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reduced graphene oxide for supercapacitor application
Accepted Manuscript Facile synthesis of layered MnWO4/reduced graphene oxide for supercapacitor application Jianhua Tang, Jianfeng Shen, Na Li, Mingxin Ye PII:
S0925-8388(15)32010-7
DOI:
10.1016/j.jallcom.2015.12.219
Reference:
JALCOM 36308
To appear in:
Journal of Alloys and Compounds
Received Date: 8 July 2015 Revised Date:
12 November 2015
Accepted Date: 26 December 2015
Please cite this article as: J. Tang, J. Shen, N. Li, M. Ye, Facile synthesis of layered MnWO4/reduced graphene oxide for supercapacitor application, Journal of Alloys and Compounds (2016), doi: 10.1016/ j.jallcom.2015.12.219. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Facile synthesis of layered MnWO4/reduced graphene oxide for supercapacitor application Jianhua Tang, Jianfeng Shen*, Na Li, Mingxin Ye ∗
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Institute of Special Materials and Technology, Fudan University, Shanghai, 200433, China
Abstract
The layered MnWO4/reduced graphene oxide (MnWO4/RGO) was prepared
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through a facile one-pot low-temperature hydrothermal route without using any templates. The structure and morphology of MnWO4/RGO nanocomposite were
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characterized through X-ray diffraction, Raman spectra, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission
electron
microscopy,
nitrogen
adsorption/desorption
and
thermo-gravimetric analysis. While its electrochemical behaviors were investigated
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using cyclic voltammograms, galvanostatic charge/discharge and electrochemical impedance spectroscopy. In the case of three electrode cells, MnWO4/RGO with 7.28 wt% RGO content fulfilled a maximum specific capacitance of 288 F g-1 at 5 mV s-1
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with the potential range from -0.35 to 0.55 V. While in the two electrode cell, it obtained a maximum specific capacitance of 109 F g-1 at 5 mV s-1 and displayed the
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cycle life of 14.9 % capacitance decline after 6000 cycles.
Keywords: Layered MnWO4, Reduced graphene oxide, Supercapacitors, Electrode materials ∗ Corresponding author: Tel.: Fax: +86-021-55664094. E-mail address: [email protected] (Mingxin Ye).
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1. Introduction In the wake of environmental pollution and depletion of traditional energy
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resource, alternative energy conversion/storage systems with high specific power and energy have attracted extensive attention [1]. Among the various emerging energy
storage systems, supercapacitors (SCs), also known as electrochemical capacitors or
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ultracapacitors, are projected to be one of the best candidates for the next-generation power device because of their high specific power density, fast charge and discharge
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rates, long cycle life, low maintenance cost, safe operation, and environmental friendliness [2, 3]. Such outstanding properties make them promising in a variety of applications, such as consumer portable electronics, mobile communications, hybrid electric vehicles, back-up power supplies, and military devices [4]. According to the
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charge-discharge mechanisms, SCs can be classified into two categories: electrical double-layer capacitors (EDLCs) and pseudocapacitors [5, 6]. EDLCs physically store energy through the reversible adsorption of the electrolyte ions onto active materials
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[7], while the capacitance of pseudocapacitors comes from reversible Faradaic reactions taking place at the electrode/electrolyte interface [8, 9]. EDLCs based on
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carbon materials have high power density and achieve excellent charge–discharge cycling
stability
but
with
relatively
low
capacitance
[10].
In
contrast,
pseudocapacitors based on transition metal oxides and conducting polymers can achieve much higher specific capacitances. However, they suffer from low electrical conductivity and poor cycling stability [11]. Therefore, increasing research efforts have been focused on combining pseudocapacitive materials and highly conductive
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materials [12]. Graphene with a two-dimensional sp2-hybridized carbon structure has attracted
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enormous attention due to its remarkable physicochemical properties such as superior electrical conductivity, high thermal conductivity, unprecedented pliability, high specific surface area and great mechanical strength [13]. It has been considered as an
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excellent electrode material for SCs [14]. Unfortunately, graphene tends to restack
face-to-face due to intensive π–π interaction between neighboring nanosheets, which
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results in severely deteriorated electrochemical performance [15]. The effective surface area of graphene is an important factor that affects the capacitance of EDLCs. Therefore, much effort has been devoted to develop strategies of avoiding the agglomeration of graphene. A major solution is to introduce a pseudocapacitive
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component which not only prevents restacking of graphene but also improves the specific capacitance [16].
As reported in some literatures, the binary metal oxide MnWO4, one of the
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tungstates, has drawn wide attention for its excellent catalytic, magnetic and electrochemical characteristics [17-20]. Usually, the binary metal oxides of tungstate
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present higher conductivity than pure metal oxide since the introduction of W atoms can greatly improve their conductivity [21]. Based on these encouraging features, MnWO4 should be a promising electrode material for SCs. Apart from the essential
properties of the electrode materials, the morphology also plays an important role in the electrochemical performance. Recently, two-dimensional electrode materials of SCs have attracted a wide range of interest owing to their large electrochemically
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active surfaces [22]. However, to our best knowledge, layered MnWO4 as electrode material for SCs has never been researched, especially the electrochemical properties
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of MnWO4 combined with graphene. Thus, layered MnWO4/reduced graphene oxide (MnWO4/RGO) nanocomposite could be synthesized to explore their potential applications as a promising electrode material for SCs.
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In this work, layered MnWO4/RGO was prepared through a facile one-pot
low-temperature hydrothermal route without using any templates. This method needs
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no requirement for harsh conditions and avoids the use of harmful surfactants or templates. In addition, the electrochemical properties of the MnWO4/RGO nanocomposite may result in potential applications for supercapacitors. 2. Experimental
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2.1 Materials
Pristine graphite was purchased from Qingdao BCSM. MnCl2·4H2O, Na2WO4·2H2O and ascorbic acid were supplied by Shanghai Chemical Reagent
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Company. They were at least of analytical reagent grade and used without further purification.
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2.2 Preparation of MnWO4 and MnWO4 /RGO nanocomposite The MnWO4/RGO nanocomposite was prepared via a one-pot low-temperature
hydrothermal method with different ratios of graphene oxide (GO) and MnWO4. The
product was named as MnWO4 /RGO-x, where x is the amount of GO. For example, the MnWO4 /RGO-75 were typically synthesized as follows: graphene oxide (GO) was synthesized with Hummers method as reported elsewhere [23]. 75 mg graphite
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oxide was exfoliated to GO in 50 mL deionized water by ultrasonication for 1 h. Then, 2 mmol MnCl2·4H2O was dissolved in the GO dispersion and stirred at 70°C for
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about 10 min. Afterwards, 20 mL of distilled water containing 2 mmol Na2WO4·2H2O was added dropwise. 300 mg ascorbic acid (ascorbic acid and GO 4: 1 in weight) was added to the above suspension and dissolved. Subsequently, the mixture was
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transferred into a Teflon-lined autoclave, and heated in an oven at 100 °C for 8 h. The
collected solid product was washed with water and ethanol three times each and dried
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in vacuum at 60 °C for 8 h. In the control experiment, similar procedure was conducted in the absence of GO and ascorbic acid. 2.3 Materials Characterizations
X-ray diffraction (XRD) powder patterns were taken on D/max-γB
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diffractometer using Cu Kα radiation. The measurement was conducted from 5º to 90º with a scan rate of 8 º min-1. Raman spectra were recorded on a Dilor LABRAM-1B multi-channel confocal micro-spectrometer by using an excitation laser wavelength of
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514 nm. X-ray photoelectron spectroscopy (XPS) was performed on XR 5 VG (UK) using a monochromatic Mg X-ray source. The XPS peaks were deconvoluted by
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using Gaussian components after a Shirley background subtraction. Fourier transform infrared spectroscopy (FTIR) spectra were recorded on a Nicolet IS10 spectrometer and samples were measured with KBr pellets. The spectrum was generated, collected 16 times, and was corrected for the background noise. The morphologies of the as-prepared materials were characterized by scanning electron microscopy (SEM, PHILIPS XL30FEG). The investigation of the structure was performed by
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transmission electron microscopy (TEM) using a JEOL 2010F (JEOL Ltd., Japan). The specific surface areas were obtained from the nitrogen adsorption/desorption
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isotherm (Micromeritics, Tristar3000 porosimeter). Thermo-gravimetric analysis (TGA) was carried out in air atmosphere with a heating rate of 10 ºC/min using a Netzsch TG 209F1 to analyze the contents of graphene.
