Direct growth of WO3 nanostructures on multi-walled carbon nanotubes for high-performance flexible all-solid-state asymmetric supercapacitor

Electrochimica Acta 308 (2019) 231e242

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Direct growth of WO3 nanostructures on multi-walled carbon nanotubes for high-performance flexible all-solid-state asymmetric supercapacitor Pragati A. Shinde, Youngho Seo, Chaiti Ray, Seong Chan Jun* Nano-Electro Mechanical Device Laboratory, School of Mechanical Engineering, Yonsei University, Seoul, 120-749, South Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 January 2019 Received in revised form 18 March 2019 Accepted 22 March 2019 Available online 25 March 2019

The rational design and development of highly conductive hierarchical nanostructured materials are of great importance to improve the electrochemical performance of supercapacitors. Great efforts have been committed to the development of positive electrodes for asymmetric supercapacitors (ASC). However, it is still necessary to develop better negative electrodes for practical applications. In present investigation, a multi-walled carbon nanotubes-tungsten trioxide (MWCNT-WO3) hybrid nanostructure is prepared as a negative electrode for ASC. The MWCNT-WO3 hybrid electrode is prepared using a simple two-step approach, which involves coating of MWCNTs on carbon cloth substrates followed by hydrothermal treatment to deposit WO3 nanorods on the MWCNT-coated carbon cloth. The MWCNT-WO3 hybrid electrode exhibits a maximum specific capacitance (areal capacitance) of 429.6 F g
1 (1.55 F cm
2) and capacity retention of 94.3% after 5000 cycles, which are higher than the 155.6 F g
1 (0.43 F cm
2) and 84.9% shown by pristine WO3 in 1 M LiClO4 electrolyte. A flexible all-solid-state ASC is self-assembled with MWCNT-WO3 as a negative electrode, MnO2 as a positive electrode, and PVA-LiClO4 as a gel electrolyte. The MnO2//MWCNT-WO3 ASC achieve specific capacitance of 145.6 F g
1 at a current of 2 mA and specific energy of 39.63 Wh kg
1 at a specific power of 546 W kg
1. Specifically, the ASC exhibits superior long-term cycling stability (77% over 10000 cycles) and excellent mechanical flexibility with less capacitance loss. These remarkable results demonstrate the potential of using MWCNT-WO3 hybrid nanostructures for the fabrication of high-performance energy storage devices. © 2019 Published by Elsevier Ltd.

Keywords: Asymmetric supercapacitor Energy density Flexibility MWCNTs WO3

1. Introduction Over the past two years, the progress in the development of highly efficient flexible energy storage devices has satisfied society's increasing energy requirements for their daily lifespan. Among the different energy storage devices, flexible supercapacitors have come into view as potential candidates in the field of energy storage as they endow with sufficient energy for application to hybrid electrical vehicles and portable electronic devices [1e3]. When a long cycling stability or variable power supply is needed for the proper functioning of different portable electronic devices, supercapacitors can replace or complement with batteries [4,5]. In hybrid electrical vehicles, supercapacitors are used in combination with batteries to provide a high level of power during

* Corresponding author. E-mail address: [email protected] (S.C. Jun). https://doi.org/10.1016/j.electacta.2019.03.159 0013-4686/© 2019 Published by Elsevier Ltd.

acceleration or retrieve breaking. Despite these favorable features, the widespread use of supercapacitors is very restricted because of their relatively lower energy density, which needs to be increased before they can be used as advanced energy storage devices. To look at the use of supercapacitors in advanced portable electronics, it is essential to boost the energy storage capability of present supercapacitors without sacrificing their other features, including their power density and long-term cycle lifetime. Extending operating potential window of a device is the most capable way to boost the energy density of supercapacitors, simply by designing an asymmetric supercapacitors (ASCs) configuration. From equation E ¼ 0.5 CV2 (where E is the energy density, C is the specific capacitance, and V is the operating potential window), the E of an ASC can be boosted by increasing the specific capacitance as well as by extending the operating potential window of the device. There have been several efforts to enhance the specific capacitance of supercapacitor electrodes, including the fabrication of nanocomposites or nanostructured materials, and hybridization [6,7].

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Generally, ASCs are constructed with two different types of electrodes with different operating potential windows (positive and negative), which assists in broadening the operating potential limit of the ASC device. In general, the operating potential limit of an ASC is nothing but the difference between the electrolyte over potential and work function of the negative and positive electrodes [8]. Thus, to fabricate an ASC with a wide potential, the selection of positive and negative electrodes is very important. In general, transition metal oxides with high theoretical specific capacitance values are used as positive electrodes, and carbon-based materials with high chemical stability but relatively low specific capacitances are used as negative electrodes [9e12]. In previous decades, considerable research attempts have been paying attention on developing positive electrodes for ASC devices. Of course, the fabrication of a proper negative electrode is also necessary for the development of high-performance ASCs. As previously mentioned, carbon-based materials are frequently used as negative electrodes of ASCs because of their low cost, environmentally friendly nature, and good conductivity. Nevertheless, the overall electrochemical performances of carbon-based materials are not comparable to those of metal oxides, which degrade the total specific capacitance and energy density of the device [13e15]. Therefore, the choice of a proper couple of positive and negative electrode materials with analogous specific capacitances is very essential to accomplish ASCs with excellent electrochemical performance. In recent years, Fe2O3 has been commonly used as a pseudocapacitive negative electrode in ASCs owing to its high negative potential range (
0.8 V) in alkaline and neutral electrolytes [16,17]. Recently, few reports on the utilization of V2O5 as a negative electrode in neutral electrolytes for ASCs have been published [18]. Among the tungsten oxides, WO3 attracted significant research focus in the early years and was used as a high-performance pseudocapacitive electrode material [19e21]. WO3 has the distinctive physico-chemical properties of excellent corrosion resistance, various oxidation states, and different negative potential ranges in acidic and neutral electrolytes, which play a main part in bringing a high energy density [22,23]. The electrochemical reactions occur at the surface of the electrode usually involve electron and ion transfers at the surface of the active material, which requires an electrode with a huge surface area and good electrical conductivity. Although WO3 has various promising features, its poor electrical conductivity limits its utilization in the development of high-performance supercapacitors. To solve this problem, a hybrid electrode prepared by integrating highly conductive carbonaceous materials (graphene, activated carbon, or carbon nanotubes) as a backbone and a pseudocapacitive WO3 as an active material may provide the required electrochemical stability and high electrical conductivity. Carbon nanotubes (CNTs) have been extensively studied for a long time and utilized in flexible energy storage devices owing to their superior properties, which include a high electrical conductivity, a huge surface area, excellent cycling stability, and exceptional mechanical strength [24e26]. Moreover, in a hybrid electrode, a 1D multi-walled CNTs (MWCNTs) intertwined network improves electrical conductivity as well as provides more electrochemically active surface area and efficient pathways for both electrons and ions. Composites of WO3 and MWCNTs are of significant interest because they have a dual charge storage mechanism, which consists of the electrical double layer capacitor (EDLC) and pseudocapacitive mechanisms. Nanostructured MWCNT-WO3 is likely to achieve a high structural stability and electrochemical performance when used as an anode for ASCs. In the present work, a hybrid electrode was prepared by combining MWCNTs and WO3 in a single electrode, which worked as an efficient anode electrode. The strong adhesion between the

