves of MnO2, GO, rGO, GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/MnO2.
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also the functional groups bonded to MnO2. As exposure to hydrazine approached 28 h, the proportion of MnO2 in the composites decreased slightly. In contrast, reduction times greater than 28 h were accompanied by a dramatic decrease in the proportion of MnO2 in the composites because of the elimination of most oxygen-containing functional groups during the reduction of GO to graphene, which is in accordance with the surface area and pore volume analysis (Table 1). Detailed surface information of GO and rGO/MnO2 at the different hydrazine reduction times examined was collected by XPS; the corresponding results are presented in Fig. 3. Fig. 3(a) shows the spectra of GO and GO/MnO2 and the deconvoluted Mn 2p core-level spectrum of the latter. Only two elements were present in the spectrum of GO, namely C and O. However, Mn signals (2p3/2, 2p1/ 2) appeared in the spectra of GO/MnO2, implying that MnO2 nanoneedles were successfully formed on the surface of the GO sheets. In the Mn 2p spectrum, the Mn 2p3/2 and 2p1/2 peaks were centered at 642.6 and 654.2 eV, respectively. Although the observed peak positions were slightly different, a consistent spin-energy
separation of 11.6 eV was observed between the Mn 2p3/2 and Mn 2p1/2 peaks. These results are also in accordance with the previously reported data for Mn 2p3/2 and Mn 2p1/2 in a MnO2 nanoneedle [25]. Fig. 3(b) shows the wide-scan spectra of GO/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, and rGO(32)/MnO2. As reduction time was increased, the intensity of the O 1s peak greatly decreased, indicating that hydrazine hydrate was effective in removing the oxygen-containing functional groups. Interestingly, the Mn 2p peak also showed a gradual decrease in intensity with increasing hydrazine reduction time. The reason for these decreases in the Mn 2p3/2 and Mn 2p1/2 peak intensity is as follows. Various oxygencontaining functional groups (i.e., epoxy, hydroxyl, carbonyl, and carboxyl) present on the surfaces and edges make the formation of an MnO2 nanoneedle on the GO surface possible. These functional groups, acting as anchor sites, enable the formation of nanostructures on the surfaces and edges of the GO sheets. However, during the hydrazine reduction process, hydrazine hydrate removes not only the functional groups on the GO sheets but also the functional groups bonded to MnO2. The atomic ratio of carbon and oxygen was obtained by calculating the ratio of the C 1s to O 1s peak areas in the corresponding XPS spectra. This ratio represents the degree of reduction, which could be altered by varying the reduction time. In order to directly compare the effects of hydrazine concentrations, the sheet resistance was measured for the pressed pellets of samples using a four-probe method. Table 2 summarizes the differences among the GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/ MnO2 composites in terms of their atomic ratio of carbon and oxygen and resulting sheet resistance. As the reduction time increased, the oxygen content of the rGO/MnO2 composite markedly decreased. The appearance of a peak corresponding to a C–N bond was observed, which increased in area because of the bond-forming events associated with hydrazones [31]. Electrical conductivity is known to be directly related to the oxygen content. Therefore, sheet resistance decreased as the hydrazine concentration was increased with increasing reduction time because it yields the lowest oxygen concentrations [26]. The FE-SEM images of GO, GO/MnO2, rGO(24)/MnO2, rGO(28)/ MnO2, rGO(32)/MnO2, and rGO(36)/MnO2 are shown in Fig. 4. Fig. 4(a) and (b) shows GO and GO/MnO2, respectively. Their comparison clearly reveals that the GO sheets were homogeneously decorated with MnO2 nanoneedles, in agreement with the XRD observations. Fig. 4(c)–(f) shows the images of rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/MnO2, respectively. No significant differences in the morphology and density of MnO2 nanoneedles in the rGO/MnO2 composites were apparent up to the 28-h reduction time. However, at reduction times greater than 28 h, MnO2 nanoneedle densities decreased dramatically with increasing time, as the strong reduction conditions effected the removal of a large amount of oxygen-containing functional groups bonded to both the MnO2 and GO sheets. These decreases in the MnO2 nanoneedle densities observed for rGO(32)/MnO2 and rGO(36)/MnO2 composites are responsible for decreases in the surface area and pore volume, resulting in slow ion diffusion
Table 2 Atomic ratio of carbon and oxygen in GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/MnO2, and their sheet resistances.
Fig. 3. XPS spectra of GO, GO/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, and rGO(32)/ MnO2. (a) Wide scan survey spectra of GO and GO/MnO2 (the inset illustrates the narrow spectra of Mn 2p peaks of the GO/MnO2 composite) and (b) wide scan survey spectra of GO/MnO2, rGO(24)/MnO2, rGO(28)/MnO2 and rGO(32)/MnO2.
GO/MnO2 rGO(20)/MnO2 rGO(24)/MnO2 rGO(28)/MnO2 rGO(32)/MnO2 rGO(36)/MnO2
C:O ratio
Sheet resistance (KO sq1)
2.51 4.22 4.31 4.46 4.68 4.83
9016 637 524 416 377 332
M. Kim et al. / Microelectronics Reliability 54 (2014) 587–594
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Fig. 4. FE-SEM images of GO, GO/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/MnO2. (a) GO, (b) GO/MnO2, (c) rGO(24)/MnO2, (d) rGO(28)/MnO2, (e) rGO(32)/MnO2 and (f) rGO(36)/MnO2.
in the charge/discharge processes. Moreover, a reduced proportion of nanoneedle MnO2 inhibits the redox reaction that generates pseudocapacitance.
Fig. 5. CV curves of GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/MnO2 electrodes at 10 mV s1.