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2.4 Electrochemical Measurements
To prepare the working electrode, 80 wt% active materials, 10 wt% carbon black
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as a conductive additive and 10 wt% polyvinylidene fluoride (PVDF) binder were mixed together and ground thoroughly. Then the mixture was further dispersed in N-methyl-2-pyrrolidone to form the homogeneous slurry which was pasted on nickel foams. Consequently, the foams were dried at 120 ºC for 12 h in a vacuum oven and
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the dried nickel foams were pressed to be a thin foil at a pressure of 10 MPa for 1 min. Electrochemical properties of the electrodes were measured in 6 M KOH electrolyte. Two- and three-electrode cells were used for the measurements. In the case of three
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electrode cells, the prepared electrode, platinum foil, and Ag/AgCl electrode were used as the working, counter, and reference electrodes, respectively. The MnWO4
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/RGO symmetric two cell system having an equal mass loading was fabricated using polypropylene
as
the
separator.
Cyclic
voltammograms(CV),
galvanostatic
charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS) were measured on an electrochemistry workstation (Autolab PGSTAT128N). 3. Results and discussion The synthesis of MnWO4/RGO nanocomposite is outlined in the Fig. 1. During
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the formation of MnWO4/RGO nanocomposite, Mn2+ ions on the surface of negatively-charged GO sheets combined with WO42- ions, then the crystallization of
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MnWO4 and the reduction of GO happened simultaneously in the one-pot hydrothermal process. Insert Fig. 1
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The crystallographic structures of GO, MnWO4 and MnWO4 /RGO-x were analyzed by X-ray powder diffraction (XRD) in Fig. 2. In the pattern of the GO, the
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strong diffraction peak at 10.5º indicates the successful oxidation of the graphite and the formation of GO. As for MnWO4 and MnWO4 /RGO-x nanocomposite, all the diffraction patterns are similar and reveal the formation of the monoclinic structure phase of MnWO4 (according to the JCPDS card No. 13-0434). The characteristic
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peaks of the MnWO4 /RGO-x totally agree with those of the MnWO4, while there is no obvious diffraction peak around 10.5º owing to the increase of the disorder degree during the reduction of the GO. During the formation of MnWO4 /RGO, the MnWO4
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in the nanocomposite prevented the stack of the RGO sheets and greatly reduced the degree of crystallization of RGO. What’s more, the characteristic diffraction peaks of
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RGO were easily shielded by the strong diffraction peaks of MnWO4. Insert Fig.2
In order to confirm the presence of RGO in MnWO4 /RGO-x, the Raman
spectra of MnWO4, GO and MnWO4 /RGO-x are shown in Fig. 3 (a). The Raman spectrum of MnWO4 exhibits an intense peak at 915 cm-1 along with some medium intensity peaks appearing at 799, 714, 507, 422, 325, 203, 173 and 129 cm-1, which
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can be attributed to the characteristic peaks of MnWO4 [20]. The characteristic D band around 1350 cm–1 is related to the presence of defects, while G band around
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1590 cm-1 corresponds to the first-order scattering of the E2g phonons of the graphitic structure of carbon atoms [24]. The D band and G band are clearly observed in all the Raman spectra of GO and MnWO4 /RGO-x. A high ratio of the intensities of the
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D and G band (ID/IG) usually indicates a high degree of disorder [25]. During the process of the GO reduction, fragmentations along the reactive sites were produced,
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resulting in smaller size but more numerous RGO. The edges of the small size RGO could act as defects and lead to a high disorder degree of the RGO, resulting in the increase of D band. What’s more, the insertion of MnWO4 into RGO nanosheets could cause a further decrease of sp2 carbon domain. Therefore, ID/IG of MnWO4
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/RGO-x was higher than that of GO. The Raman spectra analysis indicates that the MnWO4 has been successfully formed and the RGO of MnWO4 /RGO-x has a relatively high reduction degree.
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Fig. 3 (b) exhibits the FT-IR spectra of GO, MnWO4 and MnWO4/RGO-x. The oxygen-containing groups on the surface of GO, such as C-OH (3415 cm-1), C-O-C
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(1260 cm-1), the alkoxy C-O stretching peak (1056 cm-1) and carboxyl C=O and C-O
(1726 and 1398 cm-1), indicate that the GO has a relatively high degree of oxidation. In the spectrum of MnWO4, the characteristic bands are observed at 455, 592, 735,
792, and 885 cm-1, which would be related to vibration modes of MnWO4 [26]. In the spectra of MnWO4/RGO-x, the main characteristic peaks of GO almost disappear, which confirms the reduction of GO. However, some peaks of oxygen-containing
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groups still remains after reduction, which is favorable to improve the dispersion of the sample and the hydrophilicity of the electrode materials. In addition, the typical
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absorption features of MnWO4 can be observed in the spectra of MnWO4/RGO-x. Insert Fig. 3
In order to prove that the sample contained qualitative elements, X-ray
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photoelectron spectroscopy (XPS) characterization was performed to further evaluate the chemical bonding state and composition of the as-synthesized MnWO4/RGO-75.
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In the C 1s spectrum (Fig. 4(a)), the peak located at 285.52 eV is attributed to the C–C, C=C and C–H bonds. While the peaks centered at the binding energies of 286.51, 288.48 and 291.03 eV were assigned to the C-OH, C=O and O=C-O functional groups, respectively, indicating that few oxygen-containing groups are still left in the RGO
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[27]. Fig. 4 (b) is the Mn 2p region of the XPS of as-prepared MnWO4/RGO-75 sample. The binding energy of Mn 2p3/2with 640.85 eV implies the presence of Mn ions in the form of divalent state [28]. The XPS spectrum of W 4f is shown in Fig. 4
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(c). It has two peaks which correspond to W 4f7/2 and W4f5/2 at 34.41 and 36.60 eV, respectively [20]. In the spectrum of the O 1s (Fig. 4(d)), the peak with binding
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energy at 530.61 eV is due to the O2- forming oxide with manganese and tungsten
elements, while the peak located at 532.29 eV is likely attributed to surface O-H group of RGO [20, 27]. The successful formation of MnWO4 in the MnWO4/RGO-75 can be confirmed through the XPS characterization, while the C 1s spectrum certifies the existence of RGO. Insert Fig. 4
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The composition of the MnWO4/RGO-x was further studied by TGA analysis. The weight losses of RGO, MnWO4, and MnWO4/RGO-x are identified by TGA
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measurement in Fig. 5. In the TGA curve of RGO, the slight weight loss before 400 °C is attributed to the removal of surface absorbed water and the decomposition
of the few residual oxygen-containing groups of RGO. The relatively serious weight
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loss from 519 °C to 600 °C is due to the burning of RGO in air. As shown in Fig. 5, MnWO4 suffers no obvious weight loss and shows a relatively good thermal stability.