interconnected MWCNT framework and WO3 nanorods led to a high specific capacitance and electrochemical stability. The prepared MWCNT-WO3 hybrid electrode was directly applied as the negative electrode of an ASC device with MnO2 as the positive electrode. The MnO2//MWCNT-WO3 ASC device worked at an operating potential window of 1.4 V in the PVA-LiClO4 gel electrolyte and showed that the energy density of the ACS device could be improved. 2. Experimental Section 2.1. Growth of MWCNTs on carbon cloth MWCNTs were grown on pre-treated carbon cloth (CC) substrate using a simple dip-dry process. First, the CC was ultrasonically cleaned using 70% HNO3, acetone, and deionized (DI) water repeatedly, and then dried in an oven at 60  C for 12 h. The 6% MWCNT dispersion (2 mL) was mixed with DI water and 1 wt% Triton X-100 (100:1) by ultrasonication for 2 h. A piece of CC (2  4 cm2) was immersed into the above dispersion for 1 min, and the MWCNTs were adsorbed onto the CC surface as a result of the electrostatic attractive force between the CC surface and MWCNTs. Finally, the CC with the MWCNT coating was dried in an oven for 15 min, which completed one deposition cycle. The same process was repeated for 10 cycles to obtain a uniform and effectively adhered MWCNT coating on the CC substrate. Before using the MWCNT/CC substrate for the further deposition of nanomaterials, the MWCNT/CC film was ultrasonically treated for 10 min in DI water to remove any unstable and loosely attached MWCNTs from the CC surface. 2.2. Preparation of MWCNT-WO3 hybrid The WO3 was directly deposited on the obtained MWCNT/CC substrate using a one-pot hydrothermal treatment, as discussed below. Typically, 0.05 M Na2WO4$2H2O was dissolved into 40 mL of DI water, and then the pH of the solution was maintained at ~1.5 by adding 3 M HCl aqueous solution under vigorous stirring for 20 min. Subsequently, 0.1 M Na2SO4 was added to the above solution. Later, the precursor solution was transferred to a Teflon-lined stainless steel autoclave with the MWCNT/CC substrate placed vertically in the solution and subjected to a hydrothermal treatment in an oven at 180  C for 24 h. The autoclave was then allowed to naturally cool at room temperature, the MWCNT-WO3/CC film was washed many times with DI water and ultrasonicated for 5 min. At last, the film was dried in an oven at 80  C for 12 h. For comparison, pristine WO3 was directly prepared on a CC substrate following the same hydrothermal treatment without MWCNTs. The deposited weight of WO3 and MWCNT-WO3 in 1 cm2 area is ~1.2 and 1.6 mg, respectively. 2.3. Fabrication of MnO2//MWCNT-WO3 asymmetric supercapacitor device To fabricate the ASC device, two electrodes having different operating potential windows in identical electrolyte were required. Therefore, to assemble the ASC device, MnO2 electrode was prepared on the CC substrate because it has an excellent electrochemical performance and operates in a broad positive potential window. The details of the MnO2 electrode preparation are provided in Section S7 of the supplementary information (SI). The ASC supercapacitor device was configured using MnO2 as a positive electrode, MWCNT-WO3 as a negative electrode, and PVA-LiClO4 as a gel electrolyte. The PVA-LiClO4 gel electrolyte was prepared according to the following process. Typically, 4 g PVA was added in

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60 mL of DI water with constant stirring at 80  C to get a clear solution. Then, 1 M LiClO4 solution was added dropwise under stirring at room temperature to obtain a plain and viscous solution. This plain and viscous solution was used as a gel electrolyte when assembling the ASC device. Later, the electrodes were soaked in the PVA-LiClO4 gel electrolyte for 1 min and then dried in an oven at 60  C for 12 h to eradicate the water content from the gel electrolyte. Subsequently, the positive and negative electrodes were positioned in front of each other with a filter paper separator, pressed in a hydraulic pressure gauge of 1 ton, and used as MnO2// MWCNT-WO3 ASC device for further measurements. 2.4. Materials characterization The surface morphologies of the prepared materials were analyzed using a field emission scanning electron microscope (FESEM, JEOL-7800F). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) measurements were performed on a JEOL JEM-2010 electron microscope. Crystallographic analyses of the prepared materials were conducted using an X-ray diffractometer equipped with a Cu Ka (l ¼ 0.15406 nm) source. Further, the chemical composition and oxidation states of the elements present in the material were examined using X-ray photoelectron spectroscopy (XPS, VG Scientific ESCALAB250). The surface area was calculated based on the N2 adsorption/desorption measurements with the BrunauereEmmetteTeller (BET) method, and the pore size distribution was analyzed by the BarretteJoynereHalenda (BJH) method. 2.5. Electrochemical measurements

ð I Vdt mV 2

ð I Vdt CA ¼

AV 2

(2)

where Cs is the specific capacitance (F g
1), CA is the areal capacitance (F cm
2), V is the potential window, I represents the current density (A), A is the area (cm2) and m is the mass of active material on CC substrate for 1 cm2 area, which is calculated from the following formula:

m ¼ m2
m1

(3)

where m1 is the mass of CC before deposition and m2 is the mass of CC after deposition. The specific energy (SE, Wh kg
1) and specific power (SP, W kg
1) of the ASC device were calculated from discharge curve from the following formulae:

SE ¼

0:5  Cs  DV 2 3:6

SP ¼

SE  3600 Td

(4)

5

where, Cs is the specific capacitance obtained from Equation (1), DV (V) is the voltage window and Td (s) is the discharge time.

