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urchin-like manganese dioxide composite and high performance as supercapacitor electrode
Electrochimica Acta 69 (2012) 112–119
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Synthesis of reduced graphene nanosheet/urchin-like manganese dioxide composite and high performance as supercapacitor electrode Wanlu Yang a , Zan Gao a , Jun Wang a,b,∗ , Bin Wang a , Qi Liu a , Zhanshuang Li a , Tom Mann a , Piaoping Yang a,b , Milin Zhang a,b , Lianhe Liu a,b a b
Key Laboratory of Superlight Material and Surface Technology, Ministry of Education, Harbin Engineering University, 150001, PR China Institute of Advanced Marine Materials, Harbin Engineering University, 150001, PR China
a r t i c l e
i n f o
Article history: Received 28 September 2011 Received in revised form 21 February 2012 Accepted 22 February 2012 Available online 1 March 2012 Keywords: Graphene nanosheets Urchin-like manganese dioxide Composite electrode Supercapacitor
a b s t r a c t Reduced graphene nanosheet/urchin-like manganese dioxide (GNS/MnO2 ) composite for a supercapacitor electrode has been fabricated by a mild hydrothermal route. Following investigation by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), we propose an in situ formation of MnO2 nanoparticles onto graphene nanosheets. The unique structure greatly increases specific surface area of the composite and the utilization of MnO2 . The electrochemical performance of the electrode is analyzed by cyclic voltammetry, electrochemical impedance spectrometry and chronopotentiometry. Results show that the GNS/MnO2 composite exhibits a maximum specific capacitance of 263 F g−1 and an excellent cycle life with capacity retention of about 99% after 500 cycles. The method provides a facile and straightforward approach to deposit MnO2 nanoparticles onto graphene sheets; it could be readily extended to the preparation of other classes of hybrids based on GNS sheets for technological application. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction In modern society, due to environmental issues and depleting fossil fuels, the increasing demand for renewable energy sources has stimulated intense research on energy storage and conversion from alternative energy sources [1]. Supercapacitors (SCs), as charge-storage devices exhibiting high-power density, excellent reversibility and cycle-ability, are considered as promising candidates for energy storage [2,3]. Energy can be stored in supercapacitors by means of either ion adsorption at the electrode/electrolyte interface (namely, electrical double-layer capacitors, EDLCs) or fast and reversible faradic reactions (namely, pseudocapacitors) [4,5]. To date, carbon materials (activated carbons, carbon aerogels, carbon nanotubes, carbon fabrics, and reduced graphene oxide) [6–12], transition metal oxides (RuO2 , MnO2 , Co3 O4 , NiO) [13–16] and conducting polymers (polyaniline, polypyrrole, and polythiophene) [17–20] have been identified as the most promising materials for SCs. However, each material has its unique advantages and disadvantages for SC application. For example, carbon materials have outstanding electrical properties, long life-cycles and beneficial mechanical properties, but low
∗ Corresponding author at: Key Laboratory of Superlight Material and Surface Technology, Ministry of Education, Harbin Engineering University, 150001, PR China. Tel.: +86 451 8253 3026; fax: +86 451 8253 3026. E-mail address: [email protected] (J. Wang). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2012.02.081
specific capacitance (5–200 F g−1 ). Transition metal oxides and conducting polymers have relatively higher capacitance and fast redox kinetics, while the relatively low mechanical stability and cycle life limits its application for SCs. The challenge for research studies of binary or ternary composites of carbon materials, conducting polymers and metal oxides as materials for supercapacitors, is to effectively utilize their benefits while overcoming their disadvantages. Two-dimensional graphene as a rising star has triggered an exciting new area in the field of carbon nanoscience, due to its high surface area, electrical conductivity, high flexibility and mechanical strength [21–24]. Graphene comprises a single layer of carbon atoms arranged in a graphitic sp2 bonding configuration. This unique nanostructure holds great promise for potential applications in many technological fields such as nanoelectronics, sensors, batteries, supercapacitors, hydrogen storage, and nanocomposites [25–30]. Recently, one new perspective is to ideally utilize single-atom-thick GNS as a support to anchor functional nanomaterials to design next-generation electronic and energy conversion devices, since GNS has shown excellent electrochemical behavior as electrodes of SCs [31–33]. Graphene composites, such as GNS/Co3 O4 , GNS/ZnO, GNS/SnO2 , GNS/LDH and GNS/polyaniline, have been prepared as supercapacitors in aqueous electrolyte [34–38]. The capacitances of graphene composites are remarkably improved while maintaining high-rate electrochemical performance inherent in the two-dimensional structure of graphene, as long as the surfaces of graphene provide not only highly reversible
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pseudocapacitance but also electrochemical double-layer capacitance. GNS composites have shown their superior property as supercapacitor material; however, research on the potential of graphene/metal oxide composite materials for supercapacitors has so far been limited. Deposition of electrochemical active nanoparticles onto graphene sheets to produce supercapacitor electrode materials is therefore promising. Manganese dioxide (MnO2 ), as potential electrode material for the replacement of RuO2 in supercapacitors, has exhibited distinguishing properties owing to its high specific capacitance, environmental compatibility, and cost effectiveness [39–42]. Studies have shown that the electrochemical characteristics of MnO2 materials strongly depend on their structural parameters such as polymorphism, morphology, particle size, and bulk density [42–45]. Up to now, various crystalline structures of MnO2 including onedimensional (1-D) (nanorods, nanowires, nanobelts, nanotubes) [46–49], two-dimensional (2-D) (nanosheets, nanoflakes) [50,51] and three-dimensional (3-D) (nanospheres, nanoflowers) [52,53] have been extensively studied. There are also many reports about manganese oxide composite as supercapacitor electrodes [54,55]. However, to the best of our knowledge, the investigation of the application of 3-D nanostructured manganese dioxide, especially, 3-D nanostructured manganese dioxide composite, as electrode material still remains limited. We also know that the relatively high resistance of MnO2 seriously reduces the capacitive response speed and consequently becomes a bottleneck for its further commercial application. To improve the conductivity of MnO2 , conductive fillers, such as acetylene black, carbon black and carbon nanotubes, are usually added [56–59]. Recently, GNS as a matrix to improve the conductivity of GNS-based composite has attracted widespread attention by researchers [34]. Herein, we report a facile and green method to fabricate the GNS/urchin-like MnO2 binary nanocomposite in a mild hydrothermal system. First, graphene nanosheets are synthesized via an environmental friendly method, using glucose as a reducing agent [60], then 3-D urchin-like MnO2 nanoparticles are directly grown along conducting graphene nanosheets, and act as spacers to keep neighboring sheets separate. The graphene sheets coated with porous urchin-like MnO2 particles overlap with each other affording a three-dimensional conducting network, which facilitates fast electron transfer between the active materials and the charge collector, and improves the contact between the electrode materials and the electrolyte. The deposition of 3-D structure MnO2 directly onto graphene nanosheets as supercapacitor electrode has not been reported. The as-obtained GNS/MnO2 composite exhibited a high specific capacitance (234 F g−1 at 10 mA cm−2 ) and excellent long cycle life; the possible formation mechanism is also presented and discussed.