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In the TGA curves of MnWO4/RGO-x composite, according to the weight loss of RGO, the weight percentage of RGO of MnWO4/RGO-25 to MnWO4/RGO-100 nanocomposite is about 4.60%, 6.52%, 7.28%, and 9.45%, respectively. Insert Fig. 5
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SEM observations on the MnWO4 and MnWO4/RGO-75 are presented in Fig. 6. It can be clearly seen from Fig. 6 (a) and (b) that the MnWO4 is composed of nanoflakes, and the nanoflakes are assembled into sphere-like architecture. Obviously,
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in the presence of RGO, MnWO4/RGO-75 composite displays a different morphology from that of MnWO4. As is seen in the Fig. 6 (c), corrugated RGO nanosheets are
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found to be well expanded with the decoration of layered MnWO4 nanoflakes. However, compared to MnWO4, the MnWO4/RGO-75 exhibited a more irregular morphology. It might be explained by that the nuclei formation and crystal growth of the MnWO4 were affected by the oxygen-containing groups of GO during the reaction.
Besides, the binding effect of GO can also hinder the assembly of MnWO4. In Fig. 6 (d), the EDS pattern of MnWO4/RGO-75 containing the elements of Mn, W and O,
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could further confirm the formation of MnWO4 in the MnWO4/RGO-75. The presence of carbon element in the EDS pattern could be the further proof of the existence of
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RGO. To further examine the details of morphology, TEM image of MnWO4/RGO-75
(Fig.6 (e)) was taken. It is clearly demonstrated that the layered MnWO4 are anchored
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onto the surface of the winked transparent RGO, which is consistent with SEM results. In order to investigate specific surface area of the as-prepared samples, nitrogen were
measured.
As
determined
by
nitrogen
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adsorption-desorption
adsorption/desorption isotherm measurements (Fig. 6(f)), the specific surface areas of the MnWO4 and MnWO4/RGO-75 are 25.66 and 60.34 m2 g−1, respectively. The high BET specific surface can increase the reactive sites between electrode and electrolyte,
Insert Fig.6
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which can greatly improve the electrochemical performance of MnWO4/RGO.
In the case of three electrode cells, the cyclic voltammograms (CV) and
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galvanostatic charge-discharge (GCD) measurements were conducted in 6 M KOH within the operation voltage of -0.35–0.55 V vs. Ag/AgCl. The detailed
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electrochemical processes of MnWO4 and MnWO4 /RGO-x are illustrated by CV
measurements at the scan rates of 5 mV s-1 in Fig. S1 (a). The shapes of the CV curves of MnWO4 and MnWO4 /RGO-x are obviously different from that of the electric double-layer capacitances which shows a rectangular shape. Obviously, a pair of redox reaction peaks corresponding to the reversible faradaic processes demonstrates typical pseudocapacitive characteristics of MnWO4 and MnWO4 /RGO-x. The specific
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capacitance of the electrode can be calculated from CV curves by the equation: C=∫idV/(2vm△V)
(1)
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Where i is the current, V is the potential, v is the potential scan rate, m is the mass of the electro active material and △V is the total potential deviation of the voltage
window. Fig. S1 (b) shows the CV curves of MnWO4 /RGO-75 at different scan rates
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of 5, 10, 20, 50 and 100 mV s-1. The slight deviations in the CV curves shapes of
MnWO4 /RGO-75 are observed at relatively high scan rates, which is due to the low
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interaction between the electrolyte ions and the electrode. As shown in Fig. S1 (c), the corresponding specific capacitances calculated from the CV curves of MnWO4, MnWO4 /RGO-25, MnWO4 /RGO-50, MnWO4 /RGO-75 and MnWO4 /RGO-100 electrodes at 5 mV s-1 are 68, 133, 197, 288 and 235 F g-1, respectively. The
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improvement of the electrochemical performance of MnWO4/RGO-x can be attributed to the synergy effect between MnWO4 and RGO. The good dispersion of MnWO4 on the surface of RGO can avoid the aggregation of both MnWO4 and RGO,
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leading to higher active surface area for charge storage. What’s more, the good conductivity of RGO is benefit to the charge transfer of the electron. However,
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excessive RGO may decrease the contribution of the pseudocapacitive material of MnWO4 and lead to the aggregation of RGO. Therefore, among the MnWO4 and
MnWO4 /RGO-x, MnWO4 /RGO-75 fulfilled the highest capacitance instead of
MnWO4 /RGO-100. Fig. S2 shows the charge-discharge behaviors of MnWO4 and MnWO4 /RGO-x electrodes with a working potential window of 0.9 V. In the Fig. S2 (a), the
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charge-discharge curves of MnWO4 and MnWO4 /RGO-x show some curvature, which is due to redox transitions with the electrolyte and absorption/desorption
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process at the electrode/electrolyte interface. This corresponds to the typical redox couples in the CV curves. The derived specific capacitance results from both the
adsorption/desorption of electrolyte ions in the electrochemical double layer and the
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redox reactions on the surface of the active material. The faradic redox reaction is due to the electron transfer (Mn2+/Mn3+). The specific capacitances were calculated
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according to the following equation: C = (I·∆t) / (m·∆V)
(2)
Here, I is the discharge current, ∆t is the discharge time, m is the mass of an active material, and ∆V is the potential change during discharging.
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In the Fig. S2 (b), with the increase of the current density, the specific capacitances of MnWO4 /RGO-75 show a decreasing tendency, which suggest that parts of the electrode fail to react completely at high current density. During the high-rate
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charge-discharge process, the ionic motion in the electrolyte is limited by diffusion owing to the time constraint, therefore, only the outer active material can be used to
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store energy [29]. It is noted that the MnWO4 /RGO-75 electrode achieved a
maximum specific capacitance of 261 F g-1 at 0.5 A g-1 and could maintain 142 F g-1 even at 10 A g-1, which showed excellent rate performance. Besides, there is no
obvious iR drop even at the current density of 10 A g-1, which is due to the high rate capability and little internal resistance. The significant improvement of the specific capacitance of MnWO4 /RGO-x over MnWO4 could be credited to the existence of
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RGO which provided good conductivity and more space for electrolyte diffusion. The performances of the supercapacitor based on the MnWO4/RGO-x material
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were also measured by a two-electrode symmetrical system in 6 M KOH electrolyte. Fig. 7 (a) shows the CVs of the symmetric MnWO4/RGO-75//MnWO4 /RGO-75 supercapacitor from 0 to 1.5 V at different scan rates from 5 to 100 mV s-1. The
equation:
(3)
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C=4∫idV/(vm△V)
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specific capacitance of the electrode can be calculated from CV curves by the
Where i is the current, V is the potential, v is the potential scan rate, m is the total mass of the two electrodes and △V is the total potential deviation of the voltage window.