3. Results and discussion 3.1. Chemical principle for the fabrication of MWCNT-WO3/CC hybrid electrode

The pristine MWCNT, WO3, and MWCNT-WO3 electrodes on CC substrates were cut into 1 cm2 area and used as working electrodes. The electrochemical properties were measured in a three-electrode configuration using the cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance (EIS) techniques in 1 M LiClO4 electrolyte. A saturated calomel electrode (SCE) was used as a reference electrode and platinum wire was used as a counter electrode. All of the electrochemical test results were checked using an electrochemical workstation (ZIVE MP1). The capacitance of electrode and device was calculated from the charge-discharge curve by integrating the discharge portion from the following formulae [27]:

Cs ¼

233

(1)

In the present work, the MWCNT-WO3/CC hybrid electrode was prepared through a facile two-step process. First, MWCNTs were grown on the CC substrate, and then WO3 nanostructures were hydrothermally grown on the MWCNT/CC substrate to form the MWCNT-WO3/CC hybrid electrode. Fig. 1 shows schematics of the fabrication process for the MWCNT-WO3/CC electrode on the CC substrate. Here, Triton X-100 acts as a dispersant and sorbet. Smaller chain surfactants are more capable for dispersing carbon nanotubes [28]. The benzene ring in the structure of Triton X-100 has a high dispersing ability, which enables better MWCNT dispersion [29]. When the MWCNT suspension with Triton X-100 is ultrasonicated in DI water, some hydrophobic alkyl chains of Triton X-100 (C14H22O(C2H4O)n) come into contact with the MWCNT surface, and hydrophilic groups face toward the water. The Triton X-100 ions are easily adsorbed on the MWCNT surface, and the MWCNTs with a large aspect ratio are well-dispersed in the water.

Fig. 1. Schematic illustration of the synthesis of MWCNT-WO3 hybrid film.

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When the CC substrate is inserted into the MWCNT dispersion, the MWCNTs can easily attached to the surface of the CC because of the strong electrostatic attraction between the adsorbed Triton X-100 ions and substrate surface. Thus, the MWCNTs can be uniformly grown on the CC substrate. During the subsequent hydrothermal treatment, tungsten species are adsorbed on the MWCNT/CC surface, which results in the formation of the MWCNT-WO3/CC hybrid. It is known that the growth of nanomaterials using a hydrothermal method occurs as a result of controlled precipitation in a closed system under a controlled temperature and pressure conditions. At a specific reaction temperature, as the precursor solution transfers from a supersaturated to a saturated state, a thin film of nanomaterials is build on the surface of substrate via different intermediate steps such as nucleation, aggregation, coalescence, and finally crystal growth [30]. The chemical reactions concerned for the formation of WO3 are given as follows. In the first step, when Na2WO4$2H2O is dissolved in water, it dissociates into Naþ and þ WO2
4 ions, along with one H3O (hydronium) ion. In the next step, when the HCl solution is added, WO2
4 ions in the reaction medium react with acid to form a tungstic acid (H2WO4). During the hydrothermal treatment, as the reaction medium attains the temperature required for the decomposition of H2WO4, WO3 nuclei form in the solution. The earlier formed nuclei act as basic building blocks, and further growth occurs as a result of Ostwald ripening [31], which forms WO3 nanorods on the MWCNT/CC substrate

surface. þ Na2 WO4 :2H2 O/2Naþ þ WO2
4 þ H3 O

(6)

2Naþ þ WO2
4 þ 2HCl/H2 WO4 þ 2NaCl

(7)

H2 WO4 /WO3 þ H2 O

(8)

Thus, the MWCNT-WO3/CC hybrid nanostructure may provide a large surface area and facilitate more efficient electron and ion transportation, which assist in enhancing the electrochemical performance of the supercapacitor. In addition, the network of MWCNT nanowires provides good mechanical support to the growing active material and offers more electron transfer channels, which further assist in increasing the energy density of the electrode. 3.2. Physico-chemical characterization and electrochemical evaluation of MWCNT-WO3/CC hybrid electrode The surface morphology of the active material facilitates a major role for supercapacitors, because the electrochemical reactions occur at the surface of the active material. The morphologies of the MWCNTs, pristine WO3, and MWCNT-WO3 hybrid on CC substrates were examined by FESEM, as shown in Fig. 2(aef). Fig. S1 of the SI

Fig. 2. FESEM images of (aec) pristine WO3 and (def) MWCNT-WO3 hybrid at different magnifications, TEM and HRTEM images of (gei) pristine WO3 (inset of (i) shows FFT image) and (jel) MWCNT-WO3 hybrid (inset of (l) shows FFT image).

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shows FESEM images of the MWCNTs, with the formation of an interconnected framework of MWCNT nanowires grown uniformly over the CC substrate. The conducting network of MWCNT nanowires enables a much higher surface area for further growth of WO3 nanostructures. The MWCNTs nanowires have diameters of 10e20 nm and lengths in the range of micrometers. FESEM images of the pristine WO3 are shown in Fig. 2(aec). As shown in figures, the pristine WO3 has a nanorod-like morphology, with diameters in the range of 100e300 nm and lengths of up to 1 mm. The FESEM images of the MWCNT-WO3 hybrid show a irregular assessment of individual WO3 nanorods and some nanorod bundles over the MWCNT framework (Fig. 2(def)). The low-magnification FESEM image reveals that several nanorods attached together to form nanorod bundles over the surface of the MWCNTs. These nanorod bundles may provide a many placing sites for electrolyte ions meanwhile electrochemical reactions. In the high-magnification image, we can clearly see that several nanorods have smooth boundaries, and they are uniformly grown over the interconnected MWCNT framework to form a conformal deposition on the surface. Each nanorod has uniform space and good contact with the MWCNT surface, which is likely to provide easy electrolyte ion diffusion and afford an ultrafast electron transfer during electrochemical reactions. Here, the MWCNTs are mainly used as a conductive current collector for enhancing electrical conductivity of the active material. The WO3 nanorods and MWCNTs are in mutual contact with each other, which may provide better mechanical strength and form a highly conductive network. The different distributions of nanorods for the pristine WO3 and MWCNT-WO3 hybrid are accredited to different surface chemistries. The MWCNT-coated CC substrate offers hetero-epitaxial growth, and the CC substrate offers homo-epitaxial growth for the deposition of WO3 nanorods [32]. As the reaction proceeds, the hetero-epitaxial growth leads to different growth processes for the nanorods, which are determined by the MWCNT framework on the CC. The surface energy offered by the MWCNT framework confines the formation of a large number of nanorods into cylindrical bundles, which thus offer good adhesion and chemical stability. Such a nanostructured morphology may provide many benefits such as the enhancement of the electroactive surface area and conductivity, which are significantly required to increase the electrochemical performance of the supercapacitor electrode. Furthermore, to sustain the FESEM results, TEM analyses of the pristine WO3 and MWCNT-WO3 hybrid were performed. For these analyses, ultrasonication was used to scratch powder from the electrodes into ethanol, which was dropped on a Cu grid. The TEM image of the pristine WO3 shows a nanorod-like morphology (Fig. 2g), which is consistent with that observed by FESEM. Fig. 2h and i shows HRTEM images with an interplanar distance of 0.37 nm, which is concede with that calculated from the XRD reflection for the (001) plane (Fig. 3) of the hexagonal WO3 structure. A Fast Fourier Transform (FFT) image (inset of Fig. 2i) of a pristine WO3 nanorod reveals its single crystalline nature. The TEM image of MWCNT-WO3 hybrid (Fig. 2j) obviously depicts the even coating of WO3 nanorods on the surface of the MWCNTs. The lattice fringe from the HRTEM image shows a lattice distance of 0.62 nm, which agrees to the (100) crystal plane of the hexagonal WO3 (Fig. 2k and l). The result obtained here correlates with the XRD result with the maximum diffraction intensity for the (100) plane of the MWCNT-WO3 hybrid, which indicates the role of the MWCNTs in the growth direction of the WO3. The FFT image (inset of Fig. 2(l)) proves the single crystalline feature of the MWCNT-WO3 hybrid. These results further reveal that the MWCNTs are the more suitable current collector for the uniform crystal growth of the WO3. The MWCNT-WO3 hybrid nanocomposite is significantly accessible to electrolyte ions and provides more active sites for electrolyte