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with distilled water several times and the obtained GNS was redispersed in water for further use. Typical synthesis process of the GNS/MnO2 nanocomposite is described as follows: an amount of 1.262 g of KMnO4 was dissolved in 65 mL of deionized water and 2 mL of sulfuric acid (98 wt%) was slowly dropped into the solution, followed by the addition of 0.85 g of Cu scraps and 20 mL of the as-obtained GNS suspension (5 mg mL−1 ). After stirring for 15 min, the resultant system was transferred into a stainless steel autoclave and held at 110 ◦ C for 12 h. After cooling to room temperature naturally, the as-prepared products were filtered, washed with distilled water, and dried at 80 ◦ C for 12 h. For comparison, pure MnO2 was prepared under the same conditions without GNS. 2.2. Characterization methods The crystallographic structures of the materials were determined by a powder X-ray diffraction system (XRD, Rigaku TTR-III) equipped with Cu K␣ radiation ( = 0.15406 nm). X-ray photoelectron spectroscopy (XPS) measurements were performed using a PHI 5700 ESCA spectrometer with a monochromated Al K␣ radiation (h = 1486.6 eV). All XPS spectra were corrected by the C1s line at 284.5 eV. The microstructure of the samples was investigated by atomic force microscopy (AFM; NanoscopeIIIa), scanning electron microscopy (SEM; JEOL JSM-6480A microscope), and transmission electron microscopy (TEM; Philips CM 200 FEG, 160 kV). 2.3. Preparation of electrodes and electrochemical characterization The working electrodes were fabricated by a reported procedure [54]. Briefly, the as-prepared materials, acetylene black and polytetrafluoroethylene (PTFE) were mixed in a mass ratio of 80:15:5 and dispersed in ethanol to produce a homogeneous paste. Then the resulting mixture was coated onto the nickel foam substrate (1 cm × 1 cm) to fabricate the electrodes. The mass of the mixture onto the electrode including the electroactive material, conducting agent and binder was 15.8, 16.1, 16.3 mg for GNS, MnO2 and GNS/MnO2 electrode, respectively. The electrochemical properties of as-obtained products were investigated under a three-electrode cell. Platinum foil with the same area and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. All electrochemical measurements were carried out in 1 M Na2 SO4 aqueous electrolyte by a CHI 660D electrochemical work station and all the electrochemical data were analyzed by an appropriate standard [62]. 3. Results and discussion 3.1. Material characterization
2. Experimental 2.1. Synthesis of GNS/MnO2 composite All chemical reagents were analytical grade and used as received. Graphite oxide (GO) was synthesized from natural graphite by a modified Hummers method [61]. Exfoliation of GO was achieved by ultrasonication of the dispersion in an ultrasonic bath (KQ-500DB, 250 W). Compared with the traditional procedure using highly toxic hydrazine as reductant, glucose was used as reducing agent to prepare GNS. Typically, 2 g of glucose was added into 250 mL of homogeneous GO dispersion (0.5 mg mL−1 ), followed by stirring for 30 min. Then 1 mL aqueous ammonia solution (25 wt%) was added to the resulting dispersion. After vigorously shaking for a few minutes, the mixture was stirred for 60 min at 95 ◦ C. The resulting black dispersion was then filtered and washed
XRD patterns of GO, GNS, pure MnO2 powder and the GNS/MnO2 composite are shown in Fig. 1. The feature diffraction peak of exfoliated GO at 10.8◦ (0 0 1) with a basal spacing of 0.82 nm is observed, showing the complete oxidation of graphite to graphite oxide due to the introduction of oxygen-containing functional groups onto the graphite sheets (Fig. 1a) [63]. For the XRD pattern of GNS (Fig. 1b), the peak located at 10.8◦ disappears, while a broad diffraction peak (0 0 2) appears at 2 about 24.5◦ , revealing extensive reduction of GO and exfoliation of the layered GNS [64,65]. The basal spacing of GNS changes from 0.82 nm of GO to 0.37 nm, which is still a little larger than that of natural graphite (0.34 nm). The small amount of residual oxygen-containing groups and hydrogen may be the main reason for this difference, indicating incomplete reduction of GO to graphene. However, the residual oxygen functionalities on the reduced GO surfaces induce electrostatic repulsion stabilizing
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00
1
C KLL
O KLL
2
Intensity / a.u.
c
GNS/MnO2
O 1s
d
d=0.82nm
00
Mn 2P1/2 Mn 2P3/2
C 1s
(a) :GNS MnO2
d=0.37nm
GO
b
a 1200
10
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0
Binding Energy / eV
80
2 (degree)
(b)
GO C-O (286.6 eV)
Fig. 1. Typical XRD patterns of GO (a), GNS (b), MnO2 (c) and the prepared GNS/MnO2 (d) composite.
C-C (284.5 eV)
the graphene sheets, which most likely involve the adsorption of metal ions onto the GNS. The diffraction peaks of as-synthesized pure MnO2 and GNS/MnO2 (Fig. 1c and d) are ascribed to the wellcrystallized tetragonal phase of ␣-MnO2 (JCPDS no. 44-0141). From the XRD pattern of the GNS/MnO2 composite (Fig. 1d), diffraction peaks from the ␣-MnO2 phase are observed while the peak at 2 around 24.5◦ corresponding to the (0 0 2) crystal plane of graphene is not evident, indicating that the surfaces of graphene are fully covered by nanoscale MnO2 . This conclusion is also supported by the results of SEM and TEM. To investigate the reduction degree of GO and the composition of manganese oxide of the GNS/MnO2 composite, the samples were further characterized by XPS. Fig. 2 shows the XPS survey spectra and C1s XPS spectra of GO and the GNS/MnO2 composite. Compared with GO, the XPS spectrum of the GNS/MnO2 composite (Fig. 2a) not only exhibits a relatively low O1s peak and C1s peak, but also exhibits two peaks located at 642.1 and 653.7 eV that can be attributed to Mn 2p3/2 and Mn 2p1/2, confirming the presence of MnO2 in the composite. The peak values agree well with those reported for MnO2 , indicating a 4+ oxidation state for Mn [66]. The C1s XPS spectrum of GO (Fig. 2b) indicates a considerable degree of oxidation with three components that are assigned to carbon atoms in different functional groups: the nonoxygenated ring C (C C), the C in C O bonds (C O), and the carboxylate carbon (O C O) [67,68]. From the areas of the three C1s components of graphite oxide, we can compute that the nonoxygenated ring C is about 43% (284.5 eV), while that of the hybrid GNS/MnO2 material is about 75%. Meanwhile, the absorbance band intensities of the carbonyl carbon (C O) and the carboxylate carbon (O C O) decrease sharply for the GNS/MnO2 composite. Results indicate that most of the oxygen functional groups in GO are successfully removed due to the reduction process by glucose. Fig. 3 is a typical AFM image of exfoliated GO dispersion in water after their deposition on a freshly cleaved mica surface through a drop-casting method. The average thickness of as-prepared GO measured from the height profile of the AFM image is about 1.2 nm and corresponds to a single layer GO [69]. Compared with the theoretical values of 0.78 nm for single layer graphene, the higher thickness of GO may arise from oxygen-containing groups on the surfaces [70]. In this paper, utilizing this ideal single-atom-thick reduced graphene oxide as a two-dimensional carbon support to
Intensity / a.u.