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The calculated specific capacitances of the MnWO4/RGO-75 are 109, 98, 90, 80, and 72 F g −1 at different scan rates of 5, 10, 20, 50 and 100 mV s-1, respectively. In Fig. 7 (b), the galvanostatic charge–discharge measurements were further conducted at
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various current densities within the potential window from 0 to 1.5 V. The average discharge capacitances of the electrode were calculated according to the galvanostatic
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charge/discharge tests by the following equation: C = 4(I·∆t) / (m·∆V)
(4)
where I is the discharge current, ∆t is the discharge time, m is the total mass of the
two electrodes, ∆V is the operating cell voltage. The specific capacitances of the MnWO4/RGO-75 evaluated from the discharge curves are 101, 93, 83 and 76 F g-1 at the current densities of 0.5, 1, 2 and 5 A g-1,
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respectively. Insert Fig. 7
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Electron impedance spectroscopy (EIS) is a very powerful measurement for investigating the kinetic process of the electrode reactions and providing
complementary information about the frequency response of electrode [30]. In order
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to further research the electrochemical behavior of MnWO4 and MnWO4 /RGO-x, EIS analysis for two-electrode test cell was also conducted in the frequency range of 100
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kHz - 0.01 Hz in Fig. 8 (a). The impedance plots include a partial semicircle at the high-frequency and a straight slopping line along the imaginary axis at the low-frequency. The intercept of the arc on the real axis at very high frequency represents a combined resistance (Rs) including intrinsic resistance of the active
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materials and the ohmic resistance of electrolyte [4]. As it can be seen, the Rs of MnWO4 /RGO-x, compared to that of the MnWO4 electrode, are much smaller which shows the smaller bulk resistance. The depressed semicircle in the high-frequency
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region is attributable to a double layer capacitance (Cd) and an interfacial charge transfer resistance (Rct) occurring at the electrode-electrolyte interface [31]. The
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diameter of this semicircle corresponds to the interfacial charge transfer resistance (Rct), which is mainly associated with the faradic reactions. The Rct for MnWO4, MnWO4 /RGO-25, MnWO4 /RGO-50, MnWO4 /RGO-75 and MnWO4 /RGO-100 are
10.09, 4.17, 3.83, 3.93 and 3.69 Ω, respectively. Because of the effect of the RGO, the as-prepared MnWO4 /RGO-x achieved fast electron and ion transport, which lead to the much smaller Rct than that of MnWO4. The line in the low frequency called as
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Warburg impedance (Ws) is related to the electrolyte ions diffusion in the electrode [4]. MnWO4/RGO-x electrode shows a much more vertical line than MnWO4
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electrode, which illustrates that MnWO4/RGO-x electrode has a smaller Warburg impedance (Ws) because the electrolyte ions diffusion in the electrode occurs more
swiftly. The near vertical line the in the low-frequency region demonstrates the ideally
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capacitive behavior of MnWO4/RGO-x electrode. It can be explained that the
existence of RGO in MnWO4/RGO-x provides more space and easy access for
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electrolyte diffusion. The small Rs, Rct and Ws of MnWO4/RGO-x electrode play a very important role in improving the performance of supercapacitors. The cycling stability of the electrode is one of the most important parameters for the practical application of supercapacitors [12]. Therefore, the cycling life of the
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MnWO4 and MnWO4 /RGO-75 electrodes were conducted in the two electrode cell at a current density of 1 A g-1 for 6000 cycles in Fig. 8 (b). For the electrode with MnWO4 coating, the capacitance decreased ∼27.2% after 6000 cycles, while the
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MnWO4 /RGO-75 electrode declined ∼14.9% after 6000 cycles. The cycling stability of MnWO4 /RGO-75 electrode can owe to the presence of RGO which improves
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stability of the electrode material. Insert Fig. 8
4. Conclusion
In this work, MnWO4/RGO was prepared through a facile one-pot low-temperature hydrothermal method without using any templates. Owing to the synergy effect between MnWO4 and RGO, the MnWO4/RGO-75 nanocomposite
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fulfilled the capacitance of 288 F g-1 at 5 mV s-1 in the three electrode cell and 109 F g-1 at 5 mV s-1 in the two electrode cell. The electrochemical properties and the facile
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synthesis method of the MnWO4/RGO nanocomposite may result in potential
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applications for supercapacitors.
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functionalized carbon for ultrahigh supercapacitor performance, Nat Commun. 4
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(2013) 2923.
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List of Figure Captions Fig. 1 Schematic illustration for preparation of MnWO4/RGO in this study. Fig. 2 XRD patterns of GO, MnWO4, and MnWO4 /RGO-x nanocomposite.
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Fig. 3 Raman (a) and FTIR (b) spectra of GO, MnWO4 and MnWO4 /RGO-x nanocomposite.
Fig. 4 XPS of MnWO4/RGO-75 for the C 1s region (a), Mn 2p region (b), W 4f
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region (c) and O 1s region (d), respectively. Fig. 5 TGA curves of RGO, MnWO4, and MnWO4/RGO-x.
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Fig. 6 SEM images of MnWO4 (a, b) and MnWO4/RGO-75 (c), and the corresponding EDS patterns of MnWO4/RGO-75 (d); TEM image of MnWO4/RGO-75 (e) and nitrogen adsorption/desorption isotherms (f) of MnWO4 and MnWO4 /RGO-75. Fig. 7 CV curves (two-electrode cell) of MnWO4/RGO-75 at different scan rates of 5,
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10, 20, 50 and 100 mV s-1 (a) and galvanostatic charge-discharge curves (two-electrode cell) of MnWO4 /RGO-75 at different current densities of 0.5 A g-1, 1 A
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g-1, 2 A g-1 and 5A g-1 (b).
Fig. 8 EIS plots (two-electrode cell) of MnWO4 and MnWO4 /RGO-x (a), and cycling
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performances (two-electrode cell) of MnWO4 and MnWO4 /RGO-75 electrodes at 1 A g-1 for 6000 cycles (b).
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Fig. 1 Schematic illustration for preparation of MnWO4/RGO in this study.
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Fig. 2 XRD patterns of GO, MnWO4, and MnWO4 /RGO-x nanocomposite.
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Fig. 3 Raman (a) and FTIR (b) spectra of GO, MnWO4 and MnWO4 /RGO-x
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nanocomposite.
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Fig. 4 XPS of MnWO4/RGO-75 for the C 1s region (a), Mn 2p region (b), W 4f
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region (c) and O 1s region (d), respectively.
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Fig. 5 TGA curves of RGO, MnWO4, and MnWO4/RGO-x.
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Fig. 6 SEM images of MnWO4 (a, b) and MnWO4/RGO-75 (c), and the corresponding EDS patterns of MnWO4/RGO-75 (d); TEM image of MnWO4/RGO-75 (e) and
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nitrogen adsorption/desorption isotherms (f) of MnWO4 and MnWO4 /RGO-75.
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Fig. 7 CV curves (two-electrode cell) of MnWO4/RGO-75 at different scan rates of 5,
10, 20, 50 and 100 mV s-1 (a) and galvanostatic charge-discharge curves
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g-1, 2 A g-1 and 5A g-1 (b).
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(two-electrode cell) of MnWO4 /RGO-75 at different current densities of 0.5 A g-1, 1 A
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Fig. 8 EIS plots (two-electrode cell) of MnWO4 and MnWO4 /RGO-x (a), and cycling
performances (two-electrode cell) of MnWO4 and MnWO4 /RGO-75 electrodes at 1 A
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g-1 for 6000 cycles (b).
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Highlights (1) The MnWO4/RGO was first prepared through a facile hydrothermal route.