235

Fig. 3. The XRD patterns of (a) pristine WO3 and (b) MWCNT-WO3 hybrid films.

accommodation. This could make MWCNT-WO3 hybrid nanocomposites as potential electrode for supercapacitors, with improved electrochemical performance and chemical stability compared to the pristine WO3. An X-ray diffraction (XRD) analysis was conducted to obtain crystallographic information about the prepared samples. The XRD patterns of the pristine WO3 and MWCNT-WO3 hybrid on CC substrates are displayed in Fig. 3. The characteristic diffraction peaks in both the XRD patterns agree well to the hexagonal phase of WO3 (JCPDS: 33-1387). More importantly, the characteristic diffraction peak of the WO3 and MWCNT-WO3 at 13.95 corresponds to the (100) plane, indicating the preferential growth direction of the nanorods along the a-axis, which is in well accordance to the results of HRTEM analysis. The XRD patterns for bare MWCNTs and MWCNTs on CC substrate are shown in Fig. S2 of SI. The intensity of diffraction peak for the MWCNTs appeared at 26.04 corresponding to the (002) plane is higher than the MWCNTs on CC substrate. Moreover, the diffraction intensities of the (002) plane of the MWCNTs and different WO3 crystal planes in the MWCNT-WO3 hybrid are higher than those of the pristine WO3 nanorods. No other impurity peaks were observed in the XRD patterns, implying that the prepared materials possess high phase purity. The chemical composition and oxidation states of the elements were investigated using X-ray photoelectron spectroscopy (XPS) analysis. Fig. 4 displays the XPS spectra of the MWCNT-WO3 hybrid. The XPS survey scan spectrum (Fig. 4a) illustrates the presence of the W, O, and C elements in the MWCNT-WO3 hybrid. The emission of C 1s is seen to be intensive, suggesting the presence of CNTs in the sample. The contents of W, O and C in the MWCNT-WO3 hybrid were estimated to be 14.07, 39.52 and 46.41 at%, respectively. The atomic ratio of W:O in the MWCNT-WO3 hybrid is ~1:3. As shown in Fig. 4b, the W 4f core level spectrum is effectively split into the W 4f5/2 and W 4f7/2 orbitals of tungsten, situated at binding energies of 37.75 and 35.58 eV, respectively, confirming the presence of the W6þ oxidation state of WO3 [33]. The deconvoluted high-resolution C 1s and O 1s core level spectra are shown in Fig. 4c and d. The XPS spectra of C 1s and O 1s show the existence of oxygen containing functional groups, which are covalently aligned to the surface C atoms of the MWCNTs. The C 1s spectrum (Fig. 4c) is deconvoluted into peaks at 285.1 eV (CeC), 286.5 eV (CeO), and 289.3 eV (C]O) [34]. The O 1s core-level spectrum (Fig. 4d) shows a strong peak at

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Fig. 4. The XPS spectra of MWCNT-WO3 hybrid: (a) survey scan, (b) W 4f, (c) C 1s, (d) O 1s, (e) N2 adsorptionedesorption isotherms and (f) pore-size distribution plots for pristine WO3 and MWCNT-WO3 hybrid.

530.8 eV for lattice metal-oxygen bonding (WeO) and two peaks at 531.6 and 532.5 eV matches to surface (OeOH) and (CeO) groups. The above results suggest that the C atoms at the surface of the MWCNTs are probably bonded with the O atom of WO3. For comparison, the XPS spectra of pristine WO3 were also analyzed as shown in Fig. S3 of SI. The XPS survey scan spectrum of WO3 (Fig. S3a) is similar to that of the MWCNT-WO3 hybrid (Fig. 4a), apart from the intensity of the C 1s peak, which is lower than the previously mentioned result. Here, the low intensity C 1s peak is mainly related to the carbon in the conductive adhesive required for the XPS analysis, along with the CC substrate. The W 4f core level spectrum is shown in Fig. S3b, which represents deconvoluted peaks corresponding to W 4f5/2 and W 4f7/2 orbitals of tungsten. The deconvoluted C 1s and O 1s core-level spectra of pristine WO3 are shown in Fig. S3c, d of SI. The N2 adsorptionedesorption measurements were used to evaluate the specific surface areas and pore structures of the

pristine WO3 and MWCNT-WO3 hybrid, as shown in Fig. 4e and f. Based on the IUPAC classification, the N2 adsorptionedesorption isotherms (Fig. 4e) show typical type IV hysteresis. For the pristine WO3 sample, at less than the relative pressure of 0.65, roughly no adsorption of N2 is observed. After that, it shows adsorption as the relative pressure approaches 1.0. However, the adsorption quantity is very low. In contrast, for the MWCNT-WO3 hybrid, the N2 adsorption quantity slowly increases with the relative pressure and is much greater than that of the pristine WO3. The BET specific surface areas of the WO3 and MWCNT-WO3 samples are 26.84 and 67.54 m2 g
1, respectively. Furthermore, Fig. 4f depicts pore-size distribution plots of the pristine WO3 and MWCNT-WO3 hybrid, which were measured from the BJH method. The average BJH pore volumes of the pristine WO3 and MWCNT-WO3 hybrid are 0.115 and 0.372 cm3 g
1, respectively. As seen from Fig. 4f, the MWCNT-WO3 hybrid exhibits a wide pore-size distribution, having a pore radius range of 2e125 nm, indicating that the sample contains both