To explore potential applications for supercapacitors, the samples were fabricated into the supercapacitor electrodes and characterized by CV, galvanostatic charge/discharge experiments, and EIS measurements. Fig. 5 shows the CV curves of the GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/MnO2 electrodes at a scan rate of 10 mV s1 in 1 M Na2SO4 electrolyte. All CV curves were almost rectangular, indicating that the composites show ideal capacitive behavior. It is obvious that all the rGO/MnO2 electrodes examined showed much larger integrated areas under the CV curves than GO/MnO2, suggesting superior electrochemical performance, which can be attributed to the excellent electrical conductive properties of graphene. It should be remembered that graphene oxide cannot form electronic conductive channels because of its extremely low electrical conductivity. The integrated areas of the rGO/MnO2 electrodes gradually increased up to a reduction time of 28 h, which is attributed to the formation of electronic conductive channels by the removal of oxygen-containing functional groups [32]. Therefore, the reduced GO acts not only as a support for the formation of MnO2 particles but also provides electrically conductive channels [33]. However, as shown in Table 2, although rGO(32)/MnO2 and rGO(36)/MnO2 electrodes exhibited much lower sheet resistance than rGO(28)/MnO2, the rGO(28)/MnO2 electrode possessed the largest integrated area among the electrodes. It is presumed that this phenomenon arises because of the removal of a large amount of oxygen-containing functional groups bonded to MnO2 (that include hydroxyl, epoxy, and carboxylic groups on the GO sheets) under the strong reduction conditions. Consequently, a decrease
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in the formation of MnO2 nanoneedles occurs, as confirmed by the TGA analysis. This decreased MnO2 density leads to reduced redox reactivity as well as a decreased surface area, allowing for developing pores to act as ion-buffering reservoirs, which improves the diffusion rate of Na+ ions and generates a suitable nanoneedle alignment. This, in turn, can provide well-ordered tunnels that are convenient for the insertion/extraction of Na+ cations both into and from MnO2 [24]. Moreover, MnO2 nanoparticles can greatly reduce the diffusion lengths over which Na+ ions must migrate during the charge/discharge processes, thereby improving the electrochemical utilization of MnO2, which was further confirmed in the EIS study. Furthermore, the residual oxygen functional groups (i.e., hydroxyl, carboxyl, and epoxy) remaining on the external surface after the reduction process serve as a passage for ions to the internal surface and are thus important to the charging/discharging process. These functional groups were also reported to contribute to fast redox processes occurring at or near the electrode surface, which can provide pseudocapacitance [34–36]. The galvanostatic charge/discharge measurements of the GO/ MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/ MnO2, and rGO(36)/MnO2 electrodes were carried out in 1 M Na2SO4 over the voltage range 0.1 to 0.9 V at a current density 10 mA cm2. As illustrated in Fig. 6, during the charging and discharging steps, the charge curves for all electrodes were almost symmetric to their corresponding discharge counterparts with a slight curvature, indicating pseudocapacitive and double-layer contributions. However, when comparing the discharging curves, the rGO(28)/MnO2 electrode exhibited a much longer discharge time, which is consistent with the specific capacitance behavior because the discharging time is directly proportional to the specific capacitance of an electrode [37]. EIS experiments were carried out to further investigate the electrochemical and structural characteristics of the GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/MnO2 electrodes (Fig. 7). All Nyquist plots were semicircular over the high-frequency range and linear in the low-frequency section. The intercept for the real component at the beginning of the semicircle indicated the combined series resistance (Rs) of the electrolyte, electrode, current collectors, and the electrode/current collector contact resistance [38]. The resistances of the electrolyte and current collectors for all electrodes were
Fig. 7. Nyquist plots of GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/MnO2 electrodes.
equivalent. A major difference is the semicircle in the high frequency range, which corresponds to the charge-transfer resistance (Rct) caused by the Faradaic reactions and the double layer capacitance (Cdl) on the grain surface. The slope of the 45° portion of the curve is called the Warburg resistance (Zw) and is a result of the frequency dependence of ion diffusion/transport in the electrolyte to the electrode surface. CL is the limit capacitance. The intercepts of the rGO/MnO2 electrodes ranged from 0.6 to 1.03 O cm2, which were much lower than the value for the GO/MnO2 electrode (1.76 O cm2). Furthermore, the combined series resistance of the rGO/MnO2 electrodes exhibited a gradual decrease with increasing reduction time owing to the increase in the electrical conductivity caused by the removal of oxygen-containing functional groups during GO/MnO2 reduction. The diameter of the semicircle represents the Rct at the interface between the electrode material and electrolyte [39]. It is apparent that the values of Rct gradually decreased with increasing reduction time up to 28 h, which is in accordance with the TGA results. However, for reduction times greater than 28 h, the diameter of the semicircle decreased dramatically because of the removal of a large amount of MnO2. The vertical line at very low frequencies is caused by ion accumulation at the bottom of the pores of the electrode. The almost vertical line demonstrates good capacitive behavior without diffusion limitations [25]. The straight line at the low frequencies of the GO/ MnO2 electrode shows a gradual inclination compared to other electrodes. This indicates slower ion diffusion in the electrolyte and adsorption onto the electrode surface because of the removal of a large amount of MnO2, which can increase the specific area and form pores for ion-buffering reservoirs to improve the diffusion rate of Na+ ions and reduce the diffusion length of ions within the pseudocapacitive phase. These results also agree with the results obtained from the BET surface area and pore volume analysis. The specific capacitances (Cs) were calculated from the CV curves using the following equation [40]:
Cs ¼
Fig. 6. Galvanostatic charge/discharge curves of GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2 and rGO(36)/MnO2 electrodes at a current density of 10 mA cm2.
1 v ðV c V a Þ
Z
Vc
IðVÞdV
ð1Þ
Va
where Cs is the specific capacitance (F g1), v is the potential scan rate (mV s1), Vc Va represents the sweep potential range (V), and I(V) denotes the response current density (A g1). Fig. 8 indicates the relationships between Cs and the scan rates of the as-pre-
M. Kim et al. / Microelectronics Reliability 54 (2014) 587–594
Fig. 8. Specific capacitance of GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/ MnO2, rGO(32)/MnO2, and rGO(36)/MnO2 electrodes at different scan rates of 5, 10, 20, 50, 100, 200, 300, 400, and 500 mV s1.
593
Fig. 9. Ragone plot of the estimated specific energy and specific power of GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/MnO2 electrodes at various scan rates of 5, 10, 20, 50, 100, 200, 300, 400, and 500 mV s1.
pared GO/MnO2 and rGO/MnO2 electrodes prepared at various reduction times. The calculated Cs values of the GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/MnO2 electrodes at 10 mV s1 were 111.76, 325.18, 347.62, 371.74, 261.47, and 211.81 F g1, respectively. The rGO(28)/MnO2 electrode showed the highest specific capacitance over the entire range of scan rates. This result can be attributed to both the intrinsic EDL capacitance of graphene resulting from the hydrazine reduction process, and the pseudocapacitive reaction due to the homogeneous dispersion of the nanosized MnO2. Moreover, the Cs values for all electrodes decreased steadily with increasing scan rate owing to the reduced access of ions to the active surface, especially with relatively slow faradic reactions [41]. Power density and energy density, important parameters for the determination of electrochemical performance, were also used to characterize the GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/MnO2 electrodes. Both power density and energy density can be estimated from the following equations [42]:
E¼
1 CðDVÞ2 2
ð2Þ Fig. 10. Cycling stability of the rGO(28)/MnO2 electrode measured at 10 mV s1.