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(2) MnWO4/RGO composite show much higher specific capacitances than pure MnWO4
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(3) The electrochemical properties of MnWO4/RGO arise from the synergistic effect.
S0925-8388(15)32010-7
DOI:
10.1016/j.jallcom.2015.12.219
Reference:
JALCOM 36308
To appear in:
Journal of Alloys and Compounds
Received Date: 8 July 2015 Revised Date:
12 November 2015
Accepted Date: 26 December 2015
Please cite this article as: J. Tang, J. Shen, N. Li, M. Ye, Facile synthesis of layered MnWO4/reduced graphene oxide for supercapacitor application, Journal of Alloys and Compounds (2016), doi: 10.1016/ j.jallcom.2015.12.219. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Facile synthesis of layered MnWO4/reduced graphene oxide for supercapacitor application Jianhua Tang, Jianfeng Shen*, Na Li, Mingxin Ye ∗
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Institute of Special Materials and Technology, Fudan University, Shanghai, 200433, China
Abstract
The layered MnWO4/reduced graphene oxide (MnWO4/RGO) was prepared
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through a facile one-pot low-temperature hydrothermal route without using any templates. The structure and morphology of MnWO4/RGO nanocomposite were
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characterized through X-ray diffraction, Raman spectra, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission
electron
microscopy,
nitrogen
adsorption/desorption
and
thermo-gravimetric analysis. While its electrochemical behaviors were investigated
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using cyclic voltammograms, galvanostatic charge/discharge and electrochemical impedance spectroscopy. In the case of three electrode cells, MnWO4/RGO with 7.28 wt% RGO content fulfilled a maximum specific capacitance of 288 F g-1 at 5 mV s-1
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with the potential range from -0.35 to 0.55 V. While in the two electrode cell, it obtained a maximum specific capacitance of 109 F g-1 at 5 mV s-1 and displayed the
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cycle life of 14.9 % capacitance decline after 6000 cycles.
Keywords: Layered MnWO4, Reduced graphene oxide, Supercapacitors, Electrode materials ∗ Corresponding author: Tel.: Fax: +86-021-55664094. E-mail address: [email protected] (Mingxin Ye).
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1. Introduction In the wake of environmental pollution and depletion of traditional energy
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resource, alternative energy conversion/storage systems with high specific power and energy have attracted extensive attention [1]. Among the various emerging energy
storage systems, supercapacitors (SCs), also known as electrochemical capacitors or
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ultracapacitors, are projected to be one of the best candidates for the next-generation power device because of their high specific power density, fast charge and discharge
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rates, long cycle life, low maintenance cost, safe operation, and environmental friendliness [2, 3]. Such outstanding properties make them promising in a variety of applications, such as consumer portable electronics, mobile communications, hybrid electric vehicles, back-up power supplies, and military devices [4]. According to the
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charge-discharge mechanisms, SCs can be classified into two categories: electrical double-layer capacitors (EDLCs) and pseudocapacitors [5, 6]. EDLCs physically store energy through the reversible adsorption of the electrolyte ions onto active materials
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[7], while the capacitance of pseudocapacitors comes from reversible Faradaic reactions taking place at the electrode/electrolyte interface [8, 9]. EDLCs based on
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carbon materials have high power density and achieve excellent charge–discharge cycling
stability
but
with
relatively
low
capacitance
[10].
In
contrast,
pseudocapacitors based on transition metal oxides and conducting polymers can achieve much higher specific capacitances. However, they suffer from low electrical conductivity and poor cycling stability [11]. Therefore, increasing research efforts have been focused on combining pseudocapacitive materials and highly conductive
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materials [12]. Graphene with a two-dimensional sp2-hybridized carbon structure has attracted
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enormous attention due to its remarkable physicochemical properties such as superior electrical conductivity, high thermal conductivity, unprecedented pliability, high specific surface area and great mechanical strength [13]. It has been considered as an
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excellent electrode material for SCs [14]. Unfortunately, graphene tends to restack
face-to-face due to intensive π–π interaction between neighboring nanosheets, which
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results in severely deteriorated electrochemical performance [15]. The effective surface area of graphene is an important factor that affects the capacitance of EDLCs. Therefore, much effort has been devoted to develop strategies of avoiding the agglomeration of graphene. A major solution is to introduce a pseudocapacitive
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component which not only prevents restacking of graphene but also improves the specific capacitance [16].
As reported in some literatures, the binary metal oxide MnWO4, one of the
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tungstates, has drawn wide attention for its excellent catalytic, magnetic and electrochemical characteristics [17-20]. Usually, the binary metal oxides of tungstate
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present higher conductivity than pure metal oxide since the introduction of W atoms can greatly improve their conductivity [21]. Based on these encouraging features, MnWO4 should be a promising electrode material for SCs. Apart from the essential
properties of the electrode materials, the morphology also plays an important role in the electrochemical performance. Recently, two-dimensional electrode materials of SCs have attracted a wide range of interest owing to their large electrochemically
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active surfaces [22]. However, to our best knowledge, layered MnWO4 as electrode material for SCs has never been researched, especially the electrochemical properties
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of MnWO4 combined with graphene. Thus, layered MnWO4/reduced graphene oxide (MnWO4/RGO) nanocomposite could be synthesized to explore their potential applications as a promising electrode material for SCs.
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In this work, layered MnWO4/RGO was prepared through a facile one-pot
low-temperature hydrothermal route without using any templates. This method needs
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no requirement for harsh conditions and avoids the use of harmful surfactants or templates. In addition, the electrochemical properties of the MnWO4/RGO nanocomposite may result in potential applications for supercapacitors. 2. Experimental
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2.1 Materials
Pristine graphite was purchased from Qingdao BCSM. MnCl2·4H2O, Na2WO4·2H2O and ascorbic acid were supplied by Shanghai Chemical Reagent
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Company. They were at least of analytical reagent grade and used without further purification.
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2.2 Preparation of MnWO4 and MnWO4 /RGO nanocomposite The MnWO4/RGO nanocomposite was prepared via a one-pot low-temperature
hydrothermal method with different ratios of graphene oxide (GO) and MnWO4. The
product was named as MnWO4 /RGO-x, where x is the amount of GO. For example, the MnWO4 /RGO-75 were typically synthesized as follows: graphene oxide (GO) was synthesized with Hummers method as reported elsewhere [23]. 75 mg graphite
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oxide was exfoliated to GO in 50 mL deionized water by ultrasonication for 1 h. Then, 2 mmol MnCl2·4H2O was dissolved in the GO dispersion and stirred at 70°C for
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about 10 min. Afterwards, 20 mL of distilled water containing 2 mmol Na2WO4·2H2O was added dropwise. 300 mg ascorbic acid (ascorbic acid and GO 4: 1 in weight) was added to the above suspension and dissolved. Subsequently, the mixture was
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transferred into a Teflon-lined autoclave, and heated in an oven at 100 °C for 8 h. The
collected solid product was washed with water and ethanol three times each and dried
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in vacuum at 60 °C for 8 h. In the control experiment, similar procedure was conducted in the absence of GO and ascorbic acid. 2.3 Materials Characterizations
X-ray diffraction (XRD) powder patterns were taken on D/max-γB
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diffractometer using Cu Kα radiation. The measurement was conducted from 5º to 90º with a scan rate of 8 º min-1. Raman spectra were recorded on a Dilor LABRAM-1B multi-channel confocal micro-spectrometer by using an excitation laser wavelength of
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514 nm. X-ray photoelectron spectroscopy (XPS) was performed on XR 5 VG (UK) using a monochromatic Mg X-ray source. The XPS peaks were deconvoluted by
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using Gaussian components after a Shirley background subtraction. Fourier transform infrared spectroscopy (FTIR) spectra were recorded on a Nicolet IS10 spectrometer and samples were measured with KBr pellets. The spectrum was generated, collected 16 times, and was corrected for the background noise. The morphologies of the as-prepared materials were characterized by scanning electron microscopy (SEM, PHILIPS XL30FEG). The investigation of the structure was performed by
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transmission electron microscopy (TEM) using a JEOL 2010F (JEOL Ltd., Japan). The specific surface areas were obtained from the nitrogen adsorption/desorption
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isotherm (Micromeritics, Tristar3000 porosimeter). Thermo-gravimetric analysis (TGA) was carried out in air atmosphere with a heating rate of 10 ºC/min using a Netzsch TG 209F1 to analyze the contents of graphene.