C(O)O (288.5 eV)
C-C (284.5 eV)
GNS/MnO2
C-O (286.1 eV) C(O)O (288 eV)
296
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Binding Energy / eV Fig. 2. XPS survey spectra (a) and C1s XPS spectra (b) of GO and GNS/MnO2 composite.
Fig. 3. AFM image of exfoliated GO sheets on mica surface with height profile.
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Fig. 4. SEM (a) and TEM (b) images of pure GNS; SEM (c) and TEM (d) images of pure MnO2 (inset is the partial enlarged image of MnO2 ); SEM (e) and TEM (f) images of GNS/MnO2 composites.
anchor MnO2 nanoparticles provides a promising way to form new composite materials. Morphologies of the as-obtained products are characterized by SEM and TEM as shown in Fig. 4. Most of the graphene nanosheets are curled and entangled together with a layered structure resembling crumpled silk veil waves (Fig. 4a). GNSs agglomerate with each other through van der Waals interactions of the remained oxygen-containing functionalities on the surfaces of GNSs. The – stacking interaction between the graphene sheets can explain the formation of the agglomerates. The TEM images show that GNS has corrugations and scrollings on the edge of the graphene sheet (Fig. 4b). The image of pure MnO2 reveals that the sample has a spherical morphology with 4–6 m diameter and the products are built up of many interleaving nanorods. During the process of analyzing SEM and TEM, a few spheres with an interior cavity are also observed (Fig. 4c and d). As for the GNS/MnO2 composite (Fig. 4e and f), MnO2 nanorods grow along the surface of graphene
sheets, interconnected and reunited as urchin-like sphere later as well. Results show that mono-dispersed MnO2 microspheres densely distribute onto graphene nanosheets, so that the restacking of graphene nanosheets is effectively prevented, and the loss of their highly active surface area is therefore avoided. The TEM image of GNS/MnO2 demonstrates there are numerous pores among the interconnected MnO2 nanorods, resulting in a highly porous structure of the MnO2 layer. This open structure is favorable for the improvement of the electrochemical performance as an electrode for supercapacitors. On basis of the above results, a schematic representation of the formation process for the hybrid GNS/MnO2 material is given in Fig. 5. Previous studies show that GO sheets have their basal planes covered mostly with epoxy and hydroxyl groups, while carbonyl and carboxyl groups are located at the edges [71]. The covered functional groups act as anchor sites favoring in situ formation of nanostructures on the surfaces of GO sheets. However,
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Fig. 5. A schematic representation of the formation process of the hybrid GNS/MnO2 composite.
these oxygen-containing functional groups impair the conductivity of GO sheets to such an extent that they are not suitable for electrode materials. However, GNSs have an excellent conductivity, with ideal single-atom thick substrate for growth of functional nanomaterials to render them electrochemically active and electrically conductive. Step (a) shows the preparation of GNS based on glucose reducing GO, in which glucose as a reductant will remove most of the oxygen-containing groups. When the as-prepared GNS was mixed with the resultant system of KMnO4 solution treated with sulfuric acid and Cu scraps, metal oxide ions are adsorbed onto the surfaces GNSs by the residual oxygen-containing functional groups. Finally, urchin-like MnO2 nanoparticles grow onto the surfaces of graphene nanosheets under a hydrothermal treatment as shown in step (b). The urchin-like MnO2 nanoparticles are attached to the graphene nanosheets, which effectively prevents their restacking and keeps their highly active surface area. And, the capacity and cyclic performance of graphene-based material is largely increased, which is proved in the following electrochemical test. 3.2. Electrochemical properties The capacitive performances of the pristine GNS, MnO2 and the as-prepared GNS/MnO2 composite were evaluated by cyclic voltammetry (CV) and galvanostatic charge/discharge techniques in 1 M Na2 SO4 electrolyte as shown in Fig. 6. The potential window was chosen in the range of −0.2 to 0.8 V (vs. SCE). The CV curves of the GNS/MnO2 electrode at different scan rates from 5 to 40 mV s−1 (Fig. 6A) exhibit quasi-rectangular shapes, indicating all samples have a good capacitive behavior. No peaks were observed at different scan rates in the CV curves, proving that the electrode is charged and discharged at a pseudo-constant rate over the complete voltammetric cycle. The CV curve at the faster scan rate has a larger area than the lower scan rate one, but does not indicate a greater charge capacitance at the higher scan rate. Meanwhile, with the scan rate increasing, the effective interaction between the ions and the electrode is greatly reduced because of the resistance of metal oxide and the deviation from rectangularity of the CV becomes obviously. Fig. 6B shows the CV curves of the pure GNS, MnO2 and the GNS/MnO2 nanocomposite electrode under the scan rate of 10 mV s−1 . By comparison, the area of the CV curve for the GNS/MnO2 electrode is even larger than those for the pure GNS and MnO2 electrode, indicating higher specific capacitance and the synergistic effect of GNS and MnO2 . The constant current charge/discharge curves of GNS/MnO2 composites at different current densities (5, 10, 20 and 40 mA cm−2 ) show that during the charging and discharging steps, the charge curve of binary composites is almost symmetric to its corresponding discharge counterpart
with a small internal resistance (IR) drop, indicating the pseudocapacitive contribution along with the double layer contribution (Fig. 6C). The specific capacitance of the electrode is obtained from the following equation: Csp =
It vm
(1)
where I, t, v and m are the constant current (A), discharge time (s), the total potential difference (V) and the weight of active materials (g), respectively. Therefore, the specific capacitances of the GNS/MnO2 electrode at 5, 10, 20 and 40 mA cm−2 are 263, 227, 191, and 138 F g−1 , respectively. The results indicate that the GNS/MnO2 nanocomposite has a high rate of capacitance, which is recognized as one of the most important electrochemical properties in the application of electrodes and batteries [72]. The Csp value of the GNS/MnO2 nanocomposite electrode from Fig. 6D is about 227 F g−1 at a current density of 10 mA cm−2 , corresponding to a specific capacitance of 127, 64 F g−1 for GNS and MnO2 alone. These values are mainly consistent with the order indicated by the CVs. The advantages of GNS/MnO2 composites electrode over the pure GNS and MnO2 electrode are salient and the excellent electrochemical performances of GNS/MnO2 composites are attributed to their unique microstructure as illustrated in Fig. 5: (1) Urchin-Like MnO2 particles coating the surfaces of graphene nanosheets accumulate to form pores for ion-buffering reservoirs to improve the diffusion rate of ions within the bulk of the prepared materials; (2) The large specific surface area and the nanoscale size of MnO2 phase of the GNS/MnO2 nancomposite greatly reduce the diffusion length over which both ions and electrons must transfer during the charge/discharge process. This ensures a high utilization of the electrode nanoscale size of MnO2 particles; (3) Graphene in the composites not only act as supports for the deposition of MnO2 particles, but also construct a 3-D highly conductive current collector. The excellent interfacial contact between MnO2 and graphene facilitates fast transportation of electrons throughout the whole electrode matrix. This unique architecture enables the GNS/MnO2 nanocomposite electrode to have a large specific surface and fast electron and ion transport simultaneously, thus presenting the best electrochemical capacitive performance. The electrochemical impedance spectroscopy (EIS) analysis has been recognized as one of the principal methods examining the fundamental behavior of electrode materials for supercapacitors [73]. For further understanding, impedance of all products was measured in the frequency range of 100 KHz–0.05 Hz at open circuit potential with an ac perturbation of 5 mV (Fig. 7). The measured impedance spectra were analyzed using the complex nonlinear least-squares (CNLS) fitting method [74] on the basis of the
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Table 1 The calculated values of Re , CDL , Rct , Zw and CL through CNLS fitting of the experimental impedance spectra upon the proposed equivalent circuit in Fig. 7.