E P¼ t
ð3Þ
where P, C, DV, t, and E represent the power density (W kg1), specific capacitance based on the mass of the electroactive material (F g1), the potential window of discharge (V), discharge time (s), and energy density (Wh kg1), respectively. The Ragone plots for all electrodes at different scan rates are presented in Fig. 9. At a high scan rate of 500 mV s1, the rGO(28)/MnO2 electrode gave a calculated energy density of 41.87 Wh kg1 and power density of 75.3 kW kg1, which are the highest values for these particular parameters among all electrodes investigated. These results suggest that the synergistic effects of the EDL capacity, excellent electrical conductivity of graphene, and pseudo-capacitive reaction result in an increased surface area and a decreased diffusion path because of the homogeneously dispersed MnO2 nanoneedles. Cycle lifetime is particularly important for supercapacitor applications. Typical problems associated with the MnO2-based electrodes in aqueous electrolytic media include mechanical
expansion of MnO2 during the ion insertion/desertion process, MnO2 detachment from the electrode surfaces, and dissolution of Mn into the electrolyte [43]. A cyclic stability test of the rGO(28)/ MnO2 electrode was performed over 1000 cycles at a scan rate of 10 mV s1 and a potential window range between 0.1 and 0.9 V. Fig. 10 illustrates the specific capacitance retention as a function of cycle number. The rGO(28)/MnO2 electrode exhibited a specific capacitance loss of less than 12.4% after 1000 charge/discharge cycles, demonstrating its excellent capacity retention.
4. Conclusions Graphene/MnO2 composites were obtained via the hydrazine reduction of GO/MnO2 with various reduction times. The structural characteristics and electrochemical properties of the obtained composites were evaluated for potential supercapacitor applica-
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tions. The rGO(28)/MnO2 electrode showed the highest specific capacitance (371.74 F g1 at a scan rate of 10 mV s1) among the electrodes examined. These experimental results are attributed to the synergistic effects of the EDL capacity, excellent electrical conductivity of graphene, and homogeneously dispersed MnO2 nanoneedles. There is also an increased surface area, which can form pores for ion-buffering reservoirs and effectively decrease the diffusion path and provide pseudocapacitance. The sheet resistance of rGO(28)/MnO2 was much lower than that of rGO(24)/MnO2. Moreover, a comparison of rGO(28)/MnO2 and rGO(32)/MnO2 electrodes showed that the latter had a much lower surface area and pore volume than the former, although rGO(32)/MnO2 exhibited a lower sheet resistance value. These observations are ascribed to the removal of most oxygen-containing functional groups bonded to MnO2 because of extended hydrazine exposure, in addition to the elimination of hydroxyl, epoxy, and carboxylic groups present on the GO sheets. After 1000 cycles, the capacitance of the rGO(28)/ MnO2 electrode decreased by 12.4% from the initial value, demonstrating its excellent electrochemical stability. Therefore, a reduction time of 28 h was established as optimal for improved electrochemical properties, indicating that the hydrazine-mediated reduction of GO/MnO2 is a promising method for the synthesis of electrode materials for supercapacitors. Acknowledgements This work was supported by the Technological Innovation R&D Program (S2085171) funded by the Small and Medium Business Administration (SMBA, Korea). References [1] Subramanian V, Zhu H, Vajtai R, Ajayan PM, Wei B. Hydrothermal synthesis and pseudocapacitance properties of MnO2 nanostructures. J Phys Chem B 2005;109:20207–14. [2] Mohana AL, Estaline RF, Imran A, Ramaprabhu JS. Asymmetric flexible supercapacitor stack. Nanoscale Res Lett 2008;3:145–51. [3] Karina AC-G, Monica L-C, Nieves C-P, Pedro G-R. Nanocomposites hybrid molecular materials for application in solid-state electrochemical supercapacitors. Adv Funct Mater 2005;15:1125–33. [4] Chow J, Kopp RJ, Portney PR. Energy resources and global development. Science 2003;302:1528–31. [5] Wen Z, Wang X, Mao S, Bo Z, Kim H, Cui S, et al. Crumpled nitrogen-doped grapheme nanosheets with ultrahigh pore volume for high-performance supercapacitor. Adv Mater 2012;24:5610–6. [6] Kotz R, Carlen M. Principles and applications of electrochemical capacitors. Electrochim Acta 2000;45:2483–98. [7] Chen Z, Qin Y, Weng D, Xiao Q, Peng Y, Wang X, et al. Design and synthesis of hierarchical nanowire composites for electrochemical energy storage. Adv Funct Mater 2009;19:3420–6. [8] Bose S, Kuila T, Mishra AK, Rajasekar R, Kim NH, Lee JH. Carbon-based nanostructured materials and their composites as supercapacitor electrodes. J Mater Chem 2012;22:767–84. [9] Jiang H, Ma J, Li CZ. Mesoporous carbon incorporated metal oxide nanomaterials as supercapacitor electrodes. Adv Mater 2012;24:4197–202. [10] Simon P, Gogotsi Y. Materials for electrochemical capacitors. Nat Mater 2008;7:845–54. [11] Chou SL, Wang JZ, Chew SY, Liu HK, Dou SX. Electrodeposition of MnO2 nanowires on carbon nanotube paper as free-standing, flexible electrode for supercapacitors. Electrochem Commun 2008;10:1724–7. [12] Yang S-Y, Chang K-H, Tien H-W, Lee Y-F, Li S-M, Wang Y-S, et al. Design and tailoring of a hierarchical graphene-carbon nanotube architecture for supercapacitors. J Mater Chem 2011;21:2374–80. [13] Wang H, Casalongue HS, Liang Y, Dai H. Ni(OH)2 nanoplates grown on graphene as advanced electrochemical pseudocapacitor materials. J Am Chem Soc 2010;132:7472–7. [14] Qiu L, Yang X, Gou X, Yang W, Ma Z-F, Wallace GG, et al. Dispersing carbon nanotubes with graphene oxide in water and synergistic effects between graphene derivatives. Chem-Eur J 2010;16:10653–8.