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2.4 Electrochemical Measurements
To prepare the working electrode, 80 wt% active materials, 10 wt% carbon black
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as a conductive additive and 10 wt% polyvinylidene fluoride (PVDF) binder were mixed together and ground thoroughly. Then the mixture was further dispersed in N-methyl-2-pyrrolidone to form the homogeneous slurry which was pasted on nickel foams. Consequently, the foams were dried at 120 ºC for 12 h in a vacuum oven and
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the dried nickel foams were pressed to be a thin foil at a pressure of 10 MPa for 1 min. Electrochemical properties of the electrodes were measured in 6 M KOH electrolyte. Two- and three-electrode cells were used for the measurements. In the case of three
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electrode cells, the prepared electrode, platinum foil, and Ag/AgCl electrode were used as the working, counter, and reference electrodes, respectively. The MnWO4
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/RGO symmetric two cell system having an equal mass loading was fabricated using polypropylene
as
the
separator.
Cyclic
voltammograms(CV),
galvanostatic
charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS) were measured on an electrochemistry workstation (Autolab PGSTAT128N). 3. Results and discussion The synthesis of MnWO4/RGO nanocomposite is outlined in the Fig. 1. During
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the formation of MnWO4/RGO nanocomposite, Mn2+ ions on the surface of negatively-charged GO sheets combined with WO42- ions, then the crystallization of
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MnWO4 and the reduction of GO happened simultaneously in the one-pot hydrothermal process. Insert Fig. 1
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The crystallographic structures of GO, MnWO4 and MnWO4 /RGO-x were analyzed by X-ray powder diffraction (XRD) in Fig. 2. In the pattern of the GO, the
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strong diffraction peak at 10.5º indicates the successful oxidation of the graphite and the formation of GO. As for MnWO4 and MnWO4 /RGO-x nanocomposite, all the diffraction patterns are similar and reveal the formation of the monoclinic structure phase of MnWO4 (according to the JCPDS card No. 13-0434). The characteristic
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peaks of the MnWO4 /RGO-x totally agree with those of the MnWO4, while there is no obvious diffraction peak around 10.5º owing to the increase of the disorder degree during the reduction of the GO. During the formation of MnWO4 /RGO, the MnWO4
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in the nanocomposite prevented the stack of the RGO sheets and greatly reduced the degree of crystallization of RGO. What’s more, the characteristic diffraction peaks of
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RGO were easily shielded by the strong diffraction peaks of MnWO4. Insert Fig.2
In order to confirm the presence of RGO in MnWO4 /RGO-x, the Raman
spectra of MnWO4, GO and MnWO4 /RGO-x are shown in Fig. 3 (a). The Raman spectrum of MnWO4 exhibits an intense peak at 915 cm-1 along with some medium intensity peaks appearing at 799, 714, 507, 422, 325, 203, 173 and 129 cm-1, which
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can be attributed to the characteristic peaks of MnWO4 [20]. The characteristic D band around 1350 cm–1 is related to the presence of defects, while G band around
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1590 cm-1 corresponds to the first-order scattering of the E2g phonons of the graphitic structure of carbon atoms [24]. The D band and G band are clearly observed in all the Raman spectra of GO and MnWO4 /RGO-x. A high ratio of the intensities of the
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D and G band (ID/IG) usually indicates a high degree of disorder [25]. During the process of the GO reduction, fragmentations along the reactive sites were produced,
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resulting in smaller size but more numerous RGO. The edges of the small size RGO could act as defects and lead to a high disorder degree of the RGO, resulting in the increase of D band. What’s more, the insertion of MnWO4 into RGO nanosheets could cause a further decrease of sp2 carbon domain. Therefore, ID/IG of MnWO4
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/RGO-x was higher than that of GO. The Raman spectra analysis indicates that the MnWO4 has been successfully formed and the RGO of MnWO4 /RGO-x has a relatively high reduction degree.
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Fig. 3 (b) exhibits the FT-IR spectra of GO, MnWO4 and MnWO4/RGO-x. The oxygen-containing groups on the surface of GO, such as C-OH (3415 cm-1), C-O-C
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(1260 cm-1), the alkoxy C-O stretching peak (1056 cm-1) and carboxyl C=O and C-O
(1726 and 1398 cm-1), indicate that the GO has a relatively high degree of oxidation. In the spectrum of MnWO4, the characteristic bands are observed at 455, 592, 735,
792, and 885 cm-1, which would be related to vibration modes of MnWO4 [26]. In the spectra of MnWO4/RGO-x, the main characteristic peaks of GO almost disappear, which confirms the reduction of GO. However, some peaks of oxygen-containing
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groups still remains after reduction, which is favorable to improve the dispersion of the sample and the hydrophilicity of the electrode materials. In addition, the typical
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absorption features of MnWO4 can be observed in the spectra of MnWO4/RGO-x. Insert Fig. 3
In order to prove that the sample contained qualitative elements, X-ray
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photoelectron spectroscopy (XPS) characterization was performed to further evaluate the chemical bonding state and composition of the as-synthesized MnWO4/RGO-75.
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In the C 1s spectrum (Fig. 4(a)), the peak located at 285.52 eV is attributed to the C–C, C=C and C–H bonds. While the peaks centered at the binding energies of 286.51, 288.48 and 291.03 eV were assigned to the C-OH, C=O and O=C-O functional groups, respectively, indicating that few oxygen-containing groups are still left in the RGO
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[27]. Fig. 4 (b) is the Mn 2p region of the XPS of as-prepared MnWO4/RGO-75 sample. The binding energy of Mn 2p3/2with 640.85 eV implies the presence of Mn ions in the form of divalent state [28]. The XPS spectrum of W 4f is shown in Fig. 4
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(c). It has two peaks which correspond to W 4f7/2 and W4f5/2 at 34.41 and 36.60 eV, respectively [20]. In the spectrum of the O 1s (Fig. 4(d)), the peak with binding
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energy at 530.61 eV is due to the O2- forming oxide with manganese and tungsten
elements, while the peak located at 532.29 eV is likely attributed to surface O-H group of RGO [20, 27]. The successful formation of MnWO4 in the MnWO4/RGO-75 can be confirmed through the XPS characterization, while the C 1s spectrum certifies the existence of RGO. Insert Fig. 4
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The composition of the MnWO4/RGO-x was further studied by TGA analysis. The weight losses of RGO, MnWO4, and MnWO4/RGO-x are identified by TGA
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measurement in Fig. 5. In the TGA curve of RGO, the slight weight loss before 400 °C is attributed to the removal of surface absorbed water and the decomposition
of the few residual oxygen-containing groups of RGO. The relatively serious weight
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loss from 519 °C to 600 °C is due to the burning of RGO in air. As shown in Fig. 5, MnWO4 suffers no obvious weight loss and shows a relatively good thermal stability.