GNS MnO2 GNS/MnO2
Re ()
CDL (F)
Rct ()
ZW ()
CL (F)
0.9505 7.707 2.052
0.0009276 9.347E−5 0.000689
0.3995 1.408 0.4221
1.231 0.1152 0.7204
1.496 – 4.815
Fig. 6. (A) CV curves of GNS/MnO2 composite at different scan rates in 1.0 M NaSO4 solution; (B) CV curves of GNS, MnO2 , GNS/MnO2 at 10 mV s−1 ; (C) galvanostatic charge/discharge curves of GNS/MnO2 composite at the current density of 5 (a), 10 (b), 20 (c) and 40 (d) mA cm−2 ; (D) galvanostatic charge/discharge curves of GNS, MnO2 and GNS/MnO2 composites at 10 mA cm−2 .
Fig. 7. Nyquist plots of experimental impedance (EIS) data (scattering dot) and fitting results (solid line) for GNS, pure MnO2 and GNS/MnO2 composite electrodes. Inset shows the electrical equivalent circuit used for fitting impedance spectra (the upper-left for GNS and GNS/MnO2 , the lower-right corner for MnO2 ).
equivalent circuit, which is given in the inset of Fig. 7. At very high frequencies, the intercept at real part (Z ) is a combinational resistance comprising ionic resistance of electrolyte, intrinsic resistance of substrate, and contact resistance at the active material/current collector interface (Re ) [2]. A major difference is the semicircle in the high frequency range which corresponds to the charge transfer resistance (Rct ) caused by the Faradic reactions and the doublelayer capacitance (Cdl ) on the grain surface. The 45◦ slope portion of the curve is the Wurburg resistance (Zw ) which is a result of the frequency dependence of ionic diffusion/transport in the electrolyte and to the surface of the electrode. CL is the limit capacitance [74]. At low frequencies, the impedance plot should theoretically be a vertical line, which is parallel to the imaginary axis. In fact, the low frequency straight line always departs from that expected with a slope angle close to 90◦ due to the existence of “constant phase element” [75]. The straight lines close to 90◦ indicate a pure capacitive behavior and low diffusion resistance of ions in the structure of the electrodes. The obtained values of Re , Cdl , Rct , ZW and CL calculated from CNLS fitting of the experimental impedance spectra are presented in Table 1. Clearly, the Re of GNS/MnO2 composite electrode is smaller than that of MnO2 electrode, which demonstrates that the addition of graphene enhances the conductivity of composites. At the same time, the anchored MnO2 particles prohibit the aggregation of graphene sheets, and the graphene sheets overlap each other to form a conductive network through sheet
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Technology (2010RFXXG007), the Foundation of Harbin Engineering University (no. HEUFT07053). References
200
0.8
99%
0.6
150
E / V vs.SCE
Specific capacitance (F/g)
250
100
0.4 0.2 0.0
50
-0.2 2000
3000
4000
5000
6000
7000
8000
Time / s
0 0
100
200
300
400
500
Cycle number Fig. 8. Cyclic performance of GNS/MnO2 composite electrodes at 10 mA cm−2 in 1.0 M NaSO4 electrolyte; the inset shows charge/discharge curves of the GNS/MnO2 electrodes in potential range from −0.2 to 0.8 V at 10 mA cm−2 .