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Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Capacitance behavior of composites for supercapacitor applications prepared with different durations of graphene/nanoneedle MnO2 reduction Myeongjin Kim a, Myeongyeol Yoo a, Youngjae Yoo b, Jooheon Kim a,⇑ a b
School of Chemical Engineering & Materials Science, Chung-Ang University, Seoul 156-756, Republic of Korea Division of Advanced Materials, Korea Research Institute of Chemical Technology, Daejeon 305-600, Republic of Korea
a r t i c l e
i n f o
Article history: Received 25 September 2013 Received in revised form 5 November 2013 Accepted 11 November 2013 Available online 7 December 2013
a b s t r a c t Graphene/MnO2 composites were prepared by hydrazine hydrate-mediated reduction of graphene oxide (GO)/MnO2 at various reduction times to determine the optimal conditions for obtaining materials with excellent electrochemical performance. Variations in the oxygen-containing surface functional groups were observed as the reduction time was varied. These changes were found to affect the electrical conductivity and density of nanoneedle MnO2, which influence the surface area and significantly affect the supercapacitive performance of the composites. Morphological and microstructural characterizations of the as-prepared composites demonstrated that MnO2 was successfully formed on the GO surface and indicated the efficacy of hydrazine hydrate as a reducing agent for GO. The capacitive properties of the graphene/MnO2 electrodes prepared at a reduction time of 28 h (rGO(28)/MnO2) exhibited a low sheet-resistance value as well as a high surface area, resulting in a GO/MnO2 composite with excellent electrochemical performance (371.74 F g1 at a scan rate of 10 mV s1). It is anticipated that the formation of MnO2-based nanoneedles on GO surfaces by the demonstrated 28-h hydrazine-reduction protocol is a promising method for supercapacitor electrode fabrication. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The increasing global demands for energy along with growing concerns regarding environmental impact (air pollution, climate change, etc.) has prompted significant research on novel methods and materials for energy storage and conversion from alternative energy sources [1–4]. Supercapacitors have rapidly emerged as attractive power sources for various energy applications currently under investigation because of their high power density, moderate energy density, good operational safety, and long cycling life [5–7]. Generally, supercapacitors can be classified into one of the following two categories on the basis of their energy storage mechanism: electric double-layer capacitors (EDLCs) and pseudocapacitors employing faradic redox reactions [8,9]. The chemical composition of electrode materials plays a dominant role in the design and development of supercapacitors. Typical electrode materials for EDLCs include high-surface-area carbon, which is convenient for storing energy in double-layer structures formed on the surface. Conversely, in the case of pseudocapacitors, most electrode materials consist of conducting polymers and metal oxides, which transfer faradic charges between an electrolyte and
⇑ Corresponding author. Tel.: +82 2 820 5763; fax: +82 2 812 3495. E-mail address: [email protected] (J. Kim). 0026-2714/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.microrel.2013.11.005
electrode. Numerous materials have been investigated as possible supercapacitor electrodes, including carbon-based materials, conductive polymers, and transition metal oxides [10,11]. Graphene has attracted tremendous attention as an electrode material for EDLCs because of its large surface area (2630 m2 g1), excellent electronic conductivity, outstanding chemical stability, and good physical properties [12–19]. In contrast, metal oxide electrode materials are highly desired for pseudocapacitor applications owing to their large capacitance and fast redox kinetics. Manganese oxide (MnO2) is generally considered to be the most promising transition metal oxide for next-generation supercapacitor materials because of its high energy density, low cost, environmental friendliness, and natural abundance [20,21]. Some studies have reported the preparation of MnO2 particles with different morphologies: the results showed that by increasing the specific area of MnO2, specific capacitances can be enhanced. Chen et al. reported the synthesis of MnO2 particles with different crystallographic forms (a, c) and morphologies (needles, rods, and spindles), and investigated their electrochemical performance. They reported that the needle-like samples exhibited a higher specific capacitance than other examined forms did [22]. Likewise, Devaraj and Munichandraiah investigated the effect of the MnO2 crystallographic structure on its electrochemical capacitance properties and found that MnO2 with a and d crystallographic structures exhibited higher specific capacitances [23]. In addition,
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Chen et al. created electrode materials by combining graphene oxide (GO) with nanoneedle MnO2 structures and demonstrated that such structures can be homogeneously dispersed on GO sheets using various functional groups and that their edges act as reactive sites on GO surfaces [24]. Furthermore, they found that the specific capacitance of GO/MnO2 electrodes gradually increases with increasing MnO2 content. However, Chen et al. obtained a relatively lower specific capacitance value (197.2 F g1 at 0.2 A g1) compared to other reported graphene/MnO2 composites, because GO cannot form electronic conductive channels. To enhance the electrochemical performance of GO/MnO2 composite electrodes, Mao et al. suggested a new reaction method, whereby MnO2 nanoneedles were formed directly on graphene sheets in order to take advantage of their excellent electrical conductivity [25]. Improved electrochemical performances were expected owing to the outstanding electrical conductivity, EDL capacitance of graphene, and pseudocapacitive reactivity of MnO2. However, the specific capacitances of the graphene/MnO2 electrodes were found to decrease with increasing MnO2 content. This result was attributed to diffusion limitations, as a steeper vertical line was observed in the Nyquist plot at low frequencies with decreasing MnO2 content, primarily owing to the dispersion features of MnO2 nanoneedles. Moreover, MnO2 nanoneedles are unable to form the desired homogeneous dispersion owing to the absence of functional groups on graphene sheets. Therefore, graphene/MnO2 composites prepared with a high feed ratio of MnO2 exhibit the properties of graphene sheets with partly aggregated MnO2, both of which cannot participate in pseudocapacitive reactions. In order to obtain a graphene/MnO2 composite exhibiting the synergistic effects of EDL capacity, excellent electrical conductivity of graphene, and pseudocapacitive reactivity, we recently reported a procedure to fabricate a graphene/nanoneedle MnO2 composite as follows: First, MnO2 nanoneedles were formed on GO sheets using various functional groups to obtain a homogeneous dispersion of nanosized MnO2 particles. In the second stage, to compare the reducing effects of hydrazine hydrate and sodium borohydride, GO/MnO2 was reduced to graphene/MnO2 via an immersion method [26]. We found that the hydrazine reduction is more effective in increasing the hydrocarbon component because hydrazine reduction is known to form hydrocarbons from carbonyl groups, whereas sodium borohydride reduction forms residual hydroxyl functional groups. However, the optimum reduction conditions such as the concentration of reducing agent and reduction time were not evaluated. In this study, graphene/MnO2 composites exhibiting excellent electrochemical performances were prepared via reduction of GO/MnO2 at various reduction times using fixed concentrations of hydrazine hydrate to determine the optimal reduction time. Changes in the oxygen-containing functional groups were observed with variation in reduction times, resulting in marked effects on both electrical conductivity and MnO2 nanoneedle density, which impacts the surface area and can significantly affect the supercapacitive performance. The morphology and microstructural characteristics of graphene/MnO2 were investigated in detail. The electrochemical performances of the prepared electrodes were also investigated using cyclic voltammetry (CV), galvanostatic charge/discharge experiments, electrochemical impedance spectroscopy (EIS), and analysis of the specific capacitance and capacitance retention.