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In the TGA curves of MnWO4/RGO-x composite, according to the weight loss of RGO, the weight percentage of RGO of MnWO4/RGO-25 to MnWO4/RGO-100 nanocomposite is about 4.60%, 6.52%, 7.28%, and 9.45%, respectively. Insert Fig. 5
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SEM observations on the MnWO4 and MnWO4/RGO-75 are presented in Fig. 6. It can be clearly seen from Fig. 6 (a) and (b) that the MnWO4 is composed of nanoflakes, and the nanoflakes are assembled into sphere-like architecture. Obviously,
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in the presence of RGO, MnWO4/RGO-75 composite displays a different morphology from that of MnWO4. As is seen in the Fig. 6 (c), corrugated RGO nanosheets are
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found to be well expanded with the decoration of layered MnWO4 nanoflakes. However, compared to MnWO4, the MnWO4/RGO-75 exhibited a more irregular morphology. It might be explained by that the nuclei formation and crystal growth of the MnWO4 were affected by the oxygen-containing groups of GO during the reaction.
Besides, the binding effect of GO can also hinder the assembly of MnWO4. In Fig. 6 (d), the EDS pattern of MnWO4/RGO-75 containing the elements of Mn, W and O,
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could further confirm the formation of MnWO4 in the MnWO4/RGO-75. The presence of carbon element in the EDS pattern could be the further proof of the existence of
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RGO. To further examine the details of morphology, TEM image of MnWO4/RGO-75
(Fig.6 (e)) was taken. It is clearly demonstrated that the layered MnWO4 are anchored
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onto the surface of the winked transparent RGO, which is consistent with SEM results. In order to investigate specific surface area of the as-prepared samples, nitrogen were
measured.
As
determined
by
nitrogen
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adsorption-desorption
adsorption/desorption isotherm measurements (Fig. 6(f)), the specific surface areas of the MnWO4 and MnWO4/RGO-75 are 25.66 and 60.34 m2 g−1, respectively. The high BET specific surface can increase the reactive sites between electrode and electrolyte,
Insert Fig.6
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which can greatly improve the electrochemical performance of MnWO4/RGO.
In the case of three electrode cells, the cyclic voltammograms (CV) and
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galvanostatic charge-discharge (GCD) measurements were conducted in 6 M KOH within the operation voltage of -0.35–0.55 V vs. Ag/AgCl. The detailed
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electrochemical processes of MnWO4 and MnWO4 /RGO-x are illustrated by CV
measurements at the scan rates of 5 mV s-1 in Fig. S1 (a). The shapes of the CV curves of MnWO4 and MnWO4 /RGO-x are obviously different from that of the electric double-layer capacitances which shows a rectangular shape. Obviously, a pair of redox reaction peaks corresponding to the reversible faradaic processes demonstrates typical pseudocapacitive characteristics of MnWO4 and MnWO4 /RGO-x. The specific
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capacitance of the electrode can be calculated from CV curves by the equation: C=∫idV/(2vm△V)
(1)
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Where i is the current, V is the potential, v is the potential scan rate, m is the mass of the electro active material and △V is the total potential deviation of the voltage
window. Fig. S1 (b) shows the CV curves of MnWO4 /RGO-75 at different scan rates
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of 5, 10, 20, 50 and 100 mV s-1. The slight deviations in the CV curves shapes of
MnWO4 /RGO-75 are observed at relatively high scan rates, which is due to the low
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interaction between the electrolyte ions and the electrode. As shown in Fig. S1 (c), the corresponding specific capacitances calculated from the CV curves of MnWO4, MnWO4 /RGO-25, MnWO4 /RGO-50, MnWO4 /RGO-75 and MnWO4 /RGO-100 electrodes at 5 mV s-1 are 68, 133, 197, 288 and 235 F g-1, respectively. The
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improvement of the electrochemical performance of MnWO4/RGO-x can be attributed to the synergy effect between MnWO4 and RGO. The good dispersion of MnWO4 on the surface of RGO can avoid the aggregation of both MnWO4 and RGO,
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leading to higher active surface area for charge storage. What’s more, the good conductivity of RGO is benefit to the charge transfer of the electron. However,
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excessive RGO may decrease the contribution of the pseudocapacitive material of MnWO4 and lead to the aggregation of RGO. Therefore, among the MnWO4 and
MnWO4 /RGO-x, MnWO4 /RGO-75 fulfilled the highest capacitance instead of
MnWO4 /RGO-100. Fig. S2 shows the charge-discharge behaviors of MnWO4 and MnWO4 /RGO-x electrodes with a working potential window of 0.9 V. In the Fig. S2 (a), the
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charge-discharge curves of MnWO4 and MnWO4 /RGO-x show some curvature, which is due to redox transitions with the electrolyte and absorption/desorption
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process at the electrode/electrolyte interface. This corresponds to the typical redox couples in the CV curves. The derived specific capacitance results from both the
adsorption/desorption of electrolyte ions in the electrochemical double layer and the
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redox reactions on the surface of the active material. The faradic redox reaction is due to the electron transfer (Mn2+/Mn3+). The specific capacitances were calculated
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according to the following equation: C = (I·∆t) / (m·∆V)
(2)
Here, I is the discharge current, ∆t is the discharge time, m is the mass of an active material, and ∆V is the potential change during discharging.
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In the Fig. S2 (b), with the increase of the current density, the specific capacitances of MnWO4 /RGO-75 show a decreasing tendency, which suggest that parts of the electrode fail to react completely at high current density. During the high-rate
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charge-discharge process, the ionic motion in the electrolyte is limited by diffusion owing to the time constraint, therefore, only the outer active material can be used to
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store energy [29]. It is noted that the MnWO4 /RGO-75 electrode achieved a
maximum specific capacitance of 261 F g-1 at 0.5 A g-1 and could maintain 142 F g-1 even at 10 A g-1, which showed excellent rate performance. Besides, there is no
obvious iR drop even at the current density of 10 A g-1, which is due to the high rate capability and little internal resistance. The significant improvement of the specific capacitance of MnWO4 /RGO-x over MnWO4 could be credited to the existence of
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RGO which provided good conductivity and more space for electrolyte diffusion. The performances of the supercapacitor based on the MnWO4/RGO-x material
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were also measured by a two-electrode symmetrical system in 6 M KOH electrolyte. Fig. 7 (a) shows the CVs of the symmetric MnWO4/RGO-75//MnWO4 /RGO-75 supercapacitor from 0 to 1.5 V at different scan rates from 5 to 100 mV s-1. The
equation:
(3)
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C=4∫idV/(vm△V)
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specific capacitance of the electrode can be calculated from CV curves by the
Where i is the current, V is the potential, v is the potential scan rate, m is the total mass of the two electrodes and △V is the total potential deviation of the voltage window.