plane contact, which facilitates the fast electron transfer between the active materials and the charge collector. The cycle stability of GNS/MnO2 composites was evaluated by repeating the constant current charge/discharge test between −0.2 and 0.8 V (vs. SCE) at a current density of 10 mA cm−2 for 500 cycles. From Fig. 8, a small increase of capacitance is observed during the first 50 cycles, and the capacitance only decreases by about 1% of the initial capacitance after 500 cycles, indicating a good cycling life of the composite materials. The initial increase of capacitance can be explained as follows: at the initial stage, active materials have not been fully used. After repetitive charge/discharge cycling, the electrochemical active Mn sites inside the nickel foam electrode will be fully exposed to the electrolyte. Therefore, an increasing capacitance was displayed in the cyclic tests. The capacitance retention upon electrochemical cycling and higher specific capacitance of the composite are attributed to the flexibility of GNSs in the composite. The GNSs can not only form an open structure to improve the connection between active material and electrolyte and make full use of electrochemical active MnO2 during the charge and discharge processes, but also improve the electrical conductivity of the overall electrode due to the high conductivity of graphene. 4. Conclusions In this study, the GNS/MnO2 composite, with a unique architecture consisting of GNS nanosheets and urchin-like MnO2 nanoparticles, has been successfully synthesized by a simple hydrothermal method. The porous urchin-like structure MnO2 is composed of interconnected MnO2 nanorods, which grow directly on the surfaces of the GNSs. The special structure endows the composite with high-rate transportation of both electrolyte ions and electrons throughout the electrode matrix and with superior electrochemical utilization of MnO2 . The prepared GNS/MnO2 composite exhibits a high specific capacitance (263 F g−1 ) and excellent long cycle life, suggesting a promising potential for SCs. Acknowledgments This work was supported by the Fundamental Research Funds of the Central University (HEUCFE1107), Science and Technology Planning Project from Education Department of Heilongjiang Province (11553044), High Education Doctoral Fund (160100110010), Special Innovation Talents of Harbin Science and
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Synthesis of reduced graphene nanosheet/urchin-like manganese dioxide composite and high performance as supercapacitor electrode Wanlu Yang a , Zan Gao a , Jun Wang a,b,∗ , Bin Wang a , Qi Liu a , Zhanshuang Li a , Tom Mann a , Piaoping Yang a,b , Milin Zhang a,b , Lianhe Liu a,b a b
Key Laboratory of Superlight Material and Surface Technology, Ministry of Education, Harbin Engineering University, 150001, PR China Institute of Advanced Marine Materials, Harbin Engineering University, 150001, PR China
a r t i c l e
i n f o
Article history: Received 28 September 2011 Received in revised form 21 February 2012 Accepted 22 February 2012 Available online 1 March 2012 Keywords: Graphene nanosheets Urchin-like manganese dioxide Composite electrode Supercapacitor
a b s t r a c t Reduced graphene nanosheet/urchin-like manganese dioxide (GNS/MnO2 ) composite for a supercapacitor electrode has been fabricated by a mild hydrothermal route. Following investigation by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), we propose an in situ formation of MnO2 nanoparticles onto graphene nanosheets. The unique structure greatly increases specific surface area of the composite and the utilization of MnO2 . The electrochemical performance of the electrode is analyzed by cyclic voltammetry, electrochemical impedance spectrometry and chronopotentiometry. Results show that the GNS/MnO2 composite exhibits a maximum specific capacitance of 263 F g−1 and an excellent cycle life with capacity retention of about 99% after 500 cycles. The method provides a facile and straightforward approach to deposit MnO2 nanoparticles onto graphene sheets; it could be readily extended to the preparation of other classes of hybrids based on GNS sheets for technological application. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction In modern society, due to environmental issues and depleting fossil fuels, the increasing demand for renewable energy sources has stimulated intense research on energy storage and conversion from alternative energy sources [1]. Supercapacitors (SCs), as charge-storage devices exhibiting high-power density, excellent reversibility and cycle-ability, are considered as promising candidates for energy storage [2,3]. Energy can be stored in supercapacitors by means of either ion adsorption at the electrode/electrolyte interface (namely, electrical double-layer capacitors, EDLCs) or fast and reversible faradic reactions (namely, pseudocapacitors) [4,5]. To date, carbon materials (activated carbons, carbon aerogels, carbon nanotubes, carbon fabrics, and reduced graphene oxide) [6–12], transition metal oxides (RuO2 , MnO2 , Co3 O4 , NiO) [13–16] and conducting polymers (polyaniline, polypyrrole, and polythiophene) [17–20] have been identified as the most promising materials for SCs. However, each material has its unique advantages and disadvantages for SC application. For example, carbon materials have outstanding electrical properties, long life-cycles and beneficial mechanical properties, but low
∗ Corresponding author at: Key Laboratory of Superlight Material and Surface Technology, Ministry of Education, Harbin Engineering University, 150001, PR China. Tel.: +86 451 8253 3026; fax: +86 451 8253 3026. E-mail address: [email protected] (J. Wang). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2012.02.081
specific capacitance (5–200 F g−1 ). Transition metal oxides and conducting polymers have relatively higher capacitance and fast redox kinetics, while the relatively low mechanical stability and cycle life limits its application for SCs. The challenge for research studies of binary or ternary composites of carbon materials, conducting polymers and metal oxides as materials for supercapacitors, is to effectively utilize their benefits while overcoming their disadvantages. Two-dimensional graphene as a rising star has triggered an exciting new area in the field of carbon nanoscience, due to its high surface area, electrical conductivity, high flexibility and mechanical strength [21–24]. Graphene comprises a single layer of carbon atoms arranged in a graphitic sp2 bonding configuration. This unique nanostructure holds great promise for potential applications in many technological fields such as nanoelectronics, sensors, batteries, supercapacitors, hydrogen storage, and nanocomposites [25–30]. Recently, one new perspective is to ideally utilize single-atom-thick GNS as a support to anchor functional nanomaterials to design next-generation electronic and energy conversion devices, since GNS has shown excellent electrochemical behavior as electrodes of SCs [31–33]. Graphene composites, such as GNS/Co3 O4 , GNS/ZnO, GNS/SnO2 , GNS/LDH and GNS/polyaniline, have been prepared as supercapacitors in aqueous electrolyte [34–38]. The capacitances of graphene composites are remarkably improved while maintaining high-rate electrochemical performance inherent in the two-dimensional structure of graphene, as long as the surfaces of graphene provide not only highly reversible
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pseudocapacitance but also electrochemical double-layer capacitance. GNS composites have shown their superior property as supercapacitor material; however, research on the potential of graphene/metal oxide composite materials for supercapacitors has so far been limited. Deposition of electrochemical active nanoparticles onto graphene sheets to produce supercapacitor electrode materials is therefore promising. Manganese dioxide (MnO2 ), as potential electrode material for the replacement of RuO2 in supercapacitors, has exhibited distinguishing properties owing to its high specific capacitance, environmental compatibility, and cost effectiveness [39–42]. Studies have shown that the electrochemical characteristics of MnO2 materials strongly depend on their structural parameters such as polymorphism, morphology, particle size, and bulk density [42–45]. Up to now, various crystalline structures of MnO2 including onedimensional (1-D) (nanorods, nanowires, nanobelts, nanotubes) [46–49], two-dimensional (2-D) (nanosheets, nanoflakes) [50,51] and three-dimensional (3-D) (nanospheres, nanoflowers) [52,53] have been extensively studied. There are also many reports about manganese oxide composite as supercapacitor electrodes [54,55]. However, to the best of our knowledge, the investigation of the application of 3-D nanostructured manganese dioxide, especially, 3-D nanostructured manganese dioxide composite, as electrode material still remains limited. We also know that the relatively high resistance of MnO2 seriously reduces the capacitive response speed and consequently becomes a bottleneck for its further commercial application. To improve the conductivity of MnO2 , conductive fillers, such as acetylene black, carbon black and carbon nanotubes, are usually added [56–59]. Recently, GNS as a matrix to improve the conductivity of GNS-based composite has attracted widespread attention by researchers [34]. Herein, we report a facile and green method to fabricate the GNS/urchin-like MnO2 binary nanocomposite in a mild hydrothermal system. First, graphene nanosheets are synthesized via an environmental friendly method, using glucose as a reducing agent [60], then 3-D urchin-like MnO2 nanoparticles are directly grown along conducting graphene nanosheets, and act as spacers to keep neighboring sheets separate. The graphene sheets coated with porous urchin-like MnO2 particles overlap with each other affording a three-dimensional conducting network, which facilitates fast electron transfer between the active materials and the charge collector, and improves the contact between the electrode materials and the electrolyte. The deposition of 3-D structure MnO2 directly onto graphene nanosheets as supercapacitor electrode has not been reported. The as-obtained GNS/MnO2 composite exhibited a high specific capacitance (234 F g−1 at 10 mA cm−2 ) and excellent long cycle life; the possible formation mechanism is also presented and discussed.