order to synthesize GO/MnO2 at a ratio of 1:5, GO (0.05 g) and MnCl24H2O (0.34 g) were dispersed in isopropyl alcohol (50 mL) with ultrasonication for 30 min. Subsequently, the mixture was heated to approximately 85 °C in a water-cooled condenser with vigorous stirring. KMnO4 (0.18 g) dissolved in DI water (5 mL) was then added to the abovementioned boiling solution under vigorous stirring for 30 min. The obtained mixture was filtered and washed several times, and finally dried in a vacuum oven at 60 °C for 24 h. Hydrazine hydrate-reduced graphene/MnO2 (rGO/ MnO2) was obtained by an immersion method. In order to compare the reducing effect as a function of the reduction time, GO/MnO2 was immersed in an aqueous hydrazine hydrate (30 mM) solution for various reaction durations (20, 24, 28, 32, and 36 h). The resulting rGO/MnO2 composites were collected by filtration, washed with DI water several times, and dried in a vacuum oven at 90 °C for 24 h. The products are denoted as rGO(20)/MnO2, rGO(24)/ MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/MnO2, corresponding to the reduction times of 20, 24, 28, 32, and 36 h, respectively. 2.2. Characterization methods The X-ray crystallographic patterns of the as-prepared composites were collected using an X-ray diffractometer (XRD, New D8Advance/Bruker-AXS) at a scan rate of 1° s1 in the 2h range 5– 70° with Cu Ka1 radiation (k = 0.154056 nm). Field-emission scanning electron microscopy (FE-SEM, SIGMA, Carl Zeiss) was used to examine the morphology of the as-prepared samples. X-ray photoelectron spectroscopy (XPS) analysis was carried out using a VGMicrotech, ESCA2000 system, equipped with a spectrometer with an Mg Ka X-ray source (1253.6 eV) and a hemispherical analyzer. In the curve fitting, the Gaussian peak widths were constant in each spectrum. Thermal gravimetric analyses (TGA) were carried out on a TA instrument (TGA-2050) at a heating rate of 10 °C min1 in air. The sheet resistance was measured using a four-point probe method (ChangMin, LTD, CMT-SR2000N, Korea). Brunauer–Emmett–Teller (BET) surface areas were determined with an Autosorb-iQ 2ST/MP (Quantachrome) system using nitrogen as an adsorptive agent at 77 K. Prior to measurements, the samples were degassed in vacuum at 200 °C for 12 h. The fabrication of working electrodes was carried out as follows. The as-prepared materials, carbon black, and poly(vinylidene fluoride) (PVDF) were mixed at a mass ratio of 75:20:5 and dispersed in N-methylpyrrolidone (NMP). The resulting mixture was then coated onto a nickel foam substrate (1 cm 1 cm) and dried in a vacuum oven at 60 °C for 6 h. The loading mass of each electrode was ca. 2.7–3.2 mg. To investigate the electrochemical behavior of the as-prepared samples, cyclic voltammetry (CV), galvanostatic charge/discharge experiments, and electrochemical impedance spectroscopy (EIS) measurements were conducted in the three-electrode mode using the as-prepared samples, platinum foil, and Ag/AgCl (KCl-saturated) as the working, counter, and reference electrodes, respectively. The measurements were carried out in a 1 M Na2SO4 aqueous electrolyte at room temperature. The CV, galvanostatic charge/discharge characteristics, and EIS measurements were performed using a CHI 660C electrochemical workstation. The EIS measurements were recorded under AC voltage amplitude of 5 mV in the frequency range 105–0.1 Hz and an open circuit potential.
2. Experimental 3. Results and discussion 2.1. Synthesis of materials Graphene oxide (GO) was prepared from powdered flake graphite by a modified Hummers method as described previously [26]. In
The procedure for the formation of rGO/MnO2 composites comprises the following three steps: (i) synthesis of GO from graphite powder, (ii) formation of the MnO2 nanoneedles on GO surfaces
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and edges according to the reaction 2KMnO4 + 3MnCl2 + 2H2O ? 5MnO2 + 2KCl + 4HCl, and (iii) hydrazine hydrate-mediated chemical reduction of GO/MnO2 at varying immersion times to investigate the reducing effect as a function of reaction time. First, it was essential to confirm the introduction of MnO2 nanoneedles on the GO sheets prior to the reduction process. Fig. 1 shows the XRD patterns of the as-prepared GO and GO/MnO2, confirming the formation of nanoneedle-like structures of MnO2. The XRD pattern of GO revealed an intense peak around 2h = 11.5°, corresponding to the (0 0 1) reflection, which indicated that the interlayer spacing is much larger than that of pristine graphite owing to the introduction of oxygen-containing functional groups on the graphite sheets [27]. The diffraction peaks of the as-synthesized GO/MnO2 were similar to those of the nanotetragonal phase of a-MnO2 (JCPDS 44-0141, a = 9.7845 Å, c = 2.8630 Å), and the (0 0 1) reflection peak of layered GO is almost invisible, suggesting that homogeneous composites are indeed formed by the deposition of needle-like nanoscale MnO2 structures on the surface of graphene oxide sheets [22,24,25]. This result correlates well with the results of previous studies that showed that the diffraction peak heights decrease or the peaks disappear upon exfoliation of regular stacks of GO [28]. The formation of MnO2 nanoneedles can be attributed to a disruptive effect. As reported by Chen et al., in a DI water/isopropyl alcohol system, the coordination of H2O and isopropyl alcohol with the O atoms of MnO6 has a disruptive effect on the packing of the growing crystal species. The intermolecular hydrogen bonds formed among the coordinated H2O molecules favor the formation of highly ordered aggregates, whereas a shortage of such strong intermolecular interactions in isopropyl alcohol-coordinated species results in a disordered crystal. Therefore, the overall system has only one preferable crystal growth direction [24]. Surface areas and pore volumes are the most important factors determining the supercapacitive performance of graphene/MnO2 composites. High specific surface areas and the presence of mesopores are favorable for improving both the main pseudocapacitance of MnO2 nanoneedles and the EDL capacitance of graphene because the hydrated ions in the electrolyte are easily accessible to the exterior and interior pore surfaces [29]. Table 1 summarizes the BET surface area and pore volume behavior as a function of reduction time. In order to determine the surface area and porosity, liquid nitrogen cryosorption was performed on the as-prepared
Fig. 1. XRD patterns of GO and GO/MnO2.
Table 1 BET surface area and pore volume of GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/MnO2.