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mesopores (2e50 nm) and micropores (greater than 50). Nevertheless, the amount of mesopores is greater than the total pore volume. The prepared MWCNT-WO3 hybrid has a high specific surface area and good pore-size distribution compared to the pristine WO3 nanorods because of the combined influence of the MWCNTs and WO3 nanorods. Since MWCNT-WO3 hybrid has high surface area, it will possess more electrochemically active sites during electrochemical reactions. Moreover, the large pore volume is beneficial for easy access of the electrolyte ions into the interior of active material, which gives rise to an improved electrochemical

237

performance. To determine the suitability of the prepared electrodes in supercapacitors, the electrochemical properties were tested using the CV, GCD, and EIS techniques in a conventional three-electrode configuration. Fig. 5a shows the comparative CV curves of the MWCNT, pristine WO3, and MWCNT-WO3 hybrid electrodes measured at a constant scan rate of 100 mV s
1 in the operating potential range of
0.6 to 0 V/SCE. This figure clearly shows that all of the CV curves have a nearly rectangular shape with no obvious redox peaks, indicating that the prepared electrodes have superior

Fig. 5. (a) CV curves for MWCNT, pristine WO3, and MWCNT-WO3 hybrid electrodes at constant scan rate of 100 mV s
1, (b) GCD curves for MWCNT, pristine WO3, and MWCNTWO3 hybrid electrodes at constant current density of 2 mA cm
2, (c) CV curves and (d) GCD curves for MWCNT-WO3 hybrid electrode at various scan rates, (e) plot of specific capacitance versus current density and (f) plot of areal capacitance versus current density for pristine WO3 and MWCNT-WO3 hybrid electrodes, (g) Nyquist plots for MWCNT, pristine WO3, and MWCNT-WO3 hybrid electrodes, and (h) plots of capacity retention versus cycle number for pristine WO3 and MWCNT-WO3 hybrid electrodes.

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supercapacitive behavior [35]. Moreover, the MWCNT-WO3 hybrid electrode shows the highest specific capacitance because it has a larger volumetric area under the CV curve and high anodic and cathodic current responses in the same potential range compared to the MWCNT and pristine WO3 electrodes. The higher electrical conductivity and nanorod structure of the MWCNT-WO3 hybrid electrode provide a greater electrochemically active surface area, and the optimal pore radius permits more suitable path for electrolyte ions during electrochemical process. Herein, the MWCNTs in the prepared electrode enhance the electrical conductivity, providing a high surface area and electrochemical stability to the active electrode during electrochemical reactions. Further these results were confirmed by the GCD measurements. Fig. 5b shows GCD curves for the MWCNT, pristine WO3, and MWCNT-WO3 hybrid electrodes at a constant current density of 2 mA cm
2 in the same operating potential range. The discharge times of the electrodes have the order of MWCNT-WO3 > WO3 > MWCNT, which is similar to the results obtained from the CV measurements. The MWCNT-WO3 hybrid electrode shows a better electrochemical performance, which confirms that it possesses more active sites because of the synergistic interaction between the WO3 nanorods and MWCNTs. Although the beginnings of the discharge curves show little voltage (IR) drop, the measured IR drop is very small for the MWCNT-WO3 hybrid compared to the pristine WO3 electrode. Generally, the literature shows that the electrochemical performances of WO3-based electrodes have been tested in the H2SO4 electrolyte. Although WO3 has a good corrosion resistance property, a supercapacitor device prepared with the H2SO4 gel electrolyte shows a lower capacitive performance because of the formation of crystallites in the H2SO4 gel electrolyte. More importantly, in the present case, LiClO4 was used as the electrolyte instead of H2SO4 when measuring the electrochemical performance. As seen that the CV and GCD curves of the pristine WO3 and MWCNT-WO3 hybrid electrodes show ideal supercapacitive features, indicating a better ability to store charge in the LiClO4 electrolyte. In addition, the literature shows that the PVA-LiClO4 gel electrolyte is more stable and long-lasting compared to other gel electrolytes [36]. In the case of the WO3, a reversible charge storage process exhibited by a change in the oxidation state of W at 5þ/6þ. Surprisingly, the reversible charge storage process that occurred on the surface of the MWCNT-WO3 hybrid electrode did not show a change in the supercapacitor behavior, indicating that the structure of the MWCNT-WO3 hybrid was more stable in a neutral electrolyte. The electrochemical intercalation/deintercalation process for Liþ ions occurring at the surface of the WO3 is given by the following

reaction equation:

WO3 þ xLiþ þ xe
4Lix WO3

(9)

Moreover, to determine the rate capability of the MWCNT-WO3 hybrid electrode, CV and GCD measurements were conducted. Fig. 5c illustrates the CV curves of the MWCNT-WO3 hybrid electrode at scan rates of 5e100 mV s
1. As seen from figure, all of the CV curves show nearly rectangular shapes, which are retained still at a high scan rate of 100 mV s
1, showing the excellent electrochemical nature of the MWCNT-WO3 hybrid electrode. The nearly symmetric shapes of all the CV curves depict that the MWCNT-WO3 hybrid electrode shows highly reversible faradaic reactions. In addition, the volumetric area under the CV curve and current response also gradually increase with the scan rate, which signifies the ultrafast electron and ion transport at the applied potentials. For assessment, the CV curves of the MWCNT and pristine WO3 at different scan rates (5e100 mV s
1) in the 1 M LiClO4 electrolyte were also measured and are shown in Fig. S4 of SI. The specific capacitances of the MWCNT, pristine WO3, and MWCNT-WO3 electrodes were calculated from CV curves and plotted as a function of the scan rate in Fig. S5 of SI. The GCD curves of the MWCNT-WO3 hybrid electrode measured at different current densities of 2e10 mA cm
2 are displayed in Fig. 5d. The chargeedischarge curves reflect non-linear behavior with low coulombic efficiency. When the potential of electrode is less than 1 V, the electrode surface is covered by the solid-electrolyte interphase (SEI) layer, because of the decomposition of the organic electrolyte. This layer is thick, stable and ionically conducting. However, during chargeedischarge process, volume change of electrode due to lithium ion intercalation/deintercalation causes crushing of SEI layer, resulting in low coulombic efficiency [37,38]. Moreover, the low IR drop signifies the contribution of the MWCNTs in the electrochemical reactions. Fig. 5e and f shows the specific capacitance and areal capacitance of the pristine WO3 and MWCNT-WO3 hybrid electrodes as a function of current density. The specific capacitance and areal capacitance of the MWCNT-WO3 hybrid electrode are 429.6 F g
1 and 1.55 F cm
2, which are higher than the values of 155.58 F g
1 and 0.43 F cm
2 for the pristine WO3 at a current density of 2 mA cm
2, respectively. The drop in capacitance with a rise in current density is due to the occurrence of inaccessible interior active sites, which may not maintain the redox reactions at further increasing current densities. The specific capacitance of the MWCNT-WO3 hybrid is compared with those of earlier reports from the literature in Table 1.