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with distilled water several times and the obtained GNS was redispersed in water for further use. Typical synthesis process of the GNS/MnO2 nanocomposite is described as follows: an amount of 1.262 g of KMnO4 was dissolved in 65 mL of deionized water and 2 mL of sulfuric acid (98 wt%) was slowly dropped into the solution, followed by the addition of 0.85 g of Cu scraps and 20 mL of the as-obtained GNS suspension (5 mg mL−1 ). After stirring for 15 min, the resultant system was transferred into a stainless steel autoclave and held at 110 ◦ C for 12 h. After cooling to room temperature naturally, the as-prepared products were filtered, washed with distilled water, and dried at 80 ◦ C for 12 h. For comparison, pure MnO2 was prepared under the same conditions without GNS. 2.2. Characterization methods The crystallographic structures of the materials were determined by a powder X-ray diffraction system (XRD, Rigaku TTR-III) equipped with Cu K␣ radiation ( = 0.15406 nm). X-ray photoelectron spectroscopy (XPS) measurements were performed using a PHI 5700 ESCA spectrometer with a monochromated Al K␣ radiation (h = 1486.6 eV). All XPS spectra were corrected by the C1s line at 284.5 eV. The microstructure of the samples was investigated by atomic force microscopy (AFM; NanoscopeIIIa), scanning electron microscopy (SEM; JEOL JSM-6480A microscope), and transmission electron microscopy (TEM; Philips CM 200 FEG, 160 kV). 2.3. Preparation of electrodes and electrochemical characterization The working electrodes were fabricated by a reported procedure [54]. Briefly, the as-prepared materials, acetylene black and polytetrafluoroethylene (PTFE) were mixed in a mass ratio of 80:15:5 and dispersed in ethanol to produce a homogeneous paste. Then the resulting mixture was coated onto the nickel foam substrate (1 cm × 1 cm) to fabricate the electrodes. The mass of the mixture onto the electrode including the electroactive material, conducting agent and binder was 15.8, 16.1, 16.3 mg for GNS, MnO2 and GNS/MnO2 electrode, respectively. The electrochemical properties of as-obtained products were investigated under a three-electrode cell. Platinum foil with the same area and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. All electrochemical measurements were carried out in 1 M Na2 SO4 aqueous electrolyte by a CHI 660D electrochemical work station and all the electrochemical data were analyzed by an appropriate standard [62]. 3. Results and discussion 3.1. Material characterization
2. Experimental 2.1. Synthesis of GNS/MnO2 composite All chemical reagents were analytical grade and used as received. Graphite oxide (GO) was synthesized from natural graphite by a modified Hummers method [61]. Exfoliation of GO was achieved by ultrasonication of the dispersion in an ultrasonic bath (KQ-500DB, 250 W). Compared with the traditional procedure using highly toxic hydrazine as reductant, glucose was used as reducing agent to prepare GNS. Typically, 2 g of glucose was added into 250 mL of homogeneous GO dispersion (0.5 mg mL−1 ), followed by stirring for 30 min. Then 1 mL aqueous ammonia solution (25 wt%) was added to the resulting dispersion. After vigorously shaking for a few minutes, the mixture was stirred for 60 min at 95 ◦ C. The resulting black dispersion was then filtered and washed
XRD patterns of GO, GNS, pure MnO2 powder and the GNS/MnO2 composite are shown in Fig. 1. The feature diffraction peak of exfoliated GO at 10.8◦ (0 0 1) with a basal spacing of 0.82 nm is observed, showing the complete oxidation of graphite to graphite oxide due to the introduction of oxygen-containing functional groups onto the graphite sheets (Fig. 1a) [63]. For the XRD pattern of GNS (Fig. 1b), the peak located at 10.8◦ disappears, while a broad diffraction peak (0 0 2) appears at 2 about 24.5◦ , revealing extensive reduction of GO and exfoliation of the layered GNS [64,65]. The basal spacing of GNS changes from 0.82 nm of GO to 0.37 nm, which is still a little larger than that of natural graphite (0.34 nm). The small amount of residual oxygen-containing groups and hydrogen may be the main reason for this difference, indicating incomplete reduction of GO to graphene. However, the residual oxygen functionalities on the reduced GO surfaces induce electrostatic repulsion stabilizing
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00
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Fig. 1. Typical XRD patterns of GO (a), GNS (b), MnO2 (c) and the prepared GNS/MnO2 (d) composite.
C-C (284.5 eV)
the graphene sheets, which most likely involve the adsorption of metal ions onto the GNS. The diffraction peaks of as-synthesized pure MnO2 and GNS/MnO2 (Fig. 1c and d) are ascribed to the wellcrystallized tetragonal phase of ␣-MnO2 (JCPDS no. 44-0141). From the XRD pattern of the GNS/MnO2 composite (Fig. 1d), diffraction peaks from the ␣-MnO2 phase are observed while the peak at 2 around 24.5◦ corresponding to the (0 0 2) crystal plane of graphene is not evident, indicating that the surfaces of graphene are fully covered by nanoscale MnO2 . This conclusion is also supported by the results of SEM and TEM. To investigate the reduction degree of GO and the composition of manganese oxide of the GNS/MnO2 composite, the samples were further characterized by XPS. Fig. 2 shows the XPS survey spectra and C1s XPS spectra of GO and the GNS/MnO2 composite. Compared with GO, the XPS spectrum of the GNS/MnO2 composite (Fig. 2a) not only exhibits a relatively low O1s peak and C1s peak, but also exhibits two peaks located at 642.1 and 653.7 eV that can be attributed to Mn 2p3/2 and Mn 2p1/2, confirming the presence of MnO2 in the composite. The peak values agree well with those reported for MnO2 , indicating a 4+ oxidation state for Mn [66]. The C1s XPS spectrum of GO (Fig. 2b) indicates a considerable degree of oxidation with three components that are assigned to carbon atoms in different functional groups: the nonoxygenated ring C (C C), the C in C O bonds (C O), and the carboxylate carbon (O C O) [67,68]. From the areas of the three C1s components of graphite oxide, we can compute that the nonoxygenated ring C is about 43% (284.5 eV), while that of the hybrid GNS/MnO2 material is about 75%. Meanwhile, the absorbance band intensities of the carbonyl carbon (C O) and the carboxylate carbon (O C O) decrease sharply for the GNS/MnO2 composite. Results indicate that most of the oxygen functional groups in GO are successfully removed due to the reduction process by glucose. Fig. 3 is a typical AFM image of exfoliated GO dispersion in water after their deposition on a freshly cleaved mica surface through a drop-casting method. The average thickness of as-prepared GO measured from the height profile of the AFM image is about 1.2 nm and corresponds to a single layer GO [69]. Compared with the theoretical values of 0.78 nm for single layer graphene, the higher thickness of GO may arise from oxygen-containing groups on the surfaces [70]. In this paper, utilizing this ideal single-atom-thick reduced graphene oxide as a two-dimensional carbon support to
Intensity / a.u.