GO/MnO2 rGO(20)/MnO2 rGO(24)/MnO2 rGO(28)/MnO2 rGO(32)/MnO2 rGO(36)/MnO2
BET surface area (m2 g1)
Pore volume (cm3 g1)
114.26 112.41 109.74 105.23 74.75 38.63
0.72 0.71 0.70 0.68 0.47 0.32
rGO/MnO2 composites. The surface area and pore volume of rGO/ MnO2 decreased with increasing hydrazine reduction time. It can be assumed that removal of MnO2 nanoneedles during the reduction process decreases the surface area and pore volume because MnO2 nanoneedles are beneficial for pore formation and specific area enlargement. At reduction times greater than 28 h, the surface area and pore volume of the rGO/MnO2 composites decreased dramatically, indicating that significant changes in the composite MnO2 proportions occurred, likely because of the strong reduction conditions employed. The compositions of the as-prepared GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/MnO2 composites were further investigated by TGA (Fig. 2). The experiments were performed up to 700 °C in air at a heating rate of 10 °C min1. Under these conditions, the GO and rGO sheets were decomposed, while MnO2 was converted into Mn2O3 [30]. Fig. 2 shows the representative TGA curves of the nanoneedles of MnO2 (MnO2), GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, rGO(36)/MnO2, GO, and rGO. The typical weight losses of MnO2, GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, rGO(36)/MnO2, GO, and rGO were 12.5%, 21.9%, 23.1%, 23.9%, 24.6%, 31.2%, 34.1%, 92.1%, and 100%, respectively. Accordingly, the mass ratios of MnO2/(GO or rGO) for GO/MnO2, rGO(20)/MnO2, rGO(24)/ MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/MnO2 were 6.92:1, 6.74:1, 6.24:1, 5.84:1, 3.48:1, and 2.9:1, respectively. The mass ratios of MnO2/(GO or rGO) for rGO/MnO2 composites were observed to decrease with increasing reduction time, because hydrazine hydrate removed not only the functional groups on the GO sheets (i.e., hydroxyl, epoxy, and carboxylic groups), but
Fig. 2. TGA curves of MnO2, GO, rGO, GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/MnO2.
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also the functional groups bonded to MnO2. As exposure to hydrazine approached 28 h, the proportion of MnO2 in the composites decreased slightly. In contrast, reduction times greater than 28 h were accompanied by a dramatic decrease in the proportion of MnO2 in the composites because of the elimination of most oxygen-containing functional groups during the reduction of GO to graphene, which is in accordance with the surface area and pore volume analysis (Table 1). Detailed surface information of GO and rGO/MnO2 at the different hydrazine reduction times examined was collected by XPS; the corresponding results are presented in Fig. 3. Fig. 3(a) shows the spectra of GO and GO/MnO2 and the deconvoluted Mn 2p core-level spectrum of the latter. Only two elements were present in the spectrum of GO, namely C and O. However, Mn signals (2p3/2, 2p1/ 2) appeared in the spectra of GO/MnO2, implying that MnO2 nanoneedles were successfully formed on the surface of the GO sheets. In the Mn 2p spectrum, the Mn 2p3/2 and 2p1/2 peaks were centered at 642.6 and 654.2 eV, respectively. Although the observed peak positions were slightly different, a consistent spin-energy
separation of 11.6 eV was observed between the Mn 2p3/2 and Mn 2p1/2 peaks. These results are also in accordance with the previously reported data for Mn 2p3/2 and Mn 2p1/2 in a MnO2 nanoneedle [25]. Fig. 3(b) shows the wide-scan spectra of GO/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, and rGO(32)/MnO2. As reduction time was increased, the intensity of the O 1s peak greatly decreased, indicating that hydrazine hydrate was effective in removing the oxygen-containing functional groups. Interestingly, the Mn 2p peak also showed a gradual decrease in intensity with increasing hydrazine reduction time. The reason for these decreases in the Mn 2p3/2 and Mn 2p1/2 peak intensity is as follows. Various oxygencontaining functional groups (i.e., epoxy, hydroxyl, carbonyl, and carboxyl) present on the surfaces and edges make the formation of an MnO2 nanoneedle on the GO surface possible. These functional groups, acting as anchor sites, enable the formation of nanostructures on the surfaces and edges of the GO sheets. However, during the hydrazine reduction process, hydrazine hydrate removes not only the functional groups on the GO sheets but also the functional groups bonded to MnO2. The atomic ratio of carbon and oxygen was obtained by calculating the ratio of the C 1s to O 1s peak areas in the corresponding XPS spectra. This ratio represents the degree of reduction, which could be altered by varying the reduction time. In order to directly compare the effects of hydrazine concentrations, the sheet resistance was measured for the pressed pellets of samples using a four-probe method. Table 2 summarizes the differences among the GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/ MnO2 composites in terms of their atomic ratio of carbon and oxygen and resulting sheet resistance. As the reduction time increased, the oxygen content of the rGO/MnO2 composite markedly decreased. The appearance of a peak corresponding to a C–N bond was observed, which increased in area because of the bond-forming events associated with hydrazones [31]. Electrical conductivity is known to be directly related to the oxygen content. Therefore, sheet resistance decreased as the hydrazine concentration was increased with increasing reduction time because it yields the lowest oxygen concentrations [26]. The FE-SEM images of GO, GO/MnO2, rGO(24)/MnO2, rGO(28)/ MnO2, rGO(32)/MnO2, and rGO(36)/MnO2 are shown in Fig. 4. Fig. 4(a) and (b) shows GO and GO/MnO2, respectively. Their comparison clearly reveals that the GO sheets were homogeneously decorated with MnO2 nanoneedles, in agreement with the XRD observations. Fig. 4(c)–(f) shows the images of rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/MnO2, respectively. No significant differences in the morphology and density of MnO2 nanoneedles in the rGO/MnO2 composites were apparent up to the 28-h reduction time. However, at reduction times greater than 28 h, MnO2 nanoneedle densities decreased dramatically with increasing time, as the strong reduction conditions effected the removal of a large amount of oxygen-containing functional groups bonded to both the MnO2 and GO sheets. These decreases in the MnO2 nanoneedle densities observed for rGO(32)/MnO2 and rGO(36)/MnO2 composites are responsible for decreases in the surface area and pore volume, resulting in slow ion diffusion
Table 2 Atomic ratio of carbon and oxygen in GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/MnO2, and their sheet resistances.
Fig. 3. XPS spectra of GO, GO/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, and rGO(32)/ MnO2. (a) Wide scan survey spectra of GO and GO/MnO2 (the inset illustrates the narrow spectra of Mn 2p peaks of the GO/MnO2 composite) and (b) wide scan survey spectra of GO/MnO2, rGO(24)/MnO2, rGO(28)/MnO2 and rGO(32)/MnO2.
GO/MnO2 rGO(20)/MnO2 rGO(24)/MnO2 rGO(28)/MnO2 rGO(32)/MnO2 rGO(36)/MnO2
C:O ratio
Sheet resistance (KO sq1)
2.51 4.22 4.31 4.46 4.68 4.83
9016 637 524 416 377 332
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Fig. 4. FE-SEM images of GO, GO/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/MnO2. (a) GO, (b) GO/MnO2, (c) rGO(24)/MnO2, (d) rGO(28)/MnO2, (e) rGO(32)/MnO2 and (f) rGO(36)/MnO2.
in the charge/discharge processes. Moreover, a reduced proportion of nanoneedle MnO2 inhibits the redox reaction that generates pseudocapacitance.
Fig. 5. CV curves of GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/MnO2 electrodes at 10 mV s1.