Table 1 Comparison of specific capacitance of MWCNT-WO3 hybrid electrode with reported literature. Sr. No.

Material

Preparation method

Substrate

Electrolyte

Specific capacitance Cs (F g
1)

Ref.

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

W2N@P-CF h-WO3 WO3 h-WO3 h-WO3 WO3 WO3 WO3-WO3$0.5H2O W18O49 h-WO3$nH2O Graphene-WO3 WO3-graphene WO3-rGO WO3-graphene WO3$H2O-rGO WO3-graphene PW12-PPy MWCNT-WO3

Hydrothermal Hydrothermal Electrodeposition Hydrothermal Hydrothermal Chemical bath deposition Electrodeposition Microwave-assisted hydrothermal Solvothermal Hydrothermal Precipitation Hydrothermal Hydrothermal Hydrothermal Hydrothermal Solvothermal Chemical method Hydrothermal

Carbon fabric Carbon cloth Ti foil Stainless steel Cu Stainless steel IrO2 coated Ti substrate Graphite Ti horn Ti foil Ni foam Carbon paper Cu mesh Ni foam Ni foam Graphite Carbon cloth Carbon cloth

1 M H2SO4 2 M H2SO4 1 M H2SO4 0.5 M H2SO4 1 M Na2SO4 1 M Na2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 1 M H2SO4 1 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 2 M KOH 1 M H2SO4 0.1 M H2SO4 1 M H2SO4 1 M LiClO4

478 521 196 421.8 463 530 46 290 579 498 143.6 495 343 580 244 465 294.1 429.6

[27] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] Present work

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Moreover, to know the rate kinetics and charge transportation of the prepared electrodes, EIS data were collected in a frequency range of 0.01 Hze100 kHz at an open circuit potential of 10 mV. Fig. 5g shows the Nyquist plots of the MWCNT, pristine WO3, and MWCNT-WO3 hybrid electrodes. A Nyquist plot is generally separated into three main parts: the high-, middle-, and low-frequency regions. The high-frequency region gives idea regarding the intrinsic resistance of the active electrode and interfacial resistance of the substrate to the active material. The middle-frequency region shows diffusion of ions within the active material, and the lowfrequency region shows the diffusion resistance. Usually, the initial intercept point of a Nyquist plot to the x-axis provides the solution resistance (Rs), and the extent of semicircle arc gives the charge-transfer resistance (Rct). From the Nyquist plots, the Rs values for the MWCNT, pristine WO3, and MWCNT-WO3 hybrid electrodes are 2.89, 3.85, and 3.3 U cm
2, respectively. In addition, it is observed that, in the high-frequency region, the MWCNT, WO3, and MWCNT-WO3 hybrid electrodes display very small semicircle arcs. The magnified Nyquist plots for pristine WO3 and MWCNTWO3 hybrid electrodes are shown in Fig. S6 of SI. The Rs (0.21 U cm
2) value of the MWCNT-WO3 hybrid electrode is small compared to pristine WO3 (0.3 U cm
2) because of the increase in electrical conductivity offered by the combination of MWCNTs. The long-term cycling stability is most important criteria for the practical application of the supercapacitor. The cycling stabilities of the pristine WO3 and MWCNT-WO3 hybrid electrodes were investigated by repeating CV cycles at a scan rate of 100 m V s
1 for 5000 cycles. Fig. 5h illustrates the capacity retention values of the pristine WO3 and MWCNT-WO3 hybrid electrodes with the cycle number. Capacity retention of 84.9% and 94.3% were obtained for the pristine WO3 and MWCNT-WO3 hybrid electrodes, respectively, after 5000 cycles. As can be seen, the MWCNT-WO3 hybrid electrode exhibits a good specific capacity and cycling performance compared to the pristine WO3. During the initial stage of cycling, the specific capacitance suddenly decreases, after which the active material becomes stable up to 5000 cycles. The initial diminish in specific capacitance during cycling is credited to the electrochemical behavior of the active material. It is known that metal oxides require a period of activation time to wet the material completely by the regular circulation of electrolyte ions in the interior of the active material. The electrochemical reactions are restricted to the surface of the electrode, which consequences in a diminish in the specific capacitance. Ultimately, with the repeated cycling, a large number of electrochemically active sites take part in a reaction and preserve the stability of the material up to 5000 cycles. Because of the good electrical conductivity and reaction kinetics of the MWCNT-WO3 hybrid electrode, the tungsten species could be easily oxidized to a higher oxidation state, and therefore the specific capacity of electrode increased with the cycle number. As the above findings demonstrated that the MWCNT-WO3 hybrid electrode showed improvements in the specific capacitance, rate capability, cyclic stability, and electrode resistance, which makes it a potential candidate for supercapacitor because of the following aspects. First, the interconnected MWCNT nanowires and WO3 nanorods provided a synergistic effect by taking advantage of both the EDLCs and pseudocapacitance for the charge storage, which appreciably improved the electrochemical performance. Second, the voids present between the nanowires and nanorods assisted with the simple access of the electrolyte ions, decrease the ion diffusion length, and enhanced the cycling lifetime of the electrode. Third, the MWCNTs offered a large surface area, excellent mechanical support, stabilize the resulting nanostructure, and facilitated efficient charge transport at the electrode/electrolyte interface during electrochemical reactions. Fourth, the preparation of a binder-free electrode significantly decreased the resistance and

239

showed superior contact between the active material and current collector.