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Fig. 3. AFM image of exfoliated GO sheets on mica surface with height profile.
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Fig. 4. SEM (a) and TEM (b) images of pure GNS; SEM (c) and TEM (d) images of pure MnO2 (inset is the partial enlarged image of MnO2 ); SEM (e) and TEM (f) images of GNS/MnO2 composites.
anchor MnO2 nanoparticles provides a promising way to form new composite materials. Morphologies of the as-obtained products are characterized by SEM and TEM as shown in Fig. 4. Most of the graphene nanosheets are curled and entangled together with a layered structure resembling crumpled silk veil waves (Fig. 4a). GNSs agglomerate with each other through van der Waals interactions of the remained oxygen-containing functionalities on the surfaces of GNSs. The – stacking interaction between the graphene sheets can explain the formation of the agglomerates. The TEM images show that GNS has corrugations and scrollings on the edge of the graphene sheet (Fig. 4b). The image of pure MnO2 reveals that the sample has a spherical morphology with 4–6 m diameter and the products are built up of many interleaving nanorods. During the process of analyzing SEM and TEM, a few spheres with an interior cavity are also observed (Fig. 4c and d). As for the GNS/MnO2 composite (Fig. 4e and f), MnO2 nanorods grow along the surface of graphene
sheets, interconnected and reunited as urchin-like sphere later as well. Results show that mono-dispersed MnO2 microspheres densely distribute onto graphene nanosheets, so that the restacking of graphene nanosheets is effectively prevented, and the loss of their highly active surface area is therefore avoided. The TEM image of GNS/MnO2 demonstrates there are numerous pores among the interconnected MnO2 nanorods, resulting in a highly porous structure of the MnO2 layer. This open structure is favorable for the improvement of the electrochemical performance as an electrode for supercapacitors. On basis of the above results, a schematic representation of the formation process for the hybrid GNS/MnO2 material is given in Fig. 5. Previous studies show that GO sheets have their basal planes covered mostly with epoxy and hydroxyl groups, while carbonyl and carboxyl groups are located at the edges [71]. The covered functional groups act as anchor sites favoring in situ formation of nanostructures on the surfaces of GO sheets. However,
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Fig. 5. A schematic representation of the formation process of the hybrid GNS/MnO2 composite.
these oxygen-containing functional groups impair the conductivity of GO sheets to such an extent that they are not suitable for electrode materials. However, GNSs have an excellent conductivity, with ideal single-atom thick substrate for growth of functional nanomaterials to render them electrochemically active and electrically conductive. Step (a) shows the preparation of GNS based on glucose reducing GO, in which glucose as a reductant will remove most of the oxygen-containing groups. When the as-prepared GNS was mixed with the resultant system of KMnO4 solution treated with sulfuric acid and Cu scraps, metal oxide ions are adsorbed onto the surfaces GNSs by the residual oxygen-containing functional groups. Finally, urchin-like MnO2 nanoparticles grow onto the surfaces of graphene nanosheets under a hydrothermal treatment as shown in step (b). The urchin-like MnO2 nanoparticles are attached to the graphene nanosheets, which effectively prevents their restacking and keeps their highly active surface area. And, the capacity and cyclic performance of graphene-based material is largely increased, which is proved in the following electrochemical test. 3.2. Electrochemical properties The capacitive performances of the pristine GNS, MnO2 and the as-prepared GNS/MnO2 composite were evaluated by cyclic voltammetry (CV) and galvanostatic charge/discharge techniques in 1 M Na2 SO4 electrolyte as shown in Fig. 6. The potential window was chosen in the range of −0.2 to 0.8 V (vs. SCE). The CV curves of the GNS/MnO2 electrode at different scan rates from 5 to 40 mV s−1 (Fig. 6A) exhibit quasi-rectangular shapes, indicating all samples have a good capacitive behavior. No peaks were observed at different scan rates in the CV curves, proving that the electrode is charged and discharged at a pseudo-constant rate over the complete voltammetric cycle. The CV curve at the faster scan rate has a larger area than the lower scan rate one, but does not indicate a greater charge capacitance at the higher scan rate. Meanwhile, with the scan rate increasing, the effective interaction between the ions and the electrode is greatly reduced because of the resistance of metal oxide and the deviation from rectangularity of the CV becomes obviously. Fig. 6B shows the CV curves of the pure GNS, MnO2 and the GNS/MnO2 nanocomposite electrode under the scan rate of 10 mV s−1 . By comparison, the area of the CV curve for the GNS/MnO2 electrode is even larger than those for the pure GNS and MnO2 electrode, indicating higher specific capacitance and the synergistic effect of GNS and MnO2 . The constant current charge/discharge curves of GNS/MnO2 composites at different current densities (5, 10, 20 and 40 mA cm−2 ) show that during the charging and discharging steps, the charge curve of binary composites is almost symmetric to its corresponding discharge counterpart
with a small internal resistance (IR) drop, indicating the pseudocapacitive contribution along with the double layer contribution (Fig. 6C). The specific capacitance of the electrode is obtained from the following equation: Csp =
It vm
(1)
where I, t, v and m are the constant current (A), discharge time (s), the total potential difference (V) and the weight of active materials (g), respectively. Therefore, the specific capacitances of the GNS/MnO2 electrode at 5, 10, 20 and 40 mA cm−2 are 263, 227, 191, and 138 F g−1 , respectively. The results indicate that the GNS/MnO2 nanocomposite has a high rate of capacitance, which is recognized as one of the most important electrochemical properties in the application of electrodes and batteries [72]. The Csp value of the GNS/MnO2 nanocomposite electrode from Fig. 6D is about 227 F g−1 at a current density of 10 mA cm−2 , corresponding to a specific capacitance of 127, 64 F g−1 for GNS and MnO2 alone. These values are mainly consistent with the order indicated by the CVs. The advantages of GNS/MnO2 composites electrode over the pure GNS and MnO2 electrode are salient and the excellent electrochemical performances of GNS/MnO2 composites are attributed to their unique microstructure as illustrated in Fig. 5: (1) Urchin-Like MnO2 particles coating the surfaces of graphene nanosheets accumulate to form pores for ion-buffering reservoirs to improve the diffusion rate of ions within the bulk of the prepared materials; (2) The large specific surface area and the nanoscale size of MnO2 phase of the GNS/MnO2 nancomposite greatly reduce the diffusion length over which both ions and electrons must transfer during the charge/discharge process. This ensures a high utilization of the electrode nanoscale size of MnO2 particles; (3) Graphene in the composites not only act as supports for the deposition of MnO2 particles, but also construct a 3-D highly conductive current collector. The excellent interfacial contact between MnO2 and graphene facilitates fast transportation of electrons throughout the whole electrode matrix. This unique architecture enables the GNS/MnO2 nanocomposite electrode to have a large specific surface and fast electron and ion transport simultaneously, thus presenting the best electrochemical capacitive performance. The electrochemical impedance spectroscopy (EIS) analysis has been recognized as one of the principal methods examining the fundamental behavior of electrode materials for supercapacitors [73]. For further understanding, impedance of all products was measured in the frequency range of 100 KHz–0.05 Hz at open circuit potential with an ac perturbation of 5 mV (Fig. 7). The measured impedance spectra were analyzed using the complex nonlinear least-squares (CNLS) fitting method [74] on the basis of the
W. Yang et al. / Electrochimica Acta 69 (2012) 112–119
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Table 1 The calculated values of Re , CDL , Rct , Zw and CL through CNLS fitting of the experimental impedance spectra upon the proposed equivalent circuit in Fig. 7.