To explore potential applications for supercapacitors, the samples were fabricated into the supercapacitor electrodes and characterized by CV, galvanostatic charge/discharge experiments, and EIS measurements. Fig. 5 shows the CV curves of the GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/MnO2 electrodes at a scan rate of 10 mV s1 in 1 M Na2SO4 electrolyte. All CV curves were almost rectangular, indicating that the composites show ideal capacitive behavior. It is obvious that all the rGO/MnO2 electrodes examined showed much larger integrated areas under the CV curves than GO/MnO2, suggesting superior electrochemical performance, which can be attributed to the excellent electrical conductive properties of graphene. It should be remembered that graphene oxide cannot form electronic conductive channels because of its extremely low electrical conductivity. The integrated areas of the rGO/MnO2 electrodes gradually increased up to a reduction time of 28 h, which is attributed to the formation of electronic conductive channels by the removal of oxygen-containing functional groups [32]. Therefore, the reduced GO acts not only as a support for the formation of MnO2 particles but also provides electrically conductive channels [33]. However, as shown in Table 2, although rGO(32)/MnO2 and rGO(36)/MnO2 electrodes exhibited much lower sheet resistance than rGO(28)/MnO2, the rGO(28)/MnO2 electrode possessed the largest integrated area among the electrodes. It is presumed that this phenomenon arises because of the removal of a large amount of oxygen-containing functional groups bonded to MnO2 (that include hydroxyl, epoxy, and carboxylic groups on the GO sheets) under the strong reduction conditions. Consequently, a decrease
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in the formation of MnO2 nanoneedles occurs, as confirmed by the TGA analysis. This decreased MnO2 density leads to reduced redox reactivity as well as a decreased surface area, allowing for developing pores to act as ion-buffering reservoirs, which improves the diffusion rate of Na+ ions and generates a suitable nanoneedle alignment. This, in turn, can provide well-ordered tunnels that are convenient for the insertion/extraction of Na+ cations both into and from MnO2 [24]. Moreover, MnO2 nanoparticles can greatly reduce the diffusion lengths over which Na+ ions must migrate during the charge/discharge processes, thereby improving the electrochemical utilization of MnO2, which was further confirmed in the EIS study. Furthermore, the residual oxygen functional groups (i.e., hydroxyl, carboxyl, and epoxy) remaining on the external surface after the reduction process serve as a passage for ions to the internal surface and are thus important to the charging/discharging process. These functional groups were also reported to contribute to fast redox processes occurring at or near the electrode surface, which can provide pseudocapacitance [34–36]. The galvanostatic charge/discharge measurements of the GO/ MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/ MnO2, and rGO(36)/MnO2 electrodes were carried out in 1 M Na2SO4 over the voltage range 0.1 to 0.9 V at a current density 10 mA cm2. As illustrated in Fig. 6, during the charging and discharging steps, the charge curves for all electrodes were almost symmetric to their corresponding discharge counterparts with a slight curvature, indicating pseudocapacitive and double-layer contributions. However, when comparing the discharging curves, the rGO(28)/MnO2 electrode exhibited a much longer discharge time, which is consistent with the specific capacitance behavior because the discharging time is directly proportional to the specific capacitance of an electrode [37]. EIS experiments were carried out to further investigate the electrochemical and structural characteristics of the GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/MnO2 electrodes (Fig. 7). All Nyquist plots were semicircular over the high-frequency range and linear in the low-frequency section. The intercept for the real component at the beginning of the semicircle indicated the combined series resistance (Rs) of the electrolyte, electrode, current collectors, and the electrode/current collector contact resistance [38]. The resistances of the electrolyte and current collectors for all electrodes were
Fig. 7. Nyquist plots of GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/MnO2 electrodes.
equivalent. A major difference is the semicircle in the high frequency range, which corresponds to the charge-transfer resistance (Rct) caused by the Faradaic reactions and the double layer capacitance (Cdl) on the grain surface. The slope of the 45° portion of the curve is called the Warburg resistance (Zw) and is a result of the frequency dependence of ion diffusion/transport in the electrolyte to the electrode surface. CL is the limit capacitance. The intercepts of the rGO/MnO2 electrodes ranged from 0.6 to 1.03 O cm2, which were much lower than the value for the GO/MnO2 electrode (1.76 O cm2). Furthermore, the combined series resistance of the rGO/MnO2 electrodes exhibited a gradual decrease with increasing reduction time owing to the increase in the electrical conductivity caused by the removal of oxygen-containing functional groups during GO/MnO2 reduction. The diameter of the semicircle represents the Rct at the interface between the electrode material and electrolyte [39]. It is apparent that the values of Rct gradually decreased with increasing reduction time up to 28 h, which is in accordance with the TGA results. However, for reduction times greater than 28 h, the diameter of the semicircle decreased dramatically because of the removal of a large amount of MnO2. The vertical line at very low frequencies is caused by ion accumulation at the bottom of the pores of the electrode. The almost vertical line demonstrates good capacitive behavior without diffusion limitations [25]. The straight line at the low frequencies of the GO/ MnO2 electrode shows a gradual inclination compared to other electrodes. This indicates slower ion diffusion in the electrolyte and adsorption onto the electrode surface because of the removal of a large amount of MnO2, which can increase the specific area and form pores for ion-buffering reservoirs to improve the diffusion rate of Na+ ions and reduce the diffusion length of ions within the pseudocapacitive phase. These results also agree with the results obtained from the BET surface area and pore volume analysis. The specific capacitances (Cs) were calculated from the CV curves using the following equation [40]:
Cs ¼
Fig. 6. Galvanostatic charge/discharge curves of GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2 and rGO(36)/MnO2 electrodes at a current density of 10 mA cm2.
1 v ðV c V a Þ
Z
Vc
IðVÞdV
ð1Þ
Va
where Cs is the specific capacitance (F g1), v is the potential scan rate (mV s1), Vc Va represents the sweep potential range (V), and I(V) denotes the response current density (A g1). Fig. 8 indicates the relationships between Cs and the scan rates of the as-pre-
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Fig. 8. Specific capacitance of GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/ MnO2, rGO(32)/MnO2, and rGO(36)/MnO2 electrodes at different scan rates of 5, 10, 20, 50, 100, 200, 300, 400, and 500 mV s1.