3.3. Electrochemical evaluation of MnO2//MWCNT-WO3 asymmetric supercapacitor To determine the practical application of the MWCNT-WO3 hybrid in the progress of energy storage devices, a flexible all-solidstate ASC device was prepared. As the above results showed, the MWCNT-WO3 hybrid electrode exhibited superior performance in a negative operating potential range. Therefore, when fabricating the ASC device, MnO2 electrode was used to enlarge the operating potential window of the device. The schematics of the MnO2// MWCNT-WO3 ASC device fabrication is shown in Fig. 6. MnO2 was selected as a positive electrode because it has high theoretical specific capacitance (1350 F g
1), wide positive operating potential window (~0.8 V), low cost, and excellent electrochemical properties [55,56]. The details of the MnO2 synthesis, structural characterization, morphology, and electrochemical performance in a threeelectrode system with 1 M LiClO4 electrolyte are provided in Figs. S7eS10 of SI, respectively. The MnO2//MWCNT-WO3 ASC device was assembled using MnO2 as a positive electrode, MWCNTWO3 hybrid as a negative electrode, and PVA-LiClO4 as a gel electrolyte, and the electrochemical properties were investigated using a two-electrode configuration. Moreover, to get the optimal performance of the ASC device, the mass ratio of MnO2 to MWCNTWO3 to balance the charge qþ ¼ qe was determined from the following equation;

mþ Cs
 DE
¼ m
Csþ  DEþ

(10)

where, qþ =q
, mþ =m
, Csþ =Cs
and DEþ =DE
are the charge, mass, specific capacitance and potential window of the positive/ negative electrode, respectively. The mass ratio between positive and negative electrode was calculated to be 0.9:1.05. Both the MnO2 and MWCNT-WO3 hybrid electrodes showed stable performance at different potential windows in the LiClO4 electrolyte. The total electrochemical performance of the MnO2// MWCNT-WO3 ASC device arose as of the combined contributions of both these electrodes. Before evaluating the supercapacitor performance of the MnO2//MWCNT-WO3 ASC device, the first important task was to estimate the proper potential window for the

Fig. 6. Schematic representation of the fabrication process for flexible solid-state asymmetric supercapacitor device.

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sustainable functioning of the device. To accomplish that, CV measurements were recorded at various operating potential windows, as shown in Fig. 7a. All of the CV curves show a quasirectangular nature in respective of the different potential windows, demonstrating the excellent electrochemical behavior of the MnO2//MWCNT-WO3 ASC device. As seen from the CV curves, the operating potential window widens up to 1.4 V, after that it shows a bump, which may be due to the irreversible chemical reactions. Therefore, on the basis of potential windows of the individual electrodes and the above consequences, the MnO2//MWCNT-WO3 ASC device was able to perform reversible electrochemical reactions in the potential range of 0e1.4 V. The CV curves for the MnO2//MWCNT-WO3 ASC device were recorded at various scan rates ranging from 5 to 200 mV s
1, as displayed in Fig. 7b. All the CV curves maintained their quasi-rectangular shapes, still at the higher scan rates, representing the good electrochemical reversibility of the ASC device. Fig. 7c shows GCD curves of the MnO2// MWCNT-WO3 ASC device at various currents. On the basis of discharge times, the specific capacitance of the ASC device was calculated at various discharge currents and presented in Fig. 7d. The device delivered capacitance of 145.6 F g
1 at 2 mA. The specific energy and specific power are the main factors for a supercapacitor because they are directly related to its efficiency in practical applications. Based on the discharge curves, the specific

energy and specific power were evaluated and correlated with earlier reports in a Ragone plot, as shown in Fig. 7e. The MnO2// MWCNT-WO3 ASC device delivered maximum specific energy of 36.63 Wh kg
1 at a specific power of 546 W kg
1 and was capable of retaining specific energy of 27.6 Wh kg
1 at a specific power of 2221.3 W kg
1. Furthermore, the specific energy and specific power values attained for the MnO2//MWCNT-WO3 ASC device were compared to those from previous reports such as, rGO-WO3//AC (26.7 Wh kg
1, 6 kW kg
1) [53], NiO//Fe2O3 (12.4 Wh kg
1, 312 W kg
1) [57], WO3//AC (11.9 Wh kg
1, 210 W kg
1) [58], WO3PANI//PANI (9.72 Wh kg
1, 53 W kg
1) [59], CoWO4//AC (48 Wh kg
1, 365 W kg
1) [60], NiCo2O4‒NiWO4//AC (41.5 Wh kg
1, 760 W kg
1) [61], Co3O4@CoWO4
rGO//AC (19.99 Wh kg
1, 321 W kg
1) [62], and CNF//WO3 (35.3 Wh kg
1, 314 W kg
1) [63]. The results clearly demonstrated that the MnO2//MWCNT-WO3 ASC device showed excellent performance with superior specific energy without surrendering the other features such as a specific capacitance and specific power. The Nyquist plot of the MnO2//MWCNTWO3 ASC device is displayed in Fig. 7f. In the figure, the ideal capacitive feature is shown by a small semicircular arc in a highfrequency region and a straight line in the low-frequency region. The ASC device shows Rs and Rct values of 3.93 U cm
2 and 0.21 U cm
2, respectively. In addition, the inset of Fig. 7f shows an equivalent circuit fitted to the EIS data, which contains the Rs, Rct,

Fig. 7. (a) CV curves for MnO2//MWCNT-WO3 ASC device at different potential ranges, (b) CV curves for MnO2//MWCNT-WO3 ASC device at different scan rates, (c) GCD curves for MnO2//MWCNT-WO3 ASC device at different currents, (d) plot of specific capacitance versus current for MnO2//MWCNT-WO3 ASC device, (e) Ragone plot of specific energy versus specific power for MnO2//MWCNT-WO3 ASC device, (f) Nyquist plot for MnO2//MWCNT-WO3 ASC device (inset shows fitted equivalent circuit), (g) plot of capacity retention versus cycle number (inset shows CV curves for 1st and 10000th cycles) for MnO2//MWCNT-WO3 ASC device, (h) CV curves at different bending angles and (h) plot of capacity retention versus bending angle for MnO2//MWCNT-WO3 ASC device.