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The calculated specific capacitances of the MnWO4/RGO-75 are 109, 98, 90, 80, and 72 F g −1 at different scan rates of 5, 10, 20, 50 and 100 mV s-1, respectively. In Fig. 7 (b), the galvanostatic charge–discharge measurements were further conducted at
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various current densities within the potential window from 0 to 1.5 V. The average discharge capacitances of the electrode were calculated according to the galvanostatic
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charge/discharge tests by the following equation: C = 4(I·∆t) / (m·∆V)
(4)
where I is the discharge current, ∆t is the discharge time, m is the total mass of the
two electrodes, ∆V is the operating cell voltage. The specific capacitances of the MnWO4/RGO-75 evaluated from the discharge curves are 101, 93, 83 and 76 F g-1 at the current densities of 0.5, 1, 2 and 5 A g-1,
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respectively. Insert Fig. 7
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Electron impedance spectroscopy (EIS) is a very powerful measurement for investigating the kinetic process of the electrode reactions and providing
complementary information about the frequency response of electrode [30]. In order
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to further research the electrochemical behavior of MnWO4 and MnWO4 /RGO-x, EIS analysis for two-electrode test cell was also conducted in the frequency range of 100
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kHz - 0.01 Hz in Fig. 8 (a). The impedance plots include a partial semicircle at the high-frequency and a straight slopping line along the imaginary axis at the low-frequency. The intercept of the arc on the real axis at very high frequency represents a combined resistance (Rs) including intrinsic resistance of the active
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materials and the ohmic resistance of electrolyte [4]. As it can be seen, the Rs of MnWO4 /RGO-x, compared to that of the MnWO4 electrode, are much smaller which shows the smaller bulk resistance. The depressed semicircle in the high-frequency
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region is attributable to a double layer capacitance (Cd) and an interfacial charge transfer resistance (Rct) occurring at the electrode-electrolyte interface [31]. The
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diameter of this semicircle corresponds to the interfacial charge transfer resistance (Rct), which is mainly associated with the faradic reactions. The Rct for MnWO4, MnWO4 /RGO-25, MnWO4 /RGO-50, MnWO4 /RGO-75 and MnWO4 /RGO-100 are
10.09, 4.17, 3.83, 3.93 and 3.69 Ω, respectively. Because of the effect of the RGO, the as-prepared MnWO4 /RGO-x achieved fast electron and ion transport, which lead to the much smaller Rct than that of MnWO4. The line in the low frequency called as
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Warburg impedance (Ws) is related to the electrolyte ions diffusion in the electrode [4]. MnWO4/RGO-x electrode shows a much more vertical line than MnWO4
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electrode, which illustrates that MnWO4/RGO-x electrode has a smaller Warburg impedance (Ws) because the electrolyte ions diffusion in the electrode occurs more
swiftly. The near vertical line the in the low-frequency region demonstrates the ideally
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capacitive behavior of MnWO4/RGO-x electrode. It can be explained that the
existence of RGO in MnWO4/RGO-x provides more space and easy access for
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electrolyte diffusion. The small Rs, Rct and Ws of MnWO4/RGO-x electrode play a very important role in improving the performance of supercapacitors. The cycling stability of the electrode is one of the most important parameters for the practical application of supercapacitors [12]. Therefore, the cycling life of the
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MnWO4 and MnWO4 /RGO-75 electrodes were conducted in the two electrode cell at a current density of 1 A g-1 for 6000 cycles in Fig. 8 (b). For the electrode with MnWO4 coating, the capacitance decreased ∼27.2% after 6000 cycles, while the
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MnWO4 /RGO-75 electrode declined ∼14.9% after 6000 cycles. The cycling stability of MnWO4 /RGO-75 electrode can owe to the presence of RGO which improves
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stability of the electrode material. Insert Fig. 8
4. Conclusion
In this work, MnWO4/RGO was prepared through a facile one-pot low-temperature hydrothermal method without using any templates. Owing to the synergy effect between MnWO4 and RGO, the MnWO4/RGO-75 nanocomposite
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fulfilled the capacitance of 288 F g-1 at 5 mV s-1 in the three electrode cell and 109 F g-1 at 5 mV s-1 in the two electrode cell. The electrochemical properties and the facile
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synthesis method of the MnWO4/RGO nanocomposite may result in potential
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applications for supercapacitors.
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List of Figure Captions Fig. 1 Schematic illustration for preparation of MnWO4/RGO in this study. Fig. 2 XRD patterns of GO, MnWO4, and MnWO4 /RGO-x nanocomposite.
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Fig. 3 Raman (a) and FTIR (b) spectra of GO, MnWO4 and MnWO4 /RGO-x nanocomposite.
Fig. 4 XPS of MnWO4/RGO-75 for the C 1s region (a), Mn 2p region (b), W 4f
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region (c) and O 1s region (d), respectively. Fig. 5 TGA curves of RGO, MnWO4, and MnWO4/RGO-x.
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Fig. 6 SEM images of MnWO4 (a, b) and MnWO4/RGO-75 (c), and the corresponding EDS patterns of MnWO4/RGO-75 (d); TEM image of MnWO4/RGO-75 (e) and nitrogen adsorption/desorption isotherms (f) of MnWO4 and MnWO4 /RGO-75. Fig. 7 CV curves (two-electrode cell) of MnWO4/RGO-75 at different scan rates of 5,
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10, 20, 50 and 100 mV s-1 (a) and galvanostatic charge-discharge curves (two-electrode cell) of MnWO4 /RGO-75 at different current densities of 0.5 A g-1, 1 A
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g-1, 2 A g-1 and 5A g-1 (b).
Fig. 8 EIS plots (two-electrode cell) of MnWO4 and MnWO4 /RGO-x (a), and cycling
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performances (two-electrode cell) of MnWO4 and MnWO4 /RGO-75 electrodes at 1 A g-1 for 6000 cycles (b).
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Fig. 1 Schematic illustration for preparation of MnWO4/RGO in this study.
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Fig. 2 XRD patterns of GO, MnWO4, and MnWO4 /RGO-x nanocomposite.
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Fig. 3 Raman (a) and FTIR (b) spectra of GO, MnWO4 and MnWO4 /RGO-x
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nanocomposite.
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Fig. 4 XPS of MnWO4/RGO-75 for the C 1s region (a), Mn 2p region (b), W 4f
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region (c) and O 1s region (d), respectively.
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Fig. 5 TGA curves of RGO, MnWO4, and MnWO4/RGO-x.
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Fig. 6 SEM images of MnWO4 (a, b) and MnWO4/RGO-75 (c), and the corresponding EDS patterns of MnWO4/RGO-75 (d); TEM image of MnWO4/RGO-75 (e) and
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nitrogen adsorption/desorption isotherms (f) of MnWO4 and MnWO4 /RGO-75.
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Fig. 7 CV curves (two-electrode cell) of MnWO4/RGO-75 at different scan rates of 5,
10, 20, 50 and 100 mV s-1 (a) and galvanostatic charge-discharge curves
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g-1, 2 A g-1 and 5A g-1 (b).
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(two-electrode cell) of MnWO4 /RGO-75 at different current densities of 0.5 A g-1, 1 A
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Fig. 8 EIS plots (two-electrode cell) of MnWO4 and MnWO4 /RGO-x (a), and cycling
performances (two-electrode cell) of MnWO4 and MnWO4 /RGO-75 electrodes at 1 A
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g-1 for 6000 cycles (b).
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Highlights (1) The MnWO4/RGO was first prepared through a facile hydrothermal route.
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(2) MnWO4/RGO composite show much higher specific capacitances than pure MnWO4
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(3) The electrochemical properties of MnWO4/RGO arise from the synergistic effect.