593
Fig. 9. Ragone plot of the estimated specific energy and specific power of GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/MnO2 electrodes at various scan rates of 5, 10, 20, 50, 100, 200, 300, 400, and 500 mV s1.
pared GO/MnO2 and rGO/MnO2 electrodes prepared at various reduction times. The calculated Cs values of the GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/MnO2 electrodes at 10 mV s1 were 111.76, 325.18, 347.62, 371.74, 261.47, and 211.81 F g1, respectively. The rGO(28)/MnO2 electrode showed the highest specific capacitance over the entire range of scan rates. This result can be attributed to both the intrinsic EDL capacitance of graphene resulting from the hydrazine reduction process, and the pseudocapacitive reaction due to the homogeneous dispersion of the nanosized MnO2. Moreover, the Cs values for all electrodes decreased steadily with increasing scan rate owing to the reduced access of ions to the active surface, especially with relatively slow faradic reactions [41]. Power density and energy density, important parameters for the determination of electrochemical performance, were also used to characterize the GO/MnO2, rGO(20)/MnO2, rGO(24)/MnO2, rGO(28)/MnO2, rGO(32)/MnO2, and rGO(36)/MnO2 electrodes. Both power density and energy density can be estimated from the following equations [42]:
E¼
1 CðDVÞ2 2
ð2Þ Fig. 10. Cycling stability of the rGO(28)/MnO2 electrode measured at 10 mV s1.
E P¼ t
ð3Þ
where P, C, DV, t, and E represent the power density (W kg1), specific capacitance based on the mass of the electroactive material (F g1), the potential window of discharge (V), discharge time (s), and energy density (Wh kg1), respectively. The Ragone plots for all electrodes at different scan rates are presented in Fig. 9. At a high scan rate of 500 mV s1, the rGO(28)/MnO2 electrode gave a calculated energy density of 41.87 Wh kg1 and power density of 75.3 kW kg1, which are the highest values for these particular parameters among all electrodes investigated. These results suggest that the synergistic effects of the EDL capacity, excellent electrical conductivity of graphene, and pseudo-capacitive reaction result in an increased surface area and a decreased diffusion path because of the homogeneously dispersed MnO2 nanoneedles. Cycle lifetime is particularly important for supercapacitor applications. Typical problems associated with the MnO2-based electrodes in aqueous electrolytic media include mechanical
expansion of MnO2 during the ion insertion/desertion process, MnO2 detachment from the electrode surfaces, and dissolution of Mn into the electrolyte [43]. A cyclic stability test of the rGO(28)/ MnO2 electrode was performed over 1000 cycles at a scan rate of 10 mV s1 and a potential window range between 0.1 and 0.9 V. Fig. 10 illustrates the specific capacitance retention as a function of cycle number. The rGO(28)/MnO2 electrode exhibited a specific capacitance loss of less than 12.4% after 1000 charge/discharge cycles, demonstrating its excellent capacity retention.
4. Conclusions Graphene/MnO2 composites were obtained via the hydrazine reduction of GO/MnO2 with various reduction times. The structural characteristics and electrochemical properties of the obtained composites were evaluated for potential supercapacitor applica-
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tions. The rGO(28)/MnO2 electrode showed the highest specific capacitance (371.74 F g1 at a scan rate of 10 mV s1) among the electrodes examined. These experimental results are attributed to the synergistic effects of the EDL capacity, excellent electrical conductivity of graphene, and homogeneously dispersed MnO2 nanoneedles. There is also an increased surface area, which can form pores for ion-buffering reservoirs and effectively decrease the diffusion path and provide pseudocapacitance. The sheet resistance of rGO(28)/MnO2 was much lower than that of rGO(24)/MnO2. Moreover, a comparison of rGO(28)/MnO2 and rGO(32)/MnO2 electrodes showed that the latter had a much lower surface area and pore volume than the former, although rGO(32)/MnO2 exhibited a lower sheet resistance value. These observations are ascribed to the removal of most oxygen-containing functional groups bonded to MnO2 because of extended hydrazine exposure, in addition to the elimination of hydroxyl, epoxy, and carboxylic groups present on the GO sheets. After 1000 cycles, the capacitance of the rGO(28)/ MnO2 electrode decreased by 12.4% from the initial value, demonstrating its excellent electrochemical stability. Therefore, a reduction time of 28 h was established as optimal for improved electrochemical properties, indicating that the hydrazine-mediated reduction of GO/MnO2 is a promising method for the synthesis of electrode materials for supercapacitors. Acknowledgements This work was supported by the Technological Innovation R&D Program (S2085171) funded by the Small and Medium Business Administration (SMBA, Korea). References [1] Subramanian V, Zhu H, Vajtai R, Ajayan PM, Wei B. Hydrothermal synthesis and pseudocapacitance properties of MnO2 nanostructures. J Phys Chem B 2005;109:20207–14. [2] Mohana AL, Estaline RF, Imran A, Ramaprabhu JS. Asymmetric flexible supercapacitor stack. Nanoscale Res Lett 2008;3:145–51. [3] Karina AC-G, Monica L-C, Nieves C-P, Pedro G-R. Nanocomposites hybrid molecular materials for application in solid-state electrochemical supercapacitors. Adv Funct Mater 2005;15:1125–33. [4] Chow J, Kopp RJ, Portney PR. Energy resources and global development. Science 2003;302:1528–31. [5] Wen Z, Wang X, Mao S, Bo Z, Kim H, Cui S, et al. Crumpled nitrogen-doped grapheme nanosheets with ultrahigh pore volume for high-performance supercapacitor. Adv Mater 2012;24:5610–6. [6] Kotz R, Carlen M. Principles and applications of electrochemical capacitors. Electrochim Acta 2000;45:2483–98. [7] Chen Z, Qin Y, Weng D, Xiao Q, Peng Y, Wang X, et al. Design and synthesis of hierarchical nanowire composites for electrochemical energy storage. Adv Funct Mater 2009;19:3420–6. [8] Bose S, Kuila T, Mishra AK, Rajasekar R, Kim NH, Lee JH. Carbon-based nanostructured materials and their composites as supercapacitor electrodes. J Mater Chem 2012;22:767–84. [9] Jiang H, Ma J, Li CZ. Mesoporous carbon incorporated metal oxide nanomaterials as supercapacitor electrodes. Adv Mater 2012;24:4197–202. [10] Simon P, Gogotsi Y. Materials for electrochemical capacitors. Nat Mater 2008;7:845–54. [11] Chou SL, Wang JZ, Chew SY, Liu HK, Dou SX. Electrodeposition of MnO2 nanowires on carbon nanotube paper as free-standing, flexible electrode for supercapacitors. Electrochem Commun 2008;10:1724–7. [12] Yang S-Y, Chang K-H, Tien H-W, Lee Y-F, Li S-M, Wang Y-S, et al. Design and tailoring of a hierarchical graphene-carbon nanotube architecture for supercapacitors. J Mater Chem 2011;21:2374–80. [13] Wang H, Casalongue HS, Liang Y, Dai H. Ni(OH)2 nanoplates grown on graphene as advanced electrochemical pseudocapacitor materials. J Am Chem Soc 2010;132:7472–7. [14] Qiu L, Yang X, Gou X, Yang W, Ma Z-F, Wallace GG, et al. Dispersing carbon nanotubes with graphene oxide in water and synergistic effects between graphene derivatives. Chem-Eur J 2010;16:10653–8.
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