P.A. Shinde et al. / Electrochimica Acta 308 (2019) 231e242

Cdl, W, and constant phase element (CPE). The presence of CPE indicates the inhomogeneous accumulation of charge at the electrode/electrolyte interface owing to the non-uniform pore distribution. The small values of Rs and Rct signify the high conductivity of the ASC device. Moreover, the cycling stability of the ASC device was evaluated at a constant scan rate of 100 mV s
1 over 10000 CV cycles. Fig. 7g displays the capacity retention of the ASC device as a function of cycle number and inset figure illustrates the CV curves for 1st and 10000th cycle. The device conserves 77% of its initial capacitance value over 10000 CV cycles, representing that the stability of the ASC device is good. In this figure, it is seen that the capacity retention value of the ASC device increases in the early CV cycles and then decreases in succeeding cycles. However, this decrement is not more significant. The initial rise in capacitance signifies that the electrodes require an activation time to wet the material and open-up inner active sites during the electrochemical process. Furthermore, the flexibility of the supercapacitor device is essential for its application in various portable devices. To determine that, CV curves of the MnO2//MWCNT-WO3 ASC device were collected at various bending angles (0e180 ) at a scan rate of 100 mV s
1. These CV curves (Fig. 7h) display almost similar shape still at a bending angle of 180 without any significant distortion, demonstrating the good mechanical flexibility and stability of the device in the PVA-LiClO4 gel electrolyte. The capacity retention was also calculated at different bending angles, as shown in Fig. 7i. The ASC device retained 94.6% of its original capacitance at a bending angle of 180 . The bending performance clearly shows that the device has excellent mechanical flexibility, as well as good feasibility and strong adhesion of the active material to the substrate. In addition to this, mechanical simulation study was performed at different bending angles using von Mises stress ansys (commercial

241

FEM simulation package) to check stress distribution of the MnO2// MWCNT-WO3 ASC device, as shown in Fig. 8 (aed). The maximum stress applied for ASC device is as high as 2 MPa. As seen from figure, the stress is evenly distributed at different bending angles, even though at high bending angle of 180 . The good bending performance enables that as prepared MnO2//MWCNT-WO3 ASC device have excellent flexibility and fulfil the mechanical requirements. The superior electrochemical performance of the MnO2// MWCNT-WO3 ASC device could be the consequence of different factors, together with the synergistic contributions of both electrodes (MnO2 and MWCNT-WO3) and the interactive compatibility with the gel electrolyte. Here, a great effort was made in the preparation of the negative electrode, which was complementary to carbon-based anodes. The improved electrochemical performance of the ASC device would be a result of various factors. First, the binder-free synthesis of both electrodes eliminated the resistance that arises from the binder or additives and a dead surface area. Second, the MnO2 nanoneedles utilized a pseudocapacitive mechanism to store charge, which again assisted in improving the energy density of the ASC device. Additionally, these nanoneedles provided more electrochemically active sites during the electrochemical process. Third, the different potential windows of the MnO2 and MWCNT-WO3 hybrid electrodes in the same electrolyte offered a wide potential window for the ASC device. Overall, the proposed approach provided the potential for attaining highperformance ASC devices for future energy storage. Further, studies are continuing to enhance the performance of the ASC device in provisions of its capacitance, energy density, and longterm cycling stability.

Fig. 8. Simulation images for bending of MnO2//MWCNT-WO3 ASC device at different bending angles (a) 45 , (b) 90 , (c) 135 and (d) 180 .

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4. Conclusions In summary, a simple and binder-free hydrothermal method was adopted to fabricate an MWCNT-WO3 hybrid on carbon cloth as a high-performance anode material for an ASC device. The MWCNT-WO3 hybrids obtained through two-step method featured various advantages, including high conductivity, large number of electron conduction pathways, and highly active surface area for the storage of charge. The unique MWCNT-WO3 hybrid nanostructures provided a large specific surface area of 67.54 m2 g
1 and proper pore volume to increase the electrolyte ions access to the active material by minimizing the path length. These features led to the superb electrochemical performance of the MWCNT-WO3 hybrid electrode, with a high specific capacitance (areal capacitance) of 429.6 F g
1 (1.55 F cm
2) and capacity retention of 94.3% after 5000 cycles, which were higher than the 155.6 F g
1 (0.43 F cm
2) and 84.9% for pristine WO3. Thus, the excellent performance of the negative electrode improved the performance of the MnO2//MWCNT-WO3 ASC device. The as-assembled ASC device achieved a potential window of 1.4 V and specific capacitance of 145.6 F g
1 at 2 mA. More importantly, the ASC device demonstrated high specific energy of 39.63 Wh kg
1 at a specific power of 546 W kg
1, and superior long-term cycling stability (77% over 10000 cycles). The excellent electrochemical performance of the MnO2//MWCNT-WO3 ASC device indicated that the MWCNT-WO3 hybrid could be a capable material for the future development of high-performance energy storage devices. Conflicts of interest There are no conflicts to declare. Acknowledgements This research was partially supported by Nanomaterial Technology Development Program (NRF-2017M3A7B4041987) and the Korean Government (MSIP) (No. 2015R1A5A1037668). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.03.159. References [1] Z.H. Huang, Y. Song, D.Y. Feng, Z. Sun, X. Sun, X.X. Liu, ACS Nano 12 (2018) 3557. [2] D.P. Dubal, N.R. Chodankar, D.H. Kim, P. GomezeRomero, Chem. Soc. Rev. 47 (2018) 2065. [3] H. Zhang, Y. Qiao, Z. Lu, ACS Appl. Mater. Interfaces 8 (2016) 32317. [4] M. Yao, X. Zhao, L. Jin, F. Zhao, J. Zhang, J. Dong, Q. Zhang, Chem. Eng. J. 322 (2017) 582. [5] N.R. Chodankar, D.P. Dubal, A.C. Lokhande, A.M. Patil, J.H. Kim, C.D. Lokhande, Sci. Rep. 6 (2016) 39205. [6] D.P. Dubal, O. Ayyad, V. Ruiz, P. Gomez-Romero, Chem. Soc. Rev. 44 (2015) 1777. [7] X. Xia, Y. Zhang, D. Chao, C. Guan, Y. Zhang, L. Li, X. Ge, I.M. Bacho, J. Tu, H.J. Fan, Nanoscale 6 (2014) 5008. [8] J. Cheng, M. Sprik, Phys. Chem. Chem. Phys. 14 (2012) 11245. [9] K.V. Sankar, Y. Seo, S.C. Lee, S.C. Jun, ACS Appl. Mater. Interfaces 10 (2018) 8045. [10] A.V. Radhamani, K.M. Shareef, M.S.R. Rao, ACS Appl. Mater. Interfaces 8 (2016) 30531. [11] J. Zhao, Z. Li, M. Zhang, A. Meng, Q. Li, ACS Sustain. Chem. Eng. 4 (2016) 3598. [12] R. Kumar, P. Raj, A. Sharma, J. Mater. Chem. 4 (2016) 9831. [13] S. He, W. Chen, Nanoscale 7 (2015) 6957e6990. [14] M. Sevilla, R. Mokaya, Energy Environ. Sci. 7 (2014) 1250. [15] C. Shen, R. Li, L. Yan, Y. Shi, H. Guo, J. Zhang, Y. Lin, Z. Zhang, Y. Gong, L. Niu,

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