GNS MnO2 GNS/MnO2
Re ()
CDL (F)
Rct ()
ZW ()
CL (F)
0.9505 7.707 2.052
0.0009276 9.347E−5 0.000689
0.3995 1.408 0.4221
1.231 0.1152 0.7204
1.496 – 4.815
Fig. 6. (A) CV curves of GNS/MnO2 composite at different scan rates in 1.0 M NaSO4 solution; (B) CV curves of GNS, MnO2 , GNS/MnO2 at 10 mV s−1 ; (C) galvanostatic charge/discharge curves of GNS/MnO2 composite at the current density of 5 (a), 10 (b), 20 (c) and 40 (d) mA cm−2 ; (D) galvanostatic charge/discharge curves of GNS, MnO2 and GNS/MnO2 composites at 10 mA cm−2 .
Fig. 7. Nyquist plots of experimental impedance (EIS) data (scattering dot) and fitting results (solid line) for GNS, pure MnO2 and GNS/MnO2 composite electrodes. Inset shows the electrical equivalent circuit used for fitting impedance spectra (the upper-left for GNS and GNS/MnO2 , the lower-right corner for MnO2 ).
equivalent circuit, which is given in the inset of Fig. 7. At very high frequencies, the intercept at real part (Z ) is a combinational resistance comprising ionic resistance of electrolyte, intrinsic resistance of substrate, and contact resistance at the active material/current collector interface (Re ) [2]. A major difference is the semicircle in the high frequency range which corresponds to the charge transfer resistance (Rct ) caused by the Faradic reactions and the doublelayer capacitance (Cdl ) on the grain surface. The 45◦ slope portion of the curve is the Wurburg resistance (Zw ) which is a result of the frequency dependence of ionic diffusion/transport in the electrolyte and to the surface of the electrode. CL is the limit capacitance [74]. At low frequencies, the impedance plot should theoretically be a vertical line, which is parallel to the imaginary axis. In fact, the low frequency straight line always departs from that expected with a slope angle close to 90◦ due to the existence of “constant phase element” [75]. The straight lines close to 90◦ indicate a pure capacitive behavior and low diffusion resistance of ions in the structure of the electrodes. The obtained values of Re , Cdl , Rct , ZW and CL calculated from CNLS fitting of the experimental impedance spectra are presented in Table 1. Clearly, the Re of GNS/MnO2 composite electrode is smaller than that of MnO2 electrode, which demonstrates that the addition of graphene enhances the conductivity of composites. At the same time, the anchored MnO2 particles prohibit the aggregation of graphene sheets, and the graphene sheets overlap each other to form a conductive network through sheet
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Technology (2010RFXXG007), the Foundation of Harbin Engineering University (no. HEUFT07053). References
200
0.8
99%
0.6
150
E / V vs.SCE
Specific capacitance (F/g)
250
100
0.4 0.2 0.0
50
-0.2 2000
3000
4000
5000
6000
7000
8000
Time / s
0 0
100
200
300
400
500
Cycle number Fig. 8. Cyclic performance of GNS/MnO2 composite electrodes at 10 mA cm−2 in 1.0 M NaSO4 electrolyte; the inset shows charge/discharge curves of the GNS/MnO2 electrodes in potential range from −0.2 to 0.8 V at 10 mA cm−2 .
plane contact, which facilitates the fast electron transfer between the active materials and the charge collector. The cycle stability of GNS/MnO2 composites was evaluated by repeating the constant current charge/discharge test between −0.2 and 0.8 V (vs. SCE) at a current density of 10 mA cm−2 for 500 cycles. From Fig. 8, a small increase of capacitance is observed during the first 50 cycles, and the capacitance only decreases by about 1% of the initial capacitance after 500 cycles, indicating a good cycling life of the composite materials. The initial increase of capacitance can be explained as follows: at the initial stage, active materials have not been fully used. After repetitive charge/discharge cycling, the electrochemical active Mn sites inside the nickel foam electrode will be fully exposed to the electrolyte. Therefore, an increasing capacitance was displayed in the cyclic tests. The capacitance retention upon electrochemical cycling and higher specific capacitance of the composite are attributed to the flexibility of GNSs in the composite. The GNSs can not only form an open structure to improve the connection between active material and electrolyte and make full use of electrochemical active MnO2 during the charge and discharge processes, but also improve the electrical conductivity of the overall electrode due to the high conductivity of graphene. 4. Conclusions In this study, the GNS/MnO2 composite, with a unique architecture consisting of GNS nanosheets and urchin-like MnO2 nanoparticles, has been successfully synthesized by a simple hydrothermal method. The porous urchin-like structure MnO2 is composed of interconnected MnO2 nanorods, which grow directly on the surfaces of the GNSs. The special structure endows the composite with high-rate transportation of both electrolyte ions and electrons throughout the electrode matrix and with superior electrochemical utilization of MnO2 . The prepared GNS/MnO2 composite exhibits a high specific capacitance (263 F g−1 ) and excellent long cycle life, suggesting a promising potential for SCs. Acknowledgments This work was supported by the Fundamental Research Funds of the Central University (HEUCFE1107), Science and Technology Planning Project from Education Department of Heilongjiang Province (11553044), High Education Doctoral Fund (160100110010), Special Innovation Talents of Harbin Science